HST/COS OBSERVATIONS OF THE QUASAR HE 2347-4342: PROBING THE EPOCH OF He II PATCHY REIONIZATION AT REDSHIFTS z = 2.4-2.9
- CASA, Department of Astrophysical and Planetary Sciences, University of Colorado, 389-UCB, Boulder, CO 80309 (United States)
- Space Telescope Science Institute, Baltimore, MD 21218 (United States)
We report ultraviolet spectra of the high-redshift (z{sub em} {approx} 2.9) quasar, HE 2347 - 4342, taken by the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope. Spectra in the G130M (medium resolution, 1135-1440 A) and G140L (low resolution, 1030-2000 A) gratings exhibit patchy Gunn-Peterson absorption in the 303.78 A Ly{alpha} line of He II between z = 2.39-2.87 (G140L) and z = 2.74-2.90 (G130M). With COS, we obtain better spectral resolution, higher signal-to-noise ratio (S/N), and better determined backgrounds than previous studies, with sensitivity to abundance fractions x{sub He{sub II}} {approx} 0.01 in filaments of the cosmic web. The He II optical depths from COS are higher than those with the Far Ultraviolet Spectroscopic Explorer and range from {tau}{sub He{sub II}} {<=} 0.02 to {tau}{sub He{sub II}} {>=} 5, with a slow recovery in mean optical depth to ({tau}{sub He{sub II}}) {<=} 2 at z < 2.7. The He II/H I optical-depth ratio varies ({eta}{approx} 10-100 for 2.4 < z < 2.73 and {eta} = 5-500 for 2.75 < z < 2.89) on scales {Delta}z {approx}< 0.01 (10.8 Mpc in comoving radial distance at z = 2.8), with numerous flux-transmission windows between 1135 and 1186 A. The He II absorption extends to 1186.26 A (z = 2.905), including associated absorbers with z{sub abs} {approx} z{sub QSO} and minimal 'proximity effect' of flux transmission at the He II edge. We propose a QSO systemic redshift z{sub QSO} = 2.904 {+-} 0.002, some {Delta}z = 0.019 higher than that derived from O I {lambda}1302 emission. Three long troughs (4-10 A or 25-60 Mpc comoving distance) of strong He II absorption between z = 2.75and2.90 are uncharacteristic of the intergalactic medium if He II reionized at z{sub r} {approx} 3. Contrary to recent indirect estimates (z{sub r} = 3.2 {+-} 0.2) from H I optical depths, the epoch of He II reionization may extend to z {approx}< 2.7.
- OSTI ID:
- 21464622
- Journal Information:
- Astrophysical Journal, Vol. 722, Issue 2; Other Information: DOI: 10.1088/0004-637X/722/2/1312; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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