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Register a new userExtremely Compact Massive Galaxies at 1.7<z<3
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Authors
Fernando Buitrago
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We measure and analyse the sizes of 82 massive (M >= 10^11 M_Sun) galaxies at 1.7<z<3 utilizing deep HST NICMOS data taken in the GOODS North and South fields. Our sample provides the first statistical study of massive galaxy sizes at z>2. We split our sample into disk-like (Sersic index n<=2) and spheroid-like (Sersic index n>2) galaxies, and find that at a given stellar mass, disk-like galaxies at z~2.3 are a factor of 2.6+/-0.3 smaller than present day equal mass systems, and spheroid-like galaxies at the same redshift are 4.3+/-0.7 times smaller than comparatively massive elliptical galaxies today. We furthermore show that the stellar mass densities of very massive galaxies at z~2.5 are similar to present-day globular clusters with values ~2x10^10 M_Sun kpc^-3
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We present the results of a pilot near-infrared (NIR) spectroscopic campaign of five very massive galaxies ($log(text{M}_star/text{M}_odot)>11.45$) in the range of $1.7<z<2.7$. We measure an absorption feature redshift for one galaxy at $z_text{spec}=2.000pm0.006$. For the remaining galaxies, we combine the photometry with the continuum from the spectra to estimate continuum redshifts and stellar population properties. We define a continuum redshift ($z_{rm cont}$ ) as one in which the redshift is estimated probabilistically using EAZY from the combination of catalog photometry and the observed spectrum. We derive the uncertainties on the stellar population synthesis properties using a Monte Carlo simulation and examine the correlations between the parameters with and without the use of the spectrum in the modeling of the spectral energy distributions (SEDs). The spectroscopic constraints confirm the extreme stellar masses of the galaxies in our sample. We find that three out of five galaxies are quiescent (star formation rate of $lesssim 1 M_odot~yr^{-1}$) with low levels of dust obscuration ($A_{rm V} < 1$) , that one galaxy displays both high levels of star formation and dust obscuration (${rm SFR} approx 300 M_odot~{rm yr}^{-1}$, $A_{rm V} approx 1.7$~mag), and that the remaining galaxy has properties that are intermediate between the quiescent and star-forming populations.
[Abridged] Using public data from the NMBS and CANDELS surveys, we study the population of massive galaxies at z>3 to identify the potential progenitors of z~2 compact, massive, quiescent (CMQ) galaxies, furthering our understanding of the evolution of massive galaxies. Our work is enabled by high-resolution CANDELS images and accurate photometric redshifts, stellar masses and star formation rates (SFRs) from 37-band NMBS photometry. The total number of z>3 massive galaxies is consistent with the number of massive quiescent (MQ) galaxies at z~2, implying that the SFRs for all of these galaxies must be much lower by z~2. We discover 4 CMQ galaxies at z>3, pushing back the time for which such galaxies have been observed. However, the volume density for these galaxies is significantly less than that of galaxies at z<2 with similar masses, SFRs, and sizes, implying that additional CMQ galaxies must be created in the ~1 Gyr between z=3 and z=2. We find 5 star-forming galaxies at z~3 that are compact (Re<1.4 kpc) and have stellar mass M*>10^(10.6)Msun, likely to become members of the CMQ galaxy population at z~2. We evolve the stellar masses and SFRs of each individual z>3 galaxy adopting 5 different star formation histories (SFHs) and studying the resulting population of massive galaxies at z=2.3. We find that declining or truncated SFHs are necessary to match the observed number density of MQ galaxies at z~2, whereas a constant SFH results in a number density significantly smaller than observed. All of our assumed SFHs imply number densities of CMQ galaxies at z~2 that are consistent with the observed number density. Better agreement with the observed number density of CMQ galaxies at z~2 is obtained if merging is included in the analysis and better still if star formation quenching is assumed to shortly follow the merging event, as implied by recent models of formation of MQ galaxies.
