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Astron. Astrophys. 341, 44-57 (1999)

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1. Introduction

Gigahertz Peaked Spectrum (GPS, e.g. Spoelstra et al. 1985) radio sources are characterized by their convex radio spectra peaking at frequencies near 1 GHz. At frequencies above the peak, the radio spectrum has a power-law shape which is typical of an optically thin synchrotron emitting radio source (e.g. Pacholczyk 1970). The turn-over of the radio spectrum at low frequencies can be due to Synchrotron Self Absorption (SSA) in high density parts of a compact radio source (e.g. Scott & Readhead 1977), free-free absorption in a sheet of clouds surrounding the radio source (Bicknell, Dopita & O'Dea 1997) or ionized clouds within it (Begelman 1998).

VLBI observations (e.g. Dallacasa et al. 1995, Stanghellini et al. 1997, Snellen 1997) show that GPS sources are compact radio sources with linear sizes up to a few hundred pc and often with a symmetrical double radio structure (Compact Symmetric Objects, CSO, e.g. Wilkinson et al. 1994, Readhead et al. 1996). There are two popular scenarios to explain the origin of GPS sources. The first is that GPS sources are very young radio sources, that have just started to develop their radio lobes (Phillips & Mutel 1982). They are expected eventually to grow into large double-lobed radio sources, perhaps even into the Mpc-sized Giant Radio Galaxies (GRGs). Because small radio sources are much more numerous than large radio sources, strong negative luminosity evolution as they increase in size is required (e.g. Fanti et al. 1995, Readhead et al. 1996, O'Dea & Baum 1997). Reynolds & Begelman (1997) propose that this can be achieved by intermittency of the jet-production in the AGN on timescales of [FORMULA] yr.

The second scenario explains GPS sources as radio sources which are confined by a high density ISM (`frustrated' radio sources, e.g. van Breugel et al. 1984, O'Dea et al. 1991). In this scenario, the jet is not able to escape the dense surrounding material within its lifetime, either because it is repeatedly reflected off dense clouds or because the ram-pressure of the surrounding medium is high enough to practically stall the progress of the head of the jet. The radio source is therefore confined to a small central region.

Roughly 10% of known GPS sources show extended (kpc-scale) radio emission (Baum et al. 1990, Stanghellini et al. 1990, O'Dea 1998). Baum et al. (1990) propose that the GPS source in these sources may either result from a disruption of an ongoing jet flow (`smothered' jets), or from recurrent nuclear radio activity. In both these models, the central (GPS) radio sources are young and the extended radio emission is older and fading since it is disconnected from the jet flow. A convincing example of such a source is B0108+388, where motions of the radio components, observed with multi-epoch VLBI observations, imply that the pc-scale radio structure cannot be much older than [FORMULA] yr (Owsianik et al. 1998). The observed velocity of these components ([FORMULA]) strongly suggests that their outflow is not hampered by a high density medium. However, Carilli et al. (1998) observe strong redshifted HI absorption towards B0108+388, which, contrary to the expectation, suggests that the source actually is surrounded by a dense environment. B0108+388 also exhibits a faint kpc-scale radio structure (Baum et al. 1990, Carilli et al. 1998). This is considered as a strong indication that the nucleus is recurrently radio active.

Sources such as B0108+388 strongly suggest that some radio sources have recurrent phases of radio activity. Outstanding questions are how common recurrence is, and on what typical timescales it takes place? To answer these questions, it is important to find more of these apparently rejuvenated sources. A low-frequency search for extended radio emission on kpc-scales or larger around GPS/CSS sources should be able to accomplish this goal.

In this paper, we present new radio data of the region centered on the GPS source B1144+352, obtained with the Westerbork Synthesis Radio Telescope (WSRT) at 1.4 GHz, in addition to available radio data from the WENSS, NVSS and FIRST surveys. The WSRT radio map shows that B1144+352, apart from being a GPS source, also appears to be the host galaxy of a 1.2 Mpc large radio source. We further present unpublished ROSAT HRI data of the host galaxy, which shows it to be a luminous X-ray source. To calculate physical properties we adopt values of [FORMULA] km s-1 Mpc-1 and [FORMULA] throughout this paper. We define a radio spectral index [FORMULA] as [FORMULA].

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© European Southern Observatory (ESO) 1999

Online publication: November 26, 1998