117 research outputs found
Stellar Populations with ELTs
The star formation, mass assembly and chemical enrichment histories of
galaxies, and their present distributions of dark matter, remain encoded in
their stellar populations. Distinguishing the actual distribution functions of
stellar age, metallicity and kinematics at several locations in a range of
galaxies, sampling across Hubble types and representative environments, is the
information required for a robust description of galaxy histories. Achieving
this requires large aperture, to provide the sensitivity to reach a range of
environs and Hubble types beyond the Local Group, to provide high spatial
resolution, since the fields are crowded, and preferably with optical
performance since age-sensitivity is greatest near the main-sequence turn-off,
and metallicity-sensitivity for these warm stars is greatest in the optical.Comment: IAU Symposium No. 232, eds P. Whitelock, B. Leidundgeit & M.
Dennefel
Galactic Bulges
We review current knowledge on the structure, properties and evolution of
galactic bulges, considering particularly common preconceptions in the light of
recent observational results.Comment: in press, Annual Review Astron. Astrophys. 35 1997. Plain tex, 9
figures included. Also available by anonymous ftp at
ftp://ftp.ast.cam.ac.uk/pub/gil
HST Observations of the Field Star Population in the Large Magellanic Cloud
We present and photometry, obtained with the Hubble Space Telescope,
for stars in a field in the inner disk of the Large Magellanic
Cloud. We confirm previous results indicating that an intense star formation
event, probably corresponding to the formation of the LMC disk, occurred a few
times years ago. We find a small but real difference between our field
and one further out in the disk observed by Gallagher et al (1996): either star
formation in the inner disk commenced slightly earlier, or the stars are
slightly more metal rich. We also find evidence for a later burst, around 1 Gyr
ago, which may correspond to the formation of the LMC bar. About 5% of the
stars in our field are substantially older than either burst, and are probably
members of an old disk or halo population with age Gyr.Comment: 18 pages, 7 figures: only 3 available ellectronically - complete
copies by request from [email protected]
The 300km/s stellar stream near Segue 1: Insights From high-resolution spectroscopy of its brightest star
We present a chemical abundance analysis of 300S-1, the brightest likely
member star of the 300 km/s stream near the faint satellite galaxy Segue 1.
From a high-resolution Magellan/MIKE spectrum we determine a metallicity of
[Fe/H] = -1.46 +- 0.05 +- 0.23 (random and systematic uncertainties) for star
300S-1, and find an abundance pattern similar to typical halo stars at this
metallicity. Comparing our stellar parameters to theoretical isochrones, we
estimate a distance of 18 +- 7 kpc. Both the metallicity and distance estimates
are in good agreement with what can be inferred from comparing the SDSS
photometric data of the stream stars to globular cluster sequences. While
several other structures overlap with the stream in this part of the sky, the
combination of kinematic, chemical and distance information makes it unlikely
that these stars are associated with either the Segue 1 galaxy, the Sagittarius
stream or the Orphan stream. Streams with halo-like abundance signatures, such
as the 300 km/s stream, present another observational piece for understanding
the accretion history of the Galactic halo.Comment: 13 pages, emulateapj, accepted for publication in Ap
Deciphering The Last Major Invasion of the Milky Way Galaxy
We present first results from a spectroscopic survey of 2000 F/G stars
0.5--5kpc from the Galactic plane, obtained with the 2dF facility on the AAT.
These data show the mean rotation velocity of the thick disk about the Galactic
center a few kpc from the plane is very different than expectation, being about
100km/s, rather than the predicted ~180km/s. We propose that our sample is
dominated by stars from a disrupted satellite which merged with the disk of the
Milky Way Galaxy some 10-12Gyr ago. We do not find evidence for the many
substantial mergers expected in hierarchical clustering theories. We find yet
more evidence that the stellar halo retains kinematic substructure, indicative
of minor mergersComment: ApJ Letter in pres
Further Evidence for a Merger Origin for the Thick Disk: Galactic Stars Along Lines-of-sight to Dwarf Spheroidal Galaxies
The history of the Milky Way Galaxy is written in the properties of its
stellar populations. Here we analyse stars observed as part of surveys of local
dwarf spheroidal galaxies, but which from their kinematics are highly probable
to be non-members. The selection function -- designed to target metal-poor
giants in the dwarf galaxies, at distances of ~100kpc -- includes F-M dwarfs in
the Milky Way, at distances of up to several kpc. Thestars whose motions are
analysed here lie in the cardinal directions of Galactic longitude l ~ 270 and
l ~ 90, where the radial velocity is sensitive to the orbital rotational
velocity. We demonstrate that the faint F/G stars contain a significant
population with V_phi ~ 100km/s, similar to that found by a targeted, but
limited in areal coverage, survey of thick-disk/halo stars by Gilmore, Wyse &
Norris (2002). This value of mean orbital rotation does not match either the
canonical thick disk or the stellar halo. We argue that this population,
detected at both l ~ 270 and l ~ 90, has the expected properties of `satellite
debris' in the thick-disk/halo interface, which we interpret as remnants of the
merger that heated a pre-existing thin disk to form the thick disk.Comment: Accepted, Astrophysical Journal Letter
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