Letter to the Editor

Very luminous carbon stars in the outer disk of the Triangulum spiral galaxy

¹ School of Computational and Applied Mathematics, University of Witwatersrand, Private Bag 3, WITS 2050, South Africa e-mail: block@cam.wits.ac.za
² Mount Stromlo and Siding Spring Observatories, Research School of Astronomy and Astrophysics, Australian National University, Australia
³ Infrared Processing and Analysis Centre, 100-22, CALTECH, 770 South Wilson Ave, Pasadena, CA 91125, USA
⁴ Instituto Nacional de Astrofísica, Optica y Electrónica, Calle Luis Enrique Erro 1, 72840 Tonantzintla, Puebla, México
⁵ Washington State University, 1245 Webster Hall, Pullman, WA 99163-2814, USA
⁶ Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France

Received: 27 May 2004
Accepted: 8 July 2004

Abstract

Stars with masses in the range from about 1.3 to  pass through an evolutionary stage where they become carbon stars. In this stage, which lasts a few Myr, these stars are extremely luminous pulsating giants. They are so luminous in the near-infrared that just a few of them can double the integrated luminosity of intermediate-age (0.6 to 2 Gyr) Magellanic Cloud clusters at 2.2 microns. Astronomers routinely use such near-infrared observations to minimize the effects of dust extinction, but it is precisely in this band that carbon stars can contribute hugely. The actual contribution of carbon stars to the outer disk light of evolving spiral galaxies has not previously been morphologically investigated. Here we report new and very deep near-IR images of the Triangulum spiral galaxy M 33 = NGC 598, delineating spectacular arcs of carbon stars in its outer regions. It is these arcs which dominate the near-infrared  Fourier spectra of M 33. We present near-infrared photometry with the Hale 5-m reflector, and propose that the arcs are the signature of accretion of low metallicity gas in the outer disk of M 33.