Near-infrared and optical morphology of the dusty galaxy NGC 972

Near-infrared (NIR) and optical surface photometric analyses of the dusty galaxy NGC 972 are presented. The photometric profiles in the BVRJHK bands can be fitted with a combination of Gaussian and exponential profiles, corresponding to a starburst nucleus and a stellar disk, respectively. The exponential scale length in the B band is 2.8 times larger than in the K band, which implies a central B-band optical depth as high as 11. A bulge is absent even in the NIR bands, and hence the galaxy must be of a morphological type later than the usually adopted Sb type. Relatively low rotational velocity and high gas content also favor a later type, probably Sd, for the galaxy. Only one arm can be traced in the distribution of old stars; the second arm, however, can be traced in the distribution of dust and H II regions. Data suggest a short NIR bar, which ends inside the nuclear ring. The slowly rising nature of the rotation curve rules out a resonance origin of the nuclear ring. The ring is most likely not in the plane of the galaxy, given its circular appearance, in spite of the moderately high inclination of the galaxy. The off-planar nature of the star-forming ring, the unusually high fraction (30%) of the total mass in molecular form, the presence of a nuclear starburst, and the asymmetry of spiral arms are probably the result of a merger with a gas-rich companion galaxy.