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Register a new userSpectroscopic Confirmation of a Coma Cluster Progenitor at z ~ 2.2
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We report the spectroscopic confirmation of a new protocluster in the COSMOS field at $z$ $sim$ 2.2, COSMOS Cluster 2.2 (CC2.2), originally identified as an overdensity of narrowband selected H$alpha$ emitting candidates. With only two masks of Keck/MOSFIRE near-IR spectroscopy in both $H$ ($sim$ 1.47-1.81 $mu$m) and $K$ ($sim$ 1.92-2.40 $mu$m) bands ($sim$ 1.5 hour each), we confirm 35 unique protocluster members with at least two emission lines detected with S/N $>$ 3. Combined with 12 extra members from the zCOSMOS-deep spectroscopic survey (47 in total), we estimate a mean redshift and a line-of-sight velocity dispersion of $z_{mean}$=2.23224 $pm$ 0.00101 and $sigma_{los}$=645 $pm$ 69 km s$^{-1}$ for this protocluster, respectively. Assuming virialization and spherical symmetry for the system, we estimate a total mass of $M_{vir}$ $sim$ $(1-2) times$10$^{14}$ $M_{odot}$ for the structure. We evaluate a number density enhancement of $delta_{g}$ $sim$ 7 for this system and we argue that the structure is likely not fully virialized at $z$ $sim$ 2.2. However, in a spherical collapse model, $delta_{g}$ is expected to grow to a linear matter enhancement of $sim$ 1.9 by $z$=0, exceeding the collapse threshold of 1.69, and leading to a fully collapsed and virialized Coma-type structure with a total mass of $M_{dyn}$($z$=0) $sim$ 9.2$times$10$^{14}$ $M_{odot}$ by now. This observationally efficient confirmation suggests that large narrowband emission-line galaxy surveys, when combined with ancillary photometric data, can be used to effectively trace the large-scale structure and protoclusters at a time when they are mostly dominated by star-forming galaxies.
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We present the spectroscopic confirmation of a galaxy cluster at $z=2.095$ in the COSMOS field. This galaxy cluster was first reported in the ZFOURGE survey as harboring evolved massive galaxies using photometric redshifts derived with deep near-infrared (NIR) medium-band filters. We obtain medium resolution ($R sim$ 3600) NIR spectroscopy with MOSFIRE on the Keck 1 telescope and secure 180 redshifts in a $12times12$ region. We find a prominent spike of 57 galaxies at $z=2.095$ corresponding to the galaxy cluster. The cluster velocity dispersion is measured to be $sigma_{rm v1D}$ = 552 $pm$ 52 km/s. This is the first study of a galaxy cluster in this redshift range ($z gt 2.0$) with the combination of spectral resolution ($sim$26 km/s) and the number of confirmed members (${>}50$) needed to impose a meaningful constraint on the cluster velocity dispersion and map its members over a large field of view. Our $Lambda$CDM cosmological simulation suggests that this cluster will most likely evolve into a Virgo-like cluster with ${rm M_{vir}}{=}10^{14.4pm0.3} {rm M_odot}$ ($68%$ confidence) at $zsim$ 0. The theoretical expectation of finding such a cluster is $sim$ $4%$. Our results demonstrate the feasibility of studying galaxy clusters at $z > 2$ in the same detailed manner using multi-object NIR spectrographs as has been done in the optical in lower redshift clusters.
We present a detailed census of galaxies in and around PC217.96+32.3, a spectroscopically confirmed Coma analog at z=3.78. Diverse galaxy types identified in the field include Lya emitters (LAEs), massive star-forming galaxies, and ultra-massive galaxies (log (Mstar/Msun)>= 11) which may have already halted their star formation. The sky distribution of the star-forming galaxies suggests the presence of a significant overdensity (delta_g=8+/-2), which is spatially offset from the previously confirmed members by 3-4 Mpc to the west. Candidate quiescent and post-starburst galaxies are also found in large excess (a factor of ~ 8-15 higher surface density than the field) although their redshifts are less certain. We estimate that the total enclosed mass traced by star-forming galaxy candidates is roughly comparable to that of PC217.96+32.3 traced by the LAEs. We speculate that the true extent of PC217.96+32.3 may be larger than previously known, a half of which is missed by our LAE selection. Alternatively, the newly discovered overdensity may belong to another Coma progenitor not associated with PC217.96+32.3. Expectations from theory suggest that both scenarios are equally unlikely (<1%), particularly in the cosmic volume probed in our survey. If confirmed as a single structure, its total mass will be well in excess of Coma, making this a singularly large cosmic structure rarely seen even in large cosmological simulations. Finally, we find that the protocluster galaxies follow the same SFR-M_star scaling relation as the field galaxies, suggesting that the environmental effect at z~4 is a subtle one at best for normal star-forming galaxies.
