Pushpa Khare - Academia.edu (original) (raw)

Papers by Pushpa Khare

We have improved upon our previous search technique of systematically searching QSO spectra for n... more We have improved upon our previous search technique of systematically searching QSO spectra for narrow galactic H-alpha emission, which indicates a foreground galaxy within the Sloan Digital Sky Survey (SDSS) spectral fiber. We now search for H-alpha plus eight other galactic emission lines in the same manner. We have scanned the SDSS DR7 QSO catalog spectra searching for these emission lines. Here we present our sample which focuses on the redshift range z<0.401 where galactic H-alpha is detectable in the SDSS spectra. This has revealed 27 unique galaxies on top of QSOs (GOTOQs). We have deblended the QSOs from the respective galaxies and determined the photometric properties of these systems. We find upon deblending that most of the galaxies are primarily blue, late-type galaxies with colors in the range -0.71<(u-r)<2.07. We find a slight anti-correlation between reddening and impact parameter (E(B-V)_(g-i) vs. b). The galaxies have average star formation rates of 0.01 to...

The Damped and sub-Damped Lyman-alpha (DLA and sub-DLA) systems seen in the spectra of QSOs offer... more The Damped and sub-Damped Lyman-alpha (DLA and sub-DLA) systems seen in the spectra of QSOs offer a unique way to study the interstellar medium of high redshift galaxies. In this paper we report on new abundance determinations in a sample of 10 new systems, nine of the lesser studied sub-DLAs and one DLA, along the line of sight to seven QSOs from spectra taken with the MIKE spectrograph. Lines of Mg I, Mg II, Al II, Al III, Ca II, Mn II, Fe II, and Zn II were detected. Here, we give the column densities and equivalent widths of the observed absorption lines, as well as the abundances determined for these systems. Zn, a relatively undepleted element in the local interstellar medium (ISM) is detected in one system with a high metallicity of [Zn/H]=+0.27\pm0.18. In one other system, a high abundance based on the more depleted element Fe is seen with [Fe/H]=-0.37\pm0.13, although Zn is not detected. The N(HI)-weighted mean metallicity of these sub-DLA systems based on Fe is <[Fe/H]&...

We have analyzed high resolution data of absorption lines of Si and C in the absorption systems o... more We have analyzed high resolution data of absorption lines of Si and C in the absorption systems observed in the spectra of QSOs, in order to study the ionization state and the overabundance of Si with respect to C in the absorbers and also to study the change in these properties with redshift. No correlation is found between column density ratios of Si IV to C IV of intervening systems and redshift. The data do not provide any evidence for an abrupt change in the values of the ratio at any particular redshift unlike that for Lyman alpha forest clouds. We have compared the observed ratios of column densities of Si II and Si IV and of Si IV and C IV in different classes of absorption systems with predictions of photo ionization models for different shapes of the background radiation field. Overabundance of Si over C can be ruled out in several of the intervening systems for any shape of the background radiation. For these systems we can also rule out any contribution from the stellar ...

We have studied a sample of 1084 intervening absorption systems with 2.15< z_ab<5.2, having... more We have studied a sample of 1084 intervening absorption systems with 2.15< z_ab<5.2, having log(N_ HI) > 20.0 in the spectra of QSOs in Sloan Digital Sky Survey (SDSS) data release 7 (DR7), with the aim of understanding the nature and abundance of the dust and the chemical abundances in the DLA absorbers. Composite spectra were constructed for the full sample and several subsamples, chosen on the basis of absorber and QSO properties. Average extinction curves were obtained for the samples by comparing their geometric mean composite spectra with those of two samples of QSOs, matching in z_em and i magnitude with the DLA sample, one sample without any absorbers along their lines of sight and the other without any DLAs along their lines of sight irrespective of the presence of other absorption systems. While the average reddening in the DLA sample is small, we find definite evidence for the presence of dust in subsamples based on absorber properties, in particular the strength...

Exploring the dust content of SDSS DR7 damped Lyman alpha systems at 2.15 z ab <5.

