Abstract
The most heavily-obscured, luminous quasars might represent a specific phase of the evolution of the actively accreting supermassive black holes and their host galaxies, possibly related to mergers. We investigated a sample of the most luminous quasars at z ≈ 1 − 3 in the GOODS fields, selected in the mid-infrared band through detailed spectral energy distribution (SED) decomposition. The vast majority of these quasars (~80%) are obscured in the X-ray band and ~30% of them to such an extent, that they are undetected in some of the deepest (2 and 4 Ms) Chandra X-ray data. Although no clear relation is found between the star-formation rate of the host galaxies and the X-ray obscuration, we find a higher incidence of heavily-obscured quasars in disturbed/merging galaxies compared to the unobscured ones, thus possibly representing an earlier stage of evolution, after which the system is relaxing and becoming unobscured.
1. Introduction
The similarity between the accretion history of galaxies and supermassive black holes (SMBHs), peaking at redshift z ≈ 1 − 2 (e.g., Madau et al., 1996; Hopkins et al., 2006; Brandt and Alexander, 2015), suggests that there is a connection between the evolution of a galaxy and the black hole in their center. Such connection has also been hinted by the observed correlations between the BH mass and the velocity dispersion of the stars in the bulge (MBH − σ relation; Ferrarese and Merritt, 2000; Gebhardt et al., 2000) or with the bulge mass (MBH − Mbulge; Kormendy and Richstone, 1995; Magorrian et al., 1998). Whether this parallel evolution is simply due to a larger gas supply at high redshift, feeding both the SMBH and star formation (SF), or whether there are other processes self-regulating the SMBH and galaxy growth (e.g., AGN feedback) is still uncertain (e.g., Alexander and Hickox, 2012; Kormendy and Ho, 2013).
Studying active galactic nuclei (AGN) at all cosmic epochs is crucial to fully understand the accretion history of the SMBHs and their role in galaxy evolution. However, most of the accretion onto SMBHs is expected to be heavily obscured by dust and gas, making the identification of the most obscured AGN population very challenging, even in the deepest X-ray surveys. According to the unified model (e.g., Antonucci, 1993), AGN appear obscured due to orientation effects, when our line-of-sight crosses high column densities (NH) of circumnuclear material, the so called “torus”. However, it has been suggested that the most heavily-obscured, Compton-thick (CT; where cm−2) AGN, especially the (intrinsically) most luminous ones, could represent a particular phase of galaxy evolution, associated to a major merger, when a lot of gas and dust are funneled into the center of the galaxy, deeply hiding the active nucleus within it (Di Matteo et al., 2005; Hopkins et al., 2006; see also Alexander and Hickox, 2012, for a review). Yet, the emission reprocessed by the obscuring dust is re-emitted in the mid-infrared (MIR) band, which can therefore be used to find even the most obscured and elusive quasars.
2. Sample selection
The sample was selected from a large catalog of 24 μm-detected sources within the GOODS-Herschel North and South fields. We performed detailed SED decomposition using Spitzer 8, 16, and 24 μm and Herschel 100, 160, and 250 μm data, to separate the AGN from SF emission. We adopted the AGN and star-forming galaxy (SFG) templates described in Mullaney et al. (2011) and Del Moro et al. (2013); details of the SED fitting are described in Del Moro et al. (2016). Amongst these sources we selected the most luminous quasars in the MIR band, with rest-frame 6 μm luminosity of erg s−1, corrected for the galaxy contribution, at redshift z = 1 − 3. This selection results in a sample of 33 sources.
3. Analyses and results
3.1. AGN: heavily obscured population at z ≈ 2
To characterize the quasars in our sample, we used the deep X-ray Chandra data available in the Chandra Deep Field North (CDF-N; 2 Ms; Alexander et al., 2003) and Chandra Deep Field South (CDF-S 4 Ms; Xue et al., 2011). For details on the data reduction we refer to Alexander et al. (2003), Luo et al. (2008), and Xue et al. (2011). Of our 33 quasars, 24 (~73%) are detected in the X-ray band, while 9 (~27%) remain undetected, despite being intrinsically very luminous in the MIR band. These sources are candidates to be the most heavily obscured, CT AGN.
