We present multiwavelength observations for a large sample of microjansky radio sources detected in ultradeep 1.4 GHz maps centered on the Hubble Deep Field-North ( HDF-N ) and the Hawaii Survey Fields SSA13 and SSA22 .
Our spectroscopic redshifts for 169 radio sources reveal a flat median redshift distribution , and these sources are hosted by similarly luminous optical L ^ { \ast } galaxies , regardless of redshift .
We suggest that the absence of low optical luminosity galaxies at low redshifts , where there are no selection effects , is due to small galaxies not being efficient at retaining the cosmic rays necessary to host microjansky radio sources .
This is a serious concern for radio estimates of the local star formation rate density , as a substantial fraction of the ultraviolet luminosity density is generated by sub- L ^ { * } galaxies at low redshifts .
From our submillimeter measurements for 278 radio sources , we find error-weighted mean 850 \mu m fluxes of 1.72 \pm 0.09 mJy for the total sample , 2.37 \pm 0.13 mJy for the optically-faint ( I > 23.5 ) subsample , and 1.04 \pm 0.13 mJy for the optically-bright ( I < 23.5 ) subsample .
We significantly ( > 3 \sigma ) detect in the submillimeter 50 of the radio sources , 38 with I > 23.5 .
Spectroscopic redshifts for three of the I < 23.5 submillimeter-detected radio sources are in the range z = 1.0 –3.4 , and all show AGN signatures .
Using only the submillimeter mapped regions we find that 69 \pm 9 % of the submillimeter-detected radio population are at I > 23.5 .
We also find that 66 \pm 7 % of the S _ { 850 \mu m } > 5 mJy ( > 4 \sigma ) sources are radio-identified .
We use our spectroscopic sample to determine the evolution with redshift of the radio power , and hence the far-infrared ( FIR ) luminosity ( through the local FIR-radio correlation ) .
We find that millimetric redshift estimates at low redshifts are best made with a FIR template intermediate between a Milky Way type galaxy and a starburst galaxy , and at high redshifts with an Arp 220 template .