Physics Photo of the Week - May 12, 2023

 

M87 - Elliptical Galaxy in Virgo: Special properties

Photo by College View Observatory

The object (M87) in this photo is a distant galaxy - about 50 million light years distant - a galaxy of many billions of stars within the constellation Virgo.  At first glance it resembles a "blob" - more resembling a comet without a tail than a galaxy.   Many galaxies are much more photogenic showing grand spirals, dust lanes, and other interesting structures.  M87 is a class of galaxies called  elliptical galaxies. This elliptical galaxy is also rather ordinary in first appearance in that it appears spherical rather than a football-shaped ellipsoid.  Just a "blob" of stars without any noticeable structure unlike the Andromeda Galaxy (see PPOW for Jan. 18, 2019).

However, upon close inspection of the photograph, seen much clearer in a zoomed-in photo at right, there is an apparent "jet" or ray of light made much more visible by cropping the above image and enlarging.  This ray of light represents a jet of very energetic plasma emanating from a super-massive black hole that lurks in the center of this galaxy.  The negative image below shows the jet a bit more clearly. 

The super-massive black hole in this galaxy has a mass about 6.5 billion Suns.  Black holes have been detected by astronomers since the early 1970's.  The first black hole confirmed is known as Cygnus X-1 and its mass is "only" about 20 Suns.  The estimated size of the super massive black hole in M87 is calculated to be about 0.005 times the diameter of the parent galaxy.  The galaxy is about 150,000 light years across (about 1.5 times the diameter of the Milky Way Galaxy or its twin: the Andromeda Galaxy).  In this digital photo, M87 galaxy is about 50 pixels across.  The scale of the photo is therefore about 3000 light year/pixel.  The size of the super massive black hole in the digital photo is therefore only about 1/4 pixel (0.005 x 50 pixels).

A brief history of our knowledge of M87

M87 was catalogued as the 87th entry in Messier's catalog published in 1774 as an object to "avoid" looking at if you were looking for comets.

The ray was discovered by Heber Curtis of Lick Observatory in 1918 and described as a "curious straight ray".

Edwin Hubble in the early 1920's discovered the vast distance to the Andromeda Galaxy and some other spiral galaxies to be several million light years distant using the 100 inch Hooker Telescope studying variable stars in the galaxies.  The cycle time for the variable stars depends on a well-known formula for the absolute brightness, thus Hubble could calculate the distances to the spiral galaxies.

Hubble classified M87 as an "extra-galactic elliptical nebula".  I presume he was not able to observe the variable stars in M87.  M87 was not classified as a "galaxy" until 1956.

In 1947, during the early years of radio astronomy, a radio source was discovered to emanate from the approximate direction of M87 dubbed "Virgo A" (Stanley, G. J. and O. B. Slee, 1949, Australian Journal of Scientific Research A, 3, p. 234).  I was a pre-schooler.  Improvements in radio astronomy, especially the use of arrays of radio telescopes to highly resolve the size of radio sources using interferometry techniques in the 1960's, indicated that the Virgo A source was in the M87 galaxy.  I was in high school and college.

Burbridge (1956 reported a study of the polarization of the light from the jet. The light from the jet was circularly polarized.  This means that the jet was facing toward us at an angle and contained a very strong magnetic field.  The ions were moving with high energy in helical trajectories caused by the strong magnetic field along the axis of the jet.  In circularly polarized light, the electro-magnetic waves crests describe a helix.  This is called synchrotron radiation.  A rapidly spinning black hole would produce a strong magnetic field emanating from the spin axis.

X-rays were first detected from astronomical sources by x-ray detectors on board rockets in the 1960's.  In my first semester in graduate school at Carnegie Institute of Technology - later named Carnegie-Mellon University - I attended a fascinating colloquium on "X-Ray Astronomy" as a fascinating new field of research.  I was wondering "what possible mechanisms could produce X-rays among stars?"

In the early 1970's news broke out that an x-ray source (Cygnus X-1) consisted of a black hole orbiting in a binary system around an ordinary star.  The x-rays were found to be eclipsed periodically as the x-ray source passed behind its partner star.  Light cannot escape from inside a black hole - the escape velocity exceeds the velocity of light which is impossible according to Albert Einstein.  However, the extreme tidal conditions immediately outside the "surface" of the black hole actually rip apart the in-falling atoms.  When atoms are torn apart the repercussions of remnants bouncing back emit x-rays.  X-rays can only be detected from detectors flown from outer space.  The first evidence of detecting black holes was from x-rays.  I was a young faculty member at Warren Wilson College.

X-rays were also detected from M87.  (Lea, S. M.; Mushotzky, R.; Holt, S. S. (November 1982). "Einstein Observatory solid state spectrometer observations of M87 and the Virgo cluster". Astrophysical Journal, Part 1. 262: 24–32. Bibcode:1982ApJ...262...24L. doi:10.1086/160392. hdl:2060/19820026438. S2CID 120960432).  This is further evidence of the black hole at the center of M87.  The jet again is the expulsion of the atom fragments (ions and electrons) that were produced immediately outside the black-hole's event horizon - "the point of no return".  I began teaching introductory astronomy at Warren Wilson.

It was learned in the 1990's that virtually all galaxies harbor a massive black hole in their cores.  The masses of the unseen black holes were observed from the motion of stars near the center of the Milky Way observed in infrared light.  There is too much dust to see the stars very close to the nucleus of the Milky Way, but infrared light can see further through the dust.  Radio astronomy can see much further due to the much longer wavelengths of radio waves than light waves.  I was an elderly professor at Warren Wilson College

Finally in 2017 and 2022 the Event Horizon Telescope published images of the black holes' shadows for the M87 core and the Milky Way core respectively.  I had retired from teaching at Warren Wilson College and had built the College View Observatory.  The Event Horizon Telescope consists of a world-wide array of radio telescopes connected synchronously - or data reassembled coherently from data files that make use of the interferometry of the radio waves (1-3 mm wavelength) that can penetrate deep into the galaxies.   The widely-circulated images are reproduced below copied from Wikipedia:

The Event Horizon Telescope image of the core of M87 using 1.3 mm microwaves. The central dark spot is the shadow of M87* and is larger than the black hole's event horizon.

 

 

 

Sagittarius A* imaged by the Event Horizon Telescope in 2017, released in 2022.

Composite image of observations by the Hubble Space Telescope in visible and infrared light.  This is much higher resolution than the College View Observatory, but it is quite amazing that a medium-sized backyard telescope can obtain the jet's image that was first observed only 100 years ago, and also points to a bizarre object such as a supermassive black hole that is smaller than a single pixel.

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 Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.

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