A New z = 0 Metagalactic Ultraviolet Background Limit

International audienceWe present new integral-field spectroscopy in the outskirts of two nearby, edge-on, late-type galaxies to search for the Halpha emission that is expected from the exposure of their hydrogen gas to the metagalactic ultraviolet background (UVB). Despite the sensitivity of the VIRUS-P spectrograph on the McDonald 2.7 m telescope to low surface brightness emission and the large field of view, we do not detect Halpha to 5sigma upper limits of 6.4 × 10-19 erg s-1 cm-2 arcsec-2 in UGC 7321 and of 25 × 10-19 erg s-1 cm-2 arcsec-2 in UGC 1281 in each of the hundreds of independent spatial elements (fibers). We fit gas distribution models from overlapping 21 cm data of H I, extrapolate one scale length beyond the H I data, and estimate predicted Halpha surface brightness maps. We analyze three types of limits from the data with stacks formed from increasingly large spatial regions and compare to the model predictions: (1) single fibers, (2) convolution of the fiber grid with a Gaussian, circular kernel (10'' full width at half-maximum), and (3) the co-added spectra from a few hundred fibers over the brightest model regions. None of these methods produce a significant detection (>5sigma) with the most stringent constraints on the H I photoionization rate of Gamma(z = 0) -14 s-1 in UGC 7321 and Gamma(z = 0) -14 s-1 in UGC 1281. The UGC 7321 limit is below previous measurement limits and also below current theoretical models. Restricting the analysis to the fibers bound by the H I data leads to a comparable limit; the limit is Gamma(z = 0) -14 s-1 in UGC 7321. We discuss how a low Lyman limit escape fraction in z ~ 0 redshift star-forming galaxies might explain this lower than predicted UVB strength and the prospects of deeper data to make a direct detection