MCMC determination of the cosmic UV background at z ≃ 0 from Hα fluorescence

In a recent paper we reported on the detection of a diffuse H α 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 H α emission as fluorescence arising from hydrogen ionized by an external (i.e. extragalactic) radiation field, we estimated the UV background (UVB) intensity in terms of H I ionization rate (per ion) at z ≃ 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 refine our estimate and through an MCMC analysis derive a value for the photoionization 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 H α surface brightness. Our measurement is consistent with several recent determinations of the same quantity by a completely independent technique (i.e. flux decrement analysis of the Lyα forest), and support the notion that the low-redshift UVB is largely dominated by active galactic nuclei, possibly with no need of further contribution from star-forming galaxies