arXiv subjects
Publications and source records attributed to R. Morganti.
We present a study, done with the Australian LBA, of HI absorption for two compact radio galaxies (PKS 1549-79 and PKS 1814-63). In both the radio galaxies, the HI appears to give us information about the environment in which the radio sources are embedded, the effect that the ISM can have on the observed characteristics and the possible presence of interaction between the ISM and the radio plasma.
We briefly discuss the main differences between the HI properties of luminous and low-luminosity early-type galaxies. In luminous early-type galaxies the HI is often irregularly distributed, but in a few cases regular HI disks, of low surface density, are seen. In low-luminosity galaxies, the HI is more often in a disk with high central surface densities. This suggests a different evolution of the gas in these two groups of galaxies.
In this paper we present the results of VLBI observations of six radio galaxies belonging to the 2-Jy sample. The selected objects are 3C17, PKS 0620-52, PKS 0625-35, PKS 1318-43, PKS 1333-33 and 3C317. The first is a high power radio galaxy and the remainder are low power objects. Our observations were carried out with a set of different arrays and frequencies, and cover a range of resolutions from a few mas to a few tens of mas. Parsec-scale images are presented and discussed in the light of unification for radio loud galaxies. Estimates for the intrinsic plasma speed in these objects in the proximity of the parsec-scale core and tehir orientation to the line of sight are in agreement with the predictions from unified models and their optical and X-ray properties.
HI absorption has been detected with the WSRT against both lobes of the radio galaxy Coma A. This radio galaxy could be expanding in a particularly gas rich environment, perhaps the result from interactions/mergers between the dominant giant galaxy (associated with the radio galaxy) and less massive galaxies in the same group.
A Hubble Space Telescope snapshot survey of the B2 sample of low luminosity radio galaxies has, at present, produced V and I images of 41 objects. Together with 16 images of B2 sources taken from the HST archive, there are now high resolution optical data for 57 % of the sample. All host galaxies are luminous ellipticals, except one which is a spiral galaxy, while another one turns out to be a misidentification. We present an album of the images of the B2 radio galaxies observed so far, and give a brief description of the optical morphology of the galaxies. Dust features (in the form of disks, lanes or irregular patches) are seen in most of the galaxies of the sample, 58 %. Compact optical cores are also very common (18/57). A preliminary analysis has revealed the presence of an optical jet in three objects, indicating they can be detected in a sizeable percentage in these low luminosity radio sources. Brightness profiles of dust-free galaxies are well represented by a Nuker law and all shows the existence of a resolved shallow cusp.
Twenty-two powerful radio galaxies have been searched for HI absorption. We find the highest probability of detecting HI in absorption among narrow-line compact (or small) galaxies or galaxies with indication of richer interstellar medium (i.e. with ongoing or recent star-formation). We discuss the difficulty in the interpretation of the origin of the HI absorption due to the uncertainty in the systemic velocity of the galaxies.
We present the results of HST narrow band imaging of eleven Compact Steep Spectrum (CSS) radio sources. Five of them (3C 48, 3C 147, 3C303.1, 3C 277.1 and 4C 12.50) were part of a dedicated ``pointed'' program of deep line imaging ([OIII]). For six additional sources (3C 49, 3C 93.1, 3C 138, 3C 268.3, 3C305.1 and 3C343.1) ``snapshot'' images ([OIII] or [OII]) were taken from the HST archive. In all but one of the targets (3C 49) line emission has been detected and only in one case (3C 138) is unresolved. In four out five of the sources with deep observations, the line emission extends well beyond the size of the radio source but along the radio axis. Structures of similar surface brightness would have not been seen in the snapshot images. These emission line structures extend to scales of 10 to 30 kpc and cover a projected angle, when seen from the nucleus, of 30 to 110 degrees, indicating that the nuclear illumination is anisotropic. Photon counting arguments also support this interpretation. In six objects the radio emission extends over more than 1 arcsec. In these cases the line emission has an elongated structure, linking the nucleus to the radio-lobes, possibly tracing the path of the invisible radio jets. Nevertheless the emission line morphologies do not show the bow shocks at the extremities of the radio lobes expected if they are sources whose expansion is frustrated by a dense external medium. Our data favour the alternative model in which CSSs are the young phase of the large size radio sources. When ``pointed'' pure continuum images are available, there appears to be no alignment between radio and continuum emission which contradicts previous suggestions based on broad-band HST imaging.
