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Register a new userMOS spectroscopy of protocluster candidate galaxies at z=6.5
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The epoch corresponding to a redshift of z $sim 6.5$ is close to full re-ionisation of the Universe, and early enough to provide an intriguing environment to observe the early stage of large-scale structure formation. It is also en epoch that can be used to verify the abundance of a large population of low luminosity star-forming galaxies, that are deemed responsible for cosmic re-ionisation. Here, we present the results of follow-up multi-object spectroscopy using OSIRIS at Gran Telescopio Canarias (GTC) of 16 Ly$alpha$ emitter (LAE) candidates discovered in the Subaru/XMM Newton Deep Survey. We have securely confirmed 10 LAEs with sufficient signal-to-noise ratio of the Ly$alpha$ emission line. The inferred star formation rates of the confirmed LAEs are on the low side, within the range 0.9-4.7 M$_{odot}$ yr$^{-1}$. However, they show relatively high Ly$alpha$ rest frame equivalent widths. Finally we have shown that the mechanical energy released by the star formation episodes in these galaxies is enough to create holes in the neutral hydrogen medium such that Lyman continuum photons can escape to the intergalactic medium, thus contributing to the re-ionisation of the Universe.
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We present the results of near-infrared spectroscopic observations of the $K$-band selected candidate galaxies in the protocluster at $z=3.09$ in the SSA22 field. We observed 67 candidates with $K_{rm AB}<24$ and confirmed redshifts of the 39 galaxies at $2.0< z_{rm spec}< 3.4$. Of the 67 candidates, 24 are certainly protocluster members with $3.04leq z_{rm spec}leq 3.12$, which are massive red galaxies those have been unidentified in previous optical observations of the SSA22 protocluster. Many distant red galaxies (DRGs; $J-K_{rm AB}>1.4$), hyper extremely red objects (HEROs; $J-K_{rm AB}>2.1$), {it Spitzer} MIPS 24 $mu$m sources, active galactic nuclei (AGNs) as well as the counterparts of Ly$alpha$ blobs and the AzTEC/ASTE 1.1-mm sources in the SSA22 field are also found to be the protocluster members. The mass of the SSA22 protocluster is estimated to be $sim2-5times10^{14}~M_{odot}$ and this system is plausibly a progenitor of the most massive clusters of galaxies in the current Universe. The reddest ($J-K_{rm AB}geq 2.4$) protocluster galaxies are massive galaxies with $M_{rm star}sim10^{11}~M_{odot}$ showing quiescent star formation activities and plausibly dominated by old stellar populations. Most of these massive quiescent galaxies host moderately luminous AGNs detected by X-ray. There are no significant differences in the [O{footnotesize III}] $lambda$5007/H$beta$ emission line ratios, and [O{footnotesize III}] $lambda$5007 line widths and spatial extents of the protocluster galaxies from those of massive galaxies at $zsim2-3$ in the general field.
We present the results from ALMA CO(3-2) observations of 66 Halpha-selected galaxies in three protoclusters around radio galaxies, PKS1138-262 (z=2.16) and USS1558-003 (z=2.53), and 4C23.56 (z=2.49). The pointing areas have an overdensity of ~100 compared to a mean surface number density of galaxies in field environments. We detect CO emission line in 16 star-forming galaxies, including previously published six galaxies, to measure the molecular gas mass. In the stellar mass range of 10.5<log(Mstar/Msolar)<11.0, the protocluster galaxies have larger gas mass fractions and longer gas depletion timescales compared to the scaling relations established by field galaxies. On the other hand, the amounts of molecular gas in more massive galaxies with log(Mstar/Msolar)>11.0 are comparable in mass to the scaling relation, or smaller. Our results suggest that the environmental effects on gas properties are mass-dependent: in high-density environments, gas accretion through cosmic filaments is accelerated in less massive galaxies while this is suppressed in the most massive system.
