231 research outputs found
Lopsidedness in dwarf irregular galaxies
We quantify the amplitude of the lopsidedness, the azimuthal angular
asymmetry index, and the concentration of star forming regions, as represented
by the distribution of the H emission, in a sample of 78 late-type
irregular galaxies. We bin the observed galaxies in two groups representing
blue compact galaxies (BCDs) and low surface brightness dwarf galaxies (LSBs).
The light distribution is analysed with a novel algorithm, which allows
detection of details in the light distribution pattern.
We find that while the asymmetry of the underlying continuum light,
representing the older stellar generations, is relatively small, the H
emission is very asymmetric and is correlated in position angle with the
continuum light.
We test a model of random star formation over the extent of a galaxy by
simulating HII regions in artificial dwarf galaxies. The implication is that
random star formation over the full extent of a galaxy may be generated in LSB
dwarf-irregular galaxies but not in BCD galaxies.Comment: 42 pages, LaTex. Accepted by: MNRAS, 13 Mar 200
Mapping the Galactic Halo. V. Sgr dSph Tidal Debris 60 degrees from the Main Body
As part of the Spaghetti Project Survey (SPS) we have detected a
concentration of giant stars well above expectations for a smooth halo model.
The position (l~350, b~50) and distance (~50 kpc) of this concentration match
those of the Northern over-density detected by SDSS (Yanny et al. 2000, Ivezic
et al. 2000). We find additional evidence for structure at ~80 kpc in the same
direction. We present radial velocities for many of these stars, including the
first published results from the 6.5m Magellan telescope. The radial velocities
for stars in these structures are in excellent agreement with models of the
dynamical evolution of the Sgr dwarf tidal debris, whose center is 60 degrees
away. The metallicity of stars in these streams is lower than that of the main
body of the Sgr dwarf, which may indicate a radial metallicity gradient prior
to disruption.Comment: 10 pages, 3 figures accepted in Astrophysical Journal Letter
On the Spatial Distribution of Stellar Populations in the Large Magellanic Cloud
We measure the angular correlation function of stars in a region of the Large
Magellanic Cloud (LMC) that spans 2 degrees by 1.5 degrees. We find that the
correlation functions of stellar populations are represented well by
exponential functions of the angular separation for separations between 2 and
40 arcmin (corresponding to ~ 30 pc and 550 pc for an LMC distance of 50 kpc).
The inner boundary is set by the presence of distinct, highly correlated
structures, which are the more familiar stellar clusters, and the outer
boundary is set by the observed region's size and the presence of two principal
centers of star formation within the region. We also find that the
normalization and scale length of the correlation function changes
systematically with the mean age of the stellar population. The existence of
positive correlation at large separations (~300 pc), even in the youngest
population, argues for large-scale hierarchical structure in current star
formation. The evolution of the angular correlation toward lower normalizations
and longer scale lengths with stellar age argues for the dispersion of stars
with time. We show that a simple, stochastic, self-propagating star formation
model is qualitatively consistent with this behavior of the correlation
function.Comment: 30 pages, 13 Figures. Scheduled for publication in AJ in June 199
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