For the sources that are detected in the X-rays, we extracted the spectra using ACIS Extract (AE; Broos et al., 2010, 2012) and analyzed them using a simple absorbed power-law model (including Galactic and intrinsic absorption) to constrain the amount of NH. In Figure 1 we show the NH distribution of the sample. We find that the majority of these quasars (16/24; ~67%) are obscured by columns of cm−2, of which more than half (9/16) are heavily obscured ( cm−2). Amongst these heavily-obscured sources, we identified six of them as CT quasars from the X-ray spectral analysis, using spectral models appropriate for heavily-obscured sources, such as, PLCABS (Yaqoob, 1997) and TORUS (Brightman and Nandra, 2011). The fraction of obscured quasars in our sample reaches ~76%, and 54% of heavily-obscured quasars, if we include the X-ray-undetected sources, assuming these are the most heavily CT ones. Indeed, the comparison between the intrinsic luminosity at 6 μm and the X-ray luminosity upper limit of these sources suggests that the X-ray emission is heavily suppressed compared to the intrinsic LX − L6 μm relation found for AGN (e.g., Lutz et al., 2004; Fiore et al., 2009; Gandhi et al., 2009), making them very good candidates to be heavily CT quasars.
Figure 1
We note that amongst the X-ray undetected quasars in the sample, there is one source, #28 (see Table 1 from Del Moro et al., 2016), that is now detected in the 7 Ms CDF-S catalog (XID 28; Luo et al., 2017). This source has a very flat effective photon index of Γ < 0.93 (compared to the typical Γ ≈ 1.8 for unabsorbed AGN) and an extremely low rest-frame X-ray luminosity ( erg s−1, uncorrected for absorption) compared to its intrinsic luminosity measured in the MIR band (log ν L6 μm = 45.97 erg s−1), consistent with the upper limit reported in our analysis (Del Moro et al., 2016)1, supporting our assumption that the source might be a heavily obscured, CT quasar.
These results suggest that there is a large population of heavily-obscured, intrinsically luminous quasars at high redshift, which are very elusive even for deep X-ray surveys. These sources might constitute a special phase of the BH-galaxy evolution, where the actively growing BH is embedded in large amounts of gas and dust, possibly as a result of a recent merger.
3.2. AGN host galaxies: star-formation rates and merger fraction
To study the characteristics of the host galaxies of these MIR-luminous quasars, we investigated their star-formation rates (SFRs) and their morphologies, in particular the disturbance or distortion of their morphology, as an indication of galaxy mergers and interactions. We derived the SFR of each galaxy from its far-infrared (FIR) luminosity (or upper limit) resulting from the SED decomposition in the IR band (see section 2 and Del Moro et al., 2016), assuming a Salpeter initial mass function (Salpeter, 1955) and the relation from Kennicutt (1998). We also calculated the average SFR separately for the unobscured/moderately obscured quasars ( cm−2) and for the heavily-obscured quasars ( cm−2) in three different redshift bins (see Figure 2). To estimate the average SFRs accounting for the upper limits, we used the Kaplan-Meier (KM) product limit estimator (Kaplan and Meier, 1958), a non-parametric maximum-likelihood estimator of the distribution function (see Stanley et al., 2015, for details on the method).
Figure 2
We find that the average SFR increases with redshift, in general agreement with the SFR main sequence of galaxies (Figure 2). Moreover, although the heavily-obscured sources seem to have a slightly enhanced SFR compared to the unobscured/moderately obscured ones (especially at z ≈ 2), these differences are not statistically significant, and therefore we find no clear dependence of the amount of SF in the galaxy with the X-ray obscuration of the quasar. This suggests that the obscuration in the X-ray band is likely confined in the nuclear regions and not related to the presence of gas on larger scales (e.g., Rosario et al., 2012; Rovilos et al., 2012).
