4 Comparison of the ALMA and Herschel - ESO

eview) have also revealed a new class of sub-mm galaxies (SMGs). The host galaxies of
these SMGs are often very faint at optical/near-infrared wavelengths (eg, Dannerbauer et al.,
2004). Since the beamwidths (11” to 15”) of the bolometer arrays are large, the identifications
have often required deep radio imaging with the VLA (eg, Ivison et al., 2002). Optical
spectroscopy of these host galaxies has now determined the redshift of 73 SMGs; the mean
is = 2.2 (Chapman et al., 2005). At least 12 of these redshifts have been confirmed with
CO spectroscopy (Greve et al., 2005). The importance of CO confirmations is illustrated
by the detection of multiple velocity components in several SMGs, and the possibility that
in some cases, there may be a foreground galaxy acting as a gravitational lens. The main
difficulties in this work have been the low angular resolution of the bolometer arrays and
the limited sensitivity and bandwidth of millimetre interferometers.
The objects found so far are the most luminous and thus are very probably non-representative.
Several of these are lensed sources. Future measurements will provide a more representative
sample of detections. Herschel SPIRE and PACS will be much more sensitive than
the ground-based sub-mm bolometers, so deep, wide-field sky surveys will detect thousands
of star-forming galaxies for wavelengths near the peak of the thermal dust emission at the
median redshift. However, the spatial resolution of SPIRE and PACS is comparable to that
of the ground-based bolometer arrays, so the identification and redshift determination of
these galaxies will need to rely on other instruments. ALMA will be the main follow-up
instrument, since ALMA’s high sensitivity and receivers can cover 8 GHz wide slices of
the frequency band. This will allow to completely bypass identifications that were formerly
made in the radio and optical/near-infrared, and directly measure redshifts using the CO lines
(see Fig. 11). Simultaneous high spatial resolution continuum imaging will also allow one to
study the morphological distribution of the thermal dust emission, pinpointing the regions of
star formation in these galaxies. Very sensitive ALMA observations of these PACS/SPIRE
sources will also allow us to search for fainter molecular and atomic emission lines such as
the CI transitions at 609 µm and 370 µm. Additional possibilities are the C + line at 158 µm,
the OI lines at 146 µm and 63 µm, NII lines at 203 µm and 122 µm, and OIII lines at 88 µm
and 53 µm (see Fig. 1). The detection of the redshifted C + line in J114816.64+525150.3 (see
Fig. 10) shows that ALMA should be able to measure ionised carbon in many high redshift
sources (Maiolino et al., 2005). The combined analysis of these molecular and atomic lines
using radiative transfer models should allow a determination of the physical parameters of
the interstellar medium (ISM) such as temperature, density and chemical composition in
these high redshift objects.
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