Galaxy proto-clusters at z >~ 2 provide a direct probe of the rapid mass assembly and galaxy growth of present day massive clusters. Because of the need of precise galaxy redshifts for density mapping and the prevalence of star formation before quenc
hing, nearly all the proto-clusters known to date were confirmed by spectroscopy of galaxies with strong emission lines. Therefore, large emission-line galaxy surveys provide an efficient way to identify proto-clusters directly. Here we report the discovery of a large-scale structure at z = 2.44 in the HETDEX Pilot Survey. On a scale of a few tens of Mpc comoving, this structure shows a complex overdensity of Lya emitters (LAE), which coincides with broad-band selected galaxies in the COSMOS/UltraVISTA photometric and zCOSMOS spectroscopic catalogs, as well as overdensities of intergalactic gas revealed in the Lya absorption maps of Lee et al. (2014). We construct mock LAE catalogs to predict the cosmic evolution of this structure. We find that such an overdensity should have already broken away from the Hubble flow, and part of the structure will collapse to form a galaxy cluster with 10^14.5 +- 0.4 M_sun by z = 0. The structure contains a higher median stellar mass of broad-band selected galaxies, a boost of extended Lya nebulae, and a marginal excess of active galactic nuclei relative to the field, supporting a scenario of accelerated galaxy evolution in cluster progenitors. Based on the correlation between galaxy overdensity and the z = 0 descendant halo mass calibrated in the simulation, we predict that several hundred 1.9 < z < 3.5 proto-clusters with z = 0 mass of > 10^14.5 M_sun will be discovered in the 8.5 Gpc^3 of space surveyed by the Hobby Eberly Telescope Dark Energy Experiment.
To demonstrate the feasibility of studying the epoch of massive galaxy cluster formation in a more systematic manner using current and future galaxy surveys, we report the discovery of a large sample of proto-cluster candidates in the 1.62 deg^2 COSM
OS/UltraVISTA field traced by optical/IR selected galaxies using photometric redshifts. By comparing properly smoothed 3D galaxy density maps of the observations and a set of matched simulations incorporating the dominant observational effects (galaxy selection and photometric redshift uncertainties), we first confirm that the observed ~15 comoving Mpc scale galaxy clustering is consistent with LCDM models. Using further the relation between high-z overdensity and the present day cluster mass calibrated in these matched simulations, we found 36 candidate structures at 1.6<z<3.1, showing overdensities consistent with the progenitors of M_z=0 ~10^15 M_sun clusters. Taking into account the significant upward scattering of lower mass structures, the probabilities for the candidates to have at least M_z=0 ~10^14 M_sun are ~70%. For each structure, about 15%-40% of photometric galaxy candidates are expected to be true proto-cluster members that will merge into a cluster-scale halo by z=0. With solely photometric redshifts, we successfully rediscover two spectroscopically confirmed structures in this field, suggesting that our algorithm is robust. This work generates a large sample of uniformly-selected proto-cluster candidates, providing rich targets for spectroscopic follow-up and subsequent studies of cluster formation. Meanwhile, it demonstrates the potential for probing early cluster formation with upcoming redshift surveys such as the Hobby-Eberly Telescope Dark Energy Experiment and the Subaru Prime Focus Spectrograph survey.
For a complete picture of galaxy cluster formation, it is important that we start probing the early epoch of z~2-7 during which clusters and their galaxies first began to form. Because the study of these so-called proto-clusters is currently limited
by small number statistics, widely varying selection techniques and assumptions, we have performed a systematic study of cluster formation utilizing cosmological simulations. We use the Millennium Simulations to track the evolution of dark matter and galaxies in ~3,000 clusters from the earliest times to z=0. We define an effective radius R_e for proto-clusters and characterize their growth in size and mass. We show that the progenitor regions of galaxy clusters (M>10^14 M_sun/h) can already be identified at least up to z~5, provided that the galaxy overdensities, delta_gal, are measured on a sufficiently large scale (R_e~5-10 cMpc). We present the overdensities in matter, DM halos, and galaxies as functions of present-day cluster mass, redshift, bias, and window size that can be used to interpret the structures found in real surveys. We derive the probability that a structure having a delta_gal, defined by a set of observational selection criteria, is indeed a proto-cluster, and show how their z=0 masses can already be estimated long before virialization. Galaxy overdensity profiles as a function of radius are presented. We further show how their projected surface overdensities decrease as the uncertainties in redshift measurements increase. We provide a table of proto-cluster candidates selected from the literature, and discuss their properties in the light of our simulations predictions. This work provides the general framework that will allow us to extend the study of cluster formation out to much higher redshifts using the large number of proto-clusters that are expected to be discovered in, e.g., the upcoming HETDEX and HSC surveys.
We investigate the long-term variability exhibited by the X-ray point sources in the starburst galaxy M82. By combining 9 Chandra observations taken between 1999 and 2007, we detect 58 X-ray point sources within the D25 isophote of M82 down to a lumi
nosity of ~ 10^37 ergs/s. Of these 58 sources, we identify 3 supernova remnant candidates and one supersoft source. Twenty-six sources in M82 exhibit long-term (i.e., days to years) flux variability and 3 show long-term spectral variability. Furthermore, we classify 26 sources as variables and 10 as persistent sources. Among the total 26 variables, 17 varied by a flux ratio of > 3 and 6 are transient candidates. By comparing with other nearby galaxies, M82 shows extremely strong long-term X-ray variability that 47% of the X-ray sources are variables with a flux ratio of > 3. The strong X-ray variability of M82 suggests that the population is dominated by X-ray binaries.