Using the high-resolution optical HST images available in the GOODS-N and GOODS-S fields, as part of the GOODS and CANDELS projects (Giavalisco et al., 2004; Grogin et al., 2011), we visually inspected the morphology of these galaxies to identify signs of distortions or disturbances, which would indicate a recent galaxy merger/interaction event. We adopted a similar classification scheme to that used by Kocevski et al. (2012); see Del Moro et al. (2016, for details), separating the sources into “disturbed” and “undisturbed”. In Figure 3 we show the fraction of sources having disturbed and undisturbed morphologies over the total, dividing them again into unobscured/moderately obscured (blue squares) and heavily obscured (magenta circles), as in Figure 2. We find that a relatively high fraction of our sources shows signs of distortions/interactions (≈40%), higher than those typically found at low redshift (~15–20% at z < 1; e.g., Cisternas et al., 2011). This is in agreement with the trend of increasing major-merger fraction with redshift seen in previous works (e.g., Conselice et al., 2003; Treister and Urry, 2006; Kartaltepe et al., 2007). We find that, on average, the most-heavily obscured quasars tend to have more disturbed morphologies than the unobscured/moderately obscured ones (≈53 vs. ≈20%, respectively); although the errors on these fractions are large due to the small number of sources in our sample, the difference between the two quasar populations is significant at the 90% confidence level (Fisher exact probability test: P = 0.087). This trend is seen also in other studies (e.g., Kocevski et al., 2015; Ricci et al., 2017). We note, however, that these studies investigated samples in different redshift and/or luminosity ranges compared to ours, and therefore the actual fractions of sources with disturbed morphologies are not directly comparable. The smaller fraction of disturbed systems we found for the unobscured/moderately-obscured quasars could be interpreted within the SMBH-galaxy evolutionary models, as the unobscured quasars would represent a later stage of the evolution compared to the heavily-obscured sources and the distortion features due to mergers or interactions might have faded by the time these quasars are observed as unobscured, given the relaxation time of a galaxy is typically ~200–400 Myr (e.g., Lotz et al., 2010). On the other hand, for the most heavily-obscured quasars, which might represent a younger stage of evolution after a merger in this scenario, the signatures of the recent interactions are still evident in their hosts.
Figure 3
4. Conclusions
We have investigated the AGN and host galaxy properties of a sample of 33 quasars at z = 1 − 3 within the GOODS-Herschel fields, selected in the MIR band through detailed SED analysis to have an intrinsic AGN luminosity of erg s−1. Despite being intrinsically the most luminous quasars within these fields, ~26% of them are not detected in the deep 2 and 4 Ms Chandra X-ray data covering these sky areas.
We performed X-ray spectral analysis of the 24 X-ray-detected sources to investigate the AGN properties, and we found that the vast majority (~67%; 16/24 sources) are obscured by cm−2, with more than half of them (9/16) being heavily obscured ( cm−2). Including the X-ray undetected sources, which are likely to be the most heavily CT AGN, these fractions reach ~76% (~54% are heavily obscured). This means that there is a very large population of heavily obscured, intrinsically luminous quasars at redshift z ≈ 2, which can be revealed in the IR band, but remains (in part) undetected in the X-ray band.
We investigated the host galaxy properties of these quasars through their SFR, measured in the FIR band from SED fitting using Spitzer and Herschel data, and did not find any strong link between the amount of SF and the X-ray obscuration, possibly suggesting that the X-ray obscuration is mostly concentrated in the nuclear regions and does not depend on the presence of gas on larger scales.
We also visually classified the morphology of these quasars as disturbed or undisturbed using high-resolution HST data to identify signs of distortions/asymmetries in the galaxies. We find that a significant fraction (~40%) have disturbed morphologies, suggesting they have experienced a recent merger or interaction event. We find a larger fraction of sources with disturbed morphologies amongst the heavily-obscured quasars (~53%) rather than the unobscured/moderately-obscured ones (~20%). Our results possibly support the SMBH-galaxy evolutionary scenario where the heavily-obscured quasars represent an earlier stage of evolution after the merger, while the unobscured quasars represent a later stage of the evolution, when the system has relaxed, the signs of interaction have already faded, and the nucleus becomes unobscured.
