First results of a sensitive search for H I envelopes in a complete sample of spiral galaxies - Extensive H I near the isolated SC I NGC 628

Abstract

Preliminary results of a search with the Arecibo 1000-foot (305-m) telescope for extended H I envelopes surrounding spiral galaxies show that column densities of H I greater than or equal to 3 x 10 to the 18th/sq cm at 2-3 Holmberg radii are not common. The sample includes all galaxies of Hubble type later than S0, with optical major-axis diameter greater than 7 arcmin, and with declination 3-35 deg, and recessional velocity greater than 200 km/s. The prevalent upper limit to H I column density suggests that these outer regions (radius greater than two Holmberg radii) cannot form QSO-like Mg II absorption lines unless the abundance of Mg is at least 10 times that observed in the interstellar medium or in Population II regions of the Galaxy. The isolated Sc I galaxy NGC 628 is an exceptional case. This large, face-on galaxy shows H I emission extending asymmetrically to the SW as far as 3 Holmberg radii.