The enigma in the heart of Ursa Minor

¹ Département de Physique, Université de Montréal, CP 6128, Succursale Centre-Ville, Montréal, Québec H3C 3J7, Canada
² Osservatorio Astronomico di Roma Viale del Parco Mellini 84, 00136 Roma, Italy e-mail: battinel@oarhp1.rm.astro.it

Corresponding author: S. Demers, demers@astro.umontreal.ca

Received: 18 June 2001
Accepted: 9 August 2001

Abstract

To investigate the physical nature of the observed deficiency of stars in the very center of the Ursa Minor Dwarf Spheroidal galaxy, we recently obtained JCMT-SCUBA 850 μm photometry of this region. The observed void could be explained by ≈0.5 mag of extinction. While the amount of dust needed to produce such absorption (under certain assumptions for the dust temperature and the β exponent in the dust emissivity term) would yield a 5 mJy signal -easily detectable by SCUBA -we found no signal at 850 μm, after 256 min of integration. This finding rules out the presence of a few solar masses of cold dust cloud as the explanation for the central lack of stars observed in Ursa Minor. We discuss possible explanations.