COLOR-MAGNITUDE RELATIONS OF ACTIVE AND NON-ACTIVE GALAXIES IN THE CHANDRA DEEP FIELDS: HIGH-REDSHIFT CONSTRAINTS AND STELLAR-MASS SELECTION EFFECTS

Abstract

We extend color–magnitude relations for moderate-luminosity X-ray active galactic nucleus (AGN) hosts and non-AGN galaxies through the galaxy formation epoch ( z ≈ 1–4) in the Chandra Deep Field-North and Chandra Deep Field-South (CDF-N and CDF-S, respectively; jointly CDFs) surveys. This study was enabled by the deepest available X-ray data from the 2 Ms CDF surveys as well as complementary ultradeep multiwavelength data in these regions. We utilized analyses of color–magnitude diagrams (CMDs) to assess the role of moderate-luminosity AGNs in galaxy evolution. First, we confirm some previous results and extend them to higher redshifts, finding, for example, that (1) there is no apparent color bimodality (i.e., the lack of an obvious red sequence and blue cloud) for AGN hosts from z ≈ 0to2, but non-AGN galaxy color bimodality exists up to z ≈ 3 and the relative fraction of red-sequence galaxies generally increases as the redshift decreases (consistent with a blue-to-red migration of galaxies), (2) most AGNs reside in massive hosts and the AGN fraction rises strongly toward higher stellar mass, up to z ≈ 2–3, and (3) the colors of both AGN hosts and non-AGN galaxies become redder as the stellar mass increases, up to z ≈ 2–3. Second, we point out that, in order to obtain a complete and reliable picture, it is critical to use mass-matched samples to examine color–magnitude relations of AGN hosts and non-AGN galaxies. We show that for mass-matched samples up to z ≈ 2–3, AGN hosts lie in the same region of the CMD as non-AGN galaxies; i.e., there is no specific clustering of AGN hosts in the CMD around the red sequence, the top of the blue cloud, or the green valley in between. The AGN fraction (≈ 10%) is mostly independent of host-galaxy color, providing an indication of the duty cycle of supermassive black hole growth in typical massive galaxies. These results are in contrast to those obtained with non-mass-matched samples where there is apparent AGN clustering in the CMD and the AGN fraction generally increases as the color becomes redder. We also find, for mass-matched samples, that the star formation rates of AGN hosts are typically a factor of ≈ 2–3 larger than those of non-AGN galaxies at z ≈ 0–1, whereas this difference diminishes at z ≈ 1–3. With mass-selection effects taken into account, we find that almost all of the results obtained in this work can reasonably be explained by two main ingredients, color–mass correlation (i.e., X-ray AGNs preferentially reside in massive galaxies that generally tend to be redder than less-massive galaxies) and passive or secular evolution of galaxies. Our results show that the presence of moderate-luminosity AGN activity does not have a significant effect on the colors of galaxies and thus tightly constrain any effects from moderate-luminosity AGN feedback upon color–magnitude properties over the ≈ 80% of cosmic time during which most of galaxy formation occurred.