Robert H. Cornett

The On-Orbit Performance of the Space Telescope Imaging Spectrograph

The Space Telescope Imaging Spectrograph (STIS) was successfully installed into the Hubble Space Telescope (HST) in 1997 February, during the second HST servicing mission, STS-82. STIS is a versatile spectrograph, covering the 115-1000 nm wavelength range in a variety of spectroscopic and imaging modes that take advantage of the angular resolution, unobstructed wavelength coverage, and dark sky offered by the HST. In the months since launch, a number of performance tests and calibrations have been carried out and are continuing. These tests demonstrate that the instrument is performing very well. We present here a synopsis of the results to date.

Comparing Galaxy Morphology at Ultraviolet and Optical Wavelengths

We have undertaken an imaging survey of 34 nearby galaxies in far-ultraviolet (FUV, ~1500 A) and optical (UBVRI) passbands to characterize galaxy morphology as a function of wavelength. This sample, which includes a range of classical Hubble types from elliptical to irregular, with emphasis on spirals at low inclination angle, provides a valuable database for comparison with images of high-z galaxies whose FUV light is redshifted into the optical and near-infrared bands. Ultraviolet data are from the Ultraviolet Imaging Telescope (UIT) Astro-2 mission. We present images and surface brightness profiles for each galaxy, and we discuss the wavelength dependence of morphology for different Hubble types in the context of understanding high-z objects. In general, the dominance of young stars in the FUV produces the patchy appearance of a morphological type later than that inferred from optical images. Prominent rings and circumnuclear star formation regions are clearly evident in FUV images of spirals, while bulges, bars, and old, red stellar disks are faint to invisible at these short wavelengths. However, the magnitude of the change in apparent morphology ranges from dramatic in early-type spirals with prominent optical bulges to slight in late-type spirals and irregulars, in which young stars dominate both the UV and optical emission. Starburst galaxies with centrally concentrated, symmetric bursts display an apparent E/S0 structure in the FUV, while starbursts associated with rings or mergers produce a peculiar morphology. We briefly discuss the inadequacy of the optically defined Hubble sequence in describing FUV galaxy images and estimating morphological k-corrections, and we suggest some directions for future research with this data set.

THE ULTRAVIOLET IMAGING TELESCOPE: INSTRUMENT AND DATA CHARACTERISTICS

The Ultraviolet Imaging Telescope (\UIT) was flown as part of the \AstroMiss\ observatory on the Space Shuttle Columbia in December 1990 and again on the Space Shuttle Endeavor in March 1995. Ultraviolet (1200-3300A) images of a variety of astronomical objects, with a 40\arcmin\ field of view and a resolution of about 3\arcsec, were recorded on photographic film. The data recorded during the first flight is available to the astronomical community through the National Space Science Data Center (NSSDC); the data recorded during the second flight will soon be available as well. This paper discusses in detail the design, operation, data reduction, and calibration of \UIT, providing the user of the data with information for understanding and using the data. It also provides guidelines for analyzing other astronomical imagery made with image intensifiers and photographic film.

Ultraviolet Imaging Telescope Observations of the Magellanic Clouds

We present wide-field far-ultraviolet (FUV; 1300–1800 A) images of the Large and Small Magellanic Clouds (LMC, SMC). These data were obtained by the Ultraviolet Imaging Telescope (UIT) during the Astro-1 (1990 December 1–10) and Astro-2 (1995 March 2–18) missions; the images provide an extensive FUV mosaic of the SMC and contain numerous regions in the LMC, covering a wide range of stellar densities and current star formation activity. A total of 47 LMC/Lucke-Hodge and 37 SMC/Hodge OB associations are completely or partially included in the observed fields. FUV data can identify the hottest OB stars more easily than can optical photometry, and these stars dominate the ionizing flux, which is correlated to the observed Hα flux of the associated H II regions. Of the H II regions in the catalog of Davies, Elliott, & Meaburn (DEM), the UIT fields completely or partially include 102 DEM regions in the LMC and 74 DEM regions in the SMC. We present a catalog of FUV magnitudes derived from point-spread function photometry for 37,333 stars in the LMC (the UIT FUV magnitudes for 11,306 stars in the SMC were presented recently by Cornett et al.), with a completeness limit of mUV ≈ 15 mag and a detection limit of mUV ≈ 17.5. The average uncertainty in the photometry is ~0.1 mag. The full catalog with astrometric positions, photometry, and other information is also available from publicly accessible astronomical data archives. We divide the catalog into field stars and stars that are in DEM regions. We analyze each of these two sets of stars independently, comparing the composite UV luminosity function of our data with UV magnitudes derived from stellar evolution and atmosphere models in order to derive the underlying stellar formation parameters. We find a most probable initial mass function (IMF) slope for the LMC field stars of Γ = -1.80 ± 0.09. The statistical significance of this single slope for the LMC field stars is extremely high, though we also find some evidence for a field star IMF slope of Γ ~ -1.4, roughly equal to the Salpeter slope. However, in the case of the stars in the DEM regions (the stars in all the regions were analyzed together as a single group), we find three IMF slopes of roughly equal likelihood: Γ = -1.0, -1.6, and -2.0. No typical age for the field stars is found in our data for time periods up to a continuous star formation age of 500 Myr, which is the maximum age consistent with the completeness limit magnitude of the catalogs luminosity function. The best age for the collection of cluster stars was found to be t0 = 3.4 ± 1.9 Myr; this is consistent with the age expected for a collection of OB stars from many different clusters.

