[Show abstract][Hide abstract] ABSTRACT: The aim of this project is to identify low-redshift host galaxies of quasar absorption-line systems by selecting galaxies that are seen in projection onto quasar sightlines. To this end, we use the Seventh Data Release of the Sloan Digital Sky Survey to construct a parent sample of 97,489 galaxy/quasar projections at impact parameters of up to 100 kpc to the foreground galaxy. We then search the quasar spectra for absorption-line systems of Ca II and Na I within ±500 km s–1 of the galaxy's velocity. This yields 92 Ca II and 16 Na I absorption systems. We find that most of the Ca II and Na I systems are sightlines through the Galactic disk, through high-velocity cloud complexes in our halo, or Virgo Cluster sightlines. Placing constraints on the absorption line rest equivalent width significance (≥3.0σ), the local standard of rest velocity along the sightline (≥345 km s–1), and the ratio of the impact parameter to the galaxy optical radius (≤5.0), we identify four absorption-line systems that are associated with low-redshift galaxies at high confidence, consisting of two Ca II systems (one of which also shows Na I) and two Na I systems. These four systems arise in blue, ~L* r galaxies. Tables of the 108 absorption systems are provided to facilitate future follow-up.
The Astronomical Journal 09/2011; 142(4):122. · 4.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Dwarf galaxies provide opportunities for drawing inferences about the processes in the early universe by observing our "cosmological backyard"-the Local Group and its vicinity. This special issue of the open-access journal Advances in Astronomy is a snapshot of the current state of the art of dwarf-galaxy cosmology. Comment: This is the editorial paper which introduces the Special Issue on Dwarf-Galaxy Cosmology published in Advances in Astronomy. The issue contains fourteen review papers, and one original research article. All papers were peer-reviewed by a minimum of two referees. To read the Special Issue, please follow this link: http://www.hindawi.com/journals/aa/2010/si.1.html .
Advances in Astronomy 04/2010; · 1.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In an effort to study Damped Lyα (DLA) galaxies at low redshift, we have been using the Sloan Digital Sky Survey (SDSS) to identify galaxies projected onto quasi-stellar object (QSO) sight lines and to characterize their optical properties. For low-redshift galaxies, the H I 21 cm emission line can be used as an alternate tool for identifying possible DLA galaxies, since H I-emitting galaxies typically exhibit H I columns that are larger than the classical DLA limit. Here, we report on follow-up H I 21 cm emission-line observations of two DLA candidates that are both low-redshift spiral galaxies, Mrk 1456 and SDSS J211701.26–002633.7. The observations were made using the Green Bank Telescope (GBT) and Arecibo telescope, respectively. Analysis of their H I properties reveal the galaxies to be about one and two M*H I galaxies, respectively, and to have average H I mass, gas richness, and gas-mass fraction for their morphological types. We consider Mrk 1456 and SDSS J211701.26–002633.7 to be candidate DLA systems based upon the strength of the Ca II absorption lines they cause in their QSO's spectra, and impact parameters to the QSO that are smaller than the stellar disk. Compared to the small numbers of other H I detected DLA and candidate DLA galaxies, Mrk 1456 and SDSS J211701.26–002633.7 have high H I masses. Mrk 1456 and SDSS J211701.26–002633.7 have also been found to lie in galaxy groups that are high in H I gas mass compared to the group containing SBS 1543+593, the only DLA galaxy previously known to be situated in a galaxy group. When compared with the expected properties of low-z DLAs from an H I-detected sample of galaxies, Mrk 1456 and SDSS J211701.26–002633.7 fall within the ranges for impact parameter and MB ; and the H I mass distribution for the H I-detected DLAs agrees with that of the expected H I mass distribution for low-z DLAs. Our observations support galaxy-evolution models in which high-mass galaxies make up an increasing contribution to the DLA cross section at lower redshifts. We also report on the 21 cm line emission of Mrk 1457, a Seyfert galaxy observed within the beam of the GBT.
