arXiv subjects

Sandhya M. Rao

Publications and source records attributed to Sandhya M. Rao.

31 records · Page 2Linked to original sources

arXiv2004

Elemental Abundances in Two High Column Density Damped Lyman Alpha Systems at z < 1.5

We present Keck/HIRES abundance measurements and metal-line kinematic profiles of the damped Lyman alpha systems (DLAs) towards the quasars Q0933+733 (z_abs=1.479) and Q0948+433 (z_abs=1.233). These two DLAs have among the five highest HI column densities at any redshift: N(HI)=4.2E21 cm^{-2}. The metal-line data, presented here for the first time, reveal that these DLAs are noteworthy for several other reasons as well. 1) The Q0933+733 DLA exhibits simple kinematic structure with unusually narrow velocity widths as measured from its unsaturated metal lines (delta v=16 km/s). At 2.6% solar, it has the second lowest metallicity at z<2. 2) The Q0948+433 DLA has among the strongest metal-line transitions of any known DLA. The saturated SiII1808 line implies a high metallicity ([Si/H]> -1) and a significant alpha-enhancement. The strong metal lines of this DLA have made possible the detection of TiII1910, CoII2012, and MgI2026. 3) We find that the relative gas-phase abundances of both DLAs follow the general trend seen at high redshift, e.g., enhanced Zn/Fe and Si/Fe, and sub-solar Mn/Fe, indicating that there is little evolution in the nucleosynthetic patterns of DLAs down to this epoch. 4) Their high HI column densities imply that these DLAs dominate the column density-weighted cosmic mean metallicity, , of the universe at z<1.5. Using the 15 DLAs with measured metallicities in the redshift interval 0.4 = -0.89^{+0.40}_{-0.33}, where the uncertainties are 95% confidence limits.

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arXiv2004

Double-Damped Lyman Alpha Absorption: A Possible Large Neutral Hydrogen Gas Filament Near Redshift z=1

We report the discovery of two damped Ly-alpha absorption-line systems (DLAs) near redshift z=1 along a single quasar sightline (Q1727+5302) with neutral hydrogen column densities of N(HI) = (1.45\pm0.15)\times10^{21} and (2.60\pm0.20)\times10^{21} atoms/cm2. Their sightline velocity difference of 13,000 km/s corresponds to a proper separation of 106h_{70}^{-1} Mpc if interpreted as the Hubble flow (Omega_m=0.3, Omega_Lambda=0.7). The random probability of such an occurrence is significantly less than 3%. With follow-up spectroscopy, we find [Zn/H] = -0.58\pm0.15 (26.5% solar) and -1.32\pm0.28 (4.7% solar), respectively, and [Cr/H] = -1.26\pm0.15 (5.5% solar) and -1.77\pm0.28 (1.7% solar), respectively, which is evidence for depletion onto grains. Follow-up IR images show the two most likely DLA galaxy candidates to have impact parameters of 22h_{70}^{-1} kpc and 32h_{70}^{-1} kpc if near z=1. They are significantly underluminous relative to the galaxy population at z=1. To investigate the possibility of additional high-N(HI) absorbers we have searched the SDSS database for z>1 quasars within 30 arcmin of the original sightline. Five were found, and two show strong MgII-FeII absorption near z=1, consistent with classical DLA absorption approx 37% of the time, but almost always N(HI) > 10^{19} atoms/cm2. Consequently, this rare configuration of four high-N(HI) absorbers with a total sightline velocity extent of 30,600 km/s may represent a large filament-like structure stretching over a proper distance of 241h_{70}^{-1} Mpc along our sightline, and a region in space capable of harboring excessive amounts of neutral gas. Future studies of this region of the sky are encouraged.

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arXiv2003

The HII Regions of the Damped Lyman alpha Absorber SBS 1543+593

We report new imaging and spectroscopic observations of the damped Ly alpha (DLA) galaxy SBS 1543+593, a nearby dwarf galaxy whose stellar disk is intersected by the sightline 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) HII regions. Ground-based long-slit spectroscopy of the brightest HII region in the galaxy yields estimates of the star formation rate (SFR) and of chemical abundances in the galaxy's interstellar medium. We find that SBS 1543+593 exhibits a SFR $\approx 0.006 h^{-2}_{70} M_{\odot}$ yr$^{-1}$, or a SFR per unit area of $\approx 1.4 \times 10^{-4} h^{-2}_{70} M_\odot$ yr$^{-1}$ kpc$^{-2}$. We derive gas-phase abundances in the ionized gas of $12 + \log {\rm (O/H)} = 8.2 \pm 0.2$, which is about 1/3 of the solar value, and $\log {\rm (N/O)} = -1.40^{+0.2}_{-0.3}$. These values are consistent with the morphologial appearance of SBS 1543+593, an Sm dwarf of $M_B -5 \log h_{70} = -16.8\pm0.2$ and of intermediate surface brightness. SBS 1543+593 is the first {\it 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.

