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 $z\sim1$. Analyses of the brightness of these standard candles reveal that the Universe is dominated by a large cosmological constant. The recent identification of the $z\sim1.7$ 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: $\mu\sim1.4$ for the case where these elliptical galaxies have velocity dispersion $200 {\rm km/s}$. 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 $> 10$ 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 $\rm 700pc \le R \le 3500pc$ with amplitude ${\rm \sim 25 km s^{-1} kpc^{-1}}$. 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$b$ feature, has mean ${\rm [Fe/H] \approx -0.3}$ with no variation. That is, there is no detectable abundance gradient in the Galactic bulge over the galactocentric range $\rm 500pc \le R_{GC} \le 3500pc$. 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