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The strength of the lithosphere is typically modelled based on constitutive equations for steady-state flow. However, models of lithospheric flexure reveal differences in lithospheric strength that are difficult to reconcile based on such flow laws. Recent rheological data from low-temperature deformation experiments on olivine suggest that this discrepancy may be largely explained by strain hardening. Details of the mechanical data, specifically the effects of temperature-independent back stresses stored in the samples, indicate that strain hardening in olivine occurs primarily due to long-range elastic interactions between dislocations. These interpretations provided the basis for a new flow law that incorporates hardening by development of back stress. Here, we test this hypothesis by examining the microstructures of olivine samples deformed plastically at room temperature either in a deformation-DIA apparatus at differential stresses of < 4.3 GPa or in a nanoindenter at applied contact stresses of > 10.2 GPa. High-angular resolution electron backscatter diffraction maps reveal the presence of geometrically necessary dislocations with densities commonly above 10$^{14}$ m$^{-2}$ and intragranular heterogeneities in residual stress on the order of 1 GPa in both sets of samples. Scanning transmission electron micrographs reveal straight dislocations aligned along slip bands and interacting with dislocations of other types that act as obstacles. The stress heterogeneities and accumulations of dislocations along their slip planes are consistent with strain hardening resulting from long-range back-stresses acting between dislocations. These results corroborate the mechanical data in supporting the form of the new flow law for low-temperature plasticity and provide new microstructural criteria for identifying the operation of this deformation mechanism in natural samples.

We present a study exploring the nature and properties of the Circum-Galactic Medium (CGM) and its connection to the atomic gas content in the interstellar medium (ISM) of galaxies as traced by the HI 21cm line. Our sample includes 45 low-z (0.026-0.049) galaxies from the GALEX Arecibo SDSS Survey. Their CGM was probed via absorption in the spectra of background Quasi-Stellar Objects at impact parameters of 63 to 231kpc. The spectra were obtained with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope. We detected neutral hydrogen (Ly$\alpha$ absorption-lines) in the CGM of 92% of the galaxies. We find the radial profile of the CGM as traced by the Ly$\alpha$ equivalent width can be fit as an exponential with a scale length of roughly the virial radius of the dark matter halo. We found no correlation between the orientation of sightline relative to the galaxy major axis and the Ly$\alpha$ equivalent width. The velocity spread of the circumgalactic gas is consistent with that seen in the atomic gas in the interstellar medium. We find a strong correlation (99.8% confidence) between the gas fraction (M(HI)/M*) and the impact-parameter-corrected Ly$\alpha$ equivalent width. This is stronger than the analogous correlation between corrected Ly$\alpha$ equivalent width and SFR/M* (97.5% confidence). These results imply a physical connection between the HI disk and the CGM, which is on scales an order-of-magnitude larger. This is consistent with the picture in which the HI disk is nourished by accretion of gas from the CGM., Comment: 13 pages, 9 figures, and 2 tables. Submitted to ApJ

The Magellanic Stream (MS) is a well-resolved gaseous tail originating from the Magellanic Clouds. Studies of its physical properties and chemical composition are needed to understand its role in Galactic evolution. We investigate the properties of a compact HVC (CHVC 224.0-83.4-197) lying close on the sky to the MS to determine whether it is physically connected to the Stream and to examine its internal structure. Our study is based on analysis of HST/COS spectra of three QSOs (Ton S210, B0120-28, and B0117-2837) all of which pass through this single cloud at small angular separation (\lessim 0.72{\deg}), allowing us to compare physical conditions on small spatial scales. No significant variation is detected in the ionization structure from one part of the cloud to the other. Using Cloudy photoionization models, toward Ton S210 we derive elemental abundances of [C/H] = -1.21 +/- 0.11, [Si/H] = -1.16 +/- 0.11, [Al/H] = -1.19 +/- 0.17 and [O/H] = -1.12 +/- 0.22, which agree within 0.09 dex. The CHVC abundances match the 0.1 solar abundances measured along the main body of the Stream. This suggests that the CHVC (and by extension the extended network of filaments to which it belongs) has an origin in the MS. It may represent a fragment that has been removed from the Stream as it interacts with the gaseous Galactic halo., Comment: ApJ, accepted, 13 pages, 12 figures, 4 tables

