Publications (4)0 Total impact
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Article: Galaxy protocluster candidates around z ~ 2.4 radio galaxies
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ABSTRACT: We study the environments of 6 radio galaxies at 2.2 < z < 2.6 using wide-field near-infrared images. We use colour cuts to identify galaxies in this redshift range, and find that three of the radio galaxies are surrounded by significant surface overdensities of such galaxies. The excess galaxies that comprise these overdensities are strongly clustered, suggesting they are physically associated. The colour distribution of the galaxies responsible for the overdensity are consistent with those of galaxies that lie within a narrow redshift range at z ~ 2.4. Thus the excess galaxies are consistent with being companions of the radio galaxies. The overdensities have estimated masses in excess of 10^14 solar masses, and are dense enough to collapse into virizalised structures by the present day: these structures may evolve into groups or clusters of galaxies. A flux-limited sample of protocluster galaxies with K < 20.6 mag is derived by statistically subtracting the fore- and background galaxies. The colour distribution of the protocluster galaxies is bimodal, consisting of a dominant blue sequence, comprising 77 +/- 10% of the galaxies, and a poorly populated red sequence. The blue protocluster galaxies have similar colours to local star-forming irregular galaxies (U -V ~ 0.6), suggesting most protocluster galaxies are still forming stars at the observed epoch. The blue colours and lack of a dominant protocluster red sequence implies that these cluster galaxies form the bulk of their stars at z < 3. Comment: Accepted for publication in MNRAS08/2010; -
Article: Massive galaxies in high redshift proto-clusters
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ABSTRACT: Powerful radio galaxies (RGs) at high redshift have long been suspected to lie at the centre of assembling clusters, or at the very least, at a 'cross-road' of large scale filamentary structures. By targeting very red galaxies selected to lie at the same redshift as two RGs, we aim to investigate the mass assembly of proto-clusters and the formation of proto-ellipticals in these systems. At any epoch, galaxies with the reddest colours are known to trace the population of the most massive objects. In particular at high redshift, the study of massive galaxies places important constraints on the mechanisms behind galaxy formation. Furthermore, the evolution of galaxies through merging and subsequent star formation histories is accelerated in regions of high density, therefore studies of clusters at both low and high redshift can yield valuable insight into the assembly and evolution of galaxies as well as the complex structures which they make up. Using wide field infrared imaging of fields surrounding high-z RGs, our group has seen evidence that the red sequence of galaxies in clusters first begins to build up between redshifts z=2 and z=3 (Kodama et al. 2007). We present a spectroscopic campaign using FOCAS and MOIRCS on Subaru and FORS2 on the VLT, to confirm these red galaxies as proto-cluster members. We also discuss work in progress using new Spitzer data to constrain the stellar masses and star forming rates which allows us to trace the mass distribution of the clusters and study the properties of their member galaxies, revealing clues to the host environment of high-z RGs.Kodama, Tadayuki ; Yamada, Toru ; Aoki, Kentaro: Panoramic Views of Galaxy Formation and Evolution, ASP, 368-370 (2008). -
Article: Optical and near-IR spectroscopy of candidate red galaxies in two
http://dx.doi.org/10.1051/0004-6361/200912868. -
Article: Optical and near-IR spectroscopy of candidate red galaxies in two z ~ 2.5 proto-clusters
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ABSTRACT: We present a spectroscopic campaign to follow-up red colour-selected candidate massive galaxies in two high redshift proto-clusters surrounding radio galaxies. We observed a total of 57 galaxies in the field of MRC 0943-242 (z=2.93) and 33 in the field of MRC 1138-262 (z=2.16) with a mix of optical and near-infrared multi-object spectroscopy. We confirm two red galaxies in the field of MRC 1138-262 at the redshift of the radio galaxy. Based on an analysis of their spectral energy distributions, and their derived star formation rates from the Hñ and observed frame 24 üm flux, one object belongs to the class of dust-obscured star-forming red galaxies, while the other is evolved with little ongoing star formation. This result represents the first red and mainly passively evolving galaxy to be confirmed as companion galaxies in a z>2 proto-cluster. Both red galaxies in MRC 1138-262 are massive, of the order of 4-6 Ãâ 1011~Mï. They lie along a colour-magnitude relation which implies that they formed the bulk of their stellar population around z=4. In the MRC 0943-242 field we find no red galaxies at the redshift of the radio galaxy but we do confirm the effectiveness of our JHK_s selection of galaxies at 2.3<z<3.1, finding that 10 out of 18 (56%) of JHK_s-selected galaxies whose redshifts could be measured fall within this redshift range. We also serendipitously identify an interesting foreground structure of 6 galaxies at z=2.6 in the field of MRC 0943-242. This may be a proto-cluster itself, but complicates any interpretation of the red sequence build-up in MRC 0943-242 until more redshifts can be measured. Based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan, and in part on data collected with ESO's Very Large Telescope UT1/Antu, under program ID 080.A-0463(B), and with the VLT/Melipal telescope during the first X-shooter commissioning run, and ESO's New Technology Telescope under program ID 076.A-0670(B).Astronomy and Astrophysics, v.509, id.A83 (2010).
