4 Comparison of the ALMA and Herschel - ESO

of the joint formation of AGNs and their host galaxies. With a large data collection one can
classify sources, and try to model the AGN as a function of other properties of the source.
In DRSP contribution 1.51, it is proposed to image six AGNs at distances of 17 Mpc with an
angular resolution of 0.06” in the CO J = 2-1 line. The goal is to determine the molecular gas
content close to the black hole at the centre of the AGN, to estimate the mass of infalling material.
This angular resolution is equivalent to 5 pc at 17 Mpc. There would be a single pointing, with 5
kms -1 velocity resolution. The rms noise is 0.2 mJy or 1.2 K in each beam. This allows the detection
of warm CO, and in addition the continuum sensitivity is 4mJy. This limit corresponds to
an H 2
column density of 1.5 x 10 23 cm -2 if the dust temperature is 20 K, and if the dust properties
and gas-to-dust ratios are standard.
6.5 Normal Spirals and Low Surface Brightness Galaxies
The shape of the SED from the FIR to sub-mm clarifies the processes at work in distant
target objects. There are three different effects: the existence of an active nucleus, the star
formation activity of the galaxy and metallicity. Our ability to distinguish between these
in a high redshift object must rely on the amount of knowledge we have on a nearby well
understood sample. In both photometric and spectrometric modes, Herschel will map several
samples of local objects of different type, morphology and metallicity. These data will be
used to explore a large range of galaxy properties: the processes controlling the dust physics
(size, distribution, composition and evolution), the structure of the ISM, the metallicity,
the consequences of heating and cooling on dust in different metallicity environments, how
much star formation activity is dust enshrouded and the optical depth of dust to the NIR/MIR,
the relation between metals in the gas phase and those in dust as a function of metallicity.
However Herschel data alone will not be able to characterise these processes in detail. Only
high resolution measurements with ALMA will probe the detailed physical processes in the
ISM under a variety of conditions.
Even in very extended nearby galaxies such as M31 only a few regions have been imaged
with millimetre interferometers (see the review by C. Wilson, 2005). The full 2 o extent
of M31 has only been mapped with single dish instruments with a typical resolution of
~23” in the CO(1-0) line (Nieten et al., 2006). Interferometer maps of nearby galaxies
have mostly focused on Local Group members, which are close enough to resolve individual
Giant Molecular Clouds (GMCs). In the Milky Way Galaxy, GMCs represent
the main sites of star formation. Therefore, studying GMCs in external galaxies of different
morphological types and various star formation rates, will help to understand the
efficiency of star formation in different environments. For example, the mass function
of GMCs appears much steeper in M33 than it is in the Milky Way, sug gesting a different
organisation of the molecular ISM. With ALMA, it will be possible to resolve
GMCs and study their size and turbulence in galaxies out to distances of 100 Mpc.
The study of interacting galaxies will improve our knowledge of star formation triggered
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