SSA22 Protocluster Research Articles

Revisiting the Properties of X-Ray Active Galactic Nuclei in the SSA22 Protocluster: Normal Supermassive Black Hole and Host-galaxy Growth for AGNs in a z = 3.09 Overdensity

We analyze the physical properties of eight X-ray-selected active galactic nuclei (AGNs) and one candidate protoquasar system (ADF22A1) in the z = 3.09 SSA22 protocluster by fitting their X-ray-to-IR spectral energy distributions (SEDs) using our SED-fitting code, Lightning (https://www.github.com/rafaeleufrasio/lightning). We recover star formation histories (SFHs) for seven of these systems which are well fit by composite stellar population plus AGN models. We find indications that four out of nine of the SSA22 AGN systems we study have host galaxies below the main sequence, with SFR/SFRMS ≤ −0.4. The remaining SSA22 systems, including ADF22A1, are consistent with obscured supermassive black hole (SMBH) growth in star-forming galaxies. We estimate the SMBH accretion rates and masses, and compare the properties and SFHs of the nine protocluster AGN systems with X-ray-detected AGN candidates in the Chandra Deep Fields (CDF), finding that the distributions of SMBH growth rates, star formation rates (SFRs), SMBH masses, and stellar masses for the protocluster AGNs are consistent with field AGNs. We constrain the ratio between the sample-averaged SSA22 SMBH mass and CDF SMBH mass to <1.41. While the AGNs are located near the density peaks of the protocluster, we find no statistically significant trends between the AGN or host-galaxy properties and their location in the protocluster. We interpret the similarity of the protocluster and field AGN populations together with existing results as suggesting that the protocluster and field AGNs coevolve with their hosts in the same ways, while AGN-triggering events are more likely in the protocluster.

On the Nature of AGN and Star Formation Enhancement in the z = 3.1 SSA22 Protocluster: The HST WFC3 IR View

Abstract 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 (∼1.6 μm) 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 (Sérsic-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 ∼ 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 of F160W-detected protocluster and field LBGs to characterize their stellar masses and star formation histories. We find that the mean protocluster LBG is a factor of ∼2 times more massive and more attenuated than the mean z ∼ 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.

ALMA Observations of Lyα Blob 1: Multiple Major Mergers and Widely Distributed Interstellar Media

Abstract We present observations of a giant Lyα blob (LAB) in the SSA22 protocluster at z = 3.1, SSA22-LAB1, taken with the Atacama Large Millimeter/submillimeter Array. Dust continuum, along with [C ii] 158 μm and CO(4–3) line emission have been detected in LAB1, showing complex morphology and kinematics across a ∼100 kpc central region. Seven galaxies at z = 3.0987–3.1016 in the surroundings are identified in [C ii] and dust continuum emission, with two of them potential companions or tidal structures associated with the most massive galaxies. Spatially resolved [C ii] and infrared luminosity ratios for the widely distributed media (L [Cɪɪ]/L IR ≈ 10−2−10−3) suggest that the observed extended interstellar media are likely to have originated from star formation activity and the contribution from shocked gas is probably not dominant. LAB1 is found to harbor a total molecular gas mass M mol = (8.7 ± 2.0) × 1010 M ⊙, concentrated in the core region of the Lyα-emitting area. While (primarily obscured) star formation activity in the LAB1 core is one of the most plausible power sources for the Lyα emission, multiple major mergers found in the core may also play a role in making LAB1 exceptionally bright and extended in Lyα as a result of cooling radiation induced by gravitational interactions.

