4 Comparison of the ALMA and Herschel - ESO

1 Executive Summary
The Herschel Satellite and the Atacama Large Millimeter Array (ALMA) are two very large
sub-mm and far infrared (FIR) astronomy projects that are expected to come into operation
in this decade. This report contains descriptions of these instruments, emphasising the
overlaps in wavelength range and additional complementarities.
A short rationale for studying sub-mm and far infrared astronomy is given. Following this,
brief presentations of Herschel and ALMA are presented, with references to more detailed
docu ments and use cases. Emphasis is placed on the synergies between these facilities,
and the challenges of comparing data produced using both. Specific examples of projects
are given for a number of areas of astronomical research where these facilities will lead to
dramatic improvements.
This report is addressed to an audience of non-specialist astronomers who may be interested
in extending their areas of research by making use of Herschel and ALMA instruments.
ALMA has a small instantaneous field of view, but allows high angular resolution images
of selected sources. The Herschel satellite has two multi-beam bolometer systems, PACS
and SPIRE. These have larger fields of view than ALMA, but with lower angular resolutions.
Thus the SPIRE and PACS cameras provide the opportunity to cover large areas of
the sky rather quickly. Measurements with ALMA would be follow-ups, while Herschel
SPIRE and PACS can provide finding lists for ALMA, or for shorter wavelength measurements
of source emission to give complete Spectral Energy Distributions (SEDs). Since
Herschel will be above the atmosphere, measurements can be made in wavelength regions
where astronomical signals cannot reach the Earth’s surface. This is especially the case for
wavelengths shorter than 300 µm. Herschel HIFI is a heterodyne instrument, so is especially
well suited to high resolution spectroscopy of molecules such as water vapour. The number
of sources that can be measured with HIFI will be more limited than PACS and SPIRE since
HIFI is a single pixel instrument. On the Earth’s surface, most of the water vapour lines are
blocked by the atmosphere. The few that do reach the surface are nearly all strong masers.
One exception seems to be the water vapour line at 183 GHz. Six antennas of ALMA will
be equipped with receivers to image the 3 13
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transition of water vapour at 183 GHz and
the same transition from the oxygen-18 isotope of water vapour at 203 GHz. Such high
angular resolution images will complement the Herschel HIFI data.
A particularly important condition for combining ALMA and Herschel data involves a common
source sample and consistent calibration. This calibration programme will require a
fairly extensive set of Herschel and ALMA measurements, in addition to accurate models
of the calibration sources. These sources will have to be more compact planets in the outer
part of the Solar System or asteroids. For comparisons with ALMA, PACS and SPIRE
calibrations will be more complex than HIFI calibrations.
Operationally, Herschel should take the lead in initiating projects. For extragalactic sources,
PACS and SPIRE could survey large regions, providing finding surveys for ALMA. The
ALMA follow-ups could be redshift determinations, an extension of the SED’s to longer
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