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,
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. [4]
  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*.

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)
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

Thursday, May 5, 2011

Saggitarius A*: distance and mass estimates

Center of Milky Way. Credit: Stefan Gillessen, Reinhard Genzel, Frank Eisenhauer

Knowing distance to Sgr A* (Ro) is very important, because it sets the distance scale for every other distance within Milky Way. The total Galaxy's mass, the Sun's orbital velocity, and luminosities of distant stars rely upon the accurate measurement of Ro.
A variety of methods have been employed by astronomers to determine Ro. These can be separated into three broad categories:
1.    The Shapley method of using “standard candles” measurements of objects thought to be in spherical distribution around the core;
2.    Computational methods that combine observations with Galactic models to arrive at a distance;
3.    Direct measurements to objects at the Galactic center.
I will focus on recent advances in direct measurement techniques.

The recent advances in infrared astronomy, 
such as adaptive optics and imaging spectroscopy, allowed high-resolution imaging of the galactic center with 0.025" angular resolution, which corresponds to a spatial resolution less than 200 AU. With such instrumentation, the orbits of stars in vicinity of Sgr A* can be precisely measured. It is safely to say that Sgr A* provides all of the gravitational attraction in the nearby region, and the motion stars in close proximity is governed solely by this object. Therefore, direct observation of this motion can provide the mass and distance estimates for Sgr A*.

Ghez et al. (1998) using specially processed near-infrared imagery from the 10 m W. M. Keck telescope identified the position of 90 stars in within 6”x6” region near SgrA*. By following the stars over 2 years, they were able to show that those close to SgrA* have radial velocities as high as 1400 +/- 100 km/s. The high orbital velocities and closeness of the orbits to the central mass allowed Ghez estimate it as 2.6 ± 0.2 x10^6 Msun.

With the mass of the central object estimated, and several years of baseline position and velocity data in hand, Samir and Gould (1999) proposed a direct geometrical method for determination of R0, based on Kepler laws. The similar method has been used for decades to estimate the masses of, and distances to, visual binaries.

Keplerian methods
The problem with using Keplerian methods is that we do not directly measure the size of the orbits of distant stars. Instead, we measure the proper motions, which is a measurement of angle rather than length. We also need to know the orbital inclinations.
Using only the proper motions and orbital periods of the stars near Sgr A*, we could only derive the ratio mass Sgr A*/R0^3.
We have, however, one more piece of information: a Doppler shift for each star, as it moves toward or away from us, permits to directly calculate both the mass of Sgr A* and the distance R0 from Earth to Sgr A*.

Recent results

The followed continuous monitoring of the star positions proofed that stars do follow elliptical orbits with the focus close to Sgr A*. Also stars acceleration vectors were found directed to a common central gravitational source very close to the position of Sgr A* (Ghez et al. 2005). The orbital periods of observed stars are ranging from 15 to 94 years. The star with the shortest period (~ 15 years), labeled S2, has already been observed  completing its orbit.

Note that the Sun's orbits around the galaxy center  with a speed 220 km/s and  its orbital period is about 240 million years.

Schodel et al (2002) used high-resolution near-infrared imaging and spectroscopy to observe the central few light years of our Milky Way and study of the stellar dynamics in the vicinity of the compact radio source SgrA*. From a statistical analysis of the stellar proper motions and Doppler motions they inferred the presence of a compact mass of  2.6- 3.3 10^6 Msun plus the visible stellar cluster of core radius 0.34 pc, located in a region confined within ten light days of SgrA*.

Motion of a star around the galactic center,

demonstrating that Sagittarius A* is a black hole
(adapted from Schödel et al, Nature, 17 Oct 2002)

Eisenhauer, F. et al. (2005) used near IR imaging spectroscopy with astrometric accuracy of 75 mas to observe the central 30 light-days close to the galactic center. They determined radial velocities for 9 of 10 stars in the central 0.4”, and for 13 of 17 stars out to 0.7”, limiting stars magnitudes to K~16. By combining the new radial velocities with SHARP/NACO astrometry and using a global-fit technique, they derived improved three-dimensional stellar orbits for 6 S stars in the central 0.5” region. This result in the updated estimate for the distance to the Galactic center from the S2 orbit fit as Ro= 7,62 +-0,32 kpc, with a central mass of (3,61+- 0,32)x 10^6 Msun.

