The Fall of Active Galactic Nuclei and the Rise of Star-forming Galaxies: A Close Look at the Chandra Deep Field X-Ray Number Counts

By F. E. Bauer, D. M. Alexander, W. N. Brandt, D. Schneider, E. Treister, A. Hornschemeier, G. P. G. IoA, Psu, Yale, U. Chile and 1 more. 2004

243 citations recorded by the source corpus.

We investigate the X-ray number counts in the 1–2 Ms Chandra Deep Fields (CDFs) to determine the contributions of faint X-ray source populations to the extragalactic X-ray background (XRB). X-ray sources were separated into active galactic nuclei (AGNs), star-forming galaxies, and Galactic stars primarily on the basis of their X-ray–to–optical flux ratios, optical spectral classifications, X-ray spectra, and intrinsic X-ray luminosities. Number count slopes and normalizations below 2 × 10-15 ergs cm-2 s-1 were calculated in each band for all source types assuming a single power-law model. We find that AGNs continue to dominate the number counts in the 0.5–2.0 and 2–8 keV bands. At flux limits of ≈2.5 × 10-17 ergs cm-2 s-1 (0.5–2.0 keV) and ≈2.0 × 10-16 ergs cm-2 s-1 (2–8 keV), the overall AGN source densities are 7166 and 4558 sources deg-2, respectively; these are factors of ∼10–20 higher than those found in the deepest optical spectroscopic surveys. Although still a minority, the number counts of star-forming galaxies climb steeply, such that they eventually achieve source densities of 1727 sources deg-2 (0.5–2.0 keV) and 711 sources deg-2 (2–8 keV) at the CDF flux limits. The number of star-forming galaxies will likely overtake the number of AGNs at ∼1 × 10-17 ergs cm-2 s-1 (0.5–2.0 keV) and dominate the overall number counts thereafter. Adopting XRB flux densities of (7.52 ± 0.35) × 10-12 ergs cm-2 s-1 deg-2 for 0.5–2.0 keV and (1.79 ± 0.11) × 10-11 ergs cm-2 s-1 deg-2 for 2–8 keV, the CDFs resolve a total of 89.5 percent and 92.6 percent of the extragalactic 0.5–2.0 and 2–8 keV XRBs, respectively. AGNs as a whole contribute ≈83% and ≈95% to these resolved XRB fractions, respectively, whereas star-forming galaxies comprise only ≈3% and ≈2%, respectively, and Galactic stars comprise the remainder. Extrapolation of the number count slopes can easily account for the entire 0.5–2.0 and 2–8 keV XRBs to within statistical errors. We also examine the X-ray number counts as functions of intrinsic X-ray luminosity and absorption, finding that sources with L0.5–8 keV > 1043.5 ergs s-1 and NH < 1022 cm-2 are the dominant contributors to the 0.5–2.0 keV XRB flux density, whereas sources with L0.5–8 keV = 1042.5–1044.5 ergs s-1 and a broad range of absorption column densities primarily contribute to the 2–8 keV XRB flux density. This trend suggests that even less intrinsically luminous, more highly obscured AGNs may dominate the number counts at higher energies, where the XRB intensity peaks. Finally, we revisit the reported differences between the CDF-North and CDF-South number counts, finding that the two fields are consistent with each other except for sources detected at 2–8 keV below F2–8 keV ≈ 1 × 10-15 ergs cm-2 s-1, for which deviations gradually increase to ≈3.9 σ.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/45161227dc54412901c3b0562368b65f0552ebd6

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In Astronomy and Astrophysics

