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IRAS F08520−6850 (ESO 060−IG16 NE and SW): This advanced merger (stage c) is detected both by NuSTAR and Chandra. A compact point source is detected in the Chandra image, overlapping with the nucleus of the NE galaxy (Iwasawa et al. 2011). The source shows [Ne v] emission in the MIR (Inami et al. 2013), and is classified as an AGN also considering the Chandra hardness ratio (Iwasawa et al. 2011). The X-ray spectrum is well fitted by the AGN model, with the X-ray source being obscured by a line-of-sight column density of |$N_{\rm H}=1.5^{+1.0}_{-0.6}\times 10^{23}\rm \, cm^{-2}$|⁠, consistent with what was previously found by Iwasawa et al. (2011) using the Chandra data.

C Ricci, G C Privon, R W Pfeifle, L Armus, K Iwasawa, N Torres-Albà, S Satyapal, F E Bauer, E Treister, L C Ho, S Aalto, P Arévalo, L Barcos-Muñoz, V Charmandaris, T Diaz-Santos, A S Evans, T Gao, H Inami, M J Koss, G Lansbury, S T Linden, A Medling, D B Sanders, Y Song, D Stern, V U, Y Ueda, S Yamada, A hard X-ray view of luminous and ultra-luminous infrared galaxies in GOALS – I. AGN obscuration along the merger sequence, Monthly Notices of the Royal Astronomical Society, Volume 506, Issue 4, October 2021, Pages 5935–5950, https://doi.org/10.1093/mnras/stab2052

While a growing number of observations have demonstrated that the obscuration properties of AGN in mergers are very different from those of AGN in isolated galaxies (see Ramos Almeida & Ricci 2017; Hickox & Alexander 2018, for recent reviews), it is still unclear how AGN obscuration evolves in the last phases of the merger process, when the two nuclei are at a projected separation of dsep < 10 kpc. With the goal of addressing this important issue, and to increase the number of sources with dsep < 10 kpc, in this work we double, with respect to Ricci et al. (2017b), the number of U/LIRGs from the GOALS sample observed in the hard X-rays by NuSTAR. GOALS is a sample of nearby (z < 0.088) galaxies detected by the Infrared Astronomical Satellite (IRAS) revised bright Galaxy Survey (Sanders et al. 2003), which has a very wealthy collection of ancillary data across the whole multiwavelength spectrum (e.g. Howell et al. 2010; Petric et al. 2011; Stierwalt et al. 2013). Exploiting the excellent constraints on the AGN obscuration obtained by broad-band X-ray observations, we study here the relation between obscuration and merger stage, focusing in particular on the final stages of the merger process. A companion paper (Yamada et al. 2021) focuses on the physical X-ray modelling of these sources, to constrain the covering factor of the torus from X-ray spectroscopy, and on the X-ray-to-bolometric AGN luminosity ratios, to further discuss their nuclear properties in comparison with normal AGNs.

Gabriel C. et al. , 2004, in Ochsenbein F., Allen M. G., Egret D., eds, ASP Conf. Ser. Vol. 314, Astronomical Data Analysis Software and Systems (ADASS) XIII. Astron. Soc. Pac, San Francisco, p. 759

Multiwavelength tracers of AGN activity can help discover heavily obscured accreting SMBHs that cannot be identified in the X-rays. In a companion paper (Ricci et al. in preparation), we discuss in more detail these proxies of AGN activity for our sample of U/LIRGs, comparing them with the AGN X-ray emission. Considering [Ne v] as a good tracer of AGN activity (e.g. Weedman et al. 2005; Armus et al. 2006; Goulding & Alexander 2009; Petric et al. 2011), we can identify only one accreting SMBH (in the late-stage merger IRAS F23128−5919) that is not an X-ray-detected AGN. Assuming that the [Ne v] 14.32 |$\mu$|m emission is entirely due to the AGN, and invoking the [Ne v]/X-ray correlation from Satyapal et al. (2007), we would expect this source to have a bolometric AGN luminosity of |$6\times 10^{44}\rm \, erg\, s^{-1}$|⁠. Using a 2–10 keV bolometric correction of κX = 20 (e.g. Vasudevan & Fabian 2007), the source would need to be obscured by |$N_{\rm H}\gtrsim 1.9\times 10^{24}\rm \, cm^{-2}$| to have a 2–10 keV AGN luminosity consistent with the upper limit inferred by our study (Table 2), which is reasonable given the NH distribution of the other AGN.

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The bulk of the growth of SMBHs during mergers is believed to be very obscured. This has been shown by numerical simulations (e.g. Hopkins et al. 2008; Blecha et al. 2018; Kawaguchi, Yutani & Wada 2020), as well as by recent observations. Satyapal et al. (2014) have shown that post-mergers host a significantly higher fraction of mid-IR-selected AGNs than optical AGNs, which could suggest that optically obscured AGNs become prevalent in the most advanced mergers (see also Koss et al. 2010; Ellison et al. 2019; Secrest et al. 2020). Kocevski et al. (2015) have shown that heavily obscured (⁠|$N_{\rm \, H}\ge 10^{23.5}\rm \, cm^{-2}$|⁠) systems are more common in mergers than in isolated galaxies. In the local Universe, major galaxy mergers give rise to luminous infrared galaxies [LIRGS; |$L_{\rm \, IR}(8\!-\!1000\, \mu \rm m)=10^{11}\!-\!10^{12}$| L⊙] and ULIRGs that, over the past two decades, have been extensively studied in the IR, optical, and soft (0.3–10 keV) X-ray bands (e.g. Veilleux et al. 1995; Veilleux, Kim & Sanders 1999; Imanishi & Dudley 2000; Imanishi 2002; Franceschini et al. 2003; Alonso-Herrero et al. 2006, 2012; Imanishi, Dudley & Maloney 2006; Armus et al. 2007, 2009; Teng and Veilleux 2010;Nardini & Risaliti 2011; Pereira-Santaella et al. 2011; Armus, Charmandaris & Soifer 2020). Mid-IR observations have suggested the presence of a heavily buried AGN in U/LIRGs, particularly in those undergoing the final stages of mergers (e.g. Imanishi et al. 2007; Veilleux et al. 2009; Nardini et al. 2010). Hard X-ray (≥10 keV) observations can be extremely effective in detecting heavily obscured AGNs and, combined with soft X-ray observations (<10 keV), in estimating their line-of-sight column density (e.g. Burlon et al. 2011; Annuar et al. 2015; Ricci et al. 2015, 2017c; Koss et al. 2016a, b). U/LIRGs were studied in the hard X-ray band using Swift/BAT by Koss et al. (2013), who suggested that a large fraction of sources might have Compton-thick (CT) column densities. Exploiting the revolutionary capabilities of NuSTAR, the first focusing hard X-ray satellite on orbit, Ricci et al. (2017b) studied 30 nearby U/LIRGs from the Great Observatories All-sky LIRG Survey (GOALS; Armus et al. 2009) sample.1 Ricci et al. (2017b) showed that |$65^{+12}_{-13}{{\ \rm per\ cent}}$| of the AGNs in objects in late-stage mergers (i.e. with projected separations of dsep ≃ 10 kpc) are CT (⁠|$N_{\rm H}\ge 10^{24}\rm \, cm$|⁠), a fraction significantly higher than what is found for local hard X-ray-selected AGNs (⁠|$27\pm 4{{\ \rm per\ cent}}$|⁠; Ricci et al. 2015), which are typically found in non-interacting systems. Similar results have also been found by several other studies, which find that AGNs in mergers are systematically more obscured than those in isolated galaxies (e.g. Nardini & Risaliti 2011; Lanzuisi et al. 2015; Del Moro et al. 2016; Koss et al. 2016a, 2018; Satyapal et al. 2017; Dutta et al. 2018, 2019;Donley et al. 2018; Goulding et al. 2018; Pfeifle et al. 2019a, b; Secrest et al. 2020; Foord et al. 2021; Guainazzi et al. 2021). At higher luminosities and redshifts, X-ray observations of Hot DOGs have shown that these powerful AGNs are also typically very obscured (e.g. Piconcelli et al. 2015; Ricci et al. 2017a; Zappacosta et al. 2018; Toba et al. 2020).

