J. Chris Blades

Lyα Absorption Around Nearby Galaxies

We have used STIS aboard HST to search for Lyman-alpha (Lya) absorption lines in the outer regions of eight nearby galaxies using background QSOs and AGN as probes. Lya lines are detected within a few hundred km/s of the systemic velocity of the galaxy in all cases. We conclude that a background line-of-sight which passes within 26-200 h-1 kpc of a foreground galaxy is likely to intercept low column density neutral hydrogen with log N(HI) >~ 13.0. The ubiquity of detections implies a covering factor of ~ 100% for low N(HI) gas around galaxies within 200 h-1 kpc. We discuss the difficulty in trying to associate individual absorption components with the selected galaxies and their neighbors, but show that by degrading our STIS data to lower resolutions, we are able to reproduce the anti-correlation of Lya equivalent width and impact parameter found at higher redshift. We also show that the equivalent width and column density of Lya complexes (when individual components are summed over ~ 1000 km/s) correlate well with a simple estimate of the volume density of galaxies brighter than M(B) = -17.5 at the same redshift as a Lya complex. We do not reject the hypothesis that the selected galaxies are directly responsible for the observed Lya lines, but our analysis indicates that absorption by clumpy intragroup gas is an equally likely explanation. (Abriged)

The z = 0.8596 damped Ly-alpha absorbing galaxy toward PKS 0454+039

We present Hubble Space Telescope (HST) and ground-based data on the Z(sub abs) = 0.8596 metal-line absorption system along the line of sight to PKS 0454+0356. The system is a moderate-redshift damped Ly-alpha system, with N(H I) = (5.7 +/- 0.3) x 10(exp 20)/sq cm as measured from the Faint Object Spectrograph (FOS) spectrum. We also present ground-based images which we use to identify the galaxy which most probably gives rise to the damped system; the most likely candidate is relatively underluminous by QSO absorber standards M(sub B) approximately -19.0 for A(sub 0) = 0.5 and H(sub 0) = 50 km/s/Mpc) and lies approximately 8.5/h kpc in projection from the QSO sight line. Ground-based measurements of Zn II, Cr II, and Fe II absorption lines from this system allow us to infer abundances of (Zn/H) = -1.1, (Cr/H) = -1.2, and (Fe/H) = -1.2 indicating overall metallicity similar to damped systems at z is greater than 2, and that the depletion of Cr and Fe onto dust grains may be even less important than in many of the high-redshift systems of comparable metallicity. Limits previously placed on the 21 cm optical depth in the z = 0.8596 system, together with our new N(H I) measurement, suggest a very high spin temperature for the H I, T(sub s) is greater than 580 K.

Space Telescope Imaging Spectrograph Slitless Observations of Small Magellanic Cloud Planetary Nebulae: A Study on Morphology, Emission-Line Intensity, and Evolution*

A sample of 27 planetary nebulae (PNs) in the Small Magellanic Cloud (SMC) have been observed with the Hubble Space Telescope Imaging Spectrograph (HST/STIS) to determine their morphology, size, and the spatial variation of the ratios of bright emission lines. The morphologies of SMC PNs are similar to those of LMC and Galactic PNs. However, only a third of the resolved SMC PNs are asymmetric, compared to half of those in the LMC. The low-metallicity environment of the SMC seems to discourage the onset of bipolarity in PNs. We measured the line intensity, average surface brightness, and photometric radius of each nebula in Hα, Hβ, [O III] λλ4959 and 5007, [N II] λλ6548 and 6584, [S II] λλ6716 and 6731, He I λ6678, and [O I] λλ6300 and 6363. We show that the surface brightness-to-radius relationship is the same as in LMC PNs, indicating its possible use as a distance scale indicator for Galactic PNs. We determine the electron densities and the ionized masses of the nebulae where the [S II] lines were measured accurately, and we find that the SMC PNs are denser than the LMC PNs by a factor of 1.5. The average ionized mass of the SMC PNs is 0.3 M☉. We also found that the median [O III]/Hβ intensity ratio in the SMC is about half that of the corresponding LMC median. We use CLOUDY to model the dependence of the [O III]/Hβ ratio on the oxygen abundance. Our models encompass very well the average observed physical quantities. We suggest that the SMC PNs are principally cooled by the carbon lines, making it hard to study their excitation based on the optical lines at our disposal.