1. D. M. Alexander
2. P. Hopkins
3. L. Hernquist
4. W. N. Brandt
5. E. Daddi
6. Mark Dickinson
7. D. Elbaz
8. R. Dav'e

In Nuclear and High Energy Physics

1. D. M. Alexander
2. W. N. Brandt
3. G. Hasinger
4. R. Ivison
5. I. Smail
6. Luis C. Ho
7. L. Hernquist
8. P. Hopkins

Citations in the corpus

• The Extended Chandra Deep Field-South Survey: Chandra Point-Source Catalogs (2005, methodology): “We restricted our analyses to flux levels where we expect to be mostly complete based on our sensitivity maps and simulations performed by Bauer et al. (2004); this also helps to guard against Eddington bias at low flux levels.”
• Star formation in AGN hosts in GOODS-N (2010, background): “Bauer et al. (2004) have outlined for this very sample a combination of criteria to distinguish X-ray AGN from star formation dominated objects, based on X-ray luminosity, X-ray obscuring column or hardness, optical spectroscopic classifications, and X-ray/optical flux ratio.”
• A CATALOG OF X-RAY POINT SOURCES FROM TWO MEGASECONDS OF CHANDRA OBSERVATIONS OF THE GALACTIC CENTER (2008, methodology): “We calculated the sensitivity of our observations using synthetic star tests, following the basic methods described in Bauer et al. (2004) and Muno et al. (2006a; see Wang 2004 for another approach).”
• Cosmic X-ray surveys of distant active galaxies (2015, unknown): “6; e.g., Bauer et al 2004; Civano et al 2012; Lehmer et al 2012).”
• 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, methodology): “The situation is even worse, since at faint fluxes we expect a relevant contribution from normal X-ray galaxies to the counts (about 20 and 400 deg 2 at S0:5 2 ¼ 10 15 and 10 16 ergs cm 2 s 1, respectively; Ranalli et al. 2003; Bauer et al. 2004).”
• What drives the growth of black holes (2011, background): “…in star-formation activity at higher redshift (e.g., Noeske et al., 2007; Daddi et al., 2007b; Pannella et al., 2009; Rodighiero et al., 2010a; Elbaz et al., 2011). can select AGNs almost irrespective of the presence of obscuration (e.g., Bauer et al., 2004; Tozzi et al., 2006; Gilli et al., 2007).”
• Populations of X-Ray Sources in Galaxies (2006, background): “In summary, the emission from normal galaxies becomes an increasingly greater component of the X-ray emission at the deepest X-ray counts (Bauer et al. 2004; Ranalli, Comastri & Setti 2005); moreover, the hard X-ray emission is a direct diagnostic of star formation, as demonstrated by the good…”
• Enhanced star formation rates in AGN hosts with respect to inactive galaxies from PEP-Herschel observations (2012, methodology): “We refer to Bauer et al. (2004) for details on spectroscopic and photometric redshifts and X-ray properties.”
• Infrared Power-Law Galaxies in the Chandra Deep Field-South: Active Galactic Nuclei and Ultraluminous Infrared Galaxies (2006, background): “…that the IR power-law galaxies detected in X-rays with values of the photon index typical of highly absorbed or Compton-thick X-ray emission ( < 1; Bauer et al. 2004) tend to have log (2 8 keV/12 m)P 6:5, which could explain the smaller hard X-ray–to–mid-IR flux ratios if the restframe hard…”
• Evolution of the dusty infrared luminosity function from z = 0 to z = 2.3 using observations from Spitzer (2011, background): “AGNs are identified as galaxies with either LX [0.5 − 8.0 keV] ≥ 3 × 1042 erg s−1 or a hardness ratio greater than 0.8 (ratio of the counts in the 2−8 keV to 0.5−2 keV passbands) (Bauer et al. 2004).”
• BAT AGN Spectroscopic Survey. V. X-Ray Properties of the Swift/BAT 70-month AGN Catalog (2017, background): “The integrated emission of unresolved AGN gives rises to the Cosmic X-ray background (CXB; e.g., Giacconi et al. 1962; Bauer et al. 2004; Harrison et al. 2016).”
• Constraints on sterile neutrinos as dark matter candidates from the diffuse X-ray background (2005, background): “Journal compilation C© 2006 RAS, MNRAS 370, 213–218 has enabled some 90 per cent of the XRB to be resolved into point sources (Brandt & Hasinger 2005; De Luca & Molendi 2005; Bauer et al. 2004).”
• The mean star formation rate of X-ray selected active galaxies and its evolution from z~2.5: results from PEP-Herschel (2012, methodology): “For GOODS-N we used the 2 Msec Chandra Deep FieldNorth catalog of Alexander et al. (2003) and an updated version of the classification into AGN and other types of X-ray sources compiled by Bauer et al. (2004) (see also Shao et al. 2010).”
• GOODS-Herschel: the far-infrared view of star formation in active galactic nucleus host galaxies since z ≈ 3 (2011, methodology): “Redshifts and absorptioncorrected 2-10 keV luminosities (i.e.,LX) for each X-ray AGN in the CDF-N catalogue were taken from the catalogue described in Bauer et al. (2004).”
• A CHANDRA PERSPECTIVE ON GALAXY-WIDE X-RAY BINARY EMISSION AND ITS CORRELATION WITH STAR FORMATION RATE AND STELLAR MASS: NEW RESULTS FROM LUMINOUS INFRARED GALAXIES (2010, background): “…these background estimates will account for emission due to unrelated background X-ray sources (e.g., distant AGNs; see, e.g., Morretti et al. 2003; Bauer et al. 2004; Kim et al. 2007) that fall below our source detection threshold, theseestimates donot include contributions from unrelated sources…”
• THE REPEATING FAST RADIO BURST FRB 121102: MULTI-WAVELENGTH OBSERVATIONS AND ADDITIONAL BURSTS (2016, background): “7+330807 is ∼1 (Bauer et al. 2004).”
• DEEP EXTRAGALACTIC X-RAY SURVEYS (2005, background): “…fainter fluxes (up to 30–40%; see Figure 3), and this trend will continue until they become the numerically dominant source population at 0.5–2 keV fluxes of ≈ 5 × 10−18 erg cm−2 s−1 (e.g., Hornschemeier et al. 2002, 2003; Miyaji & Griffiths 2002; Ranalli, Comastri & Setti 2003; Bauer et al. 2004).”