89 research outputs found
The Integration and Test Program of the James Webb Space Telescope
With the delivery of its flight scientific instruments and the completion of all telescope optics both scheduled to occur later this year, the James Webb Space Telescope (JWST) will enter into a challenging integration and test (I&T) program. Highlights of that program include cryo-vacuum tests of the Integrated Science Instrument Module (ISIM) at Goddard Space Flight Center, ambient integration of the ISIM and the Optical Telescope Element (OTE) at Goddard, and an end-to-end cryo-vacuum test of the OTE + ISIM system at Johnson Space Center. We review the overall flow of the I&T program, highlighting the key activities and the critical verifications to be performed at each step
Far-UV Emission from Elliptical Galaxies at z=0.55
The restframe UV-to-optical flux ratio, characterizing the ``UV upturn''
phenomenon, is potentially the most sensitive tracer of age in elliptical
galaxies; models predict that it may change by orders of magnitude over the
course of a few Gyr. In order to trace the evolution of the UV upturn as a
function of redshift, we have used the far-UV camera on the Space Telescope
Imaging Spectrograph to image the galaxy cluster CL0016+16 at z=0.55. Our
25''x25'' field includes four bright elliptical galaxies, spectroscopically
confirmed to be passively evolving cluster members. The weak UV emission from
the galaxies in our image demonstrates that the UV upturn is weaker at a
lookback time 5.6 Gyr earlier than our own, as compared to measurements of the
UV upturn in cluster E and S0 galaxies at z=0 and z=0.375. These images are the
first with sufficient depth to demonstrate the fading of the UV upturn expected
at moderate redshifts. We discuss these observations and the implications for
the formation history of galaxies.Comment: 4 pages, Latex. 2 figures. Uses corrected version of emulateapj.sty
and apjfonts.sty (included). Accepted for publication in ApJ Letter
The Dearth of UV-bright Stars in M32: Implications for Stellar Evolution Theory
Using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, we have obtained deep far ultraviolet images of the compact elliptical galaxy M32. When combined with earlier near-ultraviolet images of the same field, these data enable the construction of an ultraviolet color-magnitude diagram of the hot horizontal branch (HB) population and other hot stars in late phases of stellar evolution. We find few post-asymptotic giant branch (PAGB) stars in the galaxy, implying that these stars either cross the HR diagram more rapidly than expected, and/or that they spend a significant fraction of their time enshrouded in circumstellar material. The predicted luminosity gap between the hot HB and its AGB-Manque (AGBM) progeny is less pronounced than expected, especially when compared to evolutionary tracks with enhanced helium abundances, implying that the presence of hot HB stars in this metal-rich population is not due to (Delta)Y/(Delta)Z greater than or approx. 4. Only a small fraction (approx. 2%) of the HB population is hot enough to produce significant UV emission, yet most of the W emission in this galaxy comes from the hot HB and AGBM stars, implying that PAGB stars are not a significant source of W emission even in those elliptical galaxies with a weak W excess. Subject headings: galaxies: evolution - galaxies: stellar content - galaxies: individual (M32) - stars: evolution - stars: horizontal branc
Empirical Corrections for Charge Transfer Inefficiency and Associated Centroid Shifts for STIS CCD Observations
A variety of on-orbit imaging and spectroscopic observations are used to
characterize the Charge Transfer Efficiency (CTE) of the Charge-Coupled Device
(CCD) of the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble
Space Telescope. A set of formulae is presented to correct observations of
point sources for CTE-related loss of signal. For data taken in imaging mode,
the CTE loss is parametrized in terms of the location of the source on the CCD,
the source signal level within the measurement aperture, the background level,
and the time of observation. For spectroscopic data, it is found that one
additional parameter is needed to provide an adequate calibration of the CTE
loss, namely the signal in the point spread function located between the signal
extraction box and the read-out amplifier. The effect of the latter parameter
is significant for spectra taken using the G750L or G750M gratings of STIS. The
algorithms presented here correct flux calibration inaccuracies due to CTE
losses as large as 30% to within ~ 1.5% RMS throughout the wavelength range
