A measurement of the z=0 UV background from Hα fluorescence

We report the detection of extended H alpha emission from the tip of the HI disc of the nearby edge-on galaxy UGC 7321, observed with the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope. The H alpha surface brightness fades rapidly where the HI column density drops below N-HI similar to 10(19) cm(-2), consistent with fluorescence arising at the ionization front from gas that is photoionized by the extragalactic ultraviolet background (UVB). The surface brightness measured at this location is (1.2 +/- 0.5) x 10(-19) erg s(-1) 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 H alpha wavelength. By combining the H alpha and the HI 21 cm maps with a radiative transfer calculation of an exponential disc illuminated by the UVB, we derive a value for the HI photoionization rate of Gamma(HI) similar to (6- 8) x 10(-14) s(-1). This value is consistent with transmission statistics of the Lya forest and with recent models of a UVB that is dominated by quasars.