Why Do Compact Active Galactic Nuclei at High Redshift Scintillate Less?
  The fraction of compact active galactic nuclei (AGNs) that exhibit
interstellar scintillation (ISS) at radio wavelengths, as well as their
scintillation amplitudes, have been found to decrease significantly for sources
at redshifts z > 2. This can be attributed to an increase in the angular sizes
of the \muas-scale cores or a decrease in the flux densities of the compact
\muas cores relative to that of the mas-scale components with increasing
redshift, possibly arising from (1) the space-time curvature of an expanding
Universe, (2) AGN evolution, (3) source selection biases, (4) scatter
broadening in the ionized intergalactic medium (IGM) and intervening galaxies,
or (5) gravitational lensing. We examine the frequency scaling of this redshift
dependence of ISS to determine its origin, using data from a dual-frequency
survey of ISS of 128 sources at 0 < z < 4. We present a novel method of
analysis which accounts for selection effects in the source sample. We
determine that the redshift dependence of ISS is partially linked to the
steepening of source spectral indices ({\alpha}^8.4_4.9) with redshift, caused
either by selection biases or AGN evolution, coupled with weaker ISS in the
{\alpha}^8.4_4.9 < -0.4 sources. Selecting only the -0.4 < {\alpha}^8.4_4.9 <
0.4 sources, we find that the redshift dependence of ISS is still significant,
but is not significantly steeper than the expected (1+z)^0.5 scaling of source
angular sizes due to cosmological expansion for a brightness temperature and
flux-limited sample of sources. We find no significant evidence for scatter
broadening in the IGM, ruling it out as the main cause of the redshift
dependence of ISS. We obtain an upper limit to IGM scatter broadening of <
110\muas at 4.9 GHz with 99% confidence for all lines of sight, and as low as <
8\muas for sight-lines to the most compact, \sim 10\muas sources.