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The overall fraction of CT AGNs for the sample is 16/35 (⁠|$46\pm 8{{\ \rm per\ cent}}$|2), while that of AGNs in merging galaxies is 13/30 (⁠|$44^{+8}_{-9}{{\ \rm per\ cent}}$|⁠). This is significantly higher than what is inferred for the hard X-ray-selected Swift/BAT sample overall (⁠|$27\pm 4{{\ \rm per\ cent}}$|⁠; Ricci et al. 2015, 2017c), which is mostly composed of AGNs in isolated galaxies (Section 2.3). The sample of AGNs in U/LIRGs shows a larger fraction of both heavily obscured (⁠|$N_{\rm \, H}\ge 10^{23}\rm \, cm^{-2}$|⁠; |$82_{-7}^{+5}{{\ \rm per\ cent}}$|⁠) and obscured (⁠|$N_{\rm \, H}\ge 10^{22}\rm \, cm^{-2}$|⁠; |$90^{+4}_{-6}{{\ \rm per\ cent}}$|⁠) sources than the Swift/BAT sample (⁠|$52\pm 4{{\ \rm per\ cent}}$| and |$70\pm 5{{\ \rm per\ cent}}$|⁠, respectively). This confirms the idea that the typical environment of these AGNs is different from that of AGNs in isolated galaxies, and that the obscuring medium almost fully covers the accreting SMBHs.

Alonso-Herrero A., Rieke G. H., Rieke M. J., Colina L., Pérez-González P. G., Ryder S. D., 2006, ApJ, 650, 835 10.1086/506958

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The paper is structured as follows: In Section 2, we describe our sample. In Section 3, we present the X-ray data used and the methodology for the data reduction. In Section 4, we discuss the spectral analysis of the sources. In Section 5, we discuss the relation between mergers and AGN obscuration. Our main results are summarized in Section 6. Throughout the paper, we adopt standard cosmological parameters (⁠|$H_{0}=70\rm \, km\, s^{-1}\, Mpc^{-1}$|⁠, Ωm = 0.3, and ΩΛ = 0.7). Unless otherwise stated, uncertainties are quoted at the 90 per cent confidence level.

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Observed 2–10 keV X-ray luminosities (see Table 2) versus SFR (see Table 1) for the sources of our sample divided into X-ray AGN (red empty stars) and X-ray non-AGN (black filled diamonds). The X-ray non-AGNs have 2–10 keV luminosities, consistent, or lower, than the values expected from their SFR (black continuous line, Ranalli, Comastri & Setti 2003; blue dot–dashed line, Lehmer et al. 2010), while X-ray AGNs are typically more luminous.

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Fraction of CT (⁠|$N_{\rm H}\ge 10^{24}\rm \, cm^{-2}$|⁠; top panel) and heavily obscured (⁠|$N_{\rm H}\ge 10^{23}\rm \, cm^{-2}$|⁠; bottom panel) AGNs in our sample versus the projected separation between the two nuclei. The fraction of heavily obscured sources appears to be consistently higher than Swift/BAT AGN (red continuous line; Ricci et al. 2015, 2017c), and a tentative peak in the fraction of CT AGN is found at a separation of a few kpc. Fractions are calculated following Cameron (2011), and the uncertainties quoted represent the 16th/84th quantiles of a binomial distribution, obtained using the Bayesian approach outlined in Cameron (2011).

IRAS F20551−4250 (ESO 286−IG19): This system is a late-stage merger with a single nucleus, which is clearly detected by Chandra and XMM–Newton, but it is not detected by NuSTAR. The soft X-ray emission observed in Chandra is elongated, consistent with an SF-related origin, while the hard X-ray emission is point-like, with some fainter elongation (Iwasawa et al. 2011). Our fit with the SF model leaves clear residuals around ∼6 keV. We tested our AGN model, fixing Γ = 1.8 (e.g. Ricci et al. 2017c), and found that it provides a significant improvement on the fit. However, the expected 10–24 keV luminosity from this model (⁠|$L_{10{-}24}=1.3\times 10^{42}\rm \, erg\, s^{-1}$|⁠) is above the upper limit obtained by our NuSTAR observations (⁠|$L_{10{-}24}\le 3.2\times 10^{41}\rm \, erg\, s^{-1}$|⁠). This suggests that the excess is not associated with an AGN. Including a Gaussian line to the SF model, with width fixed to 10 eV, improved the fit; the energy of the line is |$6.60^{+0.05}_{-0.06}$| keV, which suggests emission from Fe xxv.

Lehmer B. D., Alexander D. M., Bauer F. E., Brandt W. N., Goulding A. D., Jenkins L. P., Ptak A., Roberts T. P., 2010, ApJ, 724, 559 10.1088/0004-637X/724/1/559

With the goal of understanding the evolution of AGN obscuration in the final phase of a galaxy merger, we report here the results obtained by studying 60 U/LIRGs from GOALS, effectively doubling the sample observed by NuSTAR presented by Ricci et al. (2017b; see Table 2). This was done including, besides the 23 sources presented here, the 30 objects reported in Ricci et al. (2017b), and several sources reported in recent literature (e.g. Iwasawa et al. 2020; Privon et al. 2020; Yamada et al. 2020). Using X-ray spectroscopy, we identify a total of 35 AGNs in these systems, 5 of which are associated with U/LIRGs in the N stage (i.e. showing no clear sign of interactions). In the following, we will refer to objects that were not identified as AGN in the X-rays, and for which only an upper limit of the AGN X-ray luminosity is reported in Table 2, as X-ray non-AGN.

X-ray observation log. The table reports the name of the IRAS source and of the counterparts (columns 1 and 2, respectively), as well the X-ray observatory used (3), the ID (4), date (5), and exposure (6) of the observation.

IRAS F08354+2555 (NGC 2623): A hard point source is detected by Chandra coincident with the position of the advanced merger NGC 2623 (see also Torres-Albà et al. 2018). The source is also detected by both XMM–Newton and NuSTAR. As discussed in Ricci et al. (in preparation), this system shows clear [Ne v] emission from Spitzer/IRS spectra (Inami et al. 2013), which suggests that it hosts an AGN. Based on the Chandra hardness ratio, the source is also classified as a candidate obscured AGN by Torres-Albà et al. (2018). We therefore use our AGN model for the spectral fit, which was able to reproduce well the broad-band X-ray emission. We find that the AGN is only mildly obscured, and has one of the lowest column densities in our sample for an AGN in late-stage mergers (⁠|$N_{\rm H}=7.1^{+10.7}_{-3.1}\times 10^{22}\rm cm^{-2}$|⁠).

IRAS 07251−0248: Chandra shows a faint point source consistent with this advanced merger. The X-ray spectrum could be well fitted by a simple power-law model.

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Roughly half (⁠|$53^{+11}_{-12}{{\ \rm per\ cent}}$|⁠) of the AGNs in galaxies undergoing the final stages of mergers are CT. This fraction of CT sources is higher than that found in AGNs in early mergers (⁠|$33\pm 12{{\ \rm per\ cent}}$|⁠) and in local hard X-ray-selected AGNs. Considering the X-ray non-detections of objects that are identified as AGN in the IR (Section 5.4), the fraction of CT AGNs in late-stage mergers value would be higher (⁠|$69^{+8}_{-9}{{\ \rm per\ cent}}$|⁠).

Schawinski K., Simmons B. D., Urry C. M., Treister E., Glikman E., 2012, MNRAS, 425, L61 10.1111/j.1745-3933.2012.01302.x

Stage d: galaxies in the final merger stages, with the two nuclei being in a common envelope or showing only a single nucleus.

IRAS F01053−1746 (IC 1623A and IC 1623B): The Chandra image of this advanced merger system shows an extended source in the 0.3–10 keV band, which covers both galaxies. As reported by Garofali et al. (2020), the 0.3–30 keV X-ray emission can be described by the superposition of several point sources and some diffuse emission, all ascribed to SF. The source is clearly detected by NuSTAR. In order to be consistent with the XMM–Newton and NuSTAR observations, we use an extraction radius of 20 arcsec for the Chandra observation, to encompass both sources. The overall X-ray spectrum is soft, and can be well reproduced by our SF model, consistent with Garofali et al. (2020).

