3.2. Summary of Results
These results, in particular the general correlation between radio and FIR luminosity, suggest that the radio emission arising in the faint sub-mJy and microJy radio source population is largely associated with massive star formation in distant star forming galaxies. However, a significant fraction of all the sub-mJy sources (~ 30%) are also identified with low-luminosity AGN. The remaining 10-20% of the faint radio source population are assoiciated with either extremely faint (R > 25) optical identifications, or remain unidentified altogether - even in the HDF-N itself (I > 28). Note that these conclusions are dominated by the faintest (and therefore more numerous) microJy radio sources - for example, the AGN fraction increases rapidly at higher (sub-mJy) flux density limits. In addition, simply labeling sources as pure "starbursts" or pure "AGN" is a little misleading; it is quite possible (even likely) that both phenomena co-exist in some of these faint systems. When we label sources in this way, we are only identifying the dominant phenomenon that gives rise to the bulk of the observed radio emission.
Studies of nearby (nuclear) starburst galaxies such as M82 and Arp 220
give us some idea of how the radio emission arises in these star
forming galaxies. According to conventional theories, the chief
ingredients are supernova (SN) events associated with massive star
formation (see Marcaide this lecture series), and in particular, the
global acceleration of cosmic ray electrons via shocks associated with
these events. The total radio luminosity of a "normal" galaxy is
therefore a direct measure of the SN event rate, and in turn, the star
formation rate (SFR) of massive stars (e.g.
Condon 1992).
In this
scenario, the tight correlation between the FIR and radio luminosity of
star forming galaxies is explained by the FIR emission arising from the
absorption and re-radiation (via dust) of the intense uv emission also
associated with massive stars. By assuming an Initial Mass Function
(e.g. a Salpeter IMF) for the stellar population, and some scaling
factors based on local observations of the Milky Way, radio
observations provide unbiased estimates of the SFR that are largely
unaffected by extinction due to dust. The SFR inferred in this way for
M82 and Arp 220 are ~ 10 and 100
M/yr respectively.
The levels of radio and FIR emission observed for the more most distant
radio sources in the HDF implies much higher star formation rates:
~ 1000 M
/yr.