A NEWz= 0 METAGALACTIC ULTRAVIOLET BACKGROUND LIMIT

We present new integral-field spectroscopy in the outskirts of two nearby, edge-on, late-type galaxies to search for the Hα 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 Hα to 5σ 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 Hα 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 (>5σ) with the most stringent constraints on the H i photoionization rate of Γ(z = 0) < 1.7 × 10−14 s−1 in UGC 7321 and Γ(z = 0) < 14 × 10−14 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 Γ(z = 0) < 2.3 × 10−14 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.