Star formation in southern Seyfert galaxies

We have produced radio maps, using the Australia Telescope Compact Array, of the central regions of six southern type 2 Seyfert galaxies (NGC 1365, 4945, 6221, 6810, 7582 and Circinus) with circumnuclear star formation, to estimate the relative contribution of star formation activity compared to activity from the active galactic nucleus (AGN). The radio morphologies range from extended diffuse structures to compact nuclear emission, with no evidence, even in the relatively compact sources, for synchrotron self-absorption. In each case the radio to far-infrared (FIR) ratio has a value consistent with star formation, and in all but one case the radio to [Fe  II ] ratio is also consistent with star formation. We derive supernova rates and conclude that, despite the presence of a Seyfert nucleus in these galaxies, the radio, FIR and [Fe  II ] line emissions are dominated by processes associated with the circumnuclear star formation (i.e. supernova remnants and H  II regions) rather than with the AGN.     

X-ray absorption in Seyfert 2 galaxies

We have studied the correlation among X-ray absorption, optical reddening and nuclear dust morphology in Seyfert 2 galaxies. Two main conclusions emerge: (i) the Balmer decrement and the amount of X-ray absorption are anticorrelated over a wide range of column density,     – the correlation no longer applies to Compton-thick objects     , although they span a comparable range in Balmer decrement; (ii) Compton-thin Seyfert 2s seem to prefer nuclear environments, which are rich in dust on scales of hundreds of parsecs. On the other hand, Compton-thick Seyferts indifferently exhibit 'dust-poor' and 'dust-rich' environments. These results support an extension of the Seyfert unification scenario (as recently proposed by Matt ), where Compton-thick Seyfert 2s are observed through compact 'torii', whereas Compton-thin ones are obscured by dust on much larger scales.     

On core-collapse supernovae in normal and in Seyfert galaxies

This paper estimates the relative frequency of different types of core-collapse supernovae, in terms of the ratio between the number of Type Ib–Ic and of Type II supernovae. We estimate independently for all normal and Seyfert galaxies whose radial velocity is ≤14 000 km s⁻¹, and which had at least one supernova event recorded in the Asiago catalogue from 1986 January to 2000 August. We find that the ratio is  ≈0.23±0.05  in normal galaxies. This value is consistent with constant star formation rate and with a Salpeter initial mass function and an average binary rate ≈50 per cent. On the contrary, Seyfert galaxies exceed the ratio in normal galaxies by a factor ≈4 at a confidence level ≳2 σ . A caveat is that the numbers for Seyferts are still small (six of Type Ib–Ic and six of Type II supernovae discovered as yet). Assumed to be real, this excess of Type Ib/c supernovae may indicate a burst of low-age star formation  ( τ ≲20 Myr)  , a high incidence of binary systems in the inner regions  ( r ≲0.4 R ₂₅)  of Seyfert galaxies, or a top-loaded mass function.     

Stellar velocity dispersion in narrow-line Seyfert 1 galaxies

Several authors have recently explored, for narrow-line Seyfert 1 galaxies (NLS1s), the relationship between black hole mass ( M _BH) and stellar velocity dispersion (σ_*). Their results are more or less in agreement and seem to indicate that NLS1s fill the region below the fit obtained by Tremaine et al., showing a range of σ_* similar to that of Seyfert 1 galaxies, and a lower M _BH. Until now, the [O  iii ] width has been used in place of the stellar velocity dispersion, but some indications have begun to arise against the effectiveness of the gaseous kinematics in representing the bulge potential, at least in NLS1s. Bian & Zhao have stressed the urgency of producing true σ_* measurements. Here, we present new stellar velocity dispersions obtained through direct measurements of the Ca  ii absorption triplet (∼8550 Å) in the nuclei of eight NLS1 galaxies. The resulting σ_* values and a comparison with σ_[O III] confirm our suspicion that [O  iii ] typically overestimates the stellar velocity dispersion. We demonstrate that NLS1s follow the   M _BH–σ_*  relation as Seyfert 1, quasars and non-active galaxies.     

