Appeared in the May, 2005 issue of ApJL

IRS 16 SW: A New Comoving Group of Young Stars in the Central Parsec of the Milky Way

J. R. Lu, A. M. Ghez, S. D. Hornstein, M. Morris, E. E. Becklin, 2005, ApJ, 625, L51

Images and proper motions of the IRS 16 SW comoving group.


One of the most perplexing problems associated with the supermassive black hole at the center of our Galaxy is the origin of the young stars in its close vicinity. Using proper motion measurements and stellar number density counts based on 9 years of diffraction-limited K(2.2 micron)-band speckle imaging at the W. M. Keck 10-meter telescopes, we have identified a new comoving group of stars, which we call the IRS 16SW comoving group, located 1.9'' (0.08 pc, in projection) from the central black hole. Four of the five members of this comoving group have been spectroscopically identified as massive young stars, specifically He I emission-line stars and OBN stars. This is the second young comoving group within the central parsec of the Milky Way to be recognized and is the closest, by a factor of 2, in projection to the central black hole. These comoving groups may be the surviving cores of massive infalling star clusters that are undergoing disruption in the strong tidal field of the central supermassive black hole.

Figure Caption

(a): The positions of all stars in our sample (asterisks) overlaid on a map of the two-dimensional velocity dispersion (grayscale). The sizes of the asterisks represent the stars' 2.2 micron brightness. The black region is a minimum in the velocity dispersion, located ~2 arcsec from Sgr A* (black cross), and is caused by 5 comoving stars (blue), which define the newly identified IRS 16SW comoving group. (b): A K[2.2 micron]-band speckle image showing the clustering of bright sources at the position of the IRS 16SW comoving group. Group members are marked with blue Xs. (Panels): The proper motions of the IRS 16SW comoving group members. In each 0.1'' X 0.1'' panel, the stellar positions are plotted with different years of data labeled with different colors.