Resolved OH megamaser emission in the nuclear region of the ultraluminous infrared galaxy Markarian 273

We present high angular resolution MERLIN observations of the 18-cm OH maser and continuum emission associated with the active core of the ultraluminous infrared galaxy Markarian 273. The continuum emission comes from three distinct regions in the central arcsecond of the galaxy. The brightest region of emission has a double-peaked structure which is spatially coincident with similar structures observed at 6 cm and 2.2 μm. The peak of the OH maser emission is spatially coincident with the peak in the continuum. For the first time the maser emission is spatially resolved, allowing us to measure the gas motion within the central 100 pc of the galaxy. Maser emission is found in both the 1665- and 1667-MHz lines, with no systematic offset found in the spatial locations of the two lines. The brighter component of the maser emission shows ordered motion and is aligned along the axis of the double-peaked structure in the brightest continuum region. The gas motion enables us to estimate the central mass density to be 850±50 M_⊙ pc⁻³, which corresponds to a total mass of ≈1.5×10⁸ M_⊙.     

Optical, infrared and radio observations of the Seyfert galaxy Markarian 315

We report on new optical, infrared and radio observations of the Seyfert galaxy Markarian 315. We confirm the detection at all wavelengths of a secondary peak of emission, which lies ≈ 2 arcsec east of the Seyfert nucleus. Moreover, we detect a chain-like structure which surrounds the active nucleus, with peculiar behaviour westward of the nucleus. We consider different interpretations for the origin of the secondary peak emission.     

MERLIN radio observations of CfA Seyferts

We present new 1.6-GHz (18-cm) MERLIN maps of 15 Seyfert galaxies, with angular resolutions typically 0.1 to 0.3 arcsec. These and previous observations are used to investigate the properties of 19 of the 24 CfA Seyfert galaxies brighter than 2 mJy at 8.4 GHz. This is the first time a significant fraction of the CfA sample has been mapped at this frequency with subarcsecond resolution, and our observations provide the highest resolution radio maps available for several sources. We use our observations to measure the two-point spectral indices of compact radio components, and we investigate the correlation between infrared and radio emission shown by Seyfert galaxies.
Our results can be summarized as follows. Resolved structures as small as 20 pc are found in three previously unresolved radio sources, and only four sources show single, unresolved radio components. The mean 1.6 to 8.4 GHz spectral index of 31 radio components is         , and approximately 25 per cent of the components have a spectral index flatter than     . The spectral index distributions of type 1 and type 2 Seyferts are statistically indistinguishable. The cores of multiple-component sources tend to have flatter radio spectra than secondary components. The low-resolution infrared ( IRAS ) emission from Seyfert galaxies is usually dominated by kiloparsec-scale, extranuclear emission regions.     

Free–free absorption in the Seyfert nucleus of NGC 4151

We present subarcsecond MERLIN 0.4-GHz (73 cm) and 1.6-GHz (18 cm) radio measurements of the nuclear region of the Seyfert galaxy NGC 4151. By comparison with higher frequency observations, we deduce that one component (C4) shows a low-frequency turnover which we interpret as evidence for free–free absorption by ionized gas with an emission measure between 3 × 10⁵ and 10⁶ pc cm⁻⁶. The free–free absorption appears to be localized to a region ∼50 pc in extent, and we consider models in which the ionized gas may be closely associated with a neutral molecular torus.     

3C 254: MERLIN observations of a highly asymmetric quasar

Multifrequency, high-resolution radio observations of the quasar 3C 254 using the Multi-Element Radio-linked Interferometer Network (MERLIN) are presented. The quasar has a highly asymmetric radio structure, with the eastern component of the double-lobed structure being much closer to the nucleus and significantly less polarized than the western one. However, the two lobes are more symmetric in their total flux densities. The observations show the detailed structure of the hotspots which are very different on opposite sides of the radio core, reveal no radio jet and suggest that the oppositely directed jets may be intrinsically asymmetric.     

H i and radio continuum study of the isolated SBa Seyfert galaxy NGC 3783

NGC 3783 is a nearby SBa, type 1 Seyfert galaxy. We present H  i and radio continuum images of the galaxy made with the Australia Telescope Compact Array (ATCA). We find that NGC 3783 has an H  i mass of 8.4×10⁹ M_⊙, an H  i diameter of 1.9 D ₀ ( D ₀=37 kpc for h =0.5), and a nuclear depression in the H  i surface density. The H  i rotation curve is dominated by differential rotation, with little evidence of warping. The rotation curve suggests a mass-to-light ratio M L _B =7.2 and a bar-pattern speed of 19±7 km s⁻¹ kpc⁻¹. The total mass of gas in the inner 50 arcsec is ≳10 per cent of the dynamical mass, and consistent with models that require significant gas content to fuel the Seyfert nucleus. There is no evidence that the nuclear activity in NGC 3783 is being stimulated by an interaction or merger: it may be a self-generated, perhaps bar-driven, process.     

