Tuesday.
1511.08804
The stellar mass assembly of galaxies in the Illustris simulation: growth by mergers and the spatial distribution of accreted stars
Rodriguez-Gomez, et al
Use Illustris sim to study the relative contributions of in situ SF and stellar accretion to the buildup of galaxies over an unprecedentedly wide range of masses (M*=1e9-12 Msun), galaxies types, environments, and assembly histories. Find that the 'two-phase' picture of galaxy formation predicted by some models is a good approximation only for the most massive galaxies in the sim -- namely, the stellar mass growth of galaxies below a few times 1e11 Msun is dominated by in situ SF at all redshifts, while galaxies above this mass at z<1 grow primarily by accretion of stars via mergers. The fraction of the total stellar mass of galaxies at z=0 contributed by accreted stars shows a strong dependence on galaxy stellar mass, ranging from about 10% for MW-sized galaxies to over 80% for M*~1e12 Msun objects, yet with a large galaxy-to-galaxy variation. At a fixed stellar mass, elliptical galaxies and those formed at the centers of younger haloes exhibit larger fractions of ex situ stars than disc-like galaxies and those formed in older haloes. On average, ~50% of the ex situ stellar mass comes from major mergers (stellar mass ratio mu > 1/4), ~20% from minor mergers (1/10<mu<1/4), ~20% from very minor mergers (mu<1/10), and ~10% from stars that were stripped from surviving galaxies (e.g. flybys or ongoing mergers). These components are spatially segregated, with in situ stars dominated the innermost regions of galaxies, and ex situ stars being deposited at larger galactocentric distances in order of decreasing merger mass ratio. The 'transition' radius where ex situ stars begin to dominated over the in situ class decreases fro more massive galaxies and correlated strongly with the total ex situ fraction.
1511.08809
Isolated elliptical galaxies in the local Universe
Lacerna, et al
Study a sample of 89 very isolated elliptical galaxies at z<0.08 and compared their properties with elliptical galaxies located in a high-density environment such as the Coma supercluster. The aim is to probe the role of environment on the morphological transformation and quenching of elliptical galaxies as a function of mass. In addition, elucidate about the nature of a particular set of blue and SF isolated ellipticals identified here. Study physical properties of ellipticals scubas color, sSFR, galaxy size and stellar age as a function of stellar mass and environment based on SDSS data. Analyze in more detail the blue SF isolated ellipticals through photometric characterization using GALFIT and infer their SFH using STARLIGHT. Among the isolated ellipticals ~20% are blue, 8% are star-forming and ~10% are recently quenched, while among the Coma ellipticals, ~8% are blue and just <=1% are star-forming or recently quenched. There are four isolated galaxies (~4.5%) that are blue and SF at the same time. These galaxies, with masses between 7e9 and 2e10 Msun/h^2, are also the youngest galaxies with light-weighted stellar ages <= 1Gyr and exhibit bluer colors toward the galaxy center. Around 30-60% of their present-day luminosity, but only <5% of their present-day mass, is due to SF in the last 1 Gyr. The processes of morphological transformation and quenching seem to be in general independent of environment since most of elliptical galaxies are "red and dead", although the transition to the red sequence should be faster for isolated ellipticals [why?]. In some cases, the isolated environment seems to propitiate the rejuvenation of ellipticals by recent (<1 Gyr) cold gas accretion.
1511.09008
Cosmic troublemakers: the Cold Spot, the Eridanus Supervoid, and the Great Walls
Kovács, García-Bellido
The alignment of the CMB Cold Spot and the Eridanus Supervoid suggests a physical connection between these two relatively rare objects. Use galaxy catalogues with photometric (2MPZ) and spectroscopic (6dF) redshift measurements, supplemented by low-redshift compilations of cosmic voids, in order to improve the 3D mapping of the matter density in the Eridanus constellation. Find evidence for a super void with an important elongation in the LoS, effectively spanning the total redshift range z<0.3. The tomographic imaging reveals significant substructure in the Eridanus Supervoid, with a potential interpretation of a long, fully connected system of voids. Improve the analysis by extending the LoS measurements into the antipodal direction, that interestingly crosses the Northern Local Supervoid at the lowest redshifts, and intersects very rich superclusters like Hercules and Corona Borealis, in the region of the Coma and Sloan Great Walls, as a possible compensation for the large-scale matter deficit of Eridanus. Model the matter density profiles with ellipsoidal super voids, and find that large-scale structure measurements are consistent with a central matter under density delta0~-0.25, with transverse radius r0\perp~55 Mpc/h and LoS radius r0\parallel~440 Mpc/h. Based on these findings, propose a potential explanation for the Cold Spot in the CMB, and for the hot ring feature around it, as a combination of a positive primordial fluctuation and an extremely cold ISW imprint.
1511.09132
Clustering properties and halo masses for central galaxies in the local Universe
Wang, Li, Jing
Investigate the clustering and DM halo mass for a sample of ~16k central galaxies selected from SDSS/DR7 group catalog. Select subsamples of central galaxies on three 2-D planes, each formed by stellar mass (M*) and one of the other properties including optical color (g-r), surface stellar mass density (mu*), and central stellar velocity dispersion (sigma*). For each subsample, measure both the projected cross-correlation function (wp(rp)) relative to a reference galaxy sample, and an average mass of the host DM haloes (Mhalo). For comparison also estimate the wp(rp) for the full galaxy population and the subset of satellite galaxies. Find that, for central galaxies, both wp(rp) and Mhalo show strongest dependence on M*, and there is no clear dependence on other properties when stellar mass is fixed. This result provides strong support to the previously-adopted assumption that, for central galaxies, stellar mass is the galaxy property that is best indicative of the host halo mass. The full galaxy population and the subset of satellites show similar clustering properties in all cases. However, they are similar to the centrals only at high masses (M*>1e11 Msun). At low-mass (M*<1e11 Msun), the results are different: the wp(rp) increases with both sigma and g-r when M* is fixed, and depends very weakly on M* when sigma* or g-r is fixed. At fixed Mstar, the mu* shows weak correlations with clustering amplitude (and halo mass for central galaxies). The results suggest that it is necessary to consider central and satellite galaxies separately when studying the link between galaxies and DM haloes.
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