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Register a new userSpectroscopic Confirmation of two Extremely Massive Protoclusters BOSS1244 and BOSS1542 at $z=2.24$
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Dongdong Shi
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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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Massive galaxy overdensities at the peak epoch of cosmic star formation provide ideal testbeds for the formation theories of galaxies and large-scale structure. We report the confirmation of two massive galaxy overdensities at $z=2.24$, BOSS1244 and BOSS1542, selected from the MAMMOTH project using Ly$alpha$ absorption from the intergalactic medium over the scales of 15$-$30 $h^{-1}$ Mpc imprinted on the quasar spectra. We use H$alpha$ emitters (HAEs) as the density tracer and identify them using deep narrowband $H_2S1$ and broadband $K_{rm s}$ imaging data obtained with CFHT/WIRCam. In total, 244 and 223 line emitters are detected in these two fields, and $196pm 2$ and $175pm 2$ are expected to be HAEs with an H$alpha$ flux of $> 2.5times 10^{-17}$ erg s$^{-1}$ cm$^{-2}$ (corresponding to an SFR of $>$5 M$_odot$ yr$^{-1}$). The detection rate of HAE candidates suggests an overdensity factor of $delta_{rm gal}=5.6pm0.3$ and $4.9pm0.3$ over the volume of $54times32times32$ cMpc$^3$. The overdensity factor increases $2-3$ times when focusing on the high-density regions of scales $10-15$ cMpc. Interestingly, the HAE density maps reveal that BOSS1244 contains a dominant structure, while BOSS1542 manifests as a giant filamentary structure. We measure the H$alpha$ luminosity functions (HLF), finding that BOSS1244s HLF is nearly identical to that of the general field at the same epoch, while BOSS1542 shows an excess of HAEs with high H$alpha$ luminosity, indicating the presence of enhanced star formation or AGN activity. We conclude that the two massive MAMMOTH overdensities are undergoing a rapid galaxy mass assembly.
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We present Gemini and Keck spectroscopic redshifts and velocity dispersions for twenty clusters detected via the Sunyaev-Zeldovich (SZ) effect by the Planck space mission, with estimated masses in the range $2.3 times 10^{14} M_{odot} < M < 9.4 times 10^{14} M_{odot}$. Cluster members were selected for spectroscopic follow-up with Palomar, Gemini and Keck optical and (in some cases) infrared imaging. Seven cluster redshifts were measured for the first time with this observing campaign, including one of the most distant Planck clusters confirmed to date, at $z=0.782pm0.010$, PSZ2 G085.95+25.23. The spectroscopic redshift catalogs of members of each confirmed cluster are included as on-line tables. We show the galaxy redshift distributions and measure the cluster velocity dispersions. The cluster velocity dispersions obtained in this paper were used in a companion paper to measure the Planck mass bias and to constrain the cluster velocity bias.
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.
We present spectroscopic confirmation of the Pisces Overdensity, also known as Structure J, a photometric overdensity of RR Lyrae stars discovered by the Sloan Digital Sky Survey (SDSS) at an estimated photometric distance of ~85kpc. We measure radial velocities for 8 RR Lyrae stars within Pisces. We find that 5 of the 8 stars have heliocentric radial velocities within a narrow range of -87 km/s < v < -67 km/s, suggesting that the photometric overdensity is mainly due to a physically associated system, probably a dwarf galaxy or a disrupted galaxy. Two of the remaining 3 stars differ from one another by only 9 km/s, but it would be premature to identify them as a second system.
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