The X-Ray Spectral Properties of SCUBA Galaxies

By D. M. Alexander, F. E. Bauer, S. Chapman, I. Smail, A. Blain, W. N. Brandt, R. Ivison. 2005

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Deep SCUBA surveys have uncovered a large population of massive submillimeter-emitting galaxies (SMGs; f850μm ≳ 4 mJy) at z ≳ 1. Although it is generally believed that these galaxies host intense star formation activity, there is growing evidence that a substantial fraction also harbor an active galactic nucleus (AGN; i.e., an accreting super-massive black hole [SMBH]). We present here possibly the strongest evidence for this viewpoint to date: the combination of ultradeep X-ray observations (the 2 Ms Chandra Deep Field-North) and deep Keck spectroscopic data of SMGs with radio counterparts. We find that the majority (≈75%) of these radio-selected spectroscopically identified SMGs host AGN activity; the other ≈25% have X-ray properties consistent with star formation (X-ray-derived star formation rates of ≈1300-2700 M☉ yr-1). The AGNs have properties generally consistent with those of nearby luminous AGNs (Γ ≈ 1.8 ± 0.5, NH ≈ 1020-1024 cm-2, and LX ≈ 1043-1044.5 ergs s-1), and the majority (≈80%) are heavily obscured (NH ≳ 1023 cm-2). We construct composite rest-frame 2-20 keV spectra for three different obscuration classes [NH < 1023 cm-2, NH = (1-5) × 1023 cm-2, and NH > 5 × 1023 cm-2], which reveal features not seen in the individual X-ray spectra. An ≈1 keV equivalent width Fe Kα emission line is seen in the composite X-ray spectrum of the most heavily obscured AGNs, suggesting Compton-thick or near Compton-thick absorption. Even taking into account the effects of absorption, we find that the average X-ray to far-IR luminosity ratio of the AGN-classified SMGs (LX/LFIR = 0.004) is approximately 1 order of magnitude below that found for typical quasars. This result suggests that intense star formation activity (of order ≈1000 M☉ yr-1) dominates the bolometric output of these SMGs. However, we also explore the possibility that the X-ray to far-IR luminosity ratio of the AGN components is intrinsically less than that found for typical quasars and postulate that some SMGs may be AGN dominated. We investigate the implications of our results for the growth of massive black holes, discuss the prospects for deeper X-ray observations, and explore the scientific potential offered by the next generation of X-ray observatories.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/e982439a71d488c0ac0eb45c59f0d6d8fbc180c8

