arXiv subjects

Sara L. Ellison

Publications and source records attributed to Sara L. Ellison.

At least 217 records · Page 12Linked to original sources

arXiv2004

Emission Line Abundances of Absorption Selected Galaxies at z<0.5

We have obtained optical spectra of four galaxies associated with MgII QSO absorbers at redshifts 0.10 < z < 0.45. We calculate the gas-phase oxygen abundance of these galaxies using the empirical R23 strong line method. The absolute B-band magnitudes of the galaxies span -20.6 < M_B < -18.3. If the metallicities lie on the R23 upper branch (8.4 < log (O/H) + 12 < 8.9), then the metallicities of these absorption selected galaxies span the range between 0.5--1.4 solar and would be consistent with the well-known luminosity-metallicity relation for 0.10 < z < 0.45 emission-line galaxies. However, such metallicities would be 0.5--1.0 dex higher than those observed in damped Lyman alpha systems (DLAs) via absorption line measurements at similar redshifts. Conversely, the lower R23 branch calibration yields metallicities approximately 1/7 solar, consistent with the DLA absorption metallicities at low redshifts. In this case, the absorption selected galaxies would lie significantly lower than the luminosity-metallicity relation for emission-line galaxies at z<0.5. We discuss the implications and possible solutions for each scenario.

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arXiv2004

The Number Density of 0.6 < z < 1.7 MgII Systems from CORALS: Observational Biases at Intermediate Redshift

The goal of the Complete Optical and Radio Absorption Line System (CORALS) survey is to quantify the potential impact on QSO absorber statistics from dust in intervening galaxies. Dust may introduce a selection bias in surveys which are based on magnitude limited QSO samples, leading to an underestimate of absorber number density, n(z). Here we present the results of the second phase of the CORALS survey which extends our previous work on z > 1.8 damped Lyman alpha systems (DLAs) to search for strong metal line systems (candidate DLAs) in the range 0.6 < z < 1.7. We have identified 47 MgII systems with rest frame equivalent widths EW(MgII lambda 2796) > 0.3 A in our sample of 75 radio-selected quasars. Our principal and most robust result is that the n(z) of low redshift MgII systems determined for the CORALS survey is in excellent agreement with that of optically-selected, magnitude limited QSO samples. The statistically inferred number density of DLAs, n(z)=0.16^+0.08_-0.06, is consistent with other low redshift samples, although the large 1 sigma error bars permit up to a factor of 2.5 more DLAs in CORALS. However, confirmation of the DLA candidates, precise evaluation of their n(z) and measurement of their HI column densities awaits UV observations with the Hubble Space Telescope. Finally, we report an excess of intermediate redshift MgII systems observed towards bright QSOs which could be due to a lensing amplification bias. However, there is also evidence that this excess could simply be due to more sensitive EW detection limits towards brighter QSOs. We also emphasize that absorber statistics determined from magnitude limited surveys reach a steady value if the completeness limit is significantly fainter than the fiducial value of the quasar luminosity function. (Abridged).

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arXiv2003

The Sizes and Kinematic Structure of Absorption Systems Towards the Lensed Quasar APM08279+5255

We have obtained spatially resolved spectra of the z=3.911 triply imaged QSO APM08279+5255 using the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). We study the line of sight equivalent width (EW) differences and velocity shear of high and low ionization absorbers (including a damped Lyman alpha [DLA] system identified in a spatially unresolved ground based spectrum) in the three lines of sight. We find that high ionization systems (primarily CIV absorbers) do not exhibit strong EW variations on scales <0.4 kpc; their fractional EW differences are typically less than 30%. When combined with previous work on other QSO pairs, we find that the fractional variation increases steadily with separation out to at least ~100 kpc. Conversely, low ionization systems (primarily MgII absorbers) show strong variations (often > 80%) over kpc scales. A minimum radius for strong (EW > 0.3 A) MgII systems of > 1.4 kpc is inferred from absorption coincidences in all lines of sight. For weak MgII absorbers (EW < 0.3 A), a maximum likelihood analysis indicates a most probable coherence scale of 2.0 kpc for a uniform spherical geometry, with 95% confidence limits ranging between 1.5 and 4.4 kpc. Finally, for systems with weak absorption that can be confidently converted to column densities, we find constant N(CIV)/N(SiIV) across the three lines of sight. Similarly, the [Al/Fe] ratios in the z = 2.974 DLA are consistent with solar relative abundances over a transverse distance of \~0.35 kpc. (abrdiged)

