Day 630

Thursday.

1404.4049
A census of the expected properties of classical Milky Way dwarfs in Milgomian dynamics
Lüghausen, Famaey, Kroupa

Revisit the classical MW dwarf spherical satellites Draco, Sculptor, Sextans, Carina, and Fornax in the framework of MOND, following the successful predictions of Andromeda's satellite galaxies.  Use a Poisson solver with adaptive mesh refinement in order to account simultaneously for the gravitational influence of the MW and its satellites.  This allows to rigorously model the important external field effect (EFE) of MOND, which can reduce the effective acceleration significantly.  Make predictions on the M_dyn/L expected to be measured by an observer who assumes Newtonian dynamics to be valid.  Show that MIlgromian dynamics predicts typical M_dyn/L~10…50 Msun/Lsun.  The results for the most luminous ons, Fornax and Sculptor, agree well with available velocity dispersion data.  The central power law slopes of the dynamical masses agrees exceedingly well with values inferred observationally from velocity dispersion measurements.  The results for Sextans, Carina and Draco are low compared to usually quoted observational estimates, as already pointed out by Angus (2008).  For MOND to survive further observational tests in these objects, one would need that either a) previous observational findings based on velocity dispersion measurements have overestimated the dynamical mass due to e.g., binaries and contaminant outliers, b) the satellites are not in virial equilibrium due to the MW tidal field, or c) the specific theory used here does not describe the EFE correctly (e.g., the EFE could be practically negligible in some other theories), or a combination of the three.

1404.4053
Where do galaxies end? a study of hydrodynamic-simulation galaxies and their integrated properties
Stevens, Martig, Croton, Feng

In Hydrosims, often a spherical aperture defines the boundary between the galaxy and the rest of its parent (sub)halo, sometimes coupled with, or alternatively involving, the use of a sub halo finder and gas properties restrictions.  Using the suite of high-res zoom re-sims of individual haloes, and the large-scale simulation MassiveBlack-II, examine the differences in measured galaxy properties from techniques with various aperture definitions.  Perform techniques popular in the literature and present a new technique, based on the baryonic mass profiles of simulated (sub) haloes.  For the average MW-mass system, find the two most popular techniques in the literature return differences of order 30% for stellar mass, a factor of 3 for gas mass, 40% for SFR, and factors of several for gas accretion and ejection rates.  Individual cases can show variations greater than this, with the severity dependent on the concentration of a given system.  The average difference in integrated properties for a more general galaxy population are not as striking, but are still significant for stellar and gas mass.  The large differences that can occur are problematic for comparing results from various publications.  Stress the importance of both defining and justifying a technique choice and discourage using popular apertures that use an exact fraction of the virial radius, due to the unignorable variation in galaxy-to-(sub)halo size.  Finally, note that technique choice does not greatly affect simulated galaxies from lying within the scatter of observed scaling relations, but it can alter the derived best-fit slope for the Kennicutt-Schmidt relation.

1404.4069
Bending and breathing modes of the galactic disk
Widrow et al

Explore the hypothesis that a passing satellite or DM sub halo has excited coherent oscillations of the MW's stellar disk in the direction perpendicular to the Galactic mid plane.  This work is motivated by recent observations of spatially dependent bulk vertical motions within ~kpc of the Sun.  A satellite can transfer a fraction of its orbital energy to the disk stars as it plunges through the Galactic mid plane thereby heating and thickening the disk.  Bulk motions arise during the early stages of such an even when the disk is still in an unrelaxed state.  Present simple toy-model calculations and simulations of disk-satellite interactions, which show that the response of the disk depends on the relative velocity of the satellite.  When the component of the satellite's velocity perpendicular to the disk is small compared with that of the stars, the perturbation is predominantly a bending mode.  Conversely, breathing and higher order modes are excited when the vertical velocity of the satellite is larger than that of the stars.  Argue that the compression and rarefaction motions in three different surveys are in fact breathing mode perturbations of the Galactic disk.