Here we present, first results from e-VLBI observations at 18 cm on a small sample of ultra-steep spectrum sources (spectral index between 74 MHz and 325 MH

Klockner, H R

Mahmud, Mereen

Miley, G

Paragi, Zsolt

Rottgering, H

English

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PoS(10th EVN Symposium)088Probing the Nature of Ultra-Steep Spectrum RadioSourcesMehreen MahmudJIVE, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, the NetherlandsE-mail: mahmud@jive.nlZsolt Paragi1JIVE, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, the Netherlands2MTA Research Group for Physical Geodesy and Geodynamics, Penc, HungaryE-mail: zparagi@jive.nlHuub RöttgeringLeiden Observatory, Universiteit Leiden, Huygens Laboratory / J.H. Oort Building,Niels Bohrweg 2, NL-2333 CA Leiden, the NetherlandsE-mail: rottgering@strw.leidenuniv.nlHans-Rainer KlöcknerUniversity of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK& MPIfR, Auf dem Hügel 69, 53121 Bonn, GermanyE-mail: hrk@astro.ox.ac.ukGeorge MileyLeiden Observatory, Universiteit Leiden, Huygens Laboratory/ J.H. Oort Building,Niels Bohrweg 2, NL-2333 CA Leiden, the NetherlandsE-mail: miley@strw.leidenuniv.nlHere we present, first results from e-VLBI observations at 18 cm on a small sample of ultra-steepspectrum sources (spectral index between 74 MHz and 325 MHz < -1.4), that are unresolvedat the arcsecond level, in an attempt to elucidate the nature of these (as yet) poorly understoodsources. A number of possible scenarios have been previously proposed for the origin of thesesources which include: radio galaxies located near or at the epoch of reionization, young obscuredradio galaxies, steep spectrum core quasars and pulsars. Presented here are first detections onmilliarcsecond scales (positions, flux densities and radio morphologies) of two of these sources,0722+291 and 1512+470, that are a first step in understanding their radio morphologies and clas-sification. These results show that a moderate fraction of ultra-steep spectrum sources may havecompact structures that can be studied only at the highest resolution with the VLBI technique.10th European VLBI Network Symposium and EVN Users Meeting: VLBI and the new generation of radioarraysSeptember 20-24, 2010Manchester UKc© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(10th EVN Symposium)088Probing the Nature of Ultra-Steep Spectrum Radio Sources1. IntroductionRadio sources with an ultra steep radio spectrum (USS) (defined here as having α < −1.4,Sν ∝ να ) are often associated with extreme phenomenon in the Universe. USS sources with aclassical FRII type radio morphology are for a large part associated with very distant galaxies2 < z < 5.2 [4][8]. Being very massive and often located in proto-clusters, these USS sources canbe used to study the origin and evolution of massive galaxies and clusters. Recent results include (i)the finding that massive radio galaxies with masses of around 1012 M⊙ exists up to z∼ 5 [9] and (ii)the discovery of proto-clusters up to z∼ 4 with total masses as expected for progenitors of nearbyclusters (a few times 1014 M⊙ [11]). USS sources with mostly diffuse radio emission are virtuallyalways associated with clusters up to z∼ 0.5 [10]. The emitting regions can have spectacular sizesof up to a few Mpc. The most likely explanation is that the radio emitting plasma traces shocks inmerging clusters.The study of these two types of USS sources both started in the late seventies with studiesof their radio morphology. Follow-up work with large optical and X-ray telescopes subsequentlygave an understanding of their importance for studies of galaxy and cluster evolution. A thirdclass of USS objects has not yet received much attention. These are the ones that are unresolvedon arcsecond scales. The aim of these observations is to set a first step towards elucidating thenature of these objects. There are a number of possible scenarios for the origin of these objects,including: (i) radio galaxies located near or at the epoch of reionization (as suggested by the wellknown spectral index redshift correlation [5]), (ii) young obscured radio galaxies, with the earlyphase accompanied by a starburst perhaps ([7] inferred star formation rates of∼ 1000 solar massesper year), (iii) steep spectrum core quasars and (iv) pulsars.2. Sample SelectionTo obtain the sample, we correlated the 74 MHz VLSS catalog [1] with the WENSS 325 MHzcatalog [6] and selected 0.02% of about 105 VLSS sources with the required ultra steep spectrum.To obtain sources that are compact on arcsecond scales, we then selected sources that (i) wereunresolved by the FIRST 1400 MHz survey [3] and (ii) had a flux ratio between the 1400 MHzNVSS [2] and FIRST catalog in the range 0.8 to 1.2. The latter criteria ensures that FIRST surveyhas captured all the flux from the sources and therefore reliably can measure that the emissioncomes from scales less then about 5 arcsecond. The FIRST and NVSS maps were also visuallyinspected to ensure that the structures of the sources were compact and unresolved, and that therewere no nearby sources that could have led to ambiguities in the measured fluxes in the sample.3. ObservationsThe data for five USS sources was obtained at L-band (18cm) on 10 and 11 June 2010 usingthe EVN in e-VLBI mode, in a ten hour run on both days. The scheduled data rate was 1024 Mb/s.Presented in this paper are initial results for two of the