Evolution in Dust Lane Ellipticals: Detection of Accreted Molecular Gas

The detection of moderately large quantities of molecular gas in three minor axis dust lane elliptical galaxies is reported. Masses of H2 between 4 × 108 and 2 × 10 Msun have been observed in NGC 1316, NGC 1947, and NGC 5266. The distribution of the molecular gas follows the dust, which is patchy in NGC 1316, around the nucleus in NGC 1947, and in a ring in NGC 5266. The velocities and directions of motion for the CO agree with those of the ionized gas, which rotates perpendicularly to the stars. Based upon dynamical studies of interacting systems, and following related work on the origin of the H I and ionized gas, we suggest that the H2 comes from gas acquired from an external source, probably through the ingestion of a gas-rich dwarf galaxy. Since dwarfs, though typically rich in H I, are notably poor in H2, some mechanism must exist which converts the atomic into molecular gas. This may be the result of gas compression inside the ring into which the accreted object is smeared. Following the current theoretical models, the three observed galaxies are situated along a possible evolutionary sequence, with NGC 1316 in the earlier stages of gas (or satellite) accretion and NGC 5266 in a smooth, evolved configuration. The distribution of the molecular gas is entirely consistent with the models.