Michael Nicholas Fanelli

Spectral synthesis in the ultraviolet. II - Stellar populations and star formation in blue compact galaxies

An initial attempt to apply optimizing spectral synthesis techniques to the far-UV spectra of blue compact galaxies (BCGs) is presented. The far-UV absorption-line spectra of the galaxies are clearly composite, with the signatures of the main-sequence types between O3 and mid-A. Most of the low-ionization absorption lines have a stellar origin. The Si IV and C IV features in several objects have P Cygni profiles. In Haro I the strength of Si IV indicates a significant blue supergiant population. The metal-poor blue compact dwarf Mrk 209 displays weak absorption lines, evidence that the stellar component has the same low metallicity as observed in the ionized gas. Good fits to the data are obtained the technique of optimizing population synthesis. The solutions yield stellar luminosity functions which display large discontinuities, indicative of discrete star formation episodes or bursts. The amount of UV extinction is low. 97 references.

Star Formation Triggering Mechanisms in Dwarf Galaxies: The Far-Ultraviolet, Hα, and H I Morphology of Holmberg II

Far-ultraviolet (FUV), Ha, and H I observations of dwarf galaxy Holmberg II are used to investigate the means by which star formation propagates in galaxies lacking global internal triggering mechanisms such as spiral density waves. The observations trace the interaction between sites of massive star forma- tion and the neutral and ionized components of the surrounding ISM in this intrinsically simple system. Both local and large-scale triggering mechanisms related to massive star formation are seen, suggesting that feedback from massive stars is a microscopic process operating in all galaxies to a certain degree. The data emphasize the importance of local conditions in regulating star formation from evidence such as massive stars inside ionized shells, compact H II regions surrounding aging clusters, and stars formed in chains of progressing age. Surface brightness pro—les show that current activity correlates with the time-averaged level of past star formation at a given radius demonstrating a reliance on local conditions. Large-scale triggering by H I shells is supported by observations of progenitor populations as well as secondary sites of star formation associated with their dense rims. Analysis of the energy available from massive stars inside H I shells indicates that energy deposited into the ISM from supernovae and stellar winds is sufficient to account for the H I morphology. Ages of individual star-forming regions are derived using B ,H a, and FUV photometry and show both older, diUuse FUV regions and younger, compact H II regions. The distribution of ages is reconciled with the H I morphology, showing a clear preference of young regions for areas of dense H I and old regions for H I voids. Global kinematical properties may also play a role in the star formation process since diUerences in the rotation characteristics of the neutral gas disk correlate with diUerences in triggering mechanisms. Large-scale feedback from massive stars is shown to operate in regions that lack diUerential shear in the gas disk. Subject headings: galaxies: dwarfgalaxies: ISMgalaxies: individual (DDO 50) ¨ stars: formationultraviolet: galaxies

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.

Spectral synthesis in the ultraviolet. IV - A library of mean stellar groups

A library of mean UV stellar energy distributions is derived from IUE spectrophotometry of 218 stars. The spectra cover 1230-3200 A with a spectral resolution of about 6 A. They have been corrected for interstellar extinction and converted to a common flux and wavelength scale. Individual stars were combined into standard groups according to their continuum colors, observed UV spectral morphology, MK luminosity class, and metal abundance. The library consists of 56 groups: 21 dwarf(V), 8 subgiant(IV), 16 giant(III), and supergiant(I + II) groups, covering O3-M4 spectral types. A metal-poor sequence is included, containing four dwarf and two giant groups, as is a metal-enhanced sequence with a single dwarf, subgiant, and giant group. Spectral indices characterizing the continuum and several strong absorption features are examined as temperature, luminosity, and abundance diagnostics. The library is intended to serve as a basis for interpreting the composite UV spectra of a wide variety of stellar systems, e.g., elliptical galaxies, starburst systems, and high-redshift galaxies.

Far-ultraviolet Color Gradients in Early-Type Galaxies

We discuss far-UV (FUV) (1500 ?) surface photometry and FUV-B color profiles for eight E/S0 galaxies from images taken with the Ultraviolet Imaging Telescope, primarily during the Astro-2 mission. In three cases, the FUV radial profiles are more consistent with an exponential than a de Vaucouleurs function, but there is no other evidence for the presence of a disk or of young, massive stars. In all cases except M32, the FUV-B color becomes redder at larger radii. There is a wide range of internal radial FUV-B color gradients. However, we find no correlation between the FUV-B color gradients and internal metallicity gradients based on Mg absorption features. We conclude that metallicity is not the sole parameter controlling the UV upturn component in old populations.

