CO Observations of the Peculiar Galaxy NGC 7625 and Detection of Infalling Molecular Gas

Abstract

We have carried out 12CO (J = 1 — 0) emission-line observations of NGC 7625 (=Arp 212) with the Nobeyama 45-m telescope. We have found that the molecular gas is widely spread in the galaxy, and is correlated with visible dust lanes; its molecular mass is at least 1.54 × 109  M◉. The velocity of molecular gas is not fully consistent with the rotation of the stellar disk. Part of the molecular gas, corresponding to a prominent dark lane, shows a redshifted velocity with respect to the stellar disk, suggesting an infall of the molecular gas onto the stellar disk. We have estimated the present-day and past (≅ 109 yr ago) star formation rates using the far-infrared and B luminosities, respectively, and have found that these two star formation rates have the same value, as large as 8.5 M◉ yr-1. This suggests that a constant star formation rate has been maintained for the last 109 yr in this galaxy. In order to explain this high star formation rate and the velocity anomaly of CO gas, we suggest that molecular gas has been infalling onto this galaxy at a constant rate, thus enhancing star formation.