The intriguing nearly face-on southern ringed spiral galaxy NGC 4622 , the first galaxy definitively shown to have leading spiral structure , is revisited in this paper with new images from the Hubble Space Telescope ’ s WFPC2 , together with ground-based optical and near-IR imaging , and a Fabry-Perot H \alpha velocity field .
The data provide new information on the disk/bulge/halo mix , rotation curve , star formation in the galaxy , and the sense of winding of its prominent spiral arms .
Previously , we suggested that the weaker , inner single arm most likely has the leading sense , based on a numerical simulation .
Now , taking advantage of HST resolution and using de Vaucouleurs ’ standard extinction and reddening technique to determine the near side of the galaxy ’ s slightly tilted disk , we come to the more surprising conclusion that the two strong outer arms have the leading sense . We suggest that this highly unusual configuration may be the result of a past minor merger or mild tidal encounter .
Possible evidence for a minor merger is found in a short , central dust lane , although this is purely circumstantial and an unrelated interaction with a different companion could also be relevant .
The leading arms may be allowed to persist because NGC 4622 is dark halo-dominated ( i.e. , not “ maximum disk ” in the inner regions ) and displays a significantly rising rotation curve .
The new HST observations also reveal a rich globular cluster system in the galaxy .
The mean color of these clusters is ( V - I ) _ { o } = 1.04 and the specific frequency is 3.4 \pm 0.6 .
The luminosity function of these clusters confirms the membership of NGC 4622 in the Centaurus Cluster .