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Register a new userCandidate Massive Galaxies at $z sim 4$ in the Dark Energy Survey
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Added by
Pierandrea Guarnieri
Publication date
2018
fields
Physics
and research's language is
English
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Using stellar population models, we predicted that the Dark Energy Survey (DES) - due to its special combination of area (5000 deg. sq.) and depth ($i = 24.3$) - would be in the position to detect massive ($gtrsim 10^{11}$ M$_{odot}$) galaxies at $z sim 4$. We confront those theoretical calculations with the first $sim 150$ deg. sq. of DES data reaching nominal depth. From a catalogue containing $sim 5$ million sources, $sim26000$ were found to have observed-frame $g-r$ vs $r-i$ colours within the locus predicted for $z sim 4$ massive galaxies. We further removed contamination by stars and artefacts, obtaining 606 galaxies lining up by the model selection box. We obtained their photometric redshifts and physical properties by fitting model templates spanning a wide range of star formation histories, reddening and redshift. Key to constrain the models is the addition, to the optical DES bands $g$, $r$, $i$, $z$, and $Y$, of near-IR $J$, $H$, $K_{s}$ data from the Vista Hemisphere Survey. We further applied several quality cuts to the fitting results, including goodness of fit and a unimodal redshift probability distribution. We finally select 233 candidates whose photometric redshift probability distribution function peaks around $zsim4$, have high stellar masses ($log($M$^{*}$/M$_{odot})sim 11.7$ for a Salpeter IMF) and ages around 0.1 Gyr, i.e. formation redshift around 5. These properties match those of the progenitors of the most massive galaxies in the local universe. This is an ideal sample for spectroscopic follow-up to select the fraction of galaxies which is truly at high redshift. These initial results and those at the survey completion, which we shall push to higher redshifts, will set unprecedented constraints on galaxy formation, evolution, and the re-ionisation epoch.
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Using analytic modeling and simulations, we address the origin of an abundance of star-forming, clumpy, extended gas rings about massive central bodies in massive galaxies at $z !<! 4$. Rings form by high-angular-momentum streams and survive in galaxies of $M_{rm star} !>! 10^{9.5-10} M_odot$ where merger-driven spin flips and supernova feedback are ineffective. The rings survive after events of compaction to central nuggets. Ring longevity was unexpected based on inward mass transport driven by torques from violent disc instability. However, evaluating the torques from a tightly wound spiral structure, we find that the timescale for transport per orbital time is long and $propto! delta_{rm d}^{-3}$, with $delta_{rm d}$ the cold-to-total mass ratio interior to the ring. A long-lived ring forms when the ring transport is slower than its replenishment by accretion and the interior depletion by SFR, both valid for $delta_{rm d} !<! 0.3$. The central mass that lowers $delta_{rm d}$ is a compaction-driven bulge and/or dark matter, aided by the lower gas fraction at $z !<! 4$, provided that it is not too low. The ring is Toomre unstable for clump and star formation. The high-$z$ dynamic rings are not likely to arise form secular resonances or collisions. AGN feedback is not expected to affect the rings. Mock images of simulated rings through dust indicate qualitative consistency with observed rings about bulges in massive $z!sim!0.5!-!3$ galaxies, in $H_{alpha}$ and deep HST imaging. ALMA mock images indicate that $z!sim!0.5!-!1$ rings should be detectable. We quote expected observable properties of rings and their central nuggets.
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We introduce the ALMA Redshift 4 Survey (AR4S), a systematic ALMA survey of all the known galaxies with stellar mass (M*) larger than 5e10 Msun at 3.5<z<5 in the GOODS--south, UDS and COSMOS CANDELS fields. The sample we have analyzed in this paper is composed of 96 galaxies observed with ALMA at 890um (180um rest-frame) with an on-source integration time of 1.3 min per galaxy. We detected 32% of the sample at more than 3 sigma significance. Using the stacked ALMA and Herschel photometry, we derived an average dust temperature of 40+/-2 K for the whole sample, and extrapolate the Lir and SFR for all our galaxies based on their ALMA flux. We then used a forward modeling approach to estimate their intrinsic sSFR distribution, deconvolved of measurement errors and selection effects: we find a linear relation between SFR and M*, with a median sSFR=2.8+/-0.8 Gyr and a dispersion around that relation of 0.28+/-0.13 dex. This latter value is consistent with that measured at lower redshifts, which is proof that the main sequence of star-forming galaxies was already in place at z=4, at least among massive galaxies. These new constraints on the properties of the main sequence are in good agreement with the latest predictions from numerical simulations, and suggest that the bulk of star formation in galaxies is driven by the same mechanism from z=4 to the present day, that is, over at least 90% of the cosmic history. We also discuss the consequences of our results on the population of early quiescent galaxies. This paper is part of a series that will employ these new ALMA observations to explore the star formation and dust properties of the massive end of the z=4 galaxy population.
