Astronomy and Astrophysics, volume 427, 35-44 (2004/11-3)
The composite starburst/AGN nature of the superwind galaxy NGC 4666.
PERSIC M., CAPPI M., REPHAELI Y., BASSANI L., DELLA CECA R., FRANCESCHINI A., HUNT L., MALAGUTI G. and PALAZZI E.
Abstract (from CDS):
We report the discovery of a Compton-thick AGN and of intense star-formation activity in the nucleus and disk, respectively, of the nearly edge-on superwind galaxy NGC 4666. Spatially unresolved emission is detected by BeppoSAX only at energies <10keV, whereas spatially resolved emission from the whole disk is detected by XMM-Newton. A prominent (EW∼1-2keV) emission line at ∼6.4keV is detected by both instruments. From the XMM-Newton data alone the line is spectrally localized at E≃6.42 ±0.03keV, and seems to be spatially concentrated in the nuclear region of NGC 4666. This, together with the presence of a flat (Γ∼1.3) continuum in the nuclear region, suggests the existence of a strongly absorbed (i.e., Compton-thick) AGN, whose intrinsic 2-10keV luminosity is estimated to be L2–10>2x1041erg/s. At energies ≲1keV the integrated (BeppoSAX) spectrum is dominated by a ∼0.25keV thermal gas component distributed throughout the disk (resolved by XMM-Newton). At energies ∼2-10keV, the integrated spectrum is dominated by a steep (Γ>2) power-law (PL) component. The latter emission is likely due to unresolved sources with luminosity L∼1038-1039erg/s that are most likely accreting binaries (with BH masses ≤8M☉). Such binaries, which are known to dominate the X-ray point-source luminosity in nearby star-forming galaxies, have Γ∼2PL spectra in the relevant energy range. A Γ∼1.8PL contribution from Compton scattering of (the radio-emitting) relativistic electrons by the ambient FIR photons may add a truly diffuse component to the 2-10keV emission.
X-rays: galaxies - galaxies: starburst - galaxies: Seyfert - galaxies: individual: NGC 4666
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