Ultraviolet Spectral Dating of Stars and Galaxies

An echelle spectrogram (R = 30,000) of the 2300-3100 A region in the ultraviolet spectrum of the F8 V star 9 Comae is presented. The observation is used to calibrate features in the mid-ultraviolet spectra of similar stars according to age and metal content. In particular, the spectral break at 2640 A is interpreted using the spectral synthesis code SYNSPEC. We use this feature to estimate the time since the last major star formation episode in the early-type galaxy LBDS 53W091 at redshift z=1.55, whose rest-frame mid-ultraviolet spectrum, observed with the Keck Telescope, is dominated by the flux from similar stars that are at or near the main-sequence turnoff in that system (Spinrad et al.). Our result, 1 Gyr if the flux-dominating stellar population has a metallicity twice solar, or 2 Gyr for a more plausible solar metallicity, is significantly lower than the previous estimate and thereby relaxes constraints on cosmological parameters that were implied by the earlier work.

Spectral synthesis in the ultraviolet. III - The spectral morphology of normal stars in the mid-ultraviolet

The morphology of 218 mid-UV spectra of stars ranging from O through K in spectral type is examined. Several new line and continuum indices are defined and their usefulness as temperature, luminosity, and metallicity discriminants is discussed. Mid-UV stellar continua are found to be markedly affected by abundance. A UV excess, delta(2600-V), is computed which is more sensitive by a factor of 10 to (Fe/H) than is delta(U-B). The relative strength of spectral lines in the mid-UV is not as strongly affected by abundance. Mid-UV spectra are much more sensitive to the temperature of the stellar population than to either metallicity or the dwarf/giant ratio. Mg II 2800 shows unexpected behavior, displaying no sensitivity to abundance for cool stars and a reversed sensitivity in FG dwarfs such that metal-poor stars have stronger Mg II strengths at the same temperature than more metal-rich stars.

A Study of the Projected Galaxy Density around Nine Isolated Elliptical Galaxies

Using V-band imagery, we present results from a comparative density analysis of the local environments surrounding nine elliptical galaxies, with the objective of validating that these objects reside in extremely isolated environments. We have developed a technique that compares galaxy number density enhancements within well-studied clusters and loose groups with apparent clusterings of faint, predominantly uncataloged galaxies that are present in the fields of the isolated elliptical galaxy (IEG) candidates. The unique feature of this technique is the ability to combine size information of density peaks with number density values in order to place upper or lower limits on the distances of an apparent clustered structure in an IEG field. The utility of this technique is that probable physical association between an IEG and a clustered structure can be refuted without need of redshift information for the faint field sources. Of the nine IEG candidates studied, we conclude that one of the galaxies has possible membership with a loose group of Local Group density. Four other IEG candidates have environments that show density enhancements in the field that are possible loose groups with densities similar to the Leo or Dorado groups, but physical association with the IEGs is unlikely. The remaining four galaxies have environments that are confirmed to be isolated down to the detection limits that are detailed in the paper.

30 Doradus : ultraviolet and optical stellar photometry

Ultraviolet Imaging Telescope (UIT) UV magnitudes in four bands, together with optical B magnitudes, are presented for up to 314 early-type stars located in a 9.7 x 9.7 arcmin field centered on R136. The magnitudes have an rms uncertainty estimated at 0.10 mag from a comparison between the UIT magnitudes and the IUE spectra. Spectral types and E(B-V) color excesses are estimated. The mean color excesses following the two extinction curves agree well with the predictions of the two-component extinction model of Fitzpatrick and Savage (1984). However, the degree of nebular extinction is found to vary systematically by large amounts over the 30 Dor field. The minimum of nebular extinction in the central parts of the nebula suggests that dust has been expelled from this region by stellar winds. It is suggested that the form of the UV extinction curve can be understood as a consequence of the evolutionary state of the stellar population responsible for making the dust grains.