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The discovery of a correlation between the mass of supermassive black holes (SMBHs) and several properties of their host galaxies (e.g. Magorrian et al. 1998; Ferrarese & Merritt 2000; Gebhardt et al. 2000; Kormendy & Ho 2013) has suggested that the growth of SMBHs and their host galaxies are tightly connected. Mergers of galaxies are thought to be one of the most important mechanisms with which galaxies build up their stellar masses (White & Rees 1978). Both observational (e.g. Lonsdale, Persson & Matthews 1984; Joseph & Wright 1985; Armus, Heckman & Miley 1987; Clements et al. 1996; Alonso-Herrero et al. 2000; Ellison et al. 2008) and theoretical (e.g. Mihos & Hernquist 1996; Di Matteo et al. 2007) studies have shown that galaxy mergers enhance star formation (SF). Simulations have also shown that the interaction between two or more galaxies can reduce the angular momentum of the circumnuclear material (e.g. Barnes & Hernquist 1991; Blumenthal & Barnes 2018), thus providing an effective mechanism to trigger accretion on to SMBHs (e.g. Di Matteo, Springel & Hernquist 2005). Observationally, several works have confirmed this scenario. Koss et al. (2010) and Silverman et al. (2011) found a higher AGN fraction in pairs than in isolated galaxies with similar stellar masses. It has been shown that the fraction of AGN in mergers tends to increase as the separation between the two galaxies decreases (Ellison et al. 2011), and peaks after coalescence (Ellison et al. 2013). Koss et al. (2012) have shown that the average luminosity of dual AGN also increases with decreasing separation (see also Hou, Li & Liu 2020), and it is higher for the primary (i.e. more massive) component of the system (see also De Rosa et al. 2019, for a recent review). While AGNs with moderate X-ray luminosities are typically found in non-interacting disc galaxies (e.g. Koss et al. 2011; Kocevski et al. 2012; Schawinski et al. 2012), more luminous objects are commonly found in merging systems (e.g. Treister et al. 2012; Glikman et al. 2015; Hong et al. 2015). Treister et al. (2012) showed that, while for 2–10 keV AGN luminosities of |$L_{2-10}\sim 10^{41}\rm \, erg\, s^{-1}$| only a small fraction (⁠|${\lt}1{{\ \rm per\ cent}}$|⁠) of AGNs are in mergers, at |$L_{2-10}\sim 10^{46}\rm \, erg\, s^{-1}$| ∼ 70–80 per cent of the sources are found in interacting systems (see also Glikman et al. 2015). Recent evidence has suggested that hot dust obscured galaxies (Hot DOGs; Wu et al. 2012; Assef et al. 2015), which are some of the most luminous galaxies observed so far (⁠|$L_{\rm \, IR} \gt 10^{13} \mathrm{ L}_{\odot }$|⁠), are also found in mergers (e.g. Fan et al. 2016). These observations suggest that, while at low luminosities SMBH accretion is triggered by secular processes, at high luminosities mergers can play a dominant role. This is in agreement with the evolutionary scenario proposed by Sanders et al. (1988) for ultra-luminous [|$L_{\rm \, IR}(8\!-\!1000\, \mu \rm m)\ge 10^{12}\,L_{\odot }$|] infrared galaxies (ULIRGs; e.g. Sanders & Mirabel 1996; Pérez-Torres et al. 2021). In this scheme, two gas-rich disc galaxies collide, triggering SF and accretion on to the SMBH. The strong accretion on to the SMBH would lead the source to evolve first in a luminous red quasar (e.g. Urrutia, Lacy & Becker 2008; Glikman et al. 2015; LaMassa et al. 2016) and then in an unobscured blue quasar.

In this work, we have studied broad-band X-ray observations of 60 nearby U/LIRGs from the GOALS sample to understand the link between AGN obscuration and galaxy mergers. A total of 35 X-ray-detected AGNs are identified in these systems, 30 of which reside in merging galaxies. We find that

Ichikawa K., Ricci C., Ueda Y., Matsuoka K., Toba Y., Kawamuro T., Trakhtenbrot B., Koss M. J., 2017, ApJ, 835, 74 10.3847/1538-4357/835/1/74

Garmire G. P., Bautz M. W., Ford P. G., Nousek J. A., Ricker G. R. Jr., 2003, in Truemper J. E., Tananbaum H. D., eds, Proc. SPIE Conf. Ser. Vol. 4851, X-Ray and Gamma-Ray Telescopes and Instruments for Astronomy. SPIE, Bellingham, p. 28 10.1117/12.461599

IRAS 14378−3651: Chandra shows the presence of a point-like source consistent with the nucleus of the galaxy, plus some soft, extended, X-ray emission (Iwasawa et al. 2011). The source is not detected by NuSTAR, and the X-ray emission is well represented by a power-law component.

The star formation rates (SFRs) and IR luminosities were taken from Díaz-Santos et al. (2017). The SFRs were obtained based on the host galaxy IR luminosity (excluding the AGN contribution estimated by Díaz-Santos et al. 2017), using the relation reported by Murphy et al. (2011). We privileged these values rather than the more recent compilation of Shangguan et al. (2019), since it allowed us to recover the SFRs for the individual galactic nuclei. We tested the SFRs of Shangguan et al. (2019), and found results consistent with those we obtained using the aforementioned approach. For three objects in our sample, which were not reported in Díaz-Santos et al. (2017), we used values from recent literature. For NGC 1068 and NGC 1365, we used the SFRs obtained by Ichikawa et al. (2017, 2019), while for the Hickson compact Group 16 (Hickson 1982) we used the values reported in O’Sullivan et al. (2014) and Bitsakis et al. (2014).

Dividing our sample into different merger stages, excluding the N-stage galaxies, we have 13 and 17 AGNs in early- and late-stage mergers, respectively. We find that 4/13 (⁠|$33\pm 12{{\ \rm per\ cent}}$|⁠) AGNs in early mergers (i.e. objects classified as being in a or b stages) are CT, a fraction in good agreement with what is found for the Swift/BAT sample. This shows that it takes time to build up the obscuration, since at the beginning of the merger the fraction of CT AGNs is not significantly higher than the comparison sample. A large fraction of these objects (10/13 or |$74^{+11}_{-12}{{\ \rm per\ cent}}$|⁠) are heavily obscured, and almost all of them (11/13 or |$81^{+9}_{-11}{{\ \rm per\ cent}}$|⁠) are obscured. AGNs in the late phases of the merger process (i.e. having c or d stages) show a higher fraction of CT AGNs (9/17 or |$53^{+11}_{-12}{{\ \rm per\ cent}}$|⁠) than both hard X-ray-selected AGNs and AGNs in early-stage mergers. This is consistent with what was found by Ricci et al. (2017b), and Guainazzi et al. (2021) using XMM–Newton, who found that |${\sim}47{{\ \rm per\ cent}}$| of the objects in their Sloan Digital Sky Survey optically selected sample are CT. Most of the AGNs in late-stage mergers are heavily obscured (15/17 or |$85^{+7}_{-9}{{\ \rm per\ cent}}$|⁠), and all of them are obscured. The difference between early mergers, late mergers, and hard X-ray-selected AGNs is clearly illustrated in Fig. 3. AGNs in the final phases of the merger process are consistently more heavily obscured than hard X-ray-selected AGNs, and do not show the tail of objects (⁠|${\sim}30{{\ \rm per\ cent}}$|⁠) with |$\log (N_{\rm H}/\rm cm^{-2})\le 23$| found in early mergers. Interestingly, 3/5 of the U/LIRGs in the N stage are CT (⁠|$58^{+18}_{-19}{{\ \rm per\ cent}}$|⁠), and 4/5 (⁠|$74^{+14}_{-19}{{\ \rm per\ cent}}$|⁠) are heavily obscured. This could be related to the fact that several of these systems are post-mergers.