Multi-wavelength VLA observations of two extended radio galaxies, 0235-197 and 1203+043 are presented. There is some evidence from earlier studies that these two sources exhibit low frequency (<1 GHz) variability. This work shows that both sources have linear polarizations, if any, below the detection limits at 320 MHz, so we cannot explain the variability as being due to instrumental polarization effects as has been suggested for 3C159. Refractive scintillation may be the cause of the variability in 0235-197. This would require the existence of a bright, compact component in one of the hot spots seen in these observations. This is not implausible but the resolution of this observational program is insufficent to address that question. The radio source 1203+043 lacks any bright compact component thereby ruling out a refractive scintillation mechanism for its variability. Consequently, it is possible that claims of variability in this source are spurious. However, the 320 MHz VLA observations show that 1203+043 has an `X'-shaped radio structure. This is a rare morphology for the brightness distribution of a radio galaxy; the implications of this are examined.
Deep, narrow-band Halpha observations taken with the TAURUS Tunable Filter (TTF) on the 4.2m WHT telescope are presented for two nearby radio galaxies with strong jet-cloud interactions. Although the brightest emission line components are closely aligned with the radio jets --- providing nearby examples of the ``alignment effect'' most commonly observed in high redshift (z > 0.5) radio galaxies --- lower surface brightness emission line structures are detected at large distances (10's of kpc) from the radio jet axis. These latter structures cannot be reconciled with anisotropic illumination of the ISM by obscured quasar-like sources, since parts of the structures lay outside any plausible quasar ionization cones. Rather, the distribution of the emission lines around the fringes of the extended radio lobes suggests that the gas is ionized either by direct interaction with the radio components, or by the diffuse photoionizing radiation fields produced in the shocks generated in such interactions. These observations serve to emphasise that the ionizing effects of the radio components can extend far from the radio jet axes, and that deep emission line imaging observations are required to reveal the true distribution of warm gas in the host galaxies. We expect future deep imaging observations to reveal similar structures perpendicular to the radio axes in the high-z radio galaxies.
We present 21-cm HI line and 13-cm continuum observations, obtained with the Australian Long Baseline Array, of the Seyfert 2 galaxy IC 5063. This object appears to be one of the best examples of Seyfert galaxies where shocks produced by the radio plasma jet influence both the radio as well as the near-infrared emission. The picture resulting from the new observations of IC 5063 confirms and completes the one derived from previous Australia Telescope Compact Array (ATCA) lower resolution observations. A strong interaction between the radio plasma ejected from the nucleus and a molecular cloud of the ISM is occurring at the position of the western hot spot, about 0.6 kpc from the active nucleus. Because of this interaction, the gas is swept up, forming, around the radio lobe, a cocoon-like structure where the gas is moving at high speed. Due to this, part of the molecular gas is dissociated and becomes neutral or even ionised if the UV continuum produced by the shocks is hard and powerful enough. The main component detected in the VLBI HI absorption profile has a central velocity of 2786 km/s and is therefore blue-shifted 614 km/s with respect to the systemic velocity. Its peak optical depth is 5.4 %. The corresponding column density of the detected absorption, for a spin temperature of 100 K, is N_HI ~ 2x10^21 atoms cm^-2. The HI absorption properties observed in IC 5063 appear different from those observed in other Seyfert galaxies, where the HI absorption detected is attributed to undisturbed foreground gas associated with the large-scale galaxy disk.
