Helium The II Lyman Forest
Gerard Kriss STScI
What We Can Learn from Observations of He II Absorption
Use the ratio of He II and H I column densities to ascertain the shape of the ionizing spectrum. Do stars or quasars dominate the radiation field at z=2-3? Using the spectral shape in conjunction with proximity-effect measurements of the radiation field intensity, Set limits on the escape fraction of radiation from star-forming galaxies. Trace the qualitative appearance of He II reionization. This may serve as a preview of what H I reionization looks like. Look for a red damping wing on the G-P trough. Look for absorption features on the damping wing indicative of infalling clouds.
Observations of He II in the IGM
Difficult to seeat z~3, only 15% of sightlines are free of LLSs. There are only 7 known quasars with clear lines of sight. First detection of He II: Q0302-003, Jakobsen et al. (1994). First measurement of finite He II optical depth: HS1700+6416, Davidsen, Kriss & Zheng (1996). Observations of the end stages of He II reionization: HE2347-4342, Reimers et al. (1997). Other indirect indicators of He II reionization: Si IV/C IV decreases at z~3: Songaila (1998). HI line widths increase IGM temperature increases at z~3.3: Theuns et al. (2002). HI opacity decreases at z~3.3: Bernardi et al. (2003). FUSE observations resolve the He II Ly forest: Kriss et al. (2001).
FUSE Spectrum of HE2347-4342
Transparent Windows Extrapolated Continuum
(376 ks)
IGM Transparent in He II Ly
Kriss et al. 2001
IGM Optically Thick In He II Ly
FUSE He II Absorption Keck H I
Kriss et al. 2001
He II Lyman Lines are Turbulently Broadened
Zheng et al. 2004a
(NHeII/NHI) vs. Redshift (z)
Starlight
Quasar Illumination
Zheng al. et 2004a
STIS & FUSE Spectra of HS1700+6416
STIS Spectrum: G140L + G230L Best-fit continuum with E(B-V)=0.025
FUSE Spectrum: 290 ks
Fechner et al. 2006
Comparison of FUSE and Keck Spectra of HS1700+6416
Fechner et al. 2006
(NHeII/NHI) vs. Redshift in HS1700+6416
(Uncorrected for metal-line absorbers)
(Corrected for metal-line absorbers) Fechner et al. 2006
Inferred Spectral Indices Compared to QSOs
Distribution of QSO spectral indices: Telfer et al. 2002
Shull et al. 2004
Constraining Fesc for Star-forming Galaxies
Scott et al. 2001
Bianchi et al. 2001 Scott et al. 2001 Kriss & Scott 2006
He II Absorption at z=3.5 in SDSS J2346-0016
STIS Low-resolution snapshot (G140L), 600 s
Zheng et al. 2004b
A 40 ks ACS/SBC Prism spectrum was just obtained this fall. Stay Tuned!
Conclusions
FUSE observations of both HE23474342 and HS1700+6416 resolve the He II Ly forest in each object. For absorption with both He II and H I measurements (more than 90% of the observed He II features), the ratio =N(He II)/N(H I) lies bet...