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1.1. The history of baryon transformations

Although baryons contribute a negligible fraction of the global mass density of the universe, their transformations and the associated energy releases are key elements of the complex, puzzling history bringing from the primeval undifferentiated plasma to the highly structured present-day universe.

Two main driving mechanisms are able to circulate and transform baryons in astrophysical systems: one is related with stars and thermonuclear processes occurring therein, the other with gravitational contraction of gas - an important aspect of which, able to generate vast amounts of energy and producing spectacular effects in Active Galactic Nuclei and quasars, is gravitational accretion onto supermassive black holes.

Obviously, these two fundamental motors of the baryon cycle produce very different outcomes. While gravitational BH accretion irreversibly destroys baryons to produce energy, gas cycling into stars has (more beneficial) effects originating beautiful stellar systems, producing soft-energy photons, heavy elements, dust, and planetary systems in the proper amounts to bring eventually to the life.

A basic aim of the present studies of the distant universe, exploiting the current most powerful astronomical instrumentation, is indeed to clarify the history of baryon circulation, and in particular the paths through which the various different galaxy populations, which we observe in the local universe, have built their stellar content, created their hosted nuclear BH's and accumulated material in them.

While the overall story is driven by the evolving background of dark matter distribution, baryons are the observable traces of the evolving large scale structure.

The history of star formation, in particular, is a fundamental descriptor of cosmic evolution. Different cosmogonic scenarios predict very different timetables for the formation of stars and structures. For example, some models predict substantially different formation epochs for stars among the various morphological classes of galaxies, in particular between early-type and late-type galaxy systems. Some others, notably some specializations of the Cold Dark Matter-dominated models, do not.

1.2. Long-wavelength observations of galaxies: a view on the diffuse media and on the "active" phases in galaxy evolution

The build up of stellar populations in high-redshift galaxies is most usually investigated by looking at the optical/UV/near-IR emission from already formed stars in distant galaxies. The complementary approach, less frequently used, is to look at the diffuse media - atomic and molecular gas and dust - in high-z systems, and their progressive transformation into stars.

While observations of the redshifted starlight emission in the optical/near-IR can exploit large telescopes on ground and very efficient photon detectors, reliable probes of the diffuse media require longer-wavelength observations in the far-IR and sub-millimeter: a large variety of lines from atomic species and molecules in the Inter-Stellar Medium (ISM) at all ionization levels are observable there. Another fundamental component of the ISM, dust grains present in all astrophysical settings ranging from planetary disks to nuclear accretion torii around quasars, have the property to emit at these wavelengths, typically between a few µm to 1000 µm.

Observations at long- $ \lambda$ are then essential to study diffuse media in galaxies and are particularly suited [and needed] to study the early phases in galaxy evolution, when a very rich ISM is present in the forming system.

Under the generic definition of galaxy activity we indicate transient phases in the secular evolution of a galaxy during which the various transformations of the baryons undergo a significant enhancement with respect to the average rate, for reasons to be ascertained. These phenomena concern both enhanced rates of conversion of the ISM gas into stars (the starburst phenomenon), and phases of increased activity of the nuclear emission following an event of fast accretion of gas into the super-massive BH (the so-called AGN phase, reaching parossistic levels of photon production of up to $ \sim$ 1050erg/s in some high-z quasars).

As we will describe in this paper, IR and sub-mm wavelengths provide a privileged viewpoint to investigate galaxy "activity" in general, for two main reasons: (a) in many cases this $ \lambda$-interval includes a dominant fraction of the whole bolometric output of active objects; (b) at long wavelengths the screening effect of diffuse dust, present in large amounts in "active" galaxies, is no more effective and an impeded access to even the most extreme column-density regions is possible.

1.3. Observational issues

Unfortunately, the IR and sub-millimeter constitute a very difficult domain to access for astronomy: from ground this is possible only in a few narrow bands from 2.5 to 30 µm and at $ \lambda$ > 300 µm. From 30 to 300 µm observations are only possible from space platforms, the atmosphere being completely opaque.

