By Miguel Requena-Torres, Postdoc at STScI
Star formation is a beautiful event that goes on in galaxies. This phenomenon can be quiet or violent. In our own Galaxy, for example, there are quiet clouds of material that evolve slowly to form filamentary structures with growing pockets of gas and dust that eventually acquire enough density to start gravitational collapse, leading to the birth of stars. But our Galaxy also harbors violent regions where cataclysmic events inject enough energy into the surrounding medium that can trigger star formation. In the last few years, new theories have been developed to explain the formation of stars in our own Galaxy by studying the density of the clumps that could potentially become gravitationally bound. The parameter of interest is the Jean’s Mass, approximately setting the limit between a clump that can support itself by its internal gas pressure and one that cannot, the latter being subject to a runaway gravitational collapse. This limiting mass seems not to change much throughout our Galaxy, except its inner region: within the inner few hundred parsecs of our Galaxy, the so-called Central Molecular Zone, the situation is quite different!
This Central Molecular Zone is a mix of hot and cold dust, molecules and atoms that are not at all in a quiescent state. The massive black hole at the center of our Galaxy seems to have produced a barred galactic potential, structuring the surrounding material in two different sets of orbits, X1 and X2, with matter moving from the outer orbit into the inner one through some interaction areas. The X1 orbits could have a twisted structure at pericenter, compressing the gas and moving it into the inner orbits. Due to the mass densities observed and the frequency of compressional events expected in the Central Molecular Zone, one would expect this region to be a perfect nursery of stars in the Galactic center. Indeed, evidence that this has been the case in the past is the presence of three of the most massive stellar cluster in the Galaxy. Due to their mass, the stars in these clusters should be young (with a lifetime of few million years).
One of these massive stellar clusters, the so-called Central cluster, is located very close to the supermassive black hole. This cluster, together with other massive stars that orbit the central engine of our Galaxy, dissociate and ionize the molecular and atomic material in the surrounding region, creating at the outer edge of it a very dense molecular structure called the Circumnuclear Disk. The Circumnuclear Disk is not a real disk, but consists of streamers of material that are rotating around the center of the Galaxy and that are probably compressed and ionized as they enter the inner 1 parsec, forming a structure called the mini-Spiral.
Figure 1: Circumnuclear disk of the Galaxy and mini-Spiral observed by the Submillimeter Array interferometer. CN emission is in green, showing the densest region in the gas streamers. The ionized material of the mini-spiral is in orange. Red and blue correspond to the shock-tracers SiO and H2CO, respectively. (Image credit: Martin et al. 2012).
The other two massive clusters, called the Arches and the Quintuplet, are located further away from the center of the Galaxy, at positive Galactic longitudes. They lie in a very interesting area, surrounded by a lot of dense material and by what looks like magnetic tubes of plasma with bright centimeter continuum emission. These clusters themselves produce very intense ionization fields that carve-in the molecular material that surrounds them.
In the last few years, the Herschel Space Observatory has been able to map the relatively warm gas in the plane of the Galaxy. These observations, using different molecular tracers, have clearly shown that in the Central Molecular Zone there is a ring of dense material around the central black hole with a radius of about 150 pc. There is still a debate regarding how big this ring-like structure really is and whether it closes-in. Its interpretation in terms of individual clouds is problematic because of the range of velocities involved. The Galactic center is a very crowded area, with material spreading in velocity from -200 km/s to 200 km/s. Most of the material at negative longitude shows negative velocity whereas material at positive longitude shows positive velocity. However, in any given region, it is possible to identify more than one component, with velocities differing by more than 50 km/s. This, together with the spread in velocity due to the presence of turbulence, complicates the identification of individual clouds.
Figure 2: Twisted disk of the Galaxy observed by the Hi-Gal Hershel survey at 250 microns. This ring-like features covers most of the Central Molecular Zone (Image credit: Molinari et al. 2011).
The only region in this ring-like structure that currently seems to be forming stars efficiently is the Sgr B2 region. With three very dense cores (S, M and N), this region shines strongly at radio and sub-millimeter wavelengths, lying at one of the edges of the large twisted ring. Each of these cores, currently in a molecular hot-core phase, will eventually form a small cluster of stars. Their current evolutionary phase is fascinating due to their chemical richness and in this regard Sgr B2 is a case of study, with new molecules being discovered there every year (the latest being isopropyl cyanide, C3H7CN, and methyl isocyanate, CH3NCO, discovered in 2014 and 2015, respectively – for more info on this fascinating new discoveries check Astrochymist at http://www.astrochymist.org/astrochymist_ism.html).
The rest of the ring-like structure seems more quiet, although there is a very dense molecular cloud, called The Brick, that has recently raised a lot of interest, having been the target of most of the radio and sub-mm facilities in the world (ALMA, SMA, VLA, APEX, Effelsberg, ATCA). These observations have shown that this region could be dense enough to form the next generation of stars. Indeed, the presence of Maser emission could be interpreted as a sign of on-going star formation, but it could also be produced by strong shocks commonly found in the Galactic center. Regardless whether or not star formation has already started, this is one of the more prominent regions that will likely form stars in the near future.
Finding star formation in the Circumnuclear Disk of our Galaxy is not an easy task. The streamers that fall into the inner ionized region were thought to harbor some pockets of star formation, however, when we observed them a few years ago using CO and the dense gas tracers HCN and HCO+, we concluded that these objects were not gravitationally bound. The material there seems to be heavily disrupted by the sheer forces that arise due to their close proximity to the central black hole. The mass of one of the regions was close to be gravitationally bound, however, this region also showed unexpected vibrational excited emission of HCN and, when accounting for its presence in the analysis, its estimated mass density decreased deeming on-going star formation in this region of the Circumnuclear Disk unlikely. More recent observations of this unique region have unveiled the presence of the shock-tracer molecule SiO. A possible explanation is that this is the location where two of the clouds in the vicinity of the high velocity streamers collide. Our new ALMA observations of CO, HCN, HCO+, CN, and tens of other molecules in the Circumnuclear Disk will soon shed some light on this fascinating structure near the heart of our Galaxy.
Another fascinating region at the Galactic center is a bubble-like structure seen in continuum and molecular emission, likely produced by a cataclysmic event. This elongated bubble is orthogonal to the twisted ring observed by Herschel. In the edges of this bubble, the material has been compressed and is possible to observe clumps in many different molecular tracers. The expected high densities of these clumps deem them as promising sites for on-going star formation. But this still needs to be confirmed as it is not clear that their densities are high enough to keep them gravitationally bound in the extreme physical environment that surrounds the monster black hole at the center of our Galaxy.
Figure 3: Composite image of the Central Molecular Zone showing the elongated bubble, outlined by the blue ellipses. Chandra (x-ray) observations are in blue, Hubble (near-IR) in yellow and Spitzer (mid-IR) in red. (Image credit: NASA).
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