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

Abstract

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 Sérsic 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 AGNs). We study separately optically luminous (LB ≈ 1010-1011 LB,☉) and faint (LB ≈ 109.3-1010 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, respectively. The AGN fraction of our optically luminous sample evolves with redshift in a manner consistent with the (1 + z)3 evolution observed in other investigations of X-ray-selected AGNs. We find that the X-ray-to-B-band mean luminosity ratio (LX/LB) for normal optically luminous galaxies does not evolve significantly over z ≈ 0.0-0.7. This lack of X-ray evolution implies a general balance between the heating and cooling of the hot gas. If transient AGN activity is largely responsible for maintaining this balance, then we infer that mechanical power must be dominating the feedback out to z ≈ 0.7. Furthermore, in this scenario the average mechanical AGN power must remain roughly constant over the last half of cosmic time. For our optically faint early-type galaxies, we find suggestive evidence that LX/LB increases with redshift over z ≈ 0.0-0.5.