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 at redshifts z ~ 8 primarily by the UV photons from stars. However, stellar spectra are not hard enough to supply sufficient numbers of photons with energies >4 Ry, required to doubly ionize helium. He ii reionization therefore took place much later, when quasars became sufficiently abundant, culminating in the completion of helium reionization at z ≈ 2.7 (Madau and Meiksin, 1994; Reimers et al., 1997; Miralda-Escudé et al., 2000; Faucher-Giguère et al., 2009; McQuinn, 2009; Worseck et al., 2011; Haardt and Madau, 2012; Compostella et al., 2013, 2014; Worseck et al., 2016). In the general picture of He ii reionization, quasars create photoionized bubbles in the IGM which expand with time and eventually overlap, leading to the present day situation that the IGM is kept in photoionization equilibrium and highly ionized by a homogeneous and uniform UV background (Bolton et al., 2006; Furlanetto and Oh, 2008; McQuinn, 2009; Furlanetto and Dixon, 2010; Furlanetto and Lidz, 2011; Haardt and Madau, 2012; Meiksin and Tittley, 2012; Compostella et al., 2013, 2014; Davies et al., 2017). Since quasars are rare but bright sources, the reionization process is rather patchy and inhomogeneous. Its morphology therefore contains extensive information about the emission properties of the quasars.

At redshift z ~ 3, the λ_rest = 304Å He ii Lyα transition is redshifted sufficiently into the far-UV (FUV) to be observable with space based telescopes, in particular the Hubble Space Telescope (HST). The usual technique is to take FUV spectra of background quasars and infer the helium ionization characteristics along the sightline from the absorption properties of the He ii Lyα forest. The presence of a foreground quasar close to such a He ii sightline allows to explore the effect of the foreground quasars ionizing radiation on the helium ionization state along the background sightline.

Figure 1 illustrates the prototype sightline of such a constellation. 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. Here, the ionizing radiation from the background quasar has already sufficiently ionized helium to allow high He ii transmission. In addition, the spectrum shows a striking transmission spike at z = 3.05. At the same redshift and with a separation of Δθ = 6′ from the background sightline Jakobsen et al. (2003) found a foreground quasar and established the picture on the He ii transverse proximity effect. In this picture, the foreground quasar photoionizes its surrounding and the background sightline intersects this ionization bubble, leading to strong He ii transmission at the position of the foreground quasar.

Observing a strong transverse proximity effect in such a constellation allows to infer a geometric limit on the age of the quasar. Since one observed the quasar and the enhanced He ii transmission at the same redshift and therefore same lookback time, the quasar has to already shine for at least the transverse light crossing time to give the photons enough time to reach the background sightline. This constellation might also give insights into the quasar emission geometry. The foreground quasar appears as unobscured Type I from Earth but its effect on the background sightline crucially depends on the obscuration properties toward the background sightline.

This illustrates the unique abilities to infer quasar properties offered by the He ii transverse proximity effect. However, up to now, the Q 0302−003 sightline represents the only strong detection of a He ii transverse proximity effect. Our aim is therefore 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.

To increase the data sample and facilitate a statistical analysis of helium reionization, we are performing a comprehensive helium reionization survey. This includes the discovery and observation of new He ii sightlines and a homogeneous and extremely careful reduction of all existing HST observations presented in Worseck et al. (2016). In addition, we conducted a dedicated optical foreground quasar survey around the 22 available He ii sightlines which is described in Schmidt et al. (2017) and shall be summarized in the following.

