  (abridged) We present multiwavelength observations for a large sample of
microjansky radio sources detected in ultradeep 1.4GHz 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*
galaxies, regardless of redshift. 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 850micron 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\pm9% of the submillimeter-detected
radio population are at I>23.5. We also find that 66\pm7% of the S850>5 mJy
(>4\sigma) sources are radio-identified. 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 Arp220 template.

Astrophysics