Day 644

Thursday.

1404.7504
The pre-processing of sub haloes in SDSS groups and clusters
Hou, Parker, Harris

Investigate pre-processing using the observed quenched fraction of group and cluster galaxies in DR7 group catalogue in 0.01<z<0.045.  Categorize group galaxies as virialized, infall or backsplash and apply a combination of the Dressler-Schectman statistic and group member velocities to identify sub haloes.  On average the fraction of galaxies that reside in sub haloes is a function of host halo mass, where more massive systems have a higher fraction of sub halo galaxies both in the overall galaxy and infall populations.  Additionally, find that between 2<r_200<3 the quiescent fraction is higher in the sub halo population wrt both the field and non-subhalo populations.  At these large radii (2<r200<3), the majority of galaxies (~80%) belong to the infall population and therefore, attribute the enhanced quenching to in falling sub halo galaxies, indicating that pre-processing has occurred in the sub halo population.  Conclude that pre-processing plays a significant role in the observed quiescent fraction, but only for the most massive (Mhalo>1e14.5 Msun) systems in the sample.

1404.7135
The effect of the environment on the stellar mass - size relation of present-day galaxies
Cebrián, Trujillo

To study how the environment can influence the relation between stellar mass and effective radius of nearby galaxies (z<0.12), use a mass-complete sample extracted from NYU-VAC.  This sample contains 232k objects with masses up to 3e11 Msun.  For every galaxy in the sample, explore the surrounding density within 2 Mpc using two different estimators of the environment.  Find that galaxies tend to be larger in the field than in high density regions.  This effect is more pronounced for late-type morphologies (~75% larger) and especially at low masses (<2e11 Msun), although early-type galaxies also show differences (~3.5%).  The environment also leaves a subtle imprint in the scatter of the stellar mass-size relation.  This scatter is larger in low density regions than in high density regions for both morphologies, on average ~3.5% larger for early-type and ~0.8% for late-type galaxies.  Late-type galaxies with low masses (<2e11 Msun) show the largest differences in the scatter among environments.  The scatter is ~20% larger in the field than in clusters for these objects.  Analysis suggest that galaxies in clusters form earlier than those in the field.  In addition, cluster galaxies seem to be originated from a more homogeneous family of progenitors.


1404.7505
On the spin-axis dynamics of a moonless Earth
Li, Btygin

The variation of a planet's obliquity is influenced by the existence of satellite with a high mass ratio.  For instance, the Earth's obliquity is stabilized by the Moon, and would undergo chaotic variations in the Moon's absence.  In turn, such variations can lead to large-scale changes in the atmospheric circulation, rendering spin-axis dynamics a central issue for understanding climate.  The relevant quantity for dynamically-forced climate change is the rate of chaotic diffusion.  Accordingly, reexamine the spin-axis evolution of a Moonless Earth within the context of a simplified perturbative framework.  Present analytical estimates of the characteristics Lyapunov coefficient as well as the chaotic diffusion rate and demonstrate that even in the absence of the Moon, the stochastic change in the Earth's obliquity is sufficiently slow to not preclude long-term habitability.  Calculations are consistent with published numerical experiments and illustrate the putative system's underlying dynamical structure in a simple and intuitive manner.