Observing Strategy

We are undertaking a volume-limited survey for H line emission from galaxies within two intervals of redshift: (interval A) and (interval B), using the 10 arcmin field of the TTF. Although the primary target of the survey is H emission, the survey will also be sensitive to emission from redshifted [OII], H, [OIII] and possibly Ly. Table 1 summarises the scanning parameters of the TTF for each interval.

  
Table 1: Scanning parameters used in the survey, (* resolution kept fixed during scanning).

The intrinsic H line strength in a quiescent galaxy can be related to the current rate of massive star formation present (Kennicutt 1983).

  
Figure 1: Calculations used to optimize survey coverage. In (b), the solid and dashed lines correspond to redshift intervals A () and B () respectively. See text for details.

Fig. 1(a) shows apparent H luminosities, , as a function of redshift for galaxies with star formation rates (SFRs) between 0.01 and 10 /yr. We have assumed km/s/Mpc, and throughout. Fig. 1(b) shows the signal-to-noise ratio (S/N) per pixel that we expect to attain using the TTF for galaxies with SFRs between 0.1 and 10 /yr. We have assumed emission from galaxies at redshifts of 0.08 (interval A; solid curve) and 0.24 (interval B; dashed curve). Typical values of dark current, read noise and sky background during bright time have been used with a combined AAT/TTF/CCD efficiency of 17 %. Since a typical star forming region in an LMC-type dwarf is about 300 pc in diameter, we expect the emission-line regions to be spatially unresolved and so Fig. 1(b) assumes 2 arcsec seeing.

As Fig. 1(b) shows SFRs as low as 0.1 /yr at z = 0.08 and 1 /yr at z = 0.24 can yield a 3 detection in only 130 s of exposure time. By way of comparison, a low-activity Sb spiral such as the Milky Way has a SFR of /yr (Smith et al. 1978) while the LMC has a rate of 0.26 M yr (Kennicutt et al. 1995). We have settled upon an exposure time of 200 s per slice as a compromise between flux limit and sky coverage. Exposures of this duration allow us to obtain H fluxes to ergs/cm/s ( W/m) as Fig. 1a shows.

© Copyright Astronomical Society of Australia 1997