@article{RichterNuzaFoxetal.2017, author = {Richter, Philipp and Nuza, S. E. and Fox, Andrew J. and Wakker, Bart P. and Lehner, N. and Ben Bekhti, Nadya and Fechner, Cora and Wendt, Martin and Howk, J. Christopher and Muzahid, S. and Ganguly, R. and Charlton, Jane C.}, title = {An HST/COS legacy survey of high-velocity ultraviolet absorption in the}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {607}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201630081}, pages = {90}, year = {2017}, abstract = {Context. The Milky Way is surrounded by large amounts of diffuse gaseous matter that connects the stellar body of our Galaxy with its large-scale Local Group (LG) environment. Aims. To characterize the absorption properties of this circumgalactic medium (CGM) and its relation to the LG we present the so-far largest survey of metal absorption in Galactic high-velocity clouds (HVCs) using archival ultraviolet (UV) spectra of extragalactic background sources. The UV data are obtained with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST) and are supplemented by 21 cm radio observations of neutral hydrogen. Methods. Along 270 sightlines we measure metal absorption in the lines of Si II, Si III, C II, and C IV and associated H I 21 cm emission in HVCs in the velocity range vertical bar v(LSR)vertical bar = 100-500 km s(-1). With this unprecedented large HVC sample we were able to improve the statistics on HVC covering fractions, ionization conditions, small-scale structure, CGM mass, and inflow rate. For the first time, we determine robustly the angular two point correlation function of the high-velocity absorbers, systematically analyze antipodal sightlines on the celestial sphere, and compare the HVC absorption characteristics with that of damped Lyman alpha absorbers (DLAs) and constrained cosmological simulations of the LG (CLUES project).}, language = {en} } @article{RichterFoxBenBekhtietal.2014, author = {Richter, Philipp and Fox, Andrew J. and Ben Bekhti, Nadya and Murphy, M. T. and Bomans, Dominik J. and Frank, S.}, title = {High-resolution absorption spectroscopy of the circumgalactic medium of the Milky Way}, series = {Astronomische Nachrichten = Astronomical notes}, volume = {335}, journal = {Astronomische Nachrichten = Astronomical notes}, number = {1}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {0004-6337}, doi = {10.1002/asna.201312013}, pages = {92 -- 98}, year = {2014}, language = {en} } @article{WinkelBenBekhtiDarmstaedteretal.2011, author = {Winkel, B. and Ben Bekhti, Nadya and Darmstaedter, V. and Floeer, L. and Kerp, J. and Richter, Philipp}, title = {The high-velocity cloud complex Galactic center negative as seen by EBHIS and GASS I. Cloud catalog and global properties}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {533}, journal = {Astronomy and astrophysics : an international weekly journal}, number = {18}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201117357}, pages = {13}, year = {2011}, abstract = {Using Milky Way data of the new Effelsberg-Bonn HI Survey (EBHIS) and the Galactic All-Sky Survey (GASS), we present a revised picture of the high-velocity cloud (HVC) complex Galactic center negative (GCN). Owing to the higher angular resolution of these surveys compared to previous studies (e.g., the Leiden Dwingeloo Survey), we resolve complex GCN into lots of individual tiny clumps, that mostly have relatively broad line widths of more than 15 km s(-1). We do not detect a diffuse extended counterpart, which is unusual for an HVC complex. In total 243 clumps were identified and parameterized which allows us to statistically analyze the data. Cold-line components (i.e.,Delta upsilon(fwhm) < 7.5 km s(-1)) are found in about 5\% only of the identified cloudlets. Our analysis reveals that complex GCN is likely built up of several subpopulations that do not share a common origin. Furthermore, complex GCN might be a prime example for warm-gas accretion onto the Milky Way, where neutral HI clouds are not stable against interaction with the Milky Way gas halo and become ionized prior to accretion.}, language = {en} } @article{BenBekhtiWinkelRichteretal.2012, author = {Ben Bekhti, Nadya and Winkel, B. and Richter, P. and Kerp, J. and Klein, U. and Murphy, M. T.