Deep HST and Spitzer imaging of a passively evolving galaxy identified in the Gemini Deep Deep Survey (GDDS) has led to the detection of a very compact cluster of massive red galaxies at a redshift of z =1.51. For galaxies without emission lines this redshift is beyond the reach of most spectroscopic studies, however, the ultra-deep Gemini spectroscopy of the GDDS program detected continuum emission from the source which made it possible to establish the z = 1.51 redshift of the galaxy. Assuming that the other cluster members have the same redshift makes this one of the most distant clusters discovered to date.
Subsequent HST and Spitzer imaging uncovered 12 galaxies within 170 kpc of the GDDS target proving that the newly discovered cluster is both compact and densely populated. This suggests that several of the cluster members will likely merge to form a single behemoth galaxy by the present day.
Since galaxy clusters were rare when the universe was less than half its current age, the discovery of a well-formed cluster at such a high redshift has implications for the hierarchical models that attempt to describe galaxy formation. Indeed, finding an example like this is difficult, but crucial, to understanding the growth of structure and probing the underlying issues of dark matter and dark energy.
For more details, see the article "A Compact Cluster of Massive Red Galaxies at a Redshift of 1.51," Patrick J. McCarthy et al., The Astrophysical Journal Letters, in press.
The GDDS is led by Pat McCarthy (Carnegie/U.S. P.I.), Bob Abraham (Toronto/Canadian P.I.) and Karl Glazebrook (Swinburne/Australian P.I.).
Figure 1: Near-infrared HST imaging reveals a large number of faint galaxies around the central luminous galaxies clustering around GDDS-12-5869
Figure 2: Color composite image (100" x 100") of the newly discovered cluster. The 'blue' channel comes from B&V images, 'green' comes from R&I images, 'red' comes from mid-IR images at 3.6 and 4.5 µm. The diameter of the white circle is 30", corresponding to 256 kpc at z = 1.51. This figure illustrates the compact clustering of very red galaxies in the vicinity of GDDS-12-5869.
Figure 3: HST imaging with NICMOS and ACS allowed the study of galaxy morphologies in the young cluster. It is plausible that several of the central galaxies will merge in less than one billion years, such that by redshift z = 0.5 this group will look like a typical bright galaxy cluster.