MCMC determination of the cosmic UV background at z similar or equal to 0 from H alpha fluorescence

In a recent paper we reported on the detection of a diffuse Ha 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 alpha 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 HI ionization rate (per ion) at z similar or equal to 0 to be in the range Gamma(HI) similar to 6 - 8 x 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 Gamma(Hi) = 7.27(-2.90)(+2.93) x 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 observedHa 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 Ly alpha 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.