total counts, this can be attributed to the reduced instrumental sensitivity at the relevant wavelengths (see Fig. 3(c)).

The space density of galaxies in the three fields may also be estimated. As alluded to in the previous section, the comoving volume being probed by our measurements varies with wavelength and magnitude. Consider for example Fig. 2(a): a bright object–magnitude 19, say–may be detected at a range of wavelengths, from around 3920 Å to 4010 Å. A fainter object at, for instance, magnitude 22 is only detected if it lies within a much smaller wavelength range: around 3940 Å to 3960 Å. Therefore, we define an ‘accessible volume’, $\mathcal{V}_n$, for each detected object $n$ within the field. $\mathcal{V}_n$ is calculated by taking the locus of points in Fig. \[fig:simresults\] occupied by a source with the observed properties and integrating over all wavelengths. The density is taken as $\rho =
1/\mathcal{V}_1 + 1/\mathcal{V}_2 + ... + 1/\mathcal{V}_n$. The results for our fields are given in Table \[tab:density\].

  ----------- ---- --------------------------------------- ----------------------------------
     Field     \#              Number density                         SFR density
                         (Mpc$^{-3}\,\times\,10^4$)         (M$_\odot\;$yr$^{-1}$Mpc$^{-3}$)
   MRC B1256   4              $22.48 \pm 11.64$                   $0.0346 \pm 0.0174$
   MRC B2158   2    $\phantom{0}9.09 \pm \phantom{0}6.52$         $0.0070 \pm 0.0049$
   BR B0019    17             $49.09 \pm 