118 research outputs found
New Models for a Triaxial Milky Way Spheroid and Effect on the Microlensing Optical Depth to the Large Magellanic Cloud
We obtain models for a triaxial Milky Way spheroid based on data by Newberg
and Yanny. The best fits to the data occur for a spheroid center that is
shifted by 3kpc from the Galactic Center. We investigate effects of the
triaxiality on the microlensing optical depth to the Large Magellanic Cloud
(LMC). The optical depth can be used to ascertain the number of Massive Compact
Halo Objects (MACHOs); a larger spheroid contribution would imply fewer Halo
MACHOs. On the one hand, the triaxiality gives rise to more spheroid mass along
the line of sight between us and the LMC and thus a larger optical depth.
However, shifting the spheroid center leads to an effect that goes in the other
direction: the best fit to the spheroid center is_away_ from the line of sight
to the LMC. As a consequence, these two effects tend to cancel so that the
change in optical depth due to the Newberg/Yanny triaxial halo is at most 50%.
After subtracting the spheroid contribution in the four models we consider, the
MACHO contribution (central value) to the mass of the Galactic Halo varies from
\~(8-20)% if all excess lensing events observed by the MACHO collaboration are
assumed to be due to MACHOs. Here the maximum is due to the original MACHO
collaboration results and the minimum is consistent with 0% at the 1 sigma
error level in the data.Comment: 26 pages, 2 figures. v2: minor revisions. v3: expanded discussion of
the local spheroid density and minor revisions to match version published in
Journal of Cosmology and Astroparticle Physics (JCAP
Identification of A-colored Stars and Structure in the Halo of the Milky Way from SDSS Commissioning Data
A sample of 4208 objects with magnitude 15 < g* < 22 and colors of main
sequence A stars has been selected from 370 square degrees of Sloan Digital Sky
Survey (SDSS) commissioning observations. The data is from two long, narrow
stripes, each with an opening angle of greater than 60 deg, at Galactic
latitudes 36 < abs(b) < 63 on the celestial equator. An examination of the
sample's distribution shows that these stars trace considerable substructure in
the halo. Large overdensities of A-colored stars in the North at (l,b,R) =
(350, 50, 46 kpc) and in the South at (157, -58, 33 kpc) and extending over
tens of degrees are present in the halo of the Milky Way. Using photometry to
separate the stars by surface gravity, both structures are shown to contain a
sequence of low surface gravity stars consistent with identification as a blue
horizontal branch (BHB). Both structures also contain a population of high
surface gravity stars two magnitudes fainter than the BHB stars, consistent
with their identification as blue stragglers (BSs). From the numbers of
detected BHB stars, lower limits to the implied mass of the structures are
6x10^6 M_sun and 2x10^6 M_sun. The fact that two such large clumps have been
detected in a survey of only 1% of the sky indicates that such structures are
not uncommon in the halo. Simple spheroidal parameters are fit to a complete
sample of the remaining unclumped BHB stars and yield (at r < 40 kpc) a fit to
a halo distribution with flattening (c/a = 0.65+/-0.2) and a density falloff
exponent of alpha = -3.2+/-0.3.Comment: AASTeX v5_0, 26 pages, 1 table, 20 figures, ApJ accepte
Cats and Dogs, Hair and A Hero: A Quintet of New Milky Way Companions
We present five new satellites of the Milky Way discovered in Sloan Digital
Sky Survey (SDSS) imaging data, four of which were followed-up with either the
Subaru or the Isaac Newton Telescopes. They include four probable new dwarf
galaxies -- one each in the constellations of Coma Berenices, Canes Venatici,
Leo and Hercules -- together with one unusually extended globular cluster,
Segue 1. We provide distances, absolute magnitudes, half-light radii and
color-magnitude diagrams for all five satellites. The morphological features of
the color-magnitude diagrams are generally well described by the ridge line of
the old, metal-poor globular cluster M92. In the last two years, a total of ten
new Milky Way satellites with effective surface brightness mu_v >~ 28 mag/sq.
