Radio observations of early-type galaxies with dust lanes

Radio and optical images of early-type galaxies with dust lanes have been analyzed in order to investigate the characteristics of the radio emission and to compare it with their properties at other frequencies. Except three galaxies, the remaining sources of our sample have diffuse radio emission, which does not extend beyond the stellar disk. The radio structures are small and weak (linear sizes in the range 2–10 kpc and radio powers in the range 5×10²⁰–5×10²¹W Hz^–1). Our preliminary results show that in a minority of cases (the most powerful radio sources) radio emission originates in the associated galactic nuclei, where a massive black hole is harboured. On the contrary, in the less powerful among our radio galaxies, sources originating from stellar phenomena may play and important role. We have classified the galaxies with respect to the dust lane morphology, comparing it with the radio emission. The sample is too small in order to reach firm conclusions, but the lack of radio sources in early-type galaxies with dust lanes along the galaxy minor axis seems to suggest that the accretion of material does not reach the conditions necessary to trigger nuclear activity.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

ROSAT X-ray observations of 3CRR radio sources

Over half the 3CRR sample of radio galaxies and quasars has been observed in X-rays with ROSAT pointed observations, and we present results from these observations, discussing many of the sources in detail. The improved spatial resolution of ROSAT over earlier missions allows a better separation of the nuclear and extended components of the X-ray emission. We investigate the relationship between nuclear X-ray and core radio luminosity, and show that our results support a model in which every radio galaxy and quasar has a beamed nuclear soft X-ray component directly related to the radio core. We report evidence for rich cluster environments around several powerful quasars. These X-ray environments are comparable to those of high-redshift radio galaxies.     

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.     

Dynamical evolution in clusters of galaxies with low-frequency radio emission

Clusters of galaxies in which radio emission at low frequencies (178 MHz) has been detected were classified on the Bautz-Morgan (BM) system according to the dominance of the brightest galaxy. Radio sources with steep low-frequency spectra occur in clusters of all BM types but more often in rich clusters; the distributions of BM types for clusters with high and low spectral indices between 38 and 178 MHz are similar. Glass copies of Mount Palomar Sky Survey plates were measured to determine the distribution of the ten brightest galaxies in clusters without dominant galaxies. Some clusters were found to have central cores of bright galaxies which may reflect mass segregation of galaxies due to dynamical friction. The bright galaxies in such cores may later merge to form dominant cD galaxies. The positions of the cD galaxies and cores of bright galaxies are often at projected distances <200 kpc from the low-frequency radio emission. The low-frequency spectrum of radio emission associated with a cD galaxy may be either steep or normal, but the low-frequency spectrum from a core of bright galaxies is usually steep. A steep spectrum may develop when a radio source is confined by hot gas in a cluster over a long period (10⁹ yr). Confinement would probably occur for radio sources associated with bright galaxies in the cores of clusters and cD galaxies in clusters. However, cD galaxies may have recurrent radio outbursts so that steep spectra are not always observed.     

High-resolution radio observations of Seyfert galaxies in the extended 12-μm sample – I. The observations

We present 8.4-GHz VLA A-configuration observations of 87 sources from the mid-infrared-selected AGN sample of Rush et al. These