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Register a new userDiscovery of Protoclusters at z~3.7 & 4.9: Embedded in Primordial Superclusters
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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 $z=4.898$, 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 $Delta z=0.05$, which is slightly larger than the size of typical protoclusters. Therefore, if they are not the progenitors of a $>10^{15},mathrm{M_odot}$ 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 $z=2mathrm{-}6$. 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.
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We identify a strong lensing galaxy in the cluster IRC 0218 (also known as XMM-LSS J02182$-$05102) that is spectroscopically confirmed to be at $z=1.62$, making it the highest-redshift strong lens galaxy known. The lens is one of the two brightest cluster galaxies and lenses a background source galaxy into an arc and a counterimage. With Hubble Space Telescope (HST) grism and Keck/LRIS spectroscopy, we measure the source redshift to be $z_{rm S}=2.26$. Using HST imaging in ACS/F475W, ACS/F814W, WFC3/F125W, and WFC3/F160W, we model the lens mass distribution with an elliptical power-law profile and account for the effects of the cluster halo and nearby galaxies. The Einstein radius is $theta_{rm E}=0.38^{+0.02}_{-0.01}$ ($3.2_{-0.1}^{+0.2}$ kpc) and the total enclosed mass is M$_{rm tot} (< theta_{rm E})=1.8^{+0.2}_{-0.1}times10^{11}~{rm M}_{odot}$. We estimate that the cluster environment contributes $sim10$% of this total mass. Assuming a Chabrier IMF, the dark matter fraction within $theta_{{rm E}}$ is $f_{rm DM}^{{rm Chab}} = 0.3_{-0.3}^{+0.1}$, while a Salpeter IMF is marginally inconsistent with the enclosed mass ($f_{rm DM}^{{rm Salp}} = -0.3_{-0.5}^{+0.2}$). The total magnification of the source is $mu_{rm tot}=2.1_{-0.3}^{+0.4}$. The source has at least one bright compact region offset from the source center. Emission from Ly$alpha$ and [O III] are likely to probe different regions in the source.
We measured metallicities for 33 z=3.4-4.2 absorption line systems drawn from a sample of H I-selected-Lyman limit systems (LLSs) identified in Sloan Digital Sky Survey (SDSS) quasar spectra and stratified based on metal line features. We obtained higher-resolution spectra with the Keck Echellette Spectrograph and Imager, selecting targets according to our stratification scheme in an effort to fully sample the LLS population metallicity distribution. We established a plausible range of H I column densities and measured column densities (or limits) for ions of carbon, silicon, and aluminum, finding ionization-corrected metallicities or upper limits. Interestingly, our ionization models were better constrained with enhanced $alpha$-to-aluminum abundances, with a median abundance ratio of [$alpha$/Al]=0.3. Measured metallicities were generally low, ranging from [M/H]=-3 to -1.68, with even lower metallicities likely for some systems with upper limits. Using survival statistics to incorporate limits, we constructed the cumulative distribution function (CDF) for LLS metallicities. Recent models of galaxy evolution propose that galaxies replenish their gas from the low-metallicity intergalactic medium (IGM) via high-density H I flows and eject enriched interstellar gas via outflows. Thus, there has been some expectation that LLSs at the peak of cosmic star formation ($zapprox3$) might have a bimodal metallicity distribution. We modeled our CDF as a mix of two Gaussian distributions, one reflecting the metallicity of the IGM and the other representative of the interstellar medium of star-forming galaxies. This bimodal distribution yielded a poor fit. A single Gaussian distribution better represented the sample with a low mean metallicity of [M/H] $approx -2.5$.
We investigate a spatially-flat cold dark matter model (with the matter density parameter $Omega_m=0.3$) with a primordial feature in the initial power spectrum. We assume that there is a bump in the power spectrum of density fluctuations at wavelengths $lambda sim 30-60h^{-1}$Mpc, which correspond to the scale of superclusters of galaxies. There are indications for such a feature in the power spectra derived from redshift surveys and also in the power spectra derived from peculiar velocities of galaxies. We study the mass function of clusters of galaxies, the power spectrum of the CMB temperature fluctuations, the rms bulk velocity and the rms peculiar velocity of clusters of galaxies. The baryon density is assumed to be consistent with the BBN value. We show that with an appropriately chosen feature in the power spectrum of density fluctuations at the scale of superclusters, the mass function of clusters, the CMB power spectrum and peculiar velocities are in good agreement with the observed data.
