The composite starburst/AGN nature of the superwind galaxy NGC 4666

¹ INAF - Osservatorio Astronomico di Trieste, via GB Tiepolo 11, 34131 Trieste, Italy e-mail: persic@ts.astro.it
² IASF/CNR – Sezione di Bologna, via P. Gobetti 101, 40129 Bologna, Italy
³ School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
⁴ CASS, University of California, San Diego, La Jolla, CA 92093, USA
⁵ INAF/Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
⁶ Dipartimento di Astronomia, Università di Padova, vicolo Osservatorio 2, 35122 Padova, Italy
⁷ IASF/CNR – Sezione di Firenze, l.go E. Fermi 5, 50125 Firenze, Italy

Received: 25 April 2003
Accepted: 15 June 2004

Abstract

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 <10 keV, whereas spatially resolved emission from the whole disk is detected by XMM-Newton. A prominent ( keV) emission line at ~6.4 keV is detected by both instruments. From the XMM-Newton data alone the line is spectrally localized at  keV, and seems to be spatially concentrated in the nuclear region of NGC 4666. This, together with the presence of a flat () continuum in the nuclear region, suggests the existence of a strongly absorbed (i.e., Compton-thick) AGN, whose intrinsic 2–10 keV luminosity is estimated to be erg s^-1. At energies 1 keV the integrated (BeppoSAX) spectrum is dominated by a ~0.25 keV thermal gas component distributed throughout the disk (resolved by XMM-Newton). At energies ~2–10 keV, the integrated spectrum is dominated by a steep () power-law (PL) component. The latter emission is likely due to unresolved sources with luminosity erg s^-1 that are most likely accreting binaries (with BH masses ≤8 ). Such binaries, which are known to dominate the X-ray point-source luminosity in nearby star-forming galaxies, have PL spectra in the relevant energy range. A PL contribution from Compton scattering of (the radio-emitting) relativistic electrons by the ambient FIR photons may add a truly diffuse component to the 2–10 keV emission.