Statements
Author contributions
AD developed the concept, performed all the analyses and wrote the manuscript. DA initiated and helped developing the concept. FB provided the X-ray spectra. ED compiled the multiwavelength catalog, which was used for the SED analysis. DK and DM provided the optical HST images, used to classify the galaxy morphology. FS calculated the mean star-formation rates.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Footnotes
1.^We note however, that the photometric redshift assumed in our analysis of this source (z = 2.55) differs from that reported in the Luo et al. (2017) catalog (z = 1.81).
References
1
AlexanderD. M.BauerF. E.BrandtW. N.SchneiderD. P.HornschemeierA. E.VignaliC.et al. (2003). The chandra deep field north survey. XIII. 2 Ms point-source catalogs. Astron. J.126, 539–574. 10.1086/376473
2
AlexanderD. M.HickoxR. C. (2012). What drives the growth of black holes?New A Rev.56, 93–121. 10.1016/j.newar.2011.11.003
3
AntonucciR. (1993). Unified models for active galactic nuclei and quasars. Annu. Rev. Astron. Astrophys.31, 473–521. 10.1146/annurev.aa.31.090193.002353
4
BrandtW. N.AlexanderD. M. (2015). Cosmic X-ray surveys of distant active galaxies. The demographics, physics, and ecology of growing supermassive black holes. Astron. Astrophys. Rev.23:1. 10.1007/s00159-014-0081-z
5
BrightmanM.NandraK. (2011). An XMM-Newton spectral survey of 12 μm selected galaxies - I. X-ray data. Month. Notices R. Astron. Soc.413, 1206–1235. 10.1111/j.1365-2966.2011.18207.x
6
BroosP.TownsleyL.GetmanK.BauerF. (2012). AE: ACIS Extract. Astrophysics Source Code Library.
7
BroosP. S.TownsleyL. K.FeigelsonE. D.GetmanK. V.BauerF. E.GarmireG. P. (2010). Innovations in the analysis of chandra-ACIS observations. Astrophys. J.714, 1582–1605. 10.1088/0004-637X/714/2/1582
8
CisternasM.JahnkeK.InskipK. J.KartaltepeJ.KoekemoerA. M.LiskerT.et al. (2011). The bulk of the black hole growth since z ~ 1 occurs in a secular universe: no major merger-AGN connection. Astrophys. J.726:57. 10.1088/0004-637X/726/2/57
9
ConseliceC. J.BershadyM. A.DickinsonM.PapovichC. (2003). A direct measurement of major galaxy mergers at z≲3. Astron. J.126, 1183–1207. 10.1086/377318
10
Del MoroA.AlexanderD. M.BauerF. E.DaddiE.KocevskiD. D.McIntoshD. H.et al. (2016). Mid-infrared luminous quasars in the GOODS-Herschel fields: a large population of heavily obscured, Compton-thick quasars at z ≈ 2. Month. Notices R. Astron. Soc.456, 2105–2125. 10.1093/mnras/stv2748
11
Del MoroA.AlexanderD. M.MullaneyJ. R.DaddiE.PannellaM.BauerF. E.et al. (2013). GOODS-Herschel: radio-excess signature of hidden AGN activity in distant star-forming galaxies. Astron. Astrophys.549:A59. 10.1051/0004-6361/201219880
12
Di MatteoT.SpringelV.HernquistL. (2005). Energy input from quasars regulates the growth and activity of black holes and their host galaxies. Nature433, 604–607. 10.1038/nature03335
13