Vacuum ultraviolet images of the Large Magellanic Cloud

Linearized, absolutely calibrated VUV images of the LMC with a resolution of about 50 arcsec are presented. The images were made by a sounding rocket payload in two bandpasses with effective wavelengths for hot stars near 1500 A and 1930 A. The flux in each bandpass is measured for the associations in the list of Lucke and Hodge (1970). The results are discussed and their relationship to the overall characteristics of star formation in the LMC are discussed. A simple model for propagating star formation in the LMC is presented whose results closely resemble the distribution of associations revealed by the VUV images. 63 references.

The Ultraviolet Imaging Telescope - Design and performance

The instrumental configuration, calibration, and operations during the first flight of the Ultraviolet Imaging Telescope on the Astro-1 mission, December 2-10, 1990, are described. The UV images of a wide variety of astronomical objects were recorded with a 40-arcmin diameter field of view. Images of targets as faint as magnitude 21 (UV) were secured with a resolution of about 3 arcsec. The optics, light baffling, and image motion compensation system are summarized, and detectors and electronic subsystems are described.

Ultraviolet imaging of old population in nearby galaxies

We analyze UV imagery of two Sb bulges and two E galaxies obtained with the Ultraviolet Imaging Telescope during the Astro-1 mission. The UV brightness of these systems is not produced by recent massive star formation. Instead it must originate in relatively low-luminosity objects, probably low-mass, post-giant-branch stars. We find extended, large-amplitude UV color gradients, which are probably related to abundance gradients within the galaxies. M32 has a color gradient opposite to the other three objects, possibly because of an intermediate-age population.

Ultraviolet Colors and Extinctions of H II Regions in the Whirlpool Galaxy (M51)

Far-UV (wavelength 1520 A), U, Hα, and R images of the interacting Sbc spiral galaxy M51 were obtained by the Ultraviolet Imaging Telescope (UIT) during the Astro-2 Spacelab mission of 1995 March and at Mount Laguna Observatory. The μ152 - μU radial gradient of over 1 mag, becoming bluer with increasing radius, is attributed primarily to a corresponding radial extinction gradient. Magnitudes in both UV bands and Hα fluxes are reported for 28 H II regions. Optical extinctions for the 28 corresponding UV sources are computed from the measured m152 - U colors by fitting to the optical extinctions of Nakai & Kuno. The estimated normalized far-UV extinction A152/E(B-V) increases with increasing Galactocentric distance (decreasing metallicity), from 5.99 to 6.54, compared with the Galactic value 8.33. The best-fit m152 - U color for no extinction, -3.07, is the color of a model solar metallicity starburst of age ~2.5 Myr with IMF slope -1.0. H II regions show decreasing observed Hα fluxes with decreasing radius, relative to the Hα fluxes predicted from the observed f152 for age 2.5 Myr, after the Hα and f152 are corrected for extinction. We attribute the increasing fraction of missing Hα flux with decreasing radius to increasing extinction in the Lyman continuum. The increasing extinction-corrected far-UV flux of the H II regions with decreasing distance to the nucleus is probably a result of the corresponding increasing column density of the interstellar gas resulting in larger mass OB associations. The estimated dust-absorbed Lyman continuum energy flux is ~0.6 times the far-infrared energy flux of M51 observed by IRAS.

Images in the rocket ultraviolet - The starburst in the nucleus of M83

Ultraviolet images of the SAB(s)c I-II galaxy M83 (NGC 5236) obtained with a rocketborne telescope in broad bandpasses centered at 1540 A and 2360 A show a bright resolved nuclear source which accounts for approximately 20 percent of the flux of the galaxy in both bandpasses. Low-resolution International Ultraviolet Explorer spectra of this source reveal an energy distribution similar to that of the starburst nucleus of NGC 7714. Strong blueshifted absorption lines can be interpreted as evidence for a nuclear wind powered by supernovae. Observations from UV, X-ray, optical, and far-infrared bandpasses are consistent with a starburst approximately one-sixth as strong as that in M82. A scaling of the M82 models of Rieke et al. (1980) predicts that the nucleus of M83 contains 10 to the 6th - 3 x 10 to the 7th solar masses in young stars and has a supernova rate of approximately 0.01 per yr.

Simulated optical images of galaxies at z about 1 using ultraviolet images of nearby galaxies

Ultraviolet sounding rocket images of several nearby galaxies are used to simulate the appearance in optical bandpasses of similar systems at redshifts z = 0.5-2.67, as observed by the WFPC on HST and by ground-based instruments. Since the morphology of galaxies is a strong function of the wavelength, the appearance of galaxies at large redshifts is subjected to a large k-correction effect. The strong dependence of monochromatic surface brightness on redshift also implies that observed morphology of distant systems will be crucially dependent on the limiting surface brightness set by the sky background. Although the angle subtended by sources depends only weakly on z, the fraction of a galaxy whose surface brightness is above the detection threshold varies strongly with z. Morphological distinctions of spiral from elliptical, spiral from irregular, and barred from unbarred types become more difficult, as does accurate evaluation of interactions or of the local environment. 32 refs.