LOFAR low-band antenna observations of the 3C295 and Bootes fields: source counts and ultra-steep spectrum sources

R. J. van Weeren (1, 2) , W. L. Williams (3) , C. Tasse (4, 5, 6) , H. J. A. Rottgering (3) , D. A. Rafferty (7) , S. van Der Tol , G. Heald (1) , G. J. White (8) , A. Shulevski , P. Best (9) , H. T. Intema , S. Bhatnagar (10) , W. Reich (11) , M. Steinmetz , S. van Velzen , T. A. Ensslin , I. Prandoni , F. de Gasperin , M. Jamrozy , G. Brunetti , M. J. Jarvis (12) , J. P. Mckean (1) , M. W. Wise (1, 13) , C. Ferrari (14) , J. Harwood , J. B. R. Oonk (1) , M. Hoeft (15) , M. Kunert-Bajraszewska , C. Horellou , O. Wucknitz (16) , A. Bonafede (17) , N. R. Mohan , A. M. M. Scaife (18) , H. -R. Klockner , I. M. van Bemmel , A. Merloni , K. T. Chyzy , D. Engels , H. Falcke (19, 11, 1) , M. Pandey-Pommier (20) , A. Alexov (1) , J. Anderson (21) , I. M. Avruch (22) , R. Beck (23) , M. E. Bell (24) , M. J. Bentum (1) , G. Bernardi , F. Breitling (21) , J. Broderick (24) , W. N. Brouw (25) , M. Bruggen (17) , H. R. Butcher (1) , B. Ciardi (26) , E. de Geus (27) , M. de Vos , A. Deller , S. Duscha , J. Eisloffel (15) , R. A. Fallows (28) , W. Frieswijk , M. A. Garrett (3, 1) , Jean-Mathias Griessmeier (29, 30) , A. W. Gunst (1) , J. P. Hamaker , T. E. Hassall (31) , J. Horandel , A. van Der Horst , M. Iacobelli (32, 33) , N. J. Jackson , E. Juette (34) , V. I. Kondratiev (1) , M. Kuniyoshi (11) , P. Maat (1) , G. Mann (21) , D. Mckay-Bukowski (35) , M. Mevius (1) , R. Morganti (25, 1) , H. Munk (1) , A. R. Offringa , E. Orru (36) , H. Paas (1) , V. N. Pandey , G. Pietka (37) , R. Pizzo (1) , A. G. Polatidis (1) , A. Renting , A. Rowlinson , D. Schwarz (38) , M. Serylak (30) , J. Sluman (1) , O. Smirnov (5) , B. W. Stappers (31) , A. Stewart (39) , J. Swinbank (13) , Michel Tagger (30) , Y. Tang , S. Thoudam (19) , C. Toribio , R. Vermeulen , C. Vocks , P. Zarka (40)

1 ASTRON - Netherlands Institute for Radio Astronomy
2 CfA - Harvard-Smithsonian Center for Astrophysics
3 Leiden Observatory [Leiden]
4 SKA South Africa
5 Rhodes University, Grahamstown
6 GEPI - Galaxies, Etoiles, Physique, Instrumentation
7 Department of Astronomy and Astrophysics [PennState]
8 SLAC - SLAC National Accelerator Laboratory
9 University of Edinburgh
10 CSA - Department of Computer Science and Automation [Bangalore]
11 MPIFR - Max-Planck-Institut für Radioastronomie
12 Charles Darwin University
13 AI PANNEKOEK - Astronomical Institute Anton Pannekoek
14 LRGP - Laboratoire Réactions et Génie des Procédés
15 TLS - Thüringer Landessternwarte Tautenburg
16 AlfA - Argelander-Institut für Astronomie
17 Jacobs University [Bremen]
18 School of Physics and Astronomy [Southampton]
19 Radboud University [Nijmegen]
20 CRAL - Centre de Recherche Astrophysique de Lyon
21 AIP - Leibniz-Institut für Astrophysik Potsdam
22 SRON - SRON Netherlands Institute for Space Research
23 LCPM - Laboratoire de Chimie Physique Moléculaire
24 University of Southampton
25 Kapteyn Astronomical Institute [Groningen]
26 Max Planck Institute for Astrophysics
27 Medstar Research Institute
28 Institute of Mathematical and Physical Sciences
29 USN - Unité Scientifique de la Station de Nançay
30 LPC2E - Laboratoire de Physique et Chimie de l'Environnement et de l'Espace
31 Jodrell Bank Centre for Astrophysics
32 Sapienza University of Rome - Dipartimento di Matematica "Guido Castelnuovo" [Roma I]
33 CMLS - Centre de Mathématiques Laurent Schwartz
34 Ruhr-Universität Bochum [Bochum]
35 University of Oulu
36 IMAPP - Applied Stochastics
37 Oxford Astrophysics
38 Interactions Son Musique Mouvement
39 LASP - Laboratory for Atmospheric and Space Physics [Boulder]
40 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique

We present LOFAR Low Band observations of the Bootes and 3C295 fields. Our images made at 34, 46, and 62 MHz reach noise levels of 12, 8, and 5 mJy beam$^{-1}$, making them the deepest images ever obtained in this frequency range. In total, we detect between 300 and 400 sources in each of these images, covering an area of 17 to 52 deg$^{2}$. From the observations we derive Euclidean-normalized differential source counts. The 62 MHz source counts agree with previous GMRT 153 MHz and VLA 74 MHz differential source counts, scaling with a spectral index of $-0.7$. We find that a spectral index scaling of $-0.5$ is required to match up the LOFAR 34 MHz source counts. This result is also in agreement with source counts from the 38 MHz 8C survey, indicating that the average spectral index of radio sources flattens towards lower frequencies. We also find evidence for spectral flattening using the individual flux measurements of sources between 34 and 1400 MHz and by calculating the spectral index averaged over the source population. To select ultra-steep spectrum ($\alpha < -1.1$) radio sources, that could be associated with massive high redshift radio galaxies, we compute spectral indices between 62 MHz, 153 MHz and 1.4 GHz for sources in the Bo\"otes field. We cross-correlate these radio sources with optical and infrared catalogues and fit the spectral energy distribution to obtain photometric redshifts. We find that most of these ultra-steep spectrum sources are located in the $ 0.7 \lesssim z \lesssim 2.5$ range.

Domains: Sciences of the Universe [physics]; Astrophysics [astro-ph]

R. J. van Weeren, W. L. Williams, C. Tasse, H. J. A. Rottgering, D. A. Rafferty, et al.. LOFAR low-band antenna observations of the 3C295 and Bootes fields: source counts and ultra-steep spectrum sources. The Astrophysical Journal, 2014, 793 (2), 22 p. ⟨10.1088/0004-637X/793/2/82⟩. ⟨insu-01284922⟩