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We present the detection of a low surface-brightness stellar counterpart to an enormous (190 kpc) ring of neutral hydrogen (H I) gas that surrounds the nearby radio galaxy B2 0648+27. This system is currently in an evolutionary stage between major merger and (radio-loud) early-type galaxy. In a previous paper we investigated in detail the timescales between merger, starburst and AGN activity in B2 0648+27, based on its unusual multi-wavelength properties (large-scale H I ring, dominating post-starburst stellar population and infra-red luminosity). In this Research Note we present deep optical B- and V-band imaging that provides further evidence for the merger origin of B2 0648+27. The host galaxy shows a distorted optical morphology and a broad tidal arm is clearly present. A low surface-brightness stellar tail or partial ring curls around more than half the host galaxy at a distance of up to 55 kpc from the centre of the galaxy, following the large-scale, ring-like H I structure that we detected previously around this system. The gas and stars in this ring are most likely tidally expelled material that slowly fell back onto the host galaxy after the merger event. There also appear to be sites of star formation within the H I ring that may have formed within the gaseous tidal debris after the merger. We argue that the observed properties of the gas and stars in B2 0648+27, as well as the apparent time-delay between the merger and the starburst event, may be the logical result of a merger between two gas-rich disk galaxies with a prominent bulge, or of a merger between an elliptical and a gas-rich spiral galaxy. There also appears to be a significant time-delay between the merger/starburst event and the current episode of radio-AGN activity.

We present new ATCA 17- and 24-GHz radio images and ESO-NTT optical spectra of the radio-loud Seyfert galaxy IC 5063, the first galaxy in which a fast (~600 km s-1) outflow of neutral hydrogen was discovered. The new radio data confirm the triple radio structure with a central, unresolved flat-spectrum core and two resolved radio lobes with steep spectral index. This implies that the previously detected fast outflow of neutral gas is occurring off-nucleus, near a radio lobe about 0.5 kpc from the core. The ionised gas shows highly complex kinematics in the region co-spatial with the radio emission. Broad and blueshifted (~500 km s-1) emission is observed in the region of the radio lobe, at the same location as the blueshifted $\ion{H}{i}$absorption. The velocity of the ionised outflow is similar to the one found in $\ion{H}{i}$. The first order correspondence between the radio and optical properties suggests that the outflow is driven by the interaction between the radio jet and the ISM. However, despite the high outflow velocities, no evidence is found for the ionisation of the gas being due to fast shocks in the region of the outflow, indicating that photoionisation from the AGN is likely to be the dominant ionisation mechanism. The outflow rate of the warm (ionised) gas is small compared to that of the cold gas, similar to what is found in other radio galaxies. The mass outflow rate associated with the $\ion{H}{i}$is in the same range as for “mild” starburst-driven superwinds in ULIRGs. However, in IC 5063, the AGN-driven outflow appears to be limited to the inner kpc region of the galaxy. The kinetic power associated with the $\ion{H}{i}$outflow is a small fraction (a few $\times$10-4) of the Eddington luminosity of the galaxy but is a significant fraction (~0.1) of the nuclear bolometric luminosity. In IC 5063, the observed outflows may have sufficient kinetic power to have a significant impact on the evolution of the ISM in the host galaxy.

We present results of a study of neutral hydrogen (H I) in a complete sample of nearby non-cluster radio galaxies. We find that radio galaxies with large amounts of extended H I ($M_{\rm H{\,\small I}} \gtrsim 10^9~M_{\odot}$) all have a compact radio source. The host galaxies of the more extended radio sources, all of Fanaroff & Riley type-I, do not contain these amounts of H I. We discuss several possible explanations for this segregation. The large-scale H I is mainly distributed in disk- and ring-like structures with sizes up to 190 kpc and masses up to $2 \times 10^{10}~M_{\odot}$. The formation of these structures could be related to past merger events, although in some cases it may also be consistent with a cold-accretion scenario.

