304 research outputs found
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
km s out to 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 , with a best fit value of . 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 Mpc. We find a Local Group
(LG) mass that is consistent with the
combined dynamical masses of M31 and the Milky Way, and a mass ratio
that rules out models where our Galaxy is more massive
than M31 with confidence. The Milky Way's circular velocity at the
solar radius is relatively high, 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 yields a
(local) Hubble constant 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
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