Abundant Molecular Gas and Inefficient Star Formation in Intracluster Regions: Ram Pressure Stripped Tail of the Norma Galaxy ESO137-001

Abstract

For the first time, we reveal large amounts of cold molecular gas in a ram-pressure-stripped tail, out to a large "intracluster" distance from the galaxy. With the Actama Pathfinder EXperiment (APEX) telescope, we have detected ¹²CO(2-1) emission corresponding to more than 10⁹ M _⊙ of H₂ in three Hα bright regions along the tail of the Norma cluster galaxy ESO 137-001, out to a projected distance of 40 kpc from the disk. ESO 137-001 has an 80 kpc long and bright X-ray tail associated with a shorter (40 kpc) and broader tail of numerous star forming H II regions. The amount of ~1.5 × 10⁸ M _⊙ of H₂ found in the most distant region is similar to molecular masses of tidal dwarf galaxies, though the standard Galactic CO-to-H₂ factor could overestimate the H₂ content. Along the tail, we find the amount of molecular gas to drop, while masses of the X-ray-emitting and diffuse ionized components stay roughly constant. Moreover, the amounts of hot and cold gas are large and similar, and together nearly account for the missing gas from the disk. We find a very low SFE (τ_dep > 10¹⁰ yr) in the stripped gas in ESO 137-001 and suggest that this is due to a low average gas density in the tail, or turbulent heating of the interstellar medium that is induced by a ram pressure shock. The unprecedented bulk of observed H₂ in the ESO 137-001 tail suggests that some stripped gas may survive ram pressure stripping in the molecular phase.

Based on observations made with ESO telescopes at La Silla Paranal Observatory under program ID 088.B-0934.