Wednesday.
1512.01253
How does the choice of observable influence our estimation of the centre of a galaxy cluster? Insights from cosmological simulations
Cui, Power, Biffi, Borgani, Knabe, Murante, Fabjan, Lewis, Poole
Galaxy clusters are an established and powerful test-bed for theories of both galaxy evolution and cosmology. Accurate interpretation of cluster observations often requires robust identification of the location of the centre. Using a statistical sample of clusters drawn from a suit of cosmo sims in which a range of galaxy formation models are explored, investigate how the location of this centre is affected by the choice of observable - stars, hot gas, or the full mass distribution as can be probed by the gravitational potential. Explore several measures of cluster centre: the minimum of the gravitational potential, which would expect to define the centre if the cluster is in dynamical equilibrium; the peak of the density; the centre of BCG; and the peak and centroid of X-ray luminosity. Find that the centre of BCG correlates more strongly with the minimum of the gravitational potential than the X-ray defined centre, while AGN feedback acts to significantly enhance the offset between the peak X-ray luminosity and minimum gravitational potential. These results highlight the importance of centre identification when interpreting clusters observations, in particular when comparing theoretical predications and observational data.
1512.02219
Vertical disc heating in Milky Way-sized galaxies in a cosmological context
Grand, Springel, Gómez, Marinucci, Pakmor, Campbell, Jenkins
Vertically extended, high velocity dispersion stellar distributions appear to be a ubiquitous feature of disc galaxies, and both internal and external mechanisms have been proposed to be the major driver of their formation. However, it is unclear to what extent each mechanism can generate such a distribution, which is likely to depend on the assembly history of the galaxy. To this end, perform 16 high res cosmos-zoom simulations of MW-sized galaxies using the state-of-the-art cosmological magneto-hydrodynamical code REPO, and analyse the evolution of the vertical kinematics of the stellar disc in connection with various heating mechanisms. Find that the bar is the dominant heating mechanism in most cases, whereas spiral arms, radial migration, and adiabatic heating from mid-plane density growth are all sub-dominant. The strongest source, though less prevalent than bars, originates from external perturbations from satellite/sub-haloes of masses log10(M/Msun)>10. However, in many simulations the orbits of newborn star particles become cooler with time, such that they dominate the shape of the age-velocity dispersion relation and overall vertical disc structure unless a strong external perturbation takes place.
1512.02236
The alignment of galaxy spin with the shear field in observations
Pahoa, Libeskind, Tempel, Hoffman, Tully, et al
Tidal torque theory suggests that galaxies gain angular momentum in the linear stage of structure formation. Such a theory predicts alignments between the spin of haloes and tidal shear field. However, NL evolution and angular momentum acquisition may later this prediction significantly. In this paper, use a reconstruction of the cosmic shear field from observed peculiar velocities combined with spin axes extracted from galaxies within 115 Mpc (~8000 km/s) from 2MRS catalog, to test whether or not galaxies appear aligned with principal axes of shear field. Although linear reconstructions of the tidal field have looked at similar issues, this is the first such study to examine galaxy alignments with velocity-shear field. Ellipticals in the 2MRS sample, show a statistically significant alignment with tow of the principal axes of the shear field. In general, elliptical galaxies have their short axis aligned with the axis of greatest compression and perpendicular to the axis of slowest compression. Spiral galaxies show no signal. Such an alignment is significantly strengthened when considering only those galaxies that are used in velocity field reconstruction. When examining such a subsample, a weak alignment with the axis of greatest compression emerges for spiral galaxies as well. This result indicates that although velocity field reconstructions still rely on family noisy and sparse data, the underlying alignment with shear field is strong enough to be visible even when small numbers of galaxies are considered - especially if those galaxies are used as constraints in the reconstruction.
1512.02342
Galaxy and Mass Assembly (GAMA): the stellar mass budget by galaxy type
Moffatt, et al
Report an expanded sample of visual morphological classifications from GAMA phase two, which now includes 7556 objects (previously 3727 in phase one). Define a local (z<0.06) sample and classify galaxies into E, S0-Sa, SB0-SBa, Sab-Scd, SBabSBcd, Sd-Irr, and "little blue spheroid" types. Using these updated classifications, derive stellar mass function fits to individual galaxy populations divided both by morphological class and more general spheroid- or disk-dominated categories with a lower mass limit of log(M*/Msun)=8 (on dex below earlier morphological mass function determinations). Find that all individual morphological classes and the combined spheroid-/bulge-dominated classes are well described by single Schechter stellar mass function forms. Find that the total stellar mass densities for individual galaxy populations and for the entire galaxy population are bounded within the stellar mass limits and derive an estimated total stellar mass density of rho_star = 2.5e8 Msun h_0.7/Mpc^3, which corresponds to an approximately 4% fraction of baryons found in stars. The mass contributions to this total stellar mass density by galaxies that are dominated by spheroidal components (E and S0-Sa classes) and by disk components (Sab-Scd and Sd-Irr classes) are approximately 70% and 30%, respectively.
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