Notes from Lecture delivered by Professor Andrea Ghez, Nobel Laureate Physics 2020, Professor of Physics and Astronomy, University of California, Los Angeles. March 22, 2021.
Does our galaxy harbor a Supermassive Black Hole at its center?
There are two perspectives as to why one might want to pursue this question:
- Does really massive Black Holes exist in our Universe?
- Testing Einstein’s General Relativity
In studies of Black Hole, there is an abstract size that we can think of – If an object’s mass is confined to its Schwarzschild radius, infact every object has a Schwarzschild radius, it’s a Black Hole. So to prove that Black Holes exist, we need to show there is some mass that is confined to its Schwarzschild radius.
In astronomy there are two types of Black Holes:
- Stellar Black Holes: Comes from – “How really massive (~30X Solar mass) stars end their lives”.
- Supermassive Black Holes: ~Million – Billion Solar mass.
Half a century ago a major question was whether or not all galaxy harbor Black Hole at their center? It turned out 90% of all galaxies have dormant Black Hole at their centre. And small fraction of all galaxies have Active Galactic Nuclei (AGN).
Choosing Milky Way for conducting experiment?
Like Milky Way is an ordinary galaxy, nothing special about it, spiral arms and dense center. Milky Way was chosen as it’s center is closest.
We have interesting vantage point – we are looking through the galactic plane towards the center. So, while we have the advantage of being closest to center of Milky Way, disadvantage is we have to look through all the starlight and dust that is in the plane of our galaxy. This dust, much like ‘smog’, is very good at absorbing light that has wavelength shorter than those dust particles. So, to overcome this hurdle, long wavelength was used – Near-infrared (~2µm).
First ever observation of center of our home galaxy was made when Karl Jansky, considered a father of radio astronomy, discovered in August 1931 that a radio signal was coming from center of the Milky Way. Later that source was known as Sagittarius A*.
Is there a Supermassive Black Hole?
Most direct way to address this question is to observe the orbits of stars around the galactic centre. By properly studying the orbits we can calculate the mass around which those stars are orbiting.
Telescope used for observations
W. M. Keck Observatory – is a two-telescope astronomical observatory at an elevation of 4,145 meters (13,600 ft) near the summit of Mauna Kea in the U.S. state of Hawaii. Both telescopes have 10 m (33 ft) aperture primary mirrors, and when completed in 1993 (Keck 1) and 1996 (Keck 2) were the largest astronomical telescopes in the world. They are currently the 3rd and 4th largest.
Benefits of using large telescopes
- More light collection/ Fainter objects can be detected/ Far objects can be seen.
- Angular resolution.
Challenges of using large telescopes
- Earth’s atmosphere – distorts image of stars at the center of galaxy.
One possible solution is getting above atmosphere – like HST, JWST. However its not feasible for large telescopes. Telescope that was used for observations had 10 meter diameter, while Hubble is only 2.4 meter. In theory that means – Keck telescope can see 44 times smaller details than HST. To reach that theoretical limit, we have to tackle the atmospheric turbulence.
Tackling atmospheric turbulence in observations
From 1995 to 2004, Speckle Imaging was used – its like taking pictures at high shutter speed that apparently freezes atmospheric turbulence. Speckle imaging is software intense solution.
Speckle Imaging –
- Computationally simple approach.
- Didn’t gave much observational data.
- Did the work of resolving stars near galactic center.
From 2005 to today, Adaptive Optics is used – its like adding another artificial turbulence that will cancel out the atmospheric turbulence, in real time. This added turbulence will have to be added continuously, its quite a tricky process to handle.
A thin physical adaptive mirror is added in the imaging system of telescope. This mirror changes shape at ~1000Hz to adapt for atmospheric turbulence.
Adaptive Optics were developed by US military, and were declassified in 1990s for astronomy and astrophysics community.
Using bright LASER beams (part of Adaptive Optics) to stimulate sodium atoms that are deposited by meteorites up in the thin layer at about 90 km in Earth’s atmosphere.
All this help to reach the diffraction limits, and further resolve the stars.
Observation – Stellar Orbits in the Central Parsec
“An animation of the stellar orbits in the central 0.5 arcsec. Images taken from the years 1995 through 2016 are used to track specific stars orbiting the proposed black hole at the center of the Galaxy.” – UCLA Galactic Center Group.
These orbits in the above animation, and a simple application of Kepler’s Laws, provide the best evidence yet for a supermassive black hole, which has a mass of 4 million times the mass of the Sun.