SIMBAD references

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_∼10¹⁹ 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 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 Γ_ 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.