1,026 research outputs found
Impact of Reionization on the Stellar Populations of Nearby Dwarf Galaxies
Cold dark matter models for galaxy formation predict that low-mass systems
will be the first sites of star formation. As these objects have shallow
gravitational potential wells, the subsequent growth of their stellar
populations may be halted by heating and gas loss due to reionization. This
effect has been suggested to have profoundly influenced properties of
present-day dwarf galaxies, including their stellar populations and even
survival as visible galaxies. In this Letter we draw on results from
quantitative studies of Local Group dwarf galaxy star formation histories,
especially for Milky Way satellites, to show that no clear signature exists for
a widespread evolutionary impact from reionization. All nearby dwarf galaxies
studied in sufficient detail contain ancient populations indistinguishable in
age from the oldest Galactic globular clusters. Ancient star formation activity
proceeded over several Gyr, and some dwarf spheroidal galaxies even experienced
fairly continuous star formation until just a few Gyr ago. Despite their
uniformly low masses, their star formation histories differ considerably. The
evolutionary histories of nearby dwarf galaxies appear to reflect influences
from a variety of local processes rather than a dominant effect from
reionization.Comment: Accepted by The Astrophysical Journal Letters. 5 pages, one figur
Hubble Space Telescope Photometry of Hodge 301: An "Old" Star Cluster in 30 Doradus
We present Hubble Space Telescope Planetary Camera UVI data for the
little-studied cluster Hodge 301 3' northwest of 30 Doradus' central ionizing
cluster R136. The average reddening of Hodge 301 is found to be =
(0.28+-0.05) mag from published infrared and ultraviolet photometry. Using two
different sets of evolutionary models, we derive an age of ~ 20-25 Myr for
Hodge 301, which makes it roughly 10 times as old as R136. Hodge 301 is the
most prominent representative of the oldest population in the 30 Dor starburst
region; a region that has undergone multiple star formation events. This range
of ages is an important consideration for the modelling of starburst regions.
Hodge 301 shows a widened upper main sequence largely caused by Be stars. We
present a list of Be star candidates. The slope of the initial mass function
for intermediate-mass main sequence stars ranging from 10 to 1.3 solar masses
is found to be -1.4+-0.1 in good agreement with a Salpeter law. There is no
indication for a truncation or change of slope of the IMF within this mass
range. In accordance with the age of Hodge 301 no obvious pre-main-sequence
stars are seen down to 1 solar mass. We estimate that up to 41+-7 stars with
more than 12 solar masses may have turned into supernovae since the formation
of the cluster. Multiple supernova explosions are the most likely origin of the
extremely violent gas motions and the diffuse X-ray emission observed in the
cluster surroundings.Comment: To appear in the Astronomical Journal (Feb 2000 issue). 16 pages in
two-column style. 9 separate figures, in part in significantly reduced
resolution for space reasons (bitmapped postscript or jpg
Oxygen abundance distributions in six late-type galaxies based on SALT spectra of HII regions
Spectra of 34 H II regions in the late-type galaxies NGC1087, NGC2967,
NGC3023, NGC4030, NGC4123, and NGC4517A were observed with the South African
Large Telescope (SALT). In all 34 H II regions, oxygen abundances were
determined through the "counterpart" method (C method). Additionally, in two H
II regions in which the auroral lines were detected oxygen abundances were
measured through the classic Te method. We also estimated the abundances in our
H II regions using the O3N2 and N2 calibrations and compared those with the
C-based abundances. With these data we examined the radial abundance
distributions in the disks of our target galaxies. We derived
surface-brightness profiles and other characteristics of the disks (the surface
brightness at the disk center and the disk scale length) in three photometric
bands for each galaxy using publicly available photometric imaging data. The
radial distributions of the oxygen abundances predicted by the relation between
abundance and disk surface brightness in the W1 band obtained for spiral
galaxies in our previous study are close to the radial distributions of the
oxygen abundances determined from the analysis of the emission line spectra for
four galaxies where this relation is applicable. Hence, when the
surface-brightness profile of a late-type galaxy is known, this parametric
relation can be used to estimate the likely present-day oxygen abundance in its
disk.Comment: 15 pages, 11 figures; Accepted for publication in Astronomy &
Astrophysic
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