We present the results of a 22 cm radio survey carried out with the A3558 complex, a chain formed by the merging ACO clusters A3556-A3558-A3562 and thetwo groups SC1327-312 and SC1323-313, located in the central region of the complex, a chain formed by the merging ACO clusters A3556-A3558-A3562 and the two groups SC1327-312 and SC1323-313, located in the central region of the Shapley Concentration. The purpose of our survey is to study the effects of cluster mergers on the statistical properties of radio galaxies and to investigate the connection between mergers and the presence of radio halos and relic sources. We found that the radio source counts in the A3558 complex are consistent with the background source counts. Furthermore, we found that no correlation exists between the local density and the radio source power, and that steep spectrum radio galaxies are not segregated in denser optical regions. The radio luminosity function for elliptical and S0 galaxies is significantly lower than that for cluster type galaxies and for those not selected to be in clusters at radio powers logP(1.4) > 22.5, implying that the probability of a galaxy becoming a radio source above this power limit is lower in the Shapley Concentration compared with any other environment. The detection of a head-tail source in the centre of A3562, coupled with careful inspection of the 20 cm NRAO VLA Sky Survey (NVSS) and of 36 cm MOST observations, allowed us to spot two extended sources in the region between A3562 and SC1329-313, i.e. a candidate radio halo at the centre of A3562, and low brightness extended emission around a 14.96 magnitude Shapley galaxy.
We present HI data cubes of four low-luminosity early-type galaxies which are currently forming stars. These galaxies have absolute magnitudes in the range M_B=-17.9 to -19.9 (H_o=50 km/s/Mpc). Their HI masses range between a few times 10^8 and a few times 10^9 M_sun and the corresponding values for M_HI/L_B are between 0.07 and 0.42, so these systems are HI rich for their morphological type. In all four galaxies, the HI is strongly centrally concentrated with high central HI surface densities, in contrast to what is typically observed in more luminous early-type galaxies. In two galaxies (NGC 802 and ESO 118-G34), the kinematics of the HI suggests that the gas is in a strongly warped disk, which we take as evidence for recent accretion of HI. In the other two galaxies (NGC 2328 and ESO 027-G21) the HI must have been part of the systems for a considerable time. The HI properties of low-luminosity early-type galaxies appear to be systematically different from those of many more luminous early-type galaxies, and we suggest that these differences are due to a different evolution of the two classes. The star formation history of these galaxies remains unclear. Their UBV colours and Halpha emission-line strengths are consistent with having formed stars at a slowly-declining rate for most of the past 10^10 years. However, the current data do not rule out a small burst of recent star formation overlaid on an older stellar population.
We present new radio images obtained with the Australia Telescope Compact Array (ATCA) and the Very Large Array (VLA) for a group of 14 galaxies belonging to the 2-Jy sample of radio sources. The new images improve the data already available on these objects and, in general, the database that we are building up on the sample. They will also be used for follow-up work where radio-optical comparison will be done. We briefly discuss the core dominance parameter (R) for the objects for which the new data have given new information and, in particular, for broad line radio galaxies (BLRG). One of the BLRG does not show a core, even at 3 cm, and this is at variance with the general tendency of BLRG to have relatively strong cores. The depolarization is also discussed for a group of small double-lobed radio galaxies.
In this paper we present the first results of a radio survey at 22 cm in the central region of the Shapley Concentration, carried out with the Australia Telescope Compact Array. In order to study the effect of merging on the statistical properties of radio galaxies, and the relation between merging and the existence of {\it relic} and {\it halo} type radio sources, we observed the two complexes of merging clusters centered on A3528 and on A3558. Our results show that the radio source counts in these regions, characterised by very high optical overdensity and major cluster mergers, do not differ from the background source counts. This suggests that the merging phenomenon does not influence the probability of a galaxy to become radio surce. Furthermore we investigated the possibility that the extended radio source J1324-3138 is a relic, applying the model recently proposed by Ensslin et al. (1998). Our analysis shows that the properties of J1324-3138 are consistent with the idea that the source is a relic located on the shock front between A3556 and a smaller group accreting onto the main cluster concentration.