Keck/LRIS multi-object spectroscopy has been carried out on 140 of some of the lowest and highest surface brightness faint (19 < R < 22) dwarf galaxy candidates in the core region of the Coma Cluster. These spectra are used to measure redshifts and establish membership for these faint dwarf populations. The primary goal of the low surface brightness sample is to test our ability to use morphological and surface brightness criteria to distinguish between Coma Cluster members and background galaxies using high resolution HST/ACS images. Candidates were rated as expected members, uncertain, or expected background. From 93 spectra, 51 dwarf galaxy members and 20 background galaxies are identified. Our morphological membership estimation success rate is ~100% for objects expected to be members and better than ~90% for galaxies expected to be in the background. We confirm that low surface brightness is a very good indicator of cluster membership. High surface brightness galaxies are almost always background with confusion arising only from the cases of the rare compact elliptical galaxies. The more problematic cases occur at intermediate surface brightness. Many of these galaxies are given uncertain membership ratings, and these were found to be members about half of the time. Including color information will improve membership determination but will fail for some of the same objects that are already mis-identified when using only surface brightness and morphology criteria. Compact elliptical galaxies with B-V colors ~0.2 magnitudes redward of the red sequence in particular require spectroscopic follow-up. In a sample of 47 high surface brightness, UCD candidates, 19 objects have redshifts which place them in the Coma Cluster. Redshift measurements are presented and the use of indirect means for establishing cluster membership is discussed.
We report the discovery of a spectroscopically-confirmed strong Lyman-$alpha$ emitter at $z=7.0281pm0.0003$, observed as part of the Reionization Cluster Lensing Survey (RELICS). This galaxy, dubbed Dichromatic Primeval Galaxy at $zsim7$ (DP7), shows two distinct components. While fairly unremarkable in terms of its ultraviolet (UV) luminosity ($sim0.3L^{ast}_{UV}$, where $L^{ast}_{UV}$ is the characteristic luminosity), DP7 has one of the highest observed Lyman-$alpha$ equivalent widths (EWs) among Lyman-$alpha$ emitters at $z>6$ ($>200$ Angstrom in the rest frame). The strong Lyman-$alpha$ emission generally suggests a young metal-poor, low-dust galaxy; however, we find that the UV slope $beta$ of the galaxy as a whole is redder than typical star-forming galaxies at these redshifts, $-1.13pm 0.84$, likely indicating, on average, a considerable amount of dust obscuration, or an older stellar population. When we measure $beta$ for the two components separately, however, we find evidence of differing UV colors, suggesting two separate stellar populations. Also, we find that Lyman-$alpha$ is spatially extended and likely larger than the galaxy size, hinting to the possible existence of a Lyman-$alpha$ halo. Rejuvenation or merging events could explain these results. Either scenario requires an extreme stellar population, possibly including a component of Population III stars, or an obscured Active Galactic Nucleus. DP7, with its low UV luminosity and high Lyman-$alpha$ EW, represents the typical galaxies that are thought to be the major contribution to the reionization of the Universe, and for this reason DP7 is an excellent target for follow-up with the James Webb Space Telescope.
Galaxies had their most significant impact on the Universe when they assembled their first generations of stars. Energetic photons emitted by young, massive stars in primeval galaxies ionized the intergalactic medium surrounding their host galaxies, cleared sight-lines along which the light of the young galaxies could escape, and fundamentally altered the physical state of the intergalactic gas in the Universe continuously until the present day. Observations of the Cosmic Microwave Background, and of galaxies and quasars at the highest redshifts, suggest that the Universe was reionised through a complex process that was completed about a billion years after the Big Bang, by redshift z~6. Detecting ionizing Ly-alpha photons from increasingly distant galaxies places important constraints on the timing, location and nature of the sources responsible for reionisation. Here we report the detection of Ly-a photons emitted less than 600 million years after the Big Bang. UDFy-38135539 is at a redshift z=8.5549+-0.0002, which is greater than those of the previously known most distant objects, at z=8.2 and z=6.97. We find that this single source is unlikely to provide enough photons to ionize the volume necessary for the emission line to escape, requiring a significant contribution from other, probably fainter galaxies nearby.
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