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Astron. Astrophys. 339, 34-40 (1998)

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6. Discussion and conclusions

The first thing that attracts attention is the large number of very bright and nearby galaxies associated with WENSS sources. Obviously, the new large-scale surveys will pick up large quantities of such nearby objects that were relatively scarce until now.

6.1. The identification content

For about 400 of the 9810 radio sources located in the restricted area of the minisurvey, used by us for identification purposes, a galaxy counterpart was found, with magnitude roughly brighter than 17-18 in the red (see Table 1). In Fig. 2 we show the distribution of flux densities over elliptical and spiral galaxies. As expected, spirals start to come in mainly below 100 mJy, reflecting the well known fact that spiral galaxies contain on average weaker radio sources than ellipticals. Using only objects with [FORMULA] mJy and [FORMULA], for which limit redshift information is essentially complete for spirals, we give in Fig. 6 the histogram of radio power. For ellipticals we assumed that, as a reasonable first approximation, their absolute magnitude can be considered constant (we took [FORMULA], for [FORMULA] km s-1Mpc-1): redshifts are still missing for about a quarter of the ellipticals.

[FIGURE] Fig. 6. Distribution of radio power at 325 MHz for galaxies of the minisurvey sample with [FORMULA] mJy and [FORMULA]. The contribution of spirals is represented by the shaded area.

6.2. The radio luminosity functions

Although the majority of galaxies in Table 1 lacks redshift information, it is possible to make a first attempt to derive the radio luminosity function of starburst galaxies and AGNs, by limiting ourselves to sources with flux density [FORMULA] mJy (radio completeness limit) and to the optically bright tail: if we select only objects with [FORMULA] all spirals and most ellipticals ([FORMULA] %) have redshift information. For the remaining ellipticals we assumed an absolute red magnitude of -22.0 (see above). If we use all galaxies, assuming a standard absolute magnitude for all spirals and ellipticals without z, we get [FORMULA] for ellipticals and [FORMULA] for spirals; therefore we may expect severe incompleteness of the spiral galaxy sample. However, with the limits [FORMULA] mJy, and [FORMULA] we get: [FORMULA] (ellipticals) and [FORMULA] (spirals), which suggests that both categories of galaxies are essentially complete. The radio luminosity function thus calculated is shown in Fig. 7. For comparison we have also plotted the RLF of starforming galaxies and of AGNs as given by Condon (1989), shifted to 325 MHz, by using an average spectral index of 0.6. The qualitative agreement is evident. (Note that the high point at very low radio power is due to the detection of NGC 6503, a very bright and nearby galaxy.)

[FIGURE] Fig. 7. The radio luminosity function of spiral and elliptical galaxies represented by open and filled circles respectively. For comparison we plot the RLF as derived by Condon (1989) for spirals (solid line) and and ellipticals (dashed line).

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998
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