IRAS F18293−3413: Chandra shows resolved X-ray emission in both the soft and hard bands (Iwasawa et al. 2011) of this minor merger, classified as stage N (Ricci et al. 2017b). The source is detected by both XMM–Newton and NuSTAR, albeit for the latter only in the 3–10 keV band. The combined spectra can be well represented by the SF model plus an additional emission feature, associated with Fe xxv (e.g. Iwasawa et al. 2009). The data show in fact an excess at ∼6.6 keV, which can be well represented by a Gaussian line with a width fixed to σ = 10 eV, and energy of |$6.64^{+0.12}_{-0.11}$| keV. The equivalent width of the line is |$360^{+181}_{-162}$| eV. A detailed analysis of Fe K α emission lines in GOALS objects will be presented in a forthcoming dedicated paper (Iwasawa et al., in preparation).

Koss M., Mushotzky R., Treister E., Veilleux S., Vasudevan R., Trippe M., 2012, ApJ, 746, L22 10.1088/2041-8205/746/2/L22

Sample of 60 U/LIRGs from GOALS with NuSTAR observations. (1) IRAS name, (2) counterparts, (3) redshift, (4) merger stage, (5) projected separation between the two nuclei in arcsec and (6) in kpc, (7) star formation rate (SFR) estimated from the IR luminosity excluding AGN contribution, and (8) |$8\!-\!1000\, \mu$|m IR luminosity. In (5) and (6), we report ‘S’ for objects for which a single nucleus is observed. The sources classified as a and b are early-stage mergers, while those in c and d are late-stage mergers. Sources in the N class are those that do not show any clear sign of merger. In the objects in which more than one SFR or IR luminosity are reported, we listed the values for both nuclei.

Iwasawa K., Sanders D. B., Evans A. S., Mazzarella J. M., Armus L., Surace J. A., 2009, ApJ, 695, L103 10.1088/0004-637X/695/1/L103

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Top panel: Histogram of the distance to the objects in the GOALS sample (blue continuous line), to our sample (red dashed line), and to the X-ray AGN in our sample (black dashed line). Middle panel: Histogram of the |$8\!-\!1000\, \mu$|m IR luminosity. Bottom panel: Histogram of the projected separation between the two nuclei of the merging galaxies.

IRAS F12112+0305: The Chandra image shows two sources, coincident with the two optical nuclei (Iwasawa et al. 2011). The combined Chandra/XMM–Newton spectrum is well fitted by our SF model. The source is not detected by NuSTAR.

IRAS F10038−3338: A compact source is detected in the Chandra image, coincident with the position of this late-stage merger galaxy (Iwasawa et al. 2011). The source is not detected in the NuSTAR observation, and the X-ray spectrum is well fitted by the SF model.

Kalberla P. M. W., Burton W. B., Hartmann D., Arnal E. M., Bajaja E., Morras R., Pöppel W. G. L., 2005, A&A, 440, 775 10.1051/0004-6361:20041864

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Column density cumulative distribution function for the AGN in our sample found in early (dashed blue line) and late mergers (continuous orange line). AGNs found in galaxies undergoing the final phases of a merger are more obscured than those in the early mergers, and than hard X-ray-selected nearby AGNs (dotted purple line; Ricci et al. 2015, 2017c).

Mori K. et al. , 2016, in den Herder J.-W. A., Takahashi T., Bautz M., eds, Proc. SPIE Conf. Ser. Vol. 9905, Space Telescopes and Instrumentation 2016: Ultraviolet to Gamma Ray. SPIE, Bellingham, p. 99051O

Weisskopf M. C., Tananbaum H. D., Van Speybroeck L. P., O’Dell S. L., 2000, in Truemper J. E., Aschenbach B., eds, Proc. SPIE Conf. Ser. Vol. 4012, X-Ray Optics, Instruments, and Missions III. SPIE, Bellingham, p. 2

The results of the spectral fitting performed here are reported in Table B1. Details on the spectral fitting approach can be found in Section 4.1.

Stage N: sources that do not appear to be in a major merger. These sources could either be post-mergers or minor mergers.

Koss M., Mushotzky R., Baumgartner W., Veilleux S., Tueller J., Markwardt C., Casey C. M., 2013, ApJ, 765, L26 10.1088/2041-8205/765/2/L26

Madsen K. K. et al. , 2018, in den Herder J.-W. A., Nikzad S., Nakazawa K., eds, Proc. SPIE Conf. Ser. Vol. 10699, Space Telescopes and Instrumentation 2018: Ultraviolet to Gamma Ray. SPIE, Bellingham, p. 106996M

Bitsakis T., Charmandaris V., Appleton P. N., Díaz-Santos T., Le Floc'h E., da Cunha E., Alatalo K., Cluver M., 2014, A&A, 565, A25 10.1051/0004-6361/201323349

Anti-glare coating is a treatment method on glass that ensures external light sources, such as bright sunlight or other interior ambient lighting, do not affect ...

IRAS F14348−1447 (NE and SW): The Chandra image shows some diffuse X-ray emission, together with two point sources, with the southern one being brighter (Iwasawa et al. 2011). The source is not detected by NuSTAR, and our SF model can well represent the X-ray spectrum. We also looked at the individual properties of the two nuclei in the Chandra observations, selecting circular regions of 2 arcsec around the sources. Due to the low signal-to-noise ratio, the two spectra are fitted with a simple power-law model.

As a comparison sample, similarly to what was done in Ricci et al. (2017b), we use AGNs reported in the Swift/BAT 70-month catalogue (Baumgartner et al. 2013), which were selected in the 14–195 keV band. Studying optical images, Koss et al. (2010) showed that only |${\sim}25{{\ \rm per\ cent}}$| of the AGNs detected by BAT are found in major mergers with a nuclear separation dsep ≲ 100 kpc. The broad-band (0.3–150 keV) X-ray spectra of these ∼840 AGNs have been analysed in detail by Ricci et al. (2017c), who reported values of the column density for |${\sim}99.8{{\ \rm per\ cent}}$| of them. The obscuration properties of the ∼730 non-blazar AGNs in the sample were discussed in Ricci et al. (2015, 2017d), who found that |$27\pm 4{{\ \rm per\ cent}}$| of the objects are CT, and |$70{{\ \rm per\ cent}}$| of them are obscured [|$\log (N_{\rm H}/\rm cm^{-2})\ge 22$|].

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Considering MIR photometry, and assuming a W1 − W2 > 0.8 threshold for AGN activity (Stern et al. 2012), we find 13 sources that are classified as AGN with WISE (including IRAS F23128−5919), but were not identified by our broad-band X-ray analysis. Of these, 11 are in the final stages (c and d) of the merger process, one is not a merger (N), and another is stage b. This would be even further evidence for the CT nature of most late-stage mergers. We use the method outlined by Pfeifle et al. (2021; see their equation 2), assuming that the 12 |$\mu$|m emission is dominated by the AGN in these sources, to get constraints on the NH needed for these sources to remain undetected by NuSTAR. We find that all the lower limits on the column density are above |${\sim}3\times 10^{24}\rm \, cm^{-2}$|⁠. Including these lower limits to our sample, a total of |$69^{+8}_{-9}{{\ \rm per\ cent}}$| of the AGNs in the final stages of mergers would be CT. Dividing this sample according to the nuclear separation, we find that the peak of the CT fraction would again be found at ∼0.4−6 kpc (⁠|$85^{+8}_{-12}{{\ \rm per\ cent}}$|⁠), while |$62^{+13}_{-14}{{\ \rm per\ cent}}$| of the AGNs in merging galaxies that show a single galactic nucleus would be CT. It should be noted that, considering these candidate AGNs, we are still able to detect with NuSTAR the accreting SMBHs that are contributing to most of the overall IR emission. Using the 12 |$\mu$|m AGN emission for the WISE-selected candidate AGNs that were not detected in the X-rays, considering a typical AGN IR spectrum (Stalevski et al. 2012, 2016), we find that accreting SMBHs would in fact contribute at most ∼40 per cent to the IR flux.

IRAS F23365+3604: Chandra shows a point-like source coinciding with the nucleus of this late merger. The source is not detected by NuSTAR, and its X-ray spectrum is well fitted by a simple power-law model.

