198 research outputs found
The effect of tides on the Fornax dwarf spheroidal galaxy
Estimates of the mass distribution and dark-matter (DM) content of dwarf
spheroidal galaxies (dSphs) are usually derived under the assumption that the
effect of the tidal field of the host galaxy is negligible over the radial
extent probed by kinematic data-sets. We assess the implications of this
assumption in the specific case of the Fornax dSph by means of N-body
simulations of a satellite orbiting around the Milky Way. We consider
observationally-motivated orbits and we tailor the initial distributions of the
satellite's stars and DM to match, at the end of the simulations, the observed
structure and kinematics of Fornax. In all our simulations the present-day
observable properties of Fornax are not significantly influenced by tidal
effects. The DM component is altered by the interaction with the Galactic field
(up to 20% of the DM mass within 1.6 kpc is lost), but the structure and
kinematics of the stellar component are only mildly affected even in the more
eccentric orbit (more than 99% of the stellar particles remain bound to the
dwarf). In the simulations that successfully reproduce Fornax's observables,
the dark-to-luminous mass ratio within 1.6 kpc is in the range 5-6, and up to
16-18 if measured within 3 kpc.Comment: 19 pages, 16 figures. Accepted for publication in MNRA
The Effect of Tidal Stripping on Composite Stellar Populations in Dwarf Spheroidal Galaxies
We use N-body simulations to study the effects of tides on the kinematical
structure of satellite galaxies orbiting a Milky Way-like potential. Our work
is motivated by observations of dwarf spheroidal galaxies in the Local Group,
for which often a distinction is possible between a cold centrally concentrated
metal rich and a hot, extended metal poor population. We find that an important
attenuation of the initial differences in the distribution of the two stellar
components occurs for orbits with small pericentric radii (r_per < 20 kpc).
This is mainly due to: i) the loss of the gravitational support provided by the
dark matter component after tidal stripping takes place, which forces a
re-configuration of the luminous components, and ii) tides preferentially
affect the more extended stellar component, leading to a net decrease in its
velocity dispersion as a response for the mass loss, which thus shrinks the
kinematical gap. We apply these ideas to the Sculptor and Carina dwarf
spheroidals. Differences in their orbits might help to explain, under the
assumption of similar initial configurations, why in the former a clear
kinematical separation between metal poor and metal rich stars is apparent,
while in Carina this segregation is significantly more subtle.Comment: 17 pages, 7 figures, Accepted for publication in Advances in
Astronomy, special issue on "Dwarf-Galaxy Cosmology
Cleaning spectroscopic samples of stars in nearby dwarf galaxies: The use of the nIR MgI line to weed out Milky Way contaminants
Dwarf galaxies provide insights on the processes of star formation and
chemical enrichment at the low end of the galaxy mass function, as well as on
the clustering of dark matter on small scales. In studies of Local Group dwarf
galaxies, spectroscopic samples of individual stars are used to derive the
internal kinematics and abundance properties of these galaxies. It is therefore
important to clean these samples from Milky Way stars, not related to the dwarf
galaxy, since they can contaminate the analysis of the properties of these
objects. Here we introduce a new diagnostic for separating Milky Way
contaminant stars -- that mainly constitute of dwarf stars -- and red giant
branch stars targeted in dwarf galaxies. As discriminator we use the trends in
the equivalent width of the nIR MgI line at 8806.8 \AA\ as a function of the
equivalent width of CaII triplet lines. This method is particularly useful for
works dealing with multi-object intermediate resolution spectroscopy focusing
in the region of the nIR CaII triplet. We use synthetic spectra to explore how
the equivalent width of these lines changes for stars with different properties
(gravity, effective temperature, metallicity) and find that a discrimination
among giants above the horizontal branch and dwarfs can be made with this
method at [Fe/H]> -2 dex. For -2 [Fe/H] -1, this method is also
valid to discriminate dwarfs and giants down to approximately one magnitude
below the horizontal branch. Using a foreground model we make predictions on
the use of this new discrimination method for nearby dwarf spheroidal galaxies,
including the ultra-faints. We subsequently use VLT/FLAMES data for the
Sextans, Sculptor and Fornax dSphs to verify the predicted theoretical trends.Comment: 14 pages, 10 figures. Accepted for publication in A&
Stellar dynamics and dark matter in Local Group dwarf galaxies
When interpreted within the standard framework of Newtonian gravity and dynamics, the kinematics of stars and gas in dwarfmgalaxies reveals that most of these systems are completely dominated by their dark matter halos. These dwarf galaxies are thus among the best astrophysical laboratories to study the structure of dark halos and the nature of dark matter. We review the properties of the dwarf galaxies of the Local Group from the point of view of stellar dynamics. After describing the observed kinematics of their stellar components and providing an overview of the dynamical modelling techniques, we look into the dark matter content and distribution of these galaxies, as inferred from the combination of observed data and dynamical models. We also briefly touch upon the prospects of using nearby dwarf galaxies as targets for indirect detection of dark matter via annihilation or decay emission
The extensive age gradient of the Carina dwarf galaxy
The evolution of small systems such as dwarf spheroidal galaxies (dSph) is
likely to have been a balance between external environmental effects and
internal processes within their own relatively shallow potential wells.
