Optical Selection of Faint AGN in the COSMOS Field

TL;DR

This paper presents a novel method for selecting faint type 1 AGN candidates in the COSMOS field using broadband photometry and morphological analysis, aiming to improve understanding of the faint end of the quasar luminosity function.

Contribution

It introduces a new selection strategy combining color and morphological criteria for faint AGN, validated with spectroscopic data, to better characterize the faint AGN population.

Findings

Effective separation of AGN from faint blue galaxies using Gini Coefficient.

Predicted AGN colors match observed distributions, aiding candidate selection.

Estimated quasar surface densities inform future spectroscopic surveys.

Abstract

We outline a strategy to select faint (i<24.5) type 1 AGN candidates down to the Seyfert/QSO boundary for spectroscopic targeting in the COSMOS field, picking candidates by their nonstellar colors in broadband ground-based photometry and morphological properties extracted from HST-ACS. AGN optical color selection has not been applied to such faint magnitudes in such a large continuous part of the sky. Hot stars are known to be the dominant contaminant for bright AGN candidate selection at z<2, but we anticipate the highest color contamination at all redshifts to be from faint starburst and compact galaxies. Morphological selection via the Gini Coefficient separates most potential AGN from these faint blue galaxies. Recent models of the quasar luminosity function are used to estimate quasar surface densities, and studies of stellar populations in the COSMOS field infer stellar contamination. We use 292 spectroscopically confirmed type 1 AGN and quasar templates to predict AGN colors with redshift, and contrast those predictions with the colors of known contaminating populations. The motivation of this study and subsequent spectroscopic follow-up is to populate and refine the faint end of the QLF where the population of type 1 AGN is presently not well known. The anticipated AGN observations will add to the ~300 already known AGN in the COSMOS field, making COSMOS a densely packed field of quasars to be used to understand supermassive black holes and probe the structure of the intergalactic medium in the intervening volume.