Jun H. D., Assef R. J., Carroll C. M., Hickox R. C., Kim Y., Lee J., Ricci C., Stern D., 2021, ApJ, 906, 21 10.3847/1538-4357/abc629

Satyapal S., Ellison S. L., McAlpine W., Hickox R. C., Patton D. R., Mendel J. T., 2014, MNRAS, 441, 1297 10.1093/mnras/stu650

IRAS F08572+3915: Only a faint detection of this double system is obtained by Chandra and XMM–Newton. Chandra shows a point-like hard X-ray component from the north-west nucleus (Iwasawa et al. 2011). The source is not detected by NuSTAR, from which we could infer an upper limit on the 10–24 keV luminosity of |$\log (L_{\rm 10{-}24}/\rm erg\, s^{-1})\le 41.13$|⁠. The combined XMM–Newton/Chandra spectra could be well fitted by a simple power-law model, which returned a very low photon index (Γ = 0.7 ± 0.3). Extending this model to higher energies would result in a 10–24 keV luminosity of |$\log (L_{\rm 10{-}24}/\rm erg\, s^{-1})=41.46$|⁠, i.e. higher than the upper limit inferred from our NuSTAR observations, which suggests that this hard X-ray component is not associated with an obscured AGN.

We analyse NuSTAR (Harrison et al. 2013) observations for 23 sources using the NuSTAR Data Analysis Software nustardas v1.9.2 within heasoft v6.27. We adopted the calibration files released on 2020 May 6 (Madsen et al. 2015). In order to extract the source spectra, we use a circle of 50 arcsec, while for the background we consider an annulus centred on the source, with inner and outer radii of 60 and 100 arcsec, respectively. In several cases, no X-ray source is detected by NuSTAR, and for these sources we follow the same approach reported in Lansbury et al. (2017) to calculate the flux upper limits. This is done using the Bayesian approach of Kraft, Burrows & Nousek (1991).

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IRAS F15250+3608: The source is detected by both Chandra and XMM–Newton, with the former showing a soft point source with a position consistent with that of the optical counterpart (Iwasawa et al. 2011). IRAS F15250+3608 is not detected by NuSTAR, and its X-ray spectrum is well fitted by the SF model.

Single-order diffraction for such a period occurs at the Littrow angle of θL = arcsin(1/3) ≅ 20º. PGL Technical Note – The Grating Equation. PDF of this ...

The merger of two or more galaxies can enhance the inflow of material from galactic scales into the close environments of active galactic nuclei (AGNs), obscuring and feeding the supermassive black hole (SMBH). Both recent simulations and observations of AGN in mergers have confirmed that mergers are related to strong nuclear obscuration. However, it is still unclear how AGN obscuration evolves in the last phases of the merger process. We study a sample of 60 luminous and ultra-luminous IR galaxies (U/LIRGs) from the GOALS sample observed by NuSTAR. We find that the fraction of AGNs that are Compton thick (CT; |$N_{\rm H}\ge 10^{24}\rm \, cm^{-2}$|⁠) peaks at |$74_{-19}^{+14}{{\ \rm per\ cent}}$| at a late merger stage, prior to coalescence, when the nuclei have projected separations (dsep) of 0.4–6 kpc. A similar peak is also observed in the median NH [|$(1.6\pm 0.5)\times 10^{24}\rm \, cm^{-2}$|]. The vast majority (⁠|$85^{+7}_{-9}{{\ \rm per\ cent}}$|⁠) of the AGNs in the final merger stages (dsep ≲ 10 kpc) are heavily obscured (⁠|$N_{\rm H}\ge 10^{23}\rm \, cm^{-2}$|⁠), and the median NH of the accreting SMBHs in our sample is systematically higher than that of local hard X-ray-selected AGN, regardless of the merger stage. This implies that these objects have very obscured nuclear environments, with the |$N_{\rm H}\ge 10^{23}\rm \, cm^{-2}$| gas almost completely covering the AGN in late mergers. CT AGNs tend to have systematically higher absorption-corrected X-ray luminosities than less obscured sources. This could either be due to an evolutionary effect, with more obscured sources accreting more rapidly because they have more gas available in their surroundings, or to a selection bias. The latter scenario would imply that we are still missing a large fraction of heavily obscured, lower luminosity (⁠|$L_{2-10}\lesssim 10^{43}\rm \, erg\, s^{-1}$|⁠) AGNs in U/LIRGs.

IRAS F13126+2453 (IC 860): The source is only faintly detected by Chandra, and is not detected by NuSTAR. Due to the low signal-to-noise ratio of the spectrum, we fit it using a simple power-law model, with the photon index fixed to Γ = 1.8.

Nakazawa K. et al. , 2018, in den Herder J.-W. A., Nikzad S., Nakazawa K., eds, Proc. SPIE Conf. Ser. Vol. 10699, Space Telescopes and Instrumentation 2018: Ultraviolet to Gamma Ray. SPIE, Bellingham, p. 106992D

The table reports the values obtained from the X-ray spectral analysis of the sources of our sample. For each source, we list (1) the IRAS name of the source, (2) the counterparts, (3) the column density of the X-ray emission associated with SF, (4) the temperature of the collisionally ionized plasma, (5) the photon index of the soft X-ray emission due to X-ray binaries or to the scattered emission from the AGN, (6) the column density and (7) the photon index of the AGN, and (8) the value of the Cash or χ2 statistics and the number of degrees of freedom (DOF). Objects in which both statistics were used to fit different spectra are reported as [C/χ2], and the value of the statistic (Stat) is the combination of the two.

Our work confirms the idea that the close environments of AGNs in U/LIRGs undergoing the final stages of the merger process are different from those of AGNs in isolated galaxies (e.g. Ricci et al. 2017b), with the former having an accreting source completely buried by obscuring material. We speculate that, due to the high density and large covering factor of the obscuring dust and gas, there might be an important fraction of lower luminosity (⁠|$L_{2-10}\lesssim 10^{43}\rm \, erg\, s^{-1}$|⁠) AGNs that we are still missing in late mergers. Extremely sensitive hard X-ray telescopes, such as those onboard the proposed missions FORCE (Mori et al. 2016; Nakazawa et al. 2018) and HEX-P (Madsen et al. 2018), would be fundamental to shed light on the accretion properties of SMBHs in these nearby systems. The strong nuclear obscuration associated with AGN in mergers, combined with the increase of galaxies in mergers with redshift (e.g. Le Fèvre et al. 2000; Conselice, Yang & Bluck 2009; Lotz et al. 2011), might contribute to the observed positive relation between the fraction of obscured sources and redshift (e.g. La Franca et al. 2005; Treister & Urry 2006; Ueda et al. 2014; Buchner et al. 2015). Athena (Nandra et al. 2013) will be a fundamental tool to assess the role of mergers in the increase of the fraction of obscured AGNs with redshift, shedding light on the properties of accreting SMBHs at z ≳ 1.

Koss M., Mushotzky R., Veilleux S., Winter L. M., Baumgartner W., Tueller J., Gehrels N., Valencic L., 2011, ApJ, 739, 57 10.1088/0004-637X/739/2/57

Median column density versus the separation between the two nuclei (Tables 1 and 2). AGNs in U/LIRGs tend to have significantly higher column densities than AGNs in nearby hard X-ray-selected AGN (red continuous line; Ricci et al. 2015, 2017c). Similarly to what was found for the fraction of CT AGNs (top panel of Fig. 4), we find a tentative peak of the column density for a projected nuclear separation of a few kpc. We conservatively considered the lower limit on NH for the objects for which this parameter could not be constrained.

