Photoionization models for extreme Lyα λ1216 and Hell λ1640 line ratios in quasar halos, and PopIII vs. AGN diagnostics

Abstract

Aims. We explore potential mechanisms to produce extremely high Lyα/HeII flux ratios, or to enhance the observed number of Lyα photons per incident ionizing photon, in extended AGN-photoionized nebulae at high-redshift. Methods. We compute models to simulate, in the low density regime, photoionization of interstellar gas by the radiation field of a luminous AGN. We explore the impact of ionization parameter, gas metallicity, ionizing spectrum, electron energy distribution, and cloud viewing angle on the relative fluxes of Lyα, HeII and other lines, and on the observed number of Lyα photons per incident ionizing photon. We compare our model results with recent observations of quasar Lyα halos at z∼3.5. Results. Low ionization parameter, a relatively soft or filtered ionizing spectrum, low gas metallicity, κ-distributed electron energies, or reflection of Lyα photons by neutral hydrogen can all result in significantly enhanced Lyα relative to other lines (≥ 10 %), with log Lyα/HeII reaching values of up to 4.6. In the cases of low gas metallicity, reflection by HI, or a hard or filtered ionizing spectrum, the observed number of Lyα photons per incident ionizing photon is itself significantly enhanced above the nominal Case B value of 0.66 due to collisional excitation, reaching values as high as 5.3 in an 'extreme case' model which combines several of these effects. We find that at low gas metallicity (e.g. Z/Z ⊙ =0.1) the production of Lyα photons is predominantly via collisional excitation rather than by recombination. In addition, we find that the collisional excitation of Lyα becomes much more efficient if the ionizing continuum spectrum has been pre-filtered through an optically thin screen of gas closer to the AGN (e.g., by a wide-angle, feedbackdriven outflow.) We also show that the Lyα and HeII emission line ratios of the z∼3.5 quasars studied by Borisova et al. (2016) are consistent with AGN-photoionization of gas with moderate to low metallicity and/or low ionization parameter, without requiring exotic ionization/excitation mechanisms such as strong line-transfer effects. In addition, we present a set of UV-optical diagnostic diagrams to distinguish between photoionization by Pop III stars and photoionization by an AGN.

Deciphering the Lyman α blob 1 with deep MUSE observations

“…As explained above, for n H = 100 cm 3 gas in region "north" only log U −4 would be compatible with a scenario where the whole blob is powered by photo-ionisation, but also at these extreme ionisation parameters the Humphrey et al (2019) model are incompatible with our observations. Thus, photo-ionisation of low-metallicity gas by an AGN or Pop-III stellar population under the assumptions made in the Humphrey et al (2019) models appears not to be a valid scenario to explain the He ii emitting zones in LAB 1.…”

“…He ii/Lyα and C iv/Lyα ratios comparable to our measurements have been found in extended Lyα halos around high-z radio galaxies and AGNs, and these ratios can be explained by some photo-ionisation models. However, our ratios are not as extreme as the ratios that would be expected from quasar photoionisation of extremely metal-deficient gas at very low ionisation parameters (Humphrey et al 2019).…”

“…We remark also that for U < −2.5 (outside of the displayed range in Fig. 14, see Humphrey et al 2019), the He ii/Lyα ratios also start to decrease significantly.…”

“…Detections of He ii emitting zones within the extended Lyα nebulae surrounding radio quiet quasars or in LABs not directly associated with an AGN appear less frequent and more difficult to interpret (Scarlata et al 2009b;Prescott et al 2009;Cai et al 2017;Cantalupo et al 2019;Humphrey et al 2019). Different explanations have been put forward, for example, Scarlata et al (2009b) favouring cooling-radiation in their system or Prescott et al (2009) arguing for photo-ionisation by an AGN or an extremely metal poor stellar population in their object, while Cai et al (2017) reason that their extended He ii zone is powered by shocks.…”

“…Schaerer et al 2013) or in the surroundings of an AGN (e.g. Humphrey et al 2019). Analysing relative line strengths and comparing the spatial distribution of the He ii and C iv emitting gas to the positions of potential ionising sources within blob may help to distinguish between photo-ionisation and cooling-radiation scenarios.…”