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Constraints on Cold HI in the Halo of NGC 3079 from Absorption Measurements of Q0957+561

Published online by Cambridge University Press:  05 March 2013

Judith A. Irwin
Queen's University, Kingston, Ontario K7L 3N6, Canada; National Centre for Radio Astrophysics, Pune University Campus, Post Bag 3, Pune 411 007, India
Lawrence M. Widrow
Queen's University, Kingston, Ontario K7L 3N6, Canada;
Jayanne English
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA;
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We perform the first observational test of dark matter in the form of cold (3 K) fractal clouds, as described by Pfenniger et al. (1994) and Pfenniger & Combes (1994). This is accomplished by probing for HI absorption in the halo of NGC 3079 against the background quasar, Q 0957+561, which is separated from the centre of NGC 3079 by 64 kpc, in projection. No absorption is detected to a limit of 3ΔTb/(–Tc) = 0·01. We have considered models for HI + H2 clouds characterised by the cloud radius and fractal dimension. Using the upper limit on absorption, we have ruled out a limited but interesting region of this parameter space. The observations do not rule out the possibility that all the dark matter could be hidden in the form of cold fractal clouds. By contrast, if the gas is diffuse with unity filling factors, then HI cannot constitute more than ∼ 10−5, by mass, of the galaxy's dark matter.

Research Article
Copyright © Astronomical Society of Australia 1999


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