Search

We present the cross-correlation between 151 luminous quasars (MUV < -26) and 179 protocluster candidates at z ~ 3.8, extracted from the Wide imaging survey (~121 deg2) performed as part of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). We find that only two out of 151 quasars reside in regions that are more overdense compared to the average field at >4 σ. The distributions of the distances between quasars and the nearest protoclusters and the significance of the overdensity at the positions of quasars are statistically identical to those found for g-dropout galaxies, suggesting that quasars tend to reside in almost the same environment as star-forming galaxies at this redshift. Using stacking analysis, we find that the average density of g-dropout galaxies around quasars is slightly higher than that around g-dropout galaxies on 1.0-2.5 pMpc scales, while at <0.5 pMpc that around quasars tends to be lower. We also find that quasars with higher UV luminosity or with more massive black holes tend to avoid the most overdense regions, and that the quasar near-zone sizes are anti-correlated with overdensity. These findings are consistent with a scenario in which luminous quasars at z ~ 4 reside in structures that are less massive than those expected for the progenitors of today's rich clusters of galaxies, and possibly that luminous quasarsmay be suppressing star formation in their close vicinity. [ABSTRACT FROM AUTHOR]

We conduct a systematic search for galaxy protoclusters at z ~ 3.8 based on the latest internal data release (S16A) of the Hyper Suprime-Cam Subaru strategic program (HSC-SSP). In the Wide layer of the HSC-SSP, we investigate the large-scale projected sky distribution of g-dropout galaxies over an area of 121 deg2, and identify 216 large-scale overdense regions (>4 σ overdensity significance) that are likely protocluster candidates. Of these, 37 are located within 8΄ (3.4 physical Mpc) of other protocluster candidates of higher overdensity, and are expected to merge into a single massive structure by z = 0. Therefore, we find 179 unique protocluster candidates in our survey. A cosmological simulation that includes projection effects predicts that more than 76% of these candidates will evolve into galaxy clusters with halo masses of at least 1014 M⊙ by z = 0. The unprecedented size of our protocluster candidate catalog allows us to perform, for the first time, an angular clustering analysis of the systematic sample of protocluster candidates. We find a correlation length of 35.0 h-1 Mpc. The relation between correlation length and number density of z ~ 3.8 protocluster candidates is consistent with the prediction of the ΛCDM model, and the correlation length is similar to that of rich clusters in the local universe. This result suggests that our protocluster candidates are tracing similar spatial structures to those expected from the progenitors of rich clusters, and enhances the confidence that our method for identifying protoclusters at high redshifts is robust. In years to come, our protocluster search will be extended to the entire HSC-SSP Wide sky coverage of ~ 1400 deg2 to probe cluster formation over a wide redshift range of z ~ 2-6. [ABSTRACT FROM AUTHOR]

We discuss the host galaxy metallicity distribution of all long gamma-ray bursts (GRBs) whose redshifts are known to be <0.4, including newly obtained spectroscopic datasets of the host galaxies of GRB060614, 090417B, and 130427A. We compare the metallicity distribution of the low-redshift sample to the model predictions, and constrain the relation between metallicity and GRB occurrence. We take account of spatial variation of metallicities among star-forming regions within a galaxy. We found that the models in which only low-metallicity stars produce GRBs with a sharp cutoff of GRB production efficiency around 12+log(O/H) ∼ 8.3 can reproduce the observed distribution well, while the models with no metallicity dependence are not consistent with the observations.We also discuss possible sampling biases we may suffer by collecting long GRBs whose redshifts are known, presenting the photometric observations of the host galaxy of GRB111225A at z = 0.297 whose redshift has been undetermined until ∼2.3 yr after the burst. [ABSTRACT FROM AUTHOR]

