A&A 526, A148 (2011) Astronomy DOI: 10.1051/0004-6361/201015302 & c ESO 2011 Astrophysics Dying radio galaxies in clusters M. Murgia1,P.Parma2, K.-H. Mack2,H.R.deRuiter2,R.Fanti2,F.Govoni1, A. Tarchi1, S. Giacintucci3, and M. Markevitch3 1 INAF – Osservatorio Astronomico di Cagliari, Loc. Poggio dei Pini, Strada 54, 09012 Capoterra (CA), Italy e-mail: [email protected] 2 INAF – Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy 3 Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA Received 30 June 2010 / Accepted 29 October 2010 ABSTRACT Aims. We present a study of five “dying” nearby (z ≤ 0.2) radio galaxies belonging to both the WENSS minisurvey and the B2 bright catalogs WNB1734+6407, WNB1829+6911, WNB1851+5707, B2 0120+33, and B2 1610+29. Methods. These sources have been selected on the basis of their extremely steep broad-band radio spectra, which strongly indicates that either these objects belong to the rare class of dying radio galaxies or we are observing “fossil” radio plasma remaining from a previous instance of nuclear activity. We derive the relative duration of the dying phase from the fit of a synchrotron radiative model to the radio spectra of the sources. Results. The modeling of the integrated spectra and the deep spectral index images obtained with the VLA confirmed that in these sources the central engine has ceased to be active for a significant fraction of their lifetime, although their extended lobes have not yet completely faded away. We found that WNB1851+5707 is in reality composed of two distinct dying galaxies, which appear blended together as a single source in the WENSS. In the cases of WNB1829+6911 and B2 0120+33, the fossil radio lobes are seen in conjunction with a currently active core. A very faint core is also detected in a MERLIN image of WNB1851+5707a, one of the two dying sources composing WNB1851+5707. We found that all sources in our sample are located (at least in projection) at the center of an X-ray emitting cluster. Conclusions. Our results suggest that the duration of the dying phase for a radio source in a cluster can be significantly higher than that of a radio galaxy in the field, although no firm conclusions can be drawn because of the small number statistics involved. The simplest interpretation of the tendency for dying galaxies to be found in clusters is that the low-frequency radio emission from the fading radio lobes lasts longer if their expansion is somewhat reduced or even stopped. Another possibility is that the occurrence of dying sources is higher in galaxy clusters. We argue that radio sources in dense environments, such as the center of cooling core clusters, may have a peculiar accretion mode which results in a bursting duty cycle sequence of active and quiescent periods. This result could have important implications for theories of the life cycles of radio sources and AGN feedback in clusters of galaxies but awaits confirmation from future observations of larger, statistically significant, samples of objects. Key words. radio continuum: galaxies – galaxies: active – galaxies: clusters: general 1. Introduction such sources was B2 0924+30, which was illustrated by Cordey (1987). It is also possible that radio galaxies may be active in- Dying radio galaxies represent an interesting, but still largely termittently or even that jets flicker before eventually going off unexplored, stage of the active galactic nuclei evolution. During completely. In this scenario, one expects to observe fossil radio their active stage, which may last several 107 years, the strong ra- plasma remaining from an earlier active epoch, along with newly dio sources associated with elliptical galaxies are supplied with restarting jets. The best case of fossil radio lobes seen with a cur- energy from active galactic nuclei via plasma beams or jets. rently active galaxy is 3C 338 (see e.g. Gentile et al. 2007). The Owing to the continuous accumulation of new particles, the total very steep spectrum lobes of this source are clearly disconnected spectra of the active radio sources are usually well approximated from the currently active jets. by a power law over a wide range of frequencies. The injection We classify a radio source as dying not only if the fossil lobes of energy also sustains the growth of these radio sources that are detached from the AGN and there is no evidence of nuclear is governed by the balance between the internal pressure in the activity (trivial case), but also if some kind of nuclear activity is radio lobes and the pressure in the hot X-ray emitting external present but the fossil lobes still dominate the total source’s radio medium into which they must expand (Scheuer 