Intergalactic H ii Regions Discovered in SINGG

Abstract

A number of very small isolated H II regions have been discovered at projected distances up to 30 kpc from their nearest galaxy. These H II regions appear as tiny emission-line objects in narrowband images obtained by the NOAO Survey for Ionization in Neutral Gas Galaxies (SINGG). We present spectroscopic confirmation of four isolated H II regions in two systems; both systems have tidal H I features. The results are consistent with stars forming in interactive debris as a result of cloud-cloud collisions. The Hα luminosities of the isolated H II regions are equivalent to the ionizing flux of only a few O stars each. They are most likely ionized by stars formed in situ and represent atypical star formation in the low-density environment of the outer parts of galaxies. A small but finite intergalactic star formation rate will enrich and ionize the surrounding medium. In one system, NGC 1533, we calculate a star formation rate of 1.5 × 10 -3 M ⊙ yr -1 , resulting in a metal enrichment of ∼1 × 10 -3 solar for the continuous formation of stars. Such systems may have been more common in the past and a similar enrichment level is measured for the "metallicity floor" in damped Lyα absorption systems.