We present the results of an halo occupation distribution (HOD) analysis of star-forming galaxies at z ~ 2. We obtained high-quality angular correlation functions based on a large sgzK sample, which enabled us to carry out the HOD analysis. The mean halo mass and the HODmass parameters are found to increase monotonically with increasing K-band magnitude, suggesting that more luminous galaxies reside in more massive dark haloes. The luminosity dependence of the HOD mass parameters was found to be the same as in the local Universe; however, the masses were larger than in the local Universe over all ranges of magnitude. This implies that galaxies at z ~ 2 tend to form in more massive dark haloes than in the local Universe, a process known as downsizing. By analysing the dark halo mass evolution using the extended Press-Schechter formalism and the number evolution of satellite galaxies in a dark halo, we find that faint Lyman break galaxies at z ~ 4 could evolve into the faintest sgzKs (22.0 < K ≤ 23.0) at z ~ 2 and into the Milky-Way-like galaxies or elliptical galaxies in the local Universe, whereas the most luminous sgzKs (18.0 ≤ K ≤ 21.0) could evolve into the most massive systems in the local Universe. The stellar-to-halo mass ratio (SHMR) of the sgzKs was found to be consistent with the prediction of the model, except that the SHMR of the faintest sgzKs was smaller than the prediction at z ~ 2. This discrepancy may be explained by the confinement of our samples to star-forming galaxies. [ABSTRACT FROM AUTHOR]

We present the results of clustering analysis on z ~ 2 star-forming galaxies. By combining our data with data from publicly available archives, we collect g-, zB/z- and K-band imaging data over 5.2 deg2, which represents the largest area BzK/gzK survey. We apply colour corrections to translate our filter set to those used in the original BzK selection for the gzK selection. Because of the wide survey area, we obtain a sample of 41 112 star-forming gzK galaxies at z ~ 2 (sgzK galaxies) down to KAB < 23.0, and we determine high-quality two-point angular correlation functions (ACFs). Our ACFs show an apparent excess from power-law behaviour at small angular scale (≲ 0°.01), which corresponds to the virial radius of a dark halo at z ~ 2 with a mass of ~1013 MΘ. We find that the correlation lengths are consistent with the previous estimates over the whole magnitude range; however, our results are evaluated with a smaller margin of error than that in previous studies. The large amount of data enables us to determine ACFs differentially depending on the luminosity of the subset of the data. The mean halo mass of faint sgzK galaxies (22.0 < K ≤ 23.0) was found to be 〈Mh〉=(1.32-0.12+0.09)×1012h-1 MΘ, whereas bright sgzK galaxies (18.0 ≤ K ≤ 21.0) were found to reside in dark haloes with a mass of 〈Mh〉=(3.26-1.02+1.23)×1013h-1 MΘ. [ABSTRACT FROM AUTHOR]

We have carried out follow-up spectroscopy on three overdense regions of g- and r-dropout galaxies in the Canada–France–Hawaii Telescope Legacy Survey Deep Fields, finding two new protoclusters at z = 4.898 and 3.721 and a possible protocluster at z = 3.834. The z = 3.721 protocluster overlaps with a previously identified protocluster at z = 3.675. The redshift separation between these two protoclusters is Δz = 0.05, which is slightly larger than the size of typical protoclusters. Therefore, if they are not the progenitors of a >1015M☉ halo, they would grow into closely located independent halos like a supercluster. The other protocluster at z = 4.898 is also surrounded by smaller galaxy groups. These systems including protoclusters and neighboring groups are regarded as the early phase of superclusters. We quantify the spatial distribution of member galaxies of the protoclusters at z = 3.675 and 3.721 by fitting triaxial ellipsoids, finding a tentative difference: one has a pancake-like shape, while the other is filamentary. This could indicate that these two protoclusters are in different stages of formation. We investigate the relation between redshift and the velocity dispersion of protoclusters, including other protoclusters from the literature, in order to compare their dynamical states. Although there is no significant systematic trend in the velocity dispersions of protoclusters with redshift, the distribution is skewed to higher velocity dispersion over the redshift range of z = 2–6. This could be interpreted as two phases of cluster formation, one dominated by the steady accretion of galaxies and the other by the merging between group-size halos, perhaps depending on the surrounding large-scale environments. [ABSTRACT FROM AUTHOR]