The Astronomical Journal 11/2009; 138(6):1714. · 4.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The core of scientific research is turning new ideas into reality. From the school science fair to the search for the secrets of dark energy, high-quality research consists of scientific investigation constrained within the scope of a well-defined project. Large or small, generously funded or just scraping by,scientific projects use time, money, and information to turn ideas into plans, plans into action, and action into results. While we, as a community, do much to educate students in the techniques of research, we do not systematically train students in the nature and organization of scientific projects or in the techniques of project management. We propose a two-pronged attack to address this issue in the next decade. First, to generate a broad base of future scientists who have a basic familiarity with the ideas of projects, we propose that the community develop standards for the content of a project design and management course in astronomy and astrophysics. Second, to train future scientists to assume leadership roles in new investigations in astronomy and astrophysics, we propose that the community develop standards for graduate programs in the area of research project leadership.
[Show abstract][Hide abstract] ABSTRACT: Wind variability in OB stars may be ubiquitous, and a connection between projected stellar rotation velocity and wind activity is well established. However, the origin of this connection is unknown. To probe the nature of the rotation connection, several of the attendees at the workshop on Instability and Variability of Hot-Star Winds drafted an IUE observing proposal. The goal of this program was to follow three stars for several rotations to determine whether the rotation connection is correlative or causal. The stars selected for monitoring all have rotation periods ≤5 days. They were HD 50896 (WN5), HD 64760 (B0.5 Ib), and HD 66811 [ζ Pup; O4 If(n)]. During 16 days of nearly continuous observations in 1995 January (dubbed the "MEGA" campaign), 444 high-dispersion IUE spectra of these stars were obtained. This Letter presents an overview of the results of the MEGA campaign and provides an introduction to the three following Letters, which discuss the results for each star.
The Astrophysical Journal 01/2009; 452(1):L53. · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report HST/STIS spectroscopy and Gemini/GMOS-N imaging of the Damped Lyα (DLA) system toward HS 1543+5921 caused by the host star-forming galaxy (SFG) SBS 1543+593. The Gemini image shows new morphological details of this well-resolved DLA galaxy. In combination with previous optical spectra, the new UV spectra enable us to compare, for the first time, ionized and neutral gas-phase α-element abundances derived from emission- and absorption-line spectroscopy, in a bona fide DLA galaxy. The abundances we determine using emission-line diagnostics agree with those from absorption-line diagnostics. We present our results on a metallicity versus redshift diagram that combines local H II regions and SFGs with high-redshift DLAs, and discuss implications for the chemical evolution of galaxies.
The Astrophysical Journal 12/2008; 625(2):L79. · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Motivated by the question of the importance of ionization stratification in solving the "momentum problem" of Wolf-Rayet stellar winds, we have chosen a sample of 14 WN stars for a systematic study. We performed measurements of the emission line widths on ultraviolet, optical, and infrared spectra to obtain data spanning a large range of ionization potentials. We provide extensive tables of these measurements as well as line profile classifications. The presence of ionization stratification in the wind should result in a correlation between ionization potential and line width. We find most of the winds to be stratified and discuss the level of stratification found in each star. To test the importance of ionization stratification to efficient radiation-to-wind momentum transfer, we compare our empirically measured stratification strengths with two sets of theoretical performance numbers and give the correlation statistics in each case.
The Astrophysical Journal Supplement Series 12/2008; 126(2):469. · 14.14 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report new imaging and spectroscopic observations of the damped Lyα absorber (DLA) galaxy SBS 1543+593, a nearby dwarf galaxy whose stellar disk is intersected by the sight line to the bright background QSO HS 1543+5921. Hubble Space Telescope imaging observations with WFPC2 in the F450W and F702W bands are used to measure the DLA galaxy's properties and compile a catalog of its (candidate) H II regions. Ground-based long-slit spectroscopy of the brightest H II region in the galaxy yields estimates of the star formation rate (SFR) and chemical abundances in the galaxy's interstellar medium. We find that SBS 1543+593 exhibits an SFR of ≈0.006 h M☉ yr-1, or an SFR per unit area of ≈1.4 × 10-4 h M☉ yr-1 kpc-2. We derive gas-phase abundances in the ionized gas of 12 + log (O/H) = 8.2 ± 0.2, which is about of the solar value, and log (N/O) = -1.40. These values are consistent with the morphological appearance of SBS 1543+593, an Sm dwarf of MB - 5 log h70 = -16.8 ± 0.2 and intermediate surface brightness. SBS 1543+593 is the first bona fide DLA for which abundances have been measured using emission-line diagnostics. When compared with future high-resolution ultraviolet spectroscopy, our results should prove key for interpreting abundance determinations in high-redshift DLAs.