1974). luminosity1. Hence, in what follows we refer to both dying and At some point, however, the activity in the nuclei stops or 1 falls to such a low level that the plasma outflow can no longer be This choice is motivated by the following two considerations. First, sustained and the radio source is expected to undergo a period even if the nuclear component we are observing is really produced by of fading (dying phase) before it disappears completely. In the a new couple of restarting radio jets, we would never know a priori if this new activity would be able to last enough to regenerate a radio dying phase, the radio core, well-defined jets, and compact hot- galaxy similar to the previous event of radio activity of which the fossil spots will disappear because they are the structures produced by lobes represent the remains. Second, it may be not obvious to recognize continuing activity. On the other hand, the radio lobes may still whether a nuclear component is produced by the start of a new phase of remain detectable for a long time if they are subject only to ra- AGN activity or it is rather a fading radio core that is definitely turning- diative losses of the relativistic electrons. The first example of off. Article published by EDP Sciences A148, page 1 of 19 A&A 526, A148 (2011) re-starting sources if the fossil lobes are the dominant compo- Furthermore, we found that all the five sources are unambigu- nent. We suggest instead to reserve the term renewed source to ously associated with diffuse X-ray emission in the Rosat All indicate the case in which the new and the fossil components are Sky Survey (RASS). In all cases, the association is with a known roughly comparable in luminosity, as in the case of the double- cluster of galaxies. double radio sources (Schoenmakers et al. 2000; Saripalli et al. We report the results of this extensive campaign of radio ob- 2002, 2003). servations. In Sect. 2, we described the criteria of source selec- Given the comparatively short duration of the radio galaxy tion. The descriptions of the VLA and MERLIN observations are phenomenon, we could expect a large number of dying radio reported in Sects. 3 and 4, respectively. In Sect. 5, we present our sources. However, only a handful of dying radio galaxies in this analysis of the radio spectra. The X-ray environment is presented evolutionary stage are known. According to Giovannini et al. in Sect. 6. Finally, a summary of the work is given in Sect. 7. (1988), only a few percent of the radio sources in the B2 and Throughout this paper, we assume a ΛCDM cosmology with −1 −1 3C samples have the characteristics of a dying radio galaxy. We H0 = 71 km s Mpc , Ωm = 0.3, and ΩΛ = 0.7. note that these sources represent the last phase in the “life” of a radio galaxy. Therefore they must be well distinguished from the cluster radio halo/relic phenomenon, which is usually not 2. Source selection associated with an individual galaxy (see Ferrari et al. 2008,and 2.1. The spectral curvature of dying and restarting sources references therein). A possible explanation of the rarity of the dying radio galax- In the active stage, the total spectra of the radio galaxies are usu- ies may be their relatively fast spectral evolution during the fad- ally approximated well by a power law over a wide range of ing phase. Synchrotron losses and the inverse Compton scatter- frequencies. Spectral breaks at high frequencies are also often ing of the cosmic microwave background photons preferentially observed. The break frequency is due to the radiative losses of deplete the high-energy electrons. The fading lobes are expected the synchrotron electrons and can be related to the magnetic field −α to have very steep (α>1.3; S ν ∝ ν ) and convex radio spectra and age of the source. If the integrated spectrum of the radio characteristic of an electron population that has radiated away source is dominated by the emission of the radio lobes, which much of its original energy (Komissarov & Gubanov 1994). In accumulate the electrons injected by the jets, it can be modeled the absence of fresh particle injection, the high-frequency radio by a continuous injection model (CI) in which the low-frequency spectrum develops an exponential cutoff. At this point, the adi- spectral index αlow represents the injection spectral index of the abatic expansion of the radio lobes concurs to shift this spectral youngest electron populations, αinj, while the high-frequency break to lower frequencies and the source quickly disappears. spectral index is limited to αhigh ≤ αinj + 0.5 at frequencies On the other hand, if the source expansion is somehow reduced, greater than the break frequency (see e.g.