On multi-nuclei features of some Markarian galaxies

We present a spectroscopic investigation of an unusual cluster of galaxies that contains galaxies Mrk 261 and 262 which have ultraviolet excess at the same redshift of about 0.03. We also study Mrk 266, which has a peculiar optical morphology and a binary nucleus. The nuclear components in both of these samples show quite similar emission lines in their spectra to Seyfert galaxy activity characteristics. The kinematics and physical properties of the gas in Mrk 266 allow a reliable mass estimate to be made assuming that the characteristic broad emission lines arise from the photoionization of the virialized clouds by the central ionizing nucleus. However, the nuclei masses would be overestimated if the radiation pressure and/or magnetic fields contribute significantly to the dynamics, or if the outflows or winds could cause the observed linewidths to exceed those induced by the nucleus potential alone.     

On the relative number of Seyfert 1 and Seyfert 2 galaxies and the opening angle of dust torus

It is shown that the samples of Seyfert galaxies used to study different parameters of Seyferts are largely incomplete. The way of estimation of the number of the missed, not detected Seyferts is proposed. It is based on the assumption that distribution of inclinations of Seyferts should be similar to that of spiral galaxies. Addition of the estimated number of the missed Seyferts allows to deduce more correct value of the ratio of numbers of Sy2s to Sy1s, N2/N1. The more real value of the ratio N2/N1 is the one deduced by using the results of counts of bright, nearby Seyferts, and is equal to ∼4. The opening angle of the dust torus is ∼76°.     

Seyfert galaxies and the radio–far-infrared correlation

We have observed a sample of 149 Seyfert galaxies and radio-quiet quasars at 13 cm with both a 275-km radio interferometer and the 6-km compact array of the Australia Telescope. The high-resolution observations searched for the presence of compact, high-brightness-temperature radio emission from the active nucleus. The low-resolution observations measured the total radio emission from the galaxy disc and Seyfert core and lobes. From these we draw the following conclusions. (i) Seyfert galaxies that lack compact radio cores display a correlation between radio and far-infrared (FIR) emission similar to the correlation displayed by normal spirals, albeit with greater scatter. The correlation is found to be intrinsic and is not an artefact of the richness effect. (ii) A very different radio–FIR correlation is displayed by those Seyferts that harbour compact radio cores. These tend to be more radio-loud than either normal spirals or the Seyferts that lack compact cores. The compact core emission thus seems to be responsible for the generally poor radio–FIR correlation displayed by Seyfert galaxies. (iii) The radio–FIR correlation is not significantly improved by subtracting off the 0.1-arcsec (20- to 200-pc) compact radio emission from the total radio emission. This suggests that the emission from the active galactic nucleus has significant structure on scales larger than 0.1 arcsec. Perhaps these structures are the 'linear' radio features that have been seen previously in Seyfert nuclei.     

Narrow-line Seyfert 1 galaxies and the evolution of galaxies and active galaxies

Narrow Line Seyfert 1 galaxies (NLS1s) are intriguing owing to their continuum as well as emission-line properties. The observed peculiar properties of the NLS1s are believed to be as a result of an accretion rate close to the Eddington limit. As a consequence of this, for a given luminosity, NLS1s have smaller black hole (BH) masses compared with normal Seyfert galaxies. Here we argue that NLS1s might be Seyfert galaxies in their early stage of evolution and as such may be low-redshift, low-luminosity analogues of high-redshift quasars. We propose that NLS1s may reside in rejuvenated, gas-rich galaxies. We also argue in favour of collisional ionization for production of Fe  ii in active galactic nuclei.     

Variability of Seyfert galaxies in the near-infrared — II. The unusual starburst Seyfert NGC 7469

The bright Seyfert 1 and starburst galaxy NGC 7469 has been monitored at JHKL for 13 yr. Its variations have been small, amounting to less than 0.3 mag, except for an event in 1989 when the flux from its active nucleus declined to about one quarter of its normal level.   Using the technique of flux variation gradients (FVGs), the variations have been analysed, taking into account the known JHK fluxes from the star-forming ring surrounding the nucleus. By comparing the FVGs of NGC 7469 with those of other Seyfert galaxies, the nucleus of NGC 7469 is found to suffer very little reddening.   The active nucleus normally contributes more than 50 per cent of the flux measured in the K and L bands through an aperture of 12 arcsec diameter.   Infrared excesses (above the fluxes expected from stars) are observed from the circumnuclear ring at both K and L and it is suggested that they arise from warm, small particles that also emit the aromatic hydrocarbon features seen in this galaxy.