The Astrophysical Journal 12/2008; 600(2):613. · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: I report on observations of the z=0.01 dwarf galaxy SBS1543+593 which is projected onto the background QSO HS1543+5921. As a star-forming galaxy first noted in emission, this dwarf is playing a pivotal role in our understanding of high-redshift galaxy populations, because it also gives rise to a Damped Lyman Alpha system. This enabled us to analyze, for the first time, the chemical abundance of $\alpha$ elements in a Damped Lyman Alpha galaxy using both, emission and absorption diagnostics. We find that the abundances agree with one another within the observational uncertainties. I discuss the implications of this result for the interpretation of high-redshift galaxy observations. A catalog of dwarf-galaxy--QSO projections culled from the Sloan Digital Sky Survey is provided to stimulate future work. Comment: Proceedings of IAU Symp. 255
Proceedings of the International Astronomical Union 07/2008;
[Show abstract][Hide abstract] ABSTRACT: Linear polarization measurements have been a key to our understanding of the massive star system η Carinae and its surrounding Homunculus nebula. We here present the results of (linear) polarimetric imaging of η Carinae in the V band (F555W) with the Hubble Space Telescope's Wide Field Planetary Camera 2 (HST/WFPC2). The data agree well with previous ground-based measurements of the large-scale polarization across the Homunculus, allowing us to confirm that it is primarily a reflection nebula. The HST observations add information on the variation of the polarization across the lobes on small spatial scales. We provide measurements of the polarization in a variety of structures such as the jet, the skirt, the paddle, the southern ridge, and the spot, and discuss the properties of the polarization of the Homunculus on large and small spatial scales. Using a simple Mie-scattering model, we argue that of three previously proposed geometries for the three-dimensional structure of the Homunculus only the double-flask geometry represents a dust distribution which is consistent with our polarization map.
The Astronomical Journal 12/2007; 118(3):1320. · 4.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have taken advantage of the high spatial resolution attainable with the HST to map the linear polarization in the V band
across the nebulosity surrounding Eta Car. There are several new results related to polarization variations on different size
scales. First, we present a two-dimensional map of the amount and position angle of the polarization across the Homunculus.
Second, we provide measurements of the polarization within prominent features such as the “jet”, the “paddle”, the “skirt”,
and the “spot” in the south-eastern lobe. Third, we comment on polarization variations associated with the small-scale structure
that can be seen in HST images (and which gives the lobes their cauliflower-like appearance). The new data provide insight
into the three-dimensional distribution of dust about Eta Car.
[Show abstract][Hide abstract] ABSTRACT: We present Hα observations of the inner region of the HR Carinae nebula. The inner nebula looks clumpy and material appears
to be present only to the east of the central star. A jet-like filament stretches away from the central star to the southeast.
[Show abstract][Hide abstract] ABSTRACT: We address the hypothesis that high-velocity clouds correspond to the ‘missing’ dwarf galaxies of the Local Group predicted by cosmological simulations. To this end, we present optical and near-infrared photometry of five additional high-velocity clouds, one of which produces Lyman series absorption on the sight line towards the quasar Ton S210, with sufficient resolution and sensitivity to enable the detection of an associated stellar content. We do not detect significant stellar populations intrinsic to any of the five clouds. In combination with the results from our Paper I, which had yielded non-detections of stellar content in another five cases, we find that there is a 50 per cent chance of getting a null result in 10 trials if fewer than 7 per cent of all high-velocity clouds contain stars. We conclude that the population of high-velocity clouds is an unlikely repository for the ‘missing’ dwarfs of the Local Group.
Monthly Notices of the Royal Astronomical Society 01/2007; 374(3):1164 - 1168. · 5.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We review single-star photometry with the Hubble Space Telescope (HST) of five well resolved Blue Compact Dwarf (BCD) galaxies.