We report new observations of circumgalactic gas from the COS-Dwarfs survey, a systematic investigation of the gaseous halos around 43 low-mass z $\leq$ 0.1 galaxies using background QSOs observed with the Cosmic Origins Spectrograph. From the projected 1D and 2D distribution of C IV absorption, we find that C IV absorption is detected out to ~ 0.5 R$_{vir}$ of the host galaxies. The C IV absorption strength falls off radially as a power law and beyond 0.5 R$_{vir}$, no C IV absorption is detected above our sensitivity limit of ~ 50-100 m$\AA$. We find a tentative correlation between detected C IV absorption strength and star formation, paralleling the strong correlation seen in highly ionized oxygen for L~L* galaxies by the COS-Halos survey. The data imply a large carbon reservoir in the CGM of these galaxies, corresponding to a minimum carbon mass of $\gtrsim$ 1.2$\times 10^6$ $M_\odot$ out to ~ 110 kpc. This mass is comparable to the carbon mass in the ISM and more than the carbon mass currently in stars of these galaxies. The C IV absorption seen around these sub-L* galaxies can account for almost two-thirds of all $W_r$> 100 m$\AA$ C IV absorption detected at low z. Comparing the C IV covering fraction with hydrodynamical simulations, we find that an energy-driven wind model is consistent with the observations whereas a wind model of constant velocity fails to reproduce the CGM or the galaxy properties., Comment: 18 Pages, 11 Figures, ApJ 796 136

Dynamic interactions between the two Magellanic Clouds have flung large quantities of gas into the halo of the Milky Way, creating the Magellanic Stream, the Magellanic Bridge, and the Leading Arm (collectively referred to as the Magellanic System). In this third paper of a series studying the Magellanic gas in absorption, we analyze the gas ionization level using a sample of 69 Hubble Space Telescope/Cosmic Origins Spectrograph sightlines that pass through or within 30 degrees of the 21 cm-emitting regions. We find that 81% (56/69) of the sightlines show UV absorption at Magellanic velocities, indicating that the total cross section of the Magellanic System is ~11 000 square degrees, or around a quarter of the entire sky. Using observations of the Si III/Si II ratio together with Cloudy photoionization modeling, we calculate that the total mass (atomic plus ionized) of the Magellanic System is ~2.0 billion solar masses, with the ionized gas contributing over twice as much mass as the atomic gas. This is larger than the current-day interstellar H I mass of both Magellanic Clouds combined, indicating that they have lost most of their initial gas mass. If the gas in the Magellanic System survives to reach the Galactic disk over its inflow time of ~0.5-1.5 Gyr, it will represent an average inflow rate of ~3.7-6.7 solar masses per year, potentially raising the Galactic star formation rate. However, multiple signs of an evaporative interaction with the hot Galactic corona indicate that the Stream may not survive its journey to the disk fully intact, and will instead add material to (and cool) the corona., Comment: Accepted for publication in ApJ, 32 pages, 7 figures, 3 tables, Figure 1 shown at low resolution to reduce file size

We analyze the physical conditions of the cool, photoionized (T $\sim 10^4$ K) circumgalactic medium (CGM) using the COS-Halos suite of gas column density measurements for 44 gaseous halos within 160 kpc of $L \sim L^*$ galaxies at $z \sim 0.2$. These data are well described by simple photoionization models, with the gas highly ionized (n$_{\rm HII}$/n$_{\rm H} \gtrsim 99\%$) by the extragalactic ultraviolet background (EUVB). Scaling by estimates for the virial radius, R$_{\rm vir}$, we show that the ionization state (tracked by the dimensionless ionization parameter, U) increases with distance from the host galaxy. The ionization parameters imply a decreasing volume density profile n$_{\rm H}$ = (10$^{-4.2 \pm 0.25}$)(R/R$_{\rm vir})^{-0.8\pm0.3}$. Our derived gas volume densities are several orders of magnitude lower than predictions from standard two-phase models with a cool medium in pressure equilibrium with a hot, coronal medium expected in virialized halos at this mass scale. Applying the ionization corrections to the HI column densities, we estimate a lower limit to the cool gas mass M$_{\rm CGM}^{\rm cool} > 6.5 \times 10^{10}$ M$_{\odot}$ for the volume within R $<$ R$_{\rm vir}$. Allowing for an additional warm-hot, OVI-traced phase, the CGM accounts for at least half of the baryons purported to be missing from dark matter halos at the 10$^{12}$ M$_{\odot}$ scale., Comment: 19 pages, 12 Figures, and a 37-page Appendix with 36 additional figures. Accepted to ApJ June 21 2014

