MCMC determination of the cosmic UV background at $z\simeq 0$ from H$\alpha$ fluorescence

Abstract

In a recent paper (Fumagalli et al. 2017) we reported on the detection of a diffuse H$\alpha$ 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 ionised by an external (i.e., extragalactic) radiation field, we estimated the UV background (UVB) intensity in terms of HI ionisation rate (per ion) at $z\simeq 0$ to be in the range ${\Gamma }_{\mathrm{HI}}\sim 6-8\times 10^{-14}$ 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 photoionisaton rate of ${\Gamma }_{\mathrm{HI}}=7.27_{-2.90}^{+2.93}\times 10^{-14}$ 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$\alpha$ 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 (AGNs), possibly with no need of further contribution from star forming galaxies.

arXiv comment6 pages, 2 figures, submitted to MNRAS

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• How Robust are the Inferred Density and Metallicity of the Circumgalactic Medium?
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• A Bayesian inference and model selection algorithm with an optimisation scheme to infer the model noise power
• APES: Approximate Posterior Ensemble Sampler