5 Extremely red objects

5.1 The sample of EROs

Starting from the colour-magnitude diagram Fig. 6 we selected the population of extremely red objects or EROs. To give a very conservative estimate of the surface density of those objects, we applied additional selection criteria to candidates from Fig. 6. With $K < K_{{\rm compl}}$ we avoid any kind of false or spurious detection in the K-band. Only matches with a separation less than 1 arcsec from K- and R-band objects are accepted for EROs (as compared to 2'' for Fig. 6). This criterion rejects matches between one object from close, but resolved, object pairs in R and their combined, unresolved counterparts in K. Such constellations are caused by the large pixel size and resolution in K and redden the objects systematically. Since the merged K-object has a different position with respect to both single objects in R, the stronger criterion concerning separation efficiently removes such mismatches. In R we accepted for the EROs every object as counterpart, in contrast to Fig. 6 where only sources with $mag < mag_{{\rm 50}}$ were considered. While this might result in matches with non-existing sources in R, no false EROs are produced since the R-K colour of a solid detection in K can only become bluer. Finally, both authors individually checked the ERO candidates for signs of errors in detection, photometry and matching of the counterparts in R and K. Only objects confirmed by both authors are considered as EROs. As R-dropouts, objects which only have a lower limit in their R-K-colour we considered only objects with $K < K_{{\rm compl}}$. To test for errors the objects were individually checked on the K-images.

Figure 8: The colour-magnitude diagram of the EROs. Extended and point-like objects with reliable classification are marked as filled square and triangles, respectively. R-band dropouts are marked with their lower limit

5.2 The surface density of EROs

Figure 8 shows the colour-magnitude diagram of the EROs in our survey. Unfortunately the threshold for EROs in R-K is not very well defined, and the values vary from R-K > 5.0(Cimatti et al. 1999) to R-K > 6.0 (Thompson et al. 1999). Therefore all objects with R-K > 5.0 are included in Fig. 8. ERO objects without detection in R (R-dropouts) are included with their lower limit in $(R-K)_{{\rm lim}}$ (computed via $R_{{\rm compl}}-K$) in Fig. 8. In Table 9 we give the surface density of the our EROs for different limits in K-magnitude as well as colour R-K. Only EROs in sub-surveys complete down to the K-limit specified in the first column of Table 9 are taken to compute the surface densities. The area of those sub-surveys is given in the last column of Table 9 (see also Paper I). The R-dropouts were taken with their respective $(R-K)_{{\rm lim}}$ to compute the surface densities in Table 9. The number of R-dropouts is given in Table 9 within parentheses.

Only 11 EROs out of the 146 from Fig. 8 are bright enough to allow a reliable morphological classification in R. While the only extended object is marked with a filled square, the point-like sources are given as filled triangles.

The two main contributors to our EROs-population are late type stars and galaxies at high redshift (z>0.8). As the available information on morphology suggests, the bright end is dominated by stellar objects. According to Leggett (1992) a colour $R-K>5.0~{\rm mag}$is expected for stellar types M 6 and later. With typical absolute magnitudes of $M_K=9.5~{\rm mag}$ and $M_K=11.5~{\rm mag}$ for M 6-dwarfs and L-dwarfs, respectively (see Leggett 1992; Reid 1999) and we detect these objects out to a distance of $400~{\rm pc}$ and $160~{\rm pc}$.

The surface density of extragalactic EROs at the depth of our survey is completely unknown. Thompson et al. (1999) give a surface density of $0.04~{\rm arcmin}^{-2}$ down to $K\le 19.0~{\rm mag}$. This is more than 5 times higher than our value at R-K>6.0 for the total population at our limit $K\le 17.5~{\rm mag}$. The high surface density of $0.7~{\rm arcmin}^{-2}$ as given by Eisenhardt et al. (1999) in their sample down to $K\le 20.1~{\rm mag}$ gives clues to a fast decline of the density towards brighter magnitudes. Therefore only a few out of the 16 objects in the reddest and deepest interval of Table 9 might be of extragalactic origin. Deeper studies of the red population would require data with both better spatial resolution and wavelength coverage.

 

 

Table 9: The density of EROs-object as a function of limits in K-magnitude and R-K colour

mag	$\rho _{R-K>5.0}^{\ast}$	$\rho _{R-K>5.5}^{\ast}$	$\rho _{R-K>6.0}^{\ast}$	area [deg]2
K<15.0	0.09	0.06	0.03	0.93
K<16.0	0.21	0.09	0.03	0.93
K<17.0	1.43(1)	0.43(1)	0.21(1)	0.91
K<17.5	5.46(7)	1.91(4)	0.91(3)	0.61

^*The densities are given in $10^{-2}\;{\rm arcmin}^{-2}$.