39 research outputs found
A comprehensive Maximum Likelihood analysis of the structural properties of faint Milky Way satellites
We derive the structural parameters of the recently discovered very low
luminosity Milky Way satellites through a Maximum Likelihood algorithm applied
to SDSS data. For each satellite, even when only a few tens of stars are
available down to the SDSS flux limit, the algorithm yields robust estimates
and errors for the centroid, position angle, ellipticity, exponential
half-light radius and number of member stars. This latter parameter is then
used in conjunction with stellar population models of the satellites to derive
their absolute magnitudes and stellar masses, accounting for `CMD shot-noise'.
We find that faint systems are somewhat more elliptical than initially found
and ascribe that to the previous use of smoothed maps which can be dominated by
the smoothing kernel. As a result, the faintest half of the Milky Way dwarf
galaxies (M_V>-7.5) is significantly (4-sigma) flatter (e=0.47+/-0.03) than its
brightest half (M_V<-7.5, e=0.32+/-0.02). From our best models, we also
investigate whether the seemingly distorted shape of the satellites, often
taken to be a sign of tidal distortion, can be quantified. We find that, except
for tentative evidence of distortion in CVnI and UMaII, these can be completely
accounted for by Poisson scatter in the sparsely sampled systems. We consider
three scenarios that could explain the rather elongated shape of faint
satellites: rotation supported systems, stars following the shape of more
triaxial dark matter subhalos, or elongation due to tidal interaction with the
Milky Way. Although none of these is entirely satisfactory, the last one
appears the least problematic, but warrants much deeper observations to track
evidence of such tidal interaction.Comment: 20 pages, 11 figures, ApJ in press; some typos corrected, magnitude
of BooII corrected (thanks go to Shane Walsh for spotting the erroneous
original value
A Deep Survey of the Fornax dSph I: Star Formation History
Based on a deep imaging survey, we present the first homogeneous star
formation history (SFH) of the Fornax dwarf spheroidal (dSph) galaxy. We have
obtained two-filter photometry to a depth of B ~ 23 over the entire surface of
Fornax, the brightest dSph associated with the Milky Way, and derived its SFH
using a CMD-fitting technique. We show that Fornax has produced the most
complex star formation and chemical enrichment histories of all the Milky Way
dSphs. This system has supported multiple epochs of star formation. A
significant number of stars were formed in the early Universe, however the most
dominant population are the intermediate age stars. This includes a strong
burst of star formation approximately 3 to 4 Gyr ago. Significant population
gradients are also evident. Similar to other dSphs, we have found that recent
star formation was concentrated towards the centre of the system. Furthermore,
we show that the central region harboured a faster rate of chemical enrichment
than the outer parts of Fornax. At the centre, the ancient stars (age > 10 Gyr)
display a mean metallicity of [Fe/H] ~ -1.4, with evidence for three peaks in
the metallicity distribution. Overall, enrichment in Fornax has been highly
efficient: the most recent star formation burst has produced stars with close
to solar metallicity. Our results support a scenario in which Fornax
experienced an early phase of rapid chemical enrichment producing a wide range
of abundances. Star formation gradually decreased until ~4 Gyr ago, when Fornax
experienced a sudden burst of strong star formation activity accompanied by
substantial chemical enrichment. Weaker star forming events followed, and we
have found tentative evidence for stars with ages less than 100 Myr.Comment: 13 pages, 13 figures, accepted for publication in Ap
Mapping low-latitude stellar substructure with SEGUE photometry
Encircling the Milky Way at low latitudes, the Low Latitude Stream is a large
stellar structure, the origin of which is as yet unknown. As part of the SEGUE
survey, several photometric scans have been obtained that cross the Galactic
plane, spread over a longitude range of 50 to 203 degrees. These data allow a
systematic study of the structure of the Galaxy at low latitudes, where the Low
Latitude Stream resides. We apply colour-magnitude diagram fitting techniques
to map the stellar (sub)structure in these regions, enabling the detection of
overdensities with respect to smooth models. These detections can be used to
distinguish between different models of the Low Latitude Stream, and help to
shed light on the nature of the system.Comment: To appear in the proceedings of IAU Symposium 254 "The Galaxy disk in
a cosmological context", Copenhagen, June 200
HST Imaging of MEGA Microlensing Candidates in M31
We investigate /ACS and WFPC2 images at the positions of five candidate
microlensing events from a large survey of variability in M31 (MEGA). Three
closely match unresolved sources, and two produce only flux upper limits. All
are confined to regions of the color-magnitude diagram where stellar
variability is unlikely to be easily confused with microlensing. Red variable
stars cannot explain these events (although background supernova are possible
for two). If these lenses arise in M31's halo, they are due to masses (95% certainty, for a -function mass distribution),
brown dwarfs for disk lenses, and stellar masses for bulge lenses.Comment: Accepted for publication in ApJL. Higher resolution version available
at http://www.astro.columbia.edu/~patrick/hst/hst_ml.pd
Statistical analysis of probability density functions for photometric redshifts through the KiDS-ESO-DR3 galaxies
Despite the high accuracy of photometric redshifts (zphot) derived using
Machine Learning (ML) methods, the quantification of errors through reliable
and accurate Probability Density Functions (PDFs) is still an open problem.
