98 research outputs found
Vast planes of satellites in a high resolution simulation of the Local Group: comparison to Andromeda
We search for vast planes of satellites (VPoS) in a high resolution
simulation of the Local Group performed by the CLUES project, which improves
significantly the resolution of former similar studies. We use a simple method
for detecting planar configurations of satellites, and validate it on the known
plane of M31. We implement a range of prescriptions for modelling the satellite
populations, roughly reproducing the variety of recipes used in the literature,
and investigate the occurence and properties of planar structures in these
populations. The structure of the simulated satellite systems is strongly
non-random and contains planes of satellites, predominantly co-rotating, with,
in some cases, sizes comparable to the plane observed in M31 by Ibata et al..
However the latter is slightly richer in satellites, slightly thinner and has
stronger co-rotation, which makes it stand out as overall more exceptional than
the simulated planes, when compared to a random population. Although the
simulated planes we find are generally dominated by one real structure, forming
its backbone, they are also partly fortuitous and are thus not kinematically
coherent structures as a whole. Provided that the simulated and observed planes
of satellites are indeed of the same nature, our results suggest that the VPoS
of M31 is not a coherent disc and that one third to one half of its satellites
must have large proper motions perpendicular to the plane
High resolution simulations of the reionization of an isolated Milky Way - M31 galaxy pair
We present the results of a set of numerical simulations aimed at studying
reionization at galactic scale. We use a high resolution simulation of the
formation of the Milky Way-M31 system to simulate the reionization of the local
group. The reionization calculation was performed with the post-processing
radiative transfer code ATON and the underlying cosmological simulation was
performed as part of the CLUES project. We vary the source models to bracket
the range of source properties used in the literature. We investigate the
structure and propagation of the galatic ionization fronts by a visual
examination of our reionization maps. Within the progenitors we find that
reionization is patchy, and proceeds locally inside out. The process becomes
patchier with decreasing source photon output. It is generally dominated by one
major HII region and 1-4 additional isolated smaller bubbles, which eventually
overlap. Higher emissivity results in faster and earlier local reionization. In
all models, the reionization of the Milky Way and M31 are similar in duration,
i.e. between 203 Myr and 22 Myr depending on the source model, placing their
zreion between 8.4 and 13.7. In all models except the most extreme, the MW and
M31 progenitors reionize internally, ignoring each other, despite being
relatively close to each other even during the epoch of reionization. Only in
the case of strong supernova feedback suppressing star formation in haloes less
massive than 10^9 M_sun, and using our highest emissivity, we find that the MW
is reionized by M31.Comment: Accepted for publication in ApJ. 14 pages, 4 figures, 1 tabl
Breaking Boundaries in Computing in Undergraduate Courses
An important question in undergraduate curricula is that of incorporating computing into STEM courses for majors and non-majors alike. What does it mean to teach “computing” in this context? What are some of the benefits and challenges for students and instructors in such courses? This paper contributes to this important dialog by describing three undergraduate courses that have been developed and taught at Harvey Mudd College and Loyola Marymount University. Each case study describes the course objectives, implementation challenges, and assessments
The orbital poles of Milky Way satellite galaxies: a rotationally supported disc-of-satellites
Available proper motion measurements of Milky Way (MW) satellite galaxies are
used to calculate their orbital poles and projected uncertainties. These are
compared to a set of recent cold dark-matter (CDM) simulations, tailored
specifically to solve the MW satellite problem. We show that the CDM satellite
orbital poles are fully consistent with being drawn from a random distribution,
while the MW satellite orbital poles indicate that the disc-of-satellites of
the Milky Way is rotationally supported. Furthermore, the bootstrapping
analysis of the spatial distribution of theoretical CDM satellites also shows
that they are consistent with being randomly drawn. The theoretical CDM
satellite population thus shows a significantly different orbital and spatial
distribution than the MW satellites, most probably indicating that the majority
of the latter are of tidal origin rather than being DM dominated
sub-structures. A statistic is presented that can be used to test a possible
correlation of satellite galaxy orbits with their spatial distribution.Comment: Accepted for publication in Ap
Galaxy subgroups in galaxy clusters
Galaxies which fall into clusters as part of the same infall halo can retain
correlations due to their shared origin. N-body simulations are used to study
properties of such galaxy subgroups within clusters, including their richnesses
and prevalence. The sizes, densities and velocity dispersions of all subgroups
with >= 8 galaxies are found and compared to those of the host clusters. The
largest galaxy subgroup provides a preferred direction in the cluster and is
compared to other preferred directions in the cluster. Scatter in cluster mass
measurements (via five observables), along ~ 96 lines of sight, is compared to
the relation of the line of sight to this preferred direction: scatter in
cluster velocity dispersion measurements show the strongest correlation. The
Dressler-Shectman test (Dressler & Shectman 1988), is applied to these
clusters, to see whether the substructure it identifies is related to these
subgroups. The results for any specific line of sight seem noisy; however,
clusters with large subgroups tend to have a higher fraction of lines of sight
where the test detects substructure.Comment: 12 pages, final version for publication with helpful comments from
referee and others include
Bayesian physical reconstruction of initial conditions from large scale structure surveys
We present a fully probabilistic, physical model of the non-linearly evolved
density field, as probed by realistic galaxy surveys. Our model is valid in the
linear and mildly non-linear regimes and uses second order Lagrangian
perturbation theory to connect the initial conditions with the final density
field. Our parameter space consists of the 3D initial density field and our
method allows a fully Bayesian exploration of the sets of initial conditions
that are consistent with the galaxy distribution sampling the final density
field. A natural byproduct of this technique is an optimal non-linear
reconstruction of the present density and velocity fields, including a full
propagation of the observational uncertainties. A test of these methods on
simulated data mimicking the survey mask, selection function and galaxy number
of the SDSS DR7 main sample shows that this physical model gives accurate
reconstructions of the underlying present-day density and velocity fields on
scales larger than ~6 Mpc/h. Our method naturally and accurately reconstructs
non-linear features corresponding to three-point and higher order correlation
functions such as walls and filaments. Simple tests of the reconstructed
initial conditions show statistical consistency with the Gaussian simulation
inputs. Our test demonstrates that statistical approaches based on physical
models of the large scale structure distribution are now becoming feasible for
realistic current and future surveys.Comment: 20 pages, 8 figure
Cosmic queuing: galaxy satellites, building blocks and the hierarchical clustering paradigm
We study the properties of building blocks (BBs, i.e. accreted satellites)
and surviving satellites of present-day galaxies using the SAG semi-analytic
model of galaxy formation in the context of a concordance Lambda Cold Dark
Matter (LCDM) cosmology. We consider large numbers of DM halo merger trees
spanning a wide range of masses (~1x10^10 - 2.14x10^15 Msun). We find higher
metallicities for BBs with respect to surviving satellites, an effect produced
by the same processes behind the build-up of the mass-metallicity relation. We
prove that these metallicity differences arise from the higher peak height in
the density fluctuation field occupied by BBs and central galaxies which have
collapsed into a single object earlier than surviving satellites. BBs start to
form stars earlier, during the peak of the merger activity in LCDM, and
build-up half of their final stellar mass (measured at the moment of
disruption) up to four times faster than surviving satellites. Surviving
satellites keep increasing their stellar masses rather quiescently down to z~1.
The difference between the metallicities of satellites, BBs and central
galaxies depends on the host DM halo mass, in a way that can be used as a
further test for the concordance cosmology.Comment: 5 pages, 4 figures. Accepted for publication in MNRAS Letter
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