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Survey Characteristics and Observation Strategy

The availability of a wide field imager (FOV $\sim 24.5\times24.5$ arcmin) at the prime focus of an 8m telescope like the LBT, would open the new and crucial possibility to attack the study of the optical counterparts of faint radio sources in a very efficient way.

The typical sky surface densities of radio sources with S1.4GHz>1 mJy s about 0.025 arcmin-2 (Becker et al. 1997), and it increases to 0.2 arcmin-2 (Gruppioni et al. 1997, MNRAS, 286, 470) and 1 arcmin-2 (Fomalont et al. 1997,ApJ, 475,L5) for S8.4GHz>0.2 mJy and S8.4GHz>12 $\mu$Jy respectively.

A wide field imager (FOV $\sim 24.5\times24.5$ arcmin) would allow us to observe in each field about 15 sources with S8.4GHz>1 mJy. The number of sources per field would increase to about 600 for sources with S8.4GHz>12 $\mu$Jy. This would allow us to identify and study large samples of weak radio source optical counterparts complete to very faint magnitude levels and in a relatively short amount of time. Each field will be observed with all the available filters (UBVRIJ + intermediate-band filters) in order to reach very faint magnitude levels of the order of $V \approx 27.7$ and $I\approx 26.6$ (reachable in about 10000 seconds of integration time).

The efficiency and the short integrations would allow us to perform multicolor (broad-band + intermediate band) photometry of the targets, and to obtain detailed SEDs of $\sim$600 objects per field. The analysis of the SEDs would provide the photometric redshifts and a classification of the host galaxy based on the shape of the SED. In this regard, we emphasize how important it is to have both a J filter and a set of intermediate-band filters. In fact, the host galaxies of z>1mJy sources (which are thought to be high-z ellipticals) are very red ( $R-K \sim 5-6$), implying that the use of the J filter will be crucial in order to identify the optical counterpart of the radio source. In addition the faint magnitudes reachable in the UBVRI bands and with the intermediate-band filters will allow us to sample the SEDs with a good spectral coverage and to estimate the photometric redshifts with a high accuracy.

These deep optical observations will also allow us to investigate the nature of the environments where the mJy and $\mu$Jy sources live, i.e. understanding the nature (colors, SEDs) of the surrounding galaxies and investigating whether these radio sources tend to live in rich environments such as groups or clusters.

The short integration times would also allow us to increase easily the sky covered area up to several square degrees. On one hand, this would increase enormously the size and the statistical significance of the $\mu$Jy sources sample, and on the other hand it would allow to study the nature of the brighter and rarer mJy and sub-mJy sources. For example, the observation of 2 square degree area (12 fields, if FOV $=24.5\times24.5$ arcmin) would allow the statistical study of a large sample of about 1000 sources and their environments at 0.2 mJy radio flux level.


next up previous contents
Next: Extremely Red Galaxies Up: Weak Radio Galaxies Previous: Scientific Justification
Guido Buscema
1999-01-29