First, because it is difficult to accurately assess the contribution from
different sources of errors, namely internal to the method itself and from the
photometric features defining the available parameter space. Second, because
the problem of defining a robust statistical method, always able to quantify
and qualify the PDF estimation validity, is still an open issue. We present a
comparison among PDFs obtained using three different methods on the same data
set: two ML techniques, METAPHOR (Machine-learning Estimation Tool for Accurate
PHOtometric Redshifts) and ANNz2, plus the spectral energy distribution
template fitting method, BPZ. The photometric data were extracted from the KiDS
(Kilo Degree Survey) ESO Data Release 3, while the spectroscopy was obtained
from the GAMA (Galaxy and Mass Assembly) Data Release 2. The statistical
evaluation of both individual and stacked PDFs was done through quantitative
and qualitative estimators, including a dummy PDF, useful to verify whether
different statistical estimators can correctly assess PDF quality. We conclude
that, in order to quantify the reliability and accuracy of any zphot PDF
method, a combined set of statistical estimators is required.Comment: Accepted for publication by MNRAS, 20 pages, 14 figure
The enigmatic pair of dwarf galaxies Leo IV and Leo V: coincidence or common origin?
We have obtained deep photometry in two 1x1 degree fields covering the close
pair of dwarf spheroidal galaxies (dSph) Leo IV and Leo V and part of the area
in between. We find that both systems are significantly larger than indicated
by previous measurements based on shallower data and also significantly
elongated. With half-light radii of r_h=4'.6 +- 0'.8 (206 +- 36 pc) and
r_h=2'.6 +- 0'.6 (133 +- 31 pc), respectively, they are now well within the
physical size bracket of typical Milky Way dSph satellites. Their ellipticities
of epsilon ~0.5 are shared by many faint (M_V>-8) Milky Way dSphs. The large
spatial extent of our survey allows us to search for extra-tidal features with
unprecedented sensitivity. The spatial distribution of candidate red giant
branch and horizontal branch stars is found to be non-uniform at the ~3 sigma
level. This substructure is aligned along the direction connecting the two
systems, indicative of a possible `bridge' of extra-tidal material. Fitting the
stellar distribution with a linear Gaussian model yields a significance of 4
sigma for this overdensity, a most likely FWHM of ~16 arcmin and a central
surface brightness of ~32 mag arcsec^{-2}. We investigate different scenarios
to explain the close proximity of Leo IV and Leo V and the possible tidal
bridge between them. Orbit calculations demonstrate that they are unlikely to
be remnants of a single disrupted progenitor, while a comparison with
cosmological simulations shows that a chance collision between unrelated
subhalos is negligibly small. Leo IV and Leo V could, however, be a bound
`tumbling pair' if their combined mass exceeds 8 +- 4 x 10^9 M_sun. The
scenario of an internally interacting pair appears to be the most viable
explanation for this close celestial companionship. (abridged)Comment: 9 pages, 8 figures, small number of minor textual changes, accepted
for publication in Astrophysical Journa
DenseLens -- Using DenseNet ensembles and information criteria for finding and rank-ordering strong gravitational lenses,
Convolutional neural networks (CNNs) are the state-of-the-art technique for
identifying strong gravitational lenses. Although they are highly successful in
recovering genuine lens systems with a high true-positive rate, the unbalanced
nature of the data set (lens systems are rare), still leads to a high false
positive rate. For these techniques to be successful in upcoming surveys (e.g.
with Euclid) most emphasis should be set on reducing false positives, rather
than on reducing false negatives. In this paper, we introduce densely connected
neural networks (DenseNets) as the CNN architecture in a new pipeline-ensemble
model containing an ensemble of classification CNNs and regression CNNs to
classify and rank-order lenses, respectively. We show that DenseNets achieve
comparable true positive rates but considerably lower false positive rates
(when compared to residual networks; ResNets). Thus, we recommend DenseNets for
future missions involving large data sets, such as Euclid, where low false
positive rates play a key role in the automated follow-up and analysis of large
numbers of strong gravitational lens candidates when human vetting is no longer
feasibl
Mapping the stellar structure of the Milky Way thick disk and halo using SEGUE photometry
We map the stellar structure of the Galactic thick disk and halo by applying
color-magnitude diagram (CMD) fitting to photometric data from the SEGUE
survey, allowing, for the first time, a comprehensive analysis of their
structure at both high and low latitudes using uniform SDSS photometry.
Incorporating photometry of all relevant stars simultaneously, CMD fitting
bypasses the need to choose single tracer populations. Using old stellar
populations of differing metallicities as templates we obtain a sparse 3D map
of the stellar mass distribution at |Z|>1 kpc. Fitting a smooth Milky Way model
comprising exponential thin and thick disks and an axisymmetric power-law halo
allows us to constrain the structural parameters of the thick disk and halo.
The thick-disk scale height and length are well constrained at 0.75+-0.07 kpc
and 4.1+-0.4 kpc, respectively. We find a stellar halo flattening within ~25
kpc of c/a=0.88+-0.03 and a power-law index of 2.75+-0.07 (for 7<R_{GC}<~30
kpc). The model fits yield thick-disk and stellar halo densities at the solar
location of rho_{thick,sun}=10^{-2.3+-0.1} M_sun pc^{-3} and
rho_{halo,sun}=10^{-4.20+-0.05} M_sun pc^{-3}, averaging over any
substructures. Our analysis provides the first clear in situ evidence for a
radial metallicity gradient in the Milky Way's stellar halo: within R<~15 kpc
the stellar halo has a mean metallicity of [Fe/H]=-1.6, which shifts to
[Fe/H]=-2.2 at larger radii. Subtraction of the best-fit smooth and symmetric
model from the overall density maps reveals a wealth of substructures at all
latitudes, some attributable to known streams and overdensities, and some new.
A simple warp cannot account for the low latitude substructure, as
overdensities occur simultaneously above and below the Galactic plane.
(abridged)Comment: 13 pages, 10 figures, accepted for publication in Astrophysical
Journa