Implications of refining the mass of Sag A*

[Quark (Trevor)]

Implications of refining Ro
Hi All,
Andre Ghez of the UCLA Keck Galactic Centre research group has been involved in study of the Sag A region in general and the Sag A* (pronounced Sagittarius A star) region in particular for many years.
Her work has resulted in the continued refinement of the mass of Sag A*, the super massive black hole candidate at our galactic centre. Much of her work involves the study of a group of very high velocity stars in extremely close, elliptical orbits around Sag A*. The latest paper by her group was published on Aug 21st and the following link is to an abstract of that paper, the entire paper can be downloaded in PDF format from the same site.[0808.2870] Measuring Distance and Properties of the Milky Way's Central Supermassive Black Hole with Stellar Orbits
For Office 2007 you should be able to access the above link by hovering your mouse over it then Ctrl+click. If that does not work for you I have included the website below.
http://arxiv.org/abs/0808.2870
Refining the mass of Sag A* has many significant ramifications relating to cosmology and astrophysics, leading to a greater understanding of the dynamics of galaxy formation but I think the spin off relating to the refinement of the value of Ro that flows from this work is of such great significance that it deserves special mention.
The value of Ro, the distance to our galactic centre is as crucial today as was the value of the AU in the 1700’s. The refinement of the AU revealed to science the physical size of the Solar System and became a distance standard that can be considered the first step of the distance ladder.
We now rely on RR Lyrae, Cepheid and Mira variable stars, among other things, as further steps in the distance ladder however distances measured based on the period luminosity relationships of these stars rely heavily on assumptions of the absolute luminosity’s attributed to them.
The significance of the work done by the team of Andrea Ghez, using the Keplerian orbits of stars such as SO-2,(which has now been observed for a complete orbit of Sag A*) to refine the value of Ro to 8 +- 0.6 kpc is immense. This is a direct measurement requiring no assumptions for absolute luminosity or allowances for interstellar reddening. The value it provides may become a distance standard in its own right, following on from the AU, LY and PC.
The refinement of Ro means that the absolute luminosity values for the RR Lyrae, Cephied and Mira variables, in the region of Sag A* can be refined based on the direct measurement of Ro. Such refinement make the distance ladder ever more robust and our perception of the scale of the Universe more accurate.
Regards
Trevor

[Paddy (Patrick)]

Thanks for pointing out the implications Trevor. It seems a truly amazing feat to measure this, especially getting the data on the orbits of stars around galactic centre! I love the animation of the movements of the stars around Sag A * which is at

http://www.astro.ucla.edu/~ghezgroup...itsMovie.shtml

Can't quite get my head around a star moving at 8000 km per second!

[Quark (Trevor)]

Hi Paddy,
Yes, the animation of the high velocity stars in orbit around Sag * is extremely impressive. We really do live in a golden age of discovery, so many of the advances that make research such as that of Andrea Ghez possible are just not known by the general community.

This animated demonstration of the Keck view of the galactic centre with adaptive optics turned on and off is a great example of such an advance.
http://www.astro.ucla.edu/~ghezgroup.../aoMovie.shtml

Regards
Trevor