4 Comparison of the ALMA and Herschel - ESO

In addition to emission lines from the sources, ALMA might detect absorption lines against
background sources, even out to high redshifts. Such measurements allow estimates of the
densities and temperatures of the line-of-sight gas, and in addition isotopic ratios from measurement
of 13 C, 15 N, 18 O and D substitutions. To date molecular hydrogen has been detected
in nine systems, but CO has been found in only four absorption systems, of which two are
truly intervening and not associated with the background source (Curran et al., 2004). With
ALMA it would be possible to detect more molecular species in already known systems as
well as finding many new absorption line systems. With the detection of new absorption line
systems, one could determine accurate abundances of molecules over cosmic time. Such
measurements might be more difficult with Herschel due to baseline uncertainties.
In DRSP contribution 1.3.1 Spectral line survey in high-z molecular absorption systems, it is
proposed to measure Bands 3, 6, 7 and 9 completely toward 2 bright compact continuum sources
and Centaurus A. This would require 43 hours of time.
6.4 Active Galactic Nuclei
These sources show intense MIR and FIR emission in the continuum and also show strong
atomic and molecular line emission. MIR and FIR emission may originate in the central
region where it is likely that an obscuring torus is present and where star formation takes
place in the host galaxy. High resolution imaging is then fundamental to test models of the
obscuring torus. The emission of this latter is anisotropic and depends on its geometry and
size. High resolution imaging is needed to (a) disentangle star-forming regions and the
nuclear torus, (b) test the formation model of AGN and (c) investigate the joint formation
of AGN and host galaxies. AGNs could be divided into two subclasses according to the rate
of black hole growth: one would be a rapidly growing phase and the other a slowly growing
phase. Narrow line and broad line Seyfert 1 galaxies are possible candidates for these
distinct phases. Joint formation models predict that an efficient feedback provided by star
formation is at work in massive systems and halts black hole growth and star formation.
The mechanism does not work in less massive and smaller objects.
An understanding of AGN physics and formation, which probably includes a phase involving
dust obscuration and black hole growth, can be studied only by carrying out Herschel
surveys at 75 µm. Such surveys will detect the dust emission peak of AGN with redshifts
of around z = 0.7 to 1. These are the objects responsible for the hard X-ray background and
are crucial for a determination of the AGN bolometric luminosity, accretion rate and black
hole mass. At present the accretion rate is estimated from optical and X-ray observations
using very uncertain bolometric corrections.
In summary, Herschel/PACS data will define the AGN spectral energy distributions during
the dust enshrouded phase, as well as discovering obscured AGNs (so-called Type II AGNs).
The FIR SED of Type I objects will also be measured. ALMA will study the details of the
nuclear and disk emission using high angular resolution images. With such data, one can
test scenarios for AGN evolution, models for the formation of black holes and the concept
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