An HST study of galactic inerstellar zinc and chromium

We present a survey of interstellar Zn II and Cr II absorption extracted from the Hubble Space Telescope Goddard High Resolution Spectrograph (HST GHRS) data archive. We find clear evidence for an enhanced depletion of Zn from the gas phase with increasing fractional abundance of molecular hydrogen f(H2). Our lower limit to the Galactic interstellar metallicity is approximately 65% of the solar value as determined by the measured Zn abundances in the lowest f(H2) sightlines, (N(Zn)/N(H(sup 0)(sub tot)) = -0.19 +/- 0.04. The correspondingly high depletion of Cr with respect to solar (N(Cr/N(H(sup 0)(sub tot)) = -1.44 +/- 0.26 indicates that there are significant amounts of dust present in these lines of sight. The Galactic abundances of Zn and Cr in the ISM provide a fundamental reference point which is used to understand the metal enrichment and dust formation history of damped Lyman alpha QSO absorption-line systems, normally believed to arise from intervening precursors to modern disk galaxies. Although the spread in Zn abundances is large for both the local ISM and in damped Lyman alpha systems, we still find a substantial difference (factor of 4-10) in metallicity between the two sets. This survey and future observations of more distant objects which probe the full extent of the Milky Way halo provide a more complete picture of the enrichment and depletion characteristics of present-day galaxies.

Morphology and Evolution of the Large Magellanic Cloud Planetary Nebulae

The LMC is ideal for studying the coevolution of planetary nebulae (PNs) and their central stars in that the debilitating uncertainties of the Galactic PN distance scale and selection biases from attenuation by interstellar dust do not apply. We present images and analyze slitless spectra that were obtained in a survey of Large Magellanic Cloud PNs. These data on 29 targets were obtained with the Hubble Space T elescope (HST ) using the Space Telescope Imaging Spectrograph. The data permit us to determine the nebular dimensions and morphology in the monochromatic light of several emission lines, including those that have traditionally been used for morphological studies in the Galaxy : Ha ,[ NII] j6583, and [O III] j5007, plus others of varying ionization including [O I], He I, and [S II]. Together with the 31 resolved LMC PNs for which monochromatic images exist in the HST archive, these data show that the incidence of nonsymmetric nebulae, including bipolar nebulae (which is an indicator of Population I ancestry in the Galaxy), is signiÐcantly higher than that reported for the Galaxy. The onset of asymmetric features appears even in very young nebulae (with dynamical ages of D1400 yr), suggesting that at least the gross features of the nebular morphology may be more closely tied to PN formation and that subsequent shaping of the expanding envelope by the radiation Ðeld and wind from the central star may play the lesser role of amplifying these gross features. There is some evidence of evolution between two morphological types in the sense that bipolar core nebulae may evolve to pure bipolars late in the PN lifetime. Subject headings : Magellanic Clouds E planetary nebulae : general E stars : evolution

