[Paper Review] The HELLAS2XMM survey: IV. Optical identifications and the evolution of the accretion luminosity in the Universe

F. Fiore, M. Brusa+13 more|ArXiv.org|Jun 26, 2003

Astrophysical Phenomena and ObservationsPhysics and Astronomy45 references156 citations

TL;DR

This study presents optical identifications of 122 XMM-Newton X-ray sources from the HELLAS2XMM survey, revealing that ~20% exhibit extreme X-ray to optical flux ratios (X/O > 10), indicating highly obscured or luminous AGN. Combining with deep surveys, it finds high-luminosity AGN (log L_{2-10keV} > 44.5) peak in number density at z ≈ 1–2 and evolve differently than low-luminosity AGN, with comparable contributions to accretion luminosity density between z=2 and 4.

ABSTRACT

We present results from the photometric and spectroscopic identification of 122 X-ray sources recently discovered by XMM-Newton in the 2-10 keV band (the HELLAS2XMM 1dF sample). Their flux cover the range 8E-15-4E-13 cgs and the total area surveyed is 0.9 deg2. About 20% of the hard X-ray selected sources have an X-ray to optical flux ratio (X/O) ten times or more higher than that of optically selected AGN. Unlike the faint sources found in the ultra-deep Chandra and XMM-Newton surveys, which reach X-ray (and optical) fluxes more than one order of magnitude lower than the HELLAS2XMM survey sources, many of the extreme X/O sources in our sample have R<=25 and are therefore accessible to optical spectroscopy. We report the identification of 13 sources with X/O>10: 8 are narrow line QSO (i.e. QSO2), four are broad line QSO. We use a combined sample of 317 hard X-ray selected sources (HELLAS2XMM 1dF, CDFN 1Msec, SSA13 and Lockman Hole flux limited samples), 221 with measured z, to evaluate the cosmological evolution of the hard X-ray source's number and luminosity densities. Looking backward in time, the low luminosity sources (logL(2-10keV) = 43-44 erg/s) increase in number at a rate different than the high luminosity sources (logL(2-10keV)>44.5 erg/s), reaching a maximum around z=1 and then levelling off beyond z=2. This translates into an accretion driven luminosity density which is dominated by sources with logL(2-10keV) < 44.5 erg/s up to at least z=1, while the contribution of the same sources and of those with logL(2-10keV)>44.5 erg/s appear to be comparable between z=2 and 4.

Motivation & Objective

• To identify and characterize X-ray sources from the HELLAS2XMM survey using optical photometry and spectroscopy.
• To investigate the nature of sources with extreme X/O flux ratios, particularly those with X/O > 10, which may indicate obscured or highly luminous AGN.
• To measure the cosmological evolution of hard X-ray selected AGN by combining data from shallow (HELLAS2XMM) and deep (CDFN, CDFS, Lockman Hole) surveys.
• To constrain the luminosity and number density evolution of AGN across redshifts z=0 to z=4, especially for high-luminosity and obscured populations.
• To provide observational constraints for hierarchical clustering models of AGN and galaxy co-evolution, particularly regarding the role of obscuration and luminosity.

Proposed method

• Optical follow-up of 122 X-ray sources in the HELLAS2XMM 1dF survey using photometry and spectroscopy across 0.9 deg².
• Cross-identification of X-ray sources with optical/NIR counterparts using multi-band imaging from ESO and TNG telescopes.
• Spectroscopic redshift determination for 221 sources with measured redshifts across the combined sample of 317 hard X-ray selected sources.
• Construction of the X-ray luminosity function and number density evolution by binning sources in redshift and luminosity bins (log L_{2-10keV} = 43–44 and >44.5).
• Comparison of X/O flux ratios with R-K colors to identify Extremely Red Objects (EROs), linking high X/O to obscured or massive host galaxies.
• Use of photometric redshifts for faint, spectroscopically unidentifiable sources to extend the redshift coverage and improve luminosity function estimates.

Experimental results

Research questions

• RQ1 What fraction of hard X-ray selected sources exhibit extreme X/O flux ratios (X/O > 10), and what is their nature?
• RQ2 How do the number and luminosity densities of high-luminosity (log L_{2-10keV} > 44.5) and low-luminosity (log L_{2-10keV} = 43–44) AGN evolve with redshift?
• RQ3 Is there a luminosity-dependent evolution in the cosmic density of AGN, and how does it compare to predictions from AGN synthesis models?
• RQ4 Are obscured and unobscured AGN evolving similarly, and is obscuration correlated with luminosity?
• RQ5 Can the combined HELLAS2XMM and ultra-deep survey data provide robust constraints on the accretion luminosity density at z=2–4?

Key findings

• Approximately 20% of hard X-ray selected sources in the HELLAS2XMM survey have X/O ratios ≥10, indicating extreme X-ray dominance, with 13 such sources spectroscopically identified.
• Among the X/O > 10 sources, eight are narrow-line QSOs and four are broad-line QSOs, suggesting a link to high-luminosity type 2 Seyfert galaxies.
• The number density of low-luminosity AGN (log L_{2-10keV} = 43–44) increases slowly with lookback time, peaking around z ≈ 1 and declining beyond z ≈ 2.
• The number density of high-luminosity AGN (log L_{2-10keV} > 44.5) peaks at z ≈ 1–2 and remains relatively flat beyond z ≈ 2.
• The accretion luminosity density is dominated by sources with log L_{2-10keV} < 44.5 up to at least z = 1, while contributions from both low- and high-luminosity sources become comparable between z = 2 and 4.
• All 10 X/O > 10 sources in the Mignoli et al. sample with R-K > 5 are Extremely Red Objects, linking high X/O to massive, red, evolved hosts.