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Register a new userHidden in plain sight: a massive, dusty starburst in a galaxy protocluster at z=5.7 in the COSMOS field
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We report the serendipitous discovery of a dusty, starbursting galaxy at $z=5.667$ (hereafter called CRLE) in close physical association with the normal main-sequence galaxy HZ10 at $z=5.654$. CRLE was identified by detection of [CII], [NII] and CO(2-1) line emission, making it the highest redshift, most luminous starburst in the COSMOS field. This massive, dusty galaxy appears to be forming stars at a rate of at least 1500$,M_odot$ yr$^{-1}$ in a compact region only $sim3$ kpc in diameter. The dynamical and dust emission properties of CRLE suggest an ongoing merger driving the starburst, in a potentially intermediate stage relative to other known dusty galaxies at the same epoch. The ratio of [CII] to [NII] may suggest that an important ($sim15%$) contribution to the [CII] emission comes from a diffuse ionized gas component, which could be more extended than the dense, starbursting gas. CRLE appears to be located in a significant galaxy overdensity at the same redshift, potentially associated with a large-scale cosmic structure recently identified in a Lyman Alpha Emitter survey. This overdensity suggests that CRLE and HZ10 reside in a protocluster environment, offering the tantalizing opportunity to study the effect of a massive starburst on protocluster star formation. Our findings support the interpretation that a significant fraction of the earliest galaxy formation may occur from the inside out, within the central regions of the most massive halos, while rapidly evolving into the massive galaxy clusters observed in the local Universe.
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The Planck satellite has identified more than 2000 protocluster candidates with extreme star formation rates (SFRs). Here, we present the spectroscopic identification of a Planck-selected protocluster located in the Cosmos field, PHz G237.01+42.50 (G237). G237 contains a galaxy overdensity of 31 spectroscopically identified galaxies at z~2.16 (significant at 5.4 sigma) in a 10x11 region. The overdensity contains two substructures or protoclusters at <z>~2.16 and 2.195 with estimated halo masses at z=0 of ~(5-6)x10^14 Msun. The overdensity total SFR, ~4000 Msun/yr, is higher than predicted by simulations but much smaller than the SFR derived from the Planck data. The analysis of the Herschel data, in combination with the available ancillary data, shows that such a difference is due to an effect of source alignment along the line of sight that produces a 5 sigma overdensity of red Herschel sources in the field. We analyze the members UV spectra and UV-far-infrared spectral energy distributions to derive their SFR, stellar mass, and metallicity. Galaxy members include blue star-forming galaxies and AGN with SFRs and stellar masses consistent with the main sequence. AGN, identified through optical spectroscopy or X-ray data, represent a significant fraction (20+/-10%) of all members of the protocluster at z=2.16, and they are powerful enough to produce radiative feedback. The core of this protocluster, besides being denser, includes members that are, on average, more massive and star-forming and contains a larger fraction of AGN and Herschel-detected galaxies than the full sample, suggesting an environmental effect on galaxy growth. A comparison between G237 and other protoclusters in the literature at similar redshifts reveals some common traits and differences that reflect both observational biases and a diversity in intrinsic properties that is not yet fully understood.
We report a massive quiescent galaxy at $z_{rm spec}=3.0922^{+0.008}_{-0.004}$ spectroscopically confirmed at a protocluster in the SSA22 field by detecting the Balmer and Ca {footnotesize II} absorption features with multi-object spectrometer for infrared exploration (MOSFIRE) on the Keck I telescope. This is the most distant quiescent galaxy confirmed in a protocluster to date. We fit the optical to mid-infrared photometry and spectrum simultaneously with spectral energy distribution (SED) models of parametric and nonparametric star formation histories (SFH). Both models fit the observed SED well and confirm that this object is a massive quiescent galaxy with the stellar mass of $log(rm M_{star}/M_{odot}) = 11.26^{+0.03}_{-0.04}$ and $11.54^{+0.03}_{-0.00}$, and star formation rate of $rm SFR/M_{odot}~yr^{-1} <0.3$ and $=0.01^{+0.03}_{-0.01}$ for parametric and nonparametric models, respectively. The SFH from the former modeling is described as an instantaneous starburst while that of the latter modeling is longer-lived but both models agree with a sudden quenching of the star formation at $sim0.6$ Gyr ago. This massive quiescent galaxy is confirmed in an extremely dense group of galaxies predicted as a progenitor of a brightest cluster galaxy formed via multiple mergers in cosmological numerical simulations. We newly find three plausible [O III]$lambda$5007 emitters at $3.0791leq z_{rm spec}leq3.0833$ happened to be detected around the target. Two of them just between the target and its nearest massive galaxy are possible evidence of their interactions. They suggest the future strong size and stellar mass evolution of this massive quiescent galaxy via mergers.
