226 research outputs found
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
Dark influences III. Structural characterization of minor mergers of dwarf galaxies with dark satellites
In the current concordance cosmology small halos are expected to be
completely dark and can significantly perturb low-mass galaxies during minor
merger interactions. These interactions may well contribute to the diversity of
the dwarf galaxy population. Dwarf galaxies in the field are often observed to
have peculiarities in their structure, morphology, and kinematics, as well as
strong bursts of star formation without apparent cause. We aim to characterize
the signatures of minor mergers of dwarf galaxies with dark satellites to aid
their observational identification. We explore and quantify a variety of
structural, morphological, and kinematic indicators of merging dwarf galaxies
and their remnants using a suite of hydrodynamical simulations. The most
sensitive indicators of mergers with dark satellites are large asymmetries in
the gaseous and stellar distributions, enhanced central surface brightness and
starbursts, and velocity offsets and misalignments between the cold gas and
stellar components. In general, merging systems span a wide range of values of
the most commonly used indicators, while isolated objects tend to have more
confined values. Interestingly, we find in our simulations that a significantly
off-centered burst of star formation can pinpoint the location of the dark
satellite. Observational systems with such characteristics are perhaps the most
promising for unveiling the presence of the hitherto, missing satellites.Comment: 9 pages, 7 figures. Accepted in A&
Stellar feedback by radiation pressure and photoionization
The relative impact of radiation pressure and photoionization feedback from
young stars on surrounding gas is studied with hydrodynamic radiative transfer
(RT) simulations. The calculations focus on the single-scattering (direct
radiation pressure) and optically thick regime, and adopt a moment-based
RT-method implemented in the moving-mesh code AREPO. The source luminosity, gas
density profile and initial temperature are varied. At typical temperatures and
densities of molecular clouds, radiation pressure drives velocities of order
~20 km/s over 1-5 Myr; enough to unbind the smaller clouds. However, these
estimates ignore the effects of photoionization that naturally occur
concurrently. When radiation pressure and photoionization act together, the
latter is substantially more efficient, inducing velocities comparable to the
sound speed of the hot ionized medium (10-15 km/s) on timescales far shorter
than required for accumulating similar momentum with radiation pressure. This
mismatch allows photoionization to dominate the feedback as the heating and
expansion of gas lowers the central densities, further diminishing the impact
of radiation pressure. Our results indicate that a proper treatment of the
impact of young stars on the interstellar medium needs to primarily account for
their ionization power whereas direct radiation pressure appears to be a
secondary effect. This conclusion may change if extreme boosts of the radiation
pressure by photon trapping are assumed.Comment: 18 pages, 19 figures (main results presented in 13 pages, 10 figures;
extended appendix for RT tests with extra 9 figures). Accepted for
publication in MNRAS after tiny change
A box full of chocolates: The rich structure of the nearby stellar halo revealed by Gaia and RAVE
The hierarchical structure formation model predicts that stellar halos should
form, at least partly, via mergers. If this was a predominant formation channel
for the Milky Way's halo, imprints of this merger history in the form of moving
groups or streams should exist also in the vicinity of the Sun. Here we study
the kinematics of halo stars in the Solar neighbourhood using the very recent
first data release from the Gaia mission, and in particular the TGAS dataset,
in combination with data from the RAVE survey. Our aim is to determine the
amount of substructure present in the phase-space distribution of halo stars
that could be linked to merger debris. To characterise kinematic substructure,
we measure the velocity correlation function in our sample of halo (low
metallicity) stars. We also study the distribution of these stars in the space
of energy and two components of the angular momentum, in what we call
"Integrals of Motion" space. The velocity correlation function reveals
substructure in the form of an excess of pairs of stars with similar
velocities, well above that expected for a smooth distribution. Comparison to
cosmological simulations of the formation of stellar halos indicate that the
levels found are consistent with the Galactic halo having been built fully via
accretion. Similarly, the distribution of stars in the space of "Integrals of
motion" is highly complex. A strikingly high fraction (between 58% and upto
73%) of the stars that are somewhat less bound than the Sun are on (highly)
retrograde orbits. A simple comparison to Milky Way-mass galaxies in
cosmological hydrodynamical simulations suggests that less than 1% have such
prominently retrograde outer halos. We also identify several other
statistically significant structures in "Integrals of Motion" space that could
