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Register a new userA systematic search for very massive galaxies at z > 4
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Motivated by the claimed discovery of a very massive galaxy (HUDF-JD2; M~5x10^11 Msun) at extreme redshift (z = 6.5) within the Hubble Ultra Deep Field (HUDF) (Mobasher et al. 2005), we have completed a systematic search for comparably massive galaxies with z > 4 among the 2688 galaxies in our K < 23.5 (AB) catalogue within the CDFS/GOODS-South field. This search was conducted using redshift estimates based on the recently-completed, uniquely-deep 11-band imaging in this 125 square arcmin field, ~25 times larger than the NICMOS HUDF. Initial spectral fitting, based on published catalogue SExtractor photometry, led us to conclude that at least 2669 of the galaxies in our sample lie at z < 4. We carried out a detailed investigation of the 19 remaining z > 4 candidates, performing aperture photometry on all images, and including marginal detections and formal non-detections in the fitting process. This led to the rejection of a further 13 galaxies to lower redshift. Moreover, subjecting HUDF-JD2 to the same analysis, we find that it lies at z ~ 2.2, rather than the extreme redshift favoured by Mobasher et al. (2005). The 6 remaining candidates appear to be credible examples of galaxies in the redshift range z = 4 - 6, with plausible stellar ages. However, refitting with allowance for extreme values of extinction we find that, even for these objects, statistically acceptable solutions can be found at z < 3. Moreover, the recently-released Spitzer MIPS imaging in GOODS-South has revealed that 5 of our 6 final z > 4 candidates are detected at 24 microns. We conclude that there is no convincing evidence for any galaxy with M > 3 x 10^11 Msun, and z > 4 within the GOODS-South field (abridged).
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We present deep BVrI multicolor photometry in the field of the quasar BR1202-07 (z_{em}=4.694) aimed at selecting field galaxies at z>4. We compare the observed colors of the galaxies in the field with those predicted by spectral synthesis models including UV absorption by the intergalactic medium and we define a robust multicolor selection of galaxies at z>4. We provide spectroscopic confirmation of the high redshift QSO-companion galaxy (z=4.702) selected by our method. The first estimate of the surface density of galaxies in the redshift interval 4<z<4.5 is obtained for the same field, corresponding to a comoving volume density of ~ 10^{-3}$ Mpc$^{-3}. This provides a lower limit to the average star formation rate of the order of 10^{-2} Mo/yr/Mpc^{-3} at z~ 4.25.
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.
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.
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.
We present the analysis and results of a spectroscopic follow-up program of a mass-selected sample of six galaxies at 3 < z < 4 using data from Keck-NIRSPEC and VLT-Xshooter. We confirm the z > 3 redshifts for half of the sample through the detection of strong nebular emission lines, and improve the zphot accuracy for the remainder of the sample through the combination of photometry and spectra. The modeling of the emission-line-corrected spectral energy distributions (SEDs) adopting improved redshifts confirms the very large stellar masses of the sample (M_* ~ 1.5-4 x 10^11 Msun) in the first 2 Gyrs of cosmic history, with a diverse range in stellar ages, star formation rates and dust content. From the analysis of emission line luminosities and widths, and far-infrared (FIR) fluxes we confirm that >80% of the sample are hosts to luminous hidden active galactic nuclei (AGNs), with bolometric luminosities of ~10^(44-46) erg/s. We find that the MIPS 24um photometry is largely contaminated by AGN continuum, rendering the SFRs derived using only 24um photometry to be severely overestimated. By including the emission from the AGN in the modeling of the UV-to-FIR SEDs, we confirm that the presence of the AGN does not bias considerably the stellar masses (< 0.3 dex at 1sigma). We show evidence for a rapid increase of the AGN fraction from ~30% to ~60-100% over the 1 Gyr between z~2 and z~3. Although we cannot exclude some enhancement of the AGN fraction for our sample due to selection effects, the small measured [OIII] contamination to the observed K-band fluxes suggests that our sample is not significantly biased toward massive galaxies hosting AGNs.
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