Keck spectroscopy and Spitzer space telescope analysis of the outer disk of the Triangulum spiral galaxy M 33

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 Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France
⁴ Instituto Nacional de Astrofísica, Optica y Electrónica, Calle Luis Enrique Erro 1, 72840 Tonantzintla, Puebla, México
⁵ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii, USA
⁶ Institute of Geophysics and Planetary Physics and Department of Physics, University of California, Riverside, CA 92521, USA
⁷ Infrared Processing and Analysis Center, 100-22, CALTECH, 770 South Wilson Ave, Pasadena, CA 91125, USA
⁸ Washington State University, 1245 Webster Hall, Pullman, WA 99163-2814, USA
⁹ Department of Astronomy, University of Minnesota, 116 Church St. SE, Minneapolis MN 55455, USA
¹⁰ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA

Received: 26 June 2006
Accepted: 15 May 2007

Abstract

In an earlier study of the spiral galaxy M 33, we photometrically identified arcs or outer spiral arms of intermediate age (0.6-2 Gyr) carbon stars precisely at the commencement of the HI-warp. Stars in the arcs were unresolved, but were likely thermally-pulsing asymptotic giant branch carbon stars. Here we present Keck I spectroscopy of seven intrinsically bright and red target stars in the outer, northern arc in M 33. The target stars have estimated visual magnitudes as faint as . Absorption bands of CN are seen in all seven spectra reported here, confirming their carbon star status. In addition, we present Keck II spectra of a small area 0.5 degree away from the centre of M 33; the target stars there are also identified as carbon stars. We also study the non-stellar PAH dust morphology of M 33 secured using IRAC on board the Spitzer Space Telescope. The Spitzer 8 μm image attests to a change of spiral phase at the start of the HI warp. The Keck spectra confirm that carbon stars may safely be identified on the basis of their red colours in the outer, low metallicity disk of M 33. We propose that the enhanced number of carbon stars in the outer arms are an indicator of recent star formation, fueled by gas accretion from the HI-warp reservoir.