Revisiting the gas kinematics in SSA22 Lyman-α Blob 1 with radiative transfer modelling in a multiphase, clumpy medium

ABSTRACT We present new observations of Lyman-α (Lyα) Blob 1 (LAB1) in the SSA22 protocluster region (z = 3.09) using the Keck Cosmic Web Imager and Keck Multi-object Spectrometer for Infrared Exploration. We have created a narrow-band Lyα image and identified several prominent features. By comparing the spatial distributions and intensities of Lyα and Hβ, we find that recombination of photo-ionized H i gas followed by resonant scattering is sufficient to explain all the observed Lyα/Hβ ratios. We further decode the spatially resolved Lyα profiles using both moment maps and radiative transfer modelling. By fitting a set of multiphase, ‘clumpy’ models to the observed Lyα profiles, we manage to reasonably constrain many parameters, namely the H i number density in the interclump medium (ICM), the cloud volume filling factor, the random velocity and outflow velocity of the clumps, the H i outflow velocity of the ICM, and the local systemic redshift. Our model has successfully reproduced the diverse Lyα morphologies, and the main results are: (1) the observed Lyα spectra require relatively few clumps per line of sight as they have significant fluxes at the line centre; (2) the velocity dispersion of the clumps yields a significant broadening of the spectra as observed; (3) the clump bulk outflow can also cause additional broadening if the H i in the ICM is optically thick; (4) and the H i in the ICM is responsible for the absorption feature close to the Lyα line centre.

Gas filaments of the cosmic web located around active galaxies in a protocluster.

Cosmological simulations predict that the Universe contains a network of intergalactic gas filaments, within which galaxies form and evolve. However, the faintness of any emission from these filaments has limited tests of this prediction. We report the detection of rest-frame ultraviolet Lyman-α radiation from multiple filaments extending more than one megaparsec between galaxies within the SSA22 protocluster at a redshift of 3.1. Intense star formation and supermassive black-hole activity is occurring within the galaxies embedded in these structures, which are the likely sources of the elevated ionizing radiation powering the observed Lyman-α emission. Our observations map the gas in filamentary structures of the type thought to fuel the growth of galaxies and black holes in massive protoclusters.

ALMA Deep Field in the SSA22 proto-cluster at z = 3

AbstractGalaxies and nuclei in dense environment at high redshift provide a good laboratory to investigate accelerated, most extreme evolution of galaxies. The SSA22 proto-cluster at z = 3.1 is known to have a three-dimensional 50 (comoving) Mpc-scale filamentary structure, traced by Lyα emitters, which makes the field a suitable target in this regard. To identify dust-obscured star-formation, a contiguous 20 arcmin2 region at the node of the cosmic structure was observed in ALMA band 6. In total 57 ALMA sources have been identified above 5σ, which makes the field one of the richest field in ALMA-identified (sub)millimeter galaxies. The follow-up spectroscopy confirmed about 20 sources as exact proto-cluster members so far. Together with high X-ray AGN fraction, our results suggest that the vigorous star formation activity and the growth of super massive black holes occurred simultaneously in the densest regions at z ∼ 3.

Understanding Large-scale Structure in the SSA22 Protocluster Region Using Cosmological Simulations∗

Abstract We investigate the nature and evolution of large-scale structure within the SSA22 protocluster region at z = 3.09 using cosmological simulations. A redshift histogram constructed from current spectroscopic observations of the SSA22 protocluster reveals two separate peaks at z = 3.065 (blue) and z = 3.095 (red). Based on these data, we report updated overdensity and mass calculations for the SSA22 protocluster. We find and for the blue and red peaks, respectively, and for the entire region. These overdensities correspond to masses of , , and for the red, blue, and total peaks, respectively. We use the Small MultiDark Planck (SMDPL) simulation to identify comparably massive protoclusters, and uncover the underlying structure and ultimate fate of the SSA22 protocluster. For this analysis, we construct mock redshift histograms for each simulated protocluster, quantitatively comparing them with the observed SSA22 data. We find that the observed double-peaked structure in the SSA22 redshift histogram corresponds not to a single coalescing cluster, but rather the proximity of a protocluster and at least one cluster progenitor. Such associations in the SMDPL simulation are easily understood within the framework of hierarchical clustering of dark matter halos. We finally find that the opportunity to observe such a phenomenon is incredibly rare, with an occurrence rate of .

