4 Comparison of the ALMA and Herschel - ESO

2 Introduction
The Herschel satellite and the Atacama Large Millimeter Array (ALMA) are two large
astronomical instruments designed to explore the “cool Universe”, that is, to study cool
gas and dust. Such cool material is associated with objects in formation or dust enshrouded
sources. These include the earliest evolutionary stages of galaxies, stars and planets; these
are deeply hidden within dust clouds where optical extinction can be extremely large. At far
infrared and sub-mm wavelengths, the extinction is not only much smaller, but we can also
directly measure the physical phenomena associated with the formation process itself.
Figure 1: The Spectral Energy Distribution (SED) of a nearby starburst galaxy, M82 (from Genzel,
1991). Above the SED, we show the total wavelength and frequency ranges covered by the Herschel
and ALMA instruments (Band 3 to 10, see Fig. 2).
Herschel will cover the wavelength ranges from 60 to 625 µm where the broadband dust
emission peaks, allowing one to measure total bolometric luminosities and to characterise
dust temperatures. ALMA will ultimately cover the range from 320 µm (~950 GHz) to 1 cm
(~30 GHz) and will nearly always measure radiation from the optically thin Rayleigh-Jeans
part of the spectrum and so can provide measurements of dust masses. Using assumed dustto-gas
ratios, it is possible to use dust masses to determine the total mass of the interstellar
medium. With ALMA one can measure transitions of molecules with permanent dipole moments.
In the sub-mm wavelength range, emission lines arise from gas with densities higher
than those emitted at longer wavelengths. These results can be used to estimate molecular
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