Cold gas accretion and the absorption
cross section of the CGM
Philipp Richter
University of Potsdam
Galaxies in Absorption, Paris, September 2014 Gas circulation processes at large scales
Gas accretion from the
intergalactic medium
fuels star formation in galaxies (gas accretion
rate  star formation
rate in galaxies). Galaxy mergers
transport large amounts of interstellar gas into the
intergalactic
evironment of galaxies.
Galactic winds and fountain‐like processes
drive out chemically enriched gas into the IGM and indicate star formation activity in galaxies. Galactic high‐velocity clouds (HVCs)
HI 21cm (LAB)
HI covering fraction
(T. Westmeier)
HVCs and neutral gas accretion rates
Estimate of the total cold gas accretion rate density in the form of HVCs for the local galaxy population: dM/dt/dV = 0.016 Msun/yr/Mpc3
Star formation rate density (Hopkins & Beacom 2006)
Milky Way‐type HVCs may contribute substantially to the cold‐gas
accretion rates of low‐redshift galaxies …. IF HVCs were typical ! CaII‐selected neutral gas structures in the Milky Way halo
HI 21cm
VLA
VLT UVES
‐ N(HI) < 8 x 1018 cm‐2
‐ Cold (T<900 K), small (pc‐scale)
(Richter et al.2005)
Extragalactic HVC analogs traced by optical and UV absorption
observer quasar intervening gas disks and HVCs Rh
VLT/UVES survey of intervening CaII absorption systems
CaII absorption @ z=0.08
23 absorbers for z=100 (!)
df
dN/dz (CaII, z<0.5) = 0.117
(dN/dz [HI disks, z=0] = 0.045)
(Richter et al. 2011; Zwaan et al. 2005)
The absorption cross section of
neutral gas in the CGM at z<0.5 within r=50 kpc is similar to that
of the Milky Way.
Absorption cross section of high‐velocity gas in the Milky Way halo
cold neutral gas traced by CaII
VLT/UVES: 408 lines of sight, 38 detections in high‐velocity CaII (fc=0.09)
red: vLSR > 100 km/s, blue: vLSR < ‐100 km/s
(Ben Bekhti et al. 2012)
Absorption cross section of high‐velocity gas in the Milky Way halo
warm neutral and ionized gas traced by SiIII
HST/COS: 243 lines of sight, 187 detections in high‐velocity SiIII (fc=0.77)
red: vLSR > 100 km/s, blue: vLSR < ‐100 km/s
(Richter et al. 2014, in prep;
also Lehner et al. 2012)
HST/COS survey of intervening SiIII absorption systems
HST/COS survey of intervening SiIII absorption systems
Doradus group
HST/COS survey of intervening SiIII absorption systems
120 SiIII absorbers along
435 QSO sightlines
df
dN/dz (SiIII, z<0.1) = 3.1
D
The absorption cross section of
SiIII at z=0 is in line with a metal‐
enriched CGM with RCGM = Rvir
and <fc> = 0.77.
(Richter et al.2014, in prep.)
Absorber‐galaxy relation for SiIII absorbers
54 intervening SiIII absorbers,  55,000 galaxies <2 deg; |v|=1000 km s‐1
control sample
Baryon and metal budget etc. yet need to be determined: WORK IN PROGRESS …
Summary
High‐velocity clouds and their distant analogs possibly contribute
significantly to the overall gas accretion rate of galaxies.
The observed number density of intervening CaII absorbers implies
that the neutral gas cross section in the halos of low‐redshift
galaxies within R=50 kpc is similar to that of the Milky Way. The dominating (in terms of cross section) gas phase in the inner
halos of low‐redshift galaxies is traced by SiIII.
The observed number density of intervening SiIII absorbers at z=0 can be reproduced by a metal‐enriched CGM that fills the halos
of galaxies up to Rvir with a covering fraction of 75 percent.