53 research outputs found
MASSCLEANcolors - Mass Dependent Integrated Colors for Stellar Clusters Derived from 30 Million Monte Carlo Simulations -
We present Monte Carlo models of open stellar clusters with the purpose of
mapping out the behavior of integrated colors with mass and age. Our cluster
simulation package allows for stochastic variations in the stellar mass
function to evaluate variations in integrated cluster properties. We find that
UBVK colors from our simulations are consistent with simple stellar population
(SSP) models, provided the cluster mass is large, Mcluster >= 10^6 M_Sun. Below
this mass, our simulations show two significant effects. First, the mean value
of the distribution of integrated colors moves away from the SSP predictions
and is less red, in the first 10^7 to 10^8 years in UBV colors, and for all
ages in (V - K). Second, the 1\sigma dispersion of observed colors increases
significantly with lower cluster mass. The former we attribute to the reduced
number of red luminous stars in most of the lower mass clusters and the later
we attribute to the increased stochastic effect of a few of these stars on
lower mass clusters. This later point was always assumed to occur, but we now
provide the first public code able to quantify this effect. We are completing a
more extensive database of magnitudes and colors as a function of stellar
cluster age and mass that will allow the determination of the correlation
coefficients among different bands, and improve estimates of cluster age and
mass from integrated photometry.Comment: 11 pages, 5 figures, submitted to Astrophysical Journal Letter
Colour jumps across the spiral arms of Hubble Ultra Deep Field galaxies
We have measured, at various wavelengths, the spiral arm pitch angles of a
sample of distant spiral galaxies from the Hubble Space Telescope eXtreme Deep
Field (XDF). According to density wave theory, we should detect colour jumps
from red-to-blue across the spiral arms. Colour jumps are a consequence of
large-scale shocks, which also generate the classic blue-to-red age/colour
gradients, and have only been detected until now in nearby spiral galaxies. Our
results indicate that colour jumps and gradients have been occurring in distant
galaxies for at least the last 8 Gyr, in agreement with density wave theory.Comment: Accepted for publication in MNRA
The Rich Globular Cluster System of Abell 1689 and the Radial Dependence of the Globular Cluster Formation Efficiency
We study the rich globular cluster (GC) system in the center of the massive
cluster of galaxies Abell 1689 (z=0.18), one of the most powerful gravitational
lenses known. With 28 HST/ACS orbits in the F814W bandpass, we reach magnitude
I_814=29 with >90% completeness and sample the brightest ~5% of the GC system.
Assuming the well-known Gaussian form of the GC luminosity function (GCLF), we
estimate a total population of N(GC_total) = 162,850 GCs within a projected
radius of 400kpc. As many as half may comprise an intracluster component. Even
with the sizable uncertainties, which mainly result from the uncertain GCLF
parameters, this is by far the largest GC system studied to date. The specific
frequency S_N is high, but not uncommon for central galaxies in massive
clusters, rising from S_N~5 near the center to ~12 at large radii. Passive
galaxy fading would increase S_N by ~20% at z=0. We construct the radial mass
profiles of the GCs, stars, intracluster gas, and lensing-derived total mass,
and we compare the mass fractions as a function of radius. The estimated mass
in GCs, M(GC_total)=3.9x10^10 Msun, is comparable to ~80% of the total stellar
mass of the Milky Way. The shape of the GC mass profile appears intermediate
between those of the stellar light and total cluster mass. Despite the extreme
nature of this system, the ratios of the GC mass to the baryonic and total
masses, and thus the GC formation efficiency, are typical of those in other
rich clusters when comparing at the same physical radii. The GC formation
efficiency is not constant, but varies with radius, in a manner that appears
similar for different clusters; we speculate on the reasons for this similarity
in profile.Comment: 13 pages, 11 figures; accepted for publication in Ap
Effects of Non-Circular Motions on Azimuthal Color Gradients
Assuming that density waves trigger star formation, and that young stars
preserve the velocity components of the molecular gas where they are born, we
analyze the effects that non-circular gas orbits have on color gradients across
spiral arms. We try two approaches, one involving semi-analytical solutions for
spiral shocks, and another with magnetohydrodynamic (MHD) numerical simulation
data. We find that, if non-circular motions are ignored, the comparison between
observed color gradients and stellar population synthesis models would in
principle yield pattern speed values that are systematically too high for
regions inside corotation, with the difference between the real and the
measured pattern speeds increasing with decreasing radius. On the other hand,
image processing and pixel averaging result in systematically lower measured
spiral pattern speed values, regardless of the kinematics of stellar orbits.
