413 research outputs found
Galactic halo stellar structures in the Triangulum-Andromeda region
This letter reports on the Galactic stellar structures that appear in the
foreground of our Canada-France-Hawaii-Telecopse/MegaCam survey of the halo of
the Andromeda galaxy. We recover the main sequence and main sequence turn-off
of the Triangulum-Andromeda structure recently found by Majewski and
collaborators at a heliocentric distance of ~20 kpc. The survey also reveals
another less populated main sequence at fainter magnitudes that could
correspond to a more distant stellar structure at ~28 kpc. Both structures are
smoothly distributed over the ~76 sq. deg. covered by the survey although the
closer one shows an increase in density by a factor of ~2 towards the
North-West. The discovery of a stellar structure behind the
Triangulum-Andromeda structure that itself appears behind the low-latitude
stream that surrounds the Galactic disk gives further evidence that the inner
halo of the Milky Way is of a spatially clumpy nature.Comment: accepted for publication in ApJL, 4 pages, 4 figures. Significant
changes including a larger dataset and a more thorough discussio
Was SN1997ff at z~1.7 magnified by gravitational lensing?
The quest for the cosmological parameters has come to fruition with the
identification of a number of supernovae at a redshift of . Analyses of
the brightness of these standard candles reveal that the Universe is dominated
by a large cosmological constant. The recent identification of the
SN1997ff in the northern Hubble Deep Field has provided further evidence for
this cosmology. Here we examine the case for gravitational lensing of SN1997ff
due to the presence of galaxies lying along our line of sight. We find that,
while the alignment of SN1997ff with foreground masses was not favorable for it
to be multiply imaged and strongly magnified, two galaxies did lie close enough
to result in significant magnification: for the case where these
elliptical galaxies have velocity dispersion . Given the small
difference between supernova brightnesses in different cosmologies, detailed
modeling of the gravitational lensing properties of the intervening matter is
therefore required before the true cosmological significance of SN1997ff can be
deduced.Comment: 3 pages, 2 figures. Accepted for publication in MNRAS. Missing
reference adde
Sagittarius: The Nearest Dwarf Galaxy
We have discovered a new Galactic satellite galaxy in the constellation of
Sagittarius. The Sagittarius dwarf galaxy is the nearest galaxy known, subtends
an angle of degrees on the sky, lies at a distance of 24 \kpc from the
Sun, \sim 16 \kpc from the centre of the Milky Way. Itis comparable in size
and luminosity to the largest dwarf spheroidal, has a well populated red
horizontal branch with a blue HB extension; a substantial carbon star
population; and a strong intermediate age stellar component with evidence of a
metallicity spread. Isodensity maps show it to be markedly elongated along a
direction pointing towards the Galactic centre and suggest that it has been
tidally distorted. The close proximity to the Galactic centre, the
morphological appearance and the radial velocity of 140 km/s indicate that this
system must have undergone at most very few close orbital encounters with the
Milky Way. It is currently undergoing strong tidal disruption prior to being
integrated into the Galaxy. Probably all of the four globular clusters, M54,
Arp 2, Ter 7 and Ter 8, are associated with the Sagittarius dwarf galaxy, and
will probably share the fate of their progenitor.Comment: MNRAS in press, 22pp uuencoded PS file, 26 printed figures available
on request from [email protected]
Kinematic outliers in the LMC: constraints on star-star microlensing
Although a decade of microlensing searches towards the Large Magellanic Cloud
(LMC) has detected 13-25 possible microlensing events, the nature and the
location of the lenses, being either halo machos or LMC stars, remains a
subject of debate. The star-star lensing models generically predict the
existence of a small population (more than about 5 percent of stars with a
spatial and kinematic distribution different from the thin, young disc of the
LMC. Here we present the results of a large spectroscopic survey of the LMC,
consisting of more than 1300 radial velocities measured accurately with the 2dF
instrument. In this large sample, no evidence is found for any extraneous
population over the expected LMC and Galactic components. Any additional,
kinematically distint, population can only be present at less than the 1
percent level. We discuss the significance of this finding for the LMC
self-lensing models.Comment: MNRAS accepted, to be published 2003. 6pp w/ 10 eps fig, references
updated to match proo
Galactic Halo substructure in the Sloan Digital Sky Survey: the ancient tidal stream from the Sagittarius dwarf galaxy
Two studies have recently reported the discovery of pronounced Halo
substructure in the Sloan Digital Sky Survey (SDSS) commissioning data. Here we
show that this Halo substructure is almost in its entirety due to the expected
tidal stream torn off the Sagittarius dwarf galaxy during the course of its
many close encounters with the Milky Way. This interpretation makes strong
predictions on the kinematics and distances of these stream stars. Comparison
of the structure in old horizontal branch stars, detected by the SDSS team,
with the carbon star structure discovered in our own survey, indicates that
this halo stream is of comparable age to the Milky Way. It would appear that
the Milky Way and the Sagittarius dwarf galaxy have been a strongly interacting
system for most of their existence. Once complete, the SDSS will provide a
unique dataset with which to constrain the dynamical evolution of the
Sagittarius dwarf galaxy, it will also strongly constrain the mass distribution
of the outer Milky Way.Comment: 7 pages, 3 figures (1 color figure chunky due to PS compression),
minor revisions,accepted by ApJ
The intrinsic ellipticity of dwarf spheroidal galaxies: constraints from the Andromeda system
We present a study of the intrinsic deprojected ellipticity distribution of
the satellite dwarf galaxies of the Andromeda galaxy, assuming that their
visible components have a prolate shape, which is a natural outcome of
simulations. Different possibilities for the orientation of the major axis of
the prolate dwarf galaxies are tested, pointing either as close as possible to
the radial direction towards the centre of Andromeda, or tangential to the
radial direction, or with a random angle in the plane that contains the major
axis and the observer. We find that the mean intrinsic axis ratio is ~ 1/2,
with small differences depending on the assumed orientation of the population.
Our deprojections also suggest that a significant fraction of the satellites, ~
10%, are tidally disrupted remnants. We find that there is no evidence of any
obvious difference in the morphology and major axis orientation between
satellites that belong to the vast thin plane of co-rotating galaxies around
Andromeda and those that do not belong to this structure.Comment: 12 pages, 17 figures, accepted for publication in MNRA
The Outer Regions of the Galactic Bulge:II Analysis
We analyse velocity, chemical abundance and spatial distribution data for
some 1500 K- and M- giants in the Galactic Bulge. The bulge has a
well-determined linear rotation curve over the range with amplitude . The oblate isotropic
bulge model of Kent (1992) is in reasonable agreement with our data. We do not
find a significant requirement for asymmetry or a bar. The metallicity
distribution of K giants in a subset of our fields, using the Mg feature,
has mean with no variation. That is, there is no
detectable abundance gradient in the Galactic bulge over the galactocentric
range . We derive the distribution function of
specific angular momentum for the bulge from our data, and compare it with
determinations for the halo, the thick disk and the thin disk from Wyse &
Gilmore (1992). We confirm that the bulge and the halo have angular momentum
distributions which are indistinguishable, as do the thick disk and the thin
disk. The bulge-halo distribution is however very different from the thick
disk-thin disk distribution. This is perhaps the strongest available clue to
the evolutionary relationships between different Galactic structural
components.Comment: MNRAS in press, 23pp incl figures, uuencoded, tarred, compresse
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