Overdensities of 24um Sources in the Vicinities of High-Redshift Radio
  Galaxies
  We present a statistical study of the environments of 63 high-redshift radio
galaxies (HzRGs) between redshifts 1<z<5.2, using the 24um, waveband of the
MIPS instrument aboard the Spitzer Space Telescope. Using a counts-in-cell
analysis, a statistically significant source overdensity is found in 1.75arcmin
radius circular cells centred on the HzRGs when compared to reference fields.
We report an average overdensity of delta (= {N}_{targets} / {N}_{reference}) =
2.2 +/- 1.2 at a flux density cut of f24um=0.3mJy. This result implies that
HzRGs are likely to lie in protoclusters of active and star-forming galaxies at
high redshift. Over 95% of our targeted HzRGs lie in higher than average
density fields. Further, 20 (32%) of our selected fields are found to be
overdense to at least a 3sigma significance, of which 9 are newly identified
protocluster candidates. We observe a weak correlation between redshift and
24um, source density, and discuss the populations being probed at different
redshifts. In our uniformly selected sample, which was designed to cover two
orders of magnitude in radio luminosity throughout z=1-4, we find that the
24um, source density does not depend on radio luminosity. We also compare this
result with recent work describing IRAC source overdensities around the same
HzRGs and find correlations between the results.
