1510.05645
Size evolution of normal and compact galaxies in the EAGLE simulation
Furlong, Bower, et al
Present the evolution of galaxy sizes, from redshift 2 to 0 , for actively SF and passive galaxies in the cosmo hydro 1003 cMpc3 simulation of the EAGLE project. Find that the sizes increase with stellar mass, but that the relation weakens with increasing redshift. Separating galaxies by their SF activity, find that passive galaxies are typically smaller than active galaxies at fixed stellar mass. These trends are consistent with those found in observations and the level of agreement between the predicted and observed size-mass relation is of order 0.1 dex for z<1 and 0.2-0.3 dex from redshift 1 to 2. Use the simulation to compare the evolution of individual galaxies to that of the population as a whole. While the evolution of the size-stellar mass relation for active galaxies provides a good proxy for the evolution of individual galaxies, the evolution of the individual passive galaxies is not well represented by the observed size-mass relation due to the evolving number density of passive galaxies. Observations of z~2 galaxies have revealed an abundance of massive red compact galaxies, that depletes below z~1. Find that a similar population forms naturally in the simulation. Comparing these galaxies to their z=0 descendants, find that all compact galaxies grow in size due to the high-redshift stars migrating outwards. Approximately 60% of the compact galaxies increase in size further due to renewed star formation and/or mergers.
1510.05650
How stellar feedback simultaneously regulates star formation and drives outflows
Hayward, Hopkins
Present an analytic model for how momentum deposition from stellar feedback simultaneously regulates SF and drives outflows in a turbulent ISM. Because the ISM is turbulent, a given patch of ISM exhibits sub-patches with a range of surface densities. The high-density patches are 'pushed' by feedback, thereby driving turbulence and self-regulating local SF. Sufficiently low-density patches, however, are accelerated to above the escape velocity before the region can self-adjust and are thus vented as outflows. In the turbulent-pressure-supported regime, when the gas fraction is >0.3, the ratio of the turbulent velocity dispersion to the circular velocity is sufficiently high that at any given time, of order half of the ISM has surface density less than the critical value and thus can be blown out on a dynamical time. The resulting outflows have a mass-loading factor (eta==Mout/M*) that is inversely proportional to the gas fraction times the circular velocity. At low gas fractions, the star formation rate needed to local self-regulation, and corresponding turbulent Mach number, decline rapidly; the ISM is 'smoother', and it is actually more difficult to drive winds with large mass-loading factors. Crucially, the model predicts that stellar-feedback-driven outflows should be suppressed at z<1 in M*>1e10 Msun galaxies. This mechanism allows massive galaxies to exhibit violent outflows at high redshifts and then 'shut down' those outflows at late times, thereby enabling the formation of a smooth, extended thin stellar disk. Provide simple fitting functions for eta that should be useful for sub-resolution and semi-analytic models.
1510.05651
The concentration dependence of the galaxy-halo connection
Lehmann, Mao, Becker, Skillman, Wechsler
Empirical methods for connecting galaxies to their DM haloes have become essential in interpreting measurements of the spatial statistics of galaxies. Among the most successful of these methods is the technique of sub halo abundance matching, which has to date been used to associated galaxy properties with a small set of halo properties. Generalize this set of halo properties to allow variable dependence on halo concentration, and parameterize the degree of concentration dependence with a single parameter. This parameter provides a smooth interpolation between abundance matching to peak halo mass and to peak halo circular velocity. Characterize the influence of this parameter on two-point clustering, the satellite fraction, and the degree of galaxy assembly bias. Also evaluate the degeneracies between the concentration dependence and the scatter in the abundance matching relation. Show that low redshift clustering measurements from SDSS prefer a moderate amount of concentration dependence --- more than would be indicated by matching galaxy luminosity to the peak halo mass, and less than would be indicated by matching to the peak halo circular velocity. Also show that these results are robust to moderate changes in cosmo params, and that the best-fit model from 2pt clustering agrees with previous measurements of the satellite fraction. Note that statistical constraints on these models have been (and still are, in most regimes) limited primarily by sample variance in the limited-size simulations, and not in the data. Discuss physical interpretations of these results and their implications for the galaxy-halo connection.
1510.05750
`Refsdal' meets Popper: comparing predictions of the re-apearance of the multiply imaged supernova behind MACS1149.5+2223
Treu et al
Supernova `Refsdal', multiply imaged by cluster MACS1149.5+2223, represents a rare opportunity to make a true blind test of model predictions in extragalactic astronomy, on a time scale that is short compared to a human lifetime In order to take advantage of this event, produce 7 GL models with 5 independent methods, based on HST Hubble Frontier Field images, along with extensive spectroscopic follow-up from HST and form the VLT. Compare the model predictions and shout that they agree reasonably well with the measured time delays and magnification ratios between the known images, even though these quantities were not used as input. This agreement is encouraging, considering that the models only provide statistical uncertainties, and do not include additional sources of uncertainties such as structure along the line of sight, cosmology, and the mass sheet degeneracy. Then present the model predictions for the other appearances of SN `Refsdal'. A further image will reach its peak in the first half of 2016, while another image appeared between 1994 and 2004. The past image would have been too faint to be detected in archival images. The future image should be approximately 1/3 as bright as the brightest known images and thus detectable in HST images, as soon as the cluster can be targeted again (beginning 2015 October 30). Soon to find out whether the predictions are correct.
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