87 research outputs found
The tidal stripping of satellites
We present an improved analytic calculation for the tidal radius of
satellites and test our results against N-body simulations.
The tidal radius in general depends upon four factors: the potential of the
host galaxy, the potential of the satellite, the orbit of the satellite and
{\it the orbit of the star within the satellite}. We demonstrate that this last
point is critical and suggest using {\it three tidal radii} to cover the range
of orbits of stars within the satellite. In this way we show explicitly that
prograde star orbits will be more easily stripped than radial orbits; while
radial orbits are more easily stripped than retrograde ones. This result has
previously been established by several authors numerically, but can now be
understood analytically. For point mass, power-law (which includes the
isothermal sphere), and a restricted class of split power law potentials our
solution is fully analytic. For more general potentials, we provide an equation
which may be rapidly solved numerically. Over short times (\simlt 1-2 Gyrs
satellite orbit), we find excellent agreement between our analytic and
numerical models. Over longer times, star orbits within the satellite are
transformed by the tidal field of the host galaxy. In a Hubble time, this
causes a convergence of the three limiting tidal radii towards the prograde
stripping radius. Beyond the prograde stripping radius, the velocity dispersion
will be tangentially anisotropic.Comment: 10 pages, 5 figures. Final version accepted for publication in MNRAS.
Some new fully analytic tidal radii have been added for power law density
profiles (including the isothermal sphere) and some split power law
The mass of dwarf spheroidal galaxies and the missing satellite problem
We present the results from a suite of N-body simulations of the tidal
stripping of two-component dwarf galaxies comprising some stars and dark
matter. We show that recent kinematic data from the local group dwarf
spheroidal (dSph) galaxies suggests that dSph galaxies must be sufficiently
massive (M) that tidal stripping is of little
importance for the stars. We discuss the implications of these massive dSph
galaxies for cosmology and galaxy formation.Comment: 4 pages, 1 figure, to appear in the proceedings of the IAUC198
"Near-Field Cosmology with Dwarf Elliptical Galaxies", H. Jerjen & B.
Binggeli (eds.). Comments welcom
First Clear Signature of an Extended Dark Matter Halo in the Draco Dwarf Spheroidal
We present the first clear evidence for an extended dark matter halo in the
Draco dwarf spheroidal galaxy based on a sample of new radial velocities for
159 giant stars out to large projected radii. Using a two parameter family of
halo models spanning a range of density profiles and velocity anisotropies, we
are able to rule out (at about the 2.5 sigma confidence level) haloes in which
mass follows light. The data strongly favor models in which the dark matter is
significantly more extended than the visible dwarf. However, haloes with
harmonic cores larger than the light distribution are also excluded. When
combined with existing measurements of the proper motion of Draco, our data
strongly suggest that Draco has not been tidally truncated within ~1 kpc. We
also show that the rising velocity dispersion at large radii represents a
serious problem for modified gravity (MOND).Comment: to be published in ApJL; 5 pages, 4 figure
Kinematically Cold Populations at Large Radii in the Draco and Ursa Minor Dwarf Spheroidals
We present projected velocity dispersion profiles for the Draco and Ursa
Minor (UMi) dwarf spheroidal galaxies based on 207 and 162 discrete stellar
velocities, respectively. Both profiles show a sharp decline in the velocity
dispersion outside ~30 arcmin (Draco) and ~40 arcmin (UMi). New, deep
photometry of Draco reveals a break in the light profile at ~25 arcmin. These
data imply the existence of a kinematically cold population in the outer parts
of both galaxies. Possible explanations of both the photometric and kinematic
data in terms of both equilibrium and non-equilibrium models are discussed in
detail. We conclude that these data challenge the picture of dSphs as simple,
isolated stellar systems.Comment: 5 pages, accepted for publication in ApJ Letter
A V and I CCD Mosaic Survey of the Ursa Minor Dwarf Spheroidal Galaxy
We discuss a Johnson-Cousins V- and I-band CCD mosaic survey of the Ursa Minor dwarf spheroidal galaxy (dSph) to V ~ 22. We covered UMi with 27 105 × 105 overlapping CCD frames, each frame consisting of two 300 s exposures in each of V and I. We also observed several regions ~3° from UMi to obtain an estimate of contamination by galaxies and Galactic stars. We report the first H-R diagram of an entire dSph. Separation of dwarf stars from foreground stars by color allows a robust estimation of the structural parameters of UMi. We examine earlier evidence of two lumps in the UMi stellar distribution. We detect a statistically significant asymmetry in the stellar distribution of UMi along the major axis. Structure in the stellar distribution of UMi might indicate a tidal origin for UMi\u27s high observed mass-to-light ratio. We demonstrate a technique for obtaining the absolute magnitude of UMi from horizontal-branch star counts compared with M92; this method of luminosity estimation is independent of the distance to UMi. We obtain MV = -8.87 ± σ , where σ = 0.14 if M92 is a perfect calibrator and σ 0.35 is an upper bound on the error arising from differences in stellar populations. Our value for MV is consistent with earlier measurements and has a smaller uncertainty
