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See also ELAIS Southern survey with ATCA

Deep VLA 20cm survey of the ISO ELAIS survey regions

from http://www.ast.cam.ac.uk/elais/vla/ on 25-Jan-99


The catalogue contains 867 sources, 44 of which have multiple component for a total of 921 components. The catalogue reports the name of the sources, the peak flux density Sp (mJy/beam), the total flux density SI in mJy, the RA and DEC (J2000), the FWHM of the major and minor axis (in arcsec), the positional angle of the major axis in degree and the off-axis value in the VLA map (in arcmin). The different components of multiple sources are labeled A, B, ... followed by a line labeled T in which flux and position of the total sources are give.
A detailed description of the catalogue is reported in the paper A Deep VLA survey at 20cm of the ISO ELAIS survey regions by Ciliegi et al. 1999, MNRAS, 302, 222. Here there is an ascii table with the full catalogue.
Paolo Ciliegi ciliegi@ast.cam.ac.uk
Last modified: Mon Jan 25 12:17:06 1999
Below text from author's TEX-file:
A Deep VLA survey at 20cm of the ISO ELAIS survey regions
P. Ciliegi, R.G. McMahon, G. Miley, C. Gruppioni,
M. Rowan-Robinson, C. Cesarsky, L. Danese, A. Franceschini,
R. Genzel, A. Lawrence, D. Lemke, S. Oliver, J-L. Puget, B. Rocca-Volmerange


We have used the Very Large Array(VLA) in C configuration to carry out a sensitive 20cm radio survey of regions of sky that have been surveyed in the Far Infra-Red over the wavelength range 5-200 microns with ISO as part of the European Large Area ISO Survey(ELAIS). As usual in surveys based on a relatively small number of overlapping VLA pointings the flux limit varies over the area surveyed. The survey has a flux limit that varies from a 5-sigma limit of 0.135mJy over an area of 0.12deg^2 to a 5-sigma limit of 1.15mJy or better over the whole region covered of 4.22 deg^2. In this paper we present the radio catalogue of 867 sources. These regions of sky have previously been surveyed to shallow flux limits at 20cm with the VLA as part of the VLA D configuration NVSS(FWHM=45 arcsec) and VLA B configuration FIRST(FWHM=5 arcsec) surveys. Our whole survey has a nominal 5 sigma flux limit a factor of 2 below that of the NVSS; 3.4 deg^2 of the survey reaches the nominal flux limit of the FIRST survey and 1.5 deg^2 reaches to 0.25 mJy, a factor of 3 below the nominal FIRST survey limit. In addition our survey is at resolution intermediate between the two surveys and thus is well suited for a comparison of the reliability and resolution dependent surface brightness effects that affect interferometric radio surveys. We have carried out a detailed comparison of the reliability of our own survey and these two independent surveys in order to assess the reliability and completeness of each survey.


The Infrared Space Observatory (ISO, Kessler et al. 1996), launched in November 1995 was the successor of the Infrared Astronomical Satellite (IRAS) and provided unparallel sensitivity in mid to far infrared wavelengths ($i.e.$ 5--200 $\mu m$). The European Large-Area ISO Survey (ELAIS, Oliver et al. 1997, Oliver et al. 1998 in preparation) is a project that used ISO to carry out a deep wide angle survey at wavelengths of 6.7, 15, 90 and 175 $\mu m$. The 6.7 and 15 $\mu m$ surveys were carried out with the ISO-CAM camera (Cesarsky et al. 1996) with the aim to reach a 5$\sigma$ sensitivity of $\sim$2mJy at 15microns. The $90\mu m$ and 175$\mu m$ surveys used the ISO-PHOT camera (Lemke et al. 1994) with the aim to reach a 5$\sigma$ sensitivity of $\sim$25mJy. At these limits, we expect ISO to be confusion limited at 90 $\mu m$ and 175 $\mu m$ by galaxies and galactic cirrus emission and hence this survey should be the deepest FIR survey possible with the satellite.
The area covered in the ELAIS survey is $\sim 13$ square degrees at 15 and 90 microns, $\sim 7$ square degrees at 6.7 microns and $\sim 3$ square degrees at 175microns.
The ELAIS survey is $\sim$50 times deeper at 5-20$\mu m$ than IRAS. Thus our survey will allow us to explore IRAS-like populations to higher redshift and possibly unveil new classes of objects or unexpected phenomena. We expect to detect thousands of galaxies, many of which will be at high redshifts and undergoing vigorous star formation. The expected large number of high-z IR galaxies should provide vital information about the star formation rate out to z=1 and possibly earlier.
The spatial resolution of ISO will be insufficient to properly identify optically faint objects. At 15 microns, the survey resolution is $\sim$ 10 arcsec and at 90 microns it will be about one arc minute. Complementary radio data will play a crucial role in identifying many of the most interesting objects, as they did in the early days of X-ray astronomy (e.g. Cyg X-1) and in more recent times for IRAS (e.g. IRAS F10214+4714 (Rowan-Robinson et al. 1991).
In this paper we report the description of the radio observations obtained in the three ISO-ELAIS survey regions in the northern celestial hemisphere (N1\_1610+5430, N2\_1636+4115 and N3\_1429+3306). The observations are made with the Very Large Array (VLA) radio telescope at 1.4GHz (20cm) in the VLA C-configuration (maximum baseline 11km) with a resolution (FWHM) of $\sim15$ arcsec. The aim of these VLA observation was to obtain an uniform covering of the ELAIS regions with a rms noise limit of $\sim$50 $\mu$Jy. These VLA observations will be essential in the optical identification phase of the ELAIS sources and in assessing the reliability of the ELAIS source lists.
Moreover, with a radio survey it will be possible to investigate the radio/far--infrared correlation in star forming galaxies to flux densities deeper than those reached by IRAS. Helou, Sofier \& Rowan-Robinson (1985) noted a strong correlation between radio and far infrared flux for star forming galaxies, valid over a very wide range of infrared luminosities, and this has been confirmed in many other studies (e.g. Wunderlich, Klein \& Wielebinski 1987; Condon, Anderson \& Helou 1991). The radio emission is interpreted as a synchrotron radiation from relativistic electron which have leaked out of supernova remnants. It is expected that this correlation should extend below the IRAS flux level since the majority of the sub-mJy radio sources have been identified with faint blue galaxies whith spectra similar to those of star forming objects (Benn et al. 1993).
In addition, combining deep radio and optical data with the ISO survey fluxes will provide information on the trivariate IR-radio-optical luminosity function and its evolution and the contribution of starburst galaxies to the sub-mJy radio source counts. The ratio of the FIR emission and radio emission will also allow is to investigate the physical origin and spatial distribution of the energy sources in the detected objects in the same way that VLA maps have been central to our understanding of the origin of IRAS sources.
Finally, this survey, due to its depth and extension, is very important also as radio survey in its own right. In fact, the selected sample is large and deep enough to constitute a statistically significant sample of sub-mJy radio sources, whose nature and characteristics are still a major topic in observational cosmology (see Windhorst, Mathis \& Neuschaefer 1990, Fomalont et al. 1991, Rowan-Robinson et al. 1993, Gruppioni et al. 1997)

