Astronomy & Astrophysics

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Issue		A&A Volume 442, Number 1, October IV 2005
Page(s)		165 - 176
Section		Extragalactic astronomy
DOI		10.1051/0004-6361:20053368

A&A 442, 165-176 (2005)
DOI: 10.1051/0004-6361:20053368

The Draco dwarf galaxy in the near-infrared

M.-R. L. Cioni¹ and H. J. Habing²

¹ SUPA, School of Physics, University of Edinburgh, IfA, Blackford Hill, Edinburgh EH9 3HJ, UK
e-mail: mrc@roe.ac.uk
² Sterrewacht Leiden, Niels Bohrweg 2, 2333 RA Leiden, The Netherlands
e-mail: habing@strw.leidenuniv.nl

(Received 4 May 2005 / Accepted 11 July 2005)

Abstract
With the William Herschel Telescope in La Palma we made IJK_s observations of an area of about $40^{\prime}\times30^{\prime}$ of the Local Group galaxy Draco. This allows us to describe Draco's late-type stellar population across the whole galaxy at a photometric level 2 mag deeper than the 2MASS survey. We detected the red giant branch (RGB) and measured the magnitude of the tip of the RGB in the three bands. From that in the I band we obtain a distance modulus of $(m-M)_0= 19.49\pm0.06{(\mathrm {stat})}\pm0.15{(\mathrm {sys})}$ , in excellent agreement with a measurement from RR Lyrae stars. The peak of the (J-K_s)₀ histogram at different M_{K_s} suggests that Draco has a mean ${\rm [Fe/H]}=-1.95\pm1.26$ while fiducial RGB tracks of Galactic globular clusters indicate a mean ${\rm [Fe/H]}=-1.33\pm0.72$ where the error corresponds to the spread around the mean value. There are significant differences between the colour-magnitude diagrams of stars in the inner, medium and outer areas of the galaxy. A metal poor (Z=0.0004) intermediate-age population (about 1.6 Gyr old) is clearly present and emerges in particular between $6^{\prime}$ and $12^{\prime}$ from the centre of the galaxy. A few additional carbon star candidates have been identified from both their location in the colour-magnitude diagram and from an indication of variability. The large scale distribution of late-type stars is smooth but irregular in shape; this points at a variation of inclination with radius.

Key words: stars: late-type -- stars: distances -- galaxies: Local Group -- galaxies: photometry -- infrared: stars -- stars: carbon

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