Assessing how strong such environmental interactions may have been is therefore
an important element in understanding the baryonic evolution of dSphs and their
derived dark matter distribution.
Here we present results from a wide-area CTIO/MOSAIC II photometric survey of
the Carina dSph, reaching down to about two magnitudes below the oldest main
sequence turn-off (MSTO). This data-set enables us to trace the structure of
Carina in detail out to very large distances from its center, and as a function
of stellar age.
We observe the presence of an extended structure made up primarily of ancient
MSTO stars, at distances between 25arcmin-60arcmin from Carina's center,
confirming results in the literature that Carina extends well beyond its
nominal tidal radius.
The large number statistics of our survey reveals features such as isophote
twists and tails that had gone undetected in other previous shallower surveys.
This is the first time that such unambiguous signs of tidal disruption have
been found in a Milky Way "classical" dwarf other than Sagittarius.
We also demonstrate the presence of a negative age gradient in Carina
directly from its MSTOs, and trace it out to very large distances from the
galaxy center. The signs of interaction with the Milky Way make it unclear
whether the age gradient was already in place before Carina underwent tidal
disruption.Comment: Accepted for publication in The Astrophysical Journal Letter
LITTLE THINGS in 3D: robust determination of the circular velocity of dwarf irregular galaxies
Dwarf Irregular galaxies (dIrrs) are the smallest stellar systems with
extended HI discs. The study of the kinematics of such discs is a powerful tool
to estimate the total matter distribution at these very small scales. In this
work, we study the HI kinematics of 17 galaxies extracted from the `Local
Irregulars That Trace Luminosity Extremes, The HI Nearby Galaxy Survey' (LITTLE
THINGS). Our approach differs significantly from previous studies in that we
directly fit 3D models (two spatial dimensions plus one spectral dimension)
using the software BAROLO, fully exploiting the information in the
HI datacubes. For each galaxy we derive the geometric parameters of the HI disc
(inclination and position angle), the radial distribution of the surface
density, the velocity-dispersion () profile and the rotation curve.
The circular velocity (V), which traces directly the galactic
potential, is then obtained by correcting the rotation curve for the asymmetric
drift. As an initial application, we show that these dIrrs lie on a baryonic
Tully-Fisher relation in excellent agreement with that seen on larger scales.
The final products of this work are high-quality, ready-to-use kinematic data
( and ) that we make publicly available. These
can be used to perform dynamical studies and improve our understanding of these
low-mass galaxies.Comment: 36 pages, 28 figures, 2 tables. Submitted to MNRAS (revised version
after the referee report). The final rotation curves can be downloaded from
http://www.filippofraternali.com/styled-9/index.htm
On the anti-correlation between pericentric distance and inner dark matter density of Milky Way's dwarf spheroidal galaxies
An anti-correlation between the central density of the dark matter halo
() and the pericentric distances () of the Milky
Way's (MW's) dwarf spheroidal galaxies (dSphs) has been reported in the
literature. The existence and origin of such anti-correlation is however
controversial, one possibility being that only the densest dSphs can survive
the tidal field towards the centre of our Galaxy. In this work, we place
particular emphasis on quantifying the statistical significance of such
anti-correlation, by using available literature data in order to explore its
robustness under different assumptions on the MW gravitational potential, and
for various derivations of and . We consider models in
which the MW is isolated and has a low () and high
() halo mass, respectively, as well as
configurations in which the MW's potential is perturbed by a Large Magellanic
Cloud (LMC) infall. We find that, while data generally support models in which
the dSphs' central DM density decreases as a function of their pericentric
radius, this anti-correlation is statistically significant at level
only in 12 of the combinations of and explored.
Moreover, including the impact of the LMC's infall onto the MW weakens or even
washes away this anti-correlation, with respect to models in which the MW is
isolated. Our results suggest that the strength and existence of such
anti-correlation is still debatable: exploring it with high-resolution
simulations including baryonic physics and different DM flavours will help us
to understand its emergence.Comment: 10 pages, 3 figures. Accepted for publication in MNRA
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