}, title = {An absorption-selected survey of neutral gas in the Milky Way halo New results based on a large sample of Ca II, Na I, and H I spectra towards QSOs}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {542}, journal = {Astronomy and astrophysics : an international weekly journal}, number = {2}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201118673}, pages = {17}, year = {2012}, abstract = {Aims. We aim at analysing systematically the distribution and physical properties of neutral and mildly ionised gas in the Milky Way halo, based on a large absorption-selected data set. Methods. Multi-wavelength studies were performed combining optical absorption line data of Ca II and Na I with follow-up H I 21-cm emission line observations along 408 sight lines towards low-and high-redshift QSOs. We made use of archival optical spectra obtained with UVES/VLT. H I data were extracted from the Effelsberg-Bonn H I survey and the Galactic All-Sky survey. For selected sight lines we obtained deeper follow-up observations using the Effelsberg 100-m telescope. Results. Ca II (Na I) halo absorbers at intermediate and high radial velocities are present in 40-55\% (20-35\%) of the sightlines, depending on the column density threshold chosen. Many halo absorbers show multi-component absorption lines, indicating the presence of sub-structure. In 65\% of the cases, absorption is associated with H I 21-cm emission. The Ca II (Na I) column density distribution function follows a power-law with a slope of beta approximate to -2.2 (-1.4). Conclusions. Our absorption-selected survey confirms our previous results that the Milky Way halo is filled with a large number of neutral gas structures whose high column density tail represents the population of common H I high-and intermediate-velocity clouds seen in 21-cm observations. We find that Na I/Ca II column density ratios in the halo absorbers are typically smaller than those in the Milky Way disc, in the gas in the Magellanic Clouds, and in damped Lyman a systems. The small ratios (prominent in particular in high-velocity components) indicate a lower level of Ca depletion onto dust grains in Milky Way halo absorbers compared to gas in discs and inner regions of galaxies.}, language = {en} } @article{RichterFoxWakkeretal.2013, author = {Richter, Philipp and Fox, Andrew J. and Wakker, Bart P. and Lehner, Nicolas and Howk, J. Christopher and Bland-Hawthorn, Joss and Ben Bekhti, Nadya and Fechner, Cora}, title = {The COS/UVES absorption survey of the magellanic stream - II. Evidence for a complex enrichment history of the stream from the fairall 9 sightline}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {772}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {2}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/772/2/111}, pages = {19}, year = {2013}, abstract = {We present a multi-wavelength study of the Magellanic Stream (MS), a massive gaseous structure in the Local Group that is believed to represent material stripped from the Magellanic Clouds. We use ultraviolet, optical and radio data obtained with HST/COS, VLT/UVES, FUSE, GASS, and ATCA to study metal abundances and physical conditions in the Stream toward the quasar Fairall 9. Line absorption in the MS from a large number of metal ions and from molecular hydrogen is detected in up to seven absorption components, indicating the presence of multi-phase gas. From the analysis of unsaturated S II absorption, in combination with a detailed photoionization model, we obtain a surprisingly high alpha abundance in the Stream toward Fairall 9 of [S/H] = -0.30 +/- 0.04 (0.50 solar). This value is five times higher than what is found along other MS sightlines based on similar COS/UVES data sets. In contrast, the measured nitrogen abundance is found to be substantially lower ([N/H] = -1.15 +/- 0.06), implying a very low [N/alpha] ratio of -0.85 dex. The substantial differences in the chemical composition of MS toward Fairall 9 compared to other sightlines point toward a complex enrichment history of the Stream. We favor a scenario, in which the gas toward Fairall 9 was locally enriched with a elements by massive stars and then was separated from the Magellanic Clouds before the delayed nitrogen enrichment from intermediate-mass stars could set in. Our results support (but do not require) the idea that there is a metal-enriched filament in the Stream toward Fairall 9 that originates in the LMC.