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arXiv2003

Damped Lyman Alpha Gas Metallicities at Redshifts z=0.9-2.0 from SDSS Spectra

Using Sloan Digital Sky Survey (SDSS) early data release spectra, we have identified 370 MgII absorption systems with MgII 2796 rest equivalent widths >= 1A and redshifts z=0.9-2.2. From our previous and ongoing HST UV spectroscopic studies, we estimate that the mean neutral hydrogen column density of a system selected in this manner is N(HI) = 3.6 +/- 1.3 x10^(20) atoms cm^(-2), which corresponds to the damped Ly-alpha (DLA) regime. We have formed high signal-to-noise ratio composite spectra using 223 of these systems with z=0.9-2.0 in order to study the strength of the ZnII and CrII absorption lines corresponding to this mean neutral hydrogen column density. After making a correction for missed DLAs, overall we find that [Zn/H] = -1.13 +/- 0.19. We find [Cr/Zn] = -0.45 +/- 0.13, which indicates that approx 65% of the Cr is depleted on to grains, but this does not correct for the missed DLAs. We have also derived Zn and Cr abundances in two kinematic regimes, and within each regime we consider two redshift intervals. We find trends which indicate that metallicities are higher in the composites where the absorption has larger velocity spreads as measured by MgII 2796 rest equivalent width. Larger velocity spreads may correspond to deeper gravitational potential wells which represent more massive and chemically evolved structures, and/or regions associated with winds from starbursting galaxies, also leading to kinematically broad structures of chemically enriched gas. Within the large velocity spread regime, we find that at lower redshifts the Zn metallicity is larger and more Cr is depleted on to grains.

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arXiv2003

Low-Redshift Damped Lyman Alpha Galaxies Toward the Quasars B2 0827+243, PKS 0952+179, PKS 1127-145, and PKS 1629+120

We present optical and near-infrared ground-based imaging results on four low-redshift damped Ly-alpha (DLA) galaxies. The corresponding DLA systems were discovered in our HST spectroscopic surveys for DLA lines in known strong MgII absorption-line systems toward the quasars B2 0827+243 (z{DLA}=0.525), PKS 0952+179 (z{DLA}=0.239), PKS 1127-145 (z{DLA}=0.313), and PKS 1629+120 (z{DLA}=0.532). The DLA galaxies span a mixture of morphological types from patchy, irregular, and low surface brightness to spiral galaxies. The luminosities range from 0.02L_K* to 1.2L_K*. We also discovered several extremely red objects (EROs) in two of these fields and discuss the possibility that they are associated with the DLA galaxies. These observations add to the small but growing list of DLA galaxies at low redshift. At the present time, 14 DLA galaxies in the redshift range 0.05 < z <1 have been studied. The distributions of DLA galaxy properties for these 14 cases are discussed and some important trends emerge. Low-luminosity dwarf galaxies with small impact parameters dominate this small sample. Also, four of the five highest column density systems, which dominate in the determination of the cosmological neutral gas mass density, arise in low surface brightness dwarf galaxies. Zwaan et al. have shown that only 15% of the neutral gas at the present epoch is contained in low surface brightness galaxies. Thus, if the low-redshift DLA galaxy trends hold up with larger samples, it would indicate that a different population of objects is responsible for the bulk of the neutral hydrogen gas in the universe at z approx 0.5. (Abridged)

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arXiv2003

Chandra ACIS-S Observations of Three Quasars with Low-Redshift Damped Ly-alpha Absorption: Constraints on the Cosmic Neutral-Gas-Phase Metallicity at Redshift z \approx 0.4

