AAS Meeting #193 - Austin, Texas, January 1999
Session 70. Star Formation and the ISM in Galaxies
Display, Friday, January 8, 1999, 9:20am-6:30pm, Exhibit Hall 1

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[70.04] Star Formation in Single Leading Arm Density Wave versus Single Tidal Arms: NGC 4622 versus NGC 4378

T. Freeman (Bevill State Comm. Coll., Walker Campus, Sumiton, AL), G. G. Byrd, G. Purcell, R. Buta (U. Alabama, Tuscaloosa), D. McCormick (Tuskegee U., Tuskegee, AL)

de Vaucouleurs (1958) found observationally that a sample of spiral galaxies had arms which trailed relative to disk rotation. Byrd et al. (1989) found an exception, the galaxy NGC 4622. Buta, Crocker and Byrd (1992) observationally verified that an inner single stellar arm occurs in NGC 4622 with the color/age position angle sequence of a leading density wave. Byrd, Freeman and Howard (1993) simulated NGC 4622's pattern with a small companion in a plunging retrograde orbit in the disk plane. This encounter created a single long-lived central leading arm. NGC 4378 is also a single armed galaxy (Rubin et al. 1978, Sandage 1961). Byrd, Freeman and Howard (1994) simulated the arm pattern of NGC 4378 by a direct planar grazing passage of a small galaxy predicting a single trailing tidal arm.

In this poster, we use new B and I images to show that NGC 4378's arm has no density wave star formation/age sequence, confirming that it is a tidal arm. We display the rotation curve created by the our simulation of the direct planar grazing passage of a small galaxy finding that it has features similar to the observed rotation curve of NGC 4378. We also use the new color observations to provide definite proof that NGC 4378's single arm does trail as the simulations predict. Preferential scattering of blue light by disk plane dust near the nucleus makes the side of a galaxy's disk which is closer to us redder (Lindblad 1941). We find this to be true even for the moderate 35 degree inclination of NGC 4378 in which the east side is redder and thus nearer. Since the arm unwinds counter-clockwise and the northern edge has a red shift relative to the nucleus, the arm trails. Not all single arms lead!

Support was provided by NSF grants: RUI 9802918, REU AST-9424226, EPSCoR RII8996152, and AST 9014137. We thank CTIO for use of their observational facilities.

The author(s) of this abstract have provided an email address for comments about the abstract: byrd@possum.astr.ua.edu

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