Accepted... Received...; in original form... We present chemical abundance measurements from high... more Accepted... Received...; in original form... We present chemical abundance measurements from high resolution observations of 7 subdamped Lyman-α absorbers and 1 damped Lyman-α system at z < 1.5. Three of these objects have high metallicity, with near or super-solar Zn abundance. Grids of Cloudy models for each system were constructed to look for possible ionization effects in these systems. For the systems in which we could constrain the ionization parameter, we find that the ionization corrections as predicted by the Cloudy models are generally small and within the typical error bars ( ∼ 0.15 dex), in general agreement with previous studies. The Al III to Al II ratio for these and other absorbers from the literature are compared, and we find that while the sub-DLAs have a larger scatter in the Al III to Al II ratios than the DLAs, there appears to be little correlation between the ratio and NH I. The relationship between the metallicity and the velocity width of the profile for ...

Most models of cosmic chemical evolution predict that the mass-weighted mean interstellar metalli... more Most models of cosmic chemical evolution predict that the mass-weighted mean interstellar metallicity of galaxies should rise with time from a low value ∼ 1/30 solar at z ∼ 3 to a nearly solar value at z = 0. In the absence of any selection effects, the damped Lyman-alpha absorbers (DLAs) in quasar spectra are expected to show such a rise in global metallicity. However, it has been difficult to determine whether or not DLAs show this effect, primarily because of the very small number of DLA metallicity measurements at low redshifts.

Quasar spectra, apart from showing the broad, redshifted emission lines, often show narrow as wel... more Quasar spectra, apart from showing the broad, redshifted emission lines, often show narrow as well as broad absorption lines. The presence of absorption lines has been known since shortly after the discovery of these objects. The spectrum of a single quasar can have more than one group of absorption lines, termed absorption line systems, each system having its characteristic redshift. Most of these systems are believed to be produced by galactic or intergalactic clouds, while those having redshifts close to that of the quasar may be produced by material intrinsic to the quasar. These systems thus offer a sensitive probe to understand the evolutionary history of the Universe over 90% of its age as well as the quasar environment. In recent times, the commissioning of large telescopes, high resolution spectrographs, space telescopes, large sky digital surveys and progress in hydro-simulations have resulted in rapid progress in this field. In this article, I will summarize some of our c...

Monthly Notices of the Royal Astronomical Society

We present chemical abundance measurements from medium resolution observations of 8 sub-damped Ly... more We present chemical abundance measurements from medium resolution observations of 8 sub-damped Lyman-alpha absorbers and 2 strong Lyman-limit systems at z < 1.5 observed with the MIKE spectrograph on the 6.5m Magellan II Clay telescope. These observations were taken as part of an ongoing project to determine abundances in z < 1.5 quasar absorption line systems (QSOALS) focusing on sub-DLA systems. Lines of Mg I, Mg II, Al II, Al III, Ca II, Mn II, Fe II, and Zn II were detected and column densities were determined. We discuss the metallicites of these absorbers based on the abundaces of Fe and Zn. Fe II lines were detected in all systems, with an average Fe abundance higher than typical abundances for DLA systems at these redshifts. We also discuss the abundances of other elements (e.g., [Fe/H], [Mn/H]), and examine relative abundances (e.g., [Mn/Fe], [Zn/Fe], etc) in these absorbers. Lastly we discuss the results of Cloudy photoionization modeling to determine the effects of ionization in these systems. This work was supported in part by NSF grant AST-0607739 to University of South Carolina.