We report on the detection of deuterated molecular hydrogen, HD, at $z = 0.18$. HD and H$_{\rm 2}$ are detected in HST/COS data of a low metallicity ($Z \sim 0.07Z_\odot$) damped Ly$\alpha$ system at $z = 0.18562$ toward QSO B0120$-$28, with log $N$(H I) = 20.50 $\pm$ 0.10. Four absorption components are clearly resolved in H$_{\rm 2}$ while two components are resolved in HD; the bulk of the molecular hydrogen is associated with the components traced by HD. We find total column densities log $N$(HD) = 14.82 $\pm$ 0.15 and log $N$(H$_{\rm 2}$) = 20.00 $\pm$ 0.10. This system has a high molecular fraction, $f$(H$_{\rm 2}$) = 0.39 $\pm$ 0.10 and a low HD to H$_{\rm 2}$ ratio, log (HD/2H$_{\rm 2}$) $= -5.5 \pm 0.2$ dex. The excitation temperature, $T_{01} = 65 \pm 2$ K, in the component containing the bulk of the molecular gas is lower than in other DLAs. These properties are unlike those in other higher redshift DLA systems known to contain HD, but are consistent with what is observed in dense clouds in the Milky Way., Comment: 18 pages, 6 figures. To be published in the Astrophysical Journal

We present the design and methods of the COS-Halos survey, a systematic investigation of the gaseous halos of 44 z = 0.15-0.35 galaxies using background QSOs observed with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope. This survey has yielded 39 spectra of z_em ~ 0.5 QSOs with S/N ~ 10-15 per resolution element. The QSO sightlines pass within 150 physical kpc of the galaxies, which span early and late types over stellar mass log M* / Msun= 9.5 - 11.5. We find that the CGM exhibits strong HI, averaging 1 Ang in Lya equivalent width out to 150 kpc, with 100% covering fraction for star-forming galaxies and 75% covering for passive galaxies. We find good agreement in column densities between this survey and previous studies over similar range of impact parameter. There is weak evidence for a difference between early- and late-type galaxies in the strength and distribution of HI. Kinematics indicate that the detected material is bound to the host galaxy, such that >~90% of the detected column density is confined within +/-200 km s^-1 of the galaxies. This material generally exists well below the halo virial temperatures at T<~ 10^5 K. We evaluate a number of possible origin scenarios for the detected material, and in the end favor a simple model in which the bulk of the detected HI arises in a bound, cool, low-density photoionized diffuse medium that is generic to all L* galaxies and may harbor a total gaseous mass comparable to galactic stellar masses., Comment: Accepted to ApJ, 87 pages, including 5 tables and 66 figures

We present the equivalent width and column density measurements for low and intermediate ionization states of the circumgalactic medium (CGM) surrounding 44 low-z, L ~ L* galaxies drawn from the COS-Halos survey. These measurements are derived from far-UV transitions observed in HST/COS and Keck/HIRES spectra of background quasars within an impact parameter R < 160 kpc to the targeted galaxies. The data show significant metal-line absorption for 33 of the 44 galaxies, including quiescent systems, revealing the common occurance of a cool (T ~ 10^{4 - 5} K), metal-enriched CGM. The detection rates and column densities derived for these metal lines decrease with increasing impact parameter, a trend we interpret as a declining metal surface density profile for the CGM. A comparison of the relative column densities of adjacent ionization states indicates the gas is predominantly ionized. The large surface density in metals demands a large reservoir of metals and gas in the cool CGM (very conservatively, M_ CGMcool > 10^9 MSun), which likely traces a distinct density and/or temperature regime from the highly-ionized CGM traced by OVI absorption. The large dispersion in absorption strengths (including non-detections) suggests the cool CGM traces a wide range of densities or a mix of local ionizing conditions. Lastly, the kinematics inferred from the metal-line profiles are consistent with the cool CGM being bound to the dark matters halos hosting the galaxies; this gas may serve as fuel for future star-formation. Future work will leverage this dataset to provide estimates on the mass, metallicity, dynamics, and origin of the cool CGM in low-z, L* galaxies., Comment: 19 pages, 15 figures, 7 Tables -- re-submitted to ApJS

