Megahertz peaked-spectrum sources in the Bo\"otes field I - a route
  towards finding high-redshift AGN?
  We present a 324.5MHz image of the NOAO Bo\"otes field that was made using
Very Large Array (VLA) P-band observations. The image has a resolution of
5.6x5.1arcsec, a radius of $2.05^\circ$ and a central noise of ~0.2mJy\beam.
Both the resolution and noise of the image are an order of magnitude better
than what was previously available at this frequency and will serve as a
valuable addition to the already extensive multiwavelength data that are
available for this field. The final source catalogue contains 1370 sources and
has a median 325 to 1400MHz spectral index of -0.72. Using a radio
colour-colour diagram of the unresolved sources in our catalogue, we identify
33 megahertz peaked-spectrum (MPS) sources. Based on the turnover frequency
linear size relation for the gigahertz peaked-spectrum (GPS) and compact
steep-spectrum (CSS) sources, we expect that the MPS sources that are compact
on scales of tens of milliarcseconds should be young radio loud active galactic
nuclei at high (z>2) redshifts. Of the 33 MPS sources, we were able to
determine redshifts for 24, with an average redshift of 1.3. Given that five of
the sources are at z>2, that the four faint sources for which we could not find
redshifts are likely at even higher redshifts and that we could only select
sources that are compact on a scale of ~5arcsec, there is encouraging evidence
that the MPS method can be used to search for high-redshift sources.
