Friday, May 27, 2011

Observing Sgr A*

Our image of Sgr A* is constrained by what we see and what we do not see (the later is even more important).
In 1974, two American radio astronomers Bruce Balick and Robert Brown discovered a compact and variable radio source that looked like a faint quasar in the center of Milky Way. Because it appeared to be inside a large, extended radio source already known as Sagittarius A, they named it Sagittarius A* (or Sgr A*).
Here I should take a short trip to the basics of radio astronomy.
First observations in radio frequencies were done by Karl Jansky in 1930. This opens a new field in astronomy and allowed observing stars and galaxies in new range of wavelengths, unveiling new features. Subsequent observations have also identified new classes of objects, such as radio galaxies, quasars, pulsars, and masers.[0]
Ground based radio-observations are limited to a range of wavelengths that can go through the Earth atmosphere - 1mm – 10m (or 30 – 300 GHz). At longer wavelengths (lower frequencies), transmission is limited by the ionosphere, which reflects waves with frequencies less than 30GHz, while at higher frequencies (at millimeter range), radio signals are prone to atmospheric absorption.
In order to observe faint sources with good angular resolution, radio telescopes need to be i) extremely large and ii) be placed in very high and dry sites, e.g. ALMA, VLA, etc.
Angular resolution is a term used to describe the ability of any imaging device to distinguish small details of an object.

Imagine two point sources to observe. They will be regarded as resolved when the distance between the principal diffraction maximum of the first source and the first minimum of the second is bigger than zero. If one considers diffraction through a circular aperture, this translates into:
sin q = 1.220 l/D,
Where
θ is the angular resolution in radians,
λ is a wavelength,
and D is the diameter of the aperture.
This means that in order to improve angular resolution one needs either smaller wavelengths (in a millimeter range) or bigger mirrors/dishes or both.

In order for a telescope to develop a clear image of a distant object, the diameter of the collecting area must be much bigger than the wavelength of its collecting radiation. For an optical telescope, the wavelengths of visible light are in the range of 380 to 780 nm, which makes telescope lenses to be meters across. To provide the same angular resolution using a radio telescope, a single dish antenna would have to be kilometers across.

With inventing the radio-interferometry technique, high-resolution images become possible via employing multiple small antennas, which are connected together to simulate the collective power of one large antenna.[1] This creates a combined telescope which size is actually the distance between the farthest antennas in the array (a baseline).[2]      With this method modern radio interferometers, such as VLBI[3], can achieve resolution up to 0.001 arcsec.
The first radio observations of the center of Milky Way were done at 20 cm wavelength and revealed a broad Rosetta-like structure composed of different elements including a remnant of supernova exposure occurred sometime within past 100 000 years. There are no fine points to be seen in these images (see also Melia, Fig 1.5).

Figure 1. This is one of the first radio images of Sgr A* taken at 20 cm covers the central  ≈ 7 parsecs. On the left side of the image, the diffuse source Sgr A-East is a supernova remnant from a star exploded several hundred years ago, fills most of the left side of this panel. The bright spiral-shaped emission toward the right-center of the panel is called Sgr A-West and comes from plasma spiraling inward to the center. Images courtesy of J.-H. Zhao
They also revealed that Sgr A* was extremely compact – less than our Solar System. However, without information from infrared and X-ray part of spectra its nature remained a mystery.
The VLA images of GC rendered at 6 cm show more detail, including 3 spiral arms 3 light-years long. And the image taken at 2cm shows the central 2 light year region, features spiral pattern of Sagittarius A West and the point-like source of radio emission known as Sagittarius A* (Melia, Fig 1.7, 1.8).
Radio observations at 7 mm and 3.5 mm have detected intrinsic structure in Sgr A*, but the spatial resolution of observations at these wavelengths is limited by interstellar scattering. The apparent size of Sgr A* is dominated by scatter broadening at frequencies up to 50 GHz and the smallest size detected (as reported by Doelemann et al., 2008) is 􏰄0.1 AU at 􏰥1.3􏰖mm.􏰂 This is less than the expected apparent size of the event horizon (as observed from 8 kpc), suggesting that most of SgrA* emission may not be centered on the black hole, but arises in the surrounding accretion flow.

