Letter to the Editor

The puzzle about the radial cut-off in galactic disks

Department of Physics, Indian Institute of Science, Bangalore 560 012, India e-mail: cjjog@physics.iisc.ernet.in

Corresponding author: C. A. Narayan, chaitra@physics.iisc.ernet.in

Received: 26 May 2003
Accepted: 8 July 2003

Abstract

The stellar disk in a spiral galaxy is believed to be truncated physically because the disk surface brightness is observed to fall faster than that for an exponential in the outer, faint regions. We review the literature associated with this phenomenon and find that a number of recent observations contradict the truncation picture. Hence we question the very existence of a physical outer cut-off in stellar disks. We show, in this paper, that the observed drop in the surface brightness profiles in fact corresponds to a negligible decrease in intensity, and that this minor change at the faint end appears to be exaggerated on a log-normal plot. Since minor deviations from a perfect exponential are common throughout the disk, we suggest that such a deviation at the faint end could easily give rise to the observed sharp drop.