Astronomy & Astrophysics

Free access article

Issue		A&A Volume 427, Number 1, November III 2004
Page(s)		35 - 44
Section		Extragalactic astronomy
DOI		10.1051/0004-6361:20041128

A&A 427, 35-44 (2004)
DOI: 10.1051/0004-6361:20041128

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

M. Persic¹, M. Cappi², Y. Rephaeli^{3, 4}, L. Bassani², R. Della Ceca⁵, A. Franceschini⁶, L. Hunt⁷, G. Malaguti² and E. Palazzi²

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 ( $EW \sim 1{-}2$ keV) emission line at ~6.4 keV is detected by both instruments. From the XMM-Newton data alone the line is spectrally localized at $E \simeq 6.42 \pm 0.03$ keV, and seems to be spatially concentrated in the nuclear region of NGC 4666. This, together with the presence of a flat ( $\Gamma\sim 1.3$ ) 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 $L_10} \ga2 \times 10^$ erg s ^-1. At energies $\la$ 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 ( $\Gamma\ga2$ ) power-law (PL) component. The latter emission is likely due to unresolved sources with luminosity $L \sim 10^{-}10^$ erg s ^-1 that are most likely accreting binaries (with BH masses $\leq$ 8 $M_\odot$ ). Such binaries, which are known to dominate the X-ray point-source luminosity in nearby star-forming galaxies, have $\Gamma\sim 2$ PL spectra in the relevant energy range. A $\Gamma\sim 1.8$ 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.

Key words: X-rays: galaxies -- galaxies: starburst -- galaxies: Seyfert -- galaxies: individual: NGC 4666

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