| Authors | van Driel, W.; Combes, F.; Casoli, F.; Gerin, M.; Nakai, N.; Miyaji, T.; Hamabe, M.; Sofue, Y.; Ichikawa, T.; Yoshida, S.; Kobayashi, Y.; Geng, F.; Minezaki, T.; Arimoto, N.; Kodama, T.; Goudfrooij, P.; Mulder, P. S.; Wakamatsu, K.; Yanagisawa, K. |
| Abstract | NGC 660 is a unique, nearby, peculiar polar ring spiral LINER galaxy with two distinct morphological and kinematic components: a spiral disk, seen almost edge on (i~70^deg^), with a major axis position angle of 45^deg^ and a diameter of ~11 kpc (D = 13 Mpc, H_0_ = 75 km s^-1^ Mpc^-1^), and an outer polar ring (P.A. 1700) with a diameter of 31 kpc, inclined on average 55^deg^ with respect to the disk major axis. It was mapped in the 21 cm HI line with a resolution of 13" x 60" and in the CO(1-0) and CO(2- 1) lines with a 12"-22" beam. B, V, R, I, J, H, K'-band images, and a long-slit Hα spectrum along the disk major axis were obtained as well. It has been morphologically classified as SBa, but our data show it has the global characteristics of a later-type (Sc?), gas-rich disk. The disk and the polar ring both have an exponential luminosity profile, with scale lengths of 1.3 and 3.9 kpc, respectively. The polar ring is blue (V-I~1.0), indicating a stellar population age of a few billion years, according to our stellar population synthesis model and the nucleus is red (V-I~1.8). Hα images show H II regions throughout the polar ring. The near-infrared images show a boxy/X shape of the bulge and 1.4 kpc long linear features along the disk major axis on both sides of the bulge. The disk was detected in radio continuum at 21 cm. It has a compact 300 mJy nuclear source and an extended (7.5 kpc diameter) component of 80 mJy. The central source has a very high radio power for a spiral galaxy, while the disk has a normal radio power. The radio spectral index is -0.57, indicating an important contribution from thermal radiation. The HI line observations show absorption against the nuclear source, and both H I and CO line data show emission from the disk and the polar ring. The H I and CO data indicate a rather flat disk rotation curve, with a rotation velocity of ~150 km s^-1^. From our Hα spectrum, taken close to the major axis, a steeper inner gradient and a considerably lower rotation velocity (~110 km s^-1^) is derived in the outer parts than from that of Benvenuti et at. (1976), if one assumes the disk to be flat and in circular rotation. This probably indicates noncircular motions in the inner disk. Using a geometrical model it was found that the ring has a slightly rising rotation curve reaching 127 km s^-1^ at 18 kpc radius ( = 1.15 R_25_), implying an M_T_/L_B_^0^ ratio of ~10M_sun_/L-sun,B_ within this radius. The total H I and H_2_ mass is about 5.4 and 3.7 10^9^ M_sun_, respectively. About 75% of the H I gas resides in the ring, and the disk has a gas content (M_HI_/L_B_^0^ = 0.3M_sun_/L_sun,B_) of about an Sc-type spiral. Though the galaxy has a LINER-type spectrum, suggesting (like other indicators) intense massive star formation in the nucleus, and though it has a rather high far-infrared luminosity (log(L_FIR_/L_sun_) = 10.30] as well as a high L_FIR_/L_B_^0^ ratio (4.7), its overall star formation efficiency, L_FIR_/M_H_2__ = 5.4L_sun_/M_sun_, is comparable to that of normal spirals of similar LFIR, and much lower than that of classical starburst galaxies. A mass model was made by fitting the luminosity profiles of the bulge, disk, and polar ring, as well as the rotation velocities in the disk, using a constant mass-to-light ratio, taking the gas mass into account. In order to fit the rotation velocities in the polar ring additional dark matter was required, however, with a dark-to-luminous mass ratio of 0.6 inside 18 kpc radius, if the halo is assumed to be spherical. The polar ring is quite massive, with a mass of about 75% of that of the disk, and it contains about 25% of the total estimated mass of the NGC 660 system. The considerable mass of the ring can explain its apparent stability through self-gravity. It was not possible to constrain the flattening of the dark halo, though, since the equatorial (disk) velocities at the radii of the polar ring are not known. In fact, this is true for all polar-ring systems, since two perpendicular streams of gas at the same radius do not form a stable situation. In the H I line the Irr? galaxy UGC 1195 was detected as well, at a projected distance of 82 kpc with a systemic velocity 74 km s^-1^ lower than NGC 660. It is gas- rich (M_HI_/L_B_^0^ = 0.4M_sun_/L_sun,B_ with an HI distribution slightly larger than the optical outlines. It has a rather low rotation velocity of 56 km s^-1^, implying M_T_/L_B_^0^ ~ 2.6M_sun_/L_sun,B_ within R = 6.4 kpc. |
