We report the detection of extended H \alpha emission from the tip of the H { \small I } disk 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 H { \small I } column density drops below N _ { HI } \sim 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 \pm 0.5 ) \times 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 H { \small I } 21 cm maps with a radiative transfer calculation of an exponential disk illuminated by the UVB , we derive a value for the H { \small I } photoionization rate of \Gamma _ { HI } \sim ( 6 - 8 ) \times 10 ^ { -14 } ~ { } s ^ { -1 } .
This value is consistent with transmission statistics of the Ly \alpha forest and with recent models of a UVB which is dominated by quasars .