Low Surface Brightness H alpha Observations of Local Intergalactic Hydrogen Clouds

Details Low Surface Brightness H alpha Observations of Local Intergalactic Hydrogen Clouds Low Surface Brightness H alpha Observations of Local Intergalactic Hydrogen Clouds

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...450l..45d&link_type=abstract

Astrophysical Journal Letters v.450, p.L45

Astronomy and Astrophysics

Astronomy

63

Intergalactic Medium

Scientific paper

We present upper limits on the local ionizing background based on a search for extended H alpha 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 A) and broadband R filters with the Burrell Schmidt telescope on Kitt Peak. We set a 95% confidence upper limit on the H alpha surface brightness for the areas of the clouds detected in H I of 1.6 x 10-19 ergs s-1 cm-2 arcsec-2 in Leo and of 3.7 x 10-19 ergs s-1 cm-2 arcsec-2 in Virgo. We limit the local ionizing background to Phi 0 < 5.0 x 103 photons s-1 cm-2 sr-1 (95%) at the location of the Leo Ring cloud and Phi 0 < 1.1 x 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 x 10-23 and 7.6 x 10-23 ergs s-1 cm-2 sr-1 Hz-1 for a nu -1/2 spectrum (1.6 times higher for a nu -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 H alpha 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 H alpha 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 ultra-faint "sheet" of H alpha emission extending from NGC 4631 to NGC 4656.

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