covered by the STIS CCD modes. This uncertainty is similar to the Poisson noise
associated with a source detected at a signal level of about 2500 electrons per
resolution element. Using bi-directional CCD readouts, centroid shifts incurred
due to CTE loss are also derived. A tight correlation is found between the CTE
loss and the centroid shift (both for imaging and spectroscopic modes), thus
enabling one to correct for both effects of imperfect charge transfer to STIS
CCD observations.Comment: 49 pages in AASTeX preprint format. 18 figures, 8 table
Far-Ultraviolet Emission from Elliptical Galaxies at z=0.33
We present far-ultraviolet (far-UV) images of the rich galaxy cluster
ZwCl1358.1+6245, taken with the Space Telescope Imaging Spectrograph on board
the Hubble Space Telescope (HST). When combined with archival HST observations,
our data provide a measurement of the UV-to-optical flux ratio in 8 early-type
galaxies at z=0.33. Because the UV flux originates in a population of evolved,
hot, horizontal branch (HB) stars, this ratio is potentially one of the most
sensitive tracers of age in old populations -- it is expected to fade rapidly
with lookback time. We find that the UV emission in these galaxies, at a
lookback time of 3.9 Gyr, is significantly weaker than it is in the current
epoch, yet similar to that in galaxies at a lookback time of 5.6 Gyr. Taken at
face value, these measurements imply different formation epochs for the massive
ellipticals in these clusters, but an alternative explanation is a "floor" in
the UV emission due to a dispersion in the parameters that govern HB
morphology.Comment: 4 pages, Latex. 2 figures. Uses corrected version of emulateapj.sty
and apjfonts.sty (included). Accepted for publication in ApJ Letter
Measurements of the Diffuse Ultraviolet Background and the Terrestrial Airglow with the Space Telescope Imaging Spectrograph
Far-UV observations in and near the Hubble Deep Fields demonstrate that the
Space Telescope Imaging Spectrograph (STIS) can potentially obtain unique and
precise measurements of the diffuse far-ultraviolet background. Although STIS
is not the ideal instrument for such measurements, high-resolution images allow
Galactic and extragalactic objects to be masked to very faint magnitudes, thus
ensuring a measurement of the truly diffuse UV signal. The programs we have
analyzed were not designed for this scientific purpose, but would be sufficient
to obtain a very sensitive measurement if it were not for a weak but
larger-than-expected signal from airglow in the STIS 1450-1900 A bandpass. Our
analysis shows that STIS far-UV crystal quartz observations taken near the limb
during orbital day can detect a faint airglow signal, most likely from NI\1493,
that is comparable to the dark rate and inseparable from the far-UV background.
Discarding all but the night data from these datasets gives a diffuse
far-ultraviolet background measurement of 501 +/- 103 ph/cm2/sec/ster/A, along
a line of sight with very low Galactic neutral hydrogen column (N_HI = 1.5E20
cm-2) and extinction (E(B-V)=0.01 mag). This result is in good agreement with
earlier measurements of the far-UV background, and should not include any
significant contribution from airglow. We present our findings as a warning to
other groups who may use the STIS far-UV camera to observe faint extended
targets, and to demonstrate how this measurement may be properly obtained with
STIS.Comment: 7 pages, Latex. 4 figures. Uses corrected version of emulateapj.sty
and apjfonts.sty (included). Accepted for publication in A
Age Constraints for an M31 Globular Cluster from Main Sequence Photometry
We present a color-magnitude diagram (CMD) of the globular cluster SKHB-312
in the Andromeda galaxy (M31), obtained with the Advanced Camera for Surveys on
the Hubble Space Telescope. The cluster was included in deep observations taken
to measure the star formation history of the M31 halo. Overcoming a very
crowded field, our photometry of SKHB-312 reaches V ~ 30.5 mag, more than 1 mag
below the main sequence turnoff. These are the first observations to allow a
direct age estimate from the turnoff in an old M31 cluster. We analyze its CMD
and luminosity function using a finely-spaced grid of isochrones that have been
calibrated using observations of Galactic clusters taken with the same camera
and filters. The luminosity difference between the subgiant and horizontal
branches is ~0.2 mag smaller in SKHB-312 than in the Galactic clusters 47 Tuc
and NGC 5927, implying SKHB-312 is 2-3 Gyr younger. A quantitative comparison
to isochrones yields an age of 10 +2.5/-1 Gyr
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