The foreground quasar survey consists of a deep narrow survey conducted on 8 m class telescopes covering the immediate vicinity of the He ii sightline and a wider survey targeting individual quasars on 4 m telescopes. The deep survey covers separations from the He ii sightline up to Δθ ≤ 10′ and reaches a limiting magnitude of r ≤ 23.5 mag. Quasar candidates were selected using deep multi-color imaging, primarily obtained with the Large Binocular Cameras at the Large Binocular Telescope (LBT/LBC, Giallongo et al., 2008; Speziali et al., 2008). The main concern here was to reach a U-band limiting magnitude of ≈ 26mag. For spectroscopic confirmation the VIsible MultiObject Spectrograph (VIMOS, Le Fèvre et al., 2003) at the Very Large Telescope (VLT) was used. To find foreground quasars with larger separations from the background sightline, which are in particular important to probe long quasar lifetimes, we conducted a wide but shallower survey on 4 m class telescopes. We selected candidates from public quasar catalogs (DiPompeo et al., 2015; Richards et al., 2015) which are based on optical imaging (SDSS) and mid-infrared photometry from the Wide-field Infrared Survey Explorer (WISE, Wright et al., 2010). Using the WISE 3.6 and 4.5μm bands allow very efficient quasar selection (Stern et al., 2012; Assef et al., 2013). Spectroscopic confirmation was done with the European Southern Observatory 3.5 m New Technology Telescope Faint Object Spectrograph and Camera (NTT/EFOSC2, Buzzoni et al., 1984) and the Calar Alto Observatory (CAHA) 3.5 m telescope TWIN spectrograph within 37 nights between November 2014 and August 2015. The wide survey reaches down to r ≤ 21.5 mag and extends out to Δθ ≈ 90′ in case the quasar candidates were bright enough to expect a measurable effect on the background sightline.

In total, our surveys discovered 121 new quasars. We complement this sample by selecting quasars from the literature and in particular the spectroscopic catalogs of the Sloan Digital Sky Survey (SDSS, York et al., 2000) and the Baryon Oscillation Spectroscopic Surveys (BOSS, Eisenstein et al., 2011; Dawson et al., 2013) twelfth data release (Alam et al., 2015; Pâris et al., 2016). For all objects we calculate the estimated He ii photoionization rate at the background sightline ΓQSOHeII assuming isotropic emission, infinite quasar lifetime and no IGM absorption. For details see Schmidt et al. (2017). Our sample contains 66 foreground quasars for which we have full spectral coverage along the background sightline and which exceed ΓQSOHeII>0.5×10-15s-1. For comparison, the He ii UV background at z ~ 3 should be approximately ΓUVBHeII≈ 1×10-15s-1(Faucher-Giguère et al., 2009; Haardt and Madau, 2012; Khrykin et al., 2016).

For the prototype object at z = 3.05 along the Q 0302−003 sightline we calculate an expected ionization rate of ΓUVBHeII≈12×10-15s-1. Thus, this object should exceed the He ii UV background by approximately one order of magnitude. Observing a strong transverse proximity effect is therefore not surprising. Within our study we find three other foreground quasars that should cause up to 50% higher He ii ionization rates at the background sightlines. The He ii spectra associated with all four of these objects are shown in Figure 2. Surprisingly, the three new objects show no evidence for strong transverse proximity effect. On the contrary, we find completely ordinary and in several cases even fully saturated He ii absorption. The three new foreground quasars are in terms of luminosity, separation from the background sightline and redshift comparable to the prototype quasar. It is therefore rather unexpected to find not even the slightest indication of a transverse proximity effect for any of them.

Possible explanations are that these quasars might be too young and therefore the ionizing radiation had not enough time to reach the background sightline. Given the transverse separations, this would point toward quasar ages below 8–15 Myr. Another possible explanation would be quasar obscuration. According to the common quasar unification models (e.g., Antonucci, 1993; Elvis, 2000), the dichotomy between Type I and Type II quasars is purely an orientation effect. Each quasar should emit only toward some part of the sky, approximately 50% according to (e.g., Brusa et al., 2010; Lusso et al., 2013; Marchesi et al., 2016), and be obscured toward the other directions. However, it seems quite unlikely that three out of four quasars are oriented in a way that no ionizing radiation at all reaches the background sightline. Without further investigation it therefore remains purely speculative why we find no strong transverse proximity effect for the three strongest foreground quasars.

