Collaborators Blair Savage, Bart Wakker (UW-Madison) Blair Savage, Bart Wakker (UW-Madison) Ken Sembach (STScI) Ken Sembach (STScI) Todd Tripp (UMass) Todd Tripp (UMass) Philipp Richter (Bonn Univ.) Philipp Richter (Bonn Univ.)

Introduction  The Ly  forest at low-z and high-z provides a powerful tool to probe the distribution and evolution of baryonic matter in the universe.  High-z Ly  forest is typically observed with 6-8 km.s -1 resolution spectra.  The spectral resolutions of UV observations for the low-z IGM were typically km.s -1.  Now, several STIS E140M (6.8 km.s -1 ) observations of low-z QSOs.

FUSE and STIS E140M Observations  Wavelength coverage: 910 to 1730Å  STIS/E140M Resolution: 6.8 km.s -1  FUSE Resolution: 20 km.s -1 z QSO S/N S/N (STIS) (FUSE) (H ) HE PG PG PG : Sembach et al. (2004) PG : Richter et al. (2004) HE FUSE and STIS Spectra (Lehner et al. 2005)

Analysis Analysis  Line IDs.  Profile fitting.  Apparent optical depth.  b and N are measured simultaneously.  Detections <3  are rejected. HE Spectra

Broad Ly  Absorption PG (Richter et al. 2004) b>40km.s -1 ; if purely thermal: T>10 5 K

The distribution and Evolution of b Hu et al. 1995, Kim et al Fraction of systems with b>50 km.s -1 three times larger at z < 0.5 than at z > 1.5, five times larger if b>70 km.s -1. Median b = 31 km.s -1 at b = 31 km.s -1 at z < 0.5 b = 26 km.s -1 at b = 26 km.s -1 at z > 1.5

Distribution and Evolution of N(H I ) Ly  Line Density: log N(H I) dN/dz log N(H I) dN/dz b ≤ 150 km.s -1 b ≤ 150 km.s -1 [13.20,16.20]114  25 [13.64,16.20] 44  10 b ≤ 40 km.s -1 b ≤ 40 km.s -1 [13.20,16.20] 77  25 [13.64,16.20] 32  10 See Weymann et al. (1998), Impey et al. (1999), Penton et al. (2004). Sample completeness: log N(H I)  13.2

The Differential Density Distribution Function f(N HI ) Z slope  > 1.5  1.5 (Tytler 87, Petitjean et al. 93, Hu et al. 95,… ) <  0.07 (Penton et al. 2004) <  0.23 (Davé &Tripp 1998) <  0.10 Present sample z<0.5

Baryon Density: Narrow Ly  Absorption Lines (b≤40 km.s -1 ): The mean gas density to the critical density is in the photoionized IGM (Schaye 2001):  (NLy  )≈2.2x10 -9 /(h  12 ) (T 4 ) 0.59  f(N HI ) (N HI ) 1/3 dN HI  12 = 0.05, H I photionization rate in s -1 (Davé & Tripp 2001). T 4 = 2.3, gas temperature in units of 10 4 K (b thermal =0.7b, Davé & Tripp 2001, and b median =28 km.s -1 ). h = 0.7, Hubble constant ( Spergel et al. 2003). f(N HI ) the differential density distribution function.

Baryon Density: Broad Ly  Absorption Lines (40<b≤150 km.s -1 ): Cosmological mass density of the BLy  absorbers in terms of today density can be written (Richter et al. 2004; Sembach et al. 2004):  (BLy  )  1.667x  f HI N HI /  X f HI is the conversion factor between H I and H, function of temperature (Sutherland & Dopita 1993). Collisional ionization equilibrium (CIE) and pure thermal broadening are assumed. BUT NO metallicity correction needed!

IGM Baryon Density Summary b log N(H I)  (Ly  )/  b b log N(H I)  (Ly  )/  b (km.s -1 ) (cm -2 ) (  b =0.044) (km.s -1 ) (cm -2 ) (  b =0.044) Ph.Ion.IGM ≤ 40 [13.20,16.20]> 0.14 Ph.Ion.IGM ≤ 40 [12.42,16.20] ≈ 0.28 (Ph.Ion.IGM≤ 150 [12.42,16.20] ~ 0.42) WHIM > 40 [13.20,16.20] > 0.21

Summary  E140M/STIS and FUSE observations reveal narrow and broad H I absorptions in low-z IGM, tracers of the warm photoionized IGM (T≤10 4 K) and WHIM (T~ K).  The Doppler parameter b increases with decreasing redshift.  A larger fraction of systems have b>40 km.s -1 at low-z than at high-z.  The observed baryonic content of the low-z IGM is enormous: photoionized 30-40%, WHIM at least 20-40%, but the shallowest, broadest H I absorptions are still to be discovered!

Concluding remarks The low redshift IGM fundamental to follow the evolution of the IGM with z. The low redshift IGM fundamental to follow the evolution of the IGM with z. We need to increase the current sample. We need to increase the current sample. Systematic search for metals. Systematic search for metals. Systematic deep galaxy redshift survey. Systematic deep galaxy redshift survey. We need We need