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arXiv2003

The Structure of High Redshift Galactic Halos

Observations of multiple or lensed QSOs at high redshift can be used to probe transverse structure in intervening absorption systems. Here we present results obtained from STIS spectroscopy of the unique triply imaged QSO APM08279+5255 at z = 3.9 and study the coherence scales of intervening low and high ionization absorbers.

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arXiv2003

The C IV Mass Density of the Universe at Redshift 5

In order to search for metals in the Lyman alpha forest at redshifts z > 4, we have obtained spectra of high S/N and resolution of three QSOs at z > 5.4 discovered by the Sloan Digital Sky Survey. These data allow us to probe to metal enrichment of the intergalactic medium at early times with higher sensitivity than previous studies. We find 16 C IV absorption systems with column densities log N(C IV) = 12.50 - 13.98 over a total redshift path Delta X = 3.29. In the redshift interval z = 4.5-5.0, where our statistics are most reliable, we deduce a comoving mass density of C IV ions Omega(C IV) = (4.3 +/- 2.5) x 10(-8) (90% confidence limits) for absorption systems with log N(C IV) > 13.0 (for an Einstein-de Sitter cosmology with h = 0.65). This value of Omega(C IV) is entirely consistent with those measured at z < 4; we confirm the earlier finding by Songaila (2001) that neither the column density distribution of C IV absorbers nor its integral show significant redshift evolution over a period of time which stretches from 1.25 to 4.5 Gyr after the big bang. This somewhat surprising conclusion may be an indication that the intergalactic medium was enriched in metals at redshifts much greater than 5, perhaps by the sources responsible for its reionization. Alternatively, the C IV systems we see may be associated with outflows from massive star-forming galaxies at later times, while the truly intergalactic metals may reside in regions of the Lyman alpha forest of lower density than those probed up to now.

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arXiv2003

The CORALS Survey: A Review and Progress Report on The Search for Dust Obscured Quasar Absorption Line Systems

We present a brief overview of the ongoing Complete Optical and Radio Absorption Line System (CORALS) survey. We review the results of our survey for z>2.2 damped Lyman alpha systems (DLAs) and present the preliminary results of an extended survey to lower redshifts (0.7<z<1.5). Although final observations are still pending, our preliminary statistics do not find an excess of absorption systems selected via MgII towards optically faint QSOs. We also briefly review the results from some spin-off projects from CORALS including the low alpha/Fe abundances of multiple DLAs and the possible excess of absorbers within 3000 km/s of the QSO redshift.

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arXiv2003

Distinct Abundance Patterns in Multiple Damped Ly-alpha Galaxies: Evidence for Truncated Star Formation?

(abridged) Following our previous work on metal abundances of a double damped Ly-alpha system with a line-of-sight separation ~2000 km/s (Ellison & Lopez 2001), we present VLT UVES abundances of 3 new systems spanning a total of \~6000 km/s at z~2.5 toward the southern QSO CTQ247. These abundances are supplemented with echelle observations of another `double' damped Ly-alpha system in the literature. We propose a definition in terms of velocity shift of the sub-class 'multiple damped Ly-alpha system', which is motivated by its possible connection with large-scale structure. We find that the abundance ratio alpha/Fe is systematically low in multiple systems compared with single systems, and with a small scatter. The same behavior is found in 2 more single DLA systems taken from the literature that show evidence of belonging to a galaxy group. After a careful investigation of possible sources of systematic errors, we conclude that the low alpha/Fe ratios in multiple DLAs have a nucleosynthetic origin. We suggest that they could be explained by reduced star formation in multiple damped Ly-alpha systems, possibly due to environmental effects.

