8 research outputs found
Galaxy Collisions - Dawn of a New Era
The study of colliding galaxies has progressed rapidly in the last few years,
driven by observations with powerful new ground and space-based instruments.
These instruments have used for detailed studies of specific nearby systems,
statistical studies of large samples of relatively nearby systems, and
increasingly large samples of high redshift systems. Following a brief summary
of the historical context, this review attempts to integrate these studies to
address the following key issues. What role do collisions play in galaxy
evolution, and how can recently discovered processes like downsizing resolve
some apparently contradictory results of high redshift studies? What is the
role of environment in galaxy collisions? How is star formation and nuclear
activity orchestrated by the large scale dynamics, before and during merger?
Are novel modes of star formation involved? What are we to make of the
association of ultraluminous X-ray sources with colliding galaxies? To what do
degree do mergers and feedback trigger long-term secular effects? How far can
we push the archaeology of individual systems to determine the nature of
precursor systems and the precise effect of the interaction? Tentative answers
to many of these questions have been suggested, and the prospects for answering
most of them in the next few decades are good.Comment: 44 pages, 9 figures, review article in press for Astrophysics Update
Vol.
Constraints on thick disc and halo parameters from HST photometry of field stars in the Galaxy
We analyse a sample of over 1000 stars from 32 fields imaged in the V and I
bands with the Wide Field and Planetary Camera, on board of the Hubble Space
Telescope. The fields are located at Galactic latitudes | b | >= 15deg and in
various directions on the sky. We consider models for the structure of the
Galaxy with different choices for the main parameters governing the shape and
luminosity function of the thick disk and stellar halo. Comparing model
predictions with the observed colour-magnitude diagram we are able to rule out
an increasing or flat stellar luminosity function at the low-luminosity end. We
also rule out large values of the vertical scale height of the thick disc, z_0,
finding it to be in the range 800 <= z_0 <= 1200 pc. As for the local density
normalization, values within the range 4 % <= n_0 <= 8 % seem to better
reproduce the data. Our data essentially rule out a flattened stellar halo (c/a
<~ 0.5) or models with both large local normalization and effective radii.Comment: 7 pages LaTeX, 9 Postscript figure
Low-mass pre--main-sequence stars in the Magellanic Clouds
[Abridged] The stellar Initial Mass Function (IMF) suggests that sub-solar
stars form in very large numbers. Most attractive places for catching low-mass
star formation in the act are young stellar clusters and associations, still
(half-)embedded in star-forming regions. The low-mass stars in such regions are
still in their pre--main-sequence (PMS) evolutionary phase. The peculiar nature
of these objects and the contamination of their samples by the evolved
populations of the Galactic disk impose demanding observational techniques for
the detection of complete numbers of PMS stars in the Milky Way. The Magellanic
Clouds, the companion galaxies to our own, demonstrate an exceptional star
formation activity. The low extinction and stellar field contamination in
star-forming regions of these galaxies imply a more efficient detection of
low-mass PMS stars than in the Milky Way, but their distance from us make the
application of special detection techniques unfeasible. Nonetheless, imaging
with the Hubble Space Telescope yield the discovery of solar and sub-solar PMS
stars in the Magellanic Clouds from photometry alone. Unprecedented numbers of
such objects are identified as the low-mass stellar content of their
star-forming regions, changing completely our picture of young stellar systems
outside the Milky Way, and extending the extragalactic stellar IMF below the
persisting threshold of a few solar masses. This review presents the recent
developments in the investigation of PMS stars in the Magellanic Clouds, with
special focus on the limitations by single-epoch photometry that can only be
circumvented by the detailed study of the observable behavior of these stars in
the color-magnitude diagram. The achieved characterization of the low-mass PMS
stars in the Magellanic Clouds allowed thus a more comprehensive understanding
of the star formation process in our neighboring galaxies.Comment: Review paper, 26 pages (in LaTeX style for Springer journals), 4
figures. Accepted for publication in Space Science Review
Constraints on the star formation history of the Large Magellanic Cloud
We present the analysis of deep colour-magnitude diagrams (CMDs) of 6 stellar fields in the LMC. The data were obtained using HST/WFPC2 in the I and V filters, reaching V=26.5. We discuss and apply a method of correcting CMDs for photometric incompleteness. A method to generate artificial CMDs based on a model star formation history is also developed. This method incorporates photometric error effects, unresolved binaries, reddening and allows use of different forms of the initial mass function and of the SFH itself. We use the Partial Models Method, as presented by Gallart and others, for CMD modelling, and include control experiments to prove its validity in a search for constraints on the Large Magellanic Cloud star formation history in different regions. Reliable star formation histories for each field are recovered by this method. In all fields, a gap in star formation with t=700 Myrs is observed. Field-to-field variations have also been observed. The two fields near the LMC bar present some significant star forming events, having formed both young (t 10 Gyrs) stars, with a clear gap from 3-6 Gyrs. Two other fields display quite similar SFHs, with increased star formation having taken place at t=2-3 Gyrs and 6 < t < 10 Gyrs. The remaining two fields present star formation histories closer to uniform, with no clear event of enhanced star formation