The duration of quasar (QSO) accretion episodes is a key quantity for distinguishing between models for the formation and growth of supermassive black holes, QSO evolution, and the potential feedback effects on their host galaxies. However, this critical timescale, often referred to as the QSO lifetime, is still uncertain by orders of magnitude t_Q~10^4-10^9 yr. The intense UV radiation from a QSO dramatically alters the ionization state of its surrounding intergalactic medium (IGM), which is observable as enhanced Lyman-alpha transmission toward the QSO in its so-called proximity zone. HST/COS spectra of far-UV bright QSOs probing the HeII Lyman-alpha forest provide a unique opportunity to make precise lifetime measurements for individual QSOs. Indeed, the size of the HeII proximity zone depends sensitively on the QSO lifetime for t_Q<30 Myr, comparable to the e-folding timescale for SMBH growth t_S=45 Myr. We have compared the sizes of HeII proximity zones to theoretical models generated by post-processing cosmological hydrodynamical simulations with 1D radiative transfer, which allows us to measure individual QSO lifetimes to 0.2 dex precision. An analysis of 24 archival HST/COS spectra has revealed a surprisingly broad distribution of lifetimes ranging from <1 to >30 Myr. These results point to complex QSO light curves that exhibit structure on a wide range of timescales. By leveraging a new sample of FUV-bright QSOs selected from GALEX, SDSS, PanSTARRS and WISE, we request 32 orbits of HST/COS to double the number of QSOs to which we can apply this exciting new technique, which will allow us to make the first measurement of the distribution of QSO lifetimes.
- Pub Date:
- May 2020