potentially be related to merger events.Comment: 19 pages, 16 figures. A&A in pres
Self-Interacting Dark Matter Subhalos in the Milky Way's Tides
We study evolution of self-interacting dark matter (SIDM) subhalos in the
Milky Way (MW) tidal field. The interaction between the subhalos and the MW's
tides lead to more diverse dark matter distribution in the inner region,
compared to their cold dark matter counterparts. We test this scenario with two
MW satellite galaxies, Draco and Fornax, opposite extremes in the inner dark
matter content, and find that they can be accommodated within the SIDM model
proposed to explain the diverse rotation curves of spiral galaxies in the
field.Comment: 6 pages, 3 figures. Updated figures and text. Accepted for
publication in PR
Stellar 3-D kinematics in the Draco dwarf spheroidal galaxy
Aims. We present the first three-dimensional internal motions for individual
stars in the Draco dwarf spheroidal galaxy. Methods. By combining first-epoch
observations and second-epoch Data Release
2 positions, we measured the proper motions of sources in the direction
of Draco. We determined the line-of-sight velocities for a sub-sample of
red giant branch stars using medium resolution spectra acquired with the DEIMOS
spectrograph at the Keck II telescope. Altogether, this resulted in a final
sample of Draco members with high-precision and accurate 3D motions, which
we present as a table in this paper. Results. Based on this high-quality
dataset, we determined the velocity dispersions at a projected distance of
pc from the centre of Draco to be
km/s, km/s and
km/s in the projected radial, tangential, and line-of-sight directions. This
results in a velocity anisotropy at
pc. Tighter constraints may be obtained using the spherical Jeans equations and
assuming constant anisotropy and Navarro-Frenk-White (NFW) mass profiles, also
based on the assumption that the 3D velocity dispersion should be lower than
of the escape velocity of the system. In this case, we constrain
the maximum circular velocity of Draco to be in the range of
km/s. The corresponding mass range is in good agreement with
previous estimates based on line-of-sight velocities only. Conclusions. Our
Jeans modelling supports the case for a cuspy dark matter profile in this
galaxy. Firmer conclusions may be drawn by applying more sophisticated models
to this dataset and with new datasets from upcoming releases.Comment: 12 pages, 15 figures, 3 tables. Accepted for publication by A&
The impact of baryonic discs on the shapes and profiles of self-interacting dark matter halos
We employ isolated N-body simulations to study the response of
self-interacting dark matter (SIDM) halos in the presence of the baryonic
potentials. Dark matter self-interactions lead to kinematic thermalization in
the inner halo, resulting in a tight correlation between the dark matter and
baryon distributions. A deep baryonic potential shortens the phase of SIDM core
expansion and triggers core contraction. This effect can be further enhanced by
a large self-scattering cross section. We find the final SIDM density profile
is sensitive to the baryonic concentration and the strength of dark matter
self-interactions. Assuming a spherical initial halo, we also study evolution
of the SIDM halo shape together with the density profile. The halo shape at
later epochs deviates from spherical symmetry due to the influence of the
non-spherical disc potential, and its significance depends on the baryonic
contribution to the total gravitational potential, relative to the dark matter
one. In addition, we construct a multi-component model for the Milky Way,
including an SIDM halo, a stellar disc and a bulge, and show it is consistent
with observations from stellar kinematics and streams.Comment: 10 pages, 8 figures, submitted to MNRAS, accepted for publication in
MNRA
Why stellar feedback promotes disc formation in simulated galaxies
We study how feedback influences baryon infall onto galaxies using
cosmological, zoom-in simulations of haloes with present mass
to . Starting
at z=4 from identical initial conditions, implementations of weak and strong
stellar feedback produce bulge- and disc-dominated galaxies, respectively.
Strong feedback favours disc formation: (1) because conversion of gas into
stars is suppressed at early times, as required by abundance matching
arguments, resulting in flat star formation histories and higher gas fractions;
(2) because 50% of the stars form in situ from recycled disc gas with angular
momentum only weakly related to that of the z=0 dark halo; (3) because
late-time gas accretion is typically an order of magnitude stronger and has
higher specific angular momentum, with recycled gas dominating over primordial
infall; (4) because 25-30% of the total accreted gas is ejected entirely before
z~1, removing primarily low angular momentum material which enriches the nearby
inter-galactic medium. Most recycled gas roughly conserves its angular
momentum, but material ejected for long times and to large radii can gain
significant angular momentum before re-accretion. These processes lower galaxy
formation efficiency in addition to promoting disc formation.Comment: 23 pages, 29 figures, accepted for publication in MNRA
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