We report new observations of circumgalactic gas in the halos of early type galaxies obtained by the COS-Halos Survey with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope. We find that detections of HI surrounding early type galaxies are typically as common and strong as around star-forming galaxies, implying that the total mass of circumgalactic material is comparable in the two populations. For early type galaxies, the covering fraction for HI absorption above 10^16 cm^2 is ~40-50% within ~150 kpc. Line widths and kinematics of the detected material show it to be cold (T ~< 10^5 K) in comparison to the virial temperature of the host halos. The implied masses of cool, photoionized CGM baryons may be up to 10^9 --- 10^11 Msun. Contrary to some theoretical expectations, strong halo HI absorbers do not disappear as part of the quenching of star-formation. Even passive galaxies retain significant reservoirs of halo baryons which could replenish the interstellar gas reservoir and eventually form stars. This halo gas may feed the diffuse and molecular gas that is frequently observed inside ETGs., Comment: 6 pages, 5 figures. Accepted to ApJL

The first systematic study of the warm gas (T=10^4-5 K) distribution across a galaxy cluster is presented using multiple background QSOs to the Virgo Cluster. We detect 25 Lya absorbers (N_HI = 10^13.1-15.4 cm^-2) in the Virgo velocity range toward 9 of 12 QSO sightlines observed with Cosmic Origin Spectrograph, with a cluster impact parameter range of 0.36-1.65 Mpc (0.23-1.05R_vir). Including 18 Lya absorbers previously detected by STIS or GHRS toward 7 of 11 background QSOs in and around the Virgo Cluster, we establish a sample of 43 absorbers towards a total of 23 background probes for studying the incidence of Lya absorbers in and around the Virgo Cluster. With these absorbers, we find: (1) warm gas is predominantly in the outskirts of the cluster and avoids the X-ray detected hot ICM. Also, Lya absorption strength increases with cluster impact parameter. (2) Lya absorbing warm gas traces cold HI emitting gas in the substructures of the Virgo Cluster. (3) Including the absorbers associated with the surrounding substructures, the warm gas covering fraction (100% for N_HI > 10^13.1 cm^-2) is in agreement with cosmological simulations. We speculate that the observed warm gas is part of large-scale gas flows feeding the cluster both the ICM and galaxies., Comment: 15 pages, 6 figures, 8 tables, ApJ in press

The circumgalactic medium (CGM) is fed by galaxy outflows and accretion of intergalactic gas, but its mass, heavy element enrichment, and relation to galaxy properties are poorly constrained by observations. In a survey of the outskirts of 42 galaxies with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope, we detected ubiquitous, large (150 kiloparsec) halos of ionized oxygen surrounding star-forming galaxies, but we find much less ionized oxygen around galaxies with little or no star formation. This ionized CGM contains a substantial mass of heavy elements and gas, perhaps far exceeding the reservoirs of gas in the galaxies themselves. It is a basic component of nearly all star-forming galaxies that is removed or transformed during the quenching of star formation and the transition to passive evolution., Comment: This paper is part of a set of three papers on circumgalactic gas observed with the Cosmic Origins Spectrograph on HST, to be published in Science, together with related papers by Tripp et al. and Lehner & Howk, in the November 18, 2011 edition. This version has not undergone final copyediting. Please see Science online for the final printed version

Outflowing winds of multiphase plasma have been proposed to regulate the buildup of galaxies, but key aspects of these outflows have not been probed with observations. Using ultraviolet absorption spectroscopy, we show that "warm-hot" plasma at 10^{5.5} K contains 10-150 times more mass than the cold gas in a poststarburst galaxy wind. This wind extends to distances >68 kiloparsecs, and at least some portion of it will escape. Moreover, the kinematical correlation of the cold and warm-hot phases indicates that the warm-hot plasma is related to the interaction of the cold matter with a hotter (unseen) phase at >>10^{6} K. Such multiphase winds can remove substantial masses and alter the evolution of poststarburst galaxies., Comment: This paper is part of a set of three papers on circumgalactic gas observed with the Cosmic Origins Spectrograph on HST, to be published in Science, together with related papers by Tumlinson et al. and Lehner & Howk, in the November 18, 2011 edition. This version has not undergone final copyediting. Please see Science online for the final printed version

