AM-1 - A very distant globular cluster

Deep B and V frames have been obtained for Arp-Madore 1 (AM-1) using the Cerro Tololo 4 m telescope and prime focus CCD camera, confirming the nature of this object as a globular cluster. In particular, the resulting color-magnitude diagram shows a sharply defined red horizontal branch at apparent magnitude m(V) = 20.93 + or - 0.04. Adopting M(V)(HB) = 0.6 then yields a galactocentric distance of 118 kpc, an amount placing AM-1 as the most distant globular cluster yet identified which may still be associated with the Galaxy. From the color of the giant branch at the horizontal branch, Fe/H is estimated as -1.8 + or - 0.3, a value which lends support to previous arguments that the abundance gradient in the outer halo is weak. In stellar content AM-1 appears similar to the other low-luminosity members of the outer halo - the Palomar clusters and dwarf spheroidals.