Chandra X-Ray Observatory (CXO) ACIS-S spectra of three quasars which lie behind three foreground damped Lyman alpha (DLA) absorbers are analyzed in order to attempt to determine the amount of photoelectric absorption due to metals present in their x-ray spectra. These absorbers are the three largest neutral hydrogen column density absorption-line systems known at low redshift (0.313 \le z_abs \le 0.524). They have HI column densities which lie between 3E21 and 5E21 atoms/cm^2. At these redshifts the amount of photoelectric absorption at x-ray energies is primarily an indicator of the oxygen abundance. Since the column densities of these systems are so high, one would expect accurate metallicity measurements of them to yield a robust estimate of the column-density-weighted cosmic neutral-gas-phase metallicity at z \approx 0.4. We consider cases where the DLA gas has solar element abundance ratios and ones with the alpha-group element abundance ratios enhanced. For the adopted assumptions, the column-density-weighted cosmic neutral-gas-phase metallicity of the non-enhanced elements (e.g. Zn) at z \approx 0.4 likely lies in the range \approx 0.04-0.38 Z_solar.

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arXiv2002

The Non-Damped Nature of Twelve Low-Redshift Damped Lyman Alpha Candidate Systems

Hubble Space Telescope (HST) UV spectroscopy of 12 candidate low-redshift damped Lyman alpha (DLA) systems in 11 QSOs (z=0.103 in Q0054+144, z=0.969 and z=0.987 in Q0302-223, z=0.478 in Q0454-220, z=1.476 in Q1047+550, z=1.070 in Q1206+459, z=1.228 in Q1247+267, z=0.399 in Q1318+290B, z=0.519 in Q1329+412, z=0.276 in Q1451-375, z=0.204 in Q2112+059, z=0.263 in Q2251+113) are presented; the observations demonstrate that they are not DLAs with $N(HI)\ge2\times10^{20}$ atoms cm$^{-2}$. In all cases except two the systems either do not exist or are well below the DLA threshold column density; the exceptions are a z=0.474 system in Q0454-220 which has $N(HI)=3\times10^{19}$ atoms cm$^{-2}$ and a z=1.223 system in Q1247+267 which has $N(HI)=8\times10^{19}$ atoms cm$^{-2}$. Despite the availability of data in the HST archives demonstrating that these are not suitable targets, many have unfortunately been approved for observation with Chandra, Gemini, and/or HST with the intent of doing followup work on low-redshift DLAs. Furthermore, these results indicate that the low-redshift DLA statistics derived from IUE spectra and presented by Lanzetta, Wolfe, & Turnshek (1995) and Wolfe et al. (1995) are invalid.

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arXiv2001

The Evolution of Neutral Gas in the Universe as Traced by Damped Lyman Alpha Systems

We discuss our recent results on the statistical properties of damped Lyman alpha systems (DLAs) at low redshift (z<1.65) (Rao & Turnshek 2000). Contrary to expectations, we found that the cosmological neutral gas mass density as traced by DLAs, $Ω_{DLA}$, does not evolve from redshifts $z \approx 4$ to $z \approx 0.5$ and that extrapolation to z=0 results in a value that is a factor of ~6.5 times higher than what is derived from galaxies at the current epoch using HI 21 cm emission measurements. We review the current status of HI measurements at low redshift and at the current epoch, and discuss possible causes of this discrepancy.

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arXiv2001

A Comparative Study of Damped Lyman Alpha Galaxies

We compare the properties of a sample of local (z=0) gas-rich galaxies studied in 21 cm emission to a sample of 13 low-redshift (z < 1) damped Lyman alpha (DLA) galaxies identified as the counterparts of low-redshift DLA systems found in QSO absorption-line surveys. This absorption-selected sample has average redshift = 0.5. We find that many of the properties of the two samples are comparable. However, consideration of the statistical results on all known low-redshift DLA systems indicates that there is: (1) a somewhat higher incidence and cosmological mass density for the low-redshift DLA systems in comparison to expectations at z=0 and (2) an unexpectedly high rate of occurrence of very large column density [N(HI) > 1E21 atoms/cm^2] low-redshift DLA systems; both of these results are discussed by Rao & Turnshek in these proceedings. These differences, coupled with imaging studies, suggest that there may be an excess of low-redshift DLA galaxies in the form of dwarf and/or low surface brightness galaxies. Some examples of low-redshift DLA galaxies are shown by Nestor et al. in these proceedings.

