4 Comparison of the ALMA and Herschel - ESO

6.3 High Redshift Objects
During the past decade, our knowledge of high redshift (z > 1) galaxies and active galactic
nuclei has substantially improved. One of the breakthroughs has been the study of their
thermal dust and molecular line emission. The molecular lines, which are strongest at mm
and sub-mm wave lengths, are excellent tracers of the reservoirs feeding such starburst objects.
Millimetre and sub-mm observations are possible thanks to the development of sensitive
bolometer arrays such as SCUBA on the JCMT (Holland et al., 1999), and MAMBO
on the IRAM 30 m telescope (Kreysa et al., 1998). In addition, there are additional instruments,
namely millimetre interferometers with large collecting area and more sensitive line
receivers such as the IRAM Plateau de Bure facility. Observations of high redshift objects
at millimetre wavelengths greatly benefit from the fact that the bulk of the luminosity is
shifted into the millimetre and sub-mm range. This “negative k-correction” leads to an
almost constant flux density from z = 1 to 10 (see Figure 9).
Figure 9: The predicted flux density of a dusty galaxy with a luminosity of 5 x 10 12 solar luminosities
at a temperature of 38 K is shown as a function of redshift for various wavelengths. The k-correction
(the shift of luminosity with redshift) was calculated for the parameters of the Universe shown in the
lower left of the figure. This k-correction favours detection of sources at wavelengths longer than
about 250 μm, since the flux densities are almost independent of redshift. Reproduced from Blain et
al. (2002).
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