J/A+A/648/A9ivo://CDS.VizieR/J/A+A/648/A9doi:10.26093/cds/vizier.36480009CDSMorganti R.Oosterloo T.A.Brienza M.Jurlin N.Prandoni I.Orru E.,Shabala S.S.Adams E.A.K.Adebahr B.Best P.N.Coolen A.H.W.M.,Damstra S.de Blok W.J.G.de Gasperin F.Denes H.Hardcastle M.,Hess K.M.Hut B.Kondapally R.Kutkin A.M.Loose G.M.Lucero D.M.,Maan Y.Maccagni F.M.Mingo B.Moss V.A.Mostert R.I.J.Norden M.J.,Oostrum L.C.Roettgering H.J.A.Ruiter M.Shimwell T.W.Schulz R.,Vermaas N.J.Vohl D.van der Hulst J.M.van Diepen G.M.van Leeuwen J.,Ziemke J.2021-09-16T09:12:59Z2021-04-07T10:35:46ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frradio-sourcesSupermassive black holes at the centres of galaxies can cycle through periods of activity and quiescence. Characterising the duty cycle of active galactic nuclei (AGN) is crucial for understanding the impact of the energy they release on the host galaxy. For radio AGN, this can be done by identifying dying (remnant) and restarted radio galaxies from their radio spectral properties. Using the combination of the images at 1400MHz produced by Apertif, the new phased-array feed receiver installed on the Westerbork Synthesis Radio Telescope, and images at 150MHz provided by LOFAR, we have derived resolved spectral index images (at a resolution of 15 arcsec) for all the sources within an approximately 6 deg^2^ area of the Lockman Hole region. In this way, we were able to select 15 extended radio sources with emission (partly or entirely) characterised by extremely steep spectral indices (steeper than 1.2). These objects represent cases of radio sources in the remnant or the restarted phases of their life cycle. Our findings confirm that these objects are not as rare as previously thought, suggesting a relatively fast cycle. They also show a variety of properties that can be relevant for modelling the evolution of radio galaxies. For example, the restarted activity can occur while the remnant structure from a previous phase of activity is still visible. This provides constraints on the duration of the 'off' (dying) phase. In extended remnants with ultra-steep spectra at low frequencies, the activity likely stopped a few hundred megayears ago, and they correspond to the older tail of the age distribution of radio galaxies, in agreement with the results of simulations of radio source evolution. We find remnant radio sources with a variety of structures (from double-lobed to amorphous), possibly suggesting different types of progenitors. The present work sets the stage for exploiting the powerful tool of low-frequency spectral index studies of extended sources by taking advantage of the large areas common to the LOFAR and the Apertif surveys.2021A&A...648A...9Mhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/648/A9CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicerelated-toJ/A+A/329/482 : ROSAT Deep Survey in the Lockman Hole (Hasinger+, 1998)J/PASJ/53/445 : ASCA Deep survey in Lockman Hole Field (Ishisaki+, 2001)J/A+A/393/425 : Spectral analysis of Lockman Hole (Mainieri+, 2002)J/A+A/398/901 : VLA survey at 6 cm in the Lockman Hole (Ciliegi+, 2003)J/AJ/125/2521 : 2MASS6x survey of the Lockman Hole (Beichman+, 2003)J/A+A/413/843 : Far IR survey in the Lockman Hole (Kawara+, 2004)J/A+A/427/23 : 14.3um survey in the Lockman Hole (Fadda+, 2004)J/A+A/427/773 : ISOCAM 14.3{mu}m Deep Survey in Lockman HoleJ/A+A/479/283 : XMM observations of the Lockman Hole (Brunner+, 2008)J/MNRAS/387/1037 : 610-MHz survey of Lockman Hole with GMRT (Garn+, 2008)J/MNRAS/397/281 : Radio imaging in Lockman Hole (Ibar+, 2009)J/ApJS/185/433 : SWIRE/Chandra survey in Lockman Hole Field (Wilkes+, 2009)J/ApJS/198/1 : Photometry catalogs for the Lockman Hole (Fotopoulou+, 2012)J/other/BASI/38.103 : 6Lockman Hole 10-MHz survey with GMRT. II. (Garn+, 2010)J/A+A/551/A97 : Lockman Hole/XMM VLBI observations (Middelberg+, 2013)J/MNRAS/462/2934 : Lockman Hole North 3GHz catalogue (Vernstrom+, 2016)J/MNRAS/463/2997 : Lockman Hole low-frequency radio sources (Mahony+, 2016)J/MNRAS/429/2080 : Lockman Hole 10C sources radio spectral indicesJ/ApJ/856/67 : Lockman Hole VLA 3GHz radio source catalog (Cotton+, 2018)J/MNRAS/495/4084 : Lockman Hole region 325MHz source catalog (Mazumder+, 2020)J/A+A/622/A1 : LOFAR Two-metre Sky Survey DR1 source catalogJ/A+A/648/A2 : LOFAR Two-metre Sky Survey Deep Fields DR1 (Sabater+, 2021)J/A+A/648/A3 : LOFAR Two-metre Sky Survey Deep Fields DR1 (Kondapally+, 2021)J/A+A/648/A4 : LoTSS Deep Fields DR1 photometric redshifts (Duncan+, 2021)https://lofar-surveys.org/releases.html : LOFAR Home Pagehttps://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/648/A9https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/648/A9http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/648/A9https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/648/A9https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/648/A9http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/648/A9GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/A+A/648/A9/list (Information of fits map)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/648/A9/list?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/648/A9/list?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/648/A9/list?GETtext/xml+votable180.050000true163.4992857.84550.0055555555555555566/6112https://cdsarc.cds.unistra.fr/viz-bin/moc/J/A+A/648/A9?format=asciiRadiodefaultJ/A+A/648/A9/listInformation of fits maprecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintRAJ2000Right Ascension of center (J2000)degpos.eq.ra;meta.mainDEJ2000Declination of center (J2000)degpos.eq.dec;meta.mainNxNumber of pixels along X-axismeta.numberintNyNumber of pixels along Y-axismeta.numberintObs.dateObservation datestime.epoch;obsnullableFreqObserved frequencyGHzem.freqdoublesizeSize of FITS fileKibytephys.size;meta.fitsintFileNameName of FITS file, in subdirectory fitsmeta.id;meta.fitscharTitleTitle of the FITS filemeta.title;meta.fitschar