Infrared Galaxies and Radio-quiet AGNs
Optical searches are biased towards unobscured, active and/or
starbursting galaxies (including low-mass galaxies in an active
phase). Near-infrared surveys, in turn, detect primarily
photospheric emission of late-type stars (red giants) and hence are
biased towards early-type (E/S0/Sa) galaxies. Dust, which is found in
most galaxies, includes a substantial fraction of all metals and
strongly affects the observability of galaxies at different
wavelengths. In this context, a major breakthrough in our
understanding of galaxies has been made by the IRAS survey.
Nevertheless, because the longest wavelength observed was 
m,
IRAS detected primarily the warm dust component in low redshift
galaxies, heated by the intense radiation fields from
star-forming regions. IRAS was not sensitive to either cold dust or
high-redshift galaxies.
Only future extensive sub-millimetre surveys, such as the one planned in the PLANCK mission, can determine how much of the stellar radiation in galaxies is either reprocessed by cold dust or redshifted to submillimetre wavelengths. PLANCK is expected to provide broad-band spectral information for several thousands of optical- and IRAS-selected galaxies (see Table 1.1), and also to test for the existence of galaxies optically obscured by cold dust. The observations will reveal the spectral behavior in this poorly-known frequency range. In addition to the study of energy balance in normal and starbursting galaxies, the multi-frequency sample will set constraints on dust properties (temperature, emissivity).
Statistical information on the various classes of objects will allow
the definition of templates for modeling galaxy formation and
evolution. Furthermore, a large sample of local sub-mm galaxies is
required for precise and unbiased estimates of the local luminosity
functions, a crucial step when interpreting the evolution of galaxies
at infrared wavelengths. Studies of the angular correlation function
will complement the analyses made from optical or IRAS surveys, and
could help to test the evolution of galaxy clustering with cosmic time
over a fraction of the Hubble time corresponding to
.
Figure 1.18: (Top) Estimates of integrated counts of galaxies at
= 60 µm; the latter is the optimal
wavelength for source selection according to our two extreme models,
PLE (solid line) and DLE (dashes). Both models reproduce
IRAS data. The solid square shows the counts by Hacking & Houck (1987) with
their Poisson errors. (Bottom) Predicted redshift distributions for a flux
limit of S = 150 mJy at
= 550µm, over 800 square degrees. The dashes give the PLE no-evolution
model based on the IRAS luminosity functions and theoretical
FIR/submm/mm spectra fitting IRAS 'colors" and observations of a sample of
galaxies at 1.25 mm (Franceschini & Andreani 1995). The solid line gives the
prediction for the PLE scheme.
Figure 1.18 summarizes some specific results about the
predicted galaxy counts and redshift distributions relevant to the
sub-mm observations of PLANCK, based on IRAS
luminosity functions and theoretical FIR/submm/mm spectra designed to
fit IRAS colors. There is a large uncertainty on dust emission at
sub-mm wavelengths. In order to bracket the uncertainties, two
extreme evolutionary models have been assumed based on pure luminosity
evolution (PLE) and density plus luminosity evolution (DLE).
The DLE model includes
only the warm dust seen by IRAS while the PLE model also includes a
cold dust component consistent with the 1.25 mm observations of
Franceschini & Andreani (1995). The counts and redshift
distributions are estimated with reference to the
m channel which, given the combination of
sensitivity and angular resolution, gives the largest sample of
infrared galaxies. The large difference in the predictions of these
two models at 
m, given that they agree with the IRAS counts at 
m, illustrates
how crucial submillimetre observations are in constraining
models of galaxy evolution. These models show that a
significant fraction of galaxies detected by PLANCK
are expected to lie at z>0.2, in marked contrast to the low
median redshift (
) of galaxies detected by IRAS.
Note that Figure 1.18 shows that the behavior of the counts depart
from an Euclidean law even for weakly-evolving galaxies (e.g. the
normal spirals).
The conclusion, although model dependent to some extent, is that PLANCK surveys would give access to an intermediate redshift range that is complementary to other existing all-sky catalogues.