Imaging of diffuse H i absorption structure in the SSA22 proto-cluster region at $z$ = 3.1

Using galaxies as background light sources to map intervening Lya absorption is a novel approach to study the interplay among galaxies, the circum-galactic medium (CGM), and the intergalactic medium (IGM). Introducing a new measure of z = 3.1$ HI Lya absorption relative to the cosmic mean, Delta_NB497, estimated from photometric data of star-forming galaxies at 3.3 < z < 3.5, we have made two-dimensional Delta_NB497 maps in the z = 3.1 SSA22 proto-cluster region and two control fields (SXDS and GOODS-N fields) with a spatial resolution of ~ 5 comoving Mpc. The Delta_NB497 measurements in the SSA22 field are systematically larger than those in the control fields, and this HI absorption enhancement extends more than 50 comoving Mpc. The field-averaged (i.e., ~50 comoving Mpc scale) Delta_NB497 and the overdensity of Lya emitters (LAEs) seem to be correlated, while there is no clear dependency of the Delta_NB497 on the local LAE overdensity in a few comoving Mpc scale. These results suggest that diffuse HI gas spreads out in/around the SSA22 proto-cluster. We have also found an enhancement of Delta_NB497 at a projected distance < 100 physical kpc from the nearest z = 3.1 galaxies at least in the SSA22 field, which is probably due to HI gas associated with the CGM of individual galaxies. The HI absorption enhancement in the CGM-scale tends to be weaker around galaxies with stronger Lya emission, which suggests that the Lya escape fraction from galaxies depends on hydrogen neutrality in the CGM.

SUBSTRUCTURE WITHIN THE SSA22 PROTOCLUSTER AT z ≈ 3.09*

ABSTRACT We present the results of a densely sampled spectroscopic survey of the SSA22 protocluster at z ≈ 3.09. Our sample with Keck/LRIS spectroscopy includes 106 Lyα emitters (LAEs) and 40 Lyman break galaxies (LBGs) at z = 3.05–3.12. These galaxies are contained within the 9′ × 9′ region in which the protocluster was discovered, which also hosts the maximum galaxy overdensity in the SSA22 region. The redshift histogram of our spectroscopic sample reveals two distinct peaks, at z = 3.069 (blue; 43 galaxies) and z = 3.095 (red; 103 galaxies). Furthermore, objects in the blue and red peaks are segregated on the sky, with galaxies in the blue peak concentrating toward the western half of the field. These results suggest that the blue and red redshift peaks represent two distinct structures in physical space. Although the double-peaked redshift histogram is traced in the same manner by LBGs and LAEs, and brighter and fainter galaxies, we find that 9 out of 10 X-ray AGNs in SSA22, and all 7 spectroscopically confirmed giant Lyα “blobs,” reside in the red peak. We combine our data set with sparsely sampled spectroscopy from the literature over a significantly wider area, finding preliminary evidence that the double-peaked structure in redshift space extends beyond the region of our dense spectroscopic sampling. In order to fully characterize the three-dimensional structure, dynamics, and evolution of large-scale structure in the SSA22 overdensity, we require the measurement of large samples of LAE and LBG redshifts over a significantly wider area, as well as detailed comparisons with cosmological simulations of massive cluster formation.

