arXiv:1001.2458v3 [astro-ph.CO] 29 Aug 2010 November 20, 2018 Preprint typeset using L A T E X style emulateapj v. 04/21/05 ON A METHOD TO RESOLVE THE NUCLEAR ACTIVITY IN GALAXIES AS APPLIED TO THE SEYFERT 2 GALAXY NGC 1358 ⋆ November 20, 2018 ABSTRACT Nuclear regions of galaxies generally host a mixture of components with different excitation, com- position, and kinematics. Derivation of emission line ratios and kinematics could then be misleading, if due correction is not made for the limited spatial and spectral resolutions of the observations. The aim of this paper is to demonstrate, with application to a long slit spectrum of the Seyfert 2 galaxy NGC 1358, how line intensities and velocities, together with modelling and knowledge of the point spread function, may be used to resolve the differing structures. In the situation outlined above, the observed kinematics differs for different spectral lines. From the observed intensity and velocity distributions of a number of spectral lines, and with some reasonable assumptions about structure of different subcomponents to diminish the number of free parameters, the true line ratios and velocity structures may be deduced. A preliminary solution for the nuclear structure of NGC 1358 is obtained, involving a nuclear point source and an emerging outflow of high excitation, ending with shock and postshock cloud as revealed by the velocities, as well as a nuclear emission line disk rotating in the potential of a stellar bulge and expressing a radial excitation gradient. The method results in a likely scenario for the nuclear structure of the Seyfert 2 galaxy NGC 1358. For definitive results an extrap- olation of the method to two dimensions combined with the use of integral field spectroscopy will be necessary. Subject headings: methods: data analysis – galaxies: structure – galaxies: kinematics and dynamics – galaxies: active 1. INTRODUCTION The nuclear and circumnuclear activity of galaxies gen- erally involves the interplay between a number of differ- ent components and phenomena, e.g. a central active source surrounded by an absorbing torus, a rotating cen- tral bulge, outflowing jets, flowing streams due to the action of a bar, or even merging. To separate these dif- ferent components and derive their respective line ratios and kinematic behaviour is generally difficult due to the limited spatial and spectroscopic resolution available. On the other hand, such a separation is crucial to the anal- ysis of the structures and physical processes involved in the nuclear region and their roles in galaxy evolution. Then, as to be demonstrated here, such a separation could benefit by considering the differences of the veloc- ities observed for different spectral lines and be eased by models of the activity, smoothed with the point spread function (PSF) and fitted to the observations. NGC 1358 (Fig. 1) is a barred Sa galaxy hosting an active galactic nucleus (AGN) at a heliocentric velocity of 4028 km s − 1 (Theureau et al. 1998), which is included in the distance-limited sample of Ulvestad & Wilson (1989) as a Seyfert 2. It attracted our attention because of its remarkable circumnuclear kinematics which also allows demonstrating the utility of the method introduced here. Assuming a Hubble constant of 71 km s − 1 Mpc − 1 (Spergel et al. 2003), the distance of NGC 1358 amounts to 57 Mpc. Then, 1 ′′ corresponds to 275 pc. The outer optical disk of NGC 1358, as seen in Fig. 1, including the narrow spiral arms, has a radius of 1 $^{′}$, or 16.5 kpc. The bar along a position angle of 130 ◦ has a semi-major axis length of 19 ′′ (Gerssen et al. 2003) or 5.2 kpc. The continuum peak at the nucleus shows a half power half width of 1 . $^{′′}$3. Dumas et al. (2007) derived, by in- tegral field spectroscopy, the velocity fields of the ion- ized gas in the central regions in the emission line of [OIII] λ 5007 as well as in the stellar absorption lines (the latter displayed only in a poster). The stellar ve- locity field shows a kinematic major axis in a position angle PA 11.5 ◦ with the North side receding. This is also close to the assumed PA of the line of nodes of the outer disk. The authors derive a symmetric stellar ro- tation curve out to ± 4 ′′ from the nucleus. The [OIII] velocity field, on the other hand, shows a kinematic ma- jor axis in about PA 90 ◦ with the East side receding. Narrow band imaging in [OIII] λ 5007 shows a slightly S-shaped emission line region with a maximum intensity 1 . $^{′′}$6 from the nucleus, again along a PA of about 130◦ (Dumas et al. 2007). This feature shows clearly deviat- ing negative velocities. Dumas et al. (2007) suggest the elongated nuclear emission line region to correspond to gaseous streaming due to an inner bar inside the Inner Lindblad Resonance of the outer bar. An HST/STIS spectrum covering the [SII] λ 6716, 6731 lines, within an aperture size of 0 . $^{′′}$2 centered on the nu- cleus, shows double components with the fainter compo- nent blueshifted with respect to the stronger (Rice et al. 2006). 2. OBSERVATIONS Per Olof Lindblad $^{1}$, Kambiz Fathi1 , $^{2}$, Maja Hjelm$^{1}$, Charles H. Nelson3 1 Stockholm Observatory, Department of Astronomy, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden 2 Oskar Klein center for Cosmoparticle Physics, Stockholm Uni- versity, 106 91 Stockholm, Sweden 3 Physics and Astronomy Department, Drake University, 2507 University Avenue, Des Moines, IA 50311, USA ⋆ Based on observations at the European Southern Observatory.