Establishing the stellar masses , and hence specific star-formation rates of submillimetre galaxies is crucial for determining the role of such objects in the cosmic history of galaxy/star formation .
However , there is as yet no consensus over the typical stellar masses of submillimetre galaxies , as illustrated by the widely differing results reported from recent optical-infrared studies of submillimetre galaxies with spectroscopic redshifts z \simeq 2 – 3 .
Specifically , even for the same set of submillimetre galaxies , the reported average stellar masses have ranged over an order of magnitude , from \simeq 5 \times 10 ^ { 10 } { M _ { \odot } } to \simeq 5 \times 10 ^ { 11 } { M _ { \odot } } .
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 IRAC 3 – 8 \mu m photometry from hot dust associated with an active nucleus is not the origin of the published discrepancies in derived stellar masses .
Instead , we expose in detail how inferred stellar mass depends on assumptions made in the photometric fitting , and quantify the individual and cumulative effects of different choices of initial mass function , different “ brands ” of evolutionary synthesis models , and different forms of assumed star-formation history .
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 stellar mass of luminous ( S _ { 850 } \gtrsim 5 mJy ) submillimetre galaxies is \simeq 2 \times 10 ^ { 11 } { M _ { \odot } } to within a factor \simeq 2 .
We also check and confirm that this number is perfectly reasonable in the light of the latest measurements of the dynamical masses of these objects ( \simeq 2 - 6 \times 10 ^ { 11 } { M _ { \odot } } from CO ( 1-0 ) observations ) , and the evolving stellar mass function of the overall galaxy population .
Galaxy stellar masses of this order imply that the average specific star-formation rate of submillimetre galaxies 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 stellar mass , most submillimetre galaxies simply form the top end of the ‘ main-sequence ’ of star-forming galaxies at these redshifts .
Conversely , this argues strongly against the viewpoint , frequently simply asserted in the literature , that submillimetre galaxies are extreme pathological objects , of little relevance in the cosmic history of star-formation .