The X‐Ray Evolution of Early‐Type Galaxies in the Extended Chandra Deep Field–South

We investigate the evolution over the last 6.3 Gyr of cosmic time (i.e., since z ≈ 0.7) of the average X-ray properties of early-type galaxies within the Extended Chandra Deep Field-South (E-CDF-S). Our early-type galaxy sample includes 539 objects with red-sequence colors and Sersic indices larger than n = 2.5, which were selected jointly from the COMBO-17 (Classifying Objects by Medium-Band Observations in 17 Filters) and GEMS (Galaxy Evolution from Morphologies and SEDs) surveys. We utilize the deep Chandra observations over the E-CDF-S and X-ray stacking analyses to constrain primarily the average X-ray emission from "normal" early-type galaxies (i.e., those that are not dominated by luminous active galactic nuclei (AGNs)). In our analyses, we study separately optically luminous (LB ≈ 1010−11 LB,⊙) and faint (LB ≈ 109.3−10 LB,⊙) galaxy samples, which we expect to have soft (0.5-2.0 keV) X-ray emission dominated by hot (∼1 keV) interstellar gas and low mass X-ray binary (LMXB) populations, re- spectively. We detect individually 49 (≈9%) of our galaxies in the X-ray band, and classify these sources as either normal early-type galaxies (17 galaxies) or AGN candi- dates (32 galaxies). The AGN fraction of our optically luminous samples evolves with redshift in a manner consistent with the (1 + z)3 evolution observed in other investi- gations of X-ray-selected AGNs. After removing potential AGNs from our samples, we find that the X-ray-to-B-band mean luminosity ratio (LX/LB) for optically luminous early-type galaxies does not evolve significantly over the redshift range z ≈ 0.0-0.7. This lack of X-ray evolution implies a general balance between the heating and cooling of the hot interstellar gas. If transient AGN activity is largely responsible for maintain- ing this balance, then we infer that mechanical power must be dominating the feedback