Henry A. Kobulnicky

The Metal Content of Dwarf Starburst Winds: Results from Chandra Observations of NGC 1569*

We present deep Chandra spectral imaging of the dwarf starburst galaxy NGC 1569. The unprecedented spatial resolution allows us to spatially identify the components of the integrated X-ray spectrum. Fitted spectral models require an intrinsic absorption component and higher metal abundances than previous studies indicated. Our results provide the first direct evidence for metal-enriched winds from dwarf starburst galaxies. We identify 14 X-ray point sources in NGC 1569. Most have properties consistent with those of high-mass X-ray binaries, but one is a steep-spectrum radio source that is probably a supernova remnant. The X-ray luminosity of NGC 1569 is dominated by diffuse, thermal emission from the disk (0.7 keV) and bipolar halo (0.3 keV). Photoelectric absorption from the inclined H I disk hardens the X-ray spectrum on the northern side of the disk relative to the southern side. Requiring the fitted absorption column to match the H I column measured at 21 cm implies that the metallicity of the H I disk is significantly less than solar but greater than 0.1 Z☉. Hence, much of the H I is enriched to levels comparable to the metallicity of the H II regions [O/H = 0.2(O/H)☉]. The X-ray color variations in the halo are inconsistent with a free-streaming wind and probably reveal the location of shocks created by the interaction of the wind with a gaseous halo. The X-ray spectrum of the diffuse gas presents strong emission lines from α-process elements. Fitted models require α-element abundances greater than 0.25 Zα, ☉ and ratios of α-elements to iron 2-4 times higher than the solar ratio. The best fit to the spectrum is obtained with solar mass fractions for the α-elements, 1.0 Zα, ☉, but a degeneracy between the metallicity and the spectral normalization prevents us from deriving an upper limit on the wind metallicity from the X-ray spectrum alone. We argue, however, that abundances larger than 2.0 Zα, ☉ pose awkward implications for the dynamical evolution of the wind based on our knowledge of the starburst properties. For consistency with our best-fitting abundances, the mass of interstellar gas entrained in the wind must be about 9 times the mass of stellar ejecta in the wind. Most of the oxygen carried by the wind comes from the stellar ejecta rather than entrained interstellar gas. The estimated mass of oxygen in the hot wind, 34,000 M☉, is similar to the oxygen yield of the current starburst. Apparently the wind carries nearly all the metals ejected by the starburst. These metals appear destined to contribute to the enrichment of the intergalactic medium. Much of the nucleosynthesis in NGC 1569 must have occurred during less violent periods of star formation, however, because our measurements imply that the neutral gas disk holds at least 5 times more oxygen than wind.

Prime focus imaging spectrograph for the Southern African Large Telescope: optical design

The University of Wisconsin - Madison, together with Rutgers University and the South African Astronomical Observatory, is designing and building an imaging spectrograph for the Prime Focus Instrument Package of the Southern African Large Telescope (SALT). The Prime Focus Imaging Spectrograph (PFIS) will be a versatile instrument specializing in very high throughput, low and medium resolution (R=500-12,500) imaging spectroscopy, using volume phase holographic (VPH) gratings and a double etalon Fabry-Perot interferometer, and spectropolarimetry from 320 to 900 nm. The optical design includes all transmissive optics for high efficiency and compactness. To maintain throughput in the ultraviolet, only fused silica, CaF2 and NaCl are used. As NaCl is very hygroscopic, the design uses NaCl as the inner element in sealed triplets only. For the highest possible first-order spectral resolution, the collimated beam size is 150 mm - the maximum for practical Fabry-Perot etalons. The F/2.2 camera can be articulated to tune the efficiency of the VPH gratings; a complement of six gratings (5 VPH and 1 standard transmission grating) has been designed to fill the resolution-wavelength space available to the instrument. Linear, circular, and all-stokes spectropolarimetry will be performed through the use of Pancharatnam superachromatic waveplates and a Wollaston beamsplitter.

