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Session 88 - Structure and Kinematics of Galaxies.
Oral session, Wednesday, January 17
La Condesa, Hilton

[88.03] The Maximum Disk Hypothesis and 2-D Spiral Galaxy Models

P. Palunas, T. B. Williams (Rutgers U.)

\def\kmskm s^-1 \def\haH\alpha We present an analysis of two-dimensional \ha\ velocity fields and I-band surface photometry for spiral galaxies taken from the southern sky Fabry-Perot Tully-Fisher survey (Schommer et al., 1993, AJ 105, 97). We construct axi-symmetric maximum disk mass models for 75 galaxies and examine in detail the deviations from axi-symmetry in the surface brightness and kinematics for a subsample of these galaxies.

The luminosity profiles and rotation curves are derived using consistent centers, position angles, and inclinations. The disk and bulge are deconvolved by fitting an exponential disk and a series expansion of Gaussians for the bulge directly to the I-band images. This helps constrain the deconvolution by exploiting geometric information as well as the distinct disk and bulge radial profiles. The final disk model is the surface brightness profile of the bulge-subtracted image. The photometric model is fitted to the rotation curve assuming a maximum disk and constant M/L's for the disk and bulge components. The overall structure of the photometric models reproduces the structure in the rotation curves in the majority of galaxies spanning a large range of morphologies and rotation widths from 120 \kms\ to 680 \kms. The median I-band M/L in solar units is 2.8, consistent with normal stellar populations. These results make the disk-halo conspiracy even more puzzling.

The degree to which spiral galaxy mass models can reproduce small-scale structure in rotation curves is often used as evidence to support or refute the maximum disk hypothesis. However, single-slit rotation curves sample the velocity distribution only along the major axis, and photometric profiles for inclined galaxies are also sampled most heavily near the major axis. The small-scale structure can be due to local perturbations, such as spiral arms and spiral-arm streaming motions, rather than variations in the global mass distribution. We test this hypothesis by analysing azimuthal correlations in the surface brightness distribution and velocity field.

Program listing for Wednesday