Rejuvenated radio galaxies J0041+3224 and J1835+6204: how long can the quiescent phase of nuclear activity last?

Mon. Not. R. Astron. Soc. 424, 1061–1076 (2012) doi:10.1111/j.1365-2966.2012.21279.x Rejuvenated radio galaxies J0041+3224 and J1835+6204: how long can the quiescent phase of nuclear activity last? C. Konar,1 M. J. Hardcastle,2 M. Jamrozy,3 J. H. Croston4 and S. Nandi5 1 Institute of Astronomy and Astrophysics, Academia Sinica, National Taiwan University, Taipei 10617, Taiwan 2 School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB 3 Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30244 Kraków, Poland 4 School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ 5 Aryabhatta Research Institute of Observational Sciences, Manora Peak, Nainital 263 129, India Accepted 2012 May 9. Received 2012 May 9; in original form 2012 February 10 We present radio observations of two well-known double–double radio galaxies, J0041+3224 and J1835+6204, at frequencies ranging from 150 to 8460 MHz, using both the Giant Metrewave Radio Telescope and the Very Large Array. These observations, over a large radio frequency range, enable us to determine the spectra of the inner and outer lobes. Our detailed spectral ageing analysis of their inner and outer lobes demonstrates that the outer doubles of double–double radio galaxies are created by the previous cycle of activity, while the inner doubles are due to the present cycle of activity. The (core subtracted) spectra of the inner doubles of both sources are power laws over a large frequency range. We found that the duration of the quiescent phase of J0041+3224 is between 4 and 28 per cent of the active phase of the previous activity. The outer north-western lobe of J1835+6204 has a compact hotspot and the regions of both the outer hotspots have close to power-law (rather than curved) spectra, which indicates that the outer lobes are still fed by jet material ejected in the previous episode just before the central engine stopped powering the jet. We estimate that the duration of the quiescent phase of J1835+6204 is 5 per cent of the duration of the active phase of the previous activity. Therefore, we conclude that the duration of the quiescent phase can be as short as a few per cent of the active phase in radio galaxies of this type. Key words: galaxies: active – galaxies: individual: J0041+3224 – galaxies: individual: J1835+6204 – galaxies: nuclei – radio continuum: galaxies. 1 I N T RO D U C T I O N It is established beyond doubt that jet-forming activity in radio galaxies is episodic in nature. This episodic jet-forming activity often gives rise to Double–Double Radio Galaxies (DDRGs), which are defined to be those having a pair of double radio sources with a common centre, and are thought to occur when a new epoch of jet activity is triggered in a radio galaxy with older lobes still visible from the activity of the previous epoch (Schoenmakers et al. 2000a). At present, about two dozen DDRGs are known in the literature (see Saikia & Jamrozy 2009, for a review). Although the DDRGs are more common, in fact more than two episodes of jet-forming activity are physically plausible: Brocksopp et al. (2007) reported the first example of three such episodes in a radio galaxy, and recently Hota et al. (2011) reported a possible case of a triple–double radio galaxy dubbed ‘SPECA’ (J1409−0302).  E-mail:  C 2012 The Authors C 2012 RAS Monthly Notices of the Royal Astronomical Society  The very existence of well-shaped and well-confined radio galaxy lobes suggests that there must be some medium confining the lobes either through ram pressure or thermal pressure. Even radio galaxies whose optical environments are known to be comparatively poor are found to possess X-ray-emitting hot gas environments, corresponding to group or cluster-scale X-ray luminosities, whose gas properties can now be modelled in detail (see Hardcastle & Worrall 1999, 2000; Worrall & Birkinshaw 2000; Croston et al. 2003, 2004, 2008; Evans et al. 2005; Belsole et al. 2007; Konar et al. 2009). This environment emits via thermal bremsstrahlung in the X-ray band, and can be classified as a poor cluster to group scale environment. The magnetized relativistic plasma (MRP) of the radio lobes does not mix well with the external thermal gas in the environment, as can be inferred from limits on internal depolarization of lobes at low frequencies, nor does it diffuse out completely into the IntraCluster Medium (ICM) even long after the jet stops feeding the lobes. Kaiser, Schoenmakers & Röttgering (2000) argued against the entrainment of material from the surrounding hot-gas environment through the lobe periphery. However, the observed ABSTRACT 1062 C. Konar et al. galaxies, injection index in different episodes and particle acceleration at hotspots, and dynamics of DDRGs through radio and X-ray observations will be published soon by Konar et al. (in preparation(a)), Konar & Hardcastle (in preparation) and Konar et al. (in preparation(b)), respectively. In this paper, we have carried out a multifrequency radio study of two DDRGs, namely J0041+3224 and J1835+6204. We present the Giant Meterwave Radio Telescope (GMRT) and Very Large Array (VLA) observations in Section 2, our observational results in Section 3, our spectral ageing analysis in Section 4, a discussion in Section 5 and concluding remarks in Section 6. The cosmological parameters that we have used are H 0 = 71 km s−1 Mpc−1 , M = 0.27 and vac = 0.73 (Spergel et al. 2003). In this cosmology, 1 arcsec corresponds to 5.733 kpc for the source J0041+3224 situated at an (estimated, see below) luminosity distance DL = 2486.3 Mpc. For J1835+6204, 1 arcsec corresponds to 6.207 kpc based on its luminosity distance of DL = 2955.7 Mpc. 2 O B S E RVAT I O N S A N D DATA R E D U C T I O N The images published in this paper are from the GMRT and the VLA. All GMRT data are from our observations with project code 10CKa01. All VLA data are public data from the VLA archive. The details of the observations with both telescopes are given in Table 1. The shortest baseline for the C and D configurations of the VLA is 35 m. Our target sources, J0041+3224 and J1835+6204 have sizes ∼2.8 and ∼3.7 arcmin, respectively; however, in both sources, at high frequencies, no single structure with dominant flux density is larger than 2 arcmin. As all the VLA maps (published by Saikia, Konar & Kulkarni 2006) used for flux density measurements of J0041+3224 were made from C-array data, where the largest angular size that can be mapped without loss of flux is 3 arcmin, our measurements for J0041+3224 should not be susceptible to loss of flux due to lack of short spacings in the uv coverage. Although the 8.4-GHz map of J1835+6204 (Fig. 2) looks continuous from hotspot to hotspot, this is (at least partially) due to the effects of comparatively poor resolution, which is why we do not see such continuity in the 4.8-GHz images at somewhat higher resolution. Th (...truncated)