Day 679

Saturday.

1406.3343
Saturn ring seismology: evidence for stable stratification in the deep interior of Saturn
Fuller

Seismology allows for direct observational constraints on the interior structures of stars and planets.  Recent observations of Saturn's ring system have revealed the presence of density waves within the rings excited by oscillation modes within Saturn, allowing for precise measurements of a limited set of the planet's mode frequencies.  Construct interior structure models of Saturn, compute the corresponding mode frequencies, and compare them with the observed mode frequencies.  The fundamental mode frequencies of the models match the observed frequencies (of the largest amplitude waves) to an accuracy of ~1, confirming that these waves are indeed excited by Saturn's f-modes.  [What's an f-mode?]  The presence of the lower amplitude waves (finely split in frequency from the f-modes) can only be reproduced in models containing gravity modes that propagate in a stably stratified region of the planet.  The stable stratification must exist deep within the planet near the large density gradients between the core and envelope.  The models cannot easily reproduce the observed fine splitting of the m=-3 modes, suggesting that additional effects (e.g., significant latitudinal differential rotation) may be important.

1406.3344
Satellite dwarf galaxies in a hierarchical universe: the prevalence of dwarf-dwarf major mergers
Deason, Wetzel, Garrison-Kimmel

Mergers are a common phenomenon in hierarchical structure formation, especially for massive galaxies and clusters, but their importance for dwarf galaxies in the LG remains poorly understood.  Investigate the frequency of major mergers between dwarf galaxies in the LG using the ELVIS suit of cosmo zoom-in dissipation less simulations of the MW- and M31-like host haloes.  Find that ~10% of satellite dwarf galaxies with M_star>1e6 Msun that are within the host virial radius experienced a major merger of stellar mass ratio closer than 0.1 since z=1, with a lower fraction for lower mass dwarf galaxies.  Recent merger remnants are biased towards larger radial distance and more recent virial infall times, because most recent mergers occurred shortly before crossing within the virial radius of the host halo.  Satellite-satllite mergers also occur within the host halo after virial infall, catalyzed by the large fraction of dwarf galaxies that fell in as part of a group.  The merger fraction doubles for dwarf galaxies outside of the host virial radius, so the most distant dwarf galaxies in the LG are the most likely to have experienced a recent major merger.  Discuss the implications of these results on observable dwarf merger remnants, their star formation histories, the gas content of mergers, and massive BHs in dwarf galaxies.

1406.3362
Optically selected fossil groups; X-ray observations and galaxy properties
Khosroshahi et al

[In X-ray selected fossil groups, the X-ray emission is extended.]  Combine X-ray and optical observations of 4 galaxy groups, find evidences for the presence of a diffuse extended X-ray emission beyond the optical size of the brightest group galaxy. Taking both the X-ray and the optical criteria, one of the groups is identified as a fossil group and one is ruled out because of the contamination in the earlier optical selection.  For the two remaining systems, the X-ray luminosity threshold is close to the convention known for fossil groups.  In all cases the X-ray luminosity is below the expected value from the X-ray selected fossils for a given optical luminosity of the group.  A rough estimation for the comoving number density of fossil groups is obtained and found to be in broad agreement with the estimations from observations of X-ray selected fossils and predictions of cosmo sims.

1406.3451
Universal profiles of the intracluster medium from Suzaku X-ray and Subaru weak lensing observations
Okabe, Umetsu, ... et al

Conduct a join X-ray and WL study of 4 relaxed galaxy clusters observed out to virial radii, with an aim to understand recently-discovered unexpected feature of the ICM in cluster outskirts.  Show that the average hydrostatic-to-lensing total mass ratio for the 4 clusters decreases from ~70% to ~40% as the overdensity contrast decreases from 500 to the virial value.  The average gas mass fraction from lensing total mass estimates increases with cluster radius and agrees with the cosmic mean baryon fraction within the virial radius, whereas the X-ray based gas fraction considerably exceeds the cosmic values due to underestimation of the hydrostatic mass.  Also develop a new advanced method for determining normalized cluster radial profiles for multiple X-ray observables by simultaneously taking into account both their radial dependence and multivariate scaling relations with WL masses.  Although the 4 clusters span a range of halo mass, concentration, X-ray luminosity and redshift, find that the gas entropy, pressure, temperature and density profiles are all remarkably self-similar when scaled with the lensing M200 mass and r_200 radius.  The entropy monotonically increases out to ~0.5 r200 following the accretion shock heating model K(r)~r^1.1, and flattens at >~ 0.5 r200.  The universality of the scaled entropy profiles indicates that the thermalization mechanism over the entire cluster region (>0.1 r200) is controlled by gravitation a common to all clusters, although the heating efficiency in the outskirts needs to be modified from the standard law.  The bivariate scaling functions of the gas density and temperature reveal that the flattening of the outskirts entry profile is caused by the steepening of the temperature, rather than the flattening of the gas density.