Spiral structure of the giant galaxy UGC 2885: Hα kinematics

The velocity field of the exceptionally large Sc galaxy UGC 2885 (Holmberg radius 84 h^-1 kpc, where h = H₀/100 km s^-1 Mpc^-1) has been mapped in Hα emission with the TAURUS I imaging Fabry-Perot spectrometer using the 2.5 m telescope at La Palma. The rotation curve extracted from the velocity field agrees with published data. Ripples in the velocity field around the minor axis indicate radial flows across the spiral arms. The radial flow speeds in the plane of the disk show 50-70 km s^-1 peak-to-peak variation, suggesting that a strong density wave is present in the underlying stellar disk. Such high speeds may alternatively be a natural consequence of the open arm spiral pattern. In addition, strong density waves may naturally occur in large spiral galaxies or in spiral galaxies as massive as UGC 2885. (Its mass is over 10¹² h^-1 M_⊙ within the radius to which spiral arms reach, 52 h^-1 kpc.) A strong density wave may also be necessary for the effective maintenance of the orderly, two-armed spiral pattern that is visible in the outer disk of UGC 2885, where the gas has made only about a dozen revolutions in a Hubble time.