NGC 5694 -Escaping Globular from the Galaxy?
NGC 5694 (RA = 14h 39', DEC = -26° 32', on Houston's meridian in May evenings) is located in the eastern tip of the tail of Hydra. It is a globular cluster which appears to be moving so fast that it is destined to escape from our Galaxy into intergalactic space. Discovered in 1794 by William Hershel, it is one of the most remote globulars known. It was first recognized as a globular by C.O. Lampland and Clyde Tombaugh at Lowell Observatory in 1932. Walter Baade did a photometric study of NGC 5694 using the Mt. Wilson's 60" and 100" telescopes to determine its distance. He arrived at D = 39.3 kpc from the Sun. Baade used photographic glass plates for magnitude estimates which was done visually and this introduced serious systematic errors. Its location on the far side of the galactic center (and small angular size = 2') has made magnitude estimates difficult due to the unknown amount of obscuring dust and gas. In addition, exhaustive searches for
variable stars within the cluster through 1994 failed to find any. A study done in the mid 1970's by William Harris and James Hesser from the Cerro-Tololo 4-meter reflector in Chile, gave the following results:
Distance : 26 ± 5 kpc, diameter: 15 pc
Velocity: -273 km/sec,
Mass: 105 M¤
NGC 5694's has an estimated mass equal to about 100,000 Solar masses, thus it is a small globular. Compare this to M22's estimated mass of 7 million Suns.
The lowest possible space motion for NGC 5694 is 273 km/sec. This is an unusually high space velocity. Is this velocity great enough to escape the Milky Way's gravity? For the location at NGC 5694 in the Galaxy, and assuming current mass models for the Milky Way, the escape velocity is 190 km/sec, this NGC 5694 must be leaving the Galaxy and will NEVER COME BACK. Harris and Hesser discount the "missing mass" theory (dark matter) that might bend NGC 5694's orbit back from escape. They also point out that this 273 km/sec is a lower limit to NGC 5694's true space velocity with respect to the galactic center since its transverse velocity is unknown.
Harris and Hesser examine in their paper from the Astronomical Journal (Vol. 88, p. 377, 1976) that NGC 5694 can either be a hyperbolic orbit (just passing through) or may have been "pushed" into a higher energy orbit at some point in its lifetime. Since NGC 5694 has all the usual characteristics possessed by other globulars (rich heavy-element abundances, normal color magnitude diagrams, high density structure) they postulate that it would be difficult to see how a globular would form in the isolation of intergalactic space. The cluster's orbit has been estimated to have a perigalacticon (closest approach to the galactic halo) of rp ~ 14 kpc with a minimum eccentricity of emin=0.32. (e = 1 for a parabolic orbit, like a comet). NGC 5694 is now twice as far away now from its perigalacticon point. The Magellanic Clouds appear to be the only obvious candidates with the right distance and sufficient mass to throw NGC 5694 into a hyperbolic orbit. It would be interesting to determine whether or not they could have realistically have intersected the path of NGC 5694 during their last orbital passage. Harris and Hesser also theorize that NGC 5694 may have once belonged to the cluster system of the Magellanic Clouds and not the Milky Way galaxy.
Relative positions of the Milky Way, Large and Small Magellanic Clouds and NGC 5694 are shown. NGC 5694 is completely outside the halo of the Milky Way and its outward velocity of 273 km/sec suggests it is escaping from us.
Burnham, R. Jr., 1978, Burham's Celestial Handbook, Dover Publications, New York, p 1031.
Harris, W., Hesser, J., 1976, NGC 5694: A Globular Cluster Escaping from the Galaxy?, Publications of the Astronomical Society of the Pacific, 88, p. 377-379.
Hazen, M., 1996, A Search for Variable Stars in the Globular Clusters NGC 5694 and NGC 6558, Astronomical Journal, 111, No. 3, p. 1184.
Murdin, P., Allen, D., Malin, D., Catalogue of the Universe, 1979, Book Club Associates, London, Cambridge University Press, ISBN 0-517-536161