SPECTROPOLARIMETRY CONFIRMS CENTRAL POWERING IN A Lyα NEBULA AT z = 3.09

ABSTRACT We present a follow-up study to the imaging polarimetry performed by Hayes et al. on LAB1 in the SSA22 protocluster region. Arguably the most well-known Lyα “blob,” this radio-quiet emission-line nebula likely hosts a galaxy that either is undergoing significant star formation or hosts an active galactic nucleus, or both. We obtain deep, spatially resolved spectropolarimetry of the Lyα emission and detect integrated linear polarization of 9%–13% ± 2%–3% at a distance of approximately 15 kpc north and south of the peak of the Lyα surface brightness with polarization vectors lying tangential to the galactic central source. In these same regions, we also detect a wavelength dependence in the polarization that is low at the center of the Lyα line profile and rises substantially in the wings of the profile. These polarization signatures are easily explained by a weak outflowing shell model. The spectral dependence of the polarization presented here provides a framework for future observations and interpretations of the southern portion of LAB1 in that any model for this system must be able to reproduce this particular spectral dependence. However, questions still remain for the northernmost spur of LAB1. In this region we detect total linear polarization of between 3% and 20% at the 5% significance level. Simulations predict that polarization should increase with radius for a symmetric geometry. That the northern spur does not suggests either that this region is not symmetric (which is likely) and exhibits variations in columns density or that it is kinematically distinct from the rest of LAB1 and powered by another mechanism altogether.

ALMA DEEP FIELD IN SSA22: A CONCENTRATION OF DUSTY STARBURSTS IN A z = 3.09 PROTOCLUSTER CORE

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 $z\sim3$, 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.

An extremely dense group of massive galaxies at the centre of the protocluster atz= 3.09 in the SSA22 field

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 $M_{\rm dyn}\sim1.6\pm0.3\times10^{13}~M_{\odot}$. 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.

An enhanced merger fraction within the galaxy population of the SSA22 protocluster atz = 3.1

The over-dense environments of protoclusters of galaxies in the early Universe (z>2) are expected to accelerate the evolution of galaxies, with an increased rate of stellar mass assembly and black hole accretion compared to co-eval galaxies in the average density `field'. These galaxies are destined to form the passive population of massive systems that dominate the cores of rich clusters today. While signatures of accelerated growth of galaxies in the SSA22 protocluster (z=3.1) have been observed, the mechanism driving this remains unclear. Here we show an enhanced rate of galaxy-galaxy mergers could be responsible. We morphologically classify Lyman-break Galaxies (LBGs) in the SSA22 protocluster and compare these to those of galaxies in the field at z=3.1 as either active mergers or non-merging using Hubble Space Telescope ACS/F814W imaging, probing the rest-frame ultraviolet stellar light. We measure a merger fraction of 48+/-10 per cent for LBGs in the protocluster compared to 30+/-6 per cent for the field. Although the excess is marginal, an enhanced rate of mergers in SSA22 hints that galaxy-galaxy mergers are one of the key channels driving accelerated star formation and AGN growth in protocluster environments.

A DEEP NARROWBAND IMAGING SEARCH FOR C iv AND He ii EMISSION FROM LyαBLOBS

We conduct a deep narrow-band imaging survey of 13 Ly$\alpha$ blobs (LABs) located in the SSA22 proto-cluster at z~3.1 in the CIV and HeII emission lines in an effort to constrain the physical process powering the Ly$\alpha$ emission in LABs. Our observations probe down to unprecedented surface brightness limits of 2.1 $-$ 3.4 $\times$ 10$^{-18}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$ per 1 arcsec$^2$ aperture (5$\sigma$) for the HeII$\lambda$1640 and CIV$\lambda$1549 lines, respectively. We do not detect extended HeII and CIV emission in any of the LABs, placing strong upper limits on the HeII/Ly$\alpha$ and CIV/Ly$\alpha$ line ratios, of 0.11 and 0.16, for the brightest two LABs in the field. We conduct detailed photoionization modeling of the expected line ratios and find that, although our data constitute the deepest ever observations of these lines, they are still not deep enough to rule out a scenario where the Ly$\alpha$ emission is powered by the ionizing luminosity of an obscured AGN. Our models can accommodate HeII/Ly$\alpha$ and CIV/Ly$\alpha$ ratios as low as $\simeq$0.05 and $\simeq$0.07 respectively, implying that one needs to reach surface brightness as low as 1 $-$ 1.5 $\times$ 10$^{-18}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$ (at 5$\sigma$) in order to rule out a photoionization scenario. These depths will be achievable with the new generation of image-slicing integral field units such as VLT/MUSE or Keck/KCWI. We also model the expected HeII/Ly$\alpha$ and CIV/Ly$\alpha$ in a different scenario, where Ly$\alpha$ emission is powered by shocks generated in a large-scale superwind, but find that our observational constraints can only be met for shock velocities $v_{\rm s} \gtrsim$ 250 km s$^{-1}$, which appear to be in conflict with recent observations of quiescent kinematics in LABs.

