Day 621

Thursday.

1404.0377
Galaxy and mass assembly (GAMA): the wavelength-ependent sizes and profiles and galaxies revealed by MegaMorph
Vulcani, et al

Investigate the relationship between color and structure within galaxies using a large, volume-limited sample of bright, low-z galaxies with optical to NIR imaging from GAMA survey.  Fit single-comonent, wavelength-dependent, elliptical Sersic models to all passbands simultaneously, using software developed by MegaMorph project.  Dividing sample by Sersic index and color, the recovered wavelength variations in effective radius R_e and index n reveal the internal structure, and hence formation history, of different types of galaxies.  All these trends depend on n; some have an addition dependence on galaxy color.  Late type galaxies (n_r<2.5) show a dramatic increase in Sersic index with wavelength.  This might be a result of their two-component (bulge-disk) nature, though stellar population gradients within each component and dust attenuation are likely to play a role.  All galaxies show a substantial decrease in R_e with wavelength.  This is strongest for early-types (n_r>2.5), even though they maintain constant n with wavelength, revealing that ellipticals are a superimposition of different stellar populations associated with multiple collapse and merging events.  Processes leading to structures with larger R_e must be associated with lower metallicity or younger stellar populations.  This appears to rule out the formation of young cores through dissipative gas accretion as an important mechanism on the recent lives of luminous elliptical galaxies.

1404.0402
Parameterising star formation histories
Simha, Weinberg, Conroy, Dave, … et al

Examine SFHs of galaxies in SPH simulations, compare them to parametric models that are commonly used in fitting observed galaxy SEDs, and examine the efficacy of these parametric models as practical tools for recovering the physical parameters of galaxies.  The commonly used tau-model, with SFR~exp(-t/tau), provides a poor match to the SFH of the SPH galaxies, with a mismatch between early and late SF that leads to systematic errors in predicting colors and stellar M/L ratios.  A one-parameter lin-exp model, SFR~t*exp(-t/tau), is much more successful on average, but it fails to match the late-time behavior of the bluest, most actively SF galaxies and thapssive, "read and dead" galaxies.  Introduce a 4-parameter model, which transitions from lin-exp to a linear ramp after a transition time, which describes the simulated galaxies well.  Test the ability to these parameterised models to recover (at z=0, 0.5, and 1) the stellar M/L ratios, sSFRs, and stellar population ages from the galaxy colors, computed from the full SPH SFHs using the FSPS code of Conroy+ 2009.  Fits with tau-models systematically overestimate M/L by ~0.2 dex, overestimate population ages by ~1-2 Gyr, and underestimate sSFR by ~0.05 dex.  Fits with lin-exp are less biased on average, but the 4-parameter model yeilds the best results for the full range of galaxies.  Marginalizing over the free parameters of the 4-paramter model leads to slightly larger statistical errors than 1-parameter fits but essentially removes all systematic biases, so this is the recommended procedure for fitting real galaxies.

1404.0641
Cosmological aspects of planetary habitability
Shchekinov, Safonova, Murthy

The HZ is defined as the region around a star where a planet can support liquid water on its surface, which, together with an oxygen atmosphere, is presumed to be necessary (and sufficient) to develop and sustain life on the planet.  Currently, about 20 potentially abatable planets are listed.  The most intriguing questions driving all these studies is whether planets within HZs host extraterrestrial life.  It is implicitly assumed that a planet in the HZ bears biota.  However along with the 2 usual indicators of habitability, and O atmosphere and liquid water on the surface, an additional one - the age - has to be taken into account when the question of the existence of life (or even a simple biota) on a planet it addressed.  The importance of planetary age for the existence of life as we know it follows from the fact that the primary process, the photosynthesis, is endothermic with an activation energy higher than temperatures in HZs.  Therefore on planets in HZs, due to variations in their albedo, orbits, diameters and other crucial parameters, the onset of photosynthesis may take much longer time than the planetary age.  Recently, several exoplanets orbiting Pop II stars with ages of 12-13 Gyr were discovered.  Even though these stars have low metallicity, they can still form protoplanetary clouds where the abundance of metals can be enhanced due to the action of physical fractionation.  Masses of protoplanets in such conditions can, in principle, be within Earth to super-Earth range.  These planets had enough time to develop necessary chains of chemical reactions and may thus carry life provided they are within the HZ.