}, language = {en} } @article{FoxRichterWakkeretal.2013, author = {Fox, Andrew J. and Richter, Philipp and Wakker, Bart P. and Lehner, Nicolas and Howk, J. Christopher and Ben Bekhti, Nadya and Bland-Hawthorn, Joss and Lucas, Stephen}, title = {The COS/UVES absorption survey of the magellanic stream - I. One-tenth solar abundances along the body of the stream}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {772}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {2}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/772/2/110}, pages = {16}, year = {2013}, abstract = {The Magellanic Stream (MS) is a massive and extended tail of multi-phase gas stripped out of the Magellanic Clouds and interacting with the Galactic halo. In this first paper of an ongoing program to study the Stream in absorption, we present a chemical abundance analysis based on HST/COS and VLT/UVES spectra of four active galactic nuclei (RBS 144, NGC 7714, PHL 2525, and HE 0056-3622) lying behind the MS. Two of these sightlines yield good MS metallicity measurements: toward RBS 144 we measure a low MS metallicity of [S/H] = [S II/H I] = -1.13 +/- 0.16 while toward NGC 7714 we measure [O/H] = [O I/H I] = -1.24 +/- 0.20. Taken together with the published MS metallicity toward NGC 7469, these measurements indicate a uniform abundance of approximate to 0.1 solar along the main body of the Stream. This provides strong support to a scenario in which most of the Stream was tidally stripped from the SMC approximate to 1.5-2.5 Gyr ago (a time at which the SMC had a metallicity of approximate to 0.1 solar), as predicted by several N-body simulations. However, in Paper II of this series, we report a much higher metallicity (S/H = 0.5 solar) in the inner Stream toward Fairall 9, a direction sampling a filament of the MS that Nidever et al. claim can be traced kinematically to the Large Magellanic Cloud, not the Small Magellanic Cloud. This shows that the bifurcation of the Stream is evident in its metal enrichment, as well as its spatial extent and kinematics. Finally we measure a similar low metallicity [O/H] = [O I/H I] = -1.03 +/- 0.18 in the v(LSR) = 150 km s(-1) cloud toward HE 0056-3622, which belongs to a population of anomalous velocity clouds near the south Galactic pole. This suggests these clouds are associated with the Stream or more distant structures (possibly the Sculptor Group, which lies in this direction at the same velocity), rather than tracing foreground Galactic material.}, language = {en} } @article{RichterCharltonFanganoetal.2009, author = {Richter, Philipp and Charlton, Jane C. and Fangano, Alessio P. M. and Ben Bekhti, Nadya and Masiero, Joseph R.}, title = {A population of weak metal-line absorbers surrounding the Milky Way}, issn = {0004-637X}, doi = {10.1088/0004-637x/695/2/1631}, year = {2009}, abstract = {We report on the detection of a population of weak metal-line absorbers in the halo or nearby intergalactic environment of the Milky Way. Using high-resolution ultraviolet absorption-line spectra of bright quasars (QSO) obtained with the Space Telescope Imaging Spectrograph (STIS), along six sight lines we have observed unsaturated, narrow absorption in O I and Si II, together with mildly saturated C II absorption at high radial velocities (vertical bar v(LSR)vertical bar = 100-320 km s(-1)). The measured O I column densities lie in the range N(O I) 2 x 10(14) cm(-2) implying that these structures represent Lyman limit Systems and sub-Lyman limit System with H I column densities between 10(16) and 3 x 10(18) cm(-2), thus below the detection limits of current 21 cm all-sky surveys of high-velocity clouds (HVCs). The absorbers apparently are not directly associated with any of the large high column density HVC complexes, but rather represent isolated, partly neutral gas clumps embedded in a more tenuous, ionized gaseous medium situated in the halo or nearby intergalactic environment of the Galaxy. Photoionization modeling of the observed low ion ratios suggests typical hydrogen volume densities of n(H) > 0.02 cm(-3) and characteristic thicknesses of a several parsec down to subparsec scales. For three absorbers, metallicities are constrained in the range of 0.1-1.0 solar, implying that these gaseous structures may have multiple origins inside and outside the Milky Way. Using supplementary optical absorption-line data, we find for two other absorbers Ca II/O I column-density ratios that correspond to solar Ca/O abundance ratios. This finding indicates that these clouds do not contain significant amounts of dust. This population of low column density gas clumps in the circumgalactic environment of the Milky Way is indicative of the various processes that contribute to the circulation of neutral gas in the extended halos of spiral galaxies. These processes include the accretion of gas from the intergalactic medium and satellite galaxies, galactic fountains, and outflows. We speculate that this absorber population represents the local analog of weak Mg II systems that are commonly observed in the circumgalactic environment of low- and high-redshift galaxies.}, language = {en} }