교육데이터 활용•지원 서비스

This table contains some of the results of Low-Frequency Array (LOFAR) High-Band Array (HBA) observations of the Herschel-ATLAS North Galactic Pole (NGP) survey area. The survey the authors carried out, consisting of four pointings covering around 142 deg² of sky in the frequency range 126-173 MHz, does not provide uniform noise coverage but otherwise is representative of the quality of data to be expected in the planned LOFAR wide-area surveys, and has been reduced using recently developed 'facet calibration' methods at a resolution approaching the full resolution of the data sets (~10 x 6 arcsec) and an rms off-source noise that ranges from 100 µJy beam^-1 in the center of the best fields to around 2 mJy/beam at the furthest extent of their imaging. In the reference paper, the authors describe the imaging, cataloguing and source identification processes, and present some initial science results based on a 5-sigma source catalog. These include (i) an initial look at the radio/far-infrared correlation at 150 MHz, showing that many Herschel sources are not yet detected by LOFAR; (ii) number counts at 150 MHz, including, for the first time, observational constraints on the numbers of star-forming galaxies; (iii) the 150-MHz luminosity functions for active and star-forming galaxies, which agree well with determinations at higher frequencies at low redshift, and show strong redshift evolution of the star-forming population; and (iv) some discussion of the implications of these observations for studies of radio galaxy life cycles. The NGP field was observed in four separate pointings, chosen to maximize the area of sky covered, with the LOFAR HBA as part of the Surveys Key Science project. These observations used the HBA_DUAL_INNER mode, meaning that the station beams of core and remote stations roughly matched each other and giving the widest possible field of view. The first observation, which was made early on in LOFAR operations, was of slightly longer duration (~10 h) than the others (~8 h). International stations were included in some of the observations in 2014 but were not used in any of the authors' analysis, which uses only the Dutch array. The author were interested in imaging in several separate frequency ranges (which are referred to hereafter as 'spectral windows'), since they wanted to be able to measure in-band spectral indices for detected sources. In addition, facet calibrating in different spectral windows could be done in parallel, speeding the processing up considerably. Accordingly, they chose to facet calibrate with six spectral windows, each made up of four bands and thus containing about 8 MHz of bandwidth:

 Spectral Nominal Frequency Frequency Range Window (MHz) (MHz) 1 130 126 - 134 2 138 134 - 142 3 146 142 - 150 4 154 150 - 158 5 161 158 - 166 6 169 166 - 173

The final source catalog was made by combining the four per-field catalogs. Ideally, the authors would have combined the images of each field and done source finding on a mosaicked image, but this proved computationally intractable given the very large image cubes that result from having six spectral windows. They therefore merged the catalogs by identifying the areas of sky where there is overlap between the fields and choosing those sources which are measured from the region with the best rms values. This should ensure that there are no duplicate sources in the final catalog. The final master catalogue contains 17,132 sources and is derived from images covering a total of 142.7 deg² of independently imaged sky, with widely varying sensitivity. Total HBA-band (150-MHz) flux densities of catalogued sources detected using the PYBDSM software and a 5-sigma detection threshold range from a few hundred µJy to 20 Jy, with a median of 10 mJy. The authors examined all sources in the initial master catalog for associations with sources in other surveys, for rejection as artifacts, and for optical identifications,