MERLIN imaging of the maser flare in Markarian 348

MERLIN images of Mrk 348 at 22 GHz show H₂O maser emission at 0.02–0.11 Jy , within ∼ 0.8 pc of the nucleus. This is the first direct confirmation that molecular material exists close to the Seyfert 2 nucleus. Mrk 348 was observed in 2000 May one month after Falcke et al. first identified the maser in single-dish spectra. The peak maser flux density has increased about threefold. The masing region is ≲ 0.6 pc in radius. The flux density of radio continuum emission from the core has been rising for about 2 yr. The maser–core separation is barely resolved, but at the 3 σ significance level maser and core are not coincident along the line of sight. The masers lie in the direction of the northern radio lobes and probably emanate from material shocked by a jet with velocity close to c . The correlation between the radio continuum increase and maser flare is explained as arising from high-level nuclear activity through a common excitation mechanism, although direct maser amplification of the core by masers tracing a Keplerian disc is not completely ruled out.     

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_⊙.     

A radio study of the superwind galaxy NGC 1482

We present multifrequency radio continuum as well as H  i observations of the superwind galaxy NGC 1482, with both the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA). This galaxy has a remarkable hourglass-shaped optical emission-line outflow as well as bipolar soft X-ray bubbles on opposite sides of the galactic disc. The low-frequency, lower-resolution radio observations show a smooth structure. From the non-thermal emission, we estimate the available energy in supernovae, and examine whether this would be adequate to drive the observed superwind outflow. The high-frequency, high-resolution radio image of the central starburst region located at the base of the superwind bi-cone shows one prominent peak and more extended emission with substructure. This image has been compared with the infrared, optical red continuum, Hα, and soft and hard X-ray images from Chandra to understand the nature and relationship of the various features seen at different wavelengths. The peak of the infrared emission is the only feature that is coincident with the prominent radio peak, and possibly defines the centre of the galaxy.
The H  i observations with the GMRT show two blobs of emission on opposite sides of the central region. These are rotating about the centre of the galaxy and are located at ∼2.4 kpc from it. In addition, these observations also reveal a multicomponent H  i absorption profile against the central region of the radio source, with a total width of ∼250 km s⁻¹. The extreme blue- and redshifted absorption components are at 1688 and 1942 km s⁻¹, respectively, while the peak absorption is at 1836 km s⁻¹. This is consistent with the heliocentric systemic velocity of  1850 ± 20 km s⁻¹  , estimated from a variety of observations. We discuss possible implications of these results.     

Ground-based mid infra-red observations of nearby starburst and AGN galaxies

We present preliminary results of recent spatially resolved mid infrared (Mid-IR) spectroscopy with the MICHELLE instrument on UKIRT of NGC 7469, observed as part of a larger programme to study a sample of starburst and AGN in the Mid-IR. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Detection of an H92α recombination line in the starburst galaxy NGC 660

We have used the Very Large Array (VLA) to search for the H92α radio recombination line (RRL) in four starburst galaxies. In NGC 660, the line was detected over a 17Å‐8 arcsec² region near its starburst nucleus. The line and continuum emission indicate that the RRL-emitting gas is most likely in the form of a cluster of H ii regions with a small filling factor. Using a simple model we find that the total ionized mass in the nuclear region is in the range 2–8Å‐10⁴ M⊙ and the rate of production of UV photons N _Lyc∼1–3Å‐10⁵³ s⁻¹. The ratio of H92α and Brγ line intensities in NGC 660 indicates that extinction is significant even at λ=2 μm. The velocity field of the ionized gas is consistent with a rotating disc with an average velocity gradient of ∼15 km s⁻¹ arcsec⁻¹. The dynamical mass within the central 500 pc is ∼4Å‐10⁸ M⊙ and may be about ∼6Å‐10⁷ M⊙ within the central 120 pc. No line was detected in the other galaxies (NGC 520, NGC 1614 and NGC 6946) to a 3σ limit of 300 μJy. In the starburst galaxies in which RRLs have been detected, we find that there is a rough correlation between the integrated H92α line flux density and both the total far-infrared flux density and the radio continuum emission from the central region.     

Discovery of an X-ray-selected radio-loud obscured AGN at z = 1.246

We have discovered an obscured active galaxy at redshift z  = 1.246 identified with the ROSAT X-ray source RX J1011.2+5545. We report on multiwavelength observations of this source and discuss its X-ray, optical and radio properties. This is the first X-ray-selected, obscured active galaxy at high redshift to be shown to be radio-loud, with a radio counterpart exhibiting a classical double-lobe morphology.     

3C 459: a highly asymmetric radio galaxy with a starburst

Multifrequency radio observations of the radio galaxy 3C 459 using MERLIN, VLA and the EVN and an optical Hubble Space Telescope ( HST ) image using the F702W filter are presented. The galaxy has a very asymmetric radio structure, a high infrared luminosity and a young stellar population. The eastern component of the double-lobed structure is brighter, much closer to the nucleus and is significantly less polarized than the western one. This is consistent with the jet on the eastern side interacting with dense gas, which could be due to a merged companion or dense cloud of gas. The HST image of the galaxy presented here exhibits filamentary structures and is compared with the MERLIN 5-GHz radio map. EVN observations of the prominent central component, which has a steep radio spectrum, show a strongly curved structure suggesting a bent or helical radio jet. The radio structure of 3C 459 is compared with other highly asymmetric, Fanaroff–Riley II radio sources, which are also good candidates for studying jet–cloud interactions. Such sources are usually of small linear size and it is possible that the jets are interacting with clouds of infalling gas that fuel the radio source.     

Chimneys in the starburst galaxy M82

We present MERLIN and VLA observations at 1.4 and 5 GHz of the diffuse radio emission in the centre of M82. We detect a large expanding shell of ionized gas surrounding the brightest supernova remnant 41.95+57.5 with a diameter of ∼100 pc and an expansion velocity of ∼100 km s⁻¹. We observe a 50-pc-scale 'blow-out' from this region, in the form of a 'cone' of missing 5-GHz continuum emission, which appears to be an excellent example of a galactic chimney.
On larger radio scales, we observe four chimney structures extending out to the north ∼100–200 pc along the minor axis. One of these features is almost certainly related to the 50-pc-scale blow-out from 41.95+57.5, although this is not the most prominent feature. The other features have also been traced to expulsion of material from the very centre by using an 'unsharp masked' image from 5-GHz VLA B-array observations, with the supernova remnant removed.
We interpret these chimney features as the base of the superwind, which implies that the ejection of material into the halo does not occur smoothly over the starburst region. Instead, very localized venting of hot gas is clearly in evidence.