The Kinematics of Arp 295 in Hα Emission: an Interacting Galaxy with Highly Asymmetric Rotation

We investigate Arp 295, a pair of interacting spirals at z = 0.023, using optical spectroscopy, Hα imaging, and high-resolution Hα spectroscopy with the Manchester Echelle Spectrograph. We measure scalelengths rexp = 5.24 for Arp 295a and 2.52 kpc for 295b. There is a much smaller Im galaxy, Arp 295c, associated with the larger spiral. Arp 295b is asymmetric with the disk more extended eastwards but with the brightest star-formation regions on the west side of the nucleus. The spectrum of Arp 295b shows strong emission lines with [OII]3727 prominent (equivalent width 36Å). Line ratios give an abundance 12 + log(O/H) = 8.757± 0.030, within the normal range for the luminosity MB = −20.44. We measure the total Hα line luminosities of Arp 295b and Arp 295c as 4.58 × 10 and 6.76 × 10 ergs s, corresponding to star-formation rates 3.6 and 0.53 M⊙yr . Arp 295c has a high radial velocity (approaching) of 279 km s relative to Arp 295a. To study the kinematics of Arp 295b we observe with MES using an E-W spectroscopic slit at a series of 9 positions, from the north to the south edge of the galaxy disk. We measure the maximum disk rotation velocity as 252.6 ± 9.9 km s and derive the mass inwards of 8 kpc as 9.8 × 10M⊙. The rotation curve of Arp 295b is very asymmetric. The east (approaching) side has a higher radial velocity than the west with the maximum difference at radius r = 5 arcsec from the nucleus, where it is 88 km s or a factor 1.675. The asymmetric kinematics reflect a major disturbance of the mass distribution within the disk, caused by the interaction with 295a. The slowed rotation on the western side of the nucleus may have locally enhanced star-formation.