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2011A&A...528A..12R - Astronomy and Astrophysics, volume 528A, 12-12 (2011/4-1)

The neutral gas extent of galaxies as derived from weak intervening Ca II absorbers.


Abstract (from CDS):

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≥45000) optical spectra of 304 quasars and active galactic nuclei observed with VLT/UVES. Along a total redshift path of Δz≃100 we detected 23 intervening CaII absorbers in both the CaII H & K lines, with rest frame equivalent widths Wr,3934=15-799mÅ and column densities logN(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 <zabs≥0.35 for absorbers with logN(CaII)≥11.65 (Wr,3934≥32mÅ). This is ∼2.6 times the value obtained for damped Lyman α 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 logN(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)∝N–β, with a slope of -β=-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 <fc,CaII≥0.33. Using this value and considering all galaxies with luminosities L≥0.05L* we calculate that the characteristic radial extent of (partly) neutral gas clouds with logN(HI)≥17.4 around low-redshift galaxies is RHVC≃55kpc.

Abstract Copyright:

Journal keyword(s): galaxies: halos - galaxies: formation - galaxies: ISM - intergalactic medium - quasars: absorption lines

VizieR on-line data: <Available at CDS (J/A+A/528/A12): table1.dat table2.dat tablea1.dat tablea2.dat tablea3.dat>

Simbad objects: 297

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