The meaning of WISE colours - I. The Galaxy and its satellites

Abstract

Through matches with the Sloan Digital Sky Survey (SDSS) catalogue we identify the location of various families of astronomical objects in WISE colour space. We identify reliable indicators that separate Galactic/local from extragalactic sources and concentrate here on the objects in our Galaxy and its closest satellites. We develop colour and magnitude criteria that are based only on WISE data to select asymptotic giant branch (AGB) stars with circumstellar dust shells, and separate them into O-rich and C-rich classes. With these criteria we produce an all-sky map for the count ratio of the two populations. The map reveals differences between the Galactic disc, the Magellanic Clouds and the Sgr Dwarf Spheroidal galaxy, as well as a radial gradient in the Large Magellanic Cloud (LMC) disc. We find that the C:O number ratio for dusty AGB stars increases with distance from the LMC centre about twice as fast as measured for near-IR selected samples of early AGB stars. Detailed radiative transfer models show that WISE colours are well explained by the emission of centrally heated dusty shells where the dust has standard properties of interstellar medium (ISM) grains. The segregation of different classes of objects in WISE colour space arises from differences in properties of the dust shells: those around young stellar objects have uniform density distributions while in evolved stars they have steep radial profiles.

20 pages, 21 figures, 3 tables. Published in MNRAS. Electronic table C1 available as model_tables.dat in the source archive. This version is as close as possible to the published version, and implements comments we received from the community, and corrections made in the proofs

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• The Outer Halo of the Milky Way as Probed by RR Lyr Variables from the Palomar Transient Facility
• A Remarkable Oxygen-rich Asymptotic Giant Branch Variable in the Sagittarius Dwarf Irregular Galaxy
• HI absorption towards low luminosity radio-loud AGNs of different accretion modes and WISE colours
• Quiescent and active phases in Be stars: A WISE snapshot of young galactic open clusters
• The Luminosity and Stellar Mass Functions of Red W1-W2 Galaxies
• Foreground and sensitivity analysis for broad band (2D) 21cm--Ly-alpha and 21cm--H-alpha correlation experiments probing the Epoch of Reionization
• SiO Maser Survey towards off-plane O-rich AGBs around the orbital plane of the Sagittarius Stellar Stream