SMA Research: Galactic Center

SMA Research on the Galactic Center

Ultrasmall scale (10 microarcseconds, 1x10^12 cm, 1 Schwarzschid radius)

The mass of the black hole in the center of the galaxy is about 4 x 10⁶ M_⊙, as determined from tracking the stars orbiting the galactic center (Ghez et al. 2008, Gillessen et al. 2009). The Schwarzschild radius, R_s, is therefore about 1 x 10¹² cm or 10 microarcseconds (µas) at a distance of 8.5 kpc. VLBI is the only astronomical technique capable of providing direct imaging of the accretion disk close to the black hole. The plasma scattering caused by the ISM varies as the square of the wavelength and badly blurs the image at long radio wavelengths. The scattering is only 22 µas at 230 GHz, and the intrinsic structure can be most readily discerned at frequencies higher than this. Furthermore, the emission becomes optically thin at frequencies above about 230 GHz so that the innermost regions of the accretion envelope can be probed. Thus, 230-350 GHz is probably the optimum band for the use of VLBI to make images of SgrA*. The results of the first 230 GHz VLBI on SgrA*, in 2007, are shown in figure 4.


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Figure 4: The correlated flux density vs. projected baseline from VLBI observations in April 2007 with array elements: Arizona Radio Observatory Submillimeter Telescope (ARO/SMT), Combined Array for Millimeter Astronomy (CARMA), and the James Clerk Maxwell Telescope (JCMT) in Hawaii reported by Doeleman et al. (2008). The signals from the JCMT were processed and recorded through IF and VLBI systems at the SMA. The solid line is a model for a source of diameter 37 µas observed through a plasma screen of 22 µas of scattering. The dotted line represents the model of a toroidal source, seen through the scattering screen, which clsely matches a ray tracing simulation (made by the Gammie group at the University of Illinois) shown in the inset. Note that a detection, rather than an upper limit at 3100 x 10⁶λ, would be crucial in distinguishing between these two models (Doeleman et al. 2009).

The visibility curve, after correction for interstellar scattering, is consistent with a Gaussian intensity profile with a diameter of 37 µas or a toroidal distribution of inner and outer diameters of 35 and 80 µas, respectively, approximately what would be expected from symmetric emission models incorporating gravitational lensing. The minimum observable diameter of a symmetric radio source centered on the black hole is about 5.2 R_s or about 52 µas. Hence, viable models for the emission are a torus, a density-enhanced plasmoid orbiting in the inner accretion disk, or an accretion disk with Doppler boosting of the emission on the approaching side. A large collaboration has been formed that seeks to build an Event Horizon Telescope, which is described in a white paper submitted to the Astro2010 decadal review of astronomy in the US (Doeleman et al. 2009). The baseline tracks are shown in figure 5 for near-term and futuristic arrays. Various source models and the reconstructed images with various arrays are shown in figure 6.


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Figure 5: Projected baseline tracks on SgrA* at 230 GHz for three arrays. Left (7-element array): ARO (Arizona), CARMA (California), MK (Hawaii), IRAM (Spain), Plateau de Bure (France), LMT (Mexico), ALMA (Chile); Center (10-element array): 7 elements of the left panel plus Haystack (Massachusetts), SPT (South Pole), and SEST (Chile); Right (13-element array): 10 elements of the center panel plus potential telescopes in New Zealand, South Africa, and Kenya. The baselines linking MK (Hawaii) are shown in red. The quality of images from an array is a function of how densely covered the uv plane is (Doeleman, private communication).


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Figure 6: Simulations and image reconstructions of structures surrounding the event horizon of the 4 x 10⁶ M_⊙ black hole in the galactic center. The top row shows emission models that have been convolved with the plasma scattering effect of the ISM, which is frequency dependent. Four source intensity models, all based on magneto-hydrodynamic simulations with general relativistic ray tracing, are shown in the top row. Left to right: BH spin, a = 0.9, accretion disk face on; radiatively inefficient accretion flow (RIAF), a = 0, accretion disk at 30° inclination angle; a = 0.9, i = 80°; 345 GHz, a = 0, i = 30°. The second, third and fourth rows show the simulated image results with data from the 7-, 10-, and 13-element arrays shown in figure 5. Each image is +/-138 µas on a side (about +/-14 R_s) (Doeleman, private communication).

SgrA*, because of its apparent lack of activity (i.e., low accretion rate and no detectable jet), may not be representative of supermassive black holes in galactic nuclei. Fortunately, there is a more active system in the relatively nearby galaxy M87, at a distance of 16 Mpc in the Virgo cluster of galaxies. Its nuclear black hole has a mass of about 6 x 10⁹ M_⊙ (Gebhardt and Thomas 2009) and a Schwarzschild radius of 1.8 x 10¹⁵ cm (100 AU), which subtends an angle of 7 µas. Note that its much higher mass in comparison to the galactic black hole means that the angular size of the event horizon is nearly as large as our galactic center black hole. VLBI measurements of M87 at 230 GHz were successfully carried out in April 2009 and show a component of about 50 µas in diameter. Thus, the possibility of imaging activity close to the event horizon in M87 in the coming decade is very real. Black holes in the nuclei of active galaxies are thought to supply the energy to create the spectacular jets of highly collimated relativistic matter emanating from them. However, the precise mechanism of the conversion of gravitational to electromagnetic energy is poorly understood. VLBI at submillimeter wavelengths offers the only method with sufficient angular resolution to directly image the material in the inner accretion disk where the jet is formed. Hence, it may provide the essential tool for solving this problem.

References:

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SMA Research: Galactic Center

SMA Research