MCMC determination of the cosmic UV background at <i>z</i> ? 0 from Hα fluorescence.

摘要： In a recent paper (Fumagalli et al. 2017) we reported on the detection of a di?use Hα glow in the outskirts of the nearby, edge-on disc galaxy UGC 7321 observed with the Multi Unit Spectroscopic Explorer (MUSE) at the ESO Very Large Telescope. By interpreting the Hα emission as ?uorescence arising from hydrogen ionised by an external (i.e., extragalactic) radiation ?eld, we estimated the UV background (UVB) intensity in terms of H I ionisation rate (per ion) at z (cid:39) 0 to be in the range ΓHI ～ 6 ? 8 × 10?14 s?1. In the present work, by performing radiative transfer calculations over a large set of models of the gaseous disc of UGC 7321, we re?ne our estimate and through an MCMC analysis derive a value for the photoionisaton rate of ΓHI = 7.27+2.93 ?2.90 × 10?14 s?1. In particular, our analysis demonstrates that this value is robust against large variations in the galaxy model and that the uncertainties are mainly driven by the errors associated with the observed Hα surface brightness. Our measurement is consistent with several recent determinations of the same quantity by a completely independent technique (i.e., ?ux decrement analysis of the Lyα-forest), and support the notion that the low redshift UVB is largely dominated by active galactic nuclei (AGNs), possibly with no need of further contribution from star forming galaxies.