Day 1138

Friday.

1608.05086
Testing galaxy formation models with galaxy stellar mass functions
Lim, Mo, Lan, Ménard

Compare predictions of a number of empirical models and numerical sims of galaxy formation to the conditional stellar mass functions (CSMF) of galaxies in groups of different masses obtained recently by Lan+ to test how well different models accommodate the data.  Among all the models considered, only the model of Lu+ can match the observational data; all other models fail to reproduce the faint-end upturn seen in the observations.  The CSMFs are used to update the halo-based empirical model of Lu+, and the model parameters obtained are very similar to those inferred by Lu+ from a completely different set of observational constraints.  The observational data clearly prefer a model in which SF in low-mass haloes changes behavior at a characteristic redshift z_c~2.  There is also tentative evidence that this characteristic z depends on environments, becoming z_c~4 in regions that eventually evolve into rich clusters of galaxies.  The constrained model is used to understand how galaxies form and evolve in DM haloes, and to make predictions for other statistical properties of the galaxy population, such as the stellar mass functions of galaxies at high z, the SF and stellar mass assembly histories in DM haloes.  A comparison of the model predictions with those of other empirical models shows that different models can make vastly different predictions, even though all of them are tuned to match the observed stellar mass functions of galaxies.


1608.05191
The spherical Brazil Nut Effect and its significance to asteroids
Perera et al

Many asteroids are likely rubble-piles that are a collection of smaller objects held together by gravity and possibly cohesion.  These asteroids are seismically shaken by impacts, which leads to excitation of their constituent particles.  As a result, it has been suggested that their surfaces and sub-surface interiors may be governed by a size sorting mechanism known as the Brazil Nut Effect.  Study the behavior of a model asteroid that is a spherical, self-gravitating aggregate with a binary size-distribution of particles under the action of applied seismic shaking.  Find that above a seismic threshold, larger particles rise to the surface when friction is present, in agreement with previous studies that focussed on a cylindrical and rectangular box configurations.  Unlike previous works, also find that size sorting takes place even with zero friction, though the presence of friction does aid the sorting process above the seismic threshold.  Additionally find that while strong size sorting can take place near the surface, the innermost regions remain unsorted under even the most vigorous shaking.


1608.05284
The interaction of relativistic spacecrafts with the interstellar medium
Hoang, Lazarian, Burkhart, Loeb

For a spacecraft speed nu=0.2c [speed capable of reaching the nearest star system alpha Centauri in about 20 years], find that dust bombardment can erode a surface layer of ~0.5 mm thickness after the spacecraft has swept a column density of N_H~3e17 cm-2, assuming the standard gas-to-dust ratio of the ISM.  Dust bombardment also damages the spacecraft surface by modifying the material structure through melting.  Calculate the equilibrium surface temperature due to collisional heating by gas atoms as well as the temperature profile as a function of depth into the spacecraft.  The quantitative result suggest methods for damage control, and highlight possibilities for shielding strategies and protection of the spacecraft.