Multi-frequency VLBI observations of faint gigahertz peaked spectrum sources

We present the data and analysis of VLBI observations at 1.6, 5 and 15 GHz of a sample of faint Gigahertz Peaked Spectrum (GPS) sources selected from the Westerbork Northern Sky Survey (WENSS). The 5 GHz observations involved a global array of 16 stations and yielded data on the total sample of 47 sources. A subsample of 26 GPS sources with peak frequencies > 5 GHz and/or peak flux densities > 125 mJy was observed with the VLBA at 15 GHz. A second subsample of 29 sources, with peak frequencies <5 GHz, was observed at 1.6 GHz using a 14 station global VLBI array. In this way, 44 of the 47 sources (94%) in the sample were observed above and at or below their spectral peak. Spectral decomposition allowed us to identify 3, 11, 7, and 2 objects as compact symmetric objects, compact doubles, core-jet and complex sources respectively. However, many of the sources classified as compact double or core-jet sources show only two components making their classification rather tentative. This may explain why the strong morphological dichotomy of GPS quasars and galaxies found for radio-bright GPS sources, is not as clear in this faint sample.Comment: Latex, 18 pages, 8 figures; MNRAS, accepted. The paper, with higher quality figures, may also be obtained from http://www.ast.cam.ac.uk/~snellen . Minor comments of referee incorporate

On the evolution of young radio-loud AGN

This paper describes an investigation of the early evolution of extragalactic radio sources using samples of faint and bright Gigahertz Peaked Spectrum (GPS) and Compact Steep Spectrum (CSS) radio galaxies. Correlations found between their peak frequency, peak flux density and angular size provide strong evidence that synchrotron self absorption is the cause of the spectral turnovers, and indicate that young radio sources evolve in a self-similar way. In addition, the data seem to suggest that the sources are in equipartition while they evolve. If GPS sources evolve to large size radio sources, their redshift dependent birth-functions should be the same. Therefore, since the lifetimes of radio sources are thought to be short compared to the Hubble time, the observed difference in redshift distribution between GPS and large size sources must be due to a difference in slope of their luminosity functions. We argue that this slope is strongly affected by the luminosity evolution of the individual sources. A scenario for the luminosity evolution is proposed in which GPS sources increase in luminosity and large scale radio sources decrease in luminosity with time. This evolution scenario is expected for a ram-pressure confined radio source in a surrounding medium with a King profile density. In the inner parts of the King profile, the density of the medium is constant and the radio source builds up its luminosity, but after it grows large enough the density of th e surrounding medium declines and the luminosity of the radio source decreases. A comparison of the local luminosity function (LLF) of GPS galaxies with that of extended sources is a good test for this evolution scenario [abridged].Comment: LaTeX, 11 pages, 8 figures; Accepted by MNRAS. Related papers may be found at http://www.ast.cam.ac.uk/~snellen . Valuable comments of referee incorporated. More discussion on simulation

The X-ray properties of young radio-loud AGN

We present XMM-Newton observations of a complete sample of five archetypal young radio-loud AGN, also known Gigahertz Peaked Spectrum (GPS) sources. They are among the brightest and best studied GPS/CSO sources in the sky, with radio powers in the range L_{5GHz}=10^{43-44} erg/s and with 4 sources having measured kinematic ages of 570 to 3000 yrs. All sources are detected, and have 2-10 keV luminosities from 0.5 to 4.8x10^{44} erg/s. In comparison with the general population of radio galaxies, we find that: 1) GPS galaxies show a a range in absorption column densities similar to other radio galaxies. We therefore find no evidence that GPS galaxies reside in significantly more dense circumnuclear environment, such that they could be hampered in their expansion. 2) The ratio of radio to X-ray luminosity is significantly higher than for classical radio sources. This is consistent with an evolution scenario in which young radio sources are more efficient radio emitters than large extended objects at a constant accretion power. 3) Taking the X-ray luminosity of radio sources as a measure of ionisation power, we find that GPS galaxies are significantly underluminous in their [OIII]_{5007 Angstrom}, including a weak trend with age. This is consistent with the fact that the Stroemgren sphere should still be expanding in these young objects. This would mean that here we are witnessing the birth of the narrow line region of radio-loud AGN.Comment: 11 pages, 6 figures. Accepted for publication by the MNRA