We present a study of the infrared properties for a sample of seven spectroscopically confirmed submillimeter galaxies at $z>$4.0. By combining ground-based near-infrared, Spitzer IRAC and MIPS, Herschel SPIRE, and ground-based submillimeter/millimeter photometry, we construct their Spectral Energy Distributions (SED) and a composite model to fit the SEDs. The model includes a stellar emission component at $lambda_{rm rest} <$ 3.5$ mu$m; a hot dust component peaking at $lambda_{rest} sim$ 5$,mu$m; and cold dust component which becomes significant for $lambda_{rm rest} >$ 50$,mu$m. Six objects in the sample are detected at 250 and 350$ mu$m. The dust temperatures for the sources in this sample are in the range of 40$-$80 K, and their $L_{rm FIR}$ $sim$ 10$^{13}$ L$_{odot}$ qualifies them as Hyper$-$Luminous Infrared Galaxies (HyperLIRGs). The mean FIR-radio index for this sample is around $< q > = 2.2$ indicating no radio excess in their radio emission. Most sources in the sample have 24$ mu$m detections corresponding to a rest-frame 4.5$ mu$m luminosity of Log$_{10}$(L$_{4.5}$ / L$_{odot}$) = 11 $sim$ 11.5. Their L$_{rm 4.5}$/$L_{rm FIR}$ ratios are very similar to those of starburst dominated submillimeter galaxies at $z sim$ 2. The $L_{rm CO}-L_{rm FIR}$ relation for this sample is consistent with that determined for local ULIRGs and SMGs at $z sim$ 2. We conclude that submillimeter galaxies at $z >$ 4 are hotter and more luminous in the FIR, but otherwise very similar to those at $z sim$ 2. None of these sources show any sign of the strong QSO phase being triggered.
We report the likely identification of a substantial population of massive M~10^11M_Sun galaxies at z~4 with suppressed star formation rates (SFRs), selected on rest-frame optical to near-IR colors from the FourStar Galaxy Evolution Survey. The observed spectral energy distributions show pronounced breaks, sampled by a set of near-IR medium-bandwidth filters, resulting in tightly constrained photometric redshifts. Fitting stellar population models suggests large Balmer/4000AA breaks, relatively old stellar populations, large stellar masses and low SFRs, with a median specific SFR of 2.9+/-1.8 x 10^-11/yr. Ultradeep Herschel/PACS 100micron, 160micron and Spitzer/MIPS 24micron data reveal no dust-obscured SFR activity for 15/19 (79%) galaxies. Two far-IR detected galaxies are obscured QSOs. Stacking the far-IR undetected galaxies yields no detection, consistent with the SED fit, indicating independently that the average specific SFR is at least 10x smaller than of typical star-forming galaxies at z~4. Assuming all far-IR undetected galaxies are indeed quiescent, the volume density is 1.8+/-0.7 x 10^-5Mpc^-3 to a limit of log10M/M_Sun>10.6, which is 10x and 80x lower than at z = 2 and z = 0.1. They comprise a remarkably high fraction (~35%) of z~4 massive galaxies, suggesting that suppression of star formation was efficient even at very high redshift. Given the average stellar age of 0.8Gyr and stellar mass of 0.8x10^11M_Sun, the galaxies likely started forming stars before z =5, with SFRs well in excess of 100M_Sun/yr, far exceeding that of similarly abundant UV-bright galaxies at z>4. This suggests that most of the star-formation in the progenitors of quiescent z~4 galaxies was obscured by dust.
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