We present a comprehensive abundance analysis of 20 elements for 16 new low-metallicity stars from the Chemical Abundances of Stars in the Halo (CASH) project. The abundances have been derived from both Hobby-Eberly Telescope High Resolution Spectrograph snapshot spectra (R~15,000) and corresponding high-resolution (R~35,000) Magellan MIKE spectra. The stars span a metallicity range from [Fe/H] from -2.9 to -3.9, including four new stars with [Fe/H]<-3.7. We find four stars to be carbon-enhanced metal-poor (CEMP) stars, confirming the trend of increasing [C/Fe] abundance ratios with decreasing metallicity. Two of these objects can be classified as CEMP-no stars, adding to the growing number of these objects at [Fe/H]<-3. We also find four neutron-capture enhanced stars in the sample, one of which has [Eu/Fe] of 0.8 with clear r-process signatures. These pilot sample stars are the most metal-poor ([Fe/H]<-3.0) of the brightest stars included in CASH and are used to calibrate a newly-developed, automated stellar parameter and abundance determination pipeline. This code will be used for the entire ~500 star CASH snapshot sample. We find that the pipeline results are statistically identical for snapshot spectra when compared to a traditional, manual analysis from a high-resolution spectrum., Comment: Accepted for publication in ApJ. 22 pages (emulateapj), 15 figures

We present high signal-to-noise optical spectra for 67 low-redshift (0.1 < z < 0.4) galaxies that lie within close projected distances (5 kpc < rho < 150 kpc) of 38 background UV-bright QSOs. The Keck LRIS and Magellan MagE data presented here are part of a survey that aims to construct a statistically sampled map of the physical state and metallicity of gaseous galaxy halos using the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST). We provide a detailed description of the optical data reduction and subsequent spectral analysis that allow us to derive the physical properties of this uniquely data-rich sample of galaxies. The galaxy sample is divided into 38 pre-selected L ~ L*, z ~ 0.2 "target" galaxies and 29 "bonus" galaxies that lie in close proximity to the QSO sightlines. We report galaxy spectroscopic redshifts accurate to +/- 30 km s-1, impact parameters, rest-frame colors, stellar masses, total star formation rates, and gas-phase interstellar medium oxygen abundances. When we compare the distribution of these galaxy characteristics to those of the general low-redshift population, we find good agreement. The L ~ L* galaxies in this sample span a diverse range of color (1.0 < u-r < 3.0), stellar mass (10^9.5 < M/M_sun < 10^11.5), and SFRs (0.01 - 20 M_sun yr-1). These optical data, along with the COS UV spectroscopy, comprise the backbone of our efforts to understand how halo gas properties may correlate with their host galaxy properties, and ultimately to uncover the processes that drive gas outflow and/or are influenced by gas inflow., Comment: 20 pages, 12 Figures, Submitted to ApJS

We identified 24 SiIV absorption systems with z <~ 1 from a blind survey of 49 low-redshift quasars with archival Hubble Space Telescope ultraviolet spectra. We relied solely on the characteristic wavelength separation of the doublet to automatically detect candidates. After visual inspection, we defined a sample of 20 definite (group G = 1) and 4 "highly-likely" (G = 2) doublets with rest equivalent widths W_r for both lines detected at > 3 sigma. The absorber line density of the G = 1 doublets was dN_SiIV/dX = 1.4+0.4/-0.3 for log N(Si+3) > 12.9. The best-fit power law to the G = 1 frequency distribution of column densities f(N(Si+3)) had normalization k = (1.2+0.5/-0.4) x 10^-14 cm2 and slope alpha = -1.6+0.3/-0.3. Using the power-law model of f(N(Si+3)), we measured the Si+3 mass density relative to the critical density: Omega(Si+3) = (3.7+2.8/-1.7) x 10^-8 for 13 < log N(Si+3) < 15. From Monte Carlo sampling of the distributions, we estimated our value to be a factor of 4.8+3.0/-1.9 higher than the 2 < z < 4.5 . From a simple linear fit to Omega(Si+3) over the age of the Universe, we estimated a slow and steady increase from z = 5.5 --> 0 with dOmega/dt_age = (0.61+/-0.23) x 10^-8 Gyr^-1. We compared our ionic ratios N(Si+3)/N(C+3) to a 2 < z < 4.5 sample and concluded, from survival analysis, that the two populations are similar, with median = 0.16., Comment: 18 pages, 9 figures, 4 tables, added figures and new analysis, results have changed, accepted to ApJ