The net effect is that roughly the correct pattern speeds are recovered,
although the trend of higher measured at lower radii (as expected
when non-circular motions exist but are neglected) should still be observed. We
examine the Martinez-Garcia et al. (2009) photometric data and confirm that
this is indeed the case. The comparison of the size of the systematic pattern
speed offset in the data with the predictions of the semi-analytical and MHD
models corroborates that spirals are more likely to end at Outer Lindblad
Resonance, as these authors had already found.Comment: 32 pages, 15 figures, accepted to Ap
The Gould's Belt Very Large Array Survey II: The Serpens region
We present deep (Jy) radio continuum observations of the Serpens
molecular cloud, the Serpens south cluster, and the W40 region obtained using
the Very Large Array in its A configuration. We detect a total of 146 sources,
29 of which are young stellar objects (YSOs), 2 are BV stars and 5 more are
associated with phenomena related to YSOs. Based on their radio variability and
spectral index, we propose that about 16 of the remaining 110 unclassified
sources are also YSOs. For approximately 65% of the known YSOs detected here as
radio sources, the emission is most likely non-thermal, and related to stellar
coronal activity. As also recently observed in Ophiuchus, our sample of YSOs
with X-ray counterparts lies below the fiducial G\"udel & Benz relation.
Finally, we analyze the proper motions of 9 sources in the W40 region. This
allows us to better constrain the membership of the radio sources in the
region.Comment: Accepted in The Astrophysical Journa
The relation between dynamics and star formation in barred galaxies
We analyze optical and near-infrared data of a sample of 11 barred spiral
galaxies, in order to establish a connection between star formation and
bar/spiral dynamics. We find that 22 regions located in the bars, and 20
regions in the spiral arms beyond the end of the bar present azimuthal
color/age gradients that may be attributed to star formation triggering.
Assuming a circular motion dynamic model, we compare the observed age gradient
candidates with stellar populations synthesis models. A link can then be
established with the disk dynamics that allows us to obtain parameters like the
pattern speed of the bar or spiral, as well as the positions of resonance
radii. We subsequently compare the derived pattern speeds with those expected
from theoretical and observational results in the literature (e.g., bars ending
near corotation). We find a tendency to overestimate bar pattern speeds derived
from color gradients in the bar at small radii, away from corotation; this
trend can be attributed to non-circular motions of the young stars born in the
bar region. In spiral regions, we find that ~ 50% of the color gradient
candidates are "inverse", i.e., with the direction of stellar aging contrary to
that of rotation. The other half of the gradients found in spiral arms have
stellar ages that increase in the same sense as rotation. Of the 9 objects with
gradients in both bars and spirals, six (67%) appear to have a bar and a spiral
with similar Omega_p, while three (33%) do not.Comment: Accepted for publication in Ap
The Hubble Deep Field South Flanking Fields
As part of the Hubble Deep Field South program, a set of shorter 2-orbit
observations were obtained of the area adjacent to the deep fields. The WFPC2
flanking fields cover a contiguous solid angle of 48 square arcminutes.
Parallel observations with the STIS and NICMOS instruments produce a patchwork
of additional fields with optical and near-infrared (1.6 micron) response.
Deeper parallel exposures with WFPC2 and NICMOS were obtained when STIS
observed the NICMOS deep field. These deeper fields are offset from the rest,
and an extended low surface brightness object is visible in the deeper WFPC2
flanking field. In this data paper, which serves as an archival record of the
project, we discuss the observations and data reduction, and present SExtractor
source catalogs and number counts derived from the data. Number counts are
broadly consistent with previous surveys from both ground and space. Among
other things, these flanking field observations are useful for defining slit
masks for spectroscopic follow-up over a wider area around the deep fields, for
studying large-scale structure that extends beyond the deep fields, for future
supernova searches, and for number counts and morphological studies, but their
ultimate utility will be defined by the astronomical community.Comment: 46 pages, 15 figures. Images and full catalogs available via the
HDF-S at http://www.stsci.edu/ftp/science/hdfsouth/hdfs.html at present. The
paper is accepted for the February 2003 Astronomical Journal. Full versions
of the catalogs will also be available on-line from AJ after publicatio
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