We use neutral hydrogen (H I) and optical spectroscopic observations to compare the timescales of a merger event, starburst episode and radio-AGN activity in the radio galaxy B2 0648+27. We detect a large ring-like structure of H I in emission around the early-type host galaxy of B2 0648+27($M_{\rm H{\,\small I}} = 8.5 \times 10^{9}~M_{\odot}$, diameter = 190 kpc). We interpret this as the result of a major merger that occurred ≳1.5 Gyr ago. From modelling optical long-slit spectra we find that a young stellar population of 0.3 Gyr, indicative of a past starburst event, dominates the stellar light throughout the galaxy. The off-set in time between the merger event and the starburst activity in B2 0648+27suggests that the starburst was triggered in an advanced stage of the merger, which can be explained if the gas-rich progenitor galaxies contained a bulge. Although the exact age of the radio source remains uncertain, there appears to be a significant time-delay between the merger/starburst event and the current episode of radio-AGN activity. We also observe an outflow of emission-line gas in this system, which is likely related to superwinds driven by the stars that formed during the starburst event. We argue that the radio galaxy B2 0648+27is a link in the evolutionary sequence between Ultra-Luminous Infrared Galaxies (ULIRGs) and genuine early-type galaxies.

We present high-spatial resolution 21-cm H  iVLA observations of the radio galaxy 3C 305 ($z=0.041$). These new high-resolution data show that the ∼1000  km s-1broad $\ion{H}{i}$absorption, earlier detected in low-resolution WSRT observations, is occurring against the bright, eastern radio lobe, about 1.6 kpc from the nucleus. We use new optical spectra taken with the WHT to make a detailed comparison of the kinematics of the neutral hydrogen with that of the ionised gas. The striking similarity between the complex kinematics of the two gas phases suggests that both the ionised gas and the neutral gas are part of the same outflow. Earlier studies of the ionised gas had already found evidence for a strong interaction between the radio jet and the interstellar medium at the location of the eastern radio lobe. Our results show that the fast outflow produced by this interaction also contains a component of neutral atomic hydrogen. The most likely interpretation is that the radio jet ionises the ISM and accelerates it to the high outflow velocities observed. Our observations demonstrate that, following this strong jet-cloud interaction, not all gas clouds are destroyed and that part of the gas can cool and become neutral. The mass outflow rate measured in 3C 305 is comparable, although at the lower end of the distribution, to that found in Ultra-Luminous IR galaxies. This suggests that AGN-driven outflows, and in particular jet-driven outflows, can have a similar impact on the evolution of a galaxy as starburst-driven superwinds.

The radio source 4C 12.50 has often been suggested to be a prime candidate for the link between ultraluminous infrared galaxies and young radio galaxies. A VLBI study of the neutral hydrogen in the nuclear regions of this object shows that most of the gas detected close to the systemic velocity is associated with an off-nuclear cloud (${\sim} 50$to 100 pc from the radio core) with a column density of ${\sim} 10^{22}\ T_{\rm spin}/(100\ {\rm K}$) cm-2and an $\ion{H}{1}$mass of a few times 105to 106$M_\odot$. We consider a number of possibilities to explain the results. In particular, we discuss the possibility that this cloud indicates the presence of a rich and clumpy interstellar medium in the centre, likely left over from the merger that triggered the activity and that this medium influences the growth of the radio source. The location of the cloud – at the edge of the northern radio jet/lobe – suggests that the radio jet might be interacting with a gas cloud. This interaction could be responsible for bending the young radio jet. The velocity profile of the gas is relatively broad (${\sim} 150$km s-1) and we interpret this as kinematical evidence for interaction of the radio plasma with the cloud. We also consider the model where the cloud is part of a broader circumnuclear structure. Only a limited region of this structure would have sufficient background radio brightness and large enough column depth in neutral gas to obtain detectable $\ion{H}{1}$absorption against the counterjet. The VLBI study of the neutral hydrogen in 4C 12.50 suggests that $\ion{H}{1}$detected near the systemic velocity (as it is often the case in radio galaxies) may not necessarily be connected with a circumnuclear disk or torus (as is very often assumed) but instead could be a tracer of the large-scale medium that surrounds the active nucleus and that may influence the growth of the young radio source.

A small sample of 7 southern Compact Steep Spectrum (CSS) radio sources has been selected as part of the study of a larger flux-limited complete sample of radio sources. High resolution images, using the VLBI network in the southern hemisphere and the high resolution MERLIN array, are presented for all sources in the CSS sample. The overall morphology of each source consists of well-defined double lobes but with substantial diffuse and extended components present. In the majority of cases only a fraction of the total flux density is detected on the VLBI baselines, indicating the presence of larger extended radio structures. However, all sources are unresolved at arcsecond scales and are of sub-galactic size, with linear size in the range 0.1–2 kpc. The radio properties of the sources agree well with CSS sources in other samples.

