ASTRONOMY AND ASTROPHYSICS BeppoSAX observation of NGC 3923, and the problem of the X-ray emission in E/S0 galaxies of low and medium LX/LB

We present the results of the analysis of the BeppoSAX LECS and MECS pointed observation of the E4 galaxy NGC 3923, for which previous X-ray measurements had given a medium X-ray to optical ratio LX/LB. The spectral analysis over (0.5–10) keV reveals that the best representation of the data is the superposition of two thermal components at temperatures of 0.4 keV and 6–8 keV. The total emission is roughly equally divided between the two components, over (0.5–4.5) keV. Abundances are very subsolar at the best fit, but not constrained by the data. The harder component is consistent with an origin from stellar sources; the softer component likely comes from hot gas. LX of this hot gas is not as large as expected for a global inflow, in a galaxy of an optical luminosity as high as that of NGC 3923. So, it is suggested that a substantial amount of hot gas was removed by internal agents, and that this process was helped by the flat mass distribution of the galaxy. Another possibility is that gas was lost as a consequence of the episod of interaction or merger that produced the system of shells visible in the optical band. Finally, the possible origins of the large scatter in the Xray emission shown by galaxies of similar LB are also reviewed. Like NGC 3923, many other low and medium LX/LB galaxies reside in small groups, in which the ambient medium (if present) cannot strip them of their hot gas; so, if only environmental factors are invoked to lower LX/LB, the most effective mechanism must be galaxy interactions. The lower LX/LB galaxies, though, are seen to occur across the whole range of galaxy densities. Another possibility, to remove some or all of the hot gas, appeals to mechanisms internal to the galaxies, such as heating of the gas by supernovae explosions or accretion onto central massive black holes. This has been shown to work in general, for a large range of LB, but it is not clear yet whether the hot gas abundances estimated from recent observations can be accomodated in it.