ElbazD.DickinsonM.HwangH. S.Díaz-SantosT.MagdisG.MagnelliB.et al. (2011). GOODS-Herschel: an infrared main sequence for star-forming galaxies. Astron. Astrophys.533:A119. 10.1051/0004-6361/201117239
14
FerrareseL.MerrittD. (2000). A fundamental relation between supermassive black holes and their host galaxies. Astrophys. J. Lett.539, L9–L12. 10.1086/312838
15
FioreF.PuccettiS.BrusaM.SalvatoM.ZamoraniG.AldcroftT.et al. (2009). Chasing highly obscured QSOs in the COSMOS field. Astrophys. J.693, 447–462. 10.1088/0004-637X/693/1/447
16
GandhiP.HorstH.SmetteA.HönigS.ComastriA.GilliR.et al. (2009). Resolving the mid-infrared cores of local Seyferts. Astron. Astrophys.502, 457–472. 10.1051/0004-6361/200811368
17
GebhardtK.BenderR.BowerG.DresslerA.FaberS. M.FilippenkoA. V.et al. (2000). A relationship between nuclear black hole mass and galaxy velocity dispersion. Astrophys. J. Lett.539, L13–L16. 10.1086/312840
18
GiavaliscoM.FergusonH. C.KoekemoerA. M.DickinsonM.AlexanderD. M.BauerF. E.et al. (2004). The great observatories origins deep survey: initial results from optical and near-infrared imaging. Astrophys. J. Lett.600, L93–L98. 10.1086/379232
19
GroginN. A.KocevskiD. D.FaberS. M.FergusonH. C.KoekemoerA. M.RiessA. G.et al. (2011). CANDELS: the cosmic assembly near-infrared deep extragalactic legacy survey. Astrophys. J. Suppl.197:35. 10.1088/0067-0049/197/2/35
20
HopkinsP. F.HernquistL.CoxT. J.Di MatteoT.RobertsonB.SpringelV. (2006). A unified, merger-driven model of the origin of starbursts, quasars, the cosmic X-ray background, supermassive black holes, and galaxy spheroids. Astrophys. J. Suppl.163, 1–49. 10.1086/499298
21
KaplanE. L.MeierP. (1958). Nonparametric estimation from incomplete observations. J. Am. Stat. Assoc.53, 457–481. 10.1080/01621459.1958.10501452
22
KartaltepeJ. S.SandersD. B.ScovilleN. Z.CalzettiD.CapakP.KoekemoerA.et al. (2007). Evolution of the frequency of luminous (>=L*V) close galaxy pairs at z < 1.2 in the COSMOS field. Astrophys. J. Suppl.172, 320–328. 10.1086/519953
23
KennicuttR. C.Jr. (1998). Star formation in galaxies along the hubble sequence. Annu. Rev. Astron. Astrophys.36, 189–232. 10.1146/annurev.astro.36.1.189
24
KocevskiD. D.BrightmanM.NandraK.KoekemoerA. M.SalvatoM.AirdJ.et al. (2015). Are compton-thick AGNs the missing link between mergers and black hole growth?Astrophys. J.814:104. 10.1088/0004-637X/814/2/104
25
KocevskiD. D.FaberS. M.MozenaM.KoekemoerA. M.NandraK.RangelC.et al. (2012). CANDELS: constraining the AGN-Merger connection with host morphologies at z ~ 2. Astrophys. J.744:148. 10.1088/0004-637X/744/2/148
26
KormendyJ.HoL. C. (2013). Coevolution (or not) of supermassive black holes and host galaxies. Annu. Rev. Astron. Astrophys.51, 511–653. 10.1146/annurev-astro-082708-101811
27
KormendyJ.RichstoneD. (1995). Inward bound—the search for supermassive black holes in galactic nuclei. Annu. Rev. Astron. Astrophys.33:581.
