The Red Halos of Galaxies

• Optical and near-IR surface photometry of the halos of edge-on disk galaxies and blue compact galaxies have revealed a very red spectral energy distribution, which cannot easily be reconciled with normal, metal-poor stellar populations like that in the stellar halo of the Milky Way. Using spectral evolutionary models, we demonstrate that a stellar population of low to intermediate metallicity, but with an extremely bottom-heavy initial mass function, can explain the red halos around both types of objects. Other previously suggested explanations, like nebular emission or very metal-rich stars, are shown to fail in this respect. This indicates that, if the reported halo colours are correct, halo populations dominated by low-mass stars may be a phenomenon common to galaxies of very different Hubble types. Potential tests of this hypothesis are discussed, along with its implications for the baryonic dark matter content of galaxies. We also report on the status of searches for red halos around types of galaxies for which thisphenomenon has not yet been reported (post-starbursts, ellipticals and low surface brightness galaxies).

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