Mon Notic Roy Astron Soc, 2009

Mon Notic Roy Astron Soc, 1994

A search for emission lines in foreground galaxies, in QSO spectra (zgal << zQSO) of the SDSS dat... more A search for emission lines in foreground galaxies, in QSO spectra (zgal << zQSO) of the SDSS data release 5, reveals 21 examples of QSOs shining through low redshift, foreground galaxies at low impact parameters (<10 kpc). About 74,000 QSO spectra were examined, by searching for narrow H! emission lines at z < 0.295, at a flux level greater than 5 x 10-17 ergs/cm 2 •sec, then confirming that other expected emission lines of the H II regions in the galaxy are detected. The measurements show that the galaxies have abundances of solar or above and star formation rates between 10 and 1000 solar masses per year. The galaxies were deblended from the QSOs to get colors and shapes. For cases that allow the galaxy and the QSO to be deblended, the galaxies are blue [1.0 < (u-r) < 1.9]. Extinction through the galaxies is determined from the color excesses of the QSOs. The color excesses range from 0 to 0.25 magnitudes. These extinction values are compared with the flux ratios of H! and H!, which reflect the extinction for an undetermined fraction of the sightline through each galaxy. Measurement or limits on galactic, interstellar Ca II and Na I absorption are given, from the QSO spectrum. The technique used is compared with other ways of finding QSO/galaxy pairs, for the purpose of designing an experiment to characterize the absorption properties of gas in galaxies at distances from the galaxy centers < 10 kpc.

We present elemental abundance measurements of 11 damped Lyalpha systems (DLAs) at .2 < z <... more We present elemental abundance measurements of 11 damped Lyalpha systems (DLAs) at .2 < z < 1.5 from recent observations with the Multiple Mirror Telescope. Absorption features from lines of Zn II, Cr II, Mg I, Mg II, Si II and Fe II were found and abundances were determined. We discuss the implications of our measurements for the metallicity evolution of damped Lymanalpha galaxies. This work was supported in part by the US National Science Foundation grant AST-0206197 and the NASA/STScI grant GO 9441 to the Univ. of South Carolina.

The damped Lyman-alpha (DLA) and sub-DLA quasar absorption lines provide powerful probes of the e... more The damped Lyman-alpha (DLA) and sub-DLA quasar absorption lines provide powerful probes of the evolution of metals, gas, and stars in galaxies. One major obstacle in trying to understand the evolution of DLAs and sub-DLAs has been the small number of metallicity measurements at z < 1.5, an epoch spanning \~70 % of the cosmic history. In recent surveys with the Hubble Space Telescope and Multiple Mirror Telescope, we have doubled the DLA Zn sample at z < 1.5. Combining our results with those at higher redshifts from the literature, we find that the global mean metallicity of DLAs does not rise to the solar value at low redshifts. These surprising results appear to contradict the near-solar mean metallicity observed for nearby (z ~ 0) galaxies and the predictions of cosmic chemical evolution models based on the global star formation history. Finally, we discuss direct constraints on the star formation rates (SFRs) in the absorber galaxies from our deep Fabry-Perot Ly-alpha imaging study and other emission-line studies in the literature. A large fraction of the observed heavy-element quasar absorbers at 0 < z < 3.4 appear to have SFRs substantially below the global mean SFR, consistent with the low metallicities observed in the spectroscopic studies.

The Astrophysical Journal, 1989

Journal of Astrophysics and Astronomy Supplement, Jun 1, 1995

ABSTRACT We report the discovery of a “super-damped” Lyman-alpha absorber at z_abs = 2.2 toward Q... more ABSTRACT We report the discovery of a “super-damped” Lyman-alpha absorber at z_abs = 2.2 toward QSO Q1135-0010 in the Sloan Digital Sky Survey, and follow-up VLT UVES spectroscopy. This is the highest-N(HI) QSO DLA discovered to date, with log N(HI) = 22.05 pm 0.10. We present abundances of various elements in this DLA, which imply about 1/10 solar metallicity. Our data indicate detection of Ly-alpha emission in the DLA trough, implying a star formation rate (SFR) of at least 10 solar masses per year in the absence of dust attenuation. We also discuss the detection of C II* and Si II* absorption (the first Si II* detection in an intervening QSO DLA) to estimate the SFR surface density and electron density. Overall, this is a robustly star-forming, moderately enriched absorber, but with relatively low dust depletion. Finally, we compare this absorber with DLAs toward other QSOs and gamma-ray bursts. This work was funded in part by NSF grants AST-0908890 and AST-1108830.