We report 14 and 26 protocluster candidates at z = 5.7 and 6.6 over 14 and 16 deg2 areas, respectively, selected from 2230 (259) Lyα emitters (LAEs) photometrically (spectroscopically) identified using Subaru/Hyper Suprime-Cam (HSC) deep images (Keck, Subaru, and Magellan spectra, and literature data). Six out of the 40 protocluster candidates include one to 13 spectroscopically confirmed LAEs. We conduct Monte Carlo simulations to estimate how many protocluster candidates are found by chance for randomly distributed sources, and find that the effective number of protocluster candidates at z = 5.7 (6.6) is six (five). By comparing with the cosmological Lyα radiative transfer (RT) model reproducing the LAEs with reionization effects, we find that more than half of these protocluster candidates are progenitors of present-day clusters with mass of . We then investigate the correlation between the LAE overdensity δ and the Lyα rest-frame equivalent width , because the cosmological Lyα RT model suggests that the slope of the –δ relation steepens toward the epoch of cosmic reionization (EoR), due to the existence of ionized bubbles around galaxy overdensities easing the escape of Lyα emission from the partly neutral intergalactic medium. The available HSC data suggest that the slope of the –δ correlation does not evolve from the post-reionization epoch, z = 5.7, to the EoR, z = 6.6, beyond the moderately large statistical errors. There is a possibility that we could detect the evolution of the –δ relation from z = 5.7 to 7.3 using the upcoming HSC observations that will provide large samples of LAEs at z = 6.6–7.3. [ABSTRACT FROM AUTHOR]

We present the results of a survey of the brightest UV-selected galaxies in protoclusters. These proto-brightest cluster galaxy (proto-BCG) candidates are drawn from 179 overdense regions of g-dropout galaxies at z ∼ 4 from the Hyper Suprime-Cam Subaru Strategic Program identified previously as good protocluster candidates. This study is the first to extend the systematic study of the progenitors of BCGs from z ∼ 2 to z ∼ 4. We carefully remove possible contaminants from foreground galaxies and, for each structure, select the brightest galaxy that is at least 1 mag brighter than the fifth-brightest galaxy. We select 63 proto-BCG candidates and compare their properties with those of galaxies in the field and those of other galaxies in overdense structures. The proto-BCG candidates and their surrounding galaxies have different rest-UV color (i − z) distributions to field galaxies and other galaxies in protoclusters that do not host proto-BCGs. In addition, galaxies surrounding proto-BCGs are brighter than those in protoclusters without proto-BCGs. The image stacking analysis reveals that the average effective radius of proto-BCGs is ∼28% larger than that of field galaxies. The i − z color differences suggest that proto-BCGs and their surrounding galaxies are dustier than other galaxies at z ∼ 4. These results suggest that specific environmental effects or assembly biases have already emerged in some protoclusters as early as z ∼ 4, and we suggest that proto-BCGs have different star formation histories than other galaxies in the same epoch. [ABSTRACT FROM AUTHOR]

We have carried out deep and wide field imaging observations with narrow bands, targeting 11 quasar fields to systematically study the possible photoevaporation effect of quasar radiation on surrounding low mass galaxies at z ∼ 2–3. We focused on Lyα emitters (LAEs) at the same redshifts as quasars that lie within the quasar proximity zones, where the UV radiation from the quasars is higher than the average background at that epoch. We found that LAEs with high rest-frame equivalent width of Lyα emission (EW0) of ≳150 Å with low stellar mass (≲108M⊙) are predominantly scarce in the quasar proximity zones, suggesting that quasar photoevaporation effects may be taking place. The halo mass of LAEs with EW0 > 150 Å is estimated to be either from spectral energy distribution fitting or the main sequence. Based on a hydrodynamical simulation, the predicted delay in star formation under a local UV background intensity with erg s−1 cm−2 Hz−1 sr−1 for galaxies having less than this halo mass is about >20 Myr, which is longer than the expected age of LAEs with EW0 > 150 Å. On the other hand, photoevaporation seems to be less effective around very luminous quasars, which is consistent with the idea that these are still in an early stage of activity. [ABSTRACT FROM AUTHOR]