arcsec have been discovered in SDSS data. They are less luminous, more
irregular and appear to be more metal-poor than the previously-known nine Milky
Way dwarf spheroidals. The relationship between these objects and other
populations is discussed. We note that there is a paucity of objects with
half-light radii between ~40 pc and ~ 100 pc. We conjecture that this may
represent the division between star clusters and dwarf galaxies.Comment: 10 pages, 8 figures, submitted to the Astrophysical Journa
Spectroscopic Target Selection in the Sloan Digital Sky Survey: The Quasar Sample
We describe the algorithm for selecting quasar candidates for optical
spectroscopy in the Sloan Digital Sky Survey. Quasar candidates are selected
via their non-stellar colors in "ugriz" broad-band photometry, and by matching
unresolved sources to the FIRST radio catalogs. The automated algorithm is
sensitive to quasars at all redshifts lower than z=5.8. Extended sources are
also targeted as low-redshift quasar candidates in order to investigate the
evolution of Active Galactic Nuclei (AGN) at the faint end of the luminosity
function. Nearly 95% of previously known quasars are recovered (based on 1540
quasars in 446 square degrees). The overall completeness, estimated from
simulated quasars, is expected to be over 90%, whereas the overall efficiency
(quasars:quasar candidates) is better than 65%. The selection algorithm targets
ultraviolet excess quasars to i^*=19.1 and higher-redshift (z>3) quasars to
i^*=20.2, yielding approximately 18 candidates per square degree. In addition
to selecting ``normal'' quasars, the design of the algorithm makes it sensitive
to atypical AGN such as Broad Absorption Line quasars and heavily reddened
quasars.Comment: 62 pages, 15 figures (8 color), 8 tables. Accepted by AJ. For a
version with higher quality color figures, see
http://archive.stsci.edu/sdss/quasartarget/RichardsGT_qsotarget.preprint.p
The Milky Way's Circular Velocity Curve to 60 kpc and an Estimate of the Dark Matter Halo Mass from Kinematics of ~2400 SDSS Blue Horizontal Branch Stars
We derive new constraints on the mass of the Milky Way's dark matter halo,
based on a set of halo stars from SDSS as kinematic tracers. Our sample
comprises 2401 rigorously selected Blue Horizontal-Branch (BHB) halo stars
drawn from SDSS DR-6. To interpret these distributions, we compare them to
matched mock observations drawn from two different cosmological galaxy
formation simulations designed to resemble the Milky Way, which we presume to
have an appropriate orbital distribution of halo stars. We then determine which
value of brings the observed distribution into agreement with
the corresponding distributions from the simulations. This procedure results in
an estimate of the Milky Way's circular velocity curve to kpc, which
is found to be slightly falling from the adopted value of
at the Sun's location, and implies MM. The radial dependence of , derived in
statistically independent bins, is found to be consistent with the expectations
from an NFW dark matter halo with the established stellar mass components at
its center. If we assume an NFW halo profile of characteristic concentration
holds, we can use the observations to estimate the virial mass of the Milky
Way's dark matter halo, MM, which is lower than many previous estimates. This estimate
implies that nearly 40% of the baryons within the virial radius of the Milky
Way's dark matter halo reside in the stellar components of our Galaxy. A value
for M of only M also (re-)opens the
question of whether all of the Milky Way's satellite galaxies are on bound
orbits.Comment: 42 pages, 17 figures and 3 table. Accepted by AP
The Field of Streams: Sagittarius and its Siblings
We use Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) u,g,r,i,z
photometry to study Milky Way halo substructure in the area around the North
Galactic Cap. A simple color cut (g-r < 0.4) reveals the tidal stream of the
Sagittarius dwarf spheroidal, as well as a number of other stellar structures
in the field. Two branches (A and B) of the Sagittarius stream are clearly
visible in an RGB-composite image created from 3 magnitude slices, and there is
also evidence for a still more distant wrap behind the A branch. A comparison
of these data with numerical models suggests that the shape of the Galactic
dark halo is close to spherical.Comment: ApJ (Letters), in pres
A Curious New Milky Way Satellite in Ursa Major
In this Letter, we study a localized stellar overdensity in the constellation
of Ursa Major, first identified in Sloan Digital Sky Survey (SDSS) data and
subsequently followed up with Subaru imaging. Its color-magnitude diagram (CMD)
shows a well-defined sub-giant branch, main sequence and turn-off, from which
we estimate a distance of ~30 kpc and a projected size of ~250 x 125 pc. The
CMD suggests a composite population with some range in metallicity and/or age.
Based on its extent and stellar population, we argue that this is a previously
unknown satellite galaxy of the Milky Way, hereby named Ursa Major II (UMa II)
after its constellation. Using SDSS data, we find an absolute magnitude of M_V
\~ -3.8, which would make it the faintest known satellite galaxy. UMa II's
isophotes are irregular and distorted with evidence for multiple
concentrations; this suggests that the satellite is in the process of
disruption.Comment: 4 pages, 4 figures, submitted to ApJ Letter
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