We introduce a new color-selection technique to identify high-redshift, massive galaxies that are systematically missed by Lyman-break selection. The new selection is based on the H_{160} and IRAC 4.5um bands, specifically H - [4.5] > 2.25 mag. These galaxies, dubbed HIEROs, include two major populations that can be separated with an additional J - H color. The populations are massive and dusty star-forming galaxies at z > 3 (JH-blue) and extremely dusty galaxies at z < 3 (JH-red). The 350 arcmin^2 of the GOODS-N and GOODS-S fields with the deepest HST/WFC3 and IRAC data contain 285 HIEROs down to [4.5] < 24 mag. We focus here primarily on JH-blue (z > 3) HIEROs, which have a median photometric redshift z ~4.4 and stellar massM_{*}~10^{10.6} Msun, and are much fainter in the rest-frame UV than similarly massive Lyman-break galaxies (LBGs). Their star formation rates (SFRs) reaches ~240 Msun yr^{-1} leading to a specific SFR, sSFR ~4.2 Gyr^{-1}, suggesting that the sSFRs for massive galaxies continue to grow at z > 2 but at a lower growth rate than from z=0 to z=2. With a median half-light radius of 2 kpc, including ~20% as compact as quiescent galaxies at similar redshifts, JH-blue HIEROs represent perfect star-forming progenitors of the most massive (M_{*} > 10^{11.2} Msun) compact quiescent galaxies at z ~ 3 and have the right number density. HIEROs make up ~60% of all galaxies with M_{*} > 10^{10.5} Msun identified at z > 3 from their photometric redshifts. This is five times more than LBGs with nearly no overlap between the two populations. While HIEROs make up 15-25% of the total SFR density at z ~ 4-5, they completely dominate the SFR density taking place in M_{*} >10^{10.5} Msun galaxies, and are therefore crucial to understanding the very early phase of massive galaxy formation.
Using a mass-selected ($M_{star} ge 10^{11} M_{odot}$) sample of 198 galaxies at 0 < z < 3.0 with HST/NICMOS $H_{160}$-band images from the COSMOS survey, we find evidence for the evolution of the pair fraction above z ~ 2, an epoch in which massive galaxies are believed to undergo significant structural and mass evolution. We observe that the pair fraction of massive galaxies is 0.15 pm 0.08 at 1.7 < z < 3.0, where galaxy pairs are defined as massive galaxies having a companion of flux ratio from 1:1 to 1:4 within a projected separation of 30 kpc. This is slightly lower, but still consistent with the pair fraction measured previously in other studies, and the merger fraction predicted in halo-occupation modelling. The redshift evolution of the pair fraction is described by a power law F(z) = (0.07 pm 0.04) * (1+z) ^ (0.6 pm 0.5). The merger rate is consistent with no redshift evolution, however it is difficult to constrain due to the limited sample size and the high uncertainties in the merging timescale. Based on the merger rate calculation, we estimate that a massive galaxy undergoes on average 1.1 pm 0.5 major merger from z = 3 to 0. The observed merger fraction is sufficient to explain the number density evolution of massive galaxies, but insufficient to explain the size evolution. This is a hint that mechanism(s) other than major merging may be required to increase the sizes of the massive, compact quiescent galaxies from z ~ 2 to 0.
I discuss constraints on star--formation and AGN activity in massive galaxies at z~1-3 using observations from the Spitzer Space Telescope. In particular I focus on a sample of distant red galaxies (DRGs) with J-K>2.3 in GOODS-S. Based on their ACS, ISAAC, and IRAC photometry, the DRGs have typical stellar masses >10^11 Msol. Interestingly, the majority (>50%) of these objects have 24 micron detections. If attributed to star formation, this implies SFRs of ~100-1000 Msol/yr. Thus, massive galaxies at z~1.5-3 have specific SFRs equal to or exceeding the global average value at that epoch. In contrast, galaxies with >10^11 Msol at z~0.3-0.75 have specific SFRs less than the global average, and more than an order of magnitude lower than that at z~1.5-3. Thus, the bulk of assembly of massive galaxies is largely complete by z~1.5. At the same time, based on the X-ray luminosities and near-IR colors, as many as 25% of the massive galaxies at z>1.5 host AGN, implying that the growth of supermassive black holes coincides with massive-galaxy assembly. The analysis of high-redshift galaxies depends on bolometric corrections between the observed 24 micron data and total IR luminosity. I review some of the sources of the (significant) uncertainties on these corrections, and discuss improvements for the future.
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