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Astron. Astrophys. 343, 23-32 (1999)

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5. Conclusions

We have analysed the X-ray properties of the E4 galaxy NGC 3923 over the energy range (0.5-10) keV, using BeppoSAX data. Our findings are as follows.

  1. The superposition of two thermal components of [FORMULA][FORMULA] keV, and [FORMULA][FORMULA] keV, is the most reasonable representation of the spectral data. The heavy element abundances cannot be constrained due to the uncertainties in the spectral data; at the best fit they are very subsolar (except for the cooling flow model), and much lower than the stellar mean abundance.

  2. The two components have roughly comparable fluxes in the (0.5-4.5) keV band, while the hard component amounts to [FORMULA] of the total (0.5-10) keV flux.

  3. The temperature of the softer component suggests as its origin the emission of a hot gas, the origin of the hard component is likely the integrated emission of LMXBs. In fact, in addition to the spectral shape, also the amount of the hard emission is consistent with that predicted for stellar sources in NGC 3923.

  4. [FORMULA]  is close to the kinetic temperature of the stars in NGC 3923, but some additional heating is needed. Since [FORMULA]  is less than predicted by a steady state cooling flow model, for a galaxy of an optical luminosity as high as that of NGC 3923, it is suggested that a large fraction of the stellar mass loss was removed by internal agents, such as the heating of SNIa's explosions, and that this process was helped by the flat mass distribution of the galaxy. Another possibility, that has to be explored with numerical simulations, is that a substantial amount of hot gas was lost as a consequence of the episod of interaction or merger, with a much smaller galaxy, which gave origin to the system of shells visible in the optical.

  5. The origin of the X-ray emission in galaxies of low and medium [FORMULA]/[FORMULA]  is finally reviewed. The detailed study of these galaxies is crucial to establish which factor plays the major role in lowering the amount of hot gas, and so to explain the large scatter in the [FORMULA]/[FORMULA]  plane. Is this factor to be linked to external agents, as stripping by ambient gas or interactions with other galaxies, or to internal heating mechanisms, such as SNIa explosions or accreting supermassive black holes? The galaxy discussed here is not surrounded by a dense medium, so that ram pressure stripping cannot be invoked; but galaxy interactions clearly took place, and so cannot be excluded as causes of the loss of hot gas. In general, since many other low and medium [FORMULA]/[FORMULA]  galaxies reside in small groups, in which the density of the environment is presumably low or very low, it is concluded that the most effective mechanism, in order to explain the [FORMULA]/[FORMULA]  of these galaxies always with environmental effects, must be galaxy interactions. But it is remarked that in general low or medium [FORMULA]/[FORMULA]  values are also shown by galaxies that reside in a region where the galaxy density is not particularly high, and where also galaxies of high [FORMULA]/[FORMULA]  are found.

  6. Another result emerged here is that NGC 3923 is the dominant elliptical of its group, but has just a medium value of [FORMULA]/[FORMULA]. So, an optically dominant galaxy does not always have a high hot gas content.

NGC 3923 also belongs to a program of investigating the nature of the X-ray emission in flat galaxies with very well known internal kinematics and photometry (see Pellegrini et al. 1997), for which the mass profile can be derived with accuracy. Two-dimensional numerical simulations are in program to study in detail the hot gas evolution in these galaxies, including the effect of galaxy interactions for this particular case.

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999