M54 – Globular Cluster or Galactic Nucleus?

   M54: NGC 6715, Globular Cluster in Sagittarius   RA = 18^h 55 ^m 3^s       DEC = -30° 28' 42''    

       Distance: (m - M) method = 27 ± 1 kpc,  Size ~ 3 kpc,  Radial Velocity = +140 km/sec

       Galactic Coordinates:  l = 5º, b = -15º,    Estimated Mass =   1.03 ± 0.23 x 10⁶ M_¤

 M54 was discovered by Messier on July 24, 1778. He described it as a faint nebula containing no stars. It is difficult to resolve in small telescopes despite its relatively bright m = 7.7 visual magnitude.     

In 1994 a team of astronomers using the Royal Greenwich Observatory’s UK Schmidt Telescope sky survey plates discovered a group of co-moving stars in the direction of the Galactic Center which they interpreted as a dwarf galaxy that was closer to our own Milky Way galaxy than any other known. Located in the constellation of Sagittarius, and on the far side of the Galactic Center, it was not previously seen because of the large number of foreground stars (from the Milky Way) in that direction.  The close proximity to the Milky Way suggests the Sagittarius Dwarf Galaxy (SDG) is being tidally disrupted and being absorbed by the Milky Way.

 In 1995  Bassino & Muzzio suggested through their numerical simulations that cannibalized dwarf galaxies such as the SDG could have their nuclei survive as globular clusters. This is partially due to the gravitational stability of globular clusters and their tightly packed large number of stars. These numerical models indicated a total tidal disruption at 5.5 billion years, plenty of time for M54 to survive as the SDG nucleus. (The average age of globular clusters are 10-13 billion years)

 Additional evidence linking M54 as the nucleus of the SDG is its revised distance estimate. Previous distance estimates were difficult due to the large amount of absorption in the direction near to the center of the Milky Way. Improved instrumentation and detectors above the Earth’s atmosphere have provided surveys of increased precision of the absorption levels in all parts of the sky.  The reliable distance estimate to M54 given by Sarajedini and Layden 1995 indicates its luminosity is comparable to that of Omega Centauri (NGC 5139), the brightest Miulky Way globular cluster.  

Contour outline showing location, area and size of the Sagittarius Dwarf galaxy, near the center of the Milky Way. 

 A history of distance estimates for M54 appears below:

 1964              15.1 kpc               Atlas of the Heavens Catalogue 

1979              13.0 kpc               Harris & Racine  (Galacto-centric)

1985              21.5 kpc               Sky Catalogue 2000.0

2003              21.2 kpc               Star Clusters (B.Archinal)

1995              27.5 kpc               Sarajedini and Layden

 M54 is located in the densest part (center) of the SDG & its distance is similar to SDG. So the conclusion based upon this information is that M54 is likely the surviving nucleus of the SDG which has been cannibalized by the Milky Way.

Other globular clusters that may be members of the SDG are  Terzan 7, Distance = 21.9 kpc, Terzan 8, Distance = 22.8 kpc , and Arp 2, Distance = 28.3 kpc.

         Relative positions of Milky Way, M54 and the Sagittarius Dwarf Galaxy.

REFERENCES:

 Archinal, B., Hynes, S.J., 2003, Star Clusters, Wilmann-Bell Inc., Richmond, Viginia, p. 244-245. 

 Bassino, L.P., Muzzio, J.C., 1995, Is M54 the Nucleus of the Sagittarius Galaxy?, The Observatory, 115, p. 256-257.

 Ibata, R.A., Gilmore, G., Irwin, M.J., 1994, A Dwarf Satellite Galaxy in Sagittarius, Nature, 370, p. 194-196.

 Jayawardhana, R., 1998, Our Galaxy’s Nearest Neighbor, Sky and Telescope, May 1988, p. 42-45.

 Sarajedini, A., Layden, A.C., A Photometric Study of the Globular Cluster M54 and the Sagittarius Dwarf Galaxy: Evidence for Three Distinct Populations, Astronomical Journal, 109, No.3, p. 1086-1094.