NIR SPECTROSCOPIC OBSERVATION OF MASSIVE GALAXIES IN THE PROTOCLUSTER ATz= 3.09

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 KAB < 24 and confirmed redshifts of the 39 galaxies at 2.0 < zspec < 3.4. Of the 67 candidates, 24 are certainly protocluster members with 3.04 ⩽ zspec ⩽ 3.12, which are massive red galaxies that have been unidentified in previous optical observations of the SSA22 protocluster. Many distant red galaxies (J − KAB > 1.4), hyper extremely red objects (J − KAB > 2.1), Spitzer MIPS 24 μm sources, active galactic nuclei (AGNs) as well as the counterparts of Lyα blobs and the AzTEC/ASTE 1.1 mm sources in the SSA22 field are also found to be protocluster members. The mass of the SSA22 protocluster is estimated to be ∼2–5 × 1014 M☉, and this system is plausibly a progenitor of the most massive clusters of galaxies in the current universe. The reddest (J − KAB ⩾ 2.4) protocluster galaxies are massive galaxies with Mstar ∼ 1011 M☉ 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 iii] λ5007/Hβ emission line ratios and [O iii] λ5007 line widths and spatial extents of the protocluster galaxies from those of massive galaxies at z ∼ 2–3 in the general field.

THE FORMATION OF THE MASSIVE GALAXIES IN THE SSA22z= 3.1 PROTOCLUSTER

We study the properties of K-band-selected galaxies (KAB < 24) in the z = 3.09 SSA22 protocluster field. 430 galaxies at 2.6 < zphot < 3.6 are selected as potential protocluster members in a 112 arcmin2 area based on their photometric redshifts. We find that ≈20% of the massive galaxies with stellar masses >1011 M☉ at zphot ∼ 3.1 have colors consistent with those of quiescent galaxies with ages >0.5 Gyr. This fraction increases to ≈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 24 μm sources at zphot ∼ 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 (AGNs) is also high compared with the field. The sky distribution of the quiescent galaxies, the 24 μm sources, and the X-ray AGNs show clustering around a density peak of z = 3.1 Lyα emitters. A significant fraction of the massive galaxies have already become quiescent, while 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 in the centers of the predecessors to present-day rich galaxy clusters.

A Deep Narrow Band Imaging Search for CIVλ1548Å and HeIIλ1640Å Emission from Lyman Alpha Blobs

AbstractTo study the nature of Lyα blobs (aka LABs), we conduct a deep C IV and He II narrowband imaging survey of 13 Lyα blobs located in SSA22 proto-cluster at z ~ 3.1. We reach the unprecedented sensitivity, 5σ surface brightness limit of 2.1 − 3.4 × 10−18 erg s−1 cm−2 arcsec−2 per 1 arcsec2 aperture for two emission lines. We do not detect any extended C IV and He II emission, placing strong upper limits on the He II/Lyα and C IV/Lyα line ratios. We compare our limits with data in the literature related to the nebulae associated with high-redshift radio galaxies (HzRGs) and quasars, and we recover the data by modeling the LABs as nebulosities powered by a central QSO. For further information see Arrigoni Battaia et al. (2014).