Apart form the non-detection of the proximity effect for individual quasars, we search for statistical evidence in the average He ii transmission profile around foreground quasars. We therefore select foreground quasars with ΓQSOHeII>2×10-15s-1 and stack the He ii spectra on the positions of these foreground quasars. The result is shown in Figure 3, top panel. Despite the large scatter, we find a clear enhancement in the average He ii transmission right at the location of the foreground quasars. We conduct a Monte Carlo analysis by stacking the He ii spectra on random positions and find a significance of 3.1σ for the measured transmission enhancement. In addition, we show in Schmidt et al. (2017) that the strength of this transmission enhancement roughly scales with the ionization rate of the foreground quasars and vanishes for ΓQSOHeII<2×10-15s-1 which is roughly comparable to the He ii UV background (Faucher-Giguère et al., 2009; Haardt and Madau, 2012). We are therefore confident that the observed effect is actually caused by the ionizing radiation of the foreground quasars.

Based on this statistical detection of the He ii transverse proximity effect we can investigate the quasar lifetime. Therefore, we include only foreground quasars in the stack that have a given minimum separation from the background sightline. As shown in Figure 3, the transverse proximity effect persists in the average transmission profile even for quasars with separations >25 Mlyr. The significance of the He ii transverse proximity effect in the >15 and >25 Mlyr stacks was determined to be 3.2 σ and 2.6 σ. Based on the transverse light crossing time we conclude that the quasars have to shine for at least 25 Myr.

In Schmidt et al. (2017) we have described our dedicated foreground quasar survey around 22 He ii sightlines and the discovery of 121 new quasars. 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. What remains however, is the surprising and to some degree contradictory result that our stacking analysis shows evidence for a long quasar lifetime while among the four strongest foreground quasars which all should exceed the He ii UV background by an order of magnitude and are at most 15Mlyr away from the background sightline only the previously known quasar is associated with a strong He ii transmission spike.

We would like to apply our method to an even larger dataset. However, given the capabilities of the current UV space telescopes (GALEX, HST), the discovery of many new He ii sightlines is unlikely and a good fraction of the known sightlines has now been searched for foreground quasars. In the near future, new insight into the He ii transverse proximity effect will therefore probably come from dedicated modeling. A first attempt for this is shown in Figure 4. We take outputs of a cosmological hydrodynamical simulation computed with the Eulerian grid code Nyx (Almgren et al., 2013; Lukić et al., 2015) and post-process these with a photo-ionization model that simulates a single bright foreground quasar. In this model we can explicitly vary quasar age and emission geometry. The case shown in Figure 4 is matched in quasar luminosity and sightline geometry to the Q 0302−003 z = 3.05 foreground quasar. We chose a classical bi-conical emission geometry with a half-opening angle of α = 60°, therefore illuminating 50% of the sky. The quasar is inclined by 20° and assumed to be 25 Myr old. For an observer on Earth, the low redshift parts of the background sightline appears to be illuminated first while the quasar radiation arrive at successively later times at higher redshifts. For a given quasar age, the illuminated region has a parabolic shape, open toward the observer and with the quasar at the focus. The finite quasar age limits the extend of the illuminated region toward high redshifts (larger comoving distance, right side). This is well visible in the top panel of Figure 4 which shows the simulated He ii transmission in a thin slice through the simulation. The bottom panel clearly shows the He ii transverse proximity effect along the background sightline and the substantially enhanced transmission compared to the the effect of the ΓUVBHeII≈1×10-15s-1 UV background alone, but also the effect of obscuration and finite quasar age.

With these models, we intent to investigate the combined effect of quasar obscuration and finite quasar age on the expected He ii transmission, compare them to the observed He ii spectra associated with the strongest foreground quasars and infer their age and obscuration properties. It is obvious that due to the random quasar orientation any constraints will only be of probabilistic nature and our endeavor will probably require a large Monte Carlo analysis and sophisticated statistical methods. However, given the high expected photoionization rate of the foreground quasars, it should still be possible to rule out certain parts of the parameter space and give insights if a reference model with e.g., 50% obscuration and 25 Myr lifetime is actually consistent with our observations and the non-detection of strong transmission peaks for the strongest foreground quasars.

This work is part of the Ph.D. project of TS under supervision of JH and GW. GW is responsible for the He ii FUV spectra. JP and NC provided optical observational data and data reduction tools. Cosmological hydrodynamical simulations were provided by ZL and JO.