4 Comparison of the ALMA and Herschel - ESO

7 Summary
The PACS and SPIRE bolometer systems on Herschel are well suited to surveying large
regions of the sky, whereas ALMA can provide high sensitivity, high angular resolution
images in spectral line and continuum at longer wavelengths for regions of a few arc minutes
in size. Thus, ALMA is better suited to be a follow-up instrument for Herschel surveys. Such
follow-ups could be measurements of CO rotational transitions to determine the redshifts of
sources detected in the dust continuum or broadband continuum to provide the component
of Spectral Energy Distributions (SEDs).
For spectral lines, ALMA will be complementary to Herschel because of different angular
resolutions, frequency ranges and effect of the Earth’s atmosphere. For any spectral line surveys
with Herschel, follow-up measurements with ALMA will greatly increase the scientific
value, but it must be stressed that there should be an effort to make the Herschel surveys as
complete as possible. ALMA images of many spectral lines will allow better estimates of
source sizes. With Herschel one can measure line intensities for entire sources, but ALMA
images would give estimates of the true source sizes, and thus the actual abundances of
species, exactly those needed for chemistry models. Water vapour is a case of particular
interest. High angular resolution images of the 183 GHz line of H 2
O and the 203 GHz
line of H 2
18
O in ALMA Band 5 will be useful to supplement the lower angular resolution
measurements of many lines of water vapour planned with Herschel HIFI. For protostars
and molecular clouds the abundance of H 2
O will have an important influence on the energy
balance and chemistry. In the study of protostars, the angular resolution of ALMA is needed
to determine the sizes of components and variations of abundances on scales finer than a
few arc seconds. Herschel would provide measurements of SEDs and line transitions that
cannot be done with ALMA. Similar considerations apply to envelopes of AGB stars and
Solar System objects.
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