Star Formation in the Central Molecular Zone


John Bally (University of Colorado, Boulder)




The Central Molecular Zone (CMZ) of our Galaxy, a region with a radius of < 500 pc, contains about 5% of the interstellar medium (ISM). Molecular clouds in the CMZ are orders of magnitude denser and have larger line-widths, more turbulence, and higher escape speeds than clouds in the Galactic disk. These clouds are embedded in a relatively dense and partially molecular inter-cloud-medium (ICM) and move on elliptical orbits in the potential of the central bar. These trajectories occasionally lead to cloud-cloud collisions and `mini-starbursts’.

I will review the Galactic context of the CMZ, starting with the 3 kpc `hole’ in the Milky Way’s ISM and the properties of x1 and x2 orbits around the central bar. A population of small but dense clouds identified by interferometers and by Herschel may trace the spray of gas transiting from x1 to x2 orbits. Several extreme cloud complexes, first identified as large-velocity `spikes’ in longitude-velocity diagrams, may trace the innermost, non-self-intersecting x1 orbits. Located on the outskirts of the CMZ, spike-features such as Bania’s Clump 2 at l = 3.2o do not support extensive star formation despite their large mass.  However, young massive stars and clusters are found throughout the CMZ. The youngest (< 1 Myr old) are found in the Sgr A, B1, B2, and C star-forming complexes. Slightly older (~10 Myr old) populations are found in Sgr A within 10 pc of the central black-hole and in the massive Arches and Quintuplet clusters which are in the interior of the brightest super-bubble in the Galaxy, the `Galactic Center Bubble’ (GCB). The GCB may mark the most recent multi-supernova-driven phase of a weak nuclear super-wind.  

I will discuss the evolutionary state of the distributed population of bright 8 and 24 μm sources found by Spitzer throughout the interior of the 90 pc ring identified by Herschel, how the 3 to 7 pc circum-nuclear ring/disk (CND) may be fueled, and how stars may form so close to the central black hole. Other speakers will discuss the 100 pc ring (Molinari) and the properties and cluster-forming potential of the most-extreme infrared dark cloud identified so-far in our Galaxy, G0.25 (Longmore).