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Register a new userThe Chandra Deep Protocluster Survey: Evidence for an Enhancement in the AGN Activity in the SSA22 Protocluster at z = 3.09
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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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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 report the results of $1^{prime}.5 times3^{prime}$ mapping at 1.1~mm with the Atacama Large Millimeter/submillimeter Array (ALMA) toward the central region of the $z=3.09$ SSA22 protocluster. By combining our source catalog with archival spectroscopic redshifts, we find that eight submillimeter galaxies (SMGs) with flux densities, $S_{rm 1.1~mm}=0.7-6.4$~mJy ($L_{rm IR}sim10^{12.1}-10^{13.1}L_odot$) are at $z=3.08-3.10$. Not only are these SMGs members of the protocluster but they in fact reside within the node at the junction of the 50 Mpc-scale filamentary three-dimensional structure traced by Lyman-$alpha$ emitters (LAEs) in this field. The eight SMGs account for a star formation rate density (SFRD) $sim$10 $M_odot$ yr$^{-1}$ Mpc$^{-3}$ in the node, which is two orders of magnitudes higher than the global SFRD at this redshift. We find that four of the eight SMGs host a X-ray luminous active galactic nuclei (AGN). Our results suggest that the vigorous star formation activity and the growth of super massive black holes (SMBHs) occurred simultaneously in the densest regions at $zsim3$, which may correspond to the most active historical phase of the massive galaxy population found in the core of the clusters in the present universe. Two SMGs are associated with Lyman-$alpha$ blobs (LABs), implying that the two populations coexist in high density environments for a few cases.
We investigate the prevalence of AGN in the high-redshift protocluster $rm{Cl},0218.3$-$0510$ at $z=1.62$. Using imaging from the Chandra X-ray Telescope, we find a large overdensity of AGN in the protocluster; a factor of $23pm9$ times the field density of AGN. Only half of this AGN overdensity is due to the overdensity of massive galaxies in the protocluster (a factor of $11pm2$), as we find that $17^{+6}_{-5}%$ of massive galaxies ($M_* > 10^{10},rm{M}_{odot}$) in the protocluster host an X-ray luminous AGN, compared to $8pm1%$ in the field. This corresponds to an enhancement of AGN activity in massive protocluster galaxies by a factor of $2.1pm0.7$ at $1.6sigma$ significance. We also find that the AGN overdensity is centrally concentrated, located within 3 arcmin and most pronounced within 1 arcmin of the centre of the protocluster. Our results confirm that there is a reversal in the local anti-correlation between galaxy density and AGN activity, so there is an enhancement of AGN in high-redshift protoclusters. We compare the properties of AGN in the protocluster to the field and find no significant differences in the distributions of their stellar mass, X-ray luminosity, or hardness ratio. We therefore suggest that triggering mechanisms are similar in both environments, and that the mechanisms simply occur more frequently in denser environments.
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 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.
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