Searching for Protoclusters

Luminous radio galaxies are the IR-brightest and presumably most massive galaxies at $z>2$, very probably marking the centres of forming clusters. Miley, Röttgering, Venemans, Kurk and collaborators used ESO's Very Large Telescope to find overdensities of Lyman-$\alpha$ emitters around radio galaxies at redshifts of $z>2$. They identified objects with a probable excess of Lyman-$\alpha$ emission, and carried out spectroscopy of the brightest of these candidates to establish that they were at the same distance as the radio galaxy by confirming that they had similar redshifts. The seven radio galaxies (at $z = 2.2, 2.9, 2.9, 3.1, 3.2, 4.1$ and 5.2 respectively) that they studied with sufficient sensitivity all turned out to be surrounded by an overdensity of Lyman-$\alpha$ emitters (by factors of 5 - 15) having velocity dispersions of 300 - 1000 km s$^{-1}$. The scale of these structure was larger than 3 Mpc, exceeding the 7' $\times$ 7' FORS field imaged. Structure masses were estimated at $> 10^{14} - 10^{15}$ M$_\odot$, comparable to the mass of present-day rich clusters of galaxies.

Overzier & Miley obtained deep Advanced Camera for Surveys (ACS) imaging observations of protocluster targets from their VLT Large Program. One such protocluster, associated with the radio galaxy TN J1338-1942 at $z=4.11$, showed that the overdensity of Lyman-$\alpha$ emitters surrounding the radio galaxy is accompanied by a similar overdensity of g,r,i-selected Lyman-break galaxies. The existence of such a population of Lyman-break galaxies was anticipated for some time. However, the object in question appeared to be very rich: Overzier & Miley identified approximately 90 (candidate) cluster members surrounding TN J1338-1942 (both Lyman-$\alpha$ and Lyman-break galaxies) and quantification of their HST morphological properties and spectral energy distributions is underway. ACS observations of other protoclusters between $z\sim5$ and $z\sim2$ as well as $z\sim1$ clusters from the Intermediate Redshift Cluster Program of the ACS GTO Science Team will be used to trace the evolution of the largest building blocks of the Universe, that began forming when the Universe was only 10% of its present age.

Kurk (Leiden/Arcetri), Röttgering, Miley and Pentericci (Heidelberg) conducted a detailed study of the environment of the powerful radio galaxy PKS 1138-262 (z=2.2) at optical, NIR, radio and X-ray wavelengths. They carried out deep narrow-band imaging, yielding 70 candidate Lyman-$\alpha$ emitters with an overdensity by at least a factor of two compared to the field. Their spectroscopic follow-up so far confirmed 14 galaxies to have redshifts similar to that of the radio galaxy. Deep narrow-band imaging and spectroscopy in the infrared resulted in the detection of 7 confirmed H$\alpha$-emitting galaxies. Interestingly, the distribution of H$\alpha$-galaxies seems much more concentrated than that of the Ly$\alpha$ emitting galaxies. Furthermore, the population of H$\alpha$ emitters appeared to have a brighter average K-band magnitude and a lower velocity dispersion. Kurk and coworkers surmised that the population of H$\alpha$-emitting galaxies is older and dustier and is closer to having it orbits virialized in the ``proto-cluster'' potential, implying that seeds of the morphology-density relation are already in place at $z=2.2$. As before, they estimated the mass associated with the `overdensity' to be in the range $10^{14} - 10^{15}$$M_\odot$, as expected for a progenitor of nearby massive clusters.