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Why do compact active galactic nuclei at high redshift scintillate less?

Citation

Koay, JY and MacQuart, J-P and Rickett, BJ and Bignall, HE and Jauncey, DL and Pursimo, T and Reynolds, C and Lovell, JEJ and Kedziora-Chudczer, L and Ojha, R, Why do compact active galactic nuclei at high redshift scintillate less?, Astrophysical Journal, 756, (1) Article 29. ISSN 0004-637X (2012) [Refereed Article]

Copyright Statement

Copyright 2012 The American Astronomical Society

DOI: doi:10.1088/0004-637X/756/1/29

Abstract

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 μas-scale cores or a decrease in the flux densities of the compact μas 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 (α8.44.9) with redshift, caused either by selection biases or AGN evolution, coupled with weaker ISS in the α8.44.9 < -0.4 sources. Selecting only the -0.4 < α8.44.9 < 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 μas at 4.9GHz with 99% confidence for all lines of sight and as low as ≲8 μas for sight lines to the most compact, ~10 μas sources.

Item Details

Item Type:Refereed Article
Keywords:cosmology: observations; galaxies: active; intergalactic medium; ISM: structure; quasars: general; radio continuum: ISM
Research Division:Physical Sciences
Research Group:Astronomical and Space Sciences
Research Field:Galactic Astronomy
Objective Division:Expanding Knowledge
Objective Group:Expanding Knowledge
Objective Field:Expanding Knowledge in the Physical Sciences
Author:Lovell, JEJ (Dr Jim Lovell)
ID Code:83110
Year Published:2012
Web of Science® Times Cited:7
Deposited By:Mathematics and Physics
Deposited On:2013-03-01
Last Modified:2017-11-03
Downloads:0

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