Prime Focus Imaging Spectrograph for the Southern African Large Telescope: operational modes

The Prime Focus Imaging Spectrograph (PFIS) will be the workhorse first-light instrument on the Southern African Large Telescope (SALT). Scheduled for commissioning in late 2004, PFIS is a versatile high-throughput imaging spectrograph with a complement of 5 volume-phase holographic gratings for spectroscopic programs from 3200Å to 9000Å at resolutions of R=1500 to R=6000. A magazine of 6 longslits and 30 custom laser-milled slitmasks enables single- or multi-object spectroscopy over an 8 arcminute diameter field. With the gratings stowed, a dual-etalon Fabry-Perot subsystem enables imaging spectroscopy at R=500, R=3000, and R=12,500. The polarization subsystem, consisting of a polarizing beam-splitter used in conjunction with half- and quarter-wave plates, allow linear or circular polarimetric measurements in ANY of the spectroscopic modes. Three mosaiced rapid-readout frame-transfer CCDs provide the capability for time-resolved sampling at rates in excess of 10 Hz. Combinations of these subsystems permit novel observing modes for specialized scientific programs. Examples include high-time resolution multi-object spectral polarizmetry of accreting compact objects, and Fabry-Perot polarimetry or imaging spectral polarimetry of nebulae and stellar clusters. The demands of queue-scheduled observing on a fixed-altitude telescope require that the instrument be capable of rapid reconfiguration between modes.

Near-Infrared Spectroscopy of Two Galaxies at z = 2.3 and z = 2.9: New Probes of Chemical and Dynamical Evolution at High Redshift

This study presents Keck optical and infrared spectroscopy of the rest-frame ultraviolet and optical emission lines in two Lyα-emitting galaxies at z > 2. These data provide insight on the evolution of fundamental galaxy scaling relations at early epochs, especially the luminosity-velocity and luminosity-metallicity relations. Spectral diagnostics suggest that the Coup Fourre galaxy at z = 2.3 and Lynx 2-9691, a serendipitously discovered, luminous Lyman drop galaxy at z = 2.9, are star-forming galaxies without active nuclei. Lynx 2-9691 exhibits extended [O III] emission over a diameter of greater than 28 kpc, reminiscent of the Lyα nebulae discovered near Lyman drop galaxies. We estimate star formation rates of 59 and 111 M☉ yr-1, respectively, from Balmer recombination line luminosities, 2-3 times higher than inferred from the ultraviolet continuum. The ratios of strong nebular emission lines indicate subsolar oxygen abundances in the range 8.2 < 12 + log(O/H) < 8.8 (Z = 0.25-0.95 Z☉). Interestingly, Galactic metal-rich globular clusters have similar metallicities, consistent with the idea that we could be seeing the formation of galaxies like the Milky Way at z ~ 3. The measured gas-phase oxygen abundances are greater than 4-10 times higher than the Z < 0.1 Z☉ metallicities found in damped Lyα (DLA) absorbers at similar redshifts, indicating that DLA systems trace fundamentally different environments than the vigorously star-forming objects observed here. If this intense star formation activity represents the dominant formation episodes for stars in todays spiral bulges or ellipticals, then the evolved descendants in the local universe should exhibit similarly subsolar metallicities in their dominant stellar populations that formed 8-10 Gyr ago. When these new data are combined with a sample of four other high-redshift spectroscopic results from the literature, we find that star-forming galaxies at z ~ 3 are 2-4 mag more luminous than local spiral galaxies of similar metallicity and thus are offset from the local luminosity-metallicity relation. Their kinematic line widths are σv = 65-130 km s-1, making this sample 1-3 mag more luminous than local galaxies of similar line width and mass. Less luminous Lyman drop galaxies need to be studied to see if these deviations are universal or apply to only the most luminous high-redshift galaxies.