Figure 2. Radio continuum emission at 3.6 cm from the inner few parsecs of our Galaxy. The bright point source in the center is Sagittarius A. The mini-spiral of emission around the point source is from ionized gas that is in systematic motion about Sgr A*. Image courtesy of NRAO/AUI

Figure 3. The nucleus of  Milky Way  observed with the VLA at 1.3 cm and imaged with an angular resolution of 0.1 arcsec (Zhao, J.-H., & Goss, W. M. 1998, ApJ, 499, L163). Sgr A*, the bright unresolved radio source in the middle of this image
The most radiation shown in radio-images is produced by the diffuse hot gas between the stars. In order to see stars themselves, we need to move to optical wavelengths, which cannot actually penetrate the thick gas around GC. The option is to use near IR diapason (Melia, Fig 1.9 -optical, 1.12 –near IR). Sgr A* itself emits very low in near IR, but we can see emission from nearby stars.
To transfer Sgr A*’s radio position to infrared images with reasonable accuracy, the multiple images of nine red giant star, which are bright in both the infrared and radio wavelengths, were taken and positions determined. Figure 4 shows the combined IR and radio image of the central 10” with Sgr A* circled. This allowed to determine coordinates of Sgr A* as accurately as ±0.01 arcseconds or ±80 AU.

Figure 4: Three-color composite image of the central 10”. A radio emission map wavelength 3.6 cm; rendered as red, as well as two SofI images in the Ks- (green) and J-bands (blue). The radio continuum data provided with the Very Large Array by Chris De Pree. Credit: ESO. The spectra demonstrate the wide range of stellar types found in the cluster, ranging from main sequence O stars (the star S2 near Sgr A*, oval in image), to the luminous blue variables (IRS 16 SW, lower right), early WN (middle left and WC (top right) Wolf-Rayet stars, to red supergiants (the brightest star IRS 7 at the top/middle of the image), bright asymptotic giant branch stars (IRS 9, lower left) and red giants (IRS 10 EE, top left). Credit: Nelly Mouawad.

For more than a decade, groups led by Reinhard Genzel in Germany and Andrea Ghez in the USA led combined radio and IR observations of stellar motions near GC (see Fig 5), producing estimates of the projected velocities (Eckart & Genzel 1996, 1997; Ghez et al. 1998), projected accelerations (Ghez et al. 2000, 2004; Eckart et al. 2002), and 3-dimensional orbital motions (Schodel et al. 2002, 2003; Ghez et al. 2003, 2005).

Figure 5. Stars within the 0.02 parsecs of the Galactic center orbiting an unseen mass observed for 13 years. Yearly positions of seven stars are color coded. Both curved paths and accelerations (note the non-uniform spacings between yearly points) are evident. Partial and complete elliptical orbital fits for these stars are indicated with lines. Image courtesy A. Ghez.
They found that the orbital paths of stars in vicinity of Sgr A* (0.02 pc) are almost perfect ellipses, and most of the unseen mass must be contained within a radius of about 0.0005 pc. This implies a central mass of 4x10^6 Msun with mass density of > 8°x10^15 Msun pc^-3.
Based on these estimates, the GC is now known to shelter a considerable concentration of dark mass, associated with the radio source Sgr A*. Observations at 7mm done by Bower et al. (2004) put the lower limit to the mass density of Sgr A* of 1.4 × 10^4 Msun per AU^-3. The sub-minute time scale variability in near-IR observed Yusef-Zadeh et al. (2010) put a strong constraint of 1/8 AU on the size of the region from which this variable emission arises.
Each of these observations provided a stronger and stronger case for a SMBH of 3 – 4*10^6 Msun at the centre of the Milky Way and its association with the unusual radio source Sgr A*.
References:

Bower G. et al. 2004, Science Vol. 304 no. 5671 pp. 704-708 DOI: 10.1126/science.1094023
R. Genzel, A. Eckart, T. Ott & F. Eisenhauer Mon. Not. Roy. Ast. Soc. 291 (1997) 219.
A. Eckart & R. Genzel, Nature 383 (1996) 415.
A. Eckart & R. Genzel, Mon. Not. Roy. Ast. Soc. 284 (1997) 576.
A. M. Ghez, B. L. Klein, M. Morris & E. E. Becklin Ap. J. 509 (1998) 678.
R. Genzel, C. Pichon, A. Eckart, O. E. Gerhard & T. Ott Mon. Not. Roy. Ast. Soc.317 (2000) 348.
A. M. Ghez, M. Morris, E. E. Becklin, A. Tanner & T. Kremenek Nature 407 (2000)
349.
A. Eckart, R. Genzel, T. Ott & R. Sch¨odel, Mon. Not. Roy. Ast. Soc. 331 (2002)
R. Schodel et al., Nature 419 (2002) 694.
A. M. Ghez, et al., Astronomische Nachrichten 324 (2003) 527.
R. Schoodel, T. Ott, R. Genzel, A. Eckart, N. Mouawad & T. Alexander, Ap. J. 596, 2003 1015.
A. M. Ghez et al., Ap. J. 620 (2005) 744.
F. Yusef-Zadeh, H. Bushouse, C.D. Dowell, M. Wardle, D. Roberts, C. Heinke, G. C. Bower, B. Vila Vilaro, S. Shapiro, A. Goldwurm, G. Belanger, 2006, A Multi-Wavelength Study of Sgr A*: The Role of Near-IR Flares in Production of X-ray, Soft $\gamma$-ray and Sub-millimeter Emission, arXiv:astro-ph/0510787v2
Yusef-Zadeh F., J. Miller-Jones2, D. Roberts3, M. Wardle4, M. Reid5, K. Dodds-Eden6, D. Porquet7 & N. Grosso, Multi-Wavelength Study of Sgr A*: The Short Time Scale Variability, 2010, The Galactic Center: A Window on the Nuclear Environment of Disk Galaxies. ASP Conference Series
Doeleman, S. S., Weintroub, J., Rogers, A. E. E., Plambeck, R., Freund, R., Tilanus, R. P. J. et al. 2008, Nature, 455, 70
[0] To observe objects in the radio spectrum, several techniques are used: 1) an instrument can be pointed at a certain energetic radio source to analyze its emission; or 2) to image a certain region of the sky, multiple overlapping scans are usually made and put together to form a mosaic image. The type of instruments used depends on the strength of the signal and the amount of detail needed.
[1] Radio interferometry was developed by British radio astronomer Martin Ryle, and two Australian-born radio astronomers Joseph Lade Pawsey and Ruby Payne-Scott in 1946.
[2] In order to produce a high quality image, a large variety of baselines required. For example, the Very Large Array has 27 telescopes giving 351 independent baselines at once (WikipediaRadioAstronomyWeb). It has to do with the Fourier transformation and a very detailed account of interferometry can be found in the book by Thompson, Moran and Swenson, “Interferometry and Synthesis in Radio Astronomy”, Wiley-Interscience; 2nd edition (April 2001) ISBN: 0471254924.
[3] VLBI consists of widely separated radio telescopes all around the world connected together and tuned to simultaneously observe the same object.
[4] The aperture synthesis technique uses tape recorders synchronized with atomic oscillators instead of cables.

Bower, G. (2004). Detection of the Intrinsic Size of Sagittarius A* Through Closure Amplitude Imaging Science, 304 (5671), 704-708 DOI: 10.1126/science.1094023

Doeleman, S., Weintroub, J., Rogers, A., Plambeck, R., Freund, R., Tilanus, R., Friberg, P., Ziurys, L., Moran, J., Corey, B., Young, K., Smythe, D., Titus, M., Marrone, D., Cappallo, R., Bock, D., Bower, G., Chamberlin, R., Davis, G., Krichbaum, T., Lamb, J., Maness, H., Niell, A., Roy, A., Strittmatter, P., Werthimer, D., Whitney, A., & Woody, D. (2008). Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre Nature, 455 (7209), 78-80 DOI: 10.1038/nature07245

F. Yusef-Zadeh, H. Bushouse, C. D. Dowell, M. Wardle, D. Roberts, C. Heinke, G. C. Bower, B. Vila Vilaro, S. Shapiro, A. Goldwurm, & G. Belanger (2005). A Multi-Wavelength Study of Sgr A*: The Role of Near-IR Flares in
Production of X-ray, Soft gamma\$-ray and Sub-millimeter Emission Astrophys.J.644:198-213,2006 arXiv: astro-ph/0510787v2

1 comment:

1. Very helpful and informative post, thank you Olga.