10. EXPLORATIONS OF THE DEEP UNIVERSE BY LARGE MILLIMETRIC TELESCOPES

Galaxy surveys in the sub-millimeter waveband offer a unique advantage for the exploration of the distant universe: the capability to naturally generate volume-limited samples from a flux-limited survey. This property is due to the peculiar shape of galaxy spectra in the sub-mm, with an extremely steep slope from 1 mm to 100 µm, as illustrated in Figure 2 for the prototype dusty starburst galaxy M82.

While above a few mm the luminosity is dominated by synchrotron and free-free radio emission, from 100 µm to 1 mm dust continuum emission dominates, with slopes as steep as L() (see Sect. 3). Then, as we observe at sub-mm wavelengths galaxies at larger and larger redshifts, the rest-frame flux density moves to higher and higher frequencies along a steeply increasing spectrum, and the corresponding K-correction almost completely counter-balances the cosmic dimming of the observed flux, for a source of given luminosity at z 1. The source flux keeps roughly constant with redshift up to z 10, assuming cosmic sources were already present and dusty so early.

A further related advantage of sub-mm surveys is that local galaxies emit very modestly at these wavelengths. Together with the very favorable K-correction, this implies that a sensitive sub-mm survey will avoid local objects (stars and nearby galaxies) and will select preferentially sources at high and very high redshifts: a kind of direct picture of the high-redshift universe, impossible to obtain at other frequencies, where surveys are dominated by galaxies at modest redshifts if not by galactic stars. Finally, and similarly to the ISO surveys, observing in the sub-mm has the advantage of producing samples completely unaffected by intergalactic opacity and dust extinction.

The third breakthrough event after 1996 for IR/sub-mm cosmology has come from operation of a powerful array of bolometers (SCUBA) at the focal plane of the sub-mm telescope JCMT on Mauna Kea. The success of SCUBA on JCMT was due to a combination of three crucial factors: a sensitive detector array with good multiplexing capability (37 bolometers on a field of 2 arcmin diameter, with a diffraction-limited spatial resolution of 15 arcsec), put at the focal plane of a powerful sub-mm telescope (15m dish), on a site allowing to operate at short enough wavelengths (850 µm) to exploit the very steep shape of sub-mm SED's of galaxies. For comparison, in spite of the larger collecting area, the competing bolometer array camera on the IRAM 30m telescope at Pico Veleta (Spain) is limited to work at wavelengths > 1.2 mm by the poorer, lower-altitude site, which means by itself a factor 5 penalty in the detectable source flux with respect to SCUBA/JCMT.

The latter had a long development phase (almost like a space project!), partly because of the difficulty to keep the microphonic noise within acceptable limits. But eventually, its long-sought results have come, and the instrument is providing new very exciting facts to observational cosmology.

Basically, SCUBA/JCMT has allowed to partly resolve the long- (850 µm) CIRB background into a population of faint distant, mostly high-z sources, as discussed in Sect. 12.3 below. During three years of activity, largely dedicated to deep surveys, SCUBA has discovered several tens of sub-millimetric sources, mostly at 850 µm.

Four main groups have used SCUBA for a variety of deep integrations. Smail et al. (1997, 1999) have undertaken an ingenious program exploiting distant galaxy clusters as cosmic lenses to amplify the flux of background sub-mm sources and to improve the spatial resolution at the source. Their sample includes now 17 sources brighter than S₈₅₀ = 6 mJy. Hughes et al. (1998) published a single very deep image of the HDF North containing 5 sources at S₈₅₀(4) 2 mJy.

Barger et al. (1998), while detecting only 2 sources down to 3 mJy, have carried out a very successful program of follow-up of SCUBA sources with optical telescopes on Mauna Kea. Eales et al. (1999) and Lilly et al. (1999) have published 12 sources to 3 mJy [a richer sample of 20 more sources is being published].

All these deep integrations are requiring many tens of hours each of especially good weather, which meant a substantial fraction of the JCMT observatory time. In spite of this effort, the surveyed areas (few tens of arcmin²) and number of detected sources are quite modest, which illustrates the difficulty to work from ground at these wavelengths.

The extragalactic source counts at 850 µm, reported in Figure 7, show a dramatic departure from the Euclidean law [N( > S) S^-2 in the crucial flux-density interval from 1 to 10 mJy], a clear signature of the strong evolution and high redshift of SCUBA-selected sources. Only 4 of them have been detected also at 450 µm, the sky transmission at Mauna Kea in this atmospheric channel is usually poor.

Figure 7. Integral counts at = 850 µm (see also caption to Fig. 6).

More recently, a new powerful bolometer array (MAMBO) has been put in operation on IRAM. Bertoldi et al. (2000) report the first results of observations at = 1.2 mm from a survey of 3 fields with a total area of over 300 arcmin² to a flux limit of few mJy.