Size of Mrk 231 at 10 microns

A new technique is demonstrated for using the signal from a source at a wavelength where it is strong to assist in determining its size at another wavelength. Observations of the size of the nucleus of Mrk 231 at 10 microns are consistent with a picture of a single luminosity source, perhaps obscured by dust. The variation with wavelength of the emissivity of the grains is constrained to be less than (wavelength) - 1. The measurements are inconsistent with Mrk 231 being a starburst galaxy with a luminosity density similar to that of M82. 7 references