579 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
The Recognition of Unusual Objects in the Sloan Digital Sky Survey Color System
We present 5 filter photometry of 21 carbon stars, 15 asteroids, 15
cataclysmic variables, 6 metal-poor stars, 5 Cepheids, 1775 field stars, blue
horizontal branch (BHB) stars and RR Lyrae stars in the globular clusters M 15
and M 2, two primary standards, and 19 secondary standards. The photometry was
carried out using a filter set identical to that which will be used for the
Sloan Digital Sky Survey. We find that carbon stars, CVs, R-type, J-type, and
V-type asteroids, BHB stars, and RR Lyr stars should be identifiable on the
basis of SDSS photometry alone, while Cepheids, metal-poor stars, and many
types of asteroids are indistinguishable from the stellar locus of field stars.Comment: 44 pages, 13 postscript figures. Accepted for publication in
Publications of the Astronomical Society of the Pacific, vol. 110, November
1998. Uses AAS Latex style file, version 4.
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
Detection of a population gradient in the Sagittarius Stream
We present a quantitative comparison between the Horizontal Branch morphology
in the core of the Sagittarius dwarf spheroidal galaxy (Sgr) and in a wide
field sampling a portion of its tidal stream (Sgr Stream), located tens of kpc
away from the center of the parent galaxy. We find that the Blue Horizontal
Branch (BHB) stars in that part of the Stream are five times more abundant than
in the Sgr core, relative to Red Clump stars. The difference in the ratio of
BHB to RC stars between the two fields is significant at the 4.8 sigma level.
This indicates that the old and metal-poor population of Sgr was preferentially
stripped from the galaxy in past peri-Galactic passages with respect to the
intermediate-age metal rich population that presently dominates the bound core
of Sgr, probably due to a strong radial gradient that was settled within the
galaxy before its disruption. The technique adopted in the present study allows
to trace population gradients along the whole extension of the Stream.Comment: 4 pages, 3 .ps figures (fig. 1 at low resolution); Accepted for
publication by A&A Letter
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
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