380 research outputs found
Past and present star formation in the SMC: NGC 346 and its neighborhood
In the quest of understanding how star formation occurs and propagates in the
low metallicity environment of the Small Magellanic Cloud (SMC), we acquired
deep F555W (~V), and F814W (~I) HST/ACS images of the young and massive star
forming region NGC 346. These images and their photometric analysis provide us
with a snapshot of the star formation history of the region. We find evidence
for star formation extending from ~10 Gyr in the past until ~150 Myr in the
field of the SMC. The youngest stellar population (~3 +/- 1 Myr) is associated
with the NGC 346 cluster. It includes a rich component of low mass pre-main
sequence stars mainly concentrated in a number of sub-clusters, spatially co-
located with CO clumps previously detected by Rubio et al. (2000). Within our
analysis uncertainties, these sub-clusters appear coeval with each other. The
most massive stars appear concentrated in the central sub-clusters, indicating
possible mass segregation. A number of embedded clusters are also observed.
This finding, combined with the overall wealth of dust and gas, could imply
that star formation is still active. An intermediate age star cluster, BS90,
formed ~4.3 +/-0.1 Gyr ago, is also present in the region. Thus, this region of
the SMC has supported star formation with varying levels of intensity over much
of the cosmic time.Comment: 38 pages, 13 figures, 3 tables; AJ accepte
Galaxies in the Hubble Ultra Deep Field: I. Detection, Multiband Photometry, Photometric Redshifts, and Morphology
We present aperture-matched PSF-corrected BVi'z'JH photometry and Bayesian
photometric redshifts (BPZ) for objects detected in the Hubble Ultra Deep Field
(UDF), 8,042 of which are detected at the 10-sigma level (e.g., i'<29.01 or
z'<28.43). Most of our objects are defined identically to those in the public
STScI catalogs, enabling straightforward object-by-object comparison. We have
combined detections from i', z', J+H, and B+V+i'+z' images into a single
comprehensive segmentation map. Using a new program called SExSeg we are able
to force this segmentation map into SExtractor for photometric analysis. The
resulting photometry is corrected for the wider NIC3 PSFs using our ColorPro
software. We also correct for the ACS z'-band PSF halo. The NIC3 magnitudes are
found to be too faint relative to the ACS fluxes. Based on BPZ SED fits to
objects of know spectroscopic redshift, we derived corrections of -0.30 +/-
0.03 mag in J and -0.18 +/- 0.04 mag in H. The offsets appear to be supported
by a recent recalibration of the UDF NIC3 images combined with non-linearity
measured in NICMOS itself. The UDF reveals a large population of faint blue
galaxies (presumably young starbursts), bluer than those observed in the
original Hubble Deep Fields (HDF). To accommodate these galaxies, we have added
two new starburst templates to the SED library used in previous BPZ papers. The
resulting photometric redshifts are accurate to within 0.04 * 1+z_spec out to z
< 6. Finally, we measure galaxy morphology, including Sersic index and
asymmetry. Our full catalog, software packages, and more are available at
http://adcam.pha.jhu.edu/~coe/UDF/ (abridged)Comment: Accepted by AJ. 82 pages, 35 figures, 10 tables. At
http://adcam.pha.jhu.edu/~coe/UDF/ you can find: full-resolution versions of
the paper (color; B&W compact); our full catalogs and segmentation map; our
SExSeg & ColorPro software packages; a clickable color image map of the UD
Pre-main sequence stars in the stellar association N11 in the Large Magellanic Cloud
Magellanic Clouds are of extreme importance to the study of the star
formation process in low metallicity environments. In this paper we report on
the discovery of pre-main sequence candidates and young embedded stellar
objects in N11 located in the Large Magellanic Cloud to cast light on the star
formation scenario. We would like to remind that this comparison is complicated
by the presence of a large age dispersion detected in the fields. Deep archive
HST/ACS photometry is used to derive color-magnitude diagrams of the
associations in N~11 and of the foreground field population. These data are
complemented by archive IR Spitzer data which allow the detection of young
embedded stellar objects. The spatial distribution of the pre-main sequence
candidates and young embedded stellar objects is compared with literature data
observed at different wavelengths, such as H and CO maps, and with
the distribution of OB and Herbig Ae/Be stars. The degree of clustering is
