Flat-spectrum radio sources with kpc-scale structure

Abstract

We have carried out the first systematic study of flat‐spectrum radio sources which have significant structure on angular scales 90–300 milliarcsec (mas), corresponding to linear scales of ∼ 0.5–1.5 kpc at a redshift of 0.5. The principal aim of the study was to search for multiple gravitational imaging of compact radio components with image separations smaller than that of the smallest galaxy lens system known (335 mas) and corresponding to masses appropriate to compact/dwarf galaxies and to the bulges of spiral galaxies. A secondary aim was the morphological classification of a sample of flat‐spectrum sources with kpc‐scale structure. We particularly wanted to find out the frequency of occurrence of compact symmetric objects (CSOs: linear size < 1 kpc) and medium symmetric objects (MSOs: linear size < 15 kpc) which are believed to be the precursors of the large classical double radio sources. The parent sample consisted of 1665 flat‐spectrum sources selected from two VLA surveys made with ∼ 200 mas resolution (the Jodrell Bank–VLA Astrometric Survey — JVAS — and the Cosmic Lens All‐Sky Survey — CLASS), from which we used visibility data to select sources (55 in total) with significant structure in the size range 90–300 mas. Sources with multiple compact components having a flux density ratio < 8:1 could be high‐magnification lens systems, CSO/MSOs or objects with strong jets. A step‐by‐step observational filtering process at successively higher angular resolution was employed to classify the 55‐source subsample. Initial MERLIN observations at 5 GHz (50 mas resolution) enabled us to classify ∼ 75 per cent of the sources. The remaining sources were observed with the VLBA at 5 GHz (2 mas resolution) and in a few cases with MERLIN at 22 GHz (10 mas resolution). The resulting maps show that the majority of flat‐spectrum sources with kpc‐scale structure are asymmetric core–jets. The remaining sources include 23 new CSO/MSOs, a much smaller fraction of the parent sample (1.4 per cent) than is found in VLBI surveys of flat‐spectrum sources (5–10 per cent). About half of the new CSO/MSOs constitute a hitherto unknown population: bright core CSO/MSOs. No definite high‐magnification gravitational lenses were found. The implication is that the optical depth to lensing with image separations in the range 90–300 mas is several times less than on arcsec scales.

Investigation of flat spectrum radio sources by the interplanetary scintillation method at 111 MHz

“…The study of these 55 sources is not complete without low frequency observations (∼100 MHz), as was pointed out in Augusto et al (1998), where the spectra of most sources have no data at all below ∼300 MHz. The turnovers in the spectra of compact components in these sources must be found, to give a physical meaning to all 55 sources, namely by fitting synchrotron emission spectra for them all.…”

“…We observed 50 sources from the sample of 55 sources in Augusto et al (1998) but only got scintillating flux density data for 48 (87% completeness): five sources have not been observed because of their high declinations (δ > 70 • ), resulting in too poor elongations 9 -B0205+722, B0352+825, B0817+710, B0916+718, B1241+735; two other (B0905+420 and B1003+174) were confused by nearby strong VLBI sources (B0904+417 and B1004+178, respectively), so no information about S c is available for these either.…”

“…A systematic search for dominant structure on 0.09-0.3 scales in large flat-spectrum radio source samples was made by Augusto et al (1998). Fifty-five radio sources were selected from a parent sample containing 1665 strong flat-spectrum radio sources (S 8.4 GHz > 100 mJy; α 4.85 1.4 < 0.5, S ν ∝ ν −α ).…”

“…2, we present both the data collection and reduction, while in Sect. 4 we compile the IPS 111 MHz results from observations of 48 of the 55 Augusto et al (1998) sources, for which we derive either a scintillating flux density estimate (13) or an upper limit (35). We also include in this section, as a case study, the detailed spectrum analysis for B0821+394.…”

“…Interplanetary scintillation observations of 48 of the 55 Augusto et al (1998) flat spectrum radio sources were carried out at 111 MHz using the interplanetary scintillation method on the Large Phased Array (LPA) in Russia. Due to the large size of the LPA beam (1 • × 0.5 • ) a careful inspection of all possible confusion sources was made using extant large radio surveys: 37 of the 48 sources are not confused.…”