IRAS F23128−5919 (ESO 148−IG002): This system is a late-stage merger, which is well detected by Chandra and XMM–Newton, and is not detected by NuSTAR. The Chandra image shows extended X-ray emission, which covers both galactic nuclei. The X-ray spectrum is well fitted by our SF model. Franceschini et al. (2003) report the presence of an AGN absorbed by a column density of |$N_{\rm H}\sim 7\times 10^{22}\rm \, cm^{-2}$|⁠, and Iwasawa et al. (2011) discuss that the AGN might be associated with the southern nucleus. We apply our AGN model, fixing Γ = 1.8 (e.g. Ricci et al. 2017c), finding a very similar column density (⁠|$N_{\rm H}=7^{+5}_{-2}\times 10^{22}\rm \, cm^{-2}$|⁠). However, the expected observed 10–24 keV luminosity from this model (⁠|$8.3\times 10^{41}\rm \, erg\, s^{-1}$|⁠) is higher than the upper limit inferred by our NuSTAR observations (⁠|$L_{10{-}24}\le 1.7\times 10^{41}\rm \, erg\, s^{-1}$|⁠). This implies that, if an AGN is present in this system, it is significantly more obscured than what is reported by Franceschini et al. (2003), and the observed X-ray emission of this object is thus dominated by SF. We therefore used the SF model for this object.

The data sets generated and/or analysed in this study are available from the corresponding author on reasonable request.

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We thank the referee for their useful suggestions, which helped us in improving the quality of the manuscript. We thank Chin-Shin Chang for useful comments on the manuscript. LCH was supported by the National Key Research and Development Program of China (2016YFA0400702) and the National Science Foundation of China (11721303 and 11991052). CR acknowledges support from the Fondecyt Iniciacion grant 11190831. ET acknowledges support from CATA-Basal AFB-170002, FONDECYT Regular grant 1190818, ANID Anillo ACT172033, and Millennium Nucleus NCN19_058 (TITANs). FEB acknowledges support from ANID – Millennium Science Initiative Program – ICN12_009, CATA-Basal – AFB-170002, and FONDECYT Regular – 1190818 and 1200495. SA gratefully acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 789410). VU acknowledges funding support from NASA Astrophysics Data Analysis Program (ADAP) Grant 80NSSC20K0450. AMM acknowledges support from the National Science Foundation under grant number 2009416. KI acknowledges support by the Spanish MICINN under grant Proyecto/AEI/10.13039/501100011033 and ‘Unit of excellence María de Maeztu 2020-2023’ awarded to ICCUB (CEX2019-000918-M). PA acknowledges financial support from ANID Millennium Nucleus NCN19-058 (TITANS) and the Max Planck Society through a Partner Group. HI acknowledges support from JSPS KAKENHI Grant Number JP19K23462. This work made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. We thank the NuSTAR Operations, Software and Calibration teams for support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (nustardas) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (Caltech, USA), and of the NASA/IPAC Infrared Science Archive and NASA/IPAC Extragalactic Database (NED), which are operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

The vast majority (⁠|$85^{+7}_{-9}{{\ \rm per\ cent}}$|⁠) of the AGNs in late-stage mergers are heavily obscured. This fraction is consistent with that obtained for early mergers (⁠|$74^{+11}_{-12}{{\ \rm per\ cent}}$|⁠), while it is significantly higher than for local hard X-ray-selected AGNs.

IRAS F04454−4838 (ESO 203−IG001): Neither of the two galaxies in this early merger are detected by Chandra or NuSTAR. ESO 203−IG001 is the only object not detected by Chandra in Iwasawa et al. (2011).

IRAS F19297−0406: The soft X-ray emission detected by Chandra for this late merger with a single nucleus is extended, while the hard X-ray emission is compact (Iwasawa et al. 2011). This object is not detected by NuSTAR, and we use a simple power-law model to reproduce its X-ray emission.

Ellison S. L., Viswanathan A., Patton D. R., Bottrell C., McConnachie A. W., Gwyn S., Cuillandre J.-C., 2019, MNRAS, 487, 2491 10.1093/mnras/stz1431

(1) IRAS name and (2) counterparts, observed (3) 2–10 keV and (4) 10–24 keV luminosities, intrinsic (5) 2–10 keV and (6) 10–24 keV AGN luminosities, (7) line-of-sight column densities towards the AGN, and (8) references. Luminosity upper limits are calculated based on the observed flux, and therefore could be significantly higher if the source is heavily obscured. A line-of-sight column density of |$N_{\rm \, H}=10^{24}\rm \, cm^{-2}$| (⁠|$10^{25}\rm \, cm^{-2}$|⁠) would correspond to an increase in luminosity of |$\Delta [\log (L_{2-10}/\rm erg\, s^{-1})]=1.3$| (2.8) and |$\Delta [\log (L_{10-24}/\rm erg\, s^{-1})]=0.4$| (1.9) in the 2–10 and 10–24 keV bands, respectively. The 2–10 keV AGN luminosity upper limit was extrapolated (assuming a power law with Γ = 1.8) from the upper limit on the 10–24 keV luminosity inferred by NuSTAR.

Oct 9, 2018 — Departure is the distance from the BFS to the aspheric form. Form error can be explained by how much the manufactured asphere contrasts with the ...

In this work, we analyse X-ray data obtained from the NuSTAR, Chandra, and XMM–Newton facilities, the data reduction of which we outline in Sections 3.1, 3.2, and 3.3, respectively. The extraction regions of the different instruments were selected to cover the host galaxies. We combine these with similar X-ray data previously analysed and presented in Ricci et al. (2017b) for 30 GOALS U/LIRGs, and literature constraints on several additional objects. The details of all X-ray observations analysed here are listed in Table A1.

IRAS F09111−1007: A point source was detected consistent with each of the two galaxies of this early merger. In both cases, the X-ray emission is rather faint, and it could be well fitted by a simple power-law model.

IRAS F12243−0036 (NGC 4418): Two point sources are found at a distance of ∼1.5 arcsec from each other in the Chandra images, with the eastern source being brighter above ∼2 keV (Torres-Albà et al. 2018). The source is not detected by NuSTAR, and the X-ray spectrum is well fitted with our SF model.

Fractions are calculated following Cameron (2011), and the uncertainties quoted represent the 16th/84th quantiles of a binomial distribution, obtained using the Bayesian approach outlined in Cameron (2011).

Sources in the early merger stages are classified as belonging to the a and b class, while those in late-stage mergers have been classified as being in the c or d stage (see Stierwalt et al. 2013, for details, and fig. 1 of Ricci et al. 2017b). Typically, sources in late-stage mergers are separated by dsep ≲ 11 kpc. All the sources in our sample, together with their merger stages and the projected distances between the two nuclei, are listed in Table 1, while in Fig. 1 we illustrate some of their main properties. The closest observed projected distance for systems showing at least an AGN is dsep = 0.4 kpc; therefore, we assign this distance as the minimum distance between two potential AGNs in this study. Of the 60 sources in our sample, 7 are in stage a, 8 in stage b, 13 in stage c, 21 in stage d, and 11 in stage N. This doubles the number of U/LIRGS with NuSTAR observations with respect to the sample presented in Ricci et al. (2017b), and in particular we have now observations of 34 late-stage galaxies, while only 17 were reported in Ricci et al. (2017b).

We include XMM–Newton (Jansen et al. 2001) observations for nine sources. The EPIC/PN (Strüder et al. 2001) spectra are obtained by first reducing the original data files using XMM–Newton Standard Analysis Software (SAS) version 18.0.0 (Gabriel et al. 2004), and then using the epchain task. We filter all observations to remove periods of high-background activity, by analysing the EPIC/PN background light curve in the 10–12 keV band. Finally, the spectra are extracted by using a circular region of 25 arcsec radius, while the background is extracted on the same CCD, in a region devoid of X-ray sources, using a circular region of 40 arcsec radius. None of the observations is significantly affected by pile-up.

IRAS F20550+1655 (CGCG 448−020E and CGCG 448−020W): This late merger, with two nuclei separated by 5 arcsec, is detected by both Chandra and XMM–Newton, but is not detected by NuSTAR. Chandra shows some diffuse X-ray emission, with two rather compact hard X-ray sources (Iwasawa et al. 2011). The X-ray spectrum is well fitted by the SF model.

