4 Comparison of the ALMA and Herschel - ESO

8 Recommendations for the Future
A number of conditions must be fulfilled to combine Herschel and ALMA datasets. First, the
sources must be observable with both instruments. For ALMA this condition requires that
the sources rise to more than 30 ° elevation at the ALMA site, with higher elevations even
better. Second, the calibrations for both instruments must be well determined and consistent.
For good calibrations, the signal-to-noise ratios must be excellent and the angular sizes of
the calibrators well determined. This may restrict calibrators to Solar System objects. Since
Herschel cannot observe sources close to the Sun, and the PACS and SPIRE detectors will
saturate when observing intense sources, the calibrations may have to be done using the
emission from asteroids such as Vesta, Ceres, moons of outer planets, or smaller planets
such as Uranus, Neptune or Pluto.
The datsets that will be produced by Herschel and ALMA will be so large that there may
have to be special procedures and archives to ensure optimal synergy. The analysis and
comparisons with models will have to be made on an automatic basis without a great deal of
human intervention. Such computer analysis programmes have been developed by Schöier
et al., 2005, for example, but these must be further developed to efficiently accommodate
the very large data cubes that will be produced by ALMA and Herschel in the near future.
The following sections discuss the factors to be considered in detailed plans.
8.1 Allocation of Time
The Herschel team must take the lead in initiating projects, since the Herschel satellite has a
relatively short lifetime. For any synergy, Herschel’s role is twofold. The PACS and SPIRE
bolometer cameras can be used for a rapid coverage of large regions of the sky, so these
instruments could provide finding surveys for ALMA. On average for extragalactic sources,
Herschel is limited to sources with redshifts z < 3 because of the combination of the shape of
the thermal spectrum and the k-correction. Since ALMA measures dust emission longward
of the peak of the Planck curve, such limits are much less severe. In our galaxy there are
a large number of intense sources. Thus, the PACS and SPIRE cameras can survey large
areas quickly. For both continuum and spectral lines, ALMA would image selected sources
with higher angular resolution. For Herschel HIFI, the most interesting measurements are
of the water vapour molecule. These measurements will be restricted to a limited number
of sources since HIFI is a single pixel instrument. Once again, Herschel measurements will
be first in time. Later ALMA would be used to provide images of water vapour lines using
ALMA Band 5 receivers. With other receiver bands ALMA could be used to image dust
continuum and spectral lines of other species.
Simultaneous observations are needed for rapidly time variable sources, such as comets. This
will also be the case for continuum measurements of extragalactic variable sources such as
active galactic nuclei or quasars. Simultaneous measurements seem to be less critical for
time variable water vapour sources.
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