2,388 research outputs found
Are stellar over-densities in dwarf galaxies the "smoking gun" of triaxial dark matter haloes?
We use N-body simulations to study the tidal evolution of globular clusters
(GCs) in dwarf spheroidal (dSph) galaxies. Our models adopt a cosmologically
motivated scenario in which the dSph is approximated by a static NFW halo with
a triaxial shape. For a large set of orbits and projection angles we examine
the spatial and velocity distribution of stellar debris deposited during the
complete disruption of stellar clusters. Our simulations show that such debris
appears as shells, isolated clumps and elongated over-densities at low surface
brightness (>26 mag/arcsec^2), reminiscent of substructure observed in several
MW dSphs. Such features arise from the triaxiality of the galaxy potential and
do not dissolve in time. Stellar over-densities reported in several MW dSphs
may thus be the telltale evidence of dark matter haloes being triaxial in
shape.
We explore a number of kinematic signatures that would help to validate (or
falsify) this scenario. The mean angular momentum of the cluster debris
associated with box and resonant orbits, which are absent in spherical
potentials, is null. As a result, we show that the line-of-sight velocity
distribution may exhibit a characteristic "double-peak" depending on the
oriention of the viewing angle with respect to the progenitor's orbital plane.
Kinematic surveys of dSphs may help to detect and identify substructures
associated with the disruption of stellar clusters, as well as to address the
shape of the dark matter haloes in which dSphs are embedded.Comment: 4 pages, 2 figures, to be published in the proceedings of "Hunting
for the Dark: The Hidden Side of Galaxy Formation", Malta, 19-23 Oct. 2009,
eds. V.P. Debattista & C.C. Popescu, AIP Conf. Ser., in pres
Bayesian analysis of resolved stellar spectra: application to MMT/Hectochelle Observations of the Draco dwarf spheroidal
We introduce a Bayesian method for fitting faint, resolved stellar spectra in
order to obtain simultaneous estimates of redshift and stellar-atmospheric
parameters. We apply the method to thousands of spectra---covering 5160-5280
Angs. at resolution R~20,000---that we have acquired with the MMT/Hectochelle
fibre spectrograph for red-giant and horizontal branch candidates along the
line of sight to the Milky Way's dwarf spheroidal satellite in Draco. The
observed stars subtend an area of ~4 deg^2, extending ~3 times beyond Draco's
nominal `tidal' radius. For each spectrum we tabulate the first four
moments---central value, variance, skewness and kurtosis---of posterior
probability distribution functions representing estimates of the following
physical parameters: line-of-sight velocity v_los, effective temperature
(T_eff), surface gravity (logg) and metallicity ([Fe/H]). After rejecting
low-quality measurements, we retain a new sample consisting of 2813 independent
observations of 1565 unique stars, including 1879 observations for 631 stars
with (as many as 13) repeat observations. Parameter estimates have median
random errors of sigma_{v_los}=0.88 km/s, sigma_{T_eff}=162 K, sigma_logg=0.37
dex and sigma_[Fe/H]=0.20 dex. Our estimates of physical parameters distinguish
~470 likely Draco members from interlopers in the Galactic foreground.Comment: published in Monthly Notices of the Royal Astronomical Society, all
data are publicly available at the following address:
http://www.andrew.cmu.edu/user/mgwalker/hectochelle
Measuring the slopes of mass profiles for dwarf spheroidals in triaxial CDM potentials
We generate stellar distribution functions (DFs) in triaxial haloes in order
to examine the reliability of slopes inferred by applying mass estimators of the form (i.e. assuming spherical symmetry, where and are
luminous effective radius and global velocity dispersion, respectively) to two
stellar sub-populations independently tracing the same gravitational potential.
The DFs take the form , are dynamically stable, and are generated within
triaxial potentials corresponding directly to subhaloes formed in cosmological
dark-matter-only simulations of Milky Way and galaxy cluster haloes.
Additionally, we consider the effect of different tracer number density
profiles (cuspy and cored) on the inferred slopes of mass profiles. For the
isotropic DFs considered here, we find that halo triaxiality tends to introduce
an anti-correlation between and when estimated for a variety of
viewing angles. The net effect is a negligible contribution to the systematic
error associated with the slope of the mass profile, which continues to be
dominated by a bias toward greater overestimation of masses for
more-concentrated tracer populations. We demonstrate that simple mass estimates
for two distinct tracer populations can give reliable (and cosmologically
meaningful) lower limits for , irrespective of the degree of
triaxiality or shape of the tracer number density profile.Comment: 5 pages, 4 figures, submitted to 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
A low pre-infall mass for the Carina dwarf galaxy from disequilibrium modelling
Dark matter only simulations of galaxy formation predict many more subhalos
around a Milky Way like galaxy than the number of observed satellites. Proposed
solutions require the satellites to inhabit dark matter halos with masses
between one to ten billion solar masses at the time they fell into the Milky
Way. Here we use a modelling approach, independent of cosmological simulations,
to obtain a preinfall mass of 360 (+380,-230) million solar masses for one of
the Milky Way's satellites: Carina. This determination of a low halo mass for
Carina can be accommodated within the standard model only if galaxy formation
becomes stochastic in halos below ten billion solar masses. Otherwise Carina,
the eighth most luminous Milky Way dwarf, would be expected to inhabit a
significantly more massive halo. The implication of this is that a population
of "dark dwarfs" should orbit the Milky Way: halos devoid of stars and yet more
massive than many of their visible counterparts.Comment: 5 pages, 3 figures, 1 table, and supplementary material availabl
Systemic Proper Motions of Milky Way Satellites from Stellar Redshifts: the Carina, Fornax, Sculptor and Sextans Dwarf Spheroidals
The transverse motions of nearby dwarf spheroidal (dSph) galaxies contribute
line-of-sight components that increase with angular distance from the dSph
centers, inducing detectable gradients in stellar redshift. In the absence of
an intrinsic velocity gradient (e.g., due to rotation or streaming), an
observed gradient in the heliocentric rest frame (HRF) relates simply to a
dSph's systemic proper motion (PM). Kinematic samples for the Milky Way's
brightest dSph satellites are now sufficiently large that we can use stellar
redshifts to constrain systemic PMs independently of astrometric data. Data
from our Michigan/MIKE Fiber System (MMFS) Survey reveal significant HRF
velocity gradients in Carina, Fornax and Sculptor, and no significant gradient
in Sextans. Assuming there are no intrinsic gradients, the data provide a
relatively tight constraint on the PM of Fornax,
(mu_{alpha}^{HRF},mu_{delta}^{HRF})=(+48 +/- 15,-25 +/- 14) mas/century, that
agrees with published HST astrometric measurements. Smaller data sets yield
weaker constraints in the remaining galaxies, but our Carina measurement,
(mu_{alpha}^{HRF},mu_{delta}^{HRF})=(+25 +/- 36,+16 +/- 43) mas/century, agrees
with the published astrometric value. The disagreement of our Sculptor
measurement, (mu_{alpha}^{HRF},mu_{delta}^{HRF})= (-40 +/- 29, -69 +/- 47)
mas/century, with astrometric measurements is expected if Sculptor has a
rotational component as reported by Battaglia et al. (2008). For Sextans, which
at present lacks an astrometric measurement, we measure
(mu_{alpha}^{HRF},mu_{delta}^{HRF})=(-26 +/- 41, +10 +/- 44) mas/century.Comment: Accepted for Publication by ApJ Letter
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