We surveyed the HST UV spectra of 49 low-redshift quasars for z < 1 CIV candidates, relying solely on the characteristic wavelength separation of the doublet. After consideration of the defining traits of CIV doublets (e.g., consistent line profiles, other associated transitions, etc.), we defined a sample of 38 definite (group G = 1) and five likely (G = 2) doublets with rest equivalent widths W_r for both lines detected at >= 3 sigma. We conducted Monte-Carlo completeness tests to measure the unblocked redshift (dz) and co-moving pathlength (dX) over which we were sensitive to CIV doublets of a range of equivalent widths and column densities. The absorber line density of (G = 1+2) doublets is dN/dX = 4.1+0.7/-0.6 for log N(C^+3) >= 13.2, and dN/dX has not evolved significantly since z = 5. The best-fit power-law to the G = 1 frequency distribution of column densities f(N(C^+3)) = k(N(C^+3)/N_0)^alpha_N has coefficient k = (0.67+0.18/-0.16) x 10^-14 cm^2 and exponent alpha_N = -1.50+0.17/-0.19, where N_0 = 10^14 cm^-2. Using the power-law model of f(N(C^+3)), we measured the C^+3 mass density relative to the critical density: Omega(C^+3) = (6.20+1.82/-1.52) x 10^-8 for 13 <= log N(C^+3) <= 15. This value is a 2.8+/-0.7 increase in Omega(C^+3) compared to the error-weighted mean from several 1 < z < 5 surveys for CIV absorbers. A simple linear regression to Omega(C^+3) over the age of the Universe indicates that Omega(C^+3) has slowly but steadily increased from z = 5 --> 0, with dOmega(C^+3)/dt_age = (0.42+/-0.2) x 10^-8 Gyr^-1., Comment: 61 pages; accepted by ApJ; includes changes based on referee report

We present a complete catalogue of 27 O VI absorbers at low redshift (0.12 < z < 0.5) from a blind survey of 16 QSO echelle spectra in the HST/STIS data archive. These absorbers are identified based only upon matching line profiles and the expected doublet ratio between the \lambda\lambda 1031, 1037 transitions. Subsequent searches are carried out to identify their associated transitions. Here we present all relevant absorption properties. By considering absorption components of different species which are well-aligned in velocity-space, we derive gas temperatures and non-thermal broadening values, b_{nt}. We show that in all 16 cases considered the observed line width is dominated by non-thermal motion and that gas temperatures are well below those expected for O^5+ in collisional ionization equilibrium. This result reaffirms previous findings from studies of individual lines of sight, but are at odds with expectations for a WHIM origin. At least half of the absorbers can be explained by a simple photoionization model. In addition, in some absorbers we find evidence for large variation in gas density/metallicity across components in individual absorbers. Comparisons of multiple associated metal species further show that under the assumption of the gas being photoionized by the metagalacitic background radiation field, the absorbing clouds have gas densities < -2.9 and sizes L > 1\kpc. Finally, we compare our absorber selection with the results of other independent studies., Comment: 15 pages + lots of figures and tables. ApJS accepted

We have conducted a systematic survey for intervening OVI absorbers in available echelle spectra of 16 QSOs at z_QSO = 0.17-0.57. These spectra were obtained using HST/STIS with the E140M grating. Our search uncovered a total of 27 foreground OVI absorbers with rest-frame absorption equivalent width W_r(1031) > 25mA. Ten of these QSOs exhibit strong OVI absorbers in their vicinity. Our OVI survey does not require the known presence of Lya, and the echelle resolution allows us to identify the OVI absorption doublet based on their common line centroid and known flux ratio. We estimate the total redshift survey path, \Delta z, using a series of Monte-Carlo simulations, and find that \Delta z=1.66, 2.18, and 2.42 for absorbers of strength W_r = 30, 50 and 80mA, respectively, leading to a number density of dN(W > 50mA)/dz = 6.7 +/- 1.7 and dN(W > 30mA)/dz = 10.4 +/- 2.2. In contrast, we also measure dN/dz = 27 +/- 9 for OVI absorbers of W_r > 50mA at |\Delta v|< 5000 kms from the background QSOs. Using the random sample of OVI absorbers with well characterized survey completeness, we estimate a mean cosmological mass density of the OVI gas \Omega(OVI)h = 1.7 +/- 0.3 x 10^-7. In addition, we show that <5% of OVI absorbers originate in underdense regions that do not show a significant trace of HI. Furthermore, we show that the neutral gas column N(HI) associated with these OVI absorbers spans nearly five orders of magnitude, and show moderate correlation with N(OVI). Finally, while the number density of OVI absorbers varies substantially from one sightline to another, it also appears to be inversely correlated with the number density of HI absorbers along individual lines of sight., Comment: 12 pages. ApJ accepted