Obscured star formation in Lyα blobs at z = 3.1

We present results from the AzTEC/ASTE 1.1-mm imaging survey of 35 Ly-alpha blobs (LABs) found in the SSA22 protocluster at z = 3.1. These 1.1-mm data reach an r.m.s. noise level of 0.7-1 mJy/beam, making this the largest millimetre-wave survey of LABs to date. No significant (> 3.5-sigma) emission is found in any of individual 35 LABs, and from this, we estimate 3-sigma upper limits on the far-infrared luminosity of L_FIR < 2e+12 Lsun. Stacking analysis reveals that the 1.1-mm flux density averaged over the LABs is S(1.1mm) < 0.40 mJy (3-sigma), which places a constraint of L_FIR < 4.5e+11 Lsun. This indicates that earlier 850-um measurements of the LABs may have overestimated their flux densities. Our results suggest that LABs on average have little ultra-luminous obscured star-formation, in contrast to a long-believed picture that LABs undergo an intense episode of dusty star-formation activities with star-formation rates of ~ 10^3 Msun/yr. Observations with ALMA are needed to directly study the obscured part of star-formation activity in the LABs.

The Subaru Lyα blob survey: a sample of 100-kpc Lyα blobs at z = 3

Abstract We present results of a survey for giant Lyα blobs (LABs) at z = 3 with Subaru/Suprime-Cam. We obtained Lyα imaging at z = 3.09 ± 0.03 around the SSA22 protocluster and in several blank fields. The total survey area is 2.1 deg2, corresponding to a comoving volume of 1.6 × 106 Mpc3. Using a uniform detection threshold of 1.4 × 10−18 erg s−1 cm−2 arcsec−2 for the Lyα images, we construct a sample of 14 LAB candidates with major-axis diameters larger than 100 kpc, including five previously known blobs and two known quasars. This survey triples the number of known LABs over 100 kpc. The giant LAB sample shows a possible ‘morphology—density relation’: filamentary LABs reside in average density environments as derived from compact Lyα emitters, while circular LABs reside in both average density and overdense environments. Although it is hard to examine the formation mechanisms of LABs only from the Lyα morphologies, more filamentary LABs may relate to cold gas accretion from the surrounding intergalactic medium (IGM) and more circular LABs may relate to large-scale gas outflows, which are driven by intense starbursts and/or by active galactic nucleus activities. Our survey highlights the potential usefulness of giant LABs to investigate the interactions between galaxies and the surrounding IGM from the field to overdense environments at high redshift.

SUBMILLIMETER ARRAY IDENTIFICATION OF THE MILLIMETER-SELECTED GALAXY SSA22-AzTEC1: A PROTOQUASAR IN A PROTOCLUSTER?

We present results from Submillimeter Array (SMA) 860-micron sub-arcsec astrometry and multiwavelength observations of the brightest millimeter (S_1.1mm = 8.4 mJy) source, SSA22-AzTEC1, found near the core of the SSA22 protocluster that is traced by Ly\alpha emitting galaxies at z = 3.09. We identify a 860-micron counterpart with a flux density of S_860um = 12.2 +/- 2.3 mJy and absolute positional accuracy that is better than 0.3". At the SMA position, we find radio to mid-infrared counterparts, whilst no object is found in Subaru optical and near-infrared deep images at wavelengths \le 1 micron (J > 25.4 in AB, 2\sigma). The photometric redshift estimate, using flux densities at \ge 24 microns, indicates z_phot = 3.19^{+0.26}_{-0.35}, consistent with the protocluster redshift. We then model the near-to-mid-infrared spectral energy distribution (SED) of SSA22-AzTEC1, and find that the SED modeling requires a large extinction (A_V \approx 3.4 mag) of starlight from a stellar component with M_star ~ 10^{10.9} M_sun, assuming z = 3.1. Additionally, we find a significant X-ray counterpart with a very hard spectrum (Gamma_eff = -0.34 ^{+0.57}_{-0.61}), strongly suggesting that SSA22-AzTEC1 harbors a luminous AGN (L_X ~ 3*10^{44} ergs s^{-1}) behind a large hydrogen column (N_H ~ 10^{24} cm^{-2}). The AGN, however, is responsible for only ~10% of the bolometric luminosity of the host galaxy, and therefore the star-formation activity likely dominates the submillimeter emission. It is possible that SSA22-AzTEC1 is the first example of a protoquasar growing at the bottom of the gravitational potential underlying the SSA22 protocluster.