Theoretical and numerical modeling of dark-matter halo assembly predicts that the most luminous galaxies at high redshift are surrounded by overdensities of fainter companions. We test this prediction with HST observations acquired by our Brightest of Reionizing Galaxies (BoRG) survey, which identified four very bright z~8 candidates as Y-dropout sources in four of the 23 non-contiguous WFC3 fields observed. We extend here the search for Y-dropouts to fainter luminosities (M_* galaxies with M_ABsim-20), with detections at >5sigma confidence (compared to >8sigma confidence adopted earlier) identifying 17 new candidates. We demonstrate that there is a correlation between number counts of faint and bright Y-dropouts at >99.84% confidence. Field BoRG58, which contains the best bright zsim8 candidate (M_AB=-21.3), has the most significant overdensity of faint Y-dropouts. Four new sources are located within 70arcsec (corresponding to 3.1 comoving Mpc at z=8) from the previously known brighter zsim8 candidate. The overdensity of Y-dropouts in this field has a physical origin to high confidence (p>99.975%), independent of completeness and contamination rate of the Y-dropout selection. We modeled the overdensity by means of cosmological simulations and estimate that the principal dark matter halo has mass M_hsim(4-7)x10^11Msun (sim5sigma density peak) and is surrounded by several M_hsim10^11Msun halos which could host the fainter dropouts. In this scenario, we predict that all halos will eventually merge into a M_h>2x10^14Msun galaxy cluster by z=0. Follow-up observations with ground and space based telescopes are required to secure the zsim8 nature of the overdensity, discover new members, and measure their precise redshift.
We performed a systematic spectroscopic observation of a protocluster at $z=6.01$ in the Subaru Deep Field. We took spectroscopy for all 53 $i$-dropout galaxies down to $z=27.09,mathrm{mag}$ in/around the protocluster region. From these observations, we confirmed that 28 galaxies are at $zsim6$, of which ten are clustered in a narrow redshift range of $Delta z<0.06$. To trace the evolution of this primordial structure, we applied the same $i$-dropout selection and the same overdensity measurements used in the observations to a semi-analytic model built upon the Millennium Simulation. We obtain a relation between the significance of overdensities observed at $zsim6$ and the predicted dark matter halo mass at $z=0$. This protocluster with $6sigma$ overdensity is expected to grow into a galaxy cluster with a mass of $sim5times10^{14},mathrm{M_odot}$ at $z=0$. Ten galaxies within $10,mathrm{comoving>Mpc}$ of the overdense region can, with more than an 80% probability, merge into a single dark matter halo by $z=0$. No significant differences appeared in UV and Ly$alpha$ luminosities between the protocluster and field galaxies, suggesting that this protocluster is still in the early phase of cluster formation before the onset of any obvious environmental effects. However, further observations are required to study other properties, such as stellar mass, dust, and age. We do find that galaxies tend to be in close pairs in this protocluster. These pair-like subgroups will coalesce into a single halo and grow into a more massive structure. We may witness an onset of cluster formation at $zsim6$ toward a cluster as seen in local universe.
We investigate the effects of dense environments on galaxy evolution by examining how the properties of galaxies in the z = 1.6 protocluster Cl 0218.3-0510 depend on their location. We determine galaxy properties using spectral energy distribution fitting to 14-band photometry, including data at three wavelengths that tightly bracket the Balmer and 4000A breaks of the protocluster galaxies. We find that two-thirds of the protocluster galaxies, which lie between several compact groups, are indistinguishable from field galaxies. The other third, which reside within the groups, differ significantly from the intergroup galaxies in both colour and specific star formation rate. We find that the fraction of red galaxies within the massive protocluster groups is twice that of the intergroup region. These excess red galaxies are due to enhanced fractions of both passive galaxies (1.7 times that of the intergroup region) and dusty star-forming galaxies (3 times that of the intergroup region). We infer that some protocluster galaxies are processed in the groups before the cluster collapses. These processes act to suppress star formation and change the mode of star formation from unobscured to obscured.
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