ISO observations of four active galaxies ^*

¹ Institut d'Astrophysique de Paris, 98bis Bd Arago, 75014 Paris, France
² Max-Planck Institut for Extraterrestrial Physics, 85740 Garching, Germany
³ Kapteyn Astronomical Institute, 9700 AV Groningen, The Netherlands

Corresponding author: M. Dennefeld, dennefel@iap.fr

Received: 23 December 2002
Accepted: 23 April 2003

Abstract

We present ISO PHOT-S spectra of four galaxies known or suspected to host a central AGN. Two of them are selected, among several others, from the initial Iras/Rosat sample of Boller et al. ([CITE]) because of their substantial X-ray emission, while no obvious Seyfert features was present in their optical spectra: IRAS 14201+2956 and IRAS 21582+1018. The latter, also known as Mrk 520, was bright enough to also allow SWS observations around selected neon lines, to establish its excitation. While both PHOT-S spectra are characteristic of starburst-dominated galaxies, the neon line ratios in IRAS 21582+1018  indicate the presence of a hard excitation source. Complementary optical spectra, both at low and high spectral resolution, show only a weak, broad component around H_α, establishing the presence of a central AGN which may not be detected in standard, classification spectra. Both objects are now classified as Sey 1.9 galaxies. These results show therefore that, although IR observations were expected to be able to pierce through the dusty central regions to reveal the presence of an active nucleus, the result may be ambiguous: the broad band IR energy distribution can still be dominated by starburts located in a wider circumnuclear region, and the AGN appear only in specific observations (high-excitation lines in the IR, or high-resolution optical spectra). As a complement, two other galaxies from the same initial sample were also observed with PHOT-S: the Narrow Line Seyfert galaxies (NLS1) Mrk 359 and Mrk 1388. NLS1s appear in high proportion in the Rosat/Iras sample, and in soft X-ray samples in general, and their Balmer line-widths are sometimes comparable to those of interacting, star-forming galaxies. Their ISO spectra however do not reveal the typical, strong PAH features found in the starburst galaxies and are more like those of standard Seyferts. All these observations therefore indicate that the key element is the presence or absence of a circumnuclear starburst region which, if strong enough, may completely hide the presence of a central AGN in the IR spectral energy distribution. The dust obscuration however needs to be patchy rather than complete to explain the detection of the high-excitation lines and Balmer wings in some cases. Only high-energy observations can then establish the strength of the central AGN and the amount of extinction with certainty.