The 3 mm radio continuum emission from active galaxies consists of three components:

1. (1) Synchrotron emission from the active galactic nucleus (AGN), which is over 1 Jy in 3C273 but which is not significant in most of the types of galaxy considered here.

2. (2) Free–free emission from H II regions. The flux of this in a starburst galaxy is typically of the order of 10 mJy and could be imaged with a 3 mm-capable Australia Telescope Compact Array (ATCA).

3. (3) Emission from the tail of the 50–100 K black-body spectrum of the dust. For example, the dust in Arp 220 (redshift of 0·02) at a temperature of 50 K has a flux of 30 mJy at 3 mm. Interestingly, this flux does not decrease substantially with redshift, as the decrease in brightness is compensated for by the redshifting of the steep edge of the blackbody curve, and so infrared-bright galaxies can be studied up to high redshifts with existing instruments.