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

Caruso, D. and Haardt, F. and Fumagalli, M. and Cantalupo, S. (2019) 'MCMC determination of the cosmic UV background at z ≃ 0 from Hα fluorescence.', Monthly notices of the Royal Astronomical Society., 482 (2). pp. 2833-2837.


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.

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Publisher statement:This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2018 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:30 October 2018
Date deposited:21 November 2018
Date of first online publication:01 November 2018
Date first made open access:21 November 2018

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