We present Chandra ACIS-S and ATCA radio continuum observations of the strongly lensed dusty, star-forming galaxy SPT-S J034640-5204.9 (hereafter SPT0346-52) at $z$ = 5.656. This galaxy has also been observed with ALMA, HST, Spitzer, Herschel, APEX, and the VLT. Previous observations indicate that if the infrared (IR) emission is driven by star formation, then the inferred lensing-corrected star formation rate ($sim$ 4500 $M_{sun}$ yr$^{-1}$) and star formation rate surface density $Sigma_{rm SFR}$ ($sim$ 2000 $M_{sun} {yr^{-1}} {kpc^{-2}}$) are both exceptionally high. It remained unclear from the previous data, however, whether a central active galactic nucleus (AGN) contributes appreciably to the IR luminosity. The {it Chandra} upper limit shows that SPT0346-52 is consistent with being star-formation dominated in the X-ray, and any AGN contribution to the IR emission is negligible. The ATCA radio continuum upper limits are also consistent with the FIR-to-radio correlation for star-forming galaxies with no indication of an additional AGN contribution. The observed prodigious intrinsic IR luminosity of (3.6 $pm$ 0.3) $times$ 10$^{13}$ $L_{sun}$ originates almost solely from vigorous star formation activity. With an intrinsic source size of 0.61 $pm$ 0.03 kpc, SPT0346-52 is confirmed to have one of the highest $Sigma_{SFR}$ of any known galaxy. This high $Sigma_{SFR}$, which approaches the Eddington limit for a radiation pressure supported starburst, may be explained by a combination of very high star formation efficiency and gas fraction.
We present optical and near infrared observations of GRB 080325 classified as a Dark GRB. Near-infrared observations with Subaru/MOIRCS provided a clear detection of afterglow in Ks band, although no optical counterpart was reported. The flux ratio of rest-wavelength optical to X-ray bands of the afterglow indicates that the dust extinction along the line of sight to the afterglow is Av = 2.7 - 10 mag. This large extinction is probably the major reason for optical faintness of GRB 080325. The J - Ks color of the host galaxy, (J - Ks = 1.3 in AB magnitude), is significantly redder than those for typical GRB hosts previously identified. In addition to J and Ks bands, optical images in B, Rc, i, and z bands with Subaru/Suprime-Cam were obtained at about one year after the burst, and a photometric redshift of the host is estimated to be z_{photo} = 1.9. The host luminosity is comparable to L^{*} at z sim 2 in contrast to the sub-L^{*} property of typical GRB hosts at lower redshifts. The best-fit stellar population synthesis model for the host shows that a large dust extinction (Av = 0.8 mag) attributes to the red nature of the host and that the host galaxy is massive (M_{*} = 7.0 times 10^{10} Msun) which is one of the most massive GRB hosts previously identified. By assuming that the mass-metallicity relation for star-forming galaxies at z sim 2 is applicable for the GRB host, this large stellar mass suggests the high metallicity environment around GRB 080325, consistent with inferred large extinction.
SPT0346-52 is one of the most most luminous and intensely star-forming galaxies in the universe, with L_FIR > 10^13 L_sol and Sigma_SFR ~ 4200 M_sol yr^-1 kpc^-2. In this paper, we present ~0.15 ALMA observations of the [CII]158micron emission line in this z=5.7 dusty star-forming galaxy. We use a pixellated lensing reconstruction code to spatially and kinematically resolve the source-plane [CII] and rest-frame 158 micron dust continuum structure at ~700 pc (~0.12) resolution. We discuss the [CII] deficit with a pixellated study of the L_[CII]/L_FIR ratio in the source plane. We find that individual pixels within the galaxy follow the same trend found using unresolved observations of other galaxies, indicating that the deficit arises on scales <700 pc. The lensing reconstruction reveals two spatially and kinematically separated components (~1 kpc and ~500 km s^-1 apart) connected by a bridge of gas. Both components are found to be globally unstable, with Toomre Q instability parameters << 1 everywhere. We argue that SPT0346-52 is undergoing a major merger, which is likely driving the intense and compact star formation.
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