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Radio observations of the Marano Field

Carlotta Gruppioni
e-mail: cgruppioni@astbo1.bo.cnr.it

Dipartimento di Astronomia, Universita di Bologna, Via Zamboni 33, 40126, Bologna, ITALY
Istituto di Radioastronomia, CNR, Via Gobetti 101, 40129, Bologna, ITALY

Abstract:

Radio observations of the Marano Field, carried out with the Australian Telescope Compact Array at 20 and 13 cm, are presented. We briefly describe the data reduction and explain the scientific aims of the work in progress. Excellent optical and X-ray data relative to this field are available. We plan to obtain a deep X-ray, optical and radio survey in order to study the nature of the sub-mJy population (a population of radio sources at very low radio fluxes) and the energy distribution and evolutionary properties of faint radio galaxies and AGNs.

Contents

1. Introduction

The importance of deep optical, X-ray and radio surveys of selected area of the sky is by now well recognized. Such surveys, like the Leiden-Berkeley Deep Survey (LBDS) carried out with the Westerbork Radio Telescope (at 1412 MHz) and optically identified with multicolor Mayall 4 m plates (see Windhorst et al. (1984),Windhorst et al. (1984)) and the deeper one carried out with the VLA (Windhorst et al. (1985)), produced many interesting results (see also Boyle et al. (1993)). In particular they showed the existence of a sub-mJy population whose nature is still matter of debate. This population manifests itself by a flattening of the normalized differential counts below 5 mJy which cannot be explained either by ordinary spiral and Seyfert galaxies, or by the expected giant elliptical and quasar contribution. From optical identification the sub-mJy population was shown to be principally composed of faint blue galaxies (mostly fainter than ). These objects have peculiar optical morphology indicative of interacting, starburst and merging galaxies.

To better understand the contents of these faint radio samples and study the correlation between radio, optical and X-ray properties of faint galaxies and AGNs, we obtained radio observations of the Marano Field (centered at (1950) = , (1950) = ) for which we already have excellent optical and X-ray data (see Zitelli et al. (1992),Zamorani et al. (1992)). Radio observations were carried out with the Australian Telescope Compact Array (ATCA) in two different wavelength bands: 20 and 13 cm.

Here we present the result of a preliminary analysis of the radio data. The analysis and the optical identification are still in progress. We obtained a list of sources at 13 cm from an area of square degrees, with an effective mean rms noise level of .

2. The radio data

2.1. The ATCA observations

The radio observations were carried out on 1994 January 4, 5, 6 and 7. The AT configuration was the 6 km Compact Array and the observing wavelengths were 20 and 13 cm. In order to observe the entire field covered by the optical and X-ray observations within the FWHM of the primary beam (33 arcminutes at 20 cm), four separate observations were used with different pointing positions. The pointing positions were located symmetrically around the center of the Marano Field, each being shifted by about 8 arcminutes in both right ascension and declination with respect to the center. We observed for 12 hours at each of the four different positions, for a total of 48 hours of observing time. The observations were made simultaneously at the two frequencies, with a bandwidth of 128 MHz, separated in thirty two 4 MHz channels. Calibration was carried out using the sources (primary) and (secondary).

2.2. The data reduction

For both the observing wavelengths the four observations were calibrated and analyzed separately. The data reduction was performed with AIPS. We chose channels between 7 and 27 to average into a mean value channel (called channel `0'). After an assessment of their quality, the images at the four pointing positions were combined in a single image. The combined image is a natural weighted map that covers pixels, with a pixel size of arcsec. The 13 cm wavelength data are shown in Figure 1. The analysis of the 20 cm data is still in progress. For the 13 cm data we obtained a mean RMS value in the inner field ( arcminutes radius) of , in good agreement with the theoretical value of . As a first step we obtained a preliminary source list at 13 cm, by searching the inner part of the image for pixels with a flux density of at least 5 times the local noise. The flux density and angular size of the sources were determined from a bi-dimensional gaussian fit.

  
Figure 1: Contour plot of the entire radio image at 13 cm of the Marano Field.

3. The optical data

In the last decade a large amount of optical data relative to the Marano Field has been obtained. As for photometry three sets of U, B, V and R plates (taken with the triplet corrector at the ESO 3.6 m telescope) with a limiting magnitude of about 22.5 in each band were acquired and fully analyzed. These plates cover an area of square degrees. In 1992 deep CCD exposures in the same bands were obtained. The CCD frames covered a substantial fraction of the central area of the field. From these data a sample of more than 100 quasar candidates has been selected on the basis of color-color diagrams and variability analysis. These candidates have all been observed spectroscopically, providing a sample of about 70 spectroscopically confirmed quasars with . Fifty of these quasars constitute a complete sample with in an area of about 0.3 square degrees. (see Marano et al. (1988),Zamorani et al. (1992),Zitelli et al. (1992)), almost exactly coincident with the inner, more sensitive area of the ROSAT observation (see the next paragraph).

4. The X-ray data

The X-ray survey of the Marano Field ( seconds with a limiting sensitivity of ) is one of the deepest ROSAT surveys now available. In the inner 20 arcminutes radius of the ROSAT field about 60 X-ray sources were detected. >From a cross-correlation of the X-ray and optical data about 40 % of these sources have been identified with optically selected quasars of the spectroscopic sample. A large fraction of the error boxes of the remaining X-ray sources contains quasar candidates (i.e. UVX objects) fainter than our current spectroscopic limit, while a few sources are identified with galaxies, cluster candidates and bright stars. In a number of cases the error box does not contain any obvious candidate at the limit of the plates. All these positions have been recently observed with deeper CCD observations.

5. Future developments and conclusions

With the radio, optical and X-ray data relative to the Marano Field now available, we plan to develop some interesting studies. First of all we will complete the radio data analysis in order to obtain a definitive list of sources at both 20 and 13 cm, their positions and their flux densities. This will be the starting point of the following studies:

  1. Deep radio surveys (e.g. Windhorst et al. (1985)) have shown the existence of a radio source population with very low radio fluxes (). That population, called sub-mJy population, determines a flattening of the LogN-LogS curve at radio fluxes less than 5 mJy. Using the counts of the Marano Field radio sources, we will be able to verify the existence of this sub-mJy population also in our source sample.
  2. The study of the Lockman Hole (see De Ruiter et al. (1993),Hasinger et al. (1993)) seems to suggest that the percentage of radio sources with optical counterparts brighter than the Palomar plates limit grows at the lower radio fluxes. In fact the percentage of identified objects (with POSS) is % at radio fluxes greater than , but increases to % for radio fluxes less than . With the available optical data of the Marano Field we will perform optical identifications of our radio sources, in order to verify the tendency suggested by the study of the Lockman Hole and to understand in more detail what kinds of objects constitute the sub-mJy population. There are, in fact, many hints that they are peculiar, starburst and interacting galaxies.
  3. By using the X-ray data obtained by ROSAT, together with the radio and optical ones, we plan to reach the following aims:

Acknowledgments

I would like to thank P. Parma for assistance in the radio data reduction and for useful discussions, and H.R. De Ruiter for a careful reading of the manuscript and for communications about the Lockman Hole results before publication.

References



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Wed Feb 22 17:56:38 GMT 1995