621 research outputs found
Principal Component Analysis of the Time- and Position-Dependent Point Spread Function of the Advanced Camera for Surveys
We describe the time- and position-dependent point spread function (PSF)
variation of the Wide Field Channel (WFC) of the Advanced Camera for Surveys
(ACS) with the principal component analysis (PCA) technique. The time-dependent
change is caused by the temporal variation of the focus whereas the
position-dependent PSF variation in ACS/WFC at a given focus is mainly the
result of changes in aberrations and charge diffusion across the detector,
which appear as position-dependent changes in elongation of the astigmatic core
and blurring of the PSF, respectively. Using >400 archival images of star
cluster fields, we construct a ACS PSF library covering diverse environments of
the observations (e.g., focus values). We find that interpolation of a
small number () of principal components or ``eigen-PSFs'' per exposure
can robustly reproduce the observed variation of the ellipticity and size of
the PSF. Our primary interest in this investigation is the application of this
PSF library to precision weak-lensing analyses, where accurate knowledge of the
instrument's PSF is crucial. However, the high-fidelity of the model judged
from the nice agreement with observed PSFs suggests that the model is
potentially also useful in other applications such as crowded field stellar
photometry, galaxy profile fitting, AGN studies, etc., which similarly demand a
fair knowledge of the PSFs at objects' locations. Our PSF models, applicable to
any WFC image rectified with the Lanczos3 kernel, are publicly available.Comment: Accepted to PASP. To appear in December issue. Figures are degraded
to meet the size limit. High-resolution version can be downloaded at
http://acs.pha.jhu.edu/~mkjee/acs_psf/acspsf.pd
New Tests for Disruption Mechanisms of Star Clusters: The Large and Small Magellanic Clouds
We compare the observed bivariate distribution of masses(M) and ages(t) of
star clusters in the LMC with the predicted distributions g(M,t) from 3
idealized models for the disruption of star clusters: (1)sudden mass-dependent
disruption;(2)gradual mass-dependent disruption; and (3)gradual
mass-independent disruption. The model with mass-{\em in}dependent disruption
provides a good, first-order description of these cluster populations, with
g(M,t) propto M^{beta} t^{gamma}, beta=-1.8+/-0.2 and gamma=-0.8+/-0.2, at
least for clusters with ages t<10^9 yr and masses M<10^3 M_sol (more
specifically, t<10^7(M/10^2 M_sol)^{1.3} yr). This model predicts that the
clusters should have a power-law luminosity function, dN/dL propto L^-1.8, in
agreement with observations. The first two models, on the other hand, fare
poorly when describing the observations, refuting previous claims that
mass-dependent disruption of star clusters is observed in the LMC over the
studied M-t domain. Clusters in the SMC can be described by the same g(M,t)
distribution as for the LMC, but with smaller samples and hence larger
uncertainties. The successful g(M,t) model for clusters in the Magellanic
Clouds is virtually the same as the one for clusters in the merging Antennae
galaxies, but extends the domain of validity to lower masses and to older ages.
This indicates that the dominant disruption processes are similar in these very
different galaxies over at least t<10^8 yr and possibly t<10^9 yr. The mass
functions for young clusters in the LMC are power-laws, while that for ancient
globular clusters is peaked. We show that the observed shapes of these mass
functions are consistent with expectations from the simple evaporation model
presented by McLaughlin & Fall.Comment: 46 pages, 17 figures, published ApJ, vol 711, page 126
Do young galaxies exist in the Local Universe? - Red Giant Branch detection in the metal-poor dwarf SBS 1415+437
We present results from an HST/ACS imaging study of the metal-poor blue
compact dwarf galaxy SBS 1415+437. It has been argued previously that this is a
very young galaxy that started to form stars only less than 100 Myr ago.
However, we find that the optical color-magnitude diagram prominently reveals
asymptotic giant branch and red giant branch (RGB) stars. The brightness of the
RGB tip yields a distance of 13.6 Mpc. The color of the RGB implies that its
stars must be older than approximately 1.3 Gyr, with the exact age depending on
the assumed metallicity and dust extinction. The number of RGB stars implies
that most of the stellar mass resides in this evolved population. In view of
these and other HST results for metal-poor galaxies it seems that the local
Universe simply may not contain any galaxies that are currently undergoing
their first burst of star formation.Comment: ApJ Letters, in press. For a complete version of the paper with high
resolution figures go to http://www.stsci.edu/~marel/psdir/SBS1.ps (.ps) or
http://www.stsci.edu/~marel/pdfdir/SBS1.pdf (.pdf
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
The Inner Halo of M87: A First Direct View of the Red-Giant Population
An unusually deep (V,I) imaging dataset for the Virgo supergiant M87 with the
Hubble Space Telescope ACS successfully resolves its brightest red-giant stars,
reaching M_I(lim) = -2.5. After assessing the photometric completeness and
biasses, we use this material to estimate the metallicity distribution for the
inner halo of M87, finding that the distribution is very broad and likely to
peak near [m/H] ~ -0.4 and perhaps higher. The shape of the MDF strongly
resembles that of the inner halo for the nearby giant E galaxy NGC 5128. As a
byproduct of our study, we also obtain a preliminary measurement of the
distance to M87 with the TRGB (red-giant branch tip) method; the result is
(m-M)_0 = 31.12 +- 0.14 (d = 16.7 +- 0.9 Mpc). Averaging this result with three
other recent techniques give a weighted mean d(M87) = (16.4 +- 0.5) Mpc.Comment: In press for Astronomy and Astrophysic
The Extended Main-Sequence Turn-off Clusters of the Large Magellanic Cloud - Missing links in Globular Cluster Evolution
Recent observations of intermediate age (1 - 3 Gyr) massive star clusters in
the Large Magellanic Cloud (LMC) have revealed that the majority possess
bifurcated or extended main-sequence turn-off (EMSTO) morphologies. This effect
can be understood to arise from subsequent star formation amongst the stellar
population with age differences between constituent stars amounting to 50 - 300
Myr. Age spreads of this order are similarly invoked to explain the light
element abundance variations witnessed in ancient globular clusters. In this
paper we explore the proposition that the clusters exhibiting the EMSTO
phenomenon are a general phase in the evolution of massive clusters, one that
naturally leads to the particular chemical properties of the ancient globular
cluster population.
We show that the isolation of EMSTO clusters to intermediate ages is the
consequence of observational selection effects. In our proposed scenario, the
EMSTO phenomenon is identical to that which establishes the light element
abundance variations that are ubiquitous in the ancient globular cluster
population. Our scenario makes a strong prediction: EMSTO clusters will exhibit
abundance variations in the light elements characteristic of the ancient GC
population.Comment: ApJ accepted. 33 pages, 5 figure
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