Dual-frequency observations of 140 compact, flat-spectrum active galactic nuclei for scintillation-induced variability
Koay, JY and Macquart, JP and Rickett, BJ and Bignall, HE and Lovell, JEJ and Reynolds, C and Jauncey, DL and Pursimo, T and Kedziora-Chudczer, L and Ojha, R, Dual-frequency observations of 140 compact, flat-spectrum active galactic nuclei for scintillation-induced variability, Astronomical Journal, 142, (4) pp. 1-21. ISSN 0004-6256 (2011) [Refereed Article]
The 4.9 GHz Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey detected a drop in interstellar
scintillation (ISS) for sources at redshifts z 2, indicating an apparent increase in angular diameter or a decrease
in flux density of the most compact components of these sources relative to their extended emission. This can result
from intrinsic source size effects or scatter broadening in the intergalactic medium (IGM) in excess of the expected
(1+z)1/2 angular diameter scaling of brightness temperature limited sources resulting from cosmological expansion.
We report here 4.9 GHz and 8.4 GHz observations and data analysis for a sample of 140 compact, flat-spectrum
sources which may allow us to determine the origin of this angular diameter–redshift relation by exploiting their
different wavelength dependences. In addition to using ISS as a cosmological probe, the observations provide
additional insight into source morphologies and the characteristics of ISS. As in the MASIV Survey, the variability
of the sources is found to be significantly correlated with line-of-sight Há intensities, confirming its link with ISS.
For 25 sources, time delays of about 0.15–3 days are observed between the scintillation patterns at both frequencies,
interpreted as being caused by a shift in core positions when probed at different optical depths. Significant correlation
is found between ISS amplitudes and source spectral index; in particular, a large drop in ISS amplitudes is observed
at á < −0.4 confirming that steep spectrum sources scintillate less. We detect a weakened redshift dependence of
ISS at 8.4 GHz over that at 4.9 GHz, with the mean variance at four-day timescales reduced by a factor of 1.8 in the
z > 2 sources relative to the z < 2 sources, as opposed to the factor of three decrease observed at 4.9 GHz. This
suggests scatter broadening in the IGM, but the interpretation is complicated by subtle selection effects that will be
explored further in a follow-up paper.
galaxies: active; intergalactic medium; ISM: structure; methods: data analysis; quasars: general; radio continuum: ISM