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arXiv2003

The Absorption and Emission Kinematics in the z=0.7450 Mg II Absorber Towards Q1331+17

(Abridged) We present a comparative analysis of the galaxy emission and QSO absorption kinematics of a z~0.7450 MgII system and its candidate absorbing galaxy (G5) located 3.86 arcsecs (28.3 h-1 kpc) from the QSO. We have obtained a spectrum of the galaxy candidate, previously identified as a luminous edge-on disk, and detect the [OII] 3727 A doublet at a systemic redshift of z =0.7450. From slit spectroscopy of this galaxy, we find v_rot ~> 210 km/s, and possibly as large as 350 km/s. Plotted on the same velocity scale, the systemic redshift of the galaxy coincides with the center of the absorption system, although the absorption components span more than 100 km/s in either direction. However, once the sense of the rotation is taken into account, there is no absorbing gas at the projected velocity of the disk rotation curve. This implies incompatibility with a simple disk scenario. Moreover, a re-analysis of archival HST data reveals that the galaxy is only 0.3 L*, considerably less luminous than previously reported in the literature. Analysis of the absorption lines shows evidence for superbubbles in the interstellar medium (ISM) of the absorbing galaxy, based on the striking symmetry between components and large MgI/MgII and MgI/FeII ratios, indicative of large densities. The large velocity separations between line pairings, dv ~ 150 km/s indicates that these bubbles may be powered by OB associations comparable to the largest observed at z=0 and that the gas is probably enriched to at least 1/10 solar metallicity.

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arXiv2002

The Abundances of Nitrogen and Oxygen in Damped Lyman Alpha Systems

We take a fresh look at the abundance of nitrogen in DLAs with oxygen abundances between 1/10 and 1/100 of solar. This is a metallicity regime poorly sampled in the local universe and where QSO absorbers may hold clues to both the nucleosynthetic origin of N and the chemical evolution of high redshift galaxies. We combine new VLT UVES observations with others from the literature to form a sample of 10 DLAs in which the abundances of N and of one of the two alpha-capture elements O or S have been measured. We confirm earlier reports that the (N/O) ratio exhibits a larger range of values than other ratios of heavy elements in DLAs; however, all 10 DLAs fall in the region of the (N/O) vs. (O/H) plot delimited by the primary and secondary levels of nitrogen production. Our results provide empirical evidence in support of the proposal that intermediate mass stars (with masses less than 7 solar masses) are the main source of primary nitrogen, so that its release into the ISM lags behind that of oxygen, which is produced by Type II SNae. A high proportion (40%) of the DLAs in our sample have apparently not yet attained the full primary level of N enrichment; this finding may be an indication that the metallicity regime we are studying preferentially picks out galaxies which have only recently condensed out of the IGM and begun forming stars. Alternatively, the delay in the release of N following an episode of star formation may increase with decreasing metallicity, if stars of progressively lower masses than 4 solar masses can synthesize N in their hotter interiors.

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arXiv2002

Spatially resolved STIS spectra of the gravitationally lensed BAL quasar APM08279+5255: the nature of component C and evidence for microlensing

While gravitationally lensed quasars are expected to display an odd number of images, invariably systems are observed with an even number of quasars. For this, lensing galaxies must have very small core radii; this provides strong demagnification of one of the images. High resolution imaging of the gravitationally lensed BAL quasar, APM08279+5255, reveals three point-like images. As these images possess similar colours, it has been suggested that each represents a lensed image. Here, spatially resolved spectra of the individual components, obtained with STIS on the HST, are presented, clearly revealing that each is an image of the quasar. This confirms that APM08279+5255 represents the first example of an odd-image gravitationally lensed system. The implications for the properties of the lensing galaxy are discussed. It is also found that the individual images possess spectral differences indicative of the influence of gravitational microlensing in this system.

