Title: A measurement of the z = 0 UV background from H$α$ fluorescence,
Abstract: 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.