We present WSRT and VLA radio observations of the neutral hydrogen in the radio galaxy Coma A. We detect extended $\ion{H}{I}$absorption against both radio lobes of Coma A, at distances of about 30 kpc from the centre. Coma A is the first radio galaxy in which $\ion{H}{I}$is seen in absorption at such large distances from the nucleus. The match between the velocities of the neutral hydrogen and those of the extended ionized gas suggests that they are part of the same disk-like structure of at least 60 kpc in diameter. Most likely, this gas disk is partly ionised by the bulk motion of the radio lobes expanding into it. The gas mass of this disk is at least 109$M_\odot$. The relatively regular structure of the gas disk suggests that a merger occurred involving at least one large gas-rich galaxy, at least a few times 108yr ago.

We present new ATCA 17- and 24-GHz radio images and ESO-NTT optical spectra of the radio-loud Seyfert galaxy?IC?5063, the first galaxy in which a fast (~600?kms-1) outflow of neutral hydrogen was discovered. The new radio data confirm the triple radio structure with a central, unresolved flat-spectrum core and two resolved radio lobes with steep spectral index. This implies that the previously detected fast outflow of neutral gas is occurring off-nucleus, near a radio lobe about 0.5?kpc from the core. The ionised gas shows highly complex kinematics in the region co-spatial with the radio emission. Broad and blueshifted (~500?kms-1) emission is observed in the region of the radio lobe, at the same location as the blueshifted Hi?absorption. The velocity of the ionised outflow is similar to the one found in Hi. The first order correspondence between the radio and optical properties suggests that the outflow is?driven by the interaction between the radio jet and the ISM. However, despite the high outflow velocities, no evidence is found for the ionisation of the gas being due to fast shocks in the region of the outflow, indicating that photoionisation from the AGN is likely to be the dominant ionisation mechanism. The outflow rate of the warm (ionised) gas is small compared to that of the cold gas, similar to what is found in other radio galaxies. The mass outflow rate associated with the Hi?is in the same range as for ?mild? starburst-driven superwinds in ULIRGs. However, in IC?5063, the AGN-driven outflow appears to be limited to the inner kpc region of the galaxy. The kinetic power associated with the Hi?outflow is a small fraction (a few ?10-4) of the Eddington luminosity of the galaxy but is a significant fraction (~0.1) of the nuclear bolometric luminosity. In IC?5063, the observed outflows may have sufficient kinetic power to have a significant impact on the evolution of the ISM in the host galaxy.

We present a detailed study of PKS 1814-637, a rare case of powerful radio source (P5   GHz= 4.1 × 1025W Hz-1) hosted by a disk galaxy. Optical images have been used to model the host galaxy morphology confirming it to be dominated by a strong (and warped) disk component that is observed close to edge-on to the line of sight. This is the first radio galaxy found to reside in a disk dominated galaxy with radio luminosity equivalent to powerful FRII objects. At radio wavelengths, PKS 1814-637 is about 480 pc in diameter and it is classified as a compact steep spectrum (CSS) source; such sources are usually considered to be radio sources observed in the early stages of their evolution. However, the optical and mid-IR spectroscopic properties of PKS1814-637 show more in common with Seyfert galaxies than they do with radio galaxies, with the detection of H2, and PAH emission features, along with HI and silicate absorption features, providing evidence for a rich ISM which is likely to be related to the disk morphology of the host galaxy. We argue that the interaction between the radio plasma and the rich ISM in this and similar objects may have boosted their radio emission, allowing them to more easily enter flux limited samples of radio sources. In this case, PKS 1814-637 represents a type of “imposter”: an intrinsically low power object that is selected in a radio flux limited sample because of the unusually efficient conversion of jet power into radio emission. This would make PKS 1814-637 an extreme example of the effects of jet-cloud interactions in galaxies containing a rich ISM, and perhaps a missing link between radio galaxies and radio-loud Seyfert galaxies. However, it is unlikely that jet-cloud interactions alone can account for the unusually powerful radio emission compared to Seyfert galaxies, and it is probable that the jet in PKS 1814-637 is also intrinsically more powerful than in typical Seyfert galaxies, perhaps due to a higher bulge and black hole mass. The estimated BH mass is indeed higher than the majority of Seyfert galaxies in the local Universe. We speculate that sources similar to PKS1814-637 are likely to be more common at high redshifts, because of the enhanced probability at earlier epochs of triggering radio sources in moderately massive bulges that are also gas-rich.