85 research outputs found
A Model for Clumpy Self-Enrichment in Globular Clusters
Detailed observations of globular clusters (GCs) have revealed evidence of
self-enrichment: some of the heavy elements that we see in stars today were
produced by cluster stars themselves. Moreover, GCs have internal
subpopulations with different elemental abundances, including, in some cases,
in elements such as iron that are produced by supernovae. This paper presents a
theoretical model for GC formation motivated by observations of Milky Way star
forming regions and simulations of star formation, where giant molecular clouds
fragment into multiple clumps which undergo star formation at slightly
different times. Core collapse supernovae from earlier-forming clumps can
enrich later-forming clumps to the degree that the ejecta can be retained
within the gravitational potential well, resulting in subpopulations with
different total metallicities once the clumps merge to form the final cluster.
The model matches the mass-metallicity relation seen in GC populations around
massive elliptical galaxies, and predicts metallicity spreads within clusters
in excellent agreement with those seen in Milky Way GCs, even for those whose
internal abundance spreads are so large that their entire identity as a GC is
in question. The internal metallicity spread serves as an excellent measurement
of how much self-enrichment has occurred in a cluster, a result that is very
robust to variation in the model parameters.Comment: ApJ, in press. 18 pages. Code instantiating the model is at
doi:10.5281/zenodo.125334
The impact of baryons on the direct detection of dark matter
The spatial and velocity distributions of dark matter particles in the Milky
Way Halo affect the signals expected to be observed in searches for dark
matter. Results from direct detection experiments are often analyzed assuming a
simple isothermal distribution of dark matter, the Standard Halo Model (SHM).
Yet there has been skepticism regarding the validity of this simple model due
to the complicated gravitational collapse and merger history of actual
galaxies. In this paper we compare the SHM to the results of cosmological
hydrodynamical simulations of galaxy formation to investigate whether or not
the SHM is a good representation of the true WIMP distribution in the analysis
of direct detection data. We examine two Milky Way-like galaxies from the
MaGICC cosmological simulations (a) with dark matter only and (b) with baryonic
physics included. The inclusion of baryons drives the shape of the DM halo to
become more spherical and makes the velocity distribution of dark matter
particles less anisotropic especially at large heliocentric velocities, thereby
making the SHM a better fit. We also note that we do not find a significant
disk-like rotating dark matter component in either of the two galaxy halos with
baryons that we examine, suggesting that dark disks are not a generic
prediction of cosmological hydrodynamical simulations. We conclude that in the
Solar neighborhood, the SHM is in fact a good approximation to the true dark
matter distribution in these cosmological simulations (with baryons) which are
reasonable representations of the Milky Way, and hence can also be used for the
purpose of dark matter direct detection calculations.Comment: Minor changes to match JCAP version. 21 pages, 9 figure
Inclination-Independent Galaxy Classification
We present a new method to classify galaxies from large surveys like the
Sloan Digital Sky Survey using inclination-corrected concentration,
inclination-corrected location on the color-magnitude diagram, and apparent
axis ratio. Explicitly accounting for inclination tightens the distribution of
each of these parameters and enables simple boundaries to be drawn that
delineate three different galaxy populations: Early-type galaxies, which are
red, highly concentrated, and round; Late-type galaxies, which are blue, have
low concentrations, and are disk dominated; and Intermediate-type galaxies,
which are red, have intermediate concentrations, and have disks. We have
validated our method by comparing to visual classifications of high-quality
imaging data from the Millennium Galaxy Catalogue. The inclination correction
is crucial to unveiling the previously unrecognized Intermediate class.
Intermediate-type galaxies, roughly corresponding to lenticulars and early
spirals, lie on the red sequence. The red sequence is therefore composed of two
distinct morphological types, suggesting that there are two distinct mechanisms
for transiting to the red sequence. We propose that Intermediate-type galaxies
are those that have lost their cold gas via strangulation, while Early-type
galaxies are those that have experienced a major merger that either consumed
their cold gas, or whose merger progenitors were already devoid of cold gas
(the ``dry merger'' scenario).Comment: Accepted for publication in ApJ. 7 pages in emulateap
d1005+68: A New Faint Dwarf Galaxy in the M81 Group
We present the discovery of d1005+68, a new faint dwarf galaxy in the M81
Group, using observations taken with the Subaru Hyper Suprime-Cam. d1005+68's
color-magnitude diagram is consistent with a distance of
Mpc, establishing group membership. We derive an absolute -band magnitude,
from stellar isochrone fitting, of , with a
half-light radius of pc. These place d1005+68 within
the radius-luminosity locus of Local Group and M81 satellites and among the
faintest confirmed satellites outside the Local Group. Assuming an age of 12
Gyr, d1005+68's red giant branch is best fit by an isochrone of [Fe/H] . It has a projected separation from nearby M81 satellite BK5N of only
5 kpc. As this is well within BK5N's virial radius, we speculate that d1005+68
may be a satellite of BK5N. If confirmed, this would make d1005+68 one of the
first detected satellites-of-a-satellite.Comment: 7 pages, 4 figures, 1 table, additional affiliations include
Characterizing dw1335-29, a recently discovered dwarf satellite of M83
The number, distribution, and properties of dwarf satellites are crucial
probes of the physics of galaxy formation at low masses and the response of
satellite galaxies to the tidal and gas dynamical effects of their more massive
parent.To make progress, it is necessary to augment and solidify the census of
dwarf satellites of galaxies outside the Local Group. M\"uller et al. (2015)
presented 16 dwarf galaxy candidates near M83, but lacking reliable distances,
it is unclear which candidates are M83 satellites. Using red giant branch stars
from the HST/GHOSTS survey in conjunction with ground-based images from
VLT/VIMOS, we confirm that one of the candidates, dw1335-29-- with a projected
distance of 26 kpc from M83 and a distance modulus of -- is a satellite of M83. We estimate an absolute magnitude
, an ellipticity of , a half light
radius of pc, and [Fe/H] = . Owing to
dw1335-29's somewhat irregular shape and possible young stars, we classify this
galaxy as a dwarf irregular or transition dwarf. This is curious, as with a
projected distance of 26 kpc from M83, dw1335-29 is expected to lack recent
star formation. Further study of M83's dwarf population will reveal if star
formation in its satellites is commonplace (suggesting a lack of a hot gas
envelope for M83 that would quench star formation) or rare (suggesting that
dw1335-29 has a larger M83-centric distance, and is fortuitously projected to
small radii).Comment: 7 pages, 5 figures, accepted for publication in MNRA
- β¦