Three BCDs exhibit well populated red giant branches. These galaxies are therefore at least 1-2 Gyr old, and possibly older
than 10 Gyr. For two BCDs, the available HST data do not go deep enough to reveal the red giant branch. We argue that over
a wide range of possible galaxy distances, the color-magnitude diagrams of these BCDs are nevertheless consistent with showing
the evolved descendants of intermediate- mass stars on the asymptotic giant branch. Such stars are nucleosynthetic sites of
nitrogen and carbon production. Their identification in BCDs contradicts recent conclusions that BCDs must be young galaxies
in which primary production of these elements takes place exclusively in massive stars, and implies that BCDs are unlikely
to be home to newly formed metal-free stars. Color-magnitude diagrams of BCDs thus point to difficulties in decoding star-formation
histories of starburst galaxies from the metallicities of the ionized gas.
[Show abstract][Hide abstract] ABSTRACT: The chance projection of the disk of Mrk 1456 onto a background QSO is similar to the case of SBS 1543+593-HS 1543+5921. Mrk 1456 is a luminous, late-type spiral galaxy at z~0.05. Although the QSO, SDSS J114719.90+522923.2 at z~2, has not yet been observed with ultraviolet spectroscopy, it shows strong Ca II absorption at the redshift of Mrk 1456, which gives evidence that it is a possible damped Lyalpha absorber. Spectroscopy of the star-forming nucleus of Mrk 1456 allows us to apply emission-line diagnostics to infer the chemical abundances at the center of the galaxy and to make a prediction of the expected metallicity on the sight line to the QSO.
The Astronomical Journal 01/2006; 132(5):1844-1850. · 4.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have been studying High Velocity Clouds to test the hypothesis that they are the ``missing" dwarf galaxies of the Local Group predicted in cosmological simulations. In this paper we present single-star optical and near-infrared photometry of a new sample of five Compact High Velocity Clouds, one of which produces Lyman series absorption on the sightline towards the Quasar Ton S210. We do not detect a stellar component in any of the five clouds. By combining our data with previous results based on single-star photometry we now find that searches have been conducted toward 17 High Velocity Clouds; and in none of them was a resolved stellar population detected. There is a 50% chance of getting a null result if fewer than 4% of all High Velocity Clouds contain stars. The High Velocity Clouds thus represent an unlikely source to provide the ``missing" satellite galaxies of the Milky Way which are predicted in cosmological simulations of structure formation. This result, and other deep imaging surveys, now imply that the ``missing" satellites may not have simply been overlooked observationally, indicating that there is no easy way out of the substructure problem. This study made use of data from the ESO VLT and from 2MASS. We were assisted in this project by our colleagues Dr. J. Kerp and by B. Cherinka. This study has been supported by a NASA ADP grant titled ``The Building Blocks of the Local Group or Testing Astrophysical Solutions to the Substructure Problem".
[Show abstract][Hide abstract] ABSTRACT: We investigate the chemical abundances of local star-forming galaxies which cause Damped Lyman Alpha lines. A metallicity versus redshift diagram is constructed, on which the chemical abundances of low-redshift star-forming galaxy populations are compared with those of high-redshift Damped Lyman Alpha systems. We disucss two types of experiments on individual star-forming galaxies. In the first, the Damped Lyman Alpha line is created against an internal ultraviolet light source generated by a star-forming cluster or a supernova explosion. In the second, the Damped Lyman Alpha line is seen against a background Quasar. The metallicities measured from ionized gas in the star-forming regions, and neutral gas in the Damped Lyman Alpha systems, are compared with one another on a case-by-case basis. We highlight the occurrence of the star-forming galaxy/Quasar pair SBS 1543+593/HS 1543+5921, where the emission- and absorption-line derived abundances give the same result. We argue that we therefore can in principle, interpret Damped Lyman Alpha system metallicities as an extension of star-forming galaxy metallicities to higher redshifts, supporting that gas-rich galaxies had lower chemical abundances when the were younger.
