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Register a new userA Study of Massive and Evolved Galaxies at High Redshift
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We use data taken as part of HST/WFC3 observations of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) to identify massive and evolved galaxies at 3<z<4.5. This is performed using the strength of the Balmer break feature at rest-frame 3648A, which is a diagnostic of the age of the stellar population in galaxies. Using WFC3 H-band selected catalog for the CANDELS GOODS-S field and deep multi-waveband photometry from optical (HST) to mid-infrared (Spitzer) wavelengths, we identify a population of old and evolved post-starburst galaxies based on the strength of their Balmer breaks (Balmer Break Galaxies- BBGs). The galaxies are also selected to be bright in rest-frame near-IR wavelengths and hence, massive. We identify a total of 16 BBGs. Fitting the spectral energy distribution (SED) of the BBGs show that the candidate galaxies have average estimated ages of ~800 Myr and average stellar masses of ~5x10^10 M_sun, consistent with being old and massive systems. Two of our BBG candidates are also identified by the criteria that is sensitive to star forming galaxies (LBG selection). We find a number density of ~3.2x10^-5 Mpc^-3 for the BBGs corresponding to a mass density of ~2.0x10^6 M_sun/Mpc^3 in the redshift range covering the survey. Given the old age and the passive evolution, it is argued that some of these objects formed the bulk of their mass only a few hundred million years after the Big Bang.
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We exploit the recent, wide samples of far-infrared (FIR) selected galaxies followed-up in X rays and of X-ray/optically selected active galactic nuclei (AGNs) followed-up in the FIR band, along with the classic data on AGN and stellar luminosity functions at high redshift z>1.5, to probe different stages in the coevolution of supermassive black holes (BHs) and host galaxies. The results of our analysis indicate the following scenario: (i) the star formation in the host galaxy proceeds within a heavily dust-enshrouded medium at an almost constant rate over a timescale ~0.5-1 Gyr, and then abruptly declines due to quasar feedback; over the same timescale, (ii) part of the interstellar medium loses angular momentum, reaches the circum-nuclear regions at a rate proportional to the star formation and is temporarily stored into a massive reservoir/proto-torus wherefrom it can be promptly accreted; (iii) the BH grows by accretion in a self-regulated regime with radiative power that can slightly exceed the Eddington limit L/L_Edd< 4, particularly at the highest redshifts; (iv) for massive BHs the ensuing energy feedback at its maximum exceeds the stellar one and removes the interstellar gas, thus stopping the star formation and the fueling of the reservoir; (v) afterwards, if the latter has retained enough gas, a phase of supply-limited accretion follows exponentially declining with a timescale of about 2 e-folding times. We show that the ratio of the FIR luminosity of the host galaxy to the bolometric luminosity of the AGN maps the various stages of the above sequence. Finally, we discuss how the detailed properties and the specific evolution of the reservoir can be investigated via coordinated, high-resolution observations of starforming, strongly-lensed galaxies in the (sub-)mm band with ALMA and in the X-ray band with Chandra and the next generation X-ray instruments.
Massive galaxy clusters are now found as early as 3 billion years after the Big Bang, containing stars that formed at even earlier epochs. The high-redshift progenitors of these galaxy clusters, termed protoclusters, are identified in cosmological simulations with the highest dark matter overdensities. While their observational signatures are less well defined compared to virialized clusters with a substantial hot intra-cluster medium (ICM), protoclusters are expected to contain extremely massive galaxies that can be observed as luminous starbursts. Recent claimed detections of protoclusters hosting such starbursts do not support the kind of rapid cluster core formation expected in simulations because these structures contain only a handful of starbursting galaxies spread throughout a broad structure, with poor evidence for eventual collapse into a protocluster. Here we report that the source SPT2349-56 consists of at least 14 gas-rich galaxies all lying at z = 4.31 based on sensitive observations of carbon monoxide and ionized carbon. We demonstrate that each of these galaxies is forming stars between 50 and 1000 times faster than our own Milky Way, and all are located within a projected region only $sim$ 130 kiloparsecs in diameter. This galaxy surface density is more than 10 times the average blank field value (integrated over all redshifts) and $>$1000 times the average field volume density. The velocity dispersion ($sim$ 410 km s$^{-1}$) of these galaxies and enormous gas and star formation densities suggest that this system represents a galaxy cluster core at an advanced stage of formation when the Universe was only 1.4 billion years old. A comparison with other known protoclusters at high redshifts shows that SPT2349-56 is a uniquely massive and dense system that could be building one of the most massive structures in the Universe today.
