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Register a new userSimulating the formation of a proto-cluster at z~2
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We present results from two high-resolution hydrodynamical simulations of proto-cluster regions at z~2.1. The simulations have been compared to observational results for the socalled Spiderweb galaxy system, the core of a putative proto-cluster region at z = 2.16, found around a radio galaxy. The simulated regions have been chosen so as to form a poor cluster with M200~10^14 h-1 Msun (C1) and a rich cluster with M200~2x10^15 h-1 Msun (C2) at z = 0. The simulated proto-clusters show evidence of ongoing assembly of a dominating central galaxy. The stellar mass of the brightest cluster galaxy (BCG) of the C2 system is in excess with respect to observational estimates for the Spiderweb galaxy, with a total star formation rate which is also larger than indicated by observations. We find that the projected velocities of galaxies in the C2 cluster are consistent with observations, while those measured for the poorer cluster C1 are too low compared to the observed velocities. We argue that the Spiderweb complex resemble the high-redshift progenitor of a rich galaxy cluster. Our results indicate that the included supernovae feedback is not enough to suppress star formation in these systems, supporting the need of introducing AGN feedback. According to our simulations, a diffuse atmosphere of hot gas in hydrostatic equilibrium should already be present at this redshift, and enriched at a level comparable to that of nearby galaxy clusters. The presence of this gas should be detectable with future deep X-ray observations.
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We present the spectroscopic confirmation of a $z=2.45$ proto-cluster. Its member galaxies lie within a radius of 1.4Mpc (physical) on the sky and within $Delta v pm 700$km/s along the line of sight. We estimate an overdensity of 10, suggesting that the structure has made the turn-around but is not assembled yet. Comparison to the Millennium simulation suggests that analogous structures evolve into $10^{14}-10^{15}$M$_{odot}$/h type dark matter haloes by $z=0$ qualifying the notion of proto-cluster. The search for the complete census of mock progenitor galaxies at $zsim2.5$ of these massive $z=0$ mock clusters reveals that they are widely spread over areas with a diameter of 3-20Mpc. This suggests that the optical selection of such proto-clusters can result in a rich diversity regarding their $z=0$ descendants. We also searched for signs of environmental differentiation in this proto-cluster. Whilst we see a weak trend for more massive and more quiescent galaxies in the proto-cluster, this is not statistically significant.
Numerical simulations of cosmological structure formation show that the Universes most massive clusters, and the galaxies living in those clusters, assemble rapidly at early times (2.5 < z < 4). While more than twenty proto-clusters have been observed at z > 2 based on associations of 5-40 galaxies around rare sources, the observational evidence for rapid cluster formation is weak. Here we report observations of an asymmetric, filamentary structure at z = 2.47 containing seven starbursting, submillimeter-luminous galaxies and five additional AGN within a comoving volume of 15000 Mpc$^{3}$. As the expected lifetime of both the luminous AGN and starburst phase of a galaxy is ~100 Myr, we conclude that these sources were likely triggered in rapid succession by environmental factors, or, alternatively, the duration of these cosmologically rare phenomena is much longer than prior direct measurements suggest. The stellar mass already built up in the structure is $sim10^{12}M_{odot}$ and we estimate that the cluster mass will exceed that of the Coma supercluster at $z sim 0$. The filamentary structure is in line with hierarchical growth simulations which predict that the peak of cluster activity occurs rapidly at z > 2.
