528 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
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
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
JWST/NIRSpec Observations of the Planetary Mass Companion TWA 27B
We present 1-5um spectroscopy of the young planetary mass companion TWA 27B
(2M1207B) performed with NIRSpec on board the James Webb Space Telescope. In
these data, the fundamental band of CH_4 is absent and the fundamental band of
CO is weak. The nondetection of CH_4 reinforces a previously observed trend of
weaker CH_4 with younger ages among L dwarfs, which has been attributed to
enhanced non-equilibrium chemistry among young objects. The weakness of CO may
reflect an additional atmospheric property that varies with age, such as the
temperature gradient or cloud thickness. We are able to reproduce the broad
shape of the spectrum with an ATMO cloudless model that has T=1300 K,
non-equilibrium chemistry, and a temperature gradient reduction caused by
fingering convection. However, the fundamental bands of CH_4 and CO are
somewhat stronger in the model. In addition, the model temperature of 1300 K is
higher than expected from evolutionary models given the luminosity and age of
TWA 27B (T=1200 K). Previous models of young L-type objects suggest that the
inclusion of clouds could potentially resolve these issues; it remains to be
seen whether cloudy models can provide a good fit to the 1-5um data from
NIRSpec. TWA 27B exhibits emission in Paschen transitions and the He I triplet
at 1.083um, which are signatures of accretion that provide the first evidence
of a circumstellar disk. We have used the NIRSpec data to estimate the
bolometric luminosity of TWA 27B (log L/L_sun=-4.466+/-0.014), which implies a
mass of 5-6 MJup according to evolutionary models.Comment: Astrophysical Journal Letters, in pres
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