We combined deep U-band and optical/near-infrared imaging, in order to select Lyman Break Galaxies (LBGs) at using U − V and colors in the Subaru Deep Field. The resulting sample of 5161 LBGs gives a UV luminosity function (LF) down to , with a steep faint-end slope of . We analyze UV-to-NIR energy distributions (SEDs) from optical photometry and photometry on IR median-stacked images. In the stacks, we find a systematic background depression centered on the LBGs. This results from the difficulty of finding faint galaxies in regions with higher-than-average surface densities of foreground galaxies, so we corrected for this deficit. Best-fit stellar population models for the LBG SEDs indicate stellar masses and star formation rates of and M⊙ yr−1 at , down to and yr−1 at . The faint LBGs show a ∼1 mag excess over the stellar continuum in K-band. We interpret this excess flux as redshifted [O iii] lines. The observed excesses imply equivalent widths that increase with decreasing mass, reaching Å (rest-frame). Such strong [O iii] emission is seen only in a miniscule fraction of local emission-line galaxies, but is probably universal in the faint galaxies that reionized the universe. Our halo occupation distribution analysis of the angular correlation function gives a halo mass of for the full sample of LBGs, and for the brightest half of the sample. [ABSTRACT FROM AUTHOR]

We present the results of clustering analyses of Lyman break galaxies (LBGs) at , 4, and 5 using the final data release of the Canada–France–Hawaii Telescope Legacy Survey (CFHTLS). Deep- and wide-field images of the CFHTLS Deep Survey enable us to obtain sufficiently accurate two-point angular correlation functions to apply a halo occupation distribution analysis. The mean halo masses, calculated as , increase with the stellar-mass limit of LBGs. The threshold halo mass to have a central galaxy, follows the same increasing trend as the low-z results, whereas the threshold halo mass to have a satellite galaxy, M1, shows higher values at than , over the entire stellar mass range. Satellite fractions of dropout galaxies, even at less massive halos, are found to drop sharply, from z = 2 down to less than 0.04, at . These results suggest that satellite galaxies form inefficiently within dark halos at , even for less massive satellites with . We compute stellar-to-halo mass ratios (SHMRs) assuming a main sequence of galaxies, which is found to provide SHMRs consistent with those derived from a spectral energy distribution fitting method. The observed SHMRs are in good agreement with model predictions based on the abundance-matching method, within confidence intervals. We derive observationally, for the first time, , which is the halo mass at a peak in the star-formation efficiency, at , and it shows a small increasing trend with cosmic time at . In addition, and its normalization are found to be almost unchanged during . Our study provides observational evidence that galaxy formation is ubiquitously most efficient near a halo mass of over cosmic time. [ABSTRACT FROM AUTHOR]

We present the discovery of three protoclusters at z ∼ 3–4 with spectroscopic confirmation in the Canada–France–Hawaii Telescope Legacy Survey Deep Fields. In these fields, we investigate the large-scale projected sky distribution of z ∼ 3–6 Lyman-break galaxies and identify 21 protocluster candidates from regions that are overdense at more than 4σ overdensity significance. Based on cosmological simulations, it is expected that more than 76% of these candidates will evolve into a galaxy cluster of at least a halo mass of 1014M⊙ at z = 0. We perform follow-up spectroscopy for eight of the candidates using Subaru/FOCAS, Keck II/DEIMOS, and Gemini-N/GMOS. In total we target 462 dropout candidates and obtain 138 spectroscopic redshifts. We confirm three real protoclusters at z = 3–4 with more than five members spectroscopically identified and find one to be an incidental overdense region by mere chance alignment. The other four candidate regions at z ∼ 5–6 require more spectroscopic follow-up in order to be conclusive. A z = 3.67 protocluster, which has 11 spectroscopically confirmed members, shows a remarkable core-like structure composed of a central small region (<0.5 physical Mpc) and an outskirts region (∼1.0 physical Mpc). The Lyα equivalent widths of members of the protocluster are significantly smaller than those of field galaxies at the same redshift, while there is no difference in the UV luminosity distributions. These results imply that some environmental effects start operating as early as at z ∼ 4 along with the growth of the protocluster structure. This study provides an important benchmark for our analysis of protoclusters in the upcoming Subaru/HSC imaging survey and its spectroscopic follow-up with the Subaru/PFS that will detect thousands of protoclusters up to z ∼ 6. [ABSTRACT FROM AUTHOR]