Letter to the Editor

Detection of a population gradient in the Sagittarius stream

¹ INAF - Osservatorio Astronomico di Bologna, Via Ranzani 1, 20127 Bologna, Italy e-mail: michele.bellazzini@oabo.inaf.it
² Department of Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
³ Dipartimento di Astronomia, Università di Bologna, Via Ranzani 1, 20127 Bologna, Italy
⁴ ESO - European Southern Observatory, Alonso de Cordova 3107, Santiago 19, Chile

Received: 10 July 2006
Accepted: 16 August 2006

Abstract

We present a quantitative comparison between the Horizontal Branch morphology in the core of the Sagittarius dwarf spheroidal galaxy (Sgr) and in a wide field sampling a portion of its tidal stream (Sgr Stream), located tens of kpc away from the center of the parent galaxy. We find that the Blue Horizontal Branch (BHB) stars in that part of the Stream are five times more abundant than in the Sgr core, relative to Red Clump stars. The difference in the ratio of BHB to RC stars between the two fields is significant at the  level. This indicates that the old and metal-poor population of Sgr was preferentially stripped from the galaxy in past peri-Galactic passages with respect to the intermediate-age metal rich population that presently dominates the bound core of Sgr, probably due to a strong radial gradient that was settled within the galaxy before its disruption. The technique adopted in the present study allows to trace population gradients along the whole extension of the Stream.