Left-hand panel: Cumulative distribution function of the |$8\!-\!1000\, \mu$|m IR luminosities of X-ray non-AGNs (grey dot–dashed line) and Compton-thin (red dashed line) and CT (red continuous line) AGNs, showing that CT AGNs are typically found in systems that are more luminous in the IR. Right-hand panel: Intrinsic 2–10 keV luminosity versus |$8\!-\!1000\, \mu$|m luminosity of the X-ray-detected Compton-thin (red empty stars) and CT (red filled circles) AGNs in our sample. The plot also shows the upper limit on the 2–10 keV luminosity of the sources for which an AGN was not identified in the X-rays (grey filled diamonds). These upper limits are calculated based on the observed flux, and therefore could be significantly higher if the source is heavily obscured. A line-of-sight column density of |$N_{\rm \, H}=10^{24}\rm \, cm^{-2}$| (⁠|$10^{25}\rm \, cm^{-2}$|⁠) would correspond to an increase in luminosity of |$\Delta [\log (L_{2-10}/\rm erg\, s^{-1})]=1.3$| (2.8). The figure illustrates that the CT AGNs we identify typically have higher intrinsic X-ray luminosities than the Compton-thin AGNs.

Baumgartner W. H., Tueller J., Markwardt C. B., Skinner G. K., Barthelmy S., Mushotzky R. F., Evans P. A., Gehrels N., 2013, ApJS, 207, 19 10.1088/0067-0049/207/2/19

A tentative peak in the fraction of CT AGNs is found at nuclear projected separations of dsep ∼ 0.4–6 kpc (⁠|$74_{-19}^{+14}{{\ \rm per\ cent}}$|⁠; top panel of Fig. 4). The median line-of-sight column density is also found to peak [|$(1.6\pm 0.5)\times 10^{24}\rm \, cm^{-2}$|] for a similar range of nuclear separations (Fig. 5). Considering the X-ray non-detection of objects that are identified as AGN in the IR, the CT fraction at dsep ∼ 0.4–6 kpc would be |$85^{+8}_{-12}{{\ \rm per\ cent}}$|⁠. The possible decrease at dsep ≲ 0.4 kpc both in the fraction of CT AGNs and in the median NH could be related to the effect of radiation pressure or to the fact that sources are more heavily obscured in the final phases of the merger process, and therefore we would detect preferentially the least obscured AGN.

Near-infrared (NIR) and mid-infrared (MIR) images were used to classify the sources into different merger stages. We followed what was reported by Haan et al. (2011) using HST images and, when that was not available, we considered the classification of Stierwalt et al. (2013), including the modifications proposed by Ricci et al. (2017b). Based on the morphological properties of the objects, we divided them into five different merger stages, following Stierwalt et al. (2013):

by DG Pobedimskii · 1976 — Chemical polarization of nuclei has been observed in the products of reactions between phosphites and ozone. 2. Reaction between phosphites and ozone take ...

IRAS 10565+2448: Chandra detects a point source coincident with the western member of this advanced merger (Iwasawa et al. 2011). The source is not detected by NuSTAR, and the X-ray spectrum is accurately modelled using the SF model.

The X-ray spectra of most (21/23) of the sources analysed here could be well reproduced by an SF model. For the two sources (NGC 2623 and ESO 060−IG16) that show a strong Fe K α line at 6.4 keV, or a clear excess over the SF model, we include an AGN component to account for the excess. This is done using the RXTorus model, developed using the RefleX ray-tracing platform (Paltani & Ricci 2017). The model assumes a toroidal absorber surrounding the accreting system. The inner-to-outer radius ratio of the torus is fixed to 0.5, while the inclination angle is fixed to 90○ (i.e. corresponding to an edge-on scenario). In xspec, the model is: tbabs×ztbabs×(zpow+apec+atable{RXTorus_rprc_200} +etable{RXTorus_cont}*zcutoffpl), where RXTorus_rprc_200 and RXTorus_cont are the reprocessed radiation and obscuration components, while zcutoffpl is a cut-off power-law model used for the continuum. In the latter component, the cut-off energy is fixed to 200 keV (Ricci et al. 2018). The zpow component includes contributions from both star-forming regions and from scattered X-ray emission (e.g. Ueda et al. 2007, 2015; Ricci et al. 2017c; Gupta et al. 2021). The parameters obtained by our spectral analysis are reported in Table B1, while the column densities and intrinsic AGN luminosities, for all the objects in our sample, are reported in Table 2.

Sanders D. B., Soifer B. T., Elias J. H., Madore B. F., Matthews K., Neugebauer G., Scoville N. Z., 1988, ApJ, 325, 74 10.1086/165983

The all-sky GOALS sample consists of 180 LIRGs and 22 ULIRGs, and is complete at 60 |$\mu$|m for fluxes >5.24 Jy. Objects in GOALS have been extensively studied in the IR, with a large number of observations carried out by Spitzer, Akari, and Herschel (e.g. Inami et al. 2010, 2013, 2018; Díaz-Santos et al. 2011; Petric et al. 2011; U et al. 2012, 2019; Stierwalt et al. 2013, 2014; Medling et al. 2014; Lu et al. 2017). Moreover, a large Chandra campaign provides spectroscopic coverage in the 0.3–10 keV range, as well as high-spatial resolution images in the same band (Iwasawa et al. 2011; Torres-Albà et al. 2018). Our sample includes all U/LIRGs from the GOALS sample that were observed by NuSTAR. This includes the 30 objects reported in Ricci et al. (2017b), besides sources that were recently analysed in literature studies, as well as 19 objects that have been recently observed by NuSTAR as a part of several observational campaigns led by our team (PIs: Ricci, C; Privon, G.; Armus, L.) to study SMBH accretion in the final phases of the merger process. Overall, our sample contains 60 U/LIRGs.

In the left-hand panel of Fig. 6, we show the cumulative distribution of the IR luminosity of the X-ray-detected AGN in our sample, divided into Compton-thin (blue dashed line) and CT (red continuous line). The CT AGNs tend to have higher IR luminosities and, from performing a Kolmogorov–Smirnov (KS) test between the LIR distributions of the two types of AGNs, we find a p-value of 0.01. This indicates that the IR luminosities of CT and Compton-thin AGNs are significantly different. Interestingly, CT AGNs (red filled circles in the right-hand panel of Fig. 6) are mostly found to have higher intrinsic X-ray luminosities than Compton-thin sources (empty red stars), and only one of them is found to have |$L_{2-10}\lesssim 10^{43}\rm \, erg\, s^{-1}$|⁠. A KS test between the two luminosity distributions results in a p-value of 4.6 × 10−4. While we cannot exclude that this is an evolutionary effect, where more obscured sources accrete more rapidly because they have more gas available in their surroundings, it is possible that this is related to a selection effect. In fact, even with our sensitive NuSTAR hard X-ray observations, because of the strong depletion of the X-ray flux at |$N_{\rm H}\gt 10^{24}\rm \, cm^{-2}$|⁠, it would be difficult to detect a large number of low-luminosity heavily obscured AGNs. This is particularly true if the heavily obscuring material covers most of the X-ray sources, as suggested by the very large fraction of heavily obscured sources (see also Ricci et al. 2017b), which would lead to a small fraction of the X-ray radiation reprocessed by the circumnuclear environment being able to escape the system.

Lotz J. M., Jonsson P., Cox T. J., Croton D., Primack J. R., Somerville R. S., Stewart K., 2011, ApJ, 742, 103 10.1088/0004-637X/742/2/103

CT AGNs typically have higher intrinsic (i.e. absorption-corrected) X-ray luminosities than less obscured sources. This could be either due to an evolutionary effect, with more obscured sources accreting more rapidly because they have more gas available in their surroundings, or to a selection effect. In the latter scenario, our NuSTAR observations might be unable to detect a significant fraction of heavily obscured less luminous (⁠|$L_{2-10}\lesssim 10^{43}\rm \, erg\, s^{-1}$|⁠) AGNs, while detecting most of the AGNs that contribute significantly to the energetics of these U/LIRGs (Ricci et al., in preparation).

The U/LIRGs in our sample show a higher fraction of heavily obscured (⁠|$N_{\rm H}\ge 10^{23}\rm \, cm^{-2}$|⁠; |$82^{+5}_{-7}{{\ \rm per\ cent}}$|⁠) and CT AGNs (⁠|$N_{\rm H}\ge 10^{24}\rm \, cm^{-2}$|⁠; |$46\pm 8{{\ \rm per\ cent}}$|⁠) than local hard X-ray-selected AGNs (⁠|$52\pm 4{{\ \rm per\ cent}}$| and |$27\pm 4{{\ \rm per\ cent}}$|⁠, respectively; Ricci et al. 2015, 2017c; see Fig. 3). The median line-of-sight column density towards AGN in U/LIRGs is also ∼1.5–2 orders of magnitude larger than that of hard X-ray-selected AGN (Fig. 5).