derived using the Minimal Spanning Tree method and the two point correlation
function to get insights about the formation process. A large population of
pre-main sequence candidates is found in N11. Their masses are in the range of
1.3-2 MSun for ages from 2 to 10 Myr. Young embedded stellar objects having
ages of 0.1-1 Myr are found to be intermixed with the candidate pre-main
sequence stars. The spatial distribution of the stars shows that this region is
the product of clustered star formation. No significant difference is found in
the clustering degree of young blue main sequence stars and faint pre-main
sequence candidates, suggesting that they might be part of the same formation
process. The data suggest that the star formation in the region is a
long-lasting process where stars from 0.1 to 10 Myr are widely distributed.Comment: 18 pages, 21 figures, accepted for publication in A&
Discovery of Globular Clusters in the Proto-Spiral NGC2915: Implications for Hierarchical Galaxy Evolution
We have discovered three globular clusters beyond the Holmberg radius in
Hubble Space Telescope Advanced Camera for Surveys images of the gas-rich dark
matter dominated blue compact dwarf galaxy NGC2915. The clusters, all of which
start to resolve into stars, have M_{V606} = -8.9 to -9.8 mag, significantly
brighter than the peak of the luminosity function of Milky Way globular
clusters. Their colors suggest a metallicity [Fe/H] ~ -1.9 dex, typical of
metal-poor Galactic globular clusters. The specific frequency of clusters is at
a minimum normal, compared to spiral galaxies. However, since only a small
portion of the system has been surveyed it is more likely that the luminosity
and mass normalized cluster content is higher, like that seen in elliptical
galaxies and galaxy clusters. This suggests that NGC2915 resembles a key phase
in the early hierarchical assembly of galaxies - the epoch when much of the old
stellar population has formed, but little of the stellar disk. Depending on the
subsequent interaction history, such systems could go on to build-up larger
elliptical galaxies, evolve into normal spirals, or in rare circumstances
remain suspended in their development to become systems like NGC2915.Comment: ApJ Letters accepted; 6 pages, 2 figures, 3 table
MCAO near-IR photometry of the Globular Cluster NGC 6388: MAD observations in crowded fields
Deep photometry of crowded fields, such as Galactic Globular Clusters, is
severely limited by the actual resolution of ground-based telescopes. On the
other hand, the Hubble Space Telescope does not provide the near-infrared (NIR)
filters needed to allow large color baselines. In this work we aim at
demonstrating how ground based observations can reach the required resolution
when using Multi-Conjugated Adaptive Optic (MCAO) devices in the NIR, such as
the experimental infrared camera (MAD) available on the VLT. This is
particularly important since these corrections are planned to be available on
all ground--based telescopes in the near future. We do this by combining the
infrared photometry obtained by MAD/VLT with ACS/HST optical photometry of our
scientific target, the bulge globular cluster NGC 6388, in which we imaged two
fields. In particular, we constructed color-magnitude diagrams with an
extremely wide color baseline in order to investigate the presence of multiple
stellar populations in this cluster. From the analysis of the external field,
observed with better seeing conditions, we derived the deepest optical-NIR CMD
of NGC 6388 to date. The high-precision photometry reveals that two distinct
sub-giant branches are clearly present in this cluster. We also use the CMD
from the central region to estimate the distance ((m-M)=15.33) and the
reddening (E(B-V)=0.38) for this cluster. We estimate the age to be ~11.5+/-
1.5 Gyr. The large relative-age error reflects the bimodal distribution of the
SGB stars. This study clearly demonstrates how MCAO correction in the NIR bands
implemented on ground based telescopes can complement the high-resolution
optical data from HST.Comment: 9 pages, 10 figures. Accepted for publication on A. &
Advanced Camera for Surveys Observations of Young Star Clusters in the Interacting Galaxy UGC 10214
We present the first Advanced Camera for Surveys (ACS) observations of young
star clusters in the colliding/merging galaxy UGC 10214. The observations were
made as part of the Early Release Observation (ERO) program for the newly
installed ACS during service mission SM3B for the Hubble Space Telescope (HST).
