559 research outputs found
The Milky Way's stellar halo - lumpy or triaxial?
We present minimum chi-squared fits of power law and Hernquist density
profiles to F-turnoff stars in eight 2.5 deg wide stripes of SDSS data: five in
the North Galactic Cap and three in the South Galactic cap. Portions of the
stellar Galactic halo that are known to contain large streams of tidal debris
or other lumpy structure, or that may include significant contamination from
the thick disk, are avoided. The data strongly favor a model that is not
symmetric about the Galaxy's axis of rotation. If included as a free parameter,
the best fit to the center of the spheroid is surprisingly approx 3 kpc from
the Galactic center in the direction of the Sun's motion. The model fits favor
a low value of the density of halo stars at the solar position. The alternative
to a non-axisymmetric stellar distribution is that our fits are contaminated by
previously unidentified lumpy substructure.Comment: 10 pages, 10 figs, to appear in proceedings of conference "Physics at
the end of the Galactic Cosmic Ray Spectrum", Journal of Physics: Conf.
series, eds. G. Thomson and P. Sokolsk
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
Spectroscopy of Quasar Candidates from SDSS Commissioning Data
The Sloan Digital Sky Survey has obtained images in five broad-band colors
for several hundred square degrees. We present color-color diagrams for stellar
objects, and demonstrate that quasars are easily distinguished from stars by
their distinctive colors. Follow-up spectroscopy in less than ten nights of
telescope time has yielded 22 new quasars, 9 of them at , and one with
, the second highest-redshift quasar yet known. Roughly 80% of the
high-redshift quasar candidates selected by color indeed turn out to be
high-redshift quasars.Comment: 4 pages, 3 figures, to appear in the proceedings of "After the Dark
Ages: When Galaxies were Young (the Universe at 2<z<5)", 9th Annual October
Astrophysics Conference in Marylan
Exploring asymmetric substructures of the outer disk based on the conjugate angle of the radial action
We use the conjugate angle of radial action (), the best
representation of the orbital phase, to explore the "mid-plane, north branch,
south branch" and "Monoceros area" disk structures that were previously
revealed in the LAMOST K giants (Xu et al. 2020). The former three
substructures, identified by their 3D kinematical distributions, have been
shown to be projections of the phase space spiral (resulting from
nonequilibrium phase mixing). In this work, we find that all of these
substructures associated with the phase spiral show high aggregation in
conjugate angle phase space, indicating that the clumping in conjugate angle
space is a feature of ongoing, incomplete phase mixing. We do not find the
phase spiral located in the "Monoceros area", but we do find a very
highly concentrated substructure in the quadrant of conjugate angle space with
the orbital phase from the apocenter to the guiding radius. The existence of
the clump in conjugate angle space provides a complementary way to connect the
"Monoceros area" with the direct response to a perturbation from a significant
gravitationally interactive event. Using test particle simulations, we show
that these features are analogous to disturbances caused by the impact of the
last passage of the Sagittarius dwarf spheroidal galaxy.Comment: 53pages, 35 figures, 4 Tables, ApJ accepte
A Photometric Metallicity Estimate of the Virgo Stellar Overdensity
We determine photometric metal abundance estimates for individual
main-sequence stars in the Virgo Overdensity (VOD), which covers almost 1000
deg^2 on the sky, based on a calibration of the metallicity sensitivity of
stellar isochrones in the gri filter passbands using field stars with
well-determined spectroscopic metal abundances. Despite the low precision of
the method for individual stars, we derive [Fe/H] = -2.0 +/-0.1 (internal)
+/-0.5 (systematic) for the metal abundance of the VOD from photometric
measurements of 0.7 million stars in the Northern Galactic hemisphere with
heliocentric distances from ~10 kpc to ~20 kpc. The metallicity of the VOD is
indistinguishable, within Delta [Fe/H] < 0.2, from that of field halo stars
covering the same distance range. This initial application suggests that the
SDSS gri passbands can be used to probe the properties of main-sequence stars
beyond ~10 kpc, complementing studies of nearby stars from more
metallicity-sensitive color indices that involve the u passband.Comment: 5 pages, 3 figures, Accepted for publication in ApJ Letter
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
The M33 Globular Cluster System with PAndAS Data: The Last Outer Halo Cluster?
We use CFHT/MegaCam data to search for outer halo star clusters in M33 as
part of the Pan-Andromeda Archaeological Survey (PAndAS). This work extends
previous studies out to a projected radius of 50 kpc and covers over 40 square
degrees. We find only one new unambiguous star cluster in addition to the five
previously known in the M33 outer halo (10 kpc <= r <= 50 kpc). Although we
identify 2440 cluster candidates of various degrees of confidence from our
objective image search procedure, almost all of these are likely background
contaminants, mostly faint unresolved galaxies. We measure the luminosity,
color and structural parameters of the new cluster in addition to the five
previously-known outer halo clusters. At a projected radius of 22 kpc, the new
cluster is slightly smaller, fainter and redder than all but one of the other
outer halo clusters, and has g' ~ 19.9, (g'-i') ~ 0.6, concentration parameter
c ~ 1.0, a core radius r_c ~ 3.5 pc, and a half-light radius r_h ~ 5.5 pc. For
M33 to have so few outer halo clusters compared to M31 suggests either tidal
stripping of M33's outer halo clusters by M31, or a very different, much calmer
accretion history of M33.Comment: 37 pages, 9 figures. Accepted by the Astrophysical Journa
K Corrections For Type Ia Supernovae and a Test for Spatial Variation of the Hubble Constant
Cross-filter K corrections for a sample of "normal" Type Ia supernovae (SNe)
have been calculated for a range of epochs. With appropriate filter choices,
the combined statistical and systematic K correction dispersion of the full
sample lies within 0.05 mag for redshifts z<0.7. This narrow dispersion of the
calculated K correction allows the Type Ia to be used as a cosmological probe.
We use the K corrections with observations of seven SNe at redshifts 0.3 < z
<0.5 to bound the possible difference between the locally measured Hubble
constant (H_L) and the true cosmological Hubble constant (H_0).Comment: 6 pages, 3 Postscript figures, uuencoded uses crckapb.sty and
psfig.sty. To appear in Thermonuclear Supernovae (NATO ASI), eds. R. Canal,
P. Ruiz-LaPuente, and J. Isern. Postscript version is also available at
http://www-supernova.lbl.gov
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