Intergalactic neutral hydrogen gas in the Grus quartet of galaxies

Abstract

Australia Telescope Compact Array multi-configuration mosaicing of the Grus quartet of galaxies reveals the presence of spectacular tidal structures. 2.1 × 109 M⊙ of neutral atomic hydrogen (H I) gas, i.e. 11% of all H I in the group, are found to be dragged from NGC 7582 into intergalactic space. About 1.34 × 109 M⊙ of H I gas are contained in a tidal tail emanating from the north-western disk of NGC 7582, with a projected length of about 85 kpc and width of up to 32 kpc and a relative velocity with respect to the centre of NGC 7582 of 130-140 km s -1. 7.7 × 108 M⊙ of H I reside in an intergalactic H I cloud 48 kpc West of NGC 7582, which might originate from the disk of NGC 7582 as well and has no optical counterpart in a red Digital Sky Survey (DSS) image. These observations prove that tidal stripping is occurring in the Grus quartet and that tidal features in compact groups can be potentially important contributors of metal-enriched matter to the intergalactic medium. The tidal features around NGC 7582 cover an area of about 2000 kpc2, almost doubling the group's cross-section for Lyman-α absorption of light from background sources compared to the optical extent of the member galaxies.