In any case, however, infrared observations even from space are seriously limited by several factors. The most fundamental limitation is intrinsic in the energies $ \epsilon$ of photons we are looking at: the quantum-mechanics uncertainty principle sets a boundary to the best achievable angular resolution $ \theta$ due to diffraction of photons in the primary mirror of a telescope of size D: $ \theta$[FWHM] $ \geq$ 1.4*57.3*3600$ \lambda$/D [arcsec], ($ \lambda$ = ch/$ \epsilon$). For a typical cooled space telescope of 1 meter diameter working at $ \lambda$ = 100 µm this corresponds to $ \theta$ $ \sim$ 30 arcsec. For deep surveys of high-redshift IR galaxies this limited spatial resolution implies a limiting flux detectable above the noise due to confusion of several faint sources in the same elementary sky pixel. This confusion limit sets in at flux levels corresponding to $ \sim$ 0.04 sources/area element, or 0.16 sources/arcmin2 = 570 sources/degree2 in the above example (see eq. [8.26] and further details below). On this regard, recent surveys (see Sects. 10 and 11) have revealed that the far-IR sky is very much populated by luminous extragalactic sources, which implies that confusion starts to manifest already at relatively bright fluxes for even large space observatories.

Other limiting factors for IR observations come from the difficulty to reduce the instrumental background of (even space) telescopes due to photons generated by the optics. This adds to the ambient photon backgrounds, due to Zodiacal light from interplanetary dust, dust emission from the Milky Way, and the terrestrial atmospheric emission.

The instrumental backgrounds are reduced by cooling the instrumentation, in particular for space IR observatories, but this requires either inserting the whole telescope in large dewars (ISO, SIRTF), or by passively cooling the telescope with a very efficient Sun-shielding (FIRST, NGST). All this is technologically very much demanding and tends to limit the duration of space IR missions (because of the finite reservoir of coolant) and the size of the primary photon collector.

Finally, photon detection is not as easy in the IR as it is in the optical, and limited performances are offered by bolometers in the sub-mm and by photo-conductors in the mid- and far-IR. Furthermore, the need to cool detectors to fundamental temperatures entails problems of response hysteresis and detector instabilities due to slow reaction of the electrons to the incoming signal.

1.4. These lectures

In spite of the mentioned difficulties to observe at long wavelengths, it was clear since the IRAS survey in 1984 that very important phenomena can be investigated here. Only recently, however, pioneering explorations of the high-redshift universe at these long-wavelengths have been made possible by new space and ground-based facilities, and a new important chapter of observational cosmology has been opened.

These lectures are dedicated to a preliminary assessment of some results in the field. Because of the very complex, often still elusive, nature of many of the discovered sources, and because of the complicated astrophysical processes involved, we dedicate a significant fraction of this paper to review properties of diffuse media (particularly dust) in local galaxies, and of their relation with stars (Sects. 2, 3, 4 and 5). We also devote a substantial chapter (Sect. 6) to the description of local IR starbursts and ultra-luminous IR galaxies, to improve our chances of understanding their high-redshift counterparts.

Then after a brief mention of historical (IRAS) results in the field (Sect. 7), we come to discuss in Sect. 8 the discovery and recent findings about the Cosmic Infrared Background (CIRB), in Sect. 9 the deep IR surveys by the Infrared Space Observatory (ISO), and in Sect. 10 the pioneering observations by millimetric telescopes (SCUBA, IRAM). Interpretations of the deep counts are given in Sect. 11, and the question of the nature of the fast-evolving IR source populations is addressed in Sect. 12. Sect. 13 is dedicated to discuss the global properties of the population and some constraints set by the CIRB observations. A concise summary is given in Sect. 14. A Hubble constant H0 = 50 Km/s/Mpc will be adopted unless otherwise stated.

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