Physical conditions and star formation activity in the intragroup medium of Stephan's Quintet

New multiband observations of the famous compact group of galaxies Stephan's Quintet (SQ) are presented and analyzed. These include far-infrared (FIR) images at 60 and 100 mum (ISOPHOT C100 camera), radio continuum images at 1.4 GHz (VLA B configuration) and 4.86 GHz ( VLA C configuration), and long-slit optical spectrographs (Palomar 200 telescope). With these new data, we aim to learn more about the X-ray/radio ridge in the middle of the intragroup medium (IGM) and the IGM starburst SQ-A, both of which are likely to be caused by the high-speed collision (similar to900 km s(-1)) between the intruder galaxy NGC 7318b (v = 5700 km s(-1)) and the IGM (v = 6600 km s(-1)). We found that the radio ridge has a steep nonthermal spectral index (alpha = 0.93 +/- 0.13) and an extremely low FIR-to-radio ratio index (q < 0.59). Its IR emission can be explained in terms of collisional heating of dust grains by shocked gas. The minimum-energy magnetic field strength is H-min approximate to 10 mu G. The long-slit spectra of sources in the ridge have typical emission-line ratios of shock-excited gas. The very broad line widths (>= 1000 km s(-1)) and the fact that in some cases more than two velocity systems were detected along the same line of sight provide further evidence for an ongoing collision along the ridge. The IGM starburst SQ-A has a radio spectral index alpha = 0.8 +/- 0.3 and an FIR-to-radio ratio index q = 2.0 +/- 0.4, consistent with those of star-forming regions. The optical spectra of two sources in this region, M1 (v = 6600 km s(-1)) and M2 (v = 6000 km s(-1)), have typical line ratios of H II regions. Both M1 and M2 have metallicity slightly higher than the solar value. The star formation rate estimated from the extinction-corrected H alpha luminosity of SQ-A is 1.45 M-. yr(-1), of which 1.25 M-. yr(-1) is due to the v = 6600 km s(-1) component and 0.20 M-. yr(-1) to the v = 6000 km s(-1) component.