High frequency radio SEDs and polarization fractions of sources in an
  ACT survey field
  We present flux densities and polarization percentages of 159 radio galaxies
based on nearly simultaneous VLA observations at four frequencies from 5 to
43GHz. This sample is selected from the high-frequency Australia Telescope
20GHz (AT20G) survey and consists of all sources with S_{20GHz}>40mJy in an
equatorial field of the ACT survey. For a subset of 25 of these sources we used
the GBT to obtain 90GHz data. The goals of this program are: 1) a
characterization of the spectra, polarization and variability of high
frequency-selected radio sources; 2) extrapolating from the few GHz regime to
the ~150GHz regime of the ACT survey, allowing for more accurate removal of the
radio source signal in our particular field; and 3) providing a data set that
will allow more accurate modeling of the high-frequency radio source
contamination in current and future SZ and CMB experiments. We find that, as
expected, this sample consists of flatter spectrum and more compact or
point-like sources than low frequency-selected samples. In the K-band,
variability is typically <~20%, although there are exceptions. The higher
frequency data is well suited to the detection of extreme Giga-Hertz Peak
spectrum Sources (GPS). The inclusion of the 43GHz data causes the relative
fraction of inverted spectrum sources to go down and of peaked spectrum sources
to go up when compared with the AT20G survey results. The trend largely
continues with the inclusion of the 90GHz data, although ~10% of the sources
with GBT data show a spectral upturn from 43GHz to 90GHz. The measured
polarization fractions are typically <5%, although in some cases they are
measured to be up to ~20%. For the ~40% of the sample with detected polarized
flux in all four bands, the polarization fractions typically increase with
frequency. This trend is stronger for steeper spectrum sources as well as for
the lower flux density sources.
