Low Surface Brightness Hα Observations of Local Intergalactic Hydrogen Clouds

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Abstract
We present upper limits on the local ionizing background based on a search for extended Hα emission from three nearby intergalactic H I clouds: the Leo Ring (M96 group), both the NE and SW lobes of the Haynes-Giovanelli Virgo Cloud (H I 1225+01), and the H I tidal tails associated with the NGC 4631/4656 group. These clouds were chosen to have 21 cm emission that is extended (10-100 kpc) and distant from any associated galaxy. Deep, wide-field CCD images were acquired through narrow- (~31 Å) and broadband R filters with the Burrell Schmidt telescope on Kitt Peak. We set a 95% confidence upper limit on the Hα surface brightness for the areas of the clouds detected in H I of 1.6 × 10-19 ergs s-1 cm-2 arcsec-2 in Leo and of 3.7 × 10-19 ergs s-1 cm-2 arcsec-2 in Virgo. We limit the local ionizing background to Φ0 < 5.0 × 103 photons s-1 cm-2 sr-1 (95%) at the location of the Leo Ring cloud and Φ0 < 1.1 × 104 photons s-1 cm-2 sr-1 at the Virgo cloud, assuming spherical clouds. (Limits are a factor of 2 higher for a thin face-on slab.) The limits correspond to J0 < 3.3 × 10-23 and 7.6 × 10-23 ergs s-1 cm-2 sr-1 Hz-1 for a ν-1/2 spectrum (1.6 times higher for a ν-1.4 spectrum) between 1 and 4 ryd. Such low limits suggest that quasar light, and not galactic light, dominates the ionizing background at low redshift. The Hα limit on the Leo cloud is significantly below a previously reported detection. In the field of the edge-on galaxies NGC 4631 and NGC 4656, we detect Hα from ionized gas extending nearly 16 kpc above N4631, which could have been blown out by starburst activity in the plane, and a low surface brightness companion or stellar tidal tail. This companion lies between N4631's H I tidal tails and may have played a role in creating the H I tidal tails, or it may represent star formation within the tidal tails. We also report the tentative detection of an ultrafaint "sheet" of Hα emission extending from NGC 4631 to NGC 4656.