We present radio observations for 29 southern Seyfert galaxies selected from a volume limited sample with cz<3600 km s^-1, and declination delta<0degr. Objects with declination -30degr<delta<0degr were observed with the Very Large Array (VLA) at 6 cm (4.9 GHz) and objects with delta<-30degr were observed with the Australia Telescope Compact Array (ATCA) at 3.5 cm (8.6 GHz). Both the VLA and the ATCA observations have a resolution of ~1arcsec. These new observations cover more than 50% of the southern sample with all but two of the 29 objects detected above our limit of 0.15 mJy. Combining these data with data available from the literature gives almost 85% coverage of the southern sample and further expands the radio observations of a distance limited sample by 22%. Collecting additional sources from the literature, consisting of known Seyferts with cz < 4600 km s^-1, we find that 38% of the sources are unresolved at arcsecond resolution, and 34% have linear radio structure. Our results expand upon and are consistent with earlier studies. We confirm a correlation between the size of the radio structure and the radio luminosity. A comparison between Seyfert types 1 and 2 finds that type 2s tend to have a larger linear size. There is no statistically significant difference in radio power between types 1 and 2, although all the most powerful objects appear to be Seyfert 2's. No significant difference has been found in the spectral indices.
We present new radio observations of the brighter region of the northern lobe (the Northern Middle Lobe, NML) of Centaurus A obtained at 20 cm with the Australia Telescope Compact Array. The angular resolutions are ~50 and ~130 arcsec, therefore much higher than for the previously available radio images of this region. The most interesting feature detected is a large-scale jet that connects the inner radio lobe and the NML and that is imaged for the first time. The NML itself appears as diffuse emission with a relatively bright ridge on the eastern side. The radio morphology of Centaurus A and, in particular, its NML could be the result of a precessing jet that has undergone a strong interaction with the environment at least in the northern side. The very big drop in intensity between the inner jet and the large-scale jet can be explained with a sequence of bursts of activity at different epochs in the life of the source. Alternatively (or additionally) a ``bursting bubble'' model is proposed which could also explain the good collimation of the large-scale jet. In this model, the plasma accumulated in the inner lobe would be able to ``burst'' out only through one nozzle that would be the region where the large-scale jet forms. From the comparison between the radio emission and the regions of ionized gas we find that the inner optical filament falls about 2 arcmin (~2 kpc) away from the large-scale radio jet. Thus, this filament does not seem to have experienced a direct interaction with the radio plasma. The outer filaments appear to be, in projection, closer to the radio emission, arguing for a direct interaction with the radio jet. However, also in this case a more complicated interaction than assumed so far has to be occuring.
We present high resolution, long-slit spectra of the jet-cloud interaction in the powerful southern radio galaxy PKS2250-41. We have resolved the emission lines into two main kinematic components: a broad component (FWHM \geq 900 km/s) and a narrow component (FWHM \leq 150 km/s). While the broad component is characterized by a low ionization level (with particular ly weak HeII$λ$4686 emission) and is spatially associated with the radio lobe, the narrow component is characterized by a higher ionization level and extends well beyond the radio lobe. Crucially, we measure a higher electron temperature for the broad component (T \sim 30,000 K) than for the narrow component (T \sim 15,000 K). The general line ratios and physical conditions of the two components are consistent with a model in which the broad component r epresents gas cooling behind the shock front driven by the radio jets, while the narrow component represents the AGN- or shock-photoionized precursor gas. However, uncertainties remain about the gas acceleration mechanism behind the shock front: unless the radio components are expanding unusually fast in this source, it is likely that entrainment of the warm clouds in the hot post-shock wind or radio plasma is required. The similarities between the kinematic properties of PKS2250-41 and some high redshift radio galaxies suggest that the ambient and the shocked gas have also been resolved in the more distant objects. Given the evidence that the emission line processes are affected by the interactions between the radio and the optical structures, care must be taken when interpreting the UV spectra of high redshift radio galaxies.
We summarize the HI properties of early-type galaxies, in particular the differences in HI morphologies observed in early-type galaxies of different luminosities. We find that in low-luminosity early-type galaxies the HI is almost always in a disk-like structure, with central surface densities high enough for star formation to occur. In a few luminous early-type galaxies the HI is also in a disk or in a ring-like structure, but in most luminous early-type galaxies the HI has a relatively irregular morphology. The surface densities in the HI disks in luminous early-type galaxies are lower than in the HI disks in low-luminosity early-type galaxies and no large-scale star formation should occur in these disks. We discuss these different HI characteristics in the context of other properties of early-type galaxies that correlate with luminosity.