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A measurement of the z = 0 UV background from Halpha fluorescence.

Fumagalli, Michele and Haardt, Francesco and Theuns, Tom and Morris, Simon L. and Cantalupo, Sebastiano and Madau, Piero and Fossati, Matteo (2017) 'A measurement of the z = 0 UV background from Halpha fluorescence.', Monthly notices of the Royal Astronomical Society., 467 (4). pp. 4802-4816.


We report the detection of extended Halpha emission from the tip of the HI disk of the nearby edge-on galaxy UGC 7321, observed with the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope. The Halpha surface brightness fades rapidly where the HI column density drops below N(HI) = 10^19 cm^-2 , consistent with fluorescence arising at the ionisation front from gas that is photoionized by the extragalactic ultraviolet background (UVB). The surface brightness measured at this location is (1.2 +/- 0.5)x10^-19 erg/s/cm^2/arcsec^2, where the error is mostly systematic and results from the proximity of the signal to the edge of the MUSE field of view, and from the presence of a sky line next to the redshifted Halpha wavelength. By combining the Halpha and the HI 21 cm maps with a radiative transfer calculation of an exponential disk illuminated by the UVB, we derive a value for the HI photoionization rate of Gamma ~ (6-8)x10^-14 1/s . This value is consistent with transmission statistics of the Lyalpha forest and with recent models of a UVB which is dominated by quasars.

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Publisher statement:This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:13 February 2017
Date deposited:27 March 2017
Date of first online publication:17 February 2017
Date first made open access:27 March 2017

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