How To Find A Supermassive Black Hole

Deep in the night sky, in the constellation of Sagittarius, shrouded by interstellar gas and dust, lies a mysterious object.   So enigmatic is this object, it cannot be observed directly, not even light can escape its presence.  This object found at the core of our Milky Way galaxy is of incredible mass, equaling millions of suns and yet stretches across an area smaller than our solar system.     It is of course, a         supermassive black hole.

Intrigued by the notion of these gigantic black holes silently churning away at the center of our Milky Way galaxy, astronomers discovered the first supermassive black hole (SMBH) candidate through radio telescopes in the 1970’s. (Balick & Brown 1974)  Almost a decade later the object was officially entitled Sagittarius A* (Brown 1982)

The strongest evidence for our galaxy’s own SMBH was conducted decades later by astronomers who carefully observed the motion of stars around the Milky Ways’ center over a 15 year period.  They observed that stars were orbiting in an elliptical motion around an invisible object. (Gillessen 2009)

SMBH-1

(Credit – ESO / MPE, 2011)

Image – Observations of Sagittarius A* using the European Southern Observatory’s – Very Large  Telescope have captured infra-red images of doomed stars and gas circling our galaxy’s SMBH.   The circled image identifies a gas cloud which astronomers anticipate witnessing first-hand as it falls into the event-horizon in the coming years.

By measuring the orbits of these doomed stars around our SMBH, astronomers were able to determine the mass and area of this invisible object.    With the velocity and the orbit know, it was calculated that the density of our Milky Way’s SMBH is 4 Million solar masses in an area of 44 million kilometers.   (Schödel et al. 2003)

Amazed by the discovery of Sagittarius A*, astronomers were curious to know if other galaxies hosted these supermassive objects.  Peering outside of our own Milky Way galaxy, scientists studied the spectral image of gas circling the center of distant galaxies.

Gas was moving so rapidly around their galactic centers, scientist could watch the redshift and blueshift of its motion.  As the gas was whirling towards our telescopes – the spectral lines would shift towards the blue, as they spirals away from our telescopes the spectral lines would shift to the red.

SMBH-2

(Gary Bower, Richard Green (NOAO) / STIS Instrument Definition Team, NASA, 1997)

Image – The above Spectral image shows gas hurling around the core of galaxy M84.  By accurately measuring the velocity of the gases orbit (360km a second) scientists could conclude that the mass of an object to create this velocity would equate to 300 million solar masses.

These are by no means unique cases, the universe appears to be littered with supermassive black holes. By peering back even deeper in time, astronomers using the space telescopes Hubble and Chandra X-ray, were able to identify over 30 million SMBHs, with the earliest having existed 700 million years after the big bang. (Treister 2011)

 

References

Balick B,  Brown, R 1974, ApJ, 194, 265
Brown,R.L. 1982, ApJ, 262, 110
Eso Public Image Library – http://www.eso.org/public/images/
Gillessen, S, et al. 2009, ApJ, 2009, 692, 1075-1109
Schödel R, et al.  2003 ApJ 596 1015
Treister, E , et al. 2011 Nature 474, 356–358

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