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arXiv2001

Properties of Low-Redshift Damped Lyman Alpha Galaxies

Images of five QSO fields containing six damped Lyman alpha (DLA) systems at redshifts 0.09<z<0.53 are presented. Identifications for the DLA galaxies giving rise to the DLA systems are made. The observed and modeled characteristics of the DLA galaxies are discussed. The DLA galaxies have impact parameters ranging from < 4 kpc to \approx 34 kpc and luminosities in the range \approx 0.03L* to \approx 1.3L*. Their morphologies include amorphous low surface brightness systems, a probable dwarf spiral, and luminous spirals.

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arXiv2001

Hubble Space Telescope Observations of the Gravitationally Lensed Cloverleaf Broad Absorption Line QSO H1413+1143: Imaging Polarimetry and Evidence for Microlensing of a Scattering Region

We report the results of HST WFPC2 broadband F555W and F702W photometric and F555W polarimetric observations of the "Cloverleaf" QSO H1413+1143. This is a four-component gravitationally-lensed broad absorption line (BAL) QSO. Observations were obtained at two epochs in March 1999 and June 1999 separated by about 100 days. The goal of our program was to detect any relative changes among the components and between the two epochs. Over this time baseline we detected an approximately 0.07 mag dimming in component D of the lensed image, which we interpret as evidence for microlensing. In March 1999 we find significant evidence for a difference in the relative linear polarization of component D in comparison to the other three components; in June 1999 the combined polarization of the Cloverleaf components was lower. In March 1999 the apparently microlensed component D has a rotated polarization position angle and a somewhat higher degree of polarization than the other three components. We suggest that this difference in polarization is due to microlensing magnification of part of a scattered-light (i.e. polarized) continuum-producing region. The results indicate that in the Cloverleaf the size-scale of the polarized scattered-light region exceeds about 10^{16} cm but lies interior to the region producing the broad emission lines (< 10^{18} cm).

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arXiv2000

Discovery of Damped Lyman-Alpha Systems at Redshifts Less Than 1.65 and Results on their Incidence and Cosmological Mass Density

We report results on the incidence and cosmological mass density of damped Lyman-alpha (DLA) systems at redshifts less that 1.65. We used HST and an efficient non-traditional (but unbiased) survey technique to discover DLA systems at redshifts z<1.65, where we observe the Lyman-alpha line in known MgII absorption-line systems. We uncovered 14 DLA lines including 2 serendipitously. We find that (1) The DLA absorbers are drawn almost exclusively from the population of MgII absorbers which have rest equivalent widths W(2796)>0.6A. (2) The incidence of DLA systems per unit redshift, n(DLA), is observed to decrease with decreasing redshift. (3) On the other hand, the cosmological mass density of neutral gas in low-redshift DLA absorbers, Omega(DLA), is observed to be comparable to that observed at high redshift. (4) The low-redshift DLA absorbers exhibit a significantly larger fraction of very high column density systems in comparison to determinations at both high redshift and locally.

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arXiv1998

Discovery of z=0.0912 and z=0.2212 Damped Lyman-alpha Absorption Line Systems Toward the Quasar OI 363: Limits on the Nature of Damped Lyman-alpha Galaxies

The discovery of a z_abs = 0.0912 damped Lyman-alpha absorption-line system in the HST-FOS ultraviolet spectrum of the quasar OI 363 (0738+313) is reported. This is the lowest redshift quasar damped Lyman-alpha system known. Its neutral hydrogen column density is N(HI) = 1.5(+/- 0.2) E21 atoms/cm^2, which easily exceeds the classical criterion for damped Lyman-alpha of N(HI) greater than or equal to 2E20 atoms/cm^2. Remarkably, a z_abs = 0.2212 damped system with N(HI) = 7.9(+/- 1.4) E20 atoms/cm^2 has also been discovered in the same spectrum. In the past, the standard paradigm for damped Lyman-alpha systems has been that they arise in galactic or protogalactic HI disks with low impact parameters in luminous galaxies. However, WIYN imaging of the OI 363 field shows that none of the galaxies visible in the vicinity of the quasar is a luminous gas-rich spiral with low impact parameter, either at z = 0.0912 or z = 0.2212. Thus, these damped systems are among the clearest examples yet of cases that are inconsistent with the standard damped Lyman-alpha - HI-disk paradigm.

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