3,440 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
Satellites of Simulated Galaxies: survival, merging, and their relation to the dark and stellar halos
We study the population of satellite galaxies formed in a suite of
N-body/gasdynamical simulations of galaxy formation in a LCDM universe. We find
little spatial or kinematic bias between the dark matter and the satellite
population. The velocity dispersion of the satellites is a good indicator of
the virial velocity of the halo: \sigma_{sat}/V_{vir}=0.9 +/- 0.2. Applied to
the Milky Way and M31 this gives V_{vir}^{MW}=109 +/- 22$ km/s and
V_{vir}^{M31} = 138 +/- 35 km/s, respectively, substantially lower than the
rotation speed of their disk components. The detailed kinematics of simulated
satellites and dark matter are also in good agreement. By contrast, the stellar
halo of the simulated galaxies is kinematically and spatially distinct from the
population of surviving satellites. This is because the survival of a satellite
depends on mass and on time of accretion; surviving satellites are biased
toward low-mass systems that have been recently accreted by the galaxy. Our
results support recent proposals for the origin of the systematic differences
between stars in the Galactic halo and in Galactic satellites: the elusive
``building blocks'' of the Milky Way stellar halo were on average more massive,
and were accreted (and disrupted) earlier than the population of dwarfs that
has survived self-bound until the present.Comment: 13 pages, 11 figures, MNRAS in press. Accepted version with minor
changes. Version with high resolution figures available at:
http://www.astro.uvic.ca/~lsales/SatPapers/SatPapers.htm
Cosmic M\'enage \`a Trois: The Origin of Satellite Galaxies On Extreme Orbits
We examine the orbits of satellite galaxies identified in a suite of
N-body/gasdynamical simulations of the formation of galaxies in a LCDM
universe. Most satellites follow conventional orbits; after turning around,
they accrete into their host halo and settle on orbits whose apocentric radii
are steadily eroded by dynamical friction. However, a number of outliers are
also present, we find that ~1/3 of satellites identified at are on
unorthodox orbits, with apocenters that exceed their turnaround radii. This
population of satellites on extreme orbits consists typically of the faint
member of a satellite pair that has been ejected onto a highly-energetic orbit
during its first approach to the primary. Since the concurrent accretion of
multiple satellite systems is a defining feature of hierarchical models of
galaxy formation, we speculate that this three-body ejection mechanism may be
the origin of (i) some of the newly discovered high-speed satellites around M31
(such as Andromeda XIV); (ii) some of the distant fast-receding Local Group
members, such as Leo I; and (iii) the oddly isolated dwarf spheroidals Cetus
and Tucana in the outskirts of the Local Group. Our results suggest that care
must be exercised when using the orbits of the most weakly bound satellites to
place constraints on the total mass of the Local Group.Comment: 10 pages, 6 figures, MNRAS in press. Accepted version with minor
changes. Version with high resolution figures available at:
http://www.astro.uvic.ca/~lsales/SatPapers/SatPapers.htm
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
- …