THE LEADING ARM, RINGED SPIRAL GALAXY NGC 4622 - A DIAGNOSTIC CASE OF A RETROGRADE TIDAL ENCOUNTER
The southern galaxy NGC 4622 is a rare example of a spiral with two sets of spiral arms winding in opposite senses: A single inner arm which opens counterclockwise, and two outer arms which open clockwise. The two sets of arms are linked by a nearly circular, off-centered inner ring of high contrast, and the presence of the two opposing senses means that NGC 4622 has at least one leading spiral arm, a phenomenon that up until now has never been firmly established in any galaxy. In this paper, we describe some of the basic morphologic and photometric properties of this galaxy, which has recently been suggested to have experienced a retrograde tidal encounter. Of the three observed arms, the inner arm is the prime suspect for the leading sense, based on published n-body models. We establish with BVI surface photometry that this arm is present in the stellar disk component and is not an artifact of dust absorption. The suspected leading arm begins almost-equal-to 3 kpc from the center and winds more than 360-degrees to a radius of almost-equal-to 4.4 kpc. The winding is logarithmic in projection with a pitch angle of only 4-degrees. A substantial portion of the inner ring may be made of the leading arm. A large variation in B - I color index around the ring is also found. At the average radius of the ring, the phase of the 1-theta-Fourier component in B is shifted almost-equal-to 40-degrees in the direction of rotation relative to that in I. The shift is consistent with the hypothesis that part of the ring is a one-armed density wave turning opposite the disk rotation at almost-equal-to 1.4 times the orbital speed. The galaxy may be the most definite case to date demonstrating this density wave effect because of the large difference between the leading arm pattern rate and the orbital rate. The perturbing companion has not yet been identified. We suspect that a small galaxy 1'.85 east, of unknown redshift, may be the most promising to explain not only the leading arm, but also the two trailing outer arms.