We present age estimates for the newly discovered very r-process enhanced metal-poor star HE 1523-0901 ([Fe/H]=-2.95) based on the radioactive decay of Th and U. The bright (V=11.1) giant was found amongst a sample of bright metal-poor stars selected from the Hamburg/ESO survey. From an abundance analysis of a high-resolution (R=75,000) VLT/UVES spectrum we find HE 1523-0901 to be strongly overabundant in r-process elements ([r/Fe]=1.8). The abundances of heavy neutron-capture elements (Z>56) measured in HE 1523-0901 match the scaled solar r-process pattern extremely well. We detect the strongest optical U line at 3859.57 A. For the first time, we are able to employ several different chronometers, such as the U/Th, U/Ir, Th/Eu and Th/Os ratios to measure the age of a star. The weighted average age of HE 1523-0901 is 13.2 Gyr. Several sources of uncertainties are assessed in detail., Comment: 7 pages, 2 figures. Accepted for publication in ApJ Letters

We present a catalogue of 430 Field Horizontal Branch (FHB) stars, selected from the Hamburg/ESO Survey (HES), which fortuitously align with high column density neutral hydrogen (HI) High-Velocity Cloud (HVC) gas. These stars are ideal candidates for absorption-line studies of HVCs, attempts at which have been made for almost 40 years with little success. A parent sample of 8321 HES FHB stars was used to extract HI spectra along each line-of-sight, using the HI Parkes All-Sky Survey. All lines-of-sight aligned with high velocity HI emission with peak brightness temperatures greater than 120mK were examined. The HI spectra of these 430 probes were visually screened and cross-referenced with several HVC catalogues. In a forthcoming paper, we report on the results of high-resolution spectroscopic observations of a sample of stars drawn from this catalogue., Comment: 7 pages, 4 figures. ApJS accepted. Full catalogue and all online-only images available at http://astronomy.swin.edu.au/staff/cthom/catalogue/index.html

Large samples of Field Horizontal Branch (FHB) stars make excellent tracers of the Galactic halo; by studying their kinematics, one can infer important physical properties of our Galaxy. Here we present the results of a medium-resolution spectroscopic survey of 530 FHB stars selected from the Hamburg/ESO survey. The stars have a mean distance of ~7 kpc and thus probe the inner parts of the Milky Way halo. We measure radial velocities from the spectra in order to test the model of Sommer-Larsen et al., who suggested that the velocity ellipsoid of the halo changes from radially-dominated orbits to tangentially-dominated orbits as one proceeds from the inner to the outer halo. We find that the present data are unable to discriminate between this model and a more simple isothermal ellipsoid; we suggest that additional observations towards the Galactic centre might help to differentiate them., Comment: 7 pages, 7 figures. Accepted for publication in MNRAS

We test the hypothesis that High-Velocity gas cloud Complex C is actually a high-latitude spiral arm extension in the direction of the Galactic warp, as opposed to the standard interpretation - that of a once extragalactic, but now infalling, gas cloud. A parallel Tree N-body code was employed to simulate the tidal interaction of a satellite perturber with the Milky Way. We find that a model incorporating a perturber of the mass of the Large Magellanic Cloud on a south-to-north polar orbit, crossing the disk at ~15 kpc, does yield a high- velocity, high-latitude, extension consistent with the spatial, kinematical and column density properties of Complex C. Unless this massive satellite remains undiscovered because of either a fortuitous alignment with the Galactic bulge (feasible within the framework of the model), or the lack of any associated baryonic component, we conclude that this alternative interpretation appears unlikely., Comment: 7 pages, 5 figures, accepted for publication in PASA, High resolution version is available at http://astronomy.swin.edu.au/staff/dkawata/research/papers.html