The Chandra COSMOS Survey: III. Optical and Infrared Identification of X-ray Point Sources

Abstract: The Chandra COSMOS Survey (C-COSMOS) is a large, 1.8 Ms, Chandra program that has imaged the central 0.9 deg^2 of the COSMOS field down to limiting depths of 1.9 10^-16 erg cm^-2 s-1 in the 0.5-2 keV band, 7.3 10^-16 erg cm^-2 s^-1 in the 2-10 keV band, and 5.7 10^-16 erg cm^-2 s-1 in the 0.5-10 keV band. In this paper we report the i, K and 3.6micron identifications of the 1761 X-ray point sources. We use the likelihood ratio technique to derive the association of optical/infrared counterparts for 97% of the X-ray sources. For most of the remaining 3%, the presence of multiple counterparts or the faintness of the possible counterpart prevented a unique association. For only 10 X-ray sources we were not able to associate a counterpart, mostly due to the presence of a very bright field source close by. Only 2 sources are truly empty fields. Making use of the large number of X-ray sources, we update the "classic locus" of AGN and define a new locus containing 90% of the AGN in the survey with full band luminosity >10^42 erg/s. We present the linear fit between the total i band magnitude and the X-ray flux in the soft and hard band, drawn over 2 orders of magnitude in X-ray flux, obtained using the combined C-COSMOS and XMM-COSMOS samples. We focus on the X-ray to optical flux ratio (X/O) and we test its known correlation with redshift and luminosity, and a recently introduced anti-correlation with the concentration index (C). We find a strong anti-correlation (though the dispersion is of the order of 0.5 dex) between C and X/O, computed in the hard band, and that 90% of the obscured AGN in the sample with morphological information live in galaxies with regular morphology (bulgy and disky/spiral), suggesting that secular processes govern a significant fraction of the BH growth at X-ray luminosities of 10^43- 10^44.5 erg/s.

An Overview of the COSMOS Survey: Optical and Infrared Identification of X-ray Point Sources

The Chandra COSMOS Survey (C-COSMOS) is an extensive study focused on imaging a significant portion of the COSMOS field with high precision using the Chandra X-ray Observatory. The primary objective of this study is to provide optical and infrared identifications for X-ray point sources detected in this survey. The analyzed data is critical for understanding the connection between galaxies and their central supermassive black holes (SMBHs), with implications for the co-evolution of these entities.

Survey Details and Data Acquisition

C-COSMOS covers an area of 0.9 square degrees of the COSMOS field with a total exposure of 1.8 Ms, reaching depths of 1.9 × 10(^{-16}) erg cm(^{-2}) s(^{-1}) in the soft X-ray band (0.5–2 keV), 7.3 × 10(^{-16}) erg cm(^{-2}) s(^{-1}) in the hard X-ray band (2–10 keV), and 5.7 × 10(^{-16}) erg cm(^{-2}) s(^{-1}) in the full band (0.5–10 keV). This paper reports identifications of 1761 X-ray point sources by utilizing the likelihood ratio technique to match optical/infrared counterparts in three different bands: the i-band, K-band, and the 3.6 μm band.

Methodology and Results

The likelihood ratio technique effectively associates optical and infrared counterparts with X-ray sources. The method identified counterparts for 97% of the X-ray sources with a strong correlation in the near-infrared bands. This success owes to the coupling of X-ray and near-infrared fluxes, highlighting the survey's depth and coverage as its defining strengths.

The analysis shows that 60% of these sources have spectroscopic identifications, and further classifications were achieved through photometric redshifts, indicating nearly complete identification and characterization of the X-ray sources in this field. These sources were classified into categories including Broad Line AGN (BLAGN), non-BLAGN, galaxy dominated, and stars. The vast majority of the sources have X-ray luminosities indicative of AGN activity rather than star-forming galaxies.

X-ray to Optical Properties

The study explores the X-ray to optical flux ratio (X/O), a pivotal diagnostic for AGN, examining its correlation with redshift and luminosity. The paper updates the "classic locus" for AGN characterized two decades ago, providing a new locus that incorporates 90% of the AGN in the survey based on X/O characteristics. Significant findings reveal a robust anti-correlation between the X/O ratio and the morphological concentration index, suggesting that non-mergal, secular processes facilitate a sizeable component of black hole growth at X-ray luminosities of 10(^{43})-10(^{44.5}) erg s(^{-1}).

Implications and Future Directions

This research delivers insight into the composite picture of AGN and their host galaxies over a range of redshifts. The study highlights the importance of multi-wavelength data in tracing the AGN population accurately and provides empirical evidence for secular (as opposed to merger-driven) black hole growth, a critical aspect of galaxy evolution models. Future surveys might extend these findings by integrating deeper data, particularly at near-infrared and submillimeter wavelengths, to probe even higher-redshift AGN and further refine our understanding of early SMBH development stages.

In conclusion, the C-COSMOS survey exemplifies how large, well-coordinated surveys can significantly advance our understanding of cosmic evolution by solving the intricate interconnection between galaxies and their central SMBHs. The survey's comprehensive approach acts as a bridge between various depths and areas, making it an essential component of the broader narrative on SMBH and galaxy co-evolution.