[Show abstract][Hide abstract] ABSTRACT: We investigate the chemical abundances of local star-forming galaxies which cause damped Lyman alpha lines. A metallicity versus redshift diagram was constructed, on which the chemical abundances of low-redshift star-forming galaxy populations are compared with those of high-redshift damped Lyman alpha systems. We discuss two types of experiments on individual star-forming galaxies. In the first, the damped Lyman alpha line is created against an internal ultraviolet light source generated by a star-forming cluster or a supernova explosion. In the second, the damped Lyman alpha line is seen against a background quasar. The metallicities measured from ionised gas in the star-forming regions, and neutral gas in the damped Lyman alpha systems, are compared with one another on a case-by-case basis. We highlight the occurrence of the star-forming galaxy/quasar pair SBS 1543+593/HS 1543+5921, where the emission- and absorption-line derived abundances give the same result. We argue that we therefore can in principle interpret damped Lyman alpha system metallicities as an extension of star-forming galaxy metallicities to higher redshifts, supporting the idea that gas-rich galaxies had lower chemical abundances when they were younger.
Proceedings of the International Astronomical Union 02/2005; 1:319 - 324.
[Show abstract][Hide abstract] ABSTRACT: The spectrum of the quasar PHL 1226 is known to have a strong Mg II and sub-damped Lymanalpha (sub-DLA) absorption line system with N(H I)=(5±2)×1019 cm-2 at z=0.1602. Using integral field spectra from the Potsdam Multi Aperture Spectrophotometer (PMAS) we investigate a galaxy at an impact parameter of 6.4 arcsec which is most probably responsible for the absorption lines. A fainter galaxy at a similar redshift and a slightly larger distance from the QSO is known to exist, but we assume that the absorption is caused by the more nearby galaxy. From optical Balmer lines we estimate an intrinsic reddening consistent with 0, and a moderate star formation rate of 0.5 M&sun; >yr-1, is inferred from the Halpha luminosity. Using nebular emission line ratios we find a solar oxygen abundance 12+log(O/H)=8.7±0.1 and a solar nitrogen to oxygen abundance ratio log(N/O)=-1.0±0.2. This abundance is larger than those of all known sub-DLA systems derived from analyses of metal absorption lines in quasar spectra. On the other hand, the properties are compatible with the most metal rich galaxies responsible for strong Mg II absorption systems. These two categories can be reconciled if we assume an abundance gradient similar to local galaxies. Under that assumption we predict abundances 12+log(O/H)=7.1 and log(N/O)=-1.9 for the sub-DLA cloud, which is similar to high redshift DLA and sub-DLA systems. We find evidence for a rotational velocity of ˜200 km s-1 over a length of ˜7 kpc. From the geometry and kinematics of the galaxy we estimate that the absorbing cloud does not belong to a rotating disk, but could originate in a rotating halo. Based on observations obtained at the German-Spanish Astronomical Center, Calar Alto, operated by the Max-Planck-Institut für Astronomie Heidelberg jointly with the Spanish National Commission for Astronomy.
Astronomy and Astrophysics 01/2005; 429(2):477-487. · 4.48 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Our knowledge about the chemical properties of galaxies is based on
measurements of emission lines from photo-ionized gas. The abundances of
galaxies at high-z are inferred using absorption lines arising in
neutral gas in Damped Lyman Alpha (DLA) systems. Do the results of
emission and absorption experiments agree in cases of nearby
star-forming galaxies (SFGs) causing DLAs?
Schulte-Ladbeck et al. (2004a) examined the z=0.009 DLA/SFG SBS
1543+593. We derived [O/H]II=-0.54. The bright QSO HS
1543+5921 intercepts the disk at small impact parameter. We found a
lower limit, [O/H]I>-2.14, using HST archival spectra. New
HST observations by Bowen et al. are analyzed to yield a S abundance of
[S/H]I=-0.54. Using S as a proxy for O this suggests
[O/H]I=[O/H]II (or 0.29xsolar) for one genuine
To investigate additional SFG/QSO pairs, we used the CMU-Pitt Value
Added Catalog to assemble from the SDSS DR1 a database of about 13,000
SFGs with 0< z <0.36 (Schulte-Ladbeck 2004b). We applied the
strong-line indices of Pettini & Pagel (2004), derived
O/HII for all objects, and a median O/HII ratio of
0.74xsolar with a SIQR of 0.19. An O/H vs. z diagram is constructed by
augmenting these data to z≈5 with O/HI ratios for DLAs
(Prochaska et al. 2003). We matched our SFG catalog against the SDSS QSO
catalog, then used the HST archive to determine the HI column densities
and limits on O/HI for these SFGs. The results are discussed
using the O/H vs. redshift diagram.
We acknowledge support of HST archival funding to program ID 10282.