Kinematic Evidence for Minor Mergers in Isolated, Normal Sa Galaxies

Sa galaxies are the earliest Hubble types exhibiting clear evidence of spiral structure. But while the small pitch angle of the arms in Sa's is distinctive, tightly wound spirals show a wide range of bulge size and current star formation rate, the other two criteria used to distinguish among the spiral types. Thus, the Sa class is heterogeneous, including gas-rich and gas-poor disks and large and small-bulged systems. Furthermore, in contrast to their later spiral counterparts, Sa's typically occupy higher density environments and require little or no dark matter within their optical disks.

The origin of the heterogeneity of the Sa class has been the focus of a detailed study of the morphology, environment, kinematics and dynamics of a sample of nearby, undisturbed and relatively isolated Sa galaxies. Broad-band optical imaging observations provide luminosity profiles, colors and measures of morphological asymmetry in the stellar distribution. Long-slit spectroscopy along both the major and minor axes provide details of the kinematics of both stellar and ionized gas components. Narrow-band Halpha imaging identifies the sites of current massive star formation, while HI synthesis imaging provides measures of both morphological and dynamical asymmetry in the gas disk and details of the dynamics which can be used to trace the mass.

For her Ph.D. thesis, Katie Jore studied the optical rotation curves of both gas and stars in a sample of 20 nearby, isolated Sa galaxies. About half of these objects show evidence of kinematically decoupled components. The degree and circumstances of the distinct kinematics vary from complete counterrotation of all of the gas from all/most of the stars (NGC3626, NGC4138) to nuclear gas disks decoupled from the stars (NGC5854) to anomalous velocity central gas components (NGC3623, NGC3900, NGC4772). The HI distribution in nine gas-rich Sa's was mapped with the VLA, and of those, all but NGC3623, known to be a member of the interacting Leo Triplet, show HI disks that extend more than twice as far as the optical edge. While the HI velocity fields are dominated by circular rotation, the HI disks are significantly warped, and the warps do not follow Briggs' "rules for warps" seen in many other galaxies.

Perhaps the most extreme of these in NGC 4138 in which 80% of the stars rotate counter to 20% of the stars and all of the gas (HI and HII).

While Sa's tend to be found in higher density environments, it may be very important that the present sample consists principally of relatively isolated, unbarred and undisturbed Sa galaxies. For her Cornell M.S. thesis, Katrin Hagemann applied the 2-D generalized exponential decomposition scheme developed by Giovanni Moriondo to derive I band B/D for a nearly-complete sample of northern Sa's to try to see if B/D might correlate with environment. Katrin found marginal evidence that Sa's with close neighbors show a larger range of B/D than ones with no near neighbors and that the bulge index n may vary with environment.

While the current sample is small and consists of relatively isolated objects, their optical morphology bears no hint of the disturbances responsible for the complex kinematics evident in half the galaxies. Thus, minor mergers may play an important role in producing the heterogeneity of the Sa class through the triggering of starbursts, bulge-building or the dislocation of disk gas, but the evidence of such minor events is quickly lost from the morphological appearance of the stellar distribution as evident in optical images. Morphological criteria alone thus may seriously underestimate the rate of minor-to-moderate mergers.

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