Many young star clusters can be identified in the tails of UGC 10214, with ages
ranging from ~3 Myr to 10 Myr. The extreme blue V-I (F606W-F814W) colors of the
star clusters found in the tail of UGC 10214 can only be explained if strong
emission lines are included with a young stellar population. This has been
confirmed by our Keck spectroscopy of some of these bright blue stellar knots.
The most luminous and largest of these blue knots has an absolute magnitude of
M_V = -14.45, with a half-light radius of 161 pc, and if it is a single star
cluster, would qualify as a super star cluster (SSC). Alternatively, it could
be a superposition of multiple scaled OB associations or clusters. With an
estimated age of ~ 4-5 Myr, its derived mass is < 1.3 x 10^6 solar masses. Thus
the young stellar knot is unbound and will not evolve into a normal globular
cluster. The bright blue clusters and associations are much younger than the
dynamical age of the tail, providing strong evidence that star formation occurs
in the tail long after it was ejected. UGC 10214 provides a nearby example of
processes that contributed to the formation of halos and intra-cluster media in
the distant and younger Universe.Comment: 6 pages with embedded figures, ApJ in pres
Internal Color Properties of Resolved Spheroids in the Deep HST/ACS field of UGC 10214
(Abridged) We study the internal color properties of a morphologically
selected sample of spheroidal galaxies taken from HST/ACS ERO program of UGC
10214 (``The Tadpole''). By taking advantage of the unprecedented high
resolution of the ACS in this very deep dataset we are able to characterize
spheroids at sub-arcseconds scales. Using the V_606W and I_814W bands, we
construct V-I color maps and extract color gradients for a sample of spheroids
at I_814W < 24 mag. We investigate the existence of a population of
morphologically classified spheroids which show extreme variation in their
internal color properties similar to the ones reported in the HDFs. These are
displayed as blue cores and inverse color gradients with respect to those
accounted from metallicity variations. Following the same analysis we find a
similar fraction of early-type systems (~30%-40%) that show non-homologous
internal colors, suggestive of recent star formation activity. We present two
statistics to quantify the internal color variation in galaxies and for tracing
blue cores, from which we estimate the fraction of non-homogeneous to
homogeneous internal colors as a function of redshift up to z<1.2. We find that
it can be described as about constant as a function of redshift, with a small
increase with redshift for the fraction of spheroids that present strong color
dispersions. The implications of a constant fraction at all redshifts suggests
the existence of a relatively permanent population of evolving spheroids up to
z~1. We discuss the implications of this in the context of spheroidal
formation.Comment: Fixed URL for high resolution version. 13 Pages, 10 Figures. Accepted
for Publication in ApJ. Sep 1st issue. Higher resolution version and complete
table3B at http://acs.pha.jhu.edu/~felipe/e-prints/Tadpol
The Luminosity Function of Early-Type Galaxies at z~0.75
We measure the luminosity function of morphologically selected E/S0 galaxies
from to using deep high resolution Advanced Camera for Surveys
imaging data. Our analysis covers an area of 48\Box\arcmin (8 the
area of the HDF-N) and extends 2 magnitudes deeper ( mag) than was
possible in the Deep Groth Strip Survey (DGSS). At , we find
and , and at
, we find . These luminosity
functions are similar in both shape and number density to the luminosity
function using morphological selection (e.g., DGSS), but are much steeper than
the luminosity functions of samples selected using morphological proxies like
the color or spectral energy distribution (e.g., CFRS, CADIS, or COMBO-17). The
difference is due to the `blue', , E/S0 galaxies, which make up to
of the sample at all magnitudes and an increasing proportion of faint
galaxies. We thereby demonstrate the need for {\it both morphological and
structural information} to constrain the evolution of galaxies.
We find that the `blue' E/S0 galaxies have the same average sizes and Sersic
parameters as the `red', , E/S0 galaxies at brighter luminosities
(), but are increasingly different at fainter magnitudes where
`blue' galaxies are both smaller and have lower Sersic parameters. Fits of the
colors to stellar population models suggest that most E/S0 galaxies have short
star-formation time scales ( Gyr), and that galaxies have formed at an
increasing rate from until after which there has been a
gradual decline.Comment: 39 pages, 21 figures, accepted in A
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