The satellite distribution of M31

(Abridged) The spatial distribution of the Galactic satellite system plays an important role in Galactic dynamics and cosmology, where its successful reproduction is a key test of simulations of galaxy halo formation. Here, we examine its representative nature by conducting an analysis of the 3-dimensional spatial distribution of the M31 subgroup of galaxies. We begin by a discussion of distance estimates and incompleteness concerns, before revisiting the question of membership of the M31 subgroup. Comparison of the distribution of M31 and Galactic satellites relative to the galactic disks suggests that the Galactic system is probably modestly incomplete at low latitudes by ~20%. We find that the radial distribution of satellites around M31 is more extended than the Galactic subgroup; 50% of the Galactic satellites are found within ~100kpc of the Galaxy, compared to ~200kpc for M31. We search for ``ghostly streams'' of satellites around M31, in the same way others have done for the Galaxy, and find several. The lack of M31-centric kinematic data, however, means we are unable to probe whether these streams represent real physical associations. Finally, we find that the M31 satellites are asymmetrically distributed with respect to our line-of-sight to this object, so that the majority of its satellites are on its near side with respect to our line-of-sight. We quantify this result and find it to be significant at the ~3 sigma level. Until such time as a satisfactory explanation for this finding is presented, our results warn against treating the M31 subgroup as complete, unbiased and relaxed.Comment: 15 pages, 9 figures. Accepted for publication in MNRA

Multiple dynamical components in Local Group dwarf spheroidals

The dwarf spheroidal (dSph) satellites of the Local Group have long been thought to be simple spheroids of stars embedded within extended dark matter halos. Recently, however, evidence for the presence of spatially and kinematically distinct stellar populations has been accumulating. Here, we examine the influence of such components on dynamical models of dwarf galaxies embedded in cold dark matter halos. We begin by constructing a model of Andromeda II, a dSph satellite of M31 which shows evidence for spatially distinct stellar components. We find that the two-component model predicts an overall velocity dispersion profile that remains approximately constant at $\sim 10 - 11$ km s$^{-1}$ out to $\sim 1$ kpc from the center; this is despite wide kinematic and spatial differences between the two individual components. The presence of two components may also help to explain oddities in the velocity dispersion profiles of other dSphs; we show that velocity dispersion profiles which appear to rise from the center outwards before leveling off--such as those of Leo I, Draco, and Fornax--can result from the gradual transition from a dynamically cold, concentrated component to a second, hotter, and more spatially extended one, both in equilibrium within the same dark halo. Dwarf galaxies with two stellar components generally have a leptokurtic line-of-sight velocity distribution which is well described by a double Maxwellian. Interestingly, we find that multiple equilibrium components could also provide a potential alternative origin for ``extra-tidal'' stars (normally ascribed to tidal effects) in situations where corroborating evidence for tides may be lacking.Comment: Accepted by MNRAS Letters. Revised version, with addition of new section and expanded discussio

The Orbital Ellipticity of Satellite Galaxies and the Mass of the Milky Way

We use simulations of Milky Way-sized dark matter haloes from the Aquarius Project to investigate the orbits of substructure haloes likely, according to a semi-analytic galaxy formation model, to host luminous satellites. These tend to populate the most massive subhaloes and are on more radial orbits than the majority of subhaloes found within the halo virial radius. One reason for this (mild) kinematic bias is that many low-mass subhaloes have apocentres that exceed the virial radius of the main host; they are thus excluded from subhalo samples identified within the virial boundary, reducing the number of subhalos on radial orbits. Two other factors contributing to the difference in orbital shape between dark and luminous subhaloes are their dynamical evolution after infall, which affects more markedly low-mass (dark) subhaloes, and a weak dependence of ellipticity on the redshift of first infall. The ellipticity distribution of luminous satellites exhibits little halo-to-halo scatter and it may therefore be compared fruitfully with that of Milky Way satellites. Since the latter depends sensitively on the total mass of the Milky Way we can use the predicted distribution of satellite ellipticities to place constraints on this important parameter. Using the latest estimates of position and velocity of dwarfs compiled from the literature, we find that the most likely Milky Way mass lies in the range $6 \times 10^{11} M_{\odot} < M_{200} < 3.1 \times 10^{12} M_{\odot}$, with a best fit value of $M_{200} = 1.1 \times 10^{12} M_{\odot}$. This value is consistent with Milky Way mass estimates based on dynamical tracers or the timing argument.Comment: 10 pages, 9 figures, Accepted by MNRA

A dynamical model of the local cosmic expansion

We combine the equations of motion that govern the dynamics of galaxies in the local volume with Bayesian techniques in order to fit orbits to published distances and velocities of galaxies within $\sim 3$ Mpc. We find a Local Group (LG) mass $2.3\pm 0.7\times 10^{12}{\rm M}_\odot$ that is consistent with the combined dynamical masses of M31 and the Milky Way, and a mass ratio $0.54^{+0.23}_{-0.17}$ that rules out models where our Galaxy is more massive than M31 with $\sim 95\%$ confidence. The Milky Way's circular velocity at the solar radius is relatively high, $245\pm 23$ km/s, which helps to reconcile the mass derived from the local Hubble flow with the larger value suggested by the `timing argument'. Adopting {\it Planck}'s bounds on $\Omega_\Lambda$ yields a (local) Hubble constant $H_0=67\pm 5$km/s/Mpc which is consistent with the value found on cosmological scales. Restricted N-body experiments show that substructures tend to fall onto the LG along the Milky Way-M31 axis, where the quadrupole attraction is maximum. Tests against mock data indicate that neglecting this effect slightly overestimates the LG mass without biasing the rest of model parameters. We also show that both the time-dependence of the LG potential and the cosmological constant have little impact on the observed local Hubble flow.Comment: 22 pages, 14 figures. Accepted to MNRAS. An error in the apex calculation (Appendix A) was found and has been fixed. The new constraints favour models where the Milky Way is less massive than M31. The rest of model parameters and conclusions remain unchange

Clues to the Origin of the Mass-Metallicity Relation: Dependence on Star Formation Rate and Galaxy Size

We use a sample of 43,690 galaxies selected from the Sloan Digital Sky Survey Data Release 4 to study the systematic effects of specific star formation rate (SSFR) and galaxy size (as measured by the half light radius, r_h) on the mass-metallicity relation. We find that galaxies with high SSFR or large r_h for their stellar mass have systematically lower gas phase-metallicities (by up to 0.2 dex) than galaxies with low SSFR or small r_h. We discuss possible origins for these dependencies, including galactic winds/outflows, abundance gradients, environment and star formation rate efficiencies.Comment: Accepted by ApJ Letter