Orbital motion of the Arches cluster
The measurement of the proper motion of the Arches cluster, combined with the known radial velocity, allowed the calculation of the cluster orbit. With the known projected location of the Arches on the sky, the only free parameter in these simulations is the line-of-sight distance. The orbital motion provides clues to the formation of the cluster, to its rapid, and disruptive, evolution in the Galaxy’s tidal field, and to its final fate.
2MASS JHK colour composite showing the Arches cluster with respect to the center of the Galaxy (GC). The green arrow indicates the cluster’s motion parallel to the disk of the Milky Way. Image credit: W. Brandner & 2MASS
Proper motion of the Arches cluster measured from high-spatial resolution Keck/NIRC2 and VLT/NACO K-band images covering a 4.3 year time baseline. The field reference sample in the inner bulge is shown in green, and the relative motion is indicated by the arrow.
Line of sight to Sun
Orbit simulation of the cluster motion in the gravitational potential of the Milky Way. The cluster is shown as a star at various possible line-of-sight distances relative to the central supermassive black hole (black dot). The orbit is seen from above the plane of the Galaxy, with the black hole at the origin. Solid lines represent the backwards integration to the presumed cluster origin 2.5 Myr ago. Dashed lines project the orbit by 360 degrees into the future.
A Galactic mystery: The formation of the Arches
The cluster moves too fast to be on one of the known circular or spheriodal orbits of molecular clouds in the inner Galaxy. So far, no formation scenario can explain the rapid motion of the cluster and the orientation of its orbital vectors at the same time. From the molecular clouds measured at radio wavelength, forming a cluster with the dynamical properties of the Arches seems not feasible.
Ongoing research focusses on the question:
How can such an extreme object like the Arches form near the centre of the Galaxy?