  Discerning the exact nature of the sub-mJy radio population has been
historically difficult due to the low luminosity of these sources at most
wavelengths. Using deep ground based optical follow-up and observations from
the Spitzer Space Telescope we are able to disentangle the radio-selected
Active Galactic Nuclei (AGN) and Star Forming Galaxy (SFG) populations for the
first time in a deep multi-frequency VLA/MERLIN Survey of the 13^H
XMM-Newton/Chandra Deep Field. The discrimination diagnostics include radio
morphology, radio spectral index, radio/near-IR and mid-IR/radio flux density
ratios. We are now able to calculate the extragalactic Euclidean normalised
source counts separately for AGN and SFGs. We find that while SFGs dominate at
the faintest flux densities and account for the majority of the up-turn in the
counts, AGN still make up around one quarter of the counts at ~5 uJy (1.4 GHz).
Using radio luminosity as an unobscured star formation rate (SFR) measure we
are then able to examine the comoving SFR density of the Universe up to z=3
which agrees well with measures at other wavelengths. We find a rough
correlation of SFR with stellar mass for both the sample presented here and a
sample of local radio-selected SFGs from the 6df-NVSS survey. This work also
confirms the existence of, and provides alternative evidence for, the evolution
of distribution of star formation by galaxy mass: ``downsizing''. As both these
samples are SFR-selected, this result suggests that there is a maximum SFR for
a given galaxy that depends linearly on its stellar mass. The low
``characteristic times'' (inverse specific SFR) of the SFGs in our sample are
similar to those of the 6dF-NVSS sample, implying that most of these sources
are in a current phase of enhanced star formation.