The central region of the Milky Way provides a unique laboratory for a systematic, spatially-resolved population study of evolved massive stars of various types in a relatively high metallicity environment. We have conducted a multi-wavelength data analysis of 180 such stars or candidates, most of which were drawn from a recent large-scale HST/NICMOS narrow-band Pa-a survey, plus additional 14 Wolf-Rayet stars identified in earlier ground-based spectroscopic observations of the same field. The multi-wavelength data include broad-band IR photometry measurements from HST/NICMOS, SIRIUS, 2MASS, Spitzer/IRAC, and Chandra X-ray observations. We correct for extinctions toward individual stars, improve the Pa-a line equivalent width measurements, quantify the substantial mid-IR dust emission associated with WC stars, and find X-ray counterparts. In the process, we identify 10 foreground sources, some of which may be nearby cataclysmic variables. The WN stars in the Arches and Central clusters show correlations between the Pa-a equivalent width and the adjacent continuum emission. However, the WN stars in the latter cluster are systematically dimmer than those in the Arches cluster, presumably due to the different ages of the two clusters. In the EW-magnitude plot, WNL stars, WC stars and OB supergiants roughly fall into three distinct regions. We estimate that the dust mass associated with individual WC stars in the Quintuplet cluster can reach 1e-5 M, or more than one order of magnitude larger than previous estimates. Thus WC stars could be a significant source of dust in the galaxies of the early universe. Nearly half of the evolved massive stars in the GC are located outside the three known massive stellar clusters. The ionization of several compact HII regions can be accounted for by their enclosed individual evolved massive stars, which thus likely formed in isolation or in small groups.
We investigate the properties of massive galaxies at z=1-3.5 using HST observations, ground-based near-IR imaging, and Spitzer Space Telescope observations at 3-24 micron. We identify 153 distant red galaxies (DRGs) with J-K > 2.3 mag (Vega) in the southern GOODS field. This sample is approximately complete in stellar mass for passively evolving galaxies above 10^11 solar masses and z < 3. The galaxies identified by this selection are roughly split between objects whose optical and near-IR rest-frame light is dominated by evolved stars combined with ongoing star formation, and galaxies whose light is dominated by heavily reddened starbursts. Very few of the galaxies (< 10%) have no indication of current star formation. Using SFR estimates that include the reradiated IR emission, the DRGs at z=1.5-3 with stellar masses > 10^11 solar masses have specific SFRs (SFRs per unit stellar mass) ranging from 0.2 to 10 Gyr^-1, with a mean value of ~2.4 Gyr^-1. The DRGs with stellar masses > 10^11 solar masses and 1.5 < z < 3 have integrated specific SFRs greater the global value over all galaxies. In contrast, we find that galaxies at z = 0.3-0.75 with these stellar masses have integrated specific SFRs less than the global value, and more than an order of magnitude lower than that for massive DRGs at z = 1.5-3. At z < 1, lower-mass galaxies dominate the overall cosmic mass assembly. This suggests that the bulk of star formation in massive galaxies occurs at early cosmic epochs and is largely complete by z~1.5. [Abridged]
We report results from a search for massive and evolved galaxies at z>5 in the GOODS southern field. Combining HST ACS, VLT ISAAC and Spitzer IRAC photometric data, we develop a color selection technique to identify candidates for being evolved galaxies at high redshifts. The color selection is primarily based on locating the Balmer-break using the K- and 3.6micron bands. Stellar population synthesis models are fitted to the SEDs of these galaxies to identify the final sample. We find 11 candidates with photometric redshifts in the range 4.9 < z < 6.5, dominated by an old stellar population, with ages 0.2-1.0 Gyr, and stellar masses in the range (0.5 - 5) 10^{11} Msun. The majority of the stars in these galaxies were formed at z > 9. One candidate has a spectroscopically confirmed redshift, in good agreement with our photometric redshift. The galaxies are very compact, with half-light radii in the observed K-band smaller than ~2 kpc. Seven of the 11 candidates are also detected at 24micron with the MIPS instrument on Spitzer. The 24micron emission could be interpreted as PAH emission from a dusty starburst at z~2-3, however, it is also consistent with the presence of an obscured AGN at z>5. We estimate the completeness of the Balmer break galaxy sample to be ~40%. The comoving number density of galaxies with a stellar mass >10^{11} Msun, at an average redshift z=5.2, is 3.9 10^{-5} Mpc^{-3} (no-MIPS sample: 1.4 10^{-5} Mpc^{-3}). The corresponding stellar mass density is 8 10^{6} Msun/Mpc^3 (no-MIPS sample: 6.2 10^6 Msun/Mpc^3).
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