A Dynamical Fossil in the Ursa Minor Dwarf Spheroidal Galaxy
The nearby Ursa Minor dwarf spheroidal (UMi dSph) is one of the most dark
matter dominated galaxies known, with a central mass to light ratio roughly
equal to 70. Somewhat anomalously, it appears to contain morphological
substructure in the form of a second peak in the stellar number density. It is
often argued that this substructure must be transient because it could not
survive for the > 10 Gyr age of the system, given the crossing time implied by
UMi's 8.8 km/s internal velocity dispersion. In this paper, however, we present
evidence that the substructure has a cold kinematical signature, and argue that
UMi's clumpiness could indeed be a primordial artefact. Using numerical
simulations, we demonstrate that substructure is incompatible with the cusped
dark matter haloes predicted by the prevailing Cold Dark Matter (CDM) paradigm,
but is consistent with an unbound stellar cluster sloshing back and forth
within the nearly harmonic potential of a cored dark matter halo. Thus CDM
appears to disagree with observation at the least massive, most dark matter
dominated end of the galaxy mass spectrum.Comment: Astrophysical Journal (Letters), in pres
A photometrically and kinematically distinct core in the Sextans dwarf spheroidal galaxy
We present the line of sight radial velocity dispersion profile of the
Sextans dwarf spheroidal galaxy (dSph), based on a sample of 88 stars extending
to about 1 degree (about 1.5 kpc). Like the Draco and Ursa Minor dSphs, Sextans
shows some evidence of a fall-off in the velocity dispersion at large projected
radii, with significance p=0.96. Surprisingly, the dispersion at the very
centre of Sextans is close to zero (with significance p=0.96). We present
evidence which suggests that this latter change in the stellar kinematics
coincides with changes in the stellar populations within the dSph. We discuss
possible scenarios which could lead to a kinematically and photometrically
distinct population at the centre of Sextans.Comment: 7 pages, 9 figures, accepted for publication in MNRA
The importance of tides for the Local Group dwarf spheroidals
There are two main tidal effects which can act on the Local Group dwarf
spheroidals (dSphs): tidal stripping and tidal shocking. Using N-body
simulations, we show that tidal stripping always leads to flat or rising
projected velocity dispersions beyond a critical radius; it is times
more likely, when averaging over all possible projection angles, that the
cylindrically averaged projected dispersion will rise, rather than be flat. In
contrast, the Local Group dSphs, as a class, show flat or falling projected
velocity dispersions interior to kpc. This argues for tidal stripping
being unimportant interior to kpc for most of the Local Group dSphs
observed so far.
These results have important implications for the formation of the dSphs and
for cosmology. A tidal origin for the formation of these Local Group dSphs (in
which they contain no dark matter) is strongly disfavoured. In the cosmological
context, a naive solution to the missing satellites problem is to allow only
the most massive substructure dark matter halos around the Milky Way to form
stars. It is possible for dSphs to reside within these halos (M) and have their velocity dispersions lowered through the
action of tidal shocks, but only if they have a central density core in their
dark matter, rather than a cusp. dSphs can reside within cuspy dark matter
halos if their halos are less massive (M) and therefore
have smaller central velocity dispersions initially (abridged).Comment: 15 pages, 9 figures. Accepted for publication in MNRAS. No change
from previous versio
Chemical Evolution in the Carina Dwarf Spheroidal
We present metallicities for 487 red giants in the Carina dwarf spheroidal
(dSph) galaxy that were obtained from FLAMES low-resolution Ca triplet (CaT)
spectroscopy. We find a mean [Fe/H] of -1.91 dex with an intrinsic dispersion
of 0.25 dex, whereas the full spread in metallicities is at least one dex. The
analysis of the radial distribution of metallicities reveals that an excess of
metal poor stars resides in a region of larger axis distances. These results
can constrain evolutionary models and are discussed in the context of chemical
evolution in the Carina dSph.Comment: 3 pages, 2 figures, to be published in the proceedings of the
ESO/Arcetri-workshop on "Chemical Abundances and Mixing in Stars", 13.-17.
Sep. 2004, Castiglione della Pescaia, Italy, L. Pasquini, S. Randich (eds.
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