Abstract
Establishing the stellar masses (M*), and hence specific star-formation rates
(sSFRs) of submillimetre galaxies (SMGs) is crucial for determining their role
in the cosmic history of galaxy/star formation. However, there is no consensus
over the typical M* of SMGs with the widely differing results reported from
studies of z~2-3 SMGs. Specifically, even for the same set of SMG, the reported
average M* have ranged over an order of magnitude, from 5x10^10 Mo to 5x10^11
Mo. Here we study how different methods of analysis can lead to such widely
varying results. We find that, contrary to recent claims in the literature,
potential contamination of 3-8um photometry from hot dust associated with an
active nucleus is not the origin of the discrepancies in derived M*. Instead,
we expose in detail how inferred M* depends on assumptions of intial mass
function, different evolutionary synthesis models, and different star-formation
histories. We review current observational evidence for and against these
alternatives as well as clues from the hydrodynamical simulations, and conclude
that, for the most justifiable choices of these model inputs, the average M* of
luminous (S_850>5 mJy) SMG is 2x10^11 Mo. We also check and confirm that this
number is perfectly reasonable in the light of the latest measurements of their
dynamical masses (2-6x10^11 Mo from CO(1-0) observations), and the evolving M*
function of the overall galaxy population. Stellar masses of this order imply
that the average sSFR of SMG is comparable to that of other star-forming
galaxies at z>2, at 2-3 Gyr^-1. This supports the view that, while rare
outliers may be found at any M*, most SMG simply form the top end of the
main-sequence of star-forming galaxies at these redshifts. Conversely, this
argues strongly against the viewpoint that SMG are extreme pathological
objects, of little relevance in the cosmic history of star-formation.
Description
[1108.6058] The stellar masses and specific star-formation rates of submillimetre galaxies
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