TY  - JOUR
A1  - Worseck, Gabor
A1  - Prochaska, J. Xavier
A1  - McQuinn, Matthew
A1  - Dall'Aglio, Aldo
A1  - Fechner, Cora
A1  - Hennawi, Joseph F.
A1  - Reimers, Dieter
A1  - Richter, Philipp
A1  - Wisotzki, Lutz
T1  - The end of Helium Reionization at z similar or equal to 2.7 Inferred  from cosmic variance in HST/COS He II Ly alpha Absorption spectra
JF  - The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters
N2  - We report on the detection of strongly varying intergalactic He II absorption in HST/COS spectra of two z(em) similar or equal to 3 quasars. From our homogeneous analysis of the He II absorption in these and three archival sightlines, we find a marked increase in the mean He II effective optical depth from <tau(eff, He II)> similar or equal to 1 at z similar or equal to 2.3 to <tau(eff, He II)> greater than or similar to 5 at z similar or equal to 3.2, but with a large scatter of 2 less than or similar to tau(eff, He II) less than or similar to 5 at 2.7 < z < 3 on scales of similar to 10 proper Mpc. This scatter is primarily due to fluctuations in the He II fraction and the He II-ionizing background, rather than density variations that are probed by the coeval Hi forest. Semianalytic models of He II absorption require a strong decrease in the He II-ionizing background to explain the strong increase of the absorption at z greater than or similar to 2.7, probably indicating He II reionization was incomplete at z(reion) greater than or similar to 2.7. Likewise, recent three-dimensional numerical simulations of He II reionization qualitatively agree with the observed trend only if He II reionization completes at z(reion) similar or equal to 2.7 or even below, as suggested by a large tau(eff, He II) greater than or similar to 3 in two of our five sightlines at z < 2.8. By doubling the sample size at 2.7 less than or similar to z less than or similar to 3, our newly discovered He II sightlines for the first time probe the diversity of the second epoch of reionization when helium became fully ionized.
KW  - dark ages, reionization, first stars
KW  - diffuse radiation
KW  - intergalactic medium
KW  - quasars: absorption lines
KW  - quasars: individual (SDSS J092447.36+485242.8, SDSS J110155.74+105302.3)
Y1  - 2011
U6  - https://doi.org/10.1088/2041-8205/733/2/L24
SN  - 2041-8205
VL  - 733
IS  - 2
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  -