On the lives of extra-galactic radio sources: the first 100,000 years

In this paper we discuss the early phase of radio source evolution as represented by Gigahertz Peaked Spectrum (GPS) and Compact Steep Spectrum (CSS) radio sources. Correlations between their spectral peak and angular size strongly suggest that the spectral turnovers are caused by synchrotron self absorption, and indicate that young radio sources evolve in a self similar way. We argue that the evolution of a radio source during its first 10^5 years is qualitatively very different from that during the rest of its life-time. This may be caused by the difference in the density gradient of the intra-galactic medium inside and outside the core-radius of the host galaxy.Comment: LaTeX, 7 pages & 2 figs. Invited talk at `Lifecycles of Radio Galaxies' workshop, ed J. Biretta et al., New Astronomy Reviews. More papers of the authors at http://www.ast.cam.ac.uk/~snelle

Parsec-scale radio structures in the nuclei of four Seyfert galaxies

We present 18-cm radio maps of four Seyfert nuclei, Mrk 1, Mrk 3, Mrk 231 and Mrk 463E, made with the European VLBI Network (EVN). Linear radio structures are present in three out of four sources on scales of ~100 pc to ~1 kpc, and the 20-mas beam of the EVN enables us to resolve details within the radio structures on scales of <10 pc. Mrk 3 was also imaged using MERLIN and the data combined with the EVN data to improve the sensitivity to extended emission. We find an unresolved flat-spectrum core in Mrk 3, which we identify with the hidden Seyfert 1 nucleus in this object, and we also see marked differences between the two highly-collimated radio jets emanating from the core. The western jet terminates in a bright hotspot and resembles an FRII radio structure, whilst the eastern jet has more in common with an FRI source. In Mrk 463E, we use the radio and optical structure of the source to argue that the true nucleus lies approximately 1 arcsec south of the position of the radio and optical brightness peaks, which probably represent a hotspot at the working surface of a radio jet. The EVN data also provide new evidence for a 100-pc radio jet powering the radio source in the Type 1 nucleus of Mrk 231. However, the Seyfert 2 galaxy Mrk 1 shows no evidence for radio jets down to the limits of resolution (~10 pc). We discuss the range of radio source size and morphology which can occur in the nuclei of Seyfert galaxies and the implications for Seyfert unification schemes and for radio surveys of large samples of objects.Comment: 23 pages, 7 postscript figures (supplied as separate files), uses AAS aaspp4 LaTeX style file, to appear in the 10 June 1999 issue of The Astrophysical Journa

A very brief description of LOFAR - the Low Frequency Array

LOFAR (Low Frequency Array) is an innovative radio telescope optimized for the frequency range 30-240 MHz. The telescope is realized as a phased aperture array without any moving parts. Digital beam forming allows the telescope to point to any part of the sky within a second. Transient buffering makes retrospective imaging of explosive short-term events possible. The scientific focus of LOFAR will initially be on four key science projects (KSPs): 1) detection of the formation of the very first stars and galaxies in the universe during the so-called epoch of reionization by measuring the power spectrum of the neutral hydrogen 21-cm line (Shaver et al. 1999) on the ~5' scale; 2) low-frequency surveys of the sky with of order $10^8$ expected new sources; 3) all-sky monitoring and detection of transient radio sources such as gamma-ray bursts, x-ray binaries, and exo-planets (Farrell et al. 2004); and 4) radio detection of ultra-high energy cosmic rays and neutrinos (Falcke & Gorham 2003) allowing for the first time access to particles beyond 10^21 eV (Scholten et al. 2006). Apart from the KSPs open access for smaller projects is also planned. Here we give a brief description of the telescope.Comment: 2 pages, IAU GA 2006, Highlights of Astronomy, Volume 14, K.A. van der Hucht, e