A proto-cluster core is the most massive dark matter halo (DMH) in a given proto-cluster. To reveal the galaxy formation in core regions, we search for proto-cluster cores at $zsim 2$ in $sim 1.5, mathrm{deg}^{2}$ of the COSMOS field. Using pairs of massive galaxies ($log(M_{*}/M_{odot})geq11$) as tracers of cores, we find 75 candidate cores, among which 54% are estimated to be real. A clustering analysis finds that these cores have an average DMH mass of $2.6_{-0.8}^{+0.9}times 10^{13}, M_{odot}$, or $4.0_{-1.5}^{+1.8}, times 10^{13} M_{odot}$ after contamination correction. The extended Press-Schechter model shows that their descendant mass at $z=0$ is consistent with Fornax-like or Virgo-like clusters. Moreover, using the IllustrisTNG simulation, we confirm that pairs of massive galaxies are good tracers of DMHs massive enough to be regarded as proto-cluster cores. We then derive the stellar mass function (SMF) and the quiescent fraction for member galaxies of the 75 candidate cores. We find that the core galaxies have a more top-heavy SMF than field galaxies at the same redshift, showing an excess at $log(M_{*}/M_{odot})gtrsim 10.5$. The quiescent fraction, $0.17_{-0.04}^{+0.04}$ in the mass range $9.0leq log(M_{*}/M_{odot})leq 11.0$, is about three times higher than that of field counterparts, giving an environmental quenching efficiency of $0.13_{-0.04}^{+0.04}$. These results suggest that stellar mass assembly and quenching are accelerated as early as at $zsim 2$ in proto-cluster cores.
We present SCUBA-2 450$mu$m and 850$mu$m data of the mature redshift 2 cluster CLJ1449. We combine this with archival Herschel data to explore the star forming properties of CLJ1449. Using high resolution ALMA and JVLA data we identify potentially confused galaxies, and use the Bayesian inference tool XID+ to estimate fluxes for them. Using archival optical and near infrared data with the energy-balance code CIGALE we calculate star formation rates, and stellar masses for all our cluster members, and find the star formation rate varies between 20-1600M$_{odot}$yr$^{-1}$ over the entire 3Mpc radial range. The central 0.5Mpc region itself has a total star formation rate of 800$pm$200M$_{odot}$yr$^{-1}$, which corresponds to a star formation rate density of (1.2$pm$0.3)$times$10$^{4}$M$_{odot}$yr$^{-1}$Mpc$^{-3}$, which is approximately five orders of magnitude greater than expected field values. When comparing this cluster to those at lower redshifts we find that there is an increase in star formation rate per unit volume towards the centre of the cluster. This indicates that there is indeed a reversal in the star formation/density relation in CLJ1449. Based on the radial star-formation rate density profile, we see evidence for an elevation in the star formation rate density, even out to radii of 3Mpc. At these radii the elevation could be an order of magnitude greater than field values, but the exact number cannot be determined due to ambiguity in the redshift associations. If this is the case it would imply that this cluster is still accreting material which is possibly interacting and undergoing vigorous star-formation.
Bright Ly-$alpha$ blobs (LABs) --- extended nebulae with sizes of $sim$100kpc and Ly-$alpha$ luminosities of $sim$10$^{44}$erg s$^{-1}$ --- often reside in overdensities of compact Ly-$alpha$ emitters (LAEs) that may be galaxy protoclusters. The number density, variance, and internal kinematics of LABs suggest that they themselves trace group-like halos. Here we test this hierarchical picture, presenting deep, wide-field Ly-$alpha$ narrowband imaging of a 1$^circ$ $times$ 0.5$^circ$ region around a LAB pair at $z$ = 2.3 discovered previously by a blind survey. We find 183 Ly-$alpha$ emitters, including the original LAB pair and three new LABs with Ly-$alpha$ luminosities of (0.9--1.3)$times$10$^{43}$erg s$^{-1}$ and isophotal areas of 16--24 arcsec$^2$. Using the LAEs as tracers and a new kernel density estimation method, we discover a large-scale overdensity (Bo{o}tes J1430+3522) with a surface density contrast of $delta_{Sigma}$ = 2.7, a volume density contrast of $delta$ $sim$ 10.4, and a projected diameter of $approx$ 20 comoving Mpc. Comparing with cosmological simulations, we conclude that this LAE overdensity will evolve into a present-day Coma-like cluster with $log{(M/M_odot)}$ $sim$ $15.1pm0.2$. In this and three other wide-field LAE surveys re-analyzed here, the extents and peak amplitudes of the largest LAE overdensities are similar, not increasing with survey size, implying that they were indeed the largest structures then and do evolve into rich clusters today. Intriguingly, LABs favor the outskirts of the densest LAE concentrations, i.e., intermediate LAE overdensities of $delta_Sigma = 1 - 2$. We speculate that these LABs mark infalling proto-groups being accreted by the more massive protocluster.
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