Interstellar and Circumstellar Optical and Ultraviolet Lines Toward SN 1998S

We have observed SN 1998S which exploded in NGC 3877, with the Utrecht Echelle Spectrograph (6-7 km s-1 FWHM) at the William Herschel Telescope and with the E230M echelle of the Space Telescope Imaging Spectrograph (8 km s-1 FWHM) aboard the Hubble Space Telescope. Both data sets were obtained at two epochs, separated by 19 (optical) and 7 days (UV data). From our own Galaxy we detect interstellar absorption lines of Ca II K, Fe II λλ2600, 2586, 2374, 2344, Mg I λ2852, and probably Mn II λ2576, at vLSR = -95 km s-1 arising from the outer edge of the High Velocity Cloud Complex M. We derive gas-phase abundances of [Fe/H] = -1.4 and [Mn/H] = -1.0, values which are very similar to warm disk clouds found in the local ISM. This is the first detection of manganese from a Galactic HVC, and we believe that the derived gas-phase abundances argue against the HVC material having an extragalactic origin. At the velocity of NGC 3877 we detect interstellar Mg I λ2852, Mn II λλ2576, 2594, 2606, Ca II K and Na I D2, D1 absorption lines, spanning a velocity range of -102 to +9 km s-1 from the systemic velocity of the galaxy (910 km s-1). Surprisingly, the component at -102 km s-1 is seen to increase by a factor of 1 dex in N(Na I) between 1998 March 20 and April 8, and in N(Mg I) between 1998 April 4 and April 11. Unusually, our data also show narrow Balmer, He I, and metastable UV Fe II P Cygni profiles, with a narrow absorption component superimposed on the bottom of the profiles absorption trough. Both the broad and narrow components of the optical lines are seen to increase substantially in strength between March 20 and April 8. The broad absorption covers ~350 km s-1 and is seen in Mg II λλ2796, 2803 absorption as well, although there is no evidence of narrow Mg II emission forming a P Cygni profile. There is some suggestion that this shelf has decreased in strength over 7 days between April 4 and April 11. Most of the low-ionization absorption can be understood in terms of gas corotating with the disk of NGC 3877, provided the supernova is at the back of the disk as we observe it, and the H I disk is of a similar thickness to our own Galaxy. However, the variable component seen in all the other lines, and the accompanying emission which forms the classic P Cygni profiles, most likely arise in slow-moving circumstellar outflows originating from the red supergiant progenitor of SN 1998S.

Optical Slitless Spectroscopy of Large Magellanic Cloud Planetary Nebulae: A Study of the Emission Lines and Morphology*

Hubble Space Telescope Space Telescope Imaging Spectrograph slitless spectroscopy of LMC planetary nebulae (PNs) is the ideal tool to study their morphology and their ionization structures at once. We present the results from a group of 29 PNs that have been spatially resolved, for the first time, in all the major optical lines. Images in the light of Hα, [N II], and [O III] are presented, together with line intensities, measured from the extracted one- and two-dimensional spectra. A study on the surface brightness in the different optical lines, the electron densities, the ionized masses, the excitation classes, and the extinction follows, illustrating an ideal consistence with the previous results found by us on LMC PNs. In particular, we find the surface brightness decline with the photometric radius to be the same in most emission lines. We find that asymmetric PNs form a well-defined cooling sequence in the excitation-surface brightness plane, confirming their different origin and larger progenitor mass.

Large Magellanic Cloud Planetary Nebula Morphology: Probing Stellar Populations and Evolution*

Planetary nebulae (PNe) in the Large Magellanic Cloud (LMC) offer the unique opportunity to study both the population and evolution of low- and intermediate-mass stars, by means of the morphological type of the nebula. Using observations from our LMC PN morphological survey, and including images available in the Hubble Space Telescope Data Archive and published chemical abundances, we find that asymmetry in PNe is strongly correlated with a younger stellar population, as indicated by the abundance of elements that are unaltered by stellar evolution (Ne, Ar, and S). While similar results have been obtained for Galactic PNe, this is the first demonstration of the relationship for extragalactic PNe. We also examine the relation between morphology and abundance of the products of stellar evolution. We found that asymmetric PNe have higher nitrogen and lower carbon abundances than symmetric PNe. Our two main results are broadly consistent with the predictions of stellar evolution if the progenitors of asymmetric PNe have on average larger masses than the progenitors of symmetric PNe. The results bear on the question of formation mechanisms for asymmetric PNe-specifically, that the genesis of PNe structure should relate strongly to the population type, and by inference the mass, of the progenitor star and less strongly on whether the central star is a member of a close binary system.

