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 H. J. A. Rottgering 3 D. A. Rafferty 6 S. Van Der Tol G. Heald 1 G. J. White 7 A. Shulevski P. Best 8 H. T. Intema S. Bhatnagar 9 W. Reich 10 M. Steinmetz S. Van Velzen T. A. Ensslin I. Prandoni F. de Gasperin M. Jamrozy G. Brunetti M. J. Jarvis 11 J. P. Mckean 1 M. W. Wise 1, 12 C. Ferrari 13 J. Harwood J. B. R. Oonk 1 M. Hoeft 14 M. Kunert-Bajraszewska C. Horellou O. Wucknitz 15 A. Bonafede 16 N. R. Mohan A. M. M. Scaife 17 H. -R. Klockner I. M. Van Bemmel A. Merloni K. T. Chyzy D. Engels H. Falcke 18, 10, 1 M. Pandey-Pommier 19 A. Alexov 1 J. Anderson 20 I. M. Avruch 21 R. Beck 22 M. E. Bell 23 M. J. Bentum 1 G. Bernardi 24 F. Breitling 25 J. Broderick 23 W. N. Brouw 26 M. Bruggen 16 H. R. Butcher 1 B. Ciardi 27 E. de Geus 28 M. de Vos A. Deller S. Duscha J. Eisloffel 14 R. A. Fallows 29 W. Frieswijk M. A. Garrett 3, 1 Jean-Mathias Grießmeier 30, 31 A. W. Gunst 1 J. P. Hamaker T. E. Hassall 32 J. Horandel A. Van Der Horst M. Iacobelli 33, 34 N. J. Jackson E. Juette 35 V. I. Kondratiev 1 M. Kuniyoshi 10 P. Maat 1 G. Mann 20 D. Mckay-Bukowski 36 M. Mevius 1 R. Morganti 26, 37 H. Munk 1 A. R. Offringa E. Orru 38 H. Paas 1 V. N. Pandey G. Pietka 39 R. Pizzo 1 A. G. Polatidis 1 A. Renting A. Rowlinson D. Schwarz 40 M. Serylak 31, 30 J. Sluman 1 O. Smirnov 5 B. W. Stappers 32 A. Stewart 41 J. Swinbank 12 Michel Tagger 31 Y. Tang S. Thoudam 18 C. Toribio R. Vermeulen C. Vocks P. Zarka 42
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Abstract : 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.
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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. Astrophysical Journal, American Astronomical Society, 2014, 793 (2), 22 p. ⟨10.1088/0004-637X/793/2/82⟩. ⟨insu-01284922⟩

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