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THE XMM-NEWTON WIDE-FIELD SURVEY IN THE COSMOS FIELD (XMM-COSMOS): DEMOGRAPHY AND MULTIWAVELENGTH PROPERTIES OF OBSCURED AND UNOBSCURED LUMINOUS ACTIVE GALACTIC NUCLEI (2010, background): “…2003, Civano et al. 2007), Extremely Red Objects (e.g. Alexander et al. 2002, Brusa et al. 2005, Severgnini et al. 2005), Sub Millimeter Galaxies (e.g. Alexander et al. 2005, Laird et al. 2009), high-z starforming systems (e.g. Daddi et al. 2007, Fiore et al. 2008, 2009, Treister et al. 2009b),…”
The synthesis of the cosmic X-ray background in the Chandra and XMM-Newton era (2006, background): “While it has been proposed that they could be associated with high-redshift submm sources (Alexander et al. 2005), the actual size of the obscured QSO population at high redshift is however still largely debated.”
Characterization of Scuba-2 450 μm and 850 μm selected galaxies in the COSMOS field (2013, background): “…studies of the X-ray and mid-infrared properties of SMGs have led to the conclusion that the majority of the SMGs’ bolometric energy is star formation dominated (Alexander et al. 2005; Pope et al. 2008; Coppin et al. 2008; Menéndez-Delmestre et al. 2009; Coppin et al. 2010; Laird et al. 2010).”
A survey of molecular gas in luminous sub-millimetre galaxies (2012, methodology): “Hainline et al. (2011) analysed a sample of X-ray-observed SMGs, finding that the fractional contribution to the mid-IR from non-stellar (i.e. power-law) emission provides an excellent proxy for the hard X-ray luminosity and so provides a robust AGN diagnostic.”
The n ature of submillimetre galaxies in cosmological hydrodynamic simulations (2009, background): “…mergers (e.g. di Matteo, Springel, & Hernquist 2005), observations indicate that star formation dominates the bolometric luminosity, and active galactic nuclei (AGN) provide only a minor contribution (Alexander et al. 2005; Menéndez-Delmestre et al. 2007; Pope et al. 2008; Clements et al. 2008).”
The HELLAS2XMM Survey. VII. The Hard X-Ray Luminosity Function of AGNs up to z = 4: More Absorbed AGNs at Low Luminosities and High Redshifts (2005, background): “…fraction of absorbed AGNs with redshift is in agreement with a picture in which the peak epoch of the star formation (z ¼ 1 2) corresponds to a heavily obscured rapid black hole phase, which is ultimately preceded by an unobscured quasar phase (Alexander et al. 2005a, 2005b; Hopkins et al. 2005).”
COSBO: The MAMBO 1.2 Millimeter Imaging Survey of the COSMOS Field (2007, background): “…and high-resolution CO follow-up observations (Genzel et al. 2003; Neri et al. 2003; Greve et al. 2005; Tacconi et al. 2006) of SCUBA andMAMBO sources have shown them to be massive starburst galaxies, often with coexisting active galactic nuclei (AGNs) of minor luminosity (Alexander et al. 2005).”
Enhanced star formation rates in AGN hosts with respect to inactive galaxies from PEP-Herschel observations (2012, background): “Although this degree of X-ray emission mostly traces AGN activity, it may also arise from intense star formation at the level of a few 102M⊙/yr (Ranalli et al. 2003; Alexander et al. 2005; Laird et al. 2005, 2010).”
A Herschel view of the far-infrared properties of submillimetre galaxies (2012, background): “Their infrared luminosities are mainly powered by star-formation rather than by active galactic nucleus (AGN) activity (Alexander et al. 2005; Lutz et al. 2005; Valiante eal.”
EVIDENCE FOR A CLUMPY, ROTATING GAS DISK IN A SUBMILLIMETER GALAXY AT z = 4 (2012, background): “Their huge far– infrared (FIR) luminosities (∼1013 L⊙) are believed to be primarily driven by intense (∼103M⊙ yr−1) star formation (e.g., Alexander et al. 2005, 2003), adding to their already significant stellar masses (e.g., Borys et al. 2005; Hainline et al. 2011).”
What drives the growth of black holes (2011, background): “…> 1043 erg s−1 increases from ≈ 3–10% for SFRs ≈ 30– 200 M yr−1 (i.e., equivalent to LIRGs) to ≈ 10–40% for SFRs ≈ 100–500 M yr−1 (i.e., equivalent to ULIRGs; e.g., Alexander et al. 2005; Laird et al. 2010; Symeonidis et al. 2010; Xue et al. 2010; Georgantopoulos et al. 2011; Rafferty et al. 2011).”
Energetic galaxy-wide outflows in high-redshift ultraluminous infrared galaxies hosting AGN activity (2012, background): “Using optical, mid-infrared and Xray diagnostics, high-redshift ULIRGs are found to have emission dominated by starburst activity with≈30% of SMGs also revealing significant AGN activity (e.g., Alexander et al. 2003, 2005, 2008; Chapman et al. 2005; Ivison et al. 2010; Raffertyt al.”
The stellar masses and specific star-formation rates of submillimetre galaxies (2011, background): “%) contamination of their bolometric luminosities by active galactic nuclei (AGNs; Alexander et al. 2005, 2008; Menéndez-Delmestre et al. 2007, 2009; Valiante et al. 2007; Pope et al. 2008; Hainline et al. 2009; Murphy et al. 2009; Watabe et al. 2009, M10).”
Cosmic evolution of submillimeter galaxies and their contribution to stellar mass assembly (2009, background): “…investigated the issue of AGN activity further by analysing the average q values of the following subsamples (see also Figure 6): X-ray identified (Alexander et al., 2005) AGNs: 2.32±0.06 and starbursts: 2.12±0.18; optically identified AGNs (Chapman et al., 2005): 2.27 ± 0.09; and midIR…”
Determining the Properties and Evolution of Red Galaxies from the Quasar Luminosity Function (2005, background): “This is also supported by direct observations of quasar host galaxies, which find strong evidence for simultaneous and strongly associated black hole growth and star formation at redshifts corresponding to peak quasar activity (z& 1) (e.g., Alexander et al. 2005).”
MOST SUBMILLIMETER GALAXIES ARE MAJOR MERGERS (2010, background): “Veilleux et al. (2009) find local ULIRG luminosities to have ≈ 39% AGN contribution; estimates for SMGs are more uncertain, but span a range of 8 - 35% (Swinbank et al. 2004; Alexander et al. 2005, 2008a; Lutz et al. 2005; Valiante et al. 2007; Pope et al. 2008; Menéndez-Delmestre et al. 2009).”
The Relation between Quasar and Merging Galaxy Luminosity Functions and the Merger-driven Star Formation History of the Universe (2006, background): “…buried quasars (Sanders et al. 1988b), as implied byX-ray observations of growing black holes in NGC 6240 (Komossa et al. 2003) and other ULIRGs, which are heavily obscured at optical and infrared wavelengths (Gerssen et al. 2004; Max et al. 2005; Alexander et al. 2005a, 2005b; Borys et al. 2005).”
A study of the gas–star formation relation over cosmic time (2010, background): “…meantime ISO and Spitzer spectroscopy, as well as Chandra X-ray observations have made a compelling case that most z ≥ 1 SMGs and the majority of z ∼ 0 ULIRGs are powered predominantly (but by no means exclusively) by star formation (Genzel et al. 1998; Alexander et al. 2005; Veilleux et al. 2009).”
The LABOCA survey of the Extended Chandra Deep Field South: A photometric redshift survey of submillimetre galaxies (2010, background): “If this results from the strong luminosity evolution of starburst galaxies [as opposed to obscured active galactic nuclei (AGNs); Alexander et al. 2005; Laird et al. 2010], then a significant fraction of the massive star formation (and metal production) at high redshift may be occurring in this…”