Radio and far-infrared extragalactic sources at Planck frequencies

We discuss the main uncertainties affecting estimates of small scale fluctu ations due to extragalactic sources in the Planck Surveyor frequency bands. Conservative estimates allow us to confidently conclude that, in the frequ ency range 100-200 GHz, the contaminating effect of extragalactic sources i s well below the expected anisotropy level of the cosmic microwave backgrou nd (CMB), down to angular scales of at least similar or equal to 10'. Hence , an accurate subtraction of foreground fluctuations is not critical for th e determination of the CMB power spectrum up to multipoles l similar or equ al to 1000. In any case, Planck's wide frequency coverage will allow to car efully control foreground contributions. On the other hand, the all sky sur veys at a frequencies, spanning the range 30-900 GHz, will be unique in pro viding complete samples comprising from several hundreds to many thousands of extragalactic. sources, selected in an essentially unexplored frequency interval, New classes of sources may be revealed in these data, Extremely c ompact radio sources, whose radio emission is relativistically boosted both in intensity and in frequency wilt be particularly prominent, Within this frequency region, very compact synchrotron components become optically thin ; the corresponding break frequency is a key parameter in models of the ene rgy distribution. Crucial information will be provided to understand the na ture of radio sources with strongly inverted spectra. Scenarios for the cos mological evolution of galaxies will be tested.