We report the Subaru Hyper Suprime-Cam (HSC) discovery of two Ly$alpha$ blobs (LABs), dubbed z70-1 and z49-1 at $z=6.965$ and $z=4.888$ respectively, that are Ly$alpha$ emitters with a bright ($log L_{rm Lyalpha}/{rm [erg s^{-1}]}>43.4$) and spatially-extended Ly$alpha$ emission, and present the photometric and spectroscopic properties of a total of seven LABs; the two new LABs and five previously-known LABs at $z=5.7-6.6$. The z70-1 LAB shows the extended Ly$alpha$ emission with a scale length of $1.4pm 0.2$ kpc, about three times larger than the UV continuum emission, making z70-1 the most distant LAB identified to date. All of the 7 LABs, except z49-1, exhibit no AGN signatures such as X-ray emission, {sc Nv}$lambda$1240 emission, or Ly$alpha$ line broadening, while z49-1 has a strong {sc Civ}$lambda$1548 emission line indicating an AGN on the basis of the UV-line ratio diagnostics. We carefully model the point-spread functions of the HSC images, and conduct two-component exponential profile fitting to the extended Ly$alpha$ emission of the LABs. The Ly$alpha$ scale lengths of the core (star-forming region) and the halo components are $r_{rm c}=0.6-1.2$ kpc and $r_{rm h}=2.0-13.8$ kpc, respectively. The average $r_{rm h}$ of the LABs falls on the extrapolation of the $r_{rm h}$-Ly$alpha$ luminosity relation of the Ly$alpha$ halos around VLT/MUSE star-forming galaxies at the similar redshifts, suggesting that typical LABs at $zgtrsim5$ are not special objects, but star-forming galaxies at the bright end.
We present spectroscopic confirmation of two new massive galaxy protoclusters at $z=2.24pm0.02$, BOSS1244 and BOSS1542, traced by groups of Coherently Strong Ly$alpha$ Absorption (CoSLA) systems imprinted in the absorption spectra of a number of quasars from the SDSS III and identified as overdensities of narrowband-selected H$alpha$ emitters (HAEs). Using MMT/MMIRS and LBT/LUCI near-infrared (NIR) spectroscopy, we confirm 46 and 36 HAEs in the BOSS1244 and BOSS1542 fields, respectively. BOSS1244 displays a South-West (SW) component at $z=2.230pm0.002$ and another North-East (NE) component at $z=2.246pm0.001$ with the line-of-sight velocity dispersions of $405pm202$ km s$^{-1}$ and $377pm99$ km s$^{-1}$, respectively. Interestingly, we find that the SW region of BOSS1244 contains two substructures in redshift space, likely merging to form a larger system. In contrast, BOSS1542 exhibits an extended filamentary structure with a low velocity dispersion of $247pm32$ km s$^{-1}$ at $z=2.241pm0.001$, providing a direct confirmation of a large-scale cosmic web in the early Universe. The galaxy overdensities $delta_{rm g}$ on the scale of 15 cMpc are $22.9pm4.9$, $10.9pm2.5$, and $20.5pm3.9$ for the BOSS1244 SW, BOSS1244 NE, and BOSS1542 filament, respectively. They are the most overdense galaxy protoclusters ($delta_{rm g}>20$) discovered to date at $z>2$. These systems are expected to become virialized at $zsim0$ with a total mass of $M_{rm SW}=(1.59pm0.20)times10^{15}$ $M_{odot}$, $M_{rm NE} =(0.83pm0.11)times10^{15}$ $M_{odot}$ and $M_{rm filament}=(1.42pm0.18)times10^{15}$ $M_{odot}$, respectively. Together with BOSS1441 described in Cai et al. (2017a), these extremely massive overdensities at $z=2-3$ exhibit different morphologies, indicating that they are in different assembly stages in the formation of early galaxy clusters.
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