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arXiv2002

Unusual Metal Abundances in a Pair of Damped Lyman Alpha Systems at z~2

We present high resolution spectroscopic observations of two neighbouring damped Lya systems (DLAs) along the same line of sight towards B2314-409. Due to their separation (v ~ 2000 km/s) and the high spectral resolution of the data, it is possible to fit not only the weak metal transitions, but also the separate HI absorption profiles. This has permitted, for the first time, a detailed study of metal abundances in two neighbouring galaxy-scale absorbers. The two DLAs have z_abs = 1.8573 and 1.8745 and have column densities log N(HI) = 20.9+/-0.1 and 20.1+/-0.2 respectively. We have determined abundances for a range of chemical elements, and find that BOTH absorbers towards B2314-409 have low alpha/Fe-peak abundances compared with other known DLAs. This indicates that not only has the recent star formation history of these absorbers been relatively passive, but that the group environment, or some other external factor, may have influenced this.

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arXiv2001

The CORALS Survey II: Clues to Galaxy Clustering Around QSOs from z_abs ~ z_em Damped Lyman Alpha Systems

We present a list of z_abs ~ z_em (Delta v < 3000 km/s) DLAs discovered during the CORALS survey for absorbers in a radio-selected QSO sample. On the assumption that z_abs ~ z_em DLAs are neither ejecta from the QSO, nor part of the host galaxy itself, we use the number density statistics of these DLAs to investigate galaxy clustering near the QSO redshift. We find that n(z) of z_abs z_em DLAs in our radio-selected QSO sample is ~4 times larger than the number density of intervening DLAs, implying an excess of galaxies near the QSO. This result is further supported with the inclusion of the radio-loud QSOs in the FIRST survey, although the total number of DLAs is still small (4) and the result is currently only significant at approximately the 2 sigma level. Since all of the z_abs ~ z_em DLAs we identify in CORALS are found towards optically bright (B < 20) QSOs, there is no strong evidence (based on these limited statistics) that this population suffers from a severe dust bias. We compare our results with those from an optically-selected, radio-quiet QSO sample in order to determine whether there is evidence for an excess of galaxies around radio-loud versus radio-quiet QSOs. We find that the n(z) of z_abs ~ z_em DLAs towards radio-quiet QSOs is in agreement with the number density of intervening absorbers. This result, although currently limited by the small number statistics of our survey, supports the conclusion that radio-loud quasars are found preferentially in rich galaxy environments at high redshift. Finally, we report that one of the new z_abs ~ z_em DLAs discovered by CORALS has some residual flux in the base of the Lya trough which may be due to Lya emission, either from star formation in the DLA galaxy or from gas photoionised by the QSO.

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arXiv2001

The CORALS Survey I: New Estimates of the Number Density and Gas Content of Damped Lyman Alpha Systems Free from Dust Bias

(Abridged) We present the first results from the Complete Optical and Radio Absorption Line System (CORALS) survey. We have compiled a homogeneous sample of radio-selected QSOs from the Parkes Catalogue and searched for damped Lyman alpha systems (DLAs) towards every target, irrespective of its optical magnitude. This approach circumvents selection effects -- particularly from intervening dust -- which have long been suspected to affect DLA surveys in optically-selected, magnitude-limited QSO samples. The CORALS data set consists of 66 z_em > 2.2 QSOs in which 22 DLAs with absorption redshifts 1.8 < z_abs < z_em have been identified over a total redshift interval Delta z = 55.46. In this first paper of the CORALS series we describe the sample, present intermediate resolution spectroscopy and determine the population statistics of DLAs. We deduce a value of the neutral gas mass density traced by DLAs (expressed as a fraction of the closure density) log Omega_DLA h = -2.59^{+0.17}_{-0.24}, and a number density of DLAs per unit redshift n(z) = 0.31^{+0.09}_{-0.08}, both at a mean redshift = 2.37. Taking into account the errors, we conclude that dust-induced bias in previous surveys may have led to an underestimate of these quantities by at most a factor of two and we have not uncovered a previously unrecognised population of high column density DLAs in front of faint QSOs.

