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

We report the detection of extended Hα emission from the tip of the H I disc of the nearby edge-on galaxy UGC 7321, observed with the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope. The Hα surface brightness fades rapidly where the H I column density drops below N H I ∼ 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) × 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 Hα wavelength. By combining the Hα and the H I 21 cm maps with a radiative transfer calculation of an exponential disc illuminated by the UVB, we derive a value for the H I photoionization rate of H I ∼ (6–8) × 10 −14 s −1 . This value is consistent with transmission statistics of the Lyα forest and with recent models of a UVB that is dominated by quasars.