Radio observations

Choice of observing frequency and VLA configuration

The VLA C--configuration and the observing frequency of 1.4 GHz give the optimum resolution to acquire the kind of radio data that we need. Whilst less prone to surface brightness effects , the VLA D configuration is confusion-limited at the fluxes we wish to attain (the 5 $\sigma$ confusion limit in D configuration is 0.4 mJy/beam). With the C configuration and a frequency of 1.4 GHz the synthesized beam size (Full Width at Half Power, FWHP) is $\sim$15 arcsec. The well-defined synthesized beam of the VLA should enable us to pinpoint optical identifications to 1 arcsec, except for the asymmetric multi-components sources. The frequency of 1.4 GHz was chosen because at this frequency the FWHP of the VLA primary beam is 31 arcmin. This allow us to cover the ELAIS field with a relative small number of pointing centers. In fact it is possible to obtain a mosaic map with nearly uniform sensitivity if the separation is 31 / $\sqrt{2}$ $\sim$ 22 arcmin. Moreover, at 1.4 GHz there will be contributions from both the steep and flat spectrum population of radio sources.

The Source Catalogue

Considering all the available observations we detected a total of 867 sources at $\geq$ 5 $\sigma$ level (44 of which have multiple components) over a total area of 4.222 deg$^2$. The catalogue with all the 867 sources (921 components) reports the name of the source, the peak flux density S$_P$ in mJy, the total flux density S$_I$ in mJy, the RA and DEC (J2000), the full width half maximum (FWHM) of the major and minor axes $\theta_M$ and $\theta_m$ (in arcsec), the positional angle PA of the major axis (in degrees) and the off-axis values in the VLA map (in arcmin). The different components of multiple sources are labeled ``A'', ``B'', etc., followed by a line labeled ``T'' in which flux and position for the total sources are given. For these total sources the position have been computed as the flux-weighted average position for all the components. Table 3 shows the first page of the catalogue as an example. ....


Using the Very Large Array (VLA) radio telescope, we observed at 1.4 GHz a total area of 4.222 deg$^2$ in the ISO/ELAIS regions N1 N2 and N3. The lower flux density limit reached by our observation is 0.135 mJy (at 5 $\sigma$ level) on an area of 0.118 deg$^2$, while the bulk of the observed regions are mapped with a flux density limit of 0.250 mJy (5 $\sigma$). The data were analyzed using the NRAO {\tt AIPS} reduction package. The source extraction has been carried out with the {\tt AIPS} task {\tt SAD}. The reliability of {\tt SAD} has been tested using the maps of the radio surveys $FIRST$ and $NVSS$.
Considering all the available observations, we detected a total of 867 sources at 5 $\sigma$ level, 44 of which have multiple components. These sources were used to calculate the normalized differential source counts. They provide a check on catalogue completeness and reliability plus information about source evolution. A comparison with other surveys shows a very good agreement, confirming the presence of the well-know flattening of the counts below 1 mJy, the completeness of our catalogue and the reliability of our procedure for the source extraction.
A comparison with the $FIRST$ and $NVSS$ radio surveys has confirmed the incompleteness of these two surveys near their flux limits, while a flux comparison between the three surveys has shown that our survey with the VLA array in C configuration is the best compromise between high and low resolution radio surveys. The positional errors of the radio sources are $\sim$ 2 arcsec for the fainter sources ($\sim$0.13 mJy) and $\sim$ 0.6 arcsec for the brighter sources ($>$ 10 mJy). This small value will enable us to obtain an accurate and fast optical/infrared identification of the radio sources.


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