@article{BoumaRichterWendt2021,
  author    = {Bouma, Sietske Jeltje Deirdre and Richter, Philipp and Wendt, Martin},
  title     = {The relation between Ly alpha absorbers and local galaxy filaments},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {647},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/202039786},
  pages     = {16},
  year      = {2021},
  abstract  = {Context. The intergalactic medium (IGM) is believed to contain the majority of baryons in the universe and to trace the same dark matter structure as galaxies, forming filaments and sheets. Ly alpha absorbers, which sample the neutral component of the IGM, have been extensively studied at low and high redshift, but the exact relation between Ly alpha absorption, galaxies, and the large-scale structure is observationally not well constrained.Aims. In this study, we aim at characterising the relation between Ly alpha absorbers and nearby over-dense cosmological structures (galaxy filaments) at recession velocities Delta v <= 6700 km s(-1) by using archival observational data from various instruments.Methods. We analyse 587 intervening Ly alpha absorbers in the spectra of 302 extragalactic background sources obtained with the Cosmic Origins Spectrograph (COS) installed on the Hubble Space Telescope (HST). We combine the absorption line information with galaxy data of five local galaxy filaments from the V8k catalogue.Results. Along the 91 sightlines that pass close to a filament, we identify 215 (227) Ly alpha absorption systems (components). Among these, 74 Ly alpha systems are aligned in position and velocity with the galaxy filaments, indicating that these absorbers and the galaxies trace the same large-scale structure. The filament-aligned Ly alpha absorbers have a similar to 90\% higher rate of incidence (d?/dz=189 for log N(HI) >= 13.2) and a slightly shallower column density distribution function slope (-beta=-1.47) relative to the general Ly alpha population at z=0, reflecting the filaments' matter over-density. The strongest Ly alpha absorbers are preferentially found near galaxies or close to the axis of a filament, although there is substantial scatter in this relation. Our sample of absorbers clusters more strongly around filament axes than a randomly distributed sample would do (as confirmed by a Kolmogorov-Smirnov test), but the clustering signal is less pronounced than for the galaxies in the filaments.},
  language  = {en}
}
@article{RichterWinkelWakkeretal.2018,
  author    = {Richter, Philipp and Winkel, Benjamin and Wakker, Bart P. and Pingel, N. M. and Fox, Andrew J. and Heald, G. and Walterbos, Rene A. M. and Fechner, C. and Ben Bekhti, N. and Gentile, G. and Zschaechner, Laura},
  title     = {Circumgalactic Gas at Its Extreme},
  series = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  volume    = {868},
  journal   = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  number    = {2},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {0004-637X},
  doi       = {10.3847/1538-4357/aae838},
  pages     = {12},
  year      = {2018},
  abstract  = {We present a detailed analysis of the absorption properties of one of the tidal gas streams around the "Whale" galaxy NGC 4631 in the direction of the quasar 2MASS J12421031+3214268. Our study is based on ultraviolet spectral data obtained with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST) and 21cm-data from the HALOGAS project and the Green Bank Telescope (GBT). We detect strong H I Ly alpha absorption in the velocity range +550 to +800 km s(-1) related to gas from a NGC 4631 tidal stream known as Spur 2. We measure a column density of log (N(H I/cm(-2))) = 18.68 +/- 0.15, indicating that the quasar sightline traces the outer boundary of Spur 2 as seen in the 21 cm data. Metal absorption in Spur 2 is detected in the lines of O I, C II, Si II, and Si III in a complex absorption pattern that reflects the multiphase nature of the gas. We find that the average neutral gas fraction in Spur 2 toward 2MASS J12421031+3214268 is only 14\%. This implies that ionized gas dominates the total mass of Spur 2, which then may comprise more than 10(9)M(circle dot). No significant depletion of Si is observed, showing that Spur 2 does not contain significant amounts of dust. From the measured O I/H I column density ratio, we determine an alpha abundance in Spur 2 of 0.131(-0.05)(+0.07) solar ([alpha/H] = -0.90 +/- 0.16), which is substantially lower than what is observed in the NGC 4631 disk. The low metallicity and low dust content suggest that Spur 2 represents metal-deficient gas stripped off a gas-rich satellite galaxy during a recent encounter with NGC 4631.},
  language  = {en}
}
@article{RichterWakkerFechneretal.2016,
  author    = {Richter, Philipp and Wakker, Bart P. and Fechner, Cora and Herenz, Peter and Tepper-Garcia, T. and Fox, Andrew J.},
  title     = {An HST/COS legacy survey of intervening Si III absorption in the extended gaseous halos of low-redshift galaxies},
  series = {Climate : open access journal},
  volume    = {590},
  journal   = {Climate : open access journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {1432-0746},
  doi       = {10.1051/0004-6361/201527038},
  pages     = {29},
  year      = {2016},
  abstract  = {Aims. Doubly ionized silicon (Si III) is a powerful tracer of diffuse ionized gas inside and outside of galaxies. It can be observed in the local Universe in ultraviolet (UV) absorption against bright extragalactic background sources. We here present an extensive study of intervening Si III-selected absorbers and study the properties of the warm circumgalactic medium (CGM) around low-redshift (z <= 0.1) galaxies. Methods. We analyzed the UV absorption spectra of 303 extragalactic background sources, as obtained with the Cosmic Origins Spectrograph (COS) on-board the Hubble Space Telescope (HST). We developed a geometrical model for the absorption-cross section of the CGM around the local galaxy population and compared the observed Si III absorption statistics with predictions provided by the model. We also compared redshifts and positions of the absorbers with those of similar to 64 000 galaxies using archival galaxy-survey data to investigate the relation between intervening Si III absorbers and the CGM. Results. Along a total redshift path of Delta z approximate to 24, we