Serendipitous discovery of a massive cD galaxy at z=1.096: Implications for the early formation and late evolution of cD galaxies

TL;DR

This paper reports the discovery of the most distant known cD galaxy at z=1.096, providing insights into early formation and late evolution of massive galaxies through deep imaging and analysis.

Contribution

It presents the first confirmed observation of a high-redshift cD galaxy with an extended envelope, supporting models of galaxy growth via dry mergers over cosmic time.

Findings

The galaxy's size and mass have increased by factors of ~3.4 and ~3.3 over 8 Gyrs.

The galaxy's properties align with models predicting dry mergers at z<1.

Extended cD envelopes can be observed at z>1 with deep imaging.

Abstract

We have made a serendipitous discovery of a massive cD galaxy at z=1.096 in a candidate rich cluster in the HUDF area of GOODS-South. This brightest cluster galaxy is the most distant cD galaxy confirmed to date. Ultra-deep HST/WFC3 images reveal an extended envelope starting from ~10 kpc and reaching ~70 kpc in radius along the semi-major axis. The spectral energy distributions indicate that both its inner component and outer envelope are composed of an old, passively-evolving stellar population. The cD galaxy lies on the same mass-size relation as the bulk of quiescent galaxies at similar redshifts. The cD galaxy has a higher stellar mass surface density but a similar velocity dispersion to those of more-massive, nearby cDs. If the cD galaxy is one of the progenitors of today's more massive cDs, its size and stellar mass have had to increase on average by factors of $3.4\pm1.1$ and $3.3\pm1.3$ over the past ~8 Gyrs, respectively. Such increases in size and stellar mass without being accompanied by significant increases in velocity dispersion are consistent with evolutionary scenarios driven by both major and minor dry mergers. If such cD envelopes originate from dry mergers, our discovery of even one example proves that some BCGs entered the dry merger phase at epochs earlier than z=1. Our data match theoretical models which predict that the continuance of dry mergers at z<1 can result in structures similar to those of massive cD galaxies seen today. Moreover, our discovery is a surprise given that the extreme depth of the HUDF is essential to reveal such an extended cD envelope at z>1 and, yet, the HUDF covers only a minuscule region of sky. Adding that cDs are rare, Our serendipitous discovery hints that such cDs may be more common than expected. [Abridged]