A Sample of Clusters of Extragalactic Ultracompact H II Regions

We report on the detection of optically thick free-free radio sources in the galaxies M33, NGC 253, and NGC 6946 using data in the literature. We interpret these sources as being young embedded star birth regions that are likely to be clusters of ultracompact H II regions. All 35 of the sources presented in this article have positive radio spectral indices (α > 0 for Sν ∝ να), suggesting an optically thick thermal bremsstrahlung origin from the H II region surrounding the hot stars. The estimated emission measures for these sources are EM6 cm 108 cm-6 pc, and energy requirements indicate that the sources in our sample have a range of a few to ~560 O7 V star equivalents powering their H II regions. Assuming a Salpeter initial mass function with lower and upper mass cutoffs of 1 and 100 M☉, respectively, this range in NLyc corresponds to integrated stellar masses of 0.1-60 × 103 M☉. For roughly half of the sources in our sample there is no obvious optical counterpart, which gives further support for their deeply embedded nature; for most of the remaining sources, the correspondence to an optical source is insecure owing to relative astrometric uncertainty. Their luminosities and radio spectral energy distributions are consistent with H II regions modeled as spheres of plasma with electron densities from ne ~ 1.5 × 103 to ~1.5 × 104 cm-3 and radii of ~1-7 pc. Because of the high densities required to fit the data, we suggest that the less luminous of these sources are extragalactic ultracompact H II region complexes, those of intermediate luminosity are similar to W49 in the Galaxy, and the brightest will be counterparts to 30 Doradus when they emerge from their birth material. These objects constitute the lower mass range of extragalactic ultradense H II regions, which we argue are the youngest stages of massive star cluster formation yet observed. The sample presented in this paper is beginning to fill in the continuum of objects between small associations of ultracompact H II regions and the analogous massive extragalactic clusters that may evolve into globular clusters.

The Gas Content and Kinematics of Nearby Blue Compact Galaxies: Implications for Studies at Intermediate and High Redshift

We present Arecibo 21 cm spectroscopy, Keck HIRES Hβ spectroscopy, and WIYN R-band imaging of 11 nearby blue compact galaxies with effective B-band surface brightnesses of SBe = 19.4–21.2 mag arcsec^2 and effective radii of R_ eff = 0.6–1.9 kpc. This sample was selected to test the reliability of mass estimates derived using optical emission line widths, particularly for the blue compact star-forming galaxies observed at intermediate redshifts (0.1 < z < 1). In addition, we also measure the H I content and gas depletion timescales for the nearby blue compact galaxies in an attempt to infer the present nature and possible future evolution of their intermediate-redshift analogs. We detected H I in 10 of 11 sample galaxies. They have H I masses of 0.3–4 × 10^9 M_⊙, H I line widths, W_20, of 133–249 km s^-1, dynamical masses of 0.5 to 5 × 10^10 M_⊙, gas depletion timescales, τgas, of 0.3–7 Gyr, H I mass fractions of 0.01–0.58, and mass-to-light ratios of 0.1–0.8. These values span the range of values typical of nearby H II galaxies, irregulars, and spirals. Despite the restricted morphological selection, our sample of galaxies is quite heterogeneous in terms of H I content, dynamical mass, and gas depletion timescale. Therefore, these galaxies have a variety of evolutionary paths and should look very different from each other in 5 Gyr. Those with high masses and gas depletion timescales are likely to retain their interstellar medium for future star formation, while the lower mass objects with small gas depletion timescales may be undergoing their last major event of star formation. Hence, the fading of intermediate-redshift, luminous, blue compact galaxies into NGC 205-type spheroidals is a viable evolutionary scenario, but only for the least massive, most gas-poor objects in this sample. The most consistent characteristic of our morphologically selected sample is that the ratios of H II line widths to H I 21 cm line widths, Script Script R = W20(H )/W20(H ), are systematically less than unity, with an average value of Script R = 0.66 ± 0.16, similar to findings for local H II galaxies. The simplest explanation for this result is that the ionized gas is more centrally concentrated than the neutral gas within the gravitational potential. We find that Script R is a function of line width, such that smaller line width galaxies have smaller values of Script R. Correcting optical line widths by this factor not only raises the derived masses of these galaxies, but also makes them consistent with the local luminosity–line width (Tully-Fisher) relation as well. If this ratio applies to intermediate-redshift galaxies, then the masses of intermediate-redshift, blue compact galaxies can be obtained from optical line widths after applying a small correction factor, and the proposed luminosity evolution of the Tully-Fisher relation is much smaller and more gradual than suggested by studies using optical emission line width measurements.