identify 69 intervening Si III systems that all show associated absorption from other low and high ions (e.g., H I, Si II, Si IV, C II, C IV). We derive a bias-corrected number density of dN/dz(Si III) = 2.5 +/- 0.4 for absorbers with column densities log N(Si III) > 12.2, which is similar to 3 times the number density of strong Mg II systems at z = 0. This number density matches the expected cross section of a Si III absorbing CGM around the local galaxy population with a mean covering fraction of < f(c)> = 0.69. For the majority (similar to 60 percent) of the absorbers, we identify possible host galaxies within 300 km s(-1) of the absorbers and derive impact parameters rho < 200 kpc, demonstrating that the spatial distributions of Si III absorbers and galaxies are highly correlated. Conclusions. Our study indicates that the majority of Si III-selected absorbers in our sample trace the CGM of nearby galaxies within their virial radii at a typical covering fraction of similar to 70 percent. We estimate that diffuse gas in the CGM around galaxies, as traced by Si III, contains substantially more (more than twice as much) baryonic mass than their neutral interstellar medium.},
  language  = {en}
}
@article{RichterKrauseFechneretal.2011,
  author    = {Richter, Philipp and Krause, F. and Fechner, Cora and Charlton, Jane C. and Murphy, M. T.},
  title     = {The neutral gas extent of galaxies as derived from weak intervening Ca II absorbers},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {528},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  number    = {4},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/201015566},
  pages     = {22},
  year      = {2011},
  abstract  = {We present a systematic study of weak intervening CaII absorbers at low redshift (z < 0.5), based on the analysis of archival high-resolution (R >= 45 000) optical spectra of 304 quasars and active galactic nuclei observed with VLT/UVES. Along a total redshift path of Delta z approximate to 100 we detected 23 intervening CaII absorbers in both the CaII H \& K lines, with rest frame equivalent widths W-r,W-3934 = 15-799 m angstrom and column densities log N(CaII) = 11.25-13.04 (obtained by fitting Voigt-profile components). We obtain a bias-corrected number density of weak intervening CaII absorbers of dN/dz = 0.117 +/- 0.044 at < z(abs)> = 0.35 for absorbers with log N(CaII) >= 11.65 (W-r,W-3934 >= 32 m angstrom). This is similar to 2.6 times the value obtained for damped Lyman alpha absorbers (DLAs) at low redshift. All CaII absorbers in our sample show associated absorption by other low ions such as MgII and FeII; 45 percent of them have associated NaI absorption. From ionization modelling we conclude that intervening CaII absorption with log N(CaII) >= 11.5 arises in DLAs, sub-DLAs and Lyman-limit systems (LLS) at HI column densities of log N(HI) >= 17.4. Using supplementary HI information for nine of the absorbers we find that the CaII/HI ratio decreases strongly with increasing HI column density, indicating a column-density-dependent dust depletion of Ca. The observed column density distribution function of CaII absorption components follows a relatively steep power law, f(N) proportional to N-beta, with a slope of -beta = -1.68, which again points towards an enhanced dust depletion in high column density systems. The relatively large cross section of these absorbers together with the frequent detection of CaII absorption in high-velocity clouds (HVCs) in the halo of the Milky Way suggests that a considerable fraction of the intervening CaII systems trace (partly) neutral gas structures in the halos and circumgalactic environment of galaxies (i.e., they are HVC analogs). Based on the recently measured detection rate of CaII absorption in the Milky Way HVCs we estimate that the mean (projected) CaII covering fraction of galaxies and their gaseous halos is < f(c,CaII)> = 0.33. Using this value and considering all galaxies with luminosities L >= 0.05 L-star we calculate that the characteristic radial extent of (partly) neutral gas clouds with log N(HI) >= 17.4 around low-redshift galaxies is R-HVC approximate to 55 kpc.},
  language  = {en}
}
@article{HerenzRichterCharltonetal.2013,
  author    = {Herenz, Peter and Richter, Philipp and Charlton, Jane C. and Masiero, Joseph R.},
  title     = {The milky way halo as a QSO absorption-line system new results from an HST/STIS absorption-line catalogue of galactic high-velocity clouds},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {550},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/201220531},
  pages     = {23},
  year      = {2013},
  abstract  = {We use archival UV absorption-line data from HST/STIS to statistically analyse the absorption characteristics of the high-velocity clouds (HVCs) in the Galactic halo towards more than 40 extragalactic background sources. We determine absorption covering fractions of low-and intermediate ions (Oi, Cii, Si ii, Mgii, Feii, Si iii, Civ, and Si iv) in the range f(c) = 0.20-0.70. For detailed analysis we concentrate on Si ii absorption components in HVCs, for which we investigate the distribution of column densities, b-values, and radial velocities. Combining information for Si ii and Mg II, and using a geometrical HVC model we investigate the contribution of HVCs to the absorption cross section of strong Mg ii absorbers in the local Universe. We estimate that the Galactic HVCs would contribute on average similar to 52 percent to the total strong Mg ii cross section of the Milky Way, if our Galaxy were to be observed from an exterior vantage point. We further estimate that the mean projected covering fraction of strong Mg ii absorption in the Milky Way halo and disc from an exterior vantage point is < f(c,sMgII)> = 0.31 for a halo radius of R = 61 kpc. These numbers, together with the observed number density of strong Mg ii absorbers at low redshift, indicate that the contribution of infalling gas clouds (i.e., HVC analogues) in the halos of Milky Way-type galaxies to the cross section of strong Mgii absorbers is < 34 percent. These findings are in line with the idea that outflowing gas (e. g., produced by galactic winds) in the halos of more actively star-forming galaxies dominate the absorption-cross section of strong Mgii absorbers in the local Universe.},
  language  = {en}
}