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B. Catinella, M.P. Haynes, R. Giovanelli (Cornell University)
Long-slit spectra of spiral galaxies are widely used to complement 21 cm HI observations, providing rotational velocity widths for the application of the Tully-Fisher (TF) relation. The accuracy of the velocity width measurement is of a key importance for a TF-based peculiar velocity (PV) survey, being the single dominant contribution to the TF error budget (besides the intrinsic scatter). We have used a large database of H-alpha optical rotation curves (ORCs), HI-line profiles, and I-band photometry designed for PV studies to investigate the errors and systematics associated with the velocity width measurement. A detailed comparison between different methods of measuring velocity widths from ORCs shows the importance of taking the ORC shape into account. In particular, estimating a line width from the ORC velocity histogram alone introduces systematic errors that correlate with the slope of the outer regions of the ORC. We have also studied the dependence of the extent of the H-alpha emission on the HI deficiency, a measure of the HI content. We find that HI-poor disks also appear to be H-alpha-truncated. Since rotation curves are typically rising in the outer regions of the disks, the optical histogram widths of HI-deficient galaxies are therefore systematically underestimated compared to the ones of HI-normal galaxies. A more reliable width estimate can be obtained by fitting a function to the ORC and measuring the velocity at a fixed metric distance from the center of the disk. Rotational widths measured in this way can account for systematic and environmental effects.
This work has been supported by NSF grant AST99-00695.
The author(s) of this abstract have provided an email address for comments about the abstract: bc42@cornell.edu
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.