# A measurement of the z = 0 UV background from H$\alpha$ fluorescence

## TL;DR

This study detects faint Halpha emission at the edge of a galaxy's HI disk to measure the extragalactic UV background, providing empirical constraints consistent with quasar-dominated models.

## Contribution

It presents the first direct measurement of the local UV background at z=0 using Halpha fluorescence in a nearby galaxy's HI disk.

## Key findings

- Measured Halpha surface brightness at the HI disk edge.
- Derived HI photoionization rate consistent with quasar-dominated UVB models.
- Confirmed the UVB's influence on galaxy outskirts through radiative transfer modeling.

## 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.