sources, 0722+291 and 1512+470. Bothsources were phase-referenced to close calibrators, J0736+2954 and J1500+4751 respectively. Theflux densities, spectral indices (based on the 75 and 325 MHz fluxes) and the flux ratios between2PoS(10th EVN Symposium)088Probing the Nature of Ultra-Steep Spectrum Radio SourcesTable 1: Integrated flux densities (mJy): 0722+291 & 1512+470Target α† Ratio‡ SV LSS SW ENSS SFIRST SNV SS S≀V LBI %∓0722+291 -1.44 1.09 650 76 10 9 1.9 271512+470 -1.49 0.98 1130 123 217 220 42 25†Spectral indices, calculated from the WENSS [6] and VLSS [1] survey flux densities‡Ratio of the NVSS [2] and FIRST [3] flux density measurements≀Flux density measurements from our 1.66GHz e-VLBI observations∓Estimated fraction of recovered flux density at 1.66GHzFigure 1: L-Band EVN total intensity maps of 1512+470 (left) and 0722+291 (right). The color bar on topshows the Flux distribution in millijansky. For 1512+470, the peak I contour is 9.9 mJy/Beam, the rms is∼0.36 mJy and the restoring beam is 23.88 x 22.31 mas at a position angle of 70.07◦. For 0722+291, peak Icontour is 1.88 mJy/Beam, the rms is∼ 0.085 mJy and the restoring beam is 32.15 x 26.43 mas at a positionangle of -9.95◦.FIRST and NVSS surveys for two sources presented here, are listed in Table 1. Also presented inthe table are our flux density measurements from e-VLBI observations.4. ResultsOur first results show two detections of our target sources from an initial sample of fivesources: 0722+291 and 1512+470 with total flux densities of ∼ 2 mJy and 42 mJy, respectively.0722+291 does not show clear evidence of structure on VLBI scales and is consistent with a pointsource model, albeit at significantly lower flux density than detected by the VLA. 1512+470 on theother hand is clearly resolved with a peak brightness to total flux density ratio of 1:4. Total intensitycontour maps are shown for both sources in Figure 1. We obtained lower limits for the brightnesstemperatures of 1512+470 as 5.1(1 + z)× 106K, and of 0722+291 as 1.5(1 + z)× 106K. The vis-ibilities in the amplitude versus UV-distance plot show evidence of extended structure, but these3PoS(10th EVN Symposium)088Probing the Nature of Ultra-Steep Spectrum Radio Sourcescannot be constrained well because of the limited uv-coverage in our short snapshot observations,and lack of short spacings.5. ConclusionBecause there are no redshifts available, it is difficult to classify these sources. But from ourinitial observations we can currently make some tentative conclusions. The fluxes of 1512+470from Table 1 indicate that the spectrum probably consists of a steep component between 74 and325 MHz and a self-absorbed core component that could be indicative of a restarting AGN. Wenote however that the structural appearance and low observed brightness temperature makes thisinterpretation unlikely. In case of 0722+291 we cannot exclude the AGN origin because the de-tected source component is very compact and thus Tb may be very significantly underestimated.The associated emission could also possibly be from a pulsar in the center of a nebula. We alsonote that while both sources have compact or somewhat resolved structure in the VLBI maps, thefactor of 4 difference in the flux density on arcsecond and milliarcsecond scales of 0722+291 and1512+470 indicates that there is extended emission associated with these sources. e-MERLIN ob-servations will thus be crucial for these sources, as they would allow us to image these sources onintermediate scales, with better sensitivity than VLBI to the diffuse emission. It is also importantto investigate whether the steep spectrum is associated with the compact components or with theextended emission. Multi-frequency VLBI observations would thus be useful for spectral indexanalysis. The upcoming telescopes such as LOFAR and SKA will allow us to investigate thesecompact targets with unprecedented sensitivity and resolution presently not available at the verylow frequency bands, where most of the emission in USS sources originate. Finally, from VLBIobservations of this small sample we may already conclude that ultra-steep spectrum sources rep-resent a rather diverse class of objects.References[1] Cohen A. S., Lane W. M., Cotton W. D., Kassim N. E., Lazio T. J., Perley R. A., Condon J. J.Erickson W. C., 2007, ApJ, 134, 1245[2] Condon J. J., Cotton W. D., Greisen E. W., Yin Q. F., Perley R. A., Taylor G. B. Broderick J. J., 1998,Astronomical Journal, 115, 1693[3] Becker R. H., White R. L. Helfand D. J., 1995, Astrophysical Journal, 450, 559[4] De Breuck, C., van Breugel, W., Röttgering H.J.A. & Miley, G., 2000, A & A, 143, 303[5] Krolik, J. H. & Chen, W. 1991, AJ, 102, 1659[6] Rengelink R. B., Tang, Y., de Bruyn A. G., et al. 1997, A & A, 124, 259[7] Reuland, M., Röttgering H. J. A., van Breugel, W., & De Breuck, C. 2004, MNRAS, 353, 377[8] Röttgering H. J. A., van Ojik R., Miley G. K., Chambers K. C., van Breugal W. J. M. de Koff S.,1997, A & A, 326, 505[9] Seymour, N., Stern, D., DeBreuck, C., et al. 2007, ApJ, 171, 353[10] Van Weeren R. J., Röttgering H. J. A., Bruggen M. Cohen A., 2009, A & A, 508, 75[11] Venemans, B. P., Röttgering H. J. A., Miley, G. K., et al. 2007, A & A, 461, 8234

Probing the Nature of Ultra-Steep Spectrum Radio Sources

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Probing the Nature of Ultra-Steep Spectrum Radio Sources

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