Abstract
The reionization of helium at z~3 is the final phase transition of the intergalactic medium and supposed to be driven purely by quasars. The HeII transverse proximity effect - enhanced HeII transmission in a background sightline caused by the ionizing radiation of a foreground quasar - therefore offers a unique opportunity to probe the morphology of HeII reionization and to investigate the emission properties of quasars, e.g. ionizing emissivity, lifetime and beaming geometry. We use the most-recent HST/COS far-UV dataset of 22 HeII absorption spectra and conduct our own dedicated optical spectroscopic survey to find foreground quasars around these HeII sightlines. Based on a set of 66 foreground quasars, we perform the first statistical analysis of the HeII transverse proximity effect. Despite a large object-to-object variance, our stacking analysis reveals an excess in the average HeII transmission near the foreground quasars at 3 sigma significance. This statistical evidence for the transverse proximity effect is corroborated by a clear dependence of the signal strength on the inferred HeII ionization rate at the background sightline. Our detection places, based on the transverse light crossing time, a geometrical limit on the quasar lifetime of t_Q > 25 Myr. This evidence for sustained activity of luminous quasars is relevant for the morphology of HI and HeII reionization and helps to constrain AGN triggering mechanisms, accretion physics and models of black hole mass assembly. We show how future modeling of the transverse proximity effect can additionally constrain quasar emission geometries and e.g. clarify if the large observed object-to-object variance can be explained by current models of quasar obscuration.
Highlights
The double ionization of helium, known as He II reionization, marks the final phase transition of the intergalactic medium (IGM) and is closely related to the emission properties of the quasar population that is supposed to drive He II reionization
With the addition of quasars from SDSS/BOSS we have composed a relatively large foreground quasar sample that allows for the first time a statistical analysis of the He II transverse proximity effect
By the means of a stacking analysis, we were able to find statistical evidence for the presence of a He II transverse proximity effect in the average transmission profile around 20 foreground quasars and by cutting on the transverse separation we could place a clear constraint on the quasar lifetime of >25 Myr
Summary
The double ionization of helium, known as He II reionization, marks the final phase transition of the intergalactic medium (IGM) and is closely related to the emission properties of the quasar population that is supposed to drive He II reionization. Hydrogen, according to the currently accepted picture (Haardt and Madau, 2012; Planck Collaboration et al, 2016), was reionized The He II Transverse Proximity Effect at redshifts z ∼ 8 primarily by the UV photons from stars. The bottom panel shows a Space Telescope Imaging Spectrograph (HST/STIS) FUV spectra (Heap et al, 2000) along the sightline toward the background quasars Q 0303−003 It shows over large regions Gunn-Peterson troughs (Gunn and Peterson, 1965) of saturated He II Lyα absorption, very similar to hydrogen Lyα spectra of high-redshift z > 6 quasars. Substantial He II transmission is only observed close to the background quasar, the so called line of sight proximity region. Our aim is to expand the sample, find additional foreground quasars close to He II sightlines and conduct a systematic investigation of the He II transverse proximity effect
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