28
LotzJ. M.JonssonP.CoxT. J.PrimackJ. R. (2010). The effect of mass ratio on the morphology and time-scales of disc galaxy mergers. Month. Notices R. Astron. Soc.404, 575–589. 10.1111/j.1365-2966.2010.16268.x
29
LuoB.BauerF. E.BrandtW. N.AlexanderD. M.LehmerB. D.SchneiderD. P.et al. (2008). The chandra deep field-south survey: 2 ms source catalogs. Astrophys. J. Suppl.179, 19–36. 10.1086/591248
30
LuoB.BrandtW. N.XueY. Q.LehmerB.AlexanderD. M.BauerF. E.et al. (2017). The chandra deep field-south survey: 7 ms source catalogs. Astrophys. J. Suppl.228:2. 10.3847/1538-4365/228/1/2
31
LutzD.MaiolinoR.SpoonH. W. W.MoorwoodA. F. M. (2004). The relation between AGN hard X-ray emission and mid-infrared continuum from ISO spectra: scatter and unification aspects. Astron. Astrophys.418, 465–473. 10.1051/0004-6361:20035838
32
MadauP.FergusonH. C.DickinsonM. E.GiavaliscoM.SteidelC. C.FruchterA. (1996). High-redshift galaxies in the Hubble Deep Field: colour selection and star formation history to z˜4. Month. Notices R. Astron. Soc.283, 1388–1404. 10.1093/mnras/283.4.1388
33
MagorrianJ.TremaineS.RichstoneD.BenderR.BowerG.DresslerA.et al. (1998). The demography of massive dark objects in galaxy centers. Astron. J.115, 2285–2305. 10.1086/300353
34
MullaneyJ. R.AlexanderD. M.GouldingA. D.HickoxR. C. (2011). Defining the intrinsic AGN infrared spectral energy distribution and measuring its contribution to the infrared output of composite galaxies. Month. Notices R. Astron. Soc.414, 1082–1110. 10.1111/j.1365-2966.2011.18448.x
35
RicciC.BauerF. E.TreisterE.SchawinskiK.PrivonG. C.BlechaL.et al. (2017). Growing supermassive black holes in the late stages of galaxy mergers are heavily obscured. Month. Notices R. Astron. Soc.468, 1273–1299. 10.1093/mnras/stx173
36
RosarioD. J.SantiniP.LutzD.ShaoL.MaiolinoR.AlexanderD. M.et al. (2012). The mean star formation rate of X-ray selected active galaxies and its evolution from z ~ 2.5: results from PEP-Herschel. Astron. Astrophys.545:A45. 10.1051/0004-6361/201219258
37
RovilosE.ComastriA.GilliR.GeorgantopoulosI.RanalliP.VignaliC.et al. (2012). GOODS-Herschel: ultra-deep XMM-Newton observations reveal AGN/star-formation connection. Astron. Astrophys.546:A58. 10.1051/0004-6361/201218952
38
SalpeterE. E. (1955). The luminosity function and stellar evolution. Astrophys. J.121:161.
39
StanleyF.HarrisonC. M.AlexanderD. M.SwinbankA. M.AirdJ. A.Del MoroA.et al. (2015). A remarkably flat relationship between the average star formation rate and AGN luminosity for distant X-ray AGN. Month. Notices R. Astron. Soc.453, 591–604. 10.1093/mnras/stv1678
40
TreisterE.UrryC. M. (2006). The evolution of obscuration in active galactic nuclei. Astrophys. J. Lett.652, L79–L82. 10.1086/510237
41
XueY. Q.LuoB.BrandtW. N.BauerF. E.LehmerB. D.BroosP. S.et al. (2011). The chandra deep field-south survey: 4 ms source catalogs. Astrophys. J. Suppl.195:10. 10.1088/0067-0049/195/1/10
42
YaqoobT. (1997). X-ray transmission in cold matter: nonrelativistic corrections for compton scattering. Astrophys. J.479, 184–189. 10.1086/303843
Summary
Keywords
galaxies: active, quasars: general, quasars: supermassive black holes, X-rays: galaxies, infrared: galaxies, galaxies: star formation
Citation
Del Moro A, Alexander DM, Bauer FE, Daddi E, Kocevski DD, Stanley F and McIntosh DH (2018) Luminous and Obscured Quasars and Their Host Galaxies. Front. Astron. Space Sci. 4:67. doi: 10.3389/fspas.2017.00067
Received
30 October 2017
Accepted
18 December 2017
Published
12 January 2018
Volume
4 - 2017
Edited by
Ascensión Del Olmo, Instituto de Astrofísica de Andalucía (CSIC), Spain
Reviewed by
Lorena Hernandez-Garcia, University of Valparaíso, Chile; Begoña Garcia-Lorenzo, Instituto de Astrofísica de Canarias, Spain
Updates
Copyright
© 2018 Del Moro, Alexander, Bauer, Daddi, Kocevski, Stanley and McIntosh.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Agnese Del Moro adelmoro@mpe.mpg.de
This article was submitted to Cosmology, a section of the journal Frontiers in Astronomy and Space Sciences
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.