* Astronomy

Title: Serendipitous discovery of a massive cD galaxy at z=1.096: Implications for the early formation and late evolution of cD galaxies
Authors: F. S. Liu, Yicheng Guo, David C. Koo, Jonathan R. Trump, Guillermo Barro, Hassen Yesuf, S. M. Faber, M. Giavalisco, P. Cassata, A. M. Koekemoer, L. Pentericci, M. Castellano, Edmond Cheung, Shude Mao, X. Y. Xia, Norman A. Grogin, Nimish P. Hathi, Kuang-Han Huang, Dale Kocevski, Elizabeth J. McGrath, Stijn Wuyts

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±1.1 and 3.3±1.3 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.

Read more (4915kb, PDF)

Hubble Provides First Census of Galaxies Near Cosmic Dawn

Using NASA's Hubble Space Telescope, astronomers have uncovered a previously unseen population of seven primitive galaxies that formed more than 13 billion years ago, when the universe was less than 3 percent of its present age. The deepest images to date from Hubble yield the first statistically robust sample of galaxies that tells how abundant they were close to the era when galaxies first formed. The results show a smooth decline in the number of galaxies with increasing look-back time to about 450 million years after the big bang. The observations support the idea that galaxies assembled continuously over time and also may have provided enough radiation to reheat, or reionise, the universe a few hundred million years after the big bang.
Read more

Title: The Tumultuous Formation of the Hubble Sequence at z > 1 Examined with HST/WFC3 Observations of the Hubble Ultra Deep Field
Authors: C. J. Conselice, A.F.L. Bluck, S. Ravindranath, A. Mortlock, A. Koekemoer, F. Buitrago, R. Grützbauch, S. Penny

We examine in this paper a stellar mass selected sample of galaxies at 1 < z < 3 within the Hubble Ultra Deep Field, utilising WFC3 imaging to study the rest-frame optical morphological distribution of galaxies at this epoch. We measure how apparent morphologies (disk, elliptical, peculiar) correlate with physical properties, such as quantitative structure and spectral-types. One primary result is that apparent morphology does not correlate strongly with stellar populations, nor with galaxy structure at this epoch, suggesting a chaotic formation history for Hubble types at z > 1. By using a locally defined definition of disk and elliptical galaxies based on structure and spectral-type, we find no true ellipticals at z > 2, and a fraction of 3.2±2.3% at 1.5 < z < 2. Local counterparts of disk galaxies are at a similar level of 7-10%, much lower than the 75% fraction at lower redshifts. We further compare WFC3 images with the rest-frame UV view of galaxies from ACS imaging, showing that galaxies imaged with ACS that appear peculiar often contain an 'elliptical' like morphology in WFC3. We show through several simulations that this larger fraction of elliptical-like galaxies is partially due to the courser PSF of WFC3, and that the 'elliptical' class very likely includes early-type disks. We also measure the merger history for our sample using CAS parameters, finding a redshift evolution increasing with redshift, and a peak merger fraction of ~30% at z~2 for the most massive galaxies with M_*> 10^{10} M_sol, consistent with previous results from ACS and NICMOS. We compare our results to semi-analytical model results and find a relatively good agreement between our morphological break-down and the predictions. Finally, we argue that the peculiars, ellipticals and peculiar ellipticals have similar properties, suggesting similar formation modes, likely driven by major mergers.

Read more (330kb, PDF)