Distance Dependence in the Solar Neighborhood Age-Metallicity Relation

The age-metallicity relation for F and G dwarf stars in the solar neighborhood, based on the stellar metallicity data of Edvardsson et al., shows an apparent scatter that is larger than expected considering the uncertainties in metallicities and ages. A number of theoretical models have been put forward to explain the large scatter. However, we present evidence, based on the Edvardsson et al. data, along with Hipparcos parallaxes and new age estimates, that the scatter in the age-metallicity relation depends on the distance to the stars in the sample, such that stars within 30 pc of the Sun show significantly less scatter in [Fe/H]. Stars of intermediate age from the Edvardsson et al. sample at distances 30-80 pc from the Sun are systematically more metal poor than those more nearby. We also find that the slope of the apparent age-metallicity relation is different for stars within 30 pc than for those stars more distant. These results are most likely an artifact of selection biases in the Edvardsson et al. star sample. We conclude that the intrinsic dispersion in metallicity at fixed age is less than 0.15 dex for field stars in the solar neighborhood, consistent with the less than 0.1 dex for Galactic open star clusters and the interstellar medium.

Instrumentation for high-resolution spectropolarimetry in the visible and far-ultraviolet

Linear spectropolarimetry of spectral lines is a neglected field in astronomy, largely because of the lack of instrumentation. Techniques that have been applied, but rarely, include investigation of the dynamics of scattering envelopes through the polarization of electron- or dust-scattered nebular light. Untried techniques include promising new magnetic diagnostics like the Hanle Effect in the far-ultraviolet and magnetic realignment in the visible. The University of Wisconsin Space Astronomy Lab is developing instrumentation for such investigations. In the visible, the Prime Focus Imaging Spectrograph (PFIS) is a first light instrument for the Southern African Large Telescope (SALT), which at an aperture of 11m will be the largest single telescope in the Southern Hemisphere. Scheduled for commissioning in late 2004, PFIS is a versatile high-throughput imaging spectrograph using volume-phase holographic gratings for spectroscopic programs from 320nm to 900nm at resolutions of R=500 to R=6000. A dual-etalon Fabry-Perot subsystem enables imaging spectroscopy at R=500 and R=3000 or 12,500. The polarization subsystem, consisting of a very large calcite polarizing beam-splitter used in conjunction with half- and quarter-wave Pancharatnam superachromatic plates, allow linear or circular polarimetric measurements in any of the spectroscopic modes. In the FUV, the Far-Ultraviolet SpectroPolarimeter (FUSP) is a sounding rocket payload, scheduled for its first flight in 2003, that will obtain the first high-precision spectropolarimetry from 105 - 150 nm, and the first astronomical polarimetry of any kind below 130 nm. The 50 cm primary mirror of the telescope is F/2.5. At the prime focus are the polarimetric optics, a stressed lithium fluoride rotating waveplate, followed by a synthetic diamond Brewster-angle mirror. The spectrometer uses an aberration-corrected spherical holographic grating and a UV-sensitized CCD detector, for a spectral resolution of R=1800.

Finding Signatures of the Youngest Starbursts

Embedded massive starclusters have recently been identified in several nearby galaxies by means of the radio-wave thermal bremsstrahlung emission from their surrounding HII regions. Energy requirements imply that these optically-obscured starclusters contain 500–1000 O-type stars, making them similar to the “super starclusters” observed in many dwarf starbursts and mergers. Based on their high free-free optical depth and visual extinctions of AV ≫ 10 mag, these massive “ultra-dense” HII regions (UDHIIs) are distinct signatures of the youngest, most compact super starclusters. UDHII regions may represent the earliest stages of globular cluster formation. We review the properties of presently-known UDHIIs, and we outline a pictoral evolutionary taxonomy for massive cluster formation which is analogous to the more familiar evolutionary sequence for individual stars.

HI Properties of Luminous Blue Compact Galaxies in the Local Universe

We present Arecibo 21 cm spectroscopy and Keck echelle Hβ spectroscopy of 11 local galaxies selected to resemble the blue, compact starforming galaxies which are observed at intermediate redshifts. Although measurement of the HI properties of our sample does not allow strong constraints to be placed on the evolutionary descendants of our sample, we can infer that their counterparts in the intermediate and high-redshift universe are likely to be similarly low mass and gas rich. We have found that the ratios of optical Balmer linewidths to HI 21 cm linewidths are systematically less than unity, with an average value of 0.66 ± 0.16. This fraction correlates strongly with the Hβ linewidth, concentration index and α equivalent width. A combination of these parameters could be used to apply correction factors which enable robust measurements of the masses of similar galaxies at cosmological distances.