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IRAS F17207−0014: This d-stage, single-nucleus merger is detected by Chandra, exhibiting two peaks, with the southern one being harder and coinciding with the position of the optical nucleus of the system (Iwasawa et al. 2011). The source is also detected by XMM–Newton, but it is not detected by NuSTAR. The SF model provides a good fit to the X-ray spectrum.

Details on the X-ray spectral analysis of the individual sources are reported in Appendix C. We verified whether the observed 2–10 keV luminosity is consistent with what would be expected by SF, considering the SFR of the galaxy. We used the L2−10–SFR relations of Ranalli et al. (2003) and Lehmer et al. (2010), and found that all sources in which no AGNs were identified by our analysis have luminosities consistent, or lower, than the value expected from their SFR (see Fig. 2). The large fraction of U/LIRGs located below the relation of Ranalli et al. (2003) is consistent with the flattening reported by Torres-Albà et al. (2018) for a large sample of U/LIRGs observed by Chandra, and is possibly associated with an increasing level of obscuration within the star-forming regions (Torres-Albà et al. 2018).

Chandra/ACIS (Weisskopf et al. 2000; Garmire et al. 2003) observations are available for all of the new sources of our sample. We reduce the observations following standard procedures, using ciao v.4.10. We reprocess all data sets using the chandra_repro task, and then extract the spectra using a circular region with a radius of 10 arcsec. For the background spectra, we used a circular region with the same radius, selected in region devoid of other X-ray sources. In the case of IC 1623B, due to its extended emission, we used a radius of 20 arcsec, for the source, in order to consider all the X-ray emission from the source, consistent with what was done to obtain the NuSTAR and XMM–Newton spectra; considering a smaller radius (10 arcsec) we obtained similar results for this source (i.e. no clear sign of AGN activity). For IRAS 14348−1447 and IRAS 20550+1655, we also extracted the X-ray emission from the individual nuclei, considering source regions of 1.8 and 2.0 arcsec, respectively. Among the new sources of our sample, only ESO 203−IG001 was not detected by Chandra.

Recent simulations (e.g. Blecha et al. 2018; Kawaguchi et al. 2020) have shown that the most obscured phase during the merger would correspond to small separation between the two nuclei. We divided our sample based on the projected nuclear separation, down to the scales in which two nuclei could be resolved at the distance of our sources (dsep ∼ 0.4 kpc). In Fig. 4, we show the fraction of CT (top panel) and heavily obscured (bottom panel) AGNs versus the projected separation between the two galactic nuclei. We find that the CT fraction appears to peak when the two nuclei are at a projected distance of a few kpc (⁠|$74^{+14}_{-19}{{\ \rm per\ cent}}$|⁠; dsep ∼ 0.4–6 kpc), and that the fraction of heavily obscured sources is consistently higher than that found for Swift/BAT AGN, regardless of the projected nuclear separation. Similarly to the CT fraction, a peak in the median NH [|$(1.6\pm 0.5)\times 10^{24}\rm \, cm^{-2}$|] is also observed when the merging galaxies are at a few kpc distance (Fig. 5). The difference in the median column density with the Swift/BAT AGN sample (red continuous line) is particularly striking, with the X-ray-observed U/LIRGs having a median NH ∼ 1.5–2 orders of magnitude larger. It should be noted that our U/LIRGs are frequently interacting or merging galaxy pairs, a process that increases the amount of gas within the central ∼kpc (e.g. Di Matteo et al. 2007). This could lead to the GOALS galaxies having additional obscuration on hundreds of pc to kpc scales, compared to host galaxy obscuration seen in AGN in non-merging systems. The median CO luminosity of GOALS galaxies (⁠|$2.6\times 10^9\rm \, K\, km\, s^{-1}\, pc^2$|⁠; Herrero-Illana et al. 2019) is a factor of 7 higher than the CO luminosity of host galaxies of BAT AGN (⁠|$4\times 10^8\rm \, K\, km\, s^{-1}\, pc^2$|⁠, Koss et al. 2021). Different molecular gas masses could affect the contribution of host galaxy obscuration, but not up to the CT level (e.g. Buchner, Schulze & Bauer 2017). However, due to potential variations in the CO–H2 conversion factor (e.g. Bolatto, Wolfire & Leroy 2013), it is unclear how different the total molecular gas masses are between the two samples.

This work and recent X-ray studies (e.g. Ricci et al. 2017b) show that the obscuration properties of AGN in U/LIRGs are very different from those of AGN in isolated galaxies. In particular, AGNs in late mergers are fully embedded in gas with |$N_{\rm H}\ge 10^{23}\rm \, cm^{-2}$|⁠. The most extreme of these sources could be associated with Compact Obscured Nuclei (or CONs; Aalto et al. 2015, 2019; Falstad et al. 2021), galaxies that show strong and compact vibrationally excited HCN from their nuclear regions. This emission is created by a strong 14 |$\mu$|m continuum, which could be due to strong emission from a heavily obscured AGN. The presence of obscuring material with a very high covering factor around AGN in galaxies undergoing the final phases of a merger has also been confirmed by a recent study focusing on the [O iv] 25.89 |$\mu$|m line. Yamada et al. (2019) found that the ratio between the [O iv] and the 12 |$\mu$|m AGN luminosity decreases as the merger progresses, which suggests that the covering factor of the material tends to be larger in late-stage mergers. Ricci et al. (2017d) demonstrated that, due to the presence of dusty gas (e.g. Fabian, Celotti & Erlund 2006; Fabian, Vasudevan & Gandhi 2008), radiation pressure can be very effective in reducing the covering factor of the obscuring material, by removing gas from the environment of nearby AGN already at low Eddington ratios (i.e. λEdd ∼ 10−2; see also García-Burillo et al. 2021). This process might not be as effective in mergers, where the obscuring material might be located at hundreds of parsecs from the accreting source (and therefore outside the sphere of influence of the SMBH). In these objects, the AGN would need to attain high luminosities (and considerably higher Eddington ratios) in order to remove the obscuring material (Ricci et al. 2017d; Jun et al. 2021). This might happen in the final stages of the merger process, when the accretion rate of the SMBH is expected to reach very high levels (e.g. Blecha et al. 2018; Kawaguchi et al. 2020), and it could be the cause of the tentative decrease at dsep ≲ 0.4 kpc we observe both in the fraction of CT AGNs (top panel of Fig. 4) and in the median NH (Fig. 5). Alternatively, the decrease could be due to sources being more heavily obscured in the final phases of the merger process, which would lead us to detect preferentially the least obscured AGN.

Role of the infrared cutoff in fermion damping rates. Phys Rev D Part Fields. 1993 Aug 15;48(4):1868-1870. doi: 10.1103/physrevd.48.1868.

IRAS F00344−3349 (ESO 350−IG038): This late-stage merging galaxy is not detected by NuSTAR. The Chandra image shows an extended source, comprising three knots of SF, overlying both galaxies (Torres-Albà et al. 2018). The X-ray emission from this object is soft, with no clear hard X-ray component, and the SF model can reproduce very well the overall X-ray spectrum. Since this is a very close merger (dsep = 1.1 kpc), we follow the strategy of Torres-Albà et al. (2018), and consider the X-ray emission for the whole system.

We fit the X-ray spectra of all sources starting with an SF model, which consists of a power-law component (zpow in xspec) and a collisionally ionized plasma (apec). We include Galactic absorption using the tbabs model (Wilms, Allen & McCray 2000), fixing the column density to the value reported by Kalberla et al. (2005) at the coordinates of the source. Intrinsic absorption is considered by including a ztbabs component. Overall, the SF model used is: tbabs×ztbabs×(zpow+apec). In a few cases, for which the signal-to-noise ratio is particularly low, we use a simple power-law model to fit the spectra [tbabs×ztbabs×(zpow)].