The Leading Arm, Ringed Spiral Galaxy NGC 4622 – A Diagnostic Case of a Retrograde Tidal Encounter

Abstract

Some of the basic morphologic and photometric properties of the southern galaxy NGC 4622, the only galaxy known to have at least one leading spiral arm, are described. BVI surface photometry is used to establish that the suspected leading arm is present in the stellar disk component and is not an artifact of dust absorption. It begins about 3 kpc from the center and winds more than 360 deg to a radius of about 4.4 kpc. The winding is logarithmic in projection with a pitch angle of only 4 deg. 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 found. At the average radius of the ring, the phase of the 1 Theta Fourier component in B is shifted about 40 deg 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 about 1.4 times the orbital speed.