Sagittarius A

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Sagittarius A
Astrometry
Radial velocity (Rv) 46 km/s
Details
Mass 3.7 million M
Radius R
Luminosity L
Surface gravity (log g) cgs
Temperature K
Metallicity ?
Rotation ?
Age ? years
Other designations
AX J1745.6-2900, SAGITTARIUS A, W 24, Cul 1742-28, SGR A, [DGW65] 96, EQ 1742-28, RORF 1742-289, [SKM2002] 28.
Database references
SIMBAD data

Sagittarius A (or Sgr A) is a complex radio source at the center of the Milky Way. It is located in the Sagittarius constellation, and is hidden from view at optical wavelengths by large clouds of cosmic dust in the spiral arms of the Milky Way.

It consists of three components, the supernova remnant Sagittarius A East, the spiral structure Sagittarius A West, and a very bright compact radio source at the center of the spiral, Sagittarius A*. These three overlap: Sagittarius A East is the largest, West appears off-center within East, and A* is at the center of West.

Two-Dimensional Size[edit]

The Two-Dimensional size of Sagittarius A* has been determined, based on Very Long Baseline Array observations, which helps explain the context of jet disk, and accretion disk models on the radio emission spectrum. These measurements were made through the use of a wavelength of 7mm, enabling the size to be determined. The result was that Sagittarius A* was determined to be an elliptical Gaussian with a semi-major axis size of 35.4x12.6 Rs with an inclination of 95 degrees east of north. The observations for this finding were found from the detection of NIR (near infrared) flares, and X-ray flares that were spotted by NuSTAR. Both flares come from electron excitation, not an enhanced accretion rate onto the black hole, which means that not all high energy events produce variability at radio wavelengths.[1]

Spirals of Sagittarius A[edit]

A study done with the measured parallaxes and motions of 10 massive regions in the spiral arms of Sagittarius in the Milky Way where stars are formed. Data was gathered using the BeSSeL Survey with the VLBA, and the results were synthesized to discover the physical properties of these sections (called the Galactocentric azimuth, around -2 and 65 degrees). The results were that the spiral pitch angle of the arms is 7.3 +- 1.5 degrees, and the half-width of the arms of the Milky Way were found to be .2 kpc. The nearest arm from the Sun is around 1.4 +- 0.2 kpc away.[2]

Sagittarius A East[edit]

This feature is approximately 25 light-years in width and has the attributes of a supernova remnant from an explosive event that occurred between 35,000 and 100,000 BCE. However, it would take 50 to 100 times more energy than a standard supernova explosion to create a structure of this size and energy. It is conjectured[citation needed] that Sgr A East is the remnant of the explosion of a star that was gravitationally compressed as it made a close approach to the central black hole.

Sagittarius A West[edit]

Surface brightness and velocity field of the inner part of Sagittarius A West

Sgr A West has the appearance of a three-arm spiral, from the point of view of the Earth. For this reason, it is also known as the "Minispiral". This appearance and nickname are misleading, though: the three-dimensional structure of the Minispiral is not that of a spiral. It is made of several dust and gas clouds, which orbit and fall onto Sagittarius A* at velocities as high as 1,000 kilometers per second. The surface layer of these clouds is ionized. The source of ionisation is the population of massive stars (more than one hundred OB star have been identified so far) that also occupy the central parsec.

Sgr A West is surrounded by a massive, clumpy torus of cooler molecular gas, the Circumnuclear Disk (CND). The nature and kinematics of the Northern Arm cloud of Sgr A West suggest that it once was a clump in the CND, which fell due to some perturbation, perhaps the supernova explosion responsible for Sgr A East. The Northern Arm appears as a very bright North—South ridge of emission, but it extends far to the East and can be detected as a dim extended source.

The Western Arc (outside the field of view of the image shown in the right) is interpreted as the ionized inner surface of the CND. The Eastern Arm and the Bar seem to be two additional large clouds similar to the Northern Arm, although they do not share the same orbital plane. They have been estimated to amount for about 20 solar masses each.

On top of these large scale structures (of the order of a few light-years in size), many smaller cloudlets and holes inside the large clouds can be seen. The most prominent of these perturbations is the Minicavity which is interpreted as a bubble blown inside the Northern Arm by the stellar wind of a massive star, which is not clearly identified.

Sagittarius a-element knee[edit]

The α-element distribution of the Sagittarius stream forms a narrow sequence at intermediate metallicities with a clear turn-down, consistent with the presence of an α-element ‘knee’. This is the first time this has been detected. Using a model of data, we determine that the α-knee in Sagittarius takes place at [Fe/H]=−1.27±0.05, only slightly less metal-poor than the knee in the Milky Way. This is indicative of a small number of similar galaxies that could have contributed to a buildup of the milky ways solar system.[3]

Sagittarius A*[edit]

Main article: Sagittarius A*
Astronomers have observed stars spinning around the supermassive black hole in Sagittarius A*.[4]

Astronomers now have evidence there is a supermassive black hole at the center of the galaxy.[5] Sagittarius A* (abbreviated Sgr A*) is agreed to be the most plausible candidate for the location of this supermassive black hole. The Very Large Telescope and Keck Telescope detected stars orbiting Sgr A* at speeds greater than that of any other stars in the galaxy. One star, designated S2, was calculated to orbit Sgr A* at speeds of over 5,000 kilometers per second at its closest approach.[6]

A gas cloud is expected to collide with the black hole in 2014 and provide additional information.[7]

Dark Matter[edit]

By studying N-body simulations of the Milky Way disk and the ongoing disruption of the Sagittarius dwarf satellite, we can see how studies show the effect of Sagittarius tidal debris on dark matter detection experiments.[8] We can emphasize the fact that the dark matter part of the leading tidal arm of the Sagittarius dwarf is extended more than the stellar component of the arm, considering that dark matter and stellar streams are not coaxial.[9] This fact suggests that the dark matter component of the Sagittarius debris is likely to have a significant influence on dark matter detection experiments.[10]

Neutrino lighthouse[edit]

By studying simulations of the Milky Way and the disruption of the satellite, we can see how studies show the effect of Sagittarius tidal debris on dark matter detection experiments.[11]

The Dark matter compenent of that SAG debris is likely to have part of the tidal of the Sagittarius dwarf is extended is extended more than the stellar component of the arm, considering the streams aren’t coaxial.[12]

This suggests that the dark matter of the debris is likely to influence on dark matter detection experiments.[13]

NuSTAR High Energy X-ray Observatory[edit]

Although Sagittarius A has low luminosity emissions as a supermassive black hole, due to the flaring on occasion from Sagittarius A, studies of emission on light aperture have been conducted to see the different spastic flares that occur. During the summer and fall of 2012, the NuSTAR High X-ray Observatory conducted a coordinate campaign to study flaring activity of Sagittarius A. Four flares were detected in observatory Chandra, each comprising of about a 50" radius. The method of searching Sagittarius A through flares, consisted of the Bayesian block analysis, which is using combined light curves as extraction from the raw outputs to find the correct aperture. As compared with previous observations and recorded data, Sagittarius A's measurement density for the past range is 1.25 × 1023 cm−2 to 1.85 × 1023 cm−2, covering from luminosities of each flare. Based on the volume of each flare's magnetic range, it can be concluded that an acceleration flow value of <3.0 × 1012 cm lies at the center of the black hole creating a separate axis of emission. The X-Ray variability detected by NuSTAR is also a supportive evidence pointing to the sporadic flares of emissions. Both the amplitude and reflex structure of the emission region rise and decay over the speed of plasma. Since the first two flares measured had low intrinsic value as opposed to the second two that were measured, it can be inferred that both particle acceleration takes place over the period of flare emissions. Keeping in mind the four flares, regardless of speed and decay, maintain the same 50"radius at all times as the emissions are recorded. This shows that the aperture opening of Sagittarius A at its center axis, maintain an order of continuous per-pendulum thus location and magnetic values can only vary if location is different in terms of region.[14]

References[edit]

  1. ^ Bower G, Law C, Ghez A, et al. The intrinsic two-dimensional size of sagittarius A*. Astrophysical Journal [serial online]. July 20, 2014;790(1)Available from: Scopus®, Ipswich, MA. Accessed October 23, 2014.
  2. ^ Wu Y, Sato M, Zheng X, et al. Trigonometric parallaxes of star forming regions in the Sagittarius spiral arm. [serial online]. April 17, 2014;Available from: arXiv, Ipswich, MA. Accessed October 23, 2014.
  3. ^ de Boer T, Belokurov V, Beers T, Lee Y. The α-element knee of the Sagittarius stream. Monthly Notices Of The Royal Astronomical Society [serial online]. September 2014;443(1):658-663. Available from: Academic Search Premier, Ipswich, MA. Accessed October 23, 2014.
  4. ^ "A monster in the Milky Way". ESA/Hubble Picture of the Week. Retrieved 8 October 2013. 
  5. ^ "Black hole confirmed in Milky Way". BBC. December 9, 2008. Retrieved 2008-12-10.
  6. ^ http://www.eso.org/public/news/eso0226/
  7. ^ It's Snack Time in the Cosmost By RON COWEN, New York Times, Feb 17, 2014
  8. ^ Purcell C, Zentner A, Wang M. Dark Matter Direct Search Rates in Simulations of the Milky Way and Sagittarius Stream. [serial online]. March 29, 2012;Available from: arXiv, Ipswich, MA. Accessed October 23, 2014.
  9. ^ Purcell C, Zentner A, Wang M. Dark Matter Direct Search Rates in Simulations of the Milky Way and Sagittarius Stream. [serial online]. March 29, 2012;Available from: arXiv, Ipswich, MA. Accessed October 23, 2014.
  10. ^ Purcell C, Zentner A, Wang M. Dark Matter Direct Search Rates in Simulations of the Milky Way and Sagittarius Stream. [serial online]. March 29, 2012;Available from: arXiv, Ipswich, MA. Accessed October 23, 2014.
  11. ^ Purcell C, Zentner A, Wang M. Dark Matter Direct Search Rates in Simulations of the Milky Way and Sagittarius Stream. [serial online]. March 29, 2012;Available from: arXiv, Ipswich, MA. Accessed October 23, 2014.
  12. ^ Purcell C, Zentner A, Wang M. Dark Matter Direct Search Rates in Simulations of the Milky Way and Sagittarius Stream. [serial online]. March 29, 2012;Available from: arXiv, Ipswich, MA. Accessed October 23, 2014.
  13. ^ Purcell C, Zentner A, Wang M. Dark Matter Direct Search Rates in Simulations of the Milky Way and Sagittarius Stream. [serial online]. March 29, 2012;Available from: arXiv, Ipswich, MA. Accessed October 23, 2014.
  14. ^ Barrière, N.M. ( 1 ), et al. "Nustar Detection Of High-Energy X-Ray Emission And Rapid Variability From Sagittarius A* Flares." Astrophysical Journal 786.1 (2014): Scopus®. Web. 24 Oct. 2014.

Further reading[edit]

External links[edit]