Astron. Astrophys. 340, 351-370 (1998)

1. Introduction

The nearby spiral galaxy NGC 3079 is seen almost edge-on and has been investigated in detail at many wavelengths. Relevant galaxy parameters used throughout this paper are presented in Table 1. In optical light NGC 3079 shows disturbed morphology and dust lanes. The nuclear spectra indicate LINER/Seyfert 2 activity (Heckman 1980, Ford et al. 1986). In radio, NGC 3079 stands out for anomalous filamentary and bubble-like structure extending for 3 kpc along the minor axis to both sides of a bright compact nucleus (de Bruyn 1977, Seaquist et al. 1978, Hummel et al. 1983, Duric et al. 1983, Duric & Seaquist 1988). Extended radio emission also arises from the disk of the galaxy. A somewhat smaller loop of optical H emission (Ford et al. 1986, Duric & Seaquist 1988) or a super-bubble (Veilleux et al. 1994) appears associated with the eastern radio loop. With the help of optical long slit spectra these features have been interpreted as evidence of a super-wind powered by a nuclear starburst (Heckman et al. 1990) or an active nucleus (Filippenko & Sargent 1992). Veilleux et al. (1995) have further investigated the disk-halo connection in a detailed study of the diffuse ionized medium (DIM). They find that DIM contributes 30% of the total emission of the galaxy disk within a radius of 10 kpc and that, within a radius of 5 kpc, X-shaped filaments rise for more than 4 kpc above the disk plane. Several bubbles and filaments within 1 kpc of the disk plane are interpreted as direct evidence for gas flow between the disk and halo. Within its distance, NGC 3079 is known to be one of the brightest sources of FIR continuum, CO line and H₂O maser emission (e.g. Henkel et al. 1984, Soifer et al. 1989, Irwin & Sofue 1992, Greenhill et al. 1995, Harwarden et al. 1995, Braine et al. 1997) reminiscent of molecule and dust clouds and circumnuclear starburst activity. The H I distribution has been modeled by Irwin & Seaquist (1991), and slightly resolved H I and OH absorption was discovered against the radio core (Gallimore et al. 1994, Baan & Irwin 1995) indicative of a central mass within the inner 90 pc radius of more than .

Table 1. Parameters of NGC 3079
References:
(1) Tully 1988, (2) Tully et al. 1992, (3) Irwin & Seaquist 1991, (4) Dickey & Lockman 1990

NGC 3079 is member of a group of galaxies consisting of the giant spiral, 0 5 to the NW, and NGC 3079, and two small companions, MCG 9-17-9, 6: NGC 3073 (= Mrk 131), 10´ to the SW (Irwin et al. 1987, Irwin & Seaquist 1991). MCG 9-17-9 is a small spiral of type Sb-Sc and NGC 3073 an early type (SAB0-) galaxy. While in the optical the two companions do not show signs of disturbance, in the radio NGC 3073 is found to exhibit an elongated H I tail which is aligned with the nucleus of NGC 3079, and the H I emission of MCG 9-17-9 is slightly extended in the direction of the nucleus of NGC 3079, too. An H II region spectrum coupled with deep Balmer absorption lines indicates that the stellar population of NGC 3073 is very young. Filippenko & Sargent (1992 and references therein) postulate that the copious star formation observed in NGC 3073 might have been triggered by the super-wind from NGC 3079 (see above), which, in projection, points in the direction of NGC 3073. From these observations it has been speculated by Irwin et al. that a hot inter-galaxy medium may be present in this group that might be observable in X-rays.

NGC 3079 was detected as an X-ray source by the Einstein observatory (Fabbiano et al. 1982, 1992) with a luminosity of erg s^-1 in the 0.5-4 keV range (corrected to a distance of 17.8 kpc and for Galactic foreground absorption of cm^-2, and assuming a thermal bremsstrahlung spectrum with a temperature of 5 keV). ROSAT PSPC data of the galaxy have been analyzed by Reichert et al. (1994) and Read et al. (1997). They find an unresolved nuclear point source with a luminosity in the 0.1-2 keV band of erg s^-1 and a diffuse emission component of erg s^-1 . Dahlem et al. (1998) investigate the integral galaxy spectrum using ROSAT PSPC and ASCA data, and in addition present an overlay of ROSAT HRI contours over an image of the center of the galaxy.

In this paper we present ROSAT HRI observation of NGC 3079 and a detailed reanalysis of the ROSAT PSPC data extracted from the ROSAT archive.

© European Southern Observatory (ESO) 1998

Online publication: November 9, 1998
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