92 research outputs found
The Bifurcated Age-Metallicity Relation of Milky Way Globular Clusters and its Implications For the Accretion History of the Galaxy
We use recently derived ages for 61 Milky Way (MW) globular clusters (GCs) to
show that their age-metallicity relation (AMR) can be divided into two
distinct, parallel sequences at [Fe/H] \ga -1.8. Approximately one-third of
the clusters form an offset sequence that spans the full range in age (--13 Gyr), but is more metal rich at a given age by dex in
[Fe/H]. All but one of the clusters in the offset sequence show orbital
properties that are consistent with membership in the MW disk. They are not
simply the most metal-rich GCs, which have long been known to have disk-like
kinematics, but they are the most metal-rich clusters at all ages. The slope of
the mass-metallicity relation (MMR) for galaxies implies that the offset in
metallicity of the two branches of the AMR corresponds to a mass decrement of 2
dex, suggesting host galaxy masses of M_{*} \sim 10^{7-8} \msol for GCs that
belong to the more metal-poor AMR. We suggest that the metal-rich branch of the
AMR consists of clusters that formed in-situ in the disk, while the metal-poor
GCs were formed in relatively low-mass (dwarf) galaxies and later accreted by
the MW. The observed AMR of MW disk stars, and of the LMC, SMC and WLM dwarf
galaxies are shown to be consistent with this interpretation, and the relative
distribution of implied progenitor masses for the halo GC clusters is in
excellent agreement with the MW subhalo mass function predicted by simulations.
A notable implication of the bifurcated AMR, is that the identical mean ages
and spread in ages, for the metal rich and metal poor GCs are difficult to
reconcile with an in-situ formation for the latter population.Comment: 16 pages, 9 figures, accepted for publication in MNRA
Bulge plus disc and S\'ersic decomposition catalogues for 16,908 galaxies in the SDSS Stripe 82 co-adds: A detailed study of the structural measurements
Quantitative characterization of galaxy morphology is vital in enabling
comparison of observations to predictions from galaxy formation theory.
However, without significant overlap between the observational footprints of
deep and shallow galaxy surveys, the extent to which structural measurements
for large galaxy samples are robust to image quality (e.g., depth, spatial
resolution) cannot be established. Deep images from the Sloan Digital Sky
Survey (SDSS) Stripe 82 co-adds provide a unique solution to this problem -
offering magnitudes improvement in depth with respect to SDSS Legacy
images. Having similar spatial resolution to Legacy, the co-adds make it
possible to examine the sensitivity of parametric morphologies to depth alone.
Using the Gim2D surface-brightness decomposition software, we provide public
morphology catalogs for 16,908 galaxies in the Stripe 82 co-adds. Our
methods and selection are completely consistent with the Simard et al. (2011)
and Mendel et al. (2014) photometric decompositions. We rigorously compare
measurements in the deep and shallow images. We find no systematics in total
magnitudes and sizes except for faint galaxies in the -band and the
brightest galaxies in each band. However, characterization of bulge-to-total
fractions is significantly improved in the deep images. Furthermore, statistics
used to determine whether single-S\'ersic or two-component (e.g., bulge+disc)
models are required become more bimodal in the deep images. Lastly, we show
that asymmetries are enhanced in the deep images and that the enhancement is
positively correlated with the asymmetries measured in Legacy images.Comment: 27 pages, 14 figures. MNRAS accepted. Our catalogs are available in
TXT and SQL formats at
http://orca.phys.uvic.ca/~cbottrel/share/Stripe82/Catalogs
The signature of dissipation in the mass-size relation: are bulges simply spheroids wrapped in a disc?
The relation between the stellar mass and size of a galaxy's structural
subcomponents, such as discs and spheroids, is a powerful way to understand the
processes involved in their formation. Using very large catalogues of
photometric bulge+disc structural decompositions and stellar masses from the
Sloan Digital Sky Survey Data Release Seven, we carefully define two large
subsamples of spheroids in a quantitative manner such that both samples share
similar characteristics with one important exception: the 'bulges' are embedded
in a disc and the 'pure spheroids' are galaxies with a single structural
component. Our bulge and pure spheroid subsample sizes are 76,012 and 171,243
respectively. Above a stellar mass of ~ M, the mass-size
relations of both subsamples are parallel to one another and are close to lines
of constant surface mass density. However, the relations are offset by a factor
of 1.4, which may be explained by the dominance of dissipation in their
formation processes. Whereas the size-mass relation of bulges in discs is
consistent with gas-rich mergers, pure spheroids appear to have been formed via
a combination of 'dry' and 'wet' mergers.Comment: Accepted for publication in MNRAS, 6 pages, 3 figure
Galaxy pairs in the Sloan Digital Sky Survey - IX: Merger-induced AGN activity as traced by the Wide-field Infrared Survey Explorer
Interactions between galaxies are predicted to cause gas inflows that can
potentially trigger nuclear activity. Since the inflowing material can obscure
the central regions of interacting galaxies, a potential limitation of previous
optical studies is that obscured Active Galactic Nuclei (AGNs) can be missed at
various stages along the merger sequence. We present the first large
mid-infrared study of AGNs in mergers and galaxy pairs, in order to quantify
the incidence of obscured AGNs triggered by interactions. The sample consists
of galaxy pairs and post-mergers drawn from the Sloan Digital Sky Survey that
are matched to detections by the Wide Field Infrared Sky Explorer (WISE). We
find that the fraction of AGN in the pairs, relative to a mass-, redshift- and
environment-matched control sample, increases as a function of decreasing
projected separation. This enhancement is most dramatic in the post-merger
sample, where we find a factor of 10-20 excess in the AGN fraction compared
with the control. Although this trend is in qualitative agreement with results
based on optical AGN selection, the mid-infrared selected AGN excess increases
much more dramatically in the post-mergers than is seen for optical AGN. Our
results suggest that energetically dominant optically obscured AGNs become more
prevalent in the most advanced mergers, consistent with theoretical
predictions.Comment: 8 pages, 7 figures accepted to MNRAS (with minor revisions
Bulge mass is king: The dominant role of the bulge in determining the fraction of passive galaxies in the Sloan Digital Sky Survey
We investigate the origin of galaxy bimodality by quantifying the relative
role of intrinsic and environmental drivers to the cessation (or `quenching')
of star formation in over half a million local Sloan Digital Sky Survey (SDSS)
galaxies. Our sample contains a wide variety of galaxies at z=0.02-0.2, with
stellar masses of 8 < log(M*/M_sun) < 12, spanning the entire morphological
range from pure disks to spheroids, and over four orders of magnitude in local
galaxy density and halo mass. We utilise published star formation rates and add
to this recent GIM2D photometric and stellar mass bulge + disk decompositions
from our group. We find that the passive fraction of galaxies increases steeply
with stellar mass, halo mass, and bulge mass, with a less steep dependence on
local galaxy density and bulge-to-total stellar mass ratio (B/T). At fixed
internal properties, we find that central and satellite galaxies have different
passive fraction relationships. For centrals, we conclude that there is less
variation in the passive fraction at a fixed bulge mass, than for any other
variable, including total stellar mass, halo mass, and B/T. This implies that
the quenching mechanism must be most tightly coupled to the bulge. We argue
that radio-mode AGN feedback offers the most plausible explanation of the
observed trends.Comment: Accepted to MNRAS. 32 pages, 27 figures. [This version is virtually
identical to v1
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