The Absence of Diffuse Gas around the Dwarf Spheroidal Galaxy Leo I

We have obtained spectra of three QSO/AGNs with the GHRS aboard the Hubble Space Telescope to search for absorption from low column density gas in the halo of the dwarf spheroidal (dSph) galaxy Leo I. The probe sight lines pass 2.1, 3.7, and 8.1 kpc from the center of the galaxy, but no C IV, Si II, or Si IV absorption is found at the velocity of Leo I. The absence of low-ionization species suggests that the column density of neutral hydrogen that exists within 2-4 kpc of the galaxy is N(H I) 1017 cm-2; assuming that the high-ionization lines of Si IV and C IV dominate the ionization fraction of silicon and carbon, the limit to the total hydrogen column is N(H) 1018 cm-2. Our results demonstrate that there are no dense flows of gas in or out of Leo I and that there is no evidence for tidally disrupted gas that might have accompanied the galaxys formation or evolution. However, our detection limits are insufficient to rule out the existence of a sphere or shell of ionized gas around the dSph, with a mass up to that constituting the entire galaxy. Our models show that dSph galaxies similar to Leo I are not massive enough to have halos that can contribute significantly to the metal line absorption cross section of QSO absorbers seen at high redshift.

Ultra-High-Resolution Observations of Interstellar Na I and Ca II K toward the High Galactic Latitude Star HD 28497

We present very high resolution (0.32 km s-1) spectra of interstellar Na I D1, D2, and Ca II K absorption toward HD 28497 obtained with the Ultra-High-Resolution Facility at the 3.9 m Anglo-Australian Telescope. The star is located in projection in a highly disturbed interstellar region close to a number of identified features including the high galactic latitude molecular cloud MBM 20, the large Orion-Eridanus shell, seen in Hα and H I 21 cm maps, and a filamentary loop structure between vLSR = -12 and -4 km s-1 in the Berkeley H I 21 cm survey and visible on the IRAS 100 μm map. Toward HD 28497 we detect 13 absorption components in the Na I spectra, to a column density limit of 2 × 1010 cm-2, and 10 in Ca II K over a velocity range of ~70 km s-1. Four absorption components in the Na I spectra show s-resolved hyperfine structure with b-values from 0.31 to 0.40 km s-1 and column densities from 4.0 to 14 × 1010 cm-2. If we assume the clouds represented by these components have no internal turbulent velocities, their temperatures would range between 134 and 227 K. One of these hyperfine split (hfs) components, at vLSR = -11.1 km s-1, shows significant temporal variation in equivalent width compared to earlier (1977) observations, making this the first interstellar sight line outside the Vela supernova remnant to show a time-varying component. The feature may be associated with the filamentary loop structure seen in this region. There is poor correspondence between the Na I and Ca II profiles: we do not detect narrow Ca II profiles to the four hfs Na I components, and only three of the well-resolved components have the same Ca II and Na I radial velocities and consistent b-values. One of these components, at vLSR = -30.0 km s-1, has a low Na I/Ca II ratio and arises in a region where turbulent motions dominate—properties consistent with the hypothesis that the cloud lies close to HD 28497. In general, however, the Na I and Ca II occupy different gaseous phases in the ISM. We have compared our data with 21 cm emission profiles obtained from the recent Leiden/Dwingeloo H I survey. Based on agreement in the velocities, the Na I/Ca II ratio, and the kinetic temperatures, we conclude that the component at vLSR = -7.5 km s-1 is associated with the front side of the large, expanding Orion-Eridanus shell. Unexpectedly, the molecular cloud MBM 20 is not detected either in our absorption spectra or in the H I profiles, indicating that HD 28497 lies away from the core of MBM 20. Apart from the two features at -11 and -7.5 km s-1 there is almost no agreement between the H I profiles and the optical spectra. Although we cannot rule out the possibility that most of the H I lies behind the star, this explanation seems unlikely because many of the H I features have previously been attributed to foreground phenomena. The beam sizes of the H I and the optical studies are quite different and this suggests a different explanation, namely that the physical sizes of the interstellar structures we detect in Na I and Ca II are not extensive enough to be detected in H I. If so, this raises questions about the usefulness in general of combining results obtained from H I 21 cm studies with results obtained from optical (or ultraviolet) studies of the interstellar gas.