SMA Research on the Galactic Center
 

Large Scale (100 , 4 pc)

The circumnuclear disk (CND) is perhaps the largest object in the galactic center region that is truly dynamically connected with the center. Its diameter is about 2 arcminutes and is thought to be in rotation around the galactic center. It appears that the gas in the CND may be dense enough that it is not tidally destroyed by the gravity of the black hole. The most ambitious study to date of the CND with the SMA has been Maria Montero-Castano's 25-field mosaic at 345 GHz in the compact/subcompact arrays for her thesis (Montero-Castano, Herrnstein, and Ho, 2009) and an unpublished 9-field mosaic at 230 GHz by Martin-Ruiz and colleagues. One of the images showing distribution of several molecules is shown in figure 1.


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Figure 1: Superimposed images of the emission in the 230 GHz band from three different molecular species in the galactic centers circumnuclear disk. The emission of the cyanogen radical (CN, in green) traces the disk most completely. Formaldehyde (H2CO, in blue) is mostly dissociated in the inner region and traces the outer dense gas. Silicon monoxide (SiO, in red) efficiently forms on the surface of dust grains, revealing the location of material affected by high-velocity shocks fronts. The position of the black hole is marked by the black spot. The spot's size is approximately the resolution of the array. The white and blue lines emanating from the black hole depict the intense and pervasive X-ray and UV radiation from both the black hole and the central massive star cluster. The molecular clouds in this region have to overcome extreme physical conditions of density, temperature, and disruptive forces such as strong tidal forces, turbulence, and cloud collisions. The image was made from a mosaic of nine fields in the 230 GHz band with a total extent of about 2 arcminutes and a resolution of 3 arcseconds (Figure courtesy of Sergio Martin-Ruiz).


The SMA could make a major impact on studies of the galactic center by following up this work with images and analysis of more molecules with higher sensitivity and a wider field. It is clear from figure 1 that the CND is not just a simple rotating ring. Understanding the full dynamical picture will help clarify how material is funneled toward the center. We have a real advantage here because the galactic center only reaches an elevation angle of 17° at the Plateau de Bure interferometer (latitude = 44.5°). The small size of the dishes of the SMA give it an advantage over arrays with larger elements because of the need for mosaicking.

In the cavity inside the CND lies the so-called minispiral, a three-armed structure of ionized gas. The kinematics of the minispiral can be traced by imaging the emission from the radio recombination lines (RRL) of hydrogen. Emission in the H30α line in the central array field at 230 GHz, which covers most of the minispiral, has been imaged with the SMA, as shown in figure 2.


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Figure 2: The distribution of radial velocities of the ionized gas in the so-called minispiral structure surrounding SgrA*, as determined from the SMA image of the emission from the H30α radio recombination line at 230 GHz. The angular scale markings are in arcseconds. The velocity scale (-300 km s-1 to 200 km s-1) is shown on the right. The positions of SgrA* and the prominent infrared sources (IRS 2, 5, 9W, and 13) are marked (Zhao et al. 2009).


The SMA results strengthen the idea that the gas traced by the RRL emission may be in Keplerian motion around the galactic center. However, the motions around some of the well known infrared sources associated with the minispiral are complex. The SMA can do a lot more with RRLs to sort out the dynamics of this structure and determine whether the ionizing gas is in Keplerian motion or is streaming toward the center. It is also important to study a larger field to understand the interaction of the minispiral with the CND.


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