Info

2.3.2 Spectroscopic surveys The main advantage of spectroscopy, which can be done with or without a slit, as a means to carry out emission-line surveys is that it allows one to probe a much larger spectral

Figure 2.4. The color-magnitude diagram used by Shimasaku et al. (2004) to select Lya emitters at ^ ~ 4.79. The narrow-band filter is centered at A = 7040 A, and the continuum image is obtained by summing the R-band and i-band images. The inset shows the transmittance of the filters. The lines indicate the 3a error for flat-spectrum sources (fv a v0), the flux limit of the survey, and the color sensitivity threshold for line emitters to be detected.

Figure 2.4. The color-magnitude diagram used by Shimasaku et al. (2004) to select Lya emitters at ^ ~ 4.79. The narrow-band filter is centered at A = 7040 A, and the continuum image is obtained by summing the R-band and i-band images. The inset shows the transmittance of the filters. The lines indicate the 3a error for flat-spectrum sources (fv a v0), the flux limit of the survey, and the color sensitivity threshold for line emitters to be detected.

range than narrow-band imaging, resulting in surveys that cover wider redshift and/or radial-velocity intervals. However, depending on the specific instrumental configuration of the survey, either survey area (and hence cosmic volume) or sensitivity, or both, can be significantly diminished compared with the case of narrow-band imaging.

In the case of long-slit surveys, or surveys done with masks capable of providing mul-tislits, the sensitivity can actually be higher than in the case of narrow-band imaging, because when light is dispersed only the sky background at the same wavelength as the emission line contributes to the noise, whereas in the imaging case the background emission in the whole spectral range transmitted by the filter is a source of noise, while the line flux is the same in both cases (assuming a slit width sufficiently large that the whole of the line-emitting region is observed). However, the sky-area coverage of long-slit surveys is limited to those regions viewed through the slit or the multi-slit mask, which is often minimal compared with the case of imaging. Also, the observations are more involved than imaging, and the data reduction and analysis are of medium to high complexity, depending on whether multi-object masks are involved or not.

Slitless-objective-prism surveys or grism surveys cover much larger areas of the sky than can be covered by slit or multi-slit spectroscopy. However, their sensitivity is generally much lower than in the previous two cases, because the sky background is much higher. In particular, in this case every pixel of the detector array receives the full-sky background transmitted by the telescope-instrument combination. Furthermore, because these surveys are often carried out at very low dispersion, sensitivity is further reduced by continuum dilution of the line emission. Sky-area coverage is generally less than in

001Ô-S305

Was this article helpful?

0 0

Post a comment