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Register a new userA spectroscopic sample of massive, evolved z~2 galaxies: Implications for the evolution of the mass-size relation
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Added by
Jens-Kristian Krogager
Publication date
2013
fields
Physics
and research's language is
English
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We present deep, near-infrared HST/WFC3 grism spectroscopy and imaging for a sample of 14 galaxies at z~2 selected from a mass-complete photometric catalog in the COSMOS field. By combining the grism observations with photometry in 30 bands, we derive accurate constraints on their redshifts, stellar masses, ages, dust extinction and formation redshifts. We show that the slope and scatter of the z~2 mass-size relation of quiescent galaxies is consistent with the local relation, and confirm previous findings that the sizes for a given mass are smaller by a factor of two to three. Finally, we show that the observed evolution of the mass-size relation of quiescent galaxies between z=2 and 0 can be explained by quenching of increasingly larger star-forming galaxies, at a rate dictated by the increase in the number density of quiescent galaxies with decreasing redshift. However, we find that the scatter in the mass-size relation should increase in the quenching-driven scenario in contrast to what is seen in the data. This suggests that merging is not needed to explain the evolution of the median mass-size relation of massive galaxies, but may still be required to tighten its scatter, and explain the size growth of individual z=2 galaxies quiescent galaxies.
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For the first time, we study the evolution of the stellar mass-size relation for star-forming galaxies from z ~ 4 to z ~ 7 from Hubble-WFC3/IR camera observations of the HUDF and Early Release Science (ERS) field. The sizes are measured by determining the best fit model to galaxy images in the rest-frame 2100 AA with the stellar masses estimated from SED fitting to rest-frame optical (from Spitzer/IRAC) and UV fluxes. We show that the stellar mass-size relation of Lyman-break galaxies (LBGs) persists, at least to z ~ 5, and the median size of LBGs at a given stellar mass increases towards lower redshifts. For galaxies with stellar masses of 9.5<Log(M*/Msun)<10.4 sizes evolve as $(1+z)^{-1.20pm0.11}$. This evolution is very similar for galaxies with lower stellar masses of 8.6<Log(M*/Msun)<9.5 which is $r_{e} propto (1+z)^{-1.18pm0.10}$, in agreement with simple theoretical galaxy formation models at high z. Our results are consistent with previous measurements of the LBGs mass-size relation at lower redshifts (z ~ 1-3).
The stellar mass-halo mass relation is a key constraint in all semi-analytic, numerical, and semi-empirical models of galaxy formation and evolution. However, its exact shape and redshift dependence remain debated. Several recent works support a relation in the local Universe steeper than previously thought. Based on the comparisons with a variety of data on massive central galaxies, we show that this steepening holds up to z~1, for stellar masses Mstar>2e11 Msun. Specifically, we find significant evidence for a high-mass end slope of beta>0.35-0.70, instead of the usual beta~0.20-0.30 reported by a number of previous results. When including the independent constraints from the recent BOSS clustering measurements, the data, independent of any systematic errors in stellar masses, tend to favor a model with a very small scatter (< 0.15 dex) in stellar mass at fixed halo mass, in the redshift range z < 0.8 and for Mstar>3e11 Msun, suggesting a close connection between massive galaxies and host halos even at relatively recent epochs. We discuss the implications of our results with respect to the evolution of the most massive galaxies since z~1.
Once understood as the paradigm of passively evolving objects, the discovery that massive galaxies experienced an enormous structural evolution in the last ten billion years has opened an active line of research. The most significant pending question in this field is the following: which mechanism has made galaxies to grow largely in size without altering their stellar populations properties dramatically? The most viable explanation is that massive galaxies have undergone a significant number of minor mergers which have deposited most of their material in the outer regions of the massive galaxies. This scenario, although appealing, is still far from be observationally proved since the number of satellite galaxies surrounding the massive objects appears insufficient at all redshifts. The presence also of a population of nearby massive compact galaxies with mixture stellar properties is another piece of the puzzle that still does not nicely fit within a comprehensive scheme. I will review these and other intriguing properties of the massive galaxies in this contribution.
Using the combined capabilities of the large near-infrared Palomar/DEEP-2 survey, and the superb resolution of the ACS HST camera, we explore the size evolution of 831 very massive galaxies (M*>10^{11}h_{70}^{-2}M_sun) since z~2. We split our sample according to their light concentration using the Sersic index n. At a given stellar mass, both low (n<2.5) and high (n>2.5) concentrated objects were much smaller in the past than their local massive counterparts. This evolution is particularly strong for the highly concentrated (spheroid-like) objects. At z~1.5, massive spheroid-like objects were a factor of 4(+-0.4) smaller (i.e. almost two orders of magnitudes denser) than those we see today. These small sized, high mass galaxies do not exist in the nearby Universe, suggesting that this population merged with other galaxies over several billion years to form the largest galaxies we see today.
Using deep infrared observations conducted with the MOIRCS on the Subaru Telescope in GOODS-N combined with public surveys in GOODS-S, we investigate the dependence on stellar mass, M_*, and galaxy type of the close pair fraction (5 kpc < r < 20 kpc) and implied merger rate. In common with some recent studies we find that the fraction of paired systems that could result in major mergers is low (~4%) and does not increase significantly with redshift to z~1.2, with (1+z)^{1.6 pm 1.6}. Our key finding is that massive galaxies with M_* > 1E11 Msun are more likely to host merging companions than less massive systems (M_* ~ 1E10 Msun). We find evidence for a higher pair fraction for red, spheroidal hosts compared to blue, late-type systems, in line with expectations based on clustering at small scales. So-called dry mergers between early-type galaxies represent nearly 50% of close pairs with M_* > 3E10 Msun at z~0.5, but less than 30% at z~1. This result can be explained by the increasing abundance of red, early-type galaxies at these masses. We compare the volumetric merger rate of galaxies with different masses to mass-dependent trends in galaxy evolution, finding that major mergers cannot fully account for the formation of spheroidal galaxies since z~1. In terms of mass assembly, major mergers contribute little to galaxy growth below M_* ~ 3E10 Msun but are more significant among galaxies with M_* > 1E11 Msun, 30% of which have undergone mostly dry mergers over the observed redshift range. Overall, the relatively more rapid coalescence of high mass galaxies mirrors the expected hierarchical growth of halos and is consistent with recent model predictions, even if the downsizing of star formation and morphological evolution involves additional physical processes.
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