Active galaxies exhibit several different phenomena in their nuclei. Within the central few hundred lightyears there can be a rotating accretion disk that feeds a supermassive black hole, which form the actual nucleus, surrounded by gas with high density and rapid rotation, in turn surrounded by a thick ring of absorbing interstellar dust. Highly excited gas streams outwards from the black hole, concentrated along its rotation axis. Further out is an extended region of less excited gas, while gas and dust streams inwards to the central region due to perturbations from a bar or spiral arms in the galaxy's outer disk. To distinguish all these details in a distant galaxy is difficult due to the seeing in ground based observations and the limited resolution of the telescope.

A research group at the Department of Astronomy has tried to distinguish the details in the centre of a Seyfert galaxy (a particular kind of active galaxy) with the help of an untested method which uses that different spectral lines have different velocity patterns. The method uses the dispersion image caused by the seeing of a star on the slit. A model for the different components in the galaxy's centre, modified by the dispersion image, is then fitted to the observed patterns of the intensity and radial velocity for emission lines from neutral hydrogen, excited nitrogen and highly excited oxygen. The observation material is a long-slit spectrum of the galaxy NGC 1358 obtained with ESO's 2.2 m telescope on the mountain La Silla in Chile.

As a result of this fitting the research group concludes that from a bright and unresolved highly excited source at the galaxy's centre a 1200 lightyears long jet of equally highly excited gas is launched with a line-of-sight velocity of 214 km/s towards us. Around this centre is a rotating gas disk with an outwards declining excitation and with a radius of the order of a couple of thousand lightyears. Outside the jet there is gas of lower excitation that follows the rotation of the gas disk. All this is enclosed by a central stellar bulge, where the rotation of the gas disk gives that the bulge mass can be estimated to be 14 billion solar masses.

Investigations of this kind would improve by extending it to be performed with two-dimensional spectrographs, where a spectrum is obtained in every pixel in a two-dimensional field.

Original article:
On a method to resolve the nuclear activity in galaxies as applied to the Seyfert 2 galaxy NGC1358 Contact:

Per Olof Lindblad, Tel: 08-5537 8523, po@astro.su.se
Kambiz Fathi, Tel: 08-5537 8521, kambiz@astro.su.se