4 Optical vs. near infrared

The NIR spectral range offers some advantages compared to the optical. The lines (optical rest frame) are less affected by dust extinction. Moreover, we can use the same lines that have provided most of the information about low redshift objects. The comparison is therefore easier and more reliable than comparing the UV rest frame at high redshift with the optical rest frame at low redshift. We can also use the diagnostic tools developed for low redshift objects to investigate the physical conditions and ionization state of the extended gas. Such diagnostic techniques are reasonably well understood, contrary to what happens in the UV rest frame.

The main problems we may find when observing in the NIR are due to the effects of the atmosphere and the thermal emission from the telescope. Due to atmospheric absorption some redshifts are forbidden to study certain emission lines (see Fig. 6).

On the other hand, HzRG are very faint sources and the OH bands (dominant source of sky emission in the NIR) are bright and variable compared to them so that in same cases it will be impossible to obtain enough S/N on the line of interest. Since the FWHM of the object lines is large (e.g. [OIII]$\lambda$5007$\sim40-50$ Å at z=2.5), the contamination will often be important independently of the spectral resolution we use, unless we work with emission lines that lie far from strong sky lines. This puts constrains on the object redshifts. At longer wavelengths ( $\geq2.2~\mu$m) the limitations are due to thermal background.