Multifrequency observations of the interacting galaxy NGC 4922 (UCM 1259+2934)

We present new multifrequency observations (X-ray ROSAT HRI, optical r Gunn -Thuan and near-infrared J and K imaging, and optical spectroscopy) of the interacting galaxy pair UCM 1259 + 2934 (NGC 4922A + B). These data are use d to study the origin of the extended soft X-ray emission and the star form ation history of the northern component of the interacting system. The sout hern component (NGC 4922S) shows no signs of activity, with an optical spec trum consistent with that of an elliptical galaxy, which is also supported by its morphology. The northern component (NGC 4922N) shows evidence of com posite activity; the nuclear optical line ratios are intermediate between t hose of a LINER and a Seyfert 2 galaxy. Its circumnuclear regions resemble a normal H II region. NGC 3922N can be classified as a transition object, i n the sense that its optical emission lines may be produced by both a non-s tellar continuum and hot stars. The ROSAT HRI observation shows extended soft X-ray emission which appears to peak close to the nucleus of NGC 4922N, with luminosity L-X(0.1-2.4 keV) = (2.6-1.3) x 10(41) erg s(-1), depending on the parameters of the adopted model. The extended nature of the source rules out a situation in which mo st of the X-ray emission originates from an active nucleus. The most likely mechanism responsible for the extended soft X-ray emission is star formati on activity via supernova remnants and/or massive X-ray binaries (MXRBs) fo rmed in a burst of star formation that occurred about 7 x 10(6) yr ago, plu s a large-scale supernova-driven wind. Based on the far-infrared versus X-r ay correlations for high-flux IRAS galaxies, we can conclude that the soft X-ray emission is arising from both galaxies in the system, though dominate d by the northern component. An evolutionary synthesis model is used to investigate the star formation h istory of NGC 4922N, From the extinction-corrected r - J and J - K colours we argue that between 0.1 and 1 per cent of the total stellar mass of the g alaxy was formed during the last episode of star formation. The star format ion rate (SFR) of the northern component, 6 less than or equal to SFR less than or equal to 35 M-circle dot yr(-1) as inferred from the H alpha and fa r-infrared luminosities, is higher than typical SFRs of isolated galaxies. Such a high SFR could result from an episode of star formation triggered by interaction with the southern component.