The anomalous properties of Markarian 1460

We present and discuss optical, near-infrared and H  i measurements of the galaxy Markarian 1460 at a distance of 19 Mpc in the Ursa Major Cluster. This low-luminosity ( M _B =−14) galaxy is unusual because (i) it is blue ( B − R =0.8) and has the spectrum of an H  ii galaxy, (ii) it has a light profile that is smooth and well fitted by an r ^1/4 and not an exponential function at all radii larger than the seeing, and (iii) it has an observed central brightness of about μ _B =20 mag arcsec⁻² , intermediate between those of elliptical galaxies (on the bright μ _B side) and normal low-luminosity dwarf irregular (on the low μ _B side) galaxies. No other known galaxy exhibits all these properties in conjunction. On morphological grounds this galaxy looks like a normal distant luminous elliptical galaxy, since the Fundamental Plane tells us that higher luminosity normal elliptical galaxies tend to have lower surface-brightnesses. Markarian 1460 has 2×10⁷ M_⊙ of H  i and a ratio M (H  i )/ L _B of 0.2, which is low compared to the typical values for star-forming dwarf galaxies. From the high surface-brightness and r ^1/4 profile, we infer that the baryonic component of Markarian 1460 has become self-gravitating through dissipative processes. From the colours, radio continuum, H  i and optical emission line properties, and yet smooth texture, we infer that Markarian 1460 has had significant star formation as recently as ∼1 Gyr ago but not today.     

Radio Identifications of Markarian Galaxies and the Correlation between Radio and Far-Infrared Properties

By checking DSS optical images and NVSS radio images, 782 Markarian galaxies were identified to be NVSS radio sources. A comparison of the radio luminosity at 1.4 GHz and the far-infrared (FIR) luminosity for 468 "normal" galaxies shows a tight correlation. Most of the Seyfert galaxies and quasars follow the radio-FIR relation deduced from the "normal" galaxy sample, but with a somewhat larger scatter. A total 167 Markarian galaxies, comprising 100 "normal" galaxies, 66 Seyfert galaxies and one quasar, have either excess radio emission or much lower FIR spectral index α(25μm,60μm). These galaxies may be classified as "AGN-powered". For "normal" galaxies, the average q value (defined as the log ratio between FIR and radio luminosities) is 2.3. There seems a trend for q to slightly decrease with increasing radio luminosity. This may imply that the ongoing active star formation in galaxies with higher radio luminosities is more efficient in heating the cosmic-ray electrons.     

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.     

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.     

Long-term optical and X-ray variability of the Seyfert galaxy Markarian 79

We present the results of concurrent X-ray and optical monitoring of the Seyfert 1 galaxy Mrk 79 over a period of more than 5 yr. We find that on short to medium time-scales (days to a few tens of days) the 2–10 keV X-ray and optical u - and V -band fluxes are significantly correlated, with a delay between the bands consistent with 0 d. We show that most of these variations may be well reproduced by a model where the short-term optical variations originate from reprocessing of X-rays by an optically thick accretion disc. The optical light curves, however, also display long time-scale variations over thousands of days, which are not present in the X-ray light curve. These optical variations must originate from an independent variability mechanism and we show that they can be produced by variations in the (geometrically) thin disc accretion rate as well as by varying reprocessed fractions through changes in the location of the X-ray corona.     

Very Long Baseline Array imaging of a periodic 12.2-GHz methanol maser flare in G9.62+0.20E

The class II methanol maser source G9.62+0.20E undergoes periodic flares at both 6.7 and 12.2 GHz. The flare starting in 2001 October was observed at seven epochs over three months using the Very Long Baseline Array (VLBA) at 12.2 GHz. High angular resolution images (beam size  ∼1.7 × 0.6 mas  ) were obtained, enabling us to observe changes in 16 individual maser components. It was found that while existing maser spots increased in flux density, no new spots developed and no changes in morphology were observed. This rules out any mechanism which disturbs the masing region itself, implying that the flares are caused by a change in either the seed or pump photon levels. A time delay of one to two weeks was observed between groups of maser features. These delays can be explained by light travel time between maser groups. The regularity of the flares can possibly be explained by a binary system.