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arXiv2001

Uncloaking the High Redshift Galaxy Population With The Complete Optical and Radio Absorption Line System (CORALS) Survey

We present the first results from the Complete Optical and Radio Absorption Line System (CORALS) survey for Damped Lyman Alpha (DLA) systems based on a radio-selected sample of quasars. We find that the number density and neutral gas content of CORALS DLAs is in good agreement with previous surveys. These results indicate that dust obscuration has probably not been responsible for a significant selection bias in the past, although the statistics still permit an underestimate of n(z) and Omega_DLA by up to a factor of about 2.

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arXiv2001

The First Detection of Co in a Damped Lyman Alpha System

The study of elemental abundances in Damped Lyman Alpha systems (DLAs) at high redshift represents one of our best opportunities to probe galaxy formation and chemical evolution at early times. By coupling measurements made in high z DLAs with our knowledge of abundances determined locally and with nucleosynthetic models, we can start to piece together the star formation histories of these galaxies. Here, we discuss the clues to galactic chemical evolution that may be gleaned from studying the abundance of Co in DLAs. We present high resolution echelle spectra of two QSOs, Q2206-199 and Q1223+17, both already known to exhibit intervening damped systems. These observations have resulted in the first ever detection of Co at high redshift, associated with the z= 1.92 DLA in the sightline towards Q2206-199. We find that the abundance of Co is approximately 1/4 solar and that there is a clear overabundance relative to iron, [Co/Fe] = +0.31 +/- 0.05. From the abundance of Zn, we determine that this is a relatively metal-rich DLA, with a metallicity approximately 1/3 solar. Therefore, this first detection of Co is similar to the marked overabundance relative to Fe seen in Galactic bulge and thick disk stars.

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arXiv2001

The First Detection of Cobalt in a Damped Lyman Alpha System

We present the first ever detection of Cobalt in a Damped Lyman Alpha system (DLA) at z = 1.92. In addition to providing important clues to the star formation history of these high redshift galaxies, we discuss how studying the Co abundance in DLAs may also help to constrain models of stellar nucleosynthesis in a regime not probed by Galactic stars.

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arXiv2000

An Imaging and Spectroscopic Study of the z=3.38639 Damped Lyman Alpha System in Q0201+1120: Clues to Star Formation Rate at High Redshift

We present the results of a series of imaging and spectroscopic observations aimed at identifying and studying the galaxy responsible for the z = 3.38639 damped lya system in the z = 3.61 QSO Q0201+1120. We find that the DLA is part of a concentration of matter which includes at least four galaxies (probably many more) over linear comoving dimensions, greater than 5h^-1Mpc. The absorber may be a 0.7 L* galaxy at an impact parameter of 15 h^-1 kpc, but follow-up spectroscopy is still required for positive identification. The gas is turbulent, with many absorption components distributed over approximately 270 km/s and a large spin temperature, T_s greater than 4000K. The metallicity is relatively high for this redshift, Z(DLA) approximately 1/20 Z(solar). From consideration of the relative ratios of elements which have different nucleosynthetic timescales, it would appear that the last major episode of star formation in this DLA occurred at z greater than 4.3, more than approximately 500 Myr prior to the time when we observe it.

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arXiv2000

The Enrichment History of the Intergalactic Medium: Measuring the CIV/HI Ratio in the Lyman Alpha Forest

We have obtained an exceptionally high S/N, high resolution spectrum of the gravitationally lensed quasar Q1422+231 in order to investigate the metal enrichment of the IGM. A column density distribution function (CDDF) is determined for CIV, which is well fit by a power law with index 1.44 +/- 0.05. Once a correction for incompleteness is applied, we find that the CDDF is consistent with a power law down to log N(CIV) = 11.75, a factor of 10 more sensitive than previously determined. In order to determine whether the CIV enrichment extends to even lower column density HI clouds, we find that an analysis of individual pixel optical depths is more robust than the stacking technique. The results obtained from this optical depth method are compared with analyses of simulated spectra enriched with varying CIV enrichment recipes. From these simulations, we conclude that more CIV than is currently directly detected in Q1422+231 is required to reproduce the optical depths determined from the data, consistent with the conclusions drawn from consideration of the power law distribution. (Abridged)

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