THE MOLECULAR GAS AND STAR-FORMATION IN IRAS BRIGHT EARLY-TYPE DISK GALAXIES .1. NGC-7625

We present a detailed study of the distribution of the molecular and a tomic gas an their relationships with the current star formation in NG C 7625, an Sa/S pec galaxy, using high-resolution images of Halpha, (C O)-C-12 J = 1 --> 0, radio continuum and H I emission. Single-dish obs ervations of (CO)-C-12 J = 1 --> 0 and 2 --> 1 transitions were also u sed to evaluate, respectively, the ''missing'' interferometric CO flux and the CO excitation in the central 23'' of the galaxy. NGC 7625 con tains approximately 2.4 x 10(9) M. of molecular hydrogen, confined mos tly to the central 3.5 kpc with peak H-2 surface density of 400 M. pc- 2. Its M(HI)/M(H-2) approximately 1 and M(H-2)/L(B) approximately 0.6 M./L. are among the highest found in early-type disk galaxies, making NGC 7625 as gas-rich as normal Sbc spirals. The distribution of the CO emission is asymmetric with most emission being from the northwest qu adrant of the galaxy. The bulk of the molecular gas seems to be in a r otating disk with the region of solid-body rotation extending approxim ately 1 kpc from the center, similar to the ionized gas and stellar co mponents. In the inner 3.5 kpc of the galaxy where CO emission was det ected, H-2 accounts for approximately 24% of the dynamical mass. The s urface brightness distributions of both the 20 cm radio continuum and Halpha emission, which are tracers of massive star formation, correlat e well with the CO surface brightness with the zero points and slopes agreeing with the correlations found in spiral galaxies. We derived a power-law index n = 1.0 +/- 0.2 for the Schmidt star formation law, wh ich implies that the efficiency of massive star formation is nearly co nstant and is insensitive to the molecular gas surface density. There is no sign of an active galactic nucleus contributing to the nontherma l radio emission in NGC 7625. Both the kinematics of the H-2 and the e fficiency of massive star formation suggest that star formation procee ds as efficiently as in most spiral galaxies and no spiral density wav e triggering is required. On the other hand, the distribution of H I i s more extended and does not seem to be directly related to the curren t star formation. The outer H I shows apparent counterrotation with re spect to the inner H-2 gas, which could be due to a strongly warped H I disk or the existence of an outer H I ring.