The instrumentation they used is SINFONI - a near-infrared (1.1 - 2.45 µm) integral field spectrograph connected to an adaptive optics module, installed on ESO VLT. The instrument operates with 4 gratings (J, H, K, H+K) providing a spectral resolution around 2000, 3000, 4000 in J, H, K, respectively, and 1500 in H+K. For more information about SINFONI, please refer to the following page:

After more than 16 yeas of observations, it was confirmed that stars in vicinity of Sgr A* are executing elliptical orbits, which fit well with a single mass enclosed in a very small volume at the focal position. Two stars have been observed to approach within 100 AU of the focal position, moving at nearly 10^4 km/s responding to an unseen compact mass of 4x10^6 Msun (Reid 2008, Ghez 2000, Shodel 2003).

How accurate these estimates are?

The error in the proper motion component of the stellar velocities depends primarily on the angular resolution of the detector. Due to advances in technology these errors have come down from 6-10 mas in 1998 to 1-3 mas in 2003 (Eisenhauer et al 2003).

The telescopic resolution also limits the accuracy of the stellar positions when they are very close to the central black hole or when two stars are close together in the crowded field because the two sources can blend together (Reid 2008).

Salim and Gould (1999) published a careful statistical analysis of how the error in R0 should decrease with increasing observational time - after 8 years of observation, the accuracy should be within 2.5% of the actual value, and after 10 years should be within 1%. They also suggested that the accuracy would improve with advances in technology and with larger telescopes, eventually yielding 0.2% error.

This was recently confirmed by Gillessen et al. (2009), who reported the mass estimate for Sgr A* as 4.31 +- 0.06 * 10^6 MSun with statistical error of 1.5%, with the best estimate for R0 = 8.33 +- 0.35 kpc. These estimates were results of 16 years of monitoring stellar orbits around Sgr A* using high-resolution NIR techniques with astrometric accuracy of 300 mas.

Any alternatives to SMBH?

According to Shodel et al. (2002), the only possible non-black hole explanation of huge mass of Sgr A* can be a ball of bosons of similar mass, because its radius can be only a few times greater than the Schwarzschild radius of a black hole.

Another prove for SMBH was analysis of the motion of Sgr A* relative to distance quasars. Sgr A* appears to be almost motionless moving 0.006379” per year (Reid & Brunthaler 2004). This is only possible if Sgr A* contains a huge unseen mass.

There is still a possibility that it might be a cluster of dark stars, e.g. a globular cluster. Because a typical GC to could contain 10^6 stars within a radius of 1 parsec. But there is a time constrain - dense star clusters undergo significant interactions, including core-collapse, collisions and evaporation of stars. For the cluster of stars with masses less of 1Msun (note that they should be low luminous starts, even dark ones), to achieve the current conditions close to Sgr A*, the evaporation time should be < 10^6 years, which seems too short. However, a quasi-steady state condition is possible, where stars feed the cluster from the outside at a rate comparable to the evaporation rate (Reid 2008).

  • Salim, Samir, and Gould, Andrew. “Sagittarius A* ‘Visual Binaries’: A Direct Measurement of the Galactocenric Distance.” The Astrophysical Journal 523 (1 October 1999): 633—641.
  • Ghez , A. M.; Klein, B. L.; Morris, M.; Becklin, E. E., 1998 ApJ,509:678–686
  • Eisenhauer, F. et al. 2005, "SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month". AJ. 628
  • Shodel et al., 2002, A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way, Letters to nature
  • A. M. Ghez, M. Morris, E. E. Becklin, A. Tanner & T. Kremenek Nature 407 (2000)
  • R. Schoodel, T. Ott, R. Genzel, A. Eckart, N. Mouawad & T. Alexander, Ap. J. 596, (2003) 1015.
  • M. J. Reid, K. M. Menten, R. Genzel, T. Ott, R. Schodel & Eckart, A. Ap. J. 587, (2003), 208.
  • Gillessen, S.; Eisenhauer; Trippe; Alexander; Genzel; Martins; Ott (2009). "Monitoring Stellar Orbits Around the Massive Black Hole in the Galactic Center". The Astrophysical Journal 692 (2): 1075–1109. arXiv:0810.4674. Bibcode 2009ApJ...692.1075G. doi:10.1088/0004-637X/692/2/1075.

Gillessen, S., Eisenhauer, F., Trippe, S., Alexander, T., Genzel, R., Martins, F., & Ott, T. (2009). MONITORING STELLAR ORBITS AROUND THE MASSIVE BLACK HOLE IN THE GALACTIC CENTER The Astrophysical Journal, 692 (2), 1075-1109 DOI: 10.1088/0004-637X/692/2/1075

F. Eisenhauer, R. Genzel, T. Alexander, R. Abuter, T. Paumard, T. Ott, A. Gilbert, S. Gillessen, M. Horrobin, S. Trippe, H. Bonnet, C. Dumas, N. Hubin, A. Kaufer, M. Kissler-Patig, G. Monnet, S. Stroebele, T. Szeifert, A. Eckart, R. Schoedel, & S. Zucker (2005). SINFONI in the Galactic Center: young stars and IR flares in the central
light month Astrophys.J. 628 (2005) 246-259 arXiv: astro-ph/0502129v1

Tuesday, May 3, 2011

Early stars

   The first generation of stars that formed right after the Big Bang were probably massive luminous stars made of hydrogen and very little helium, they lived short lives (about 30 million years) and after they deaths they provided the Universe with the first heavy (in astronomical terms) elements.
  It was long thought that these stars lived mostly solitary lives, or formed a very wide binary system. The modern studies suggest that these stars were not only very massive, but also fast rotating.
  Clark et al (2011), however, provided numerical simulations that show that these stars could be members of tight multiple systems. Their results show that the massive gaseous disks formed around the first rapidly rotating stars were unstable to gravitational fragmentation and could possibly fall into small binary and higher-order systems.
  All the elements heavier than helium found in later stars were formed by the first stars and recycled in later generations of stars. And however there are no first generation stars left in the nearby Universe to study, we can still reconstruct the chemical composition and masses of first stars using data from the very old stars observable now.
  Here low mass stars become very handy. Low mass stars (1Msun- 0.5 Msun), live very long lives and contain elements produced by the first generation of stars. They are observable now, gathering in globular clusters, and can be used to provide insights into the lives of the first stars.
  Chiappini of the Institute for Astrophysics in Potsdam and collaborators used the European Southern Observatory's Very Large Telescope in Chile to study the chemical composition of some of 8 oldest stars from an ancient globular cluster NGC-6522 to figure out what the first stars were like. These stars are old enough to have formed out of the original chemicals produced by the first generation.
  They found extraordinarily high levels of the heavy elements strontium (Sr) and yttrium (Y) in the surfaces of stars in NGC6522, which suggest that the first stars were both massive and very rapidly rotated to achieve the degree of mixing needed to produce these elements.
  Fast stellar rotation makes possible to mix the He-buring core and outer nuclear burning layers, which don’t normally mix in slow rotating stars. The nuclear reactions in the overlapping region leads to an enhanced production of radioactive neon, which emit neutrons that are subsequently captured by Fe and other heavy elements to produce Sr and Y.
  Chiappini et al. (2011) suggest that, given the abundance of Sr and Y, the first generation of stars could have been rotating as fast as 500 km/s. The typical values for massive stars in the Milky Way are about 100 km/s, and our Sun rotates at 2 km/s.
  Note that the rapidly rotating stars are more likely to result in gamma-ray bursts, which, in turn, would impact on the ionizing power of the first stars and impact early Universe.


Clark, P., Glover, S., Smith, R., Greif, T., Klessen, R., & Bromm, V. (2011). The Formation and Fragmentation of Disks Around Primordial Protostars Science, 331 (6020), 1040-1042 DOI: 10.1126/science.1198027

Clark, P., Glover, S., Smith, R., Greif, T., Klessen, R., & Bromm, V. (2011). The Formation and Fragmentation of Disks Around Primordial Protostars Science, 331 (6020), 1040-1042 DOI: 10.1126/science.1198027

Chiappini, C., Frischknecht, U., Meynet, G., Hirschi, R., Barbuy, B., Pignatari, M., Decressin, T., & Maeder, A. (2011). Imprints of fast-rotating massive stars in the Galactic Bulge Nature, 472 (7344), 454-457 DOI: 10.1038/nature10000

Chiappini, C., Frischknecht, U., Meynet, G., Hirschi, R., Barbuy, B., Pignatari, M., Decressin, T., & Maeder, A. (2011). Imprints of fast-rotating massive stars in the Galactic Bulge Nature, 472 (7344), 454-457 DOI: 10.1038/nature10000

Sunday, May 1, 2011

Milky Way: a Distance to the Galactic Center - 3

First attempts to estimate the distance to the galactic center

In the late 18th century William Herschel made an attempt to locate the position of Solar System in the Milky Way. His approach was that the center of the galaxy is the place with the highest concentration of stars and by locating such a region he can locate the center of our galaxy. So he looked at all directions and did not find any area of the sky that had a higher concentration of stars than any other area . Based on these observations Hershel concluded that the Earth (or rather the Solar System) must be in the center of our galaxy (HershelWeb). In the meantime, he discovered and catalogued over 2400 objects defined by him as nebulae and included in his famous 3 catalogues.
In 1906, Jacobus Kapteyn began a big project to find the size and the shape of Milky Way, which took him 16 years of intensive observations. He divided the visible sky into 206 zones, and with help of his colleagues from 40 observatories, he surveyed stars in these zones, analyzing their magnitudes, apparent brightness and proper motions. This project was the first coordinated statistical analysis in astronomy.
In 1922, the results of this study were finally published: our galaxy was 30,000 light years across, 6000 light years thick, and the Solar System located in its center. Kapteyn’s model of the Milky Way was commonly accepted as accurate for many years.
So the Earth as a member of Solar System was  again considered a special place – the center of our galaxy.
Note: while doing his study of proper motions, Kapteyn found that observed stars could be divided into two streams  one moving in almost opposite direction to another. These Kapteyn's data were the first evidence of the rotation of our Galaxy (KapteinWeb).
Harlow Shapley began his globular cluster survey in 1914, working on the largest telescope this time 60-inch giant at Mt. Wilson Observatory.
During his research, Shapley discovered the Cepheid variable stars in a large number of the globular clusters he observed. At that tome the period-luminosity relationship for Cepheids was just reported by Henrietta Swann Leavitt. Using her data Shapley determined the distances to 93 globular clusters (GCs) he observed. He found that the distribution of GCs was centered at about 15 kpc away from the Sun in the direction of the constellation Sagittarius by mapping out the three dimensional distribution of the clusters. This gave Shapley the idea that such massive objects as GCs should be centered at the galactic center. Shapley published his discovery in his "Big Galaxy" theory in 1918 (CudworthWeb).
A distribution of globular clusters around the center of Milky Way.

However Shapley's conclusions remained controversial at the beginning, they were eventually accepted by majority of astronomers, and his technique is still considered one of the primary means of determining the distance to the center of the Galaxy.
Jan Oort, while conducted research of the motion of stars in the vicinity of the Sun, found that stars exhibited differential rotation – stars closer to the center of the galaxy traveled at higher velocities than stars farther away from the center. In his paper published in 1927, he identified the center of the galaxy in constellation Sagittarius within 2° of Shapley’s estimate. The distance to the Galactic Center according to Oort was 10 kpc, which is much less than Shapley’s estimate (Oort 1970).
Similar studies in the 1970s and 1980s with much better data and absorption corrections yielded half shorter distances about 8 kpc.
Kapteyn, Shapley and Oort produce such divergent estimates of the distance to the galaxy center because at that time they did not have an important piece of information – the knowledge of interstellar extinction.
HershelWeb -
Oort, JH (1970), "Galaxies and the Universe: Properties of the universe are revealed by the rotation of galaxies and their distribution in space", Science 170 (3965): 1363–1370, 1970 Dec 25, Bibcode 1970Sci...170.1363O, doi:10.1126/science.170.3965.1363, PMID 17817459

Caldwell, J., & Coulson, I. (1987). Milky Way rotation and the distance to the galactic center from Cepheid variables The Astronomical Journal, 93 DOI: 10.1086/114393