Constraining the lifetime and opening angle of quasars using fluorescent Lyman $\alpha$ emission: the case of Q0420-388

TL;DR

This study models how quasar geometry and lifetime influence fluorescent Lyman alpha emission distribution, applying it to observations around quasar Q0420-388 to estimate quasar lifetime and opening angle, but finds conflicting evidence.

Contribution

Develops a toy model linking quasar geometry and lifetime to fluorescent emission distribution, and applies it to observational data to constrain quasar properties.

Findings

Evidence of azimuthal asymmetry consistent with bipolar radiation cone

Estimated quasar lifetime of at least 15-40 Myr depending on opening angle

Lack of expected fluorescence signatures challenges previous lifetime estimates

Abstract

A toy model is developed to understand how the spatial distribution of fluorescent emitters in the vicinity of bright quasars could be affected by the geometry of the quasar bi-conical radiation field and by its lifetime. The model is then applied to the distribution of high equivalent width Lyman $\alpha$ emitters (with rest-frame equivalent widths above 100 A, threshold used in e.g. Trainor & Steidel, 2013) identified in a deep narrow-band 36x36 arcmin$^2$ image centered on the luminous quasar Q0420-388. These emitters are found to the edge of the field and show some evidence of an azimuthal asymmetry on the sky of the type expected if the quasar is radiating in a bipolar cone. If these sources are being fluorescently illuminated by the quasar, the two most distant objects require a lifetime of at least 15 Myr for an opening angle of 60 degrees or more, increasing to more than 40 Myr if the opening angle is reduced to a minimum 30 degrees. However, few of the other expected signatures of boosted fluorescence are seen at the current survey limits, e.g. a fall off in Lyman $\alpha$ brightness, or equivalent width, with distance. Furthermore, to have most of the Lyman $\alpha$ emission of the two distant sources to be fluorescently boosted would require the quasar to have been significantly brighter in the past. This suggests that these particular sources may not be fluorescent, invalidating the above lifetime constraints. This would cast doubt on the use of this relatively low equivalent width threshold and thus on the lifetime analysis in Trainor & Steidel (2013).