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Register a new userThe Formation of the massive galaxies in the SSA22 z=3.1 protocluster
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The properties of K-band selected galaxies (K_AB<24) in the z = 3.09 SSA22 protocluster field are studied. 430 galaxies at 2.6 < z_phot < 3.6 are selected as potential protocluster members in a 112 arcmin^2 area based on their photometric redshifts. We find that approx 20% of the massive galaxies with stellar masses >10^11 M_sun at z_phot sim 3.1 have colors consistent with those of quiescent galaxies with ages > 0.5 Gyr. This fraction increases to approx 50% after correcting for unrelated foreground/background objects. We also find that 30% of the massive galaxies are heavily reddened dusty star-forming galaxies. Few such quiescent galaxies at similar redshifts are seen in typical survey fields. An excess surface density of 24mu m sources at z_phot sim 3.1 is also observed, implying the presence of dusty star-formation activity in the protocluster. Cross-correlation with the X-ray data indicates that the fraction of K-band selected protocluster galaxies hosting active galactic nuclei (AGN) is also high compared with the field. The sky distribution of the quiescent galaxies, the 24mu m sources, and the X-ray AGNs show clustering around a density peak of z=3.1 Lyalpha emitters (LAEs). A significant fraction of the massive galaxies have already become quiescent, while the dusty star-formation is still active in the SSA22 protocluster. These findings indicate that we are witnessing the formation epoch of massive early-type galaxies at the center of predecessors to present-day rich galaxy clusters.
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We examine possible environmental sources of the enhanced star formation and active galactic nucleus (AGN) activity in the $z = 3.09$ SSA22 protocluster using Hubble WFC3 F160W ($sim1.6 rm mu m$) observations of the SSA22 field, including new observations centered on eight X-ray selected protocluster AGN. To investigate the role of mergers in the observed AGN and star formation enhancement, we apply both quantitative (Sersic-fit and Gini-$M_{20}$) and visual morphological classifications to F160W images of protocluster Lyman break galaxies (LBGs) in the fields of the X-ray AGN and $z sim 3$ field LBGs in SSA22 and GOODS-N. We find no statistically significant differences between the morphologies and merger fractions of protocluster and field LBGs, though we are limited by small number statistics in the protocluster. We also fit the UV-to-near-IR spectral energy distributions (SED) of F160W-detected protocluster and field LBGs to characterize their stellar masses and star formation histories (SFH). We find that the mean protocluster LBG is by a factor of $sim2$ times more massive and more attenuated than the mean $z sim 3$ field LBG. We take our results to suggest that ongoing mergers are not more common among protocluster LBGs than field LBGs, though protocluster LBGs appear to be more massive. We speculate that the larger mass of the protocluster LBGs contributes to the enhancement of SMBH mass and accretion rate in the protocluster, which in turn drives the observed protocluster AGN enhancement.
We report the discovery of an extremely dense group of massive galaxies at the centre of the protocluster at $z=3.09$ in the SSA22 field from near-infrared spectroscopy conducted with the Multi-Object InfraRed Camera and Spectrograph (MOIRCS) equipped on the Subaru Telecope. The newly discovered group comprises seven galaxies confirmed at $z_{rm spec}approx3.09$ within 180 kpc including five massive objects with the stellar masses larger than $10^{10.5}~M_{odot}$ and is associated with a bright sub-mm source SSA22-AzTEC14. The dynamical mass of the group estimated from the line-of-sight velocity dispersion of the members is $M_{rm dyn}sim1.6pm0.3times10^{13}~M_{odot}$. Such a dense group is expected to be very rare at high redshift as we found only a few comparable systems in large-volume cosmological simulations. Such rare groups in the simulations are hosted in collapsed halos with $M_{rm vir}=10^{13.4}-10^{14.0}~M_{odot}$ and evolve into the brightest cluster galaxies (BCGs) of the most massive clusters at present. The observed AzTEC14 group at $z=3.09$ is therefore very likely to be a proto-BCG in the multiple merger phase. The observed total stellar mass of the group is $5.8^{+5.1}_{-2.0}times10^{11}~M_{odot}$. It suggests that over half the stellar mass of its descendant had been formed by $z=3$. Moreover, we identified over two members for each of the four Ly$alpha$ blobs (LABs) using our new spectroscopic data. This verifies our previous argument that many of the LABs in the SSA22 protocluster associated with multiple developed stellar components.
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 describe the results of a new, wide-field survey for z=3.1 Ly-alpha emission-line galaxies (LAEs) in the Extended Chandra Deep Field South (ECDF-S). By using a nearly top-hat 5010 Angstrom filter and complementary broadband photometry from the MUSYC survey, we identify a complete sample of 141 objects with monochromatic fluxes brighter than 2.4E-17 ergs/cm^2/s and observers-frame equivalent widths greater than ~ 80 Angstroms (i.e., 20 Angstroms in the rest-frame of Ly-alpha). The bright-end of this dataset is dominated by x-ray sources and foreground objects with GALEX detections, but when these interlopers are removed, we are still left with a sample of 130 LAE candidates, 39 of which have spectroscopic confirmations. This sample overlaps the set of objects found in an earlier ECDF-S survey, but due to our filters redder bandpass, it also includes 68 previously uncataloged sources. We confirm earlier measurements of the z=3.1 LAE emission-line luminosity function, and show that an apparent anti-correlation between equivalent width and continuum brightness is likely due to the effect of correlated errors in our heteroskedastic dataset. Finally, we compare the properties of z=3.1 LAEs to LAEs found at z=2.1. We show that in the ~1 Gyr after z~3, the LAE luminosity function evolved significantly, with L* fading by ~0.4 mag, the number density of sources with L > 1.5E42 ergs/s declining by ~50%, and the equivalent width scale-length contracting from 70^{+7}_{-5} Angstroms to 50^{+9}_{-6} Angstroms. When combined with literature results, our observations demonstrate that over the redshift range z~0 to z~4, LAEs contain less than ~10% of the star-formation rate density of the universe.
We present results from a new ultra-deep 400 ks Chandra observation of the SSA22 protocluster at z = 3.09. We have studied the X-ray properties of 234 z ~ 3 Lyman break galaxies (LBGs; protocluster and field) and 158 z = 3.09 Ly-alpha emitters (LAEs) in SSA22 to measure the influence of the high-density protocluster environment on the accretion activity of supermassive black holes (SMBHs) in these UV-selected star forming populations. We detect individually X-ray emission from active galactic nuclei (AGNs) in six LBGs and five LAEs; due to small overlap between the LBG and LAE source population, ten of these sources are unique. At least six and potentially eight of these sources are members of the protocluster. These sources have rest-frame 8-32 keV luminosities in the range of L_8-32 keV = (3-50) X 10^{43} ergs/s and an average observed-frame 2-8 keV to 0.5-2 keV band-ratio of ~0.8 (mean effective photon index of Gamma_eff = 1.1), suggesting significant absorption columns of N_H > 10^{22}-10^{24} cm^{-2}. We find that the fraction of LBGs and LAEs in the z = 3.09 protocluster harboring an AGN with L_8-32 keV > 3 X 10^{43} ergs/s is 9.5^{+12.7}_{-6.1}% and 5.1^{+6.8}_{-3.3}%, respectively. These AGN fractions are somewhat larger (by a mean factor of 6.1^{+10.3}_{-3.6}; significant at the 95% confidence level) than z ~ 3 sources found in lower-density field environments. Theoretical models imply that these results may be due to the presence of more actively growing and/or massive SMBHs in LBGs and LAEs within the protocluster compared to the field. Such a result is expected in a scenario where enhanced merger activity in the protocluster drives accelerated galaxy and SMBH growth at z > 2-3. (abridged)
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