M82 - Exploding or Starburst Galaxy?
M82 (NGC 3034, Arp 337, Cigar galaxy)
RA: 9h 55m 52.2 sec m = +8.4, Radial velocity = +240 km/sec
DEC: +69° 40' 47" Irregular Galaxy, Size: 11.3' x 4.2'
Distance = 3.3 Mpc (11 million light years)
M82 is the 82nd object in Charles Messier's 1784 catalogue of faint nebulae. It is located approximately 3.3 Mpc the Earth and is classified as an irregular galaxy because of its disordered shape. The band seen running from Figure 1, the upper-left to the lower-right of the image is due to light from stars in the M82 galaxy. This is the main galaxy disk. The filamentary features extending perpendicular to the galaxy are due to ionized hydrogen gas emitting its characteristic red light (the so-called "H alpha" line) at a wavelength of 6563 Angstroms. The filaments extend for over 3000 pc in each direction from the center of the galaxy.
This image was produced using the Japanese Subaru Telescope's Faint Object Camera and Spectrograph (FOCAS) on its first night of operation in February 2, 2000. FOCAS will also be able to take spectra of many dozens of objects in a single exposure within its 6 arc minute field of view.
Allan Sandage of the Carnegie Institute of Washington, detected outward motion in the filamentary structure away from the nucleus using long slit spectroscopy. The observed velocities was found to be 1,000 km/sec. Up until the early 1960s, it was believed that the extended H alpha emission observed was caused by a single massive explosion at the center of M82. This explosion idea for M82 was rare, since there are only a few other cases on which galaxies have large streaming away from the nucleus. (NGC 625 and NGC 1569). The date of the explosion was thought to have occurred some 1.5 million years ago. The huge radio emission seems to justify this colossal outward force. Others thought that M82 had an encounter with the neighboring galaxy M81. If this had occurred, M81 should show signs of tidal force interactions, which it doesn’t.
Quasars Ejected from M82?
In 1980, Margaret Burbidge et. al. announced the discovery of 3 quasars with identical redshifts (z » 2) within 8 arc minutes of M82 and having small angular separations. A 4th quasar was found later. They suggested a physical association with each other and possibly M82. The chances of having 4 quasars in such a small field and not being physically associated is computed to be less than 1 in 30 million!! The authors postulated that if the quasars are at their cosmological distances, then their separation is 2.8 Mpc (typical galaxy cluster sizes are » 5 Mpc). If the quasars are at the distance of M82 (3.3 Mpc), then their separation is on the order of 3.4 kpc and the distance from the nucleus of M82 is approximately 8 kpc (Our Sun is 8.5 kpc from the Milky Way nucleus). Because of the nearness to the disturbed galaxy M82, if the quasars were ejected from it, then an explanation should exist for their low velocity dispersion (nearly identical redshift velocities), And of course the nature of the ejection mechanism or explosion would have to be explained. Halton Arp sees an ejection mechanism responsible for these quasars, as they lie along a small cone on the southeast side of M82, see Figure 2. Also in Figure 2 is a radio source originating at the origin of the ejection cone, presumably as a consequence of the ejection process according to Halton Arp.
Recent higher resolution views showed large clouds of molecular hydrogen gas and many supernova remnants at the center of M82. Further observations with the 45 meter radio telescope at the National Astronomical Observatory of Japan's Nobeyama Radio Observatory showed that the molecular gas is flowing outwards from the nucleus of M82. It is now thought that this outflow is being driven by the copious formation of massive stars (called a starburst) and subsequent supernova explosions. Astronomers call such galaxy-size outflows "Superwinds". The explosion hypothesis has been abandoned by many astronomers except for Halton Arp.
In addition to providing the ejection mechanism for the material from the galaxy, the superwind heats the gas, causing it to glow with the light of H alpha emission. Studying the M82 galaxy may provide clues to galaxy evolution in general and details of the composition of intergalactic material. The Subaru Telescope had first light in January 1999 and is being continuously adjusted to improve its performance. During this time, test observations are being made with the seven first-phase instruments (including FOCAS). Use of the Subaru Telescope by the worldwide astronomical community has already started, and many exciting new scientific results are sure to follow.
Figure 1. M82, the Cigar galaxy . Exposure: 30 sec (B) , 25 sec (V), 120 sec (H alpha) dithering two frames for each color. The long streamers moving away from the nucleus are now believed to be caused by the Superwind effect. Photo from Astronomy Picture of the Day, April 4, 2000, from FOCAS Subaru 8.3 meter Telescope, NAOJ.
Figure 2. Quasars (circled) along an “ejection cone”. A radio source at the southeast side of the galaxy originating at the start of the cone appears also to have been ejected. From Arp 1987, p. 59.
Finder chart for M82. Its located just away from the pointer stars in the Big Dipper in Ursa Major.
REFERENCES:
Arp, Halton, 1987, 1987, Quasars, Redshifts and Controversies, Insterstellar Media, Berkley, CA. ISBN 0-941325-00-8, p. 59.
Burbidge, E.M., Junnkarinen, V.T., Koski, A.T., Smith H., Hoag, A.A., 1980, A Cluster of Quasi-Stellar Objects Near M82, Astrophysical Journal, 242, L55-57.
Condon, J.J., 1983, Strong Radio Sources in Bright Spiral Galaxies. III. Disk Emission, Astrophysical Journal Suplement Series, 53, p. 459-495.
Pedlar, A., Muxlow, T.W.B., Garrett, M.A., Diamond, P., Wills, K.A., Wilkinson, P.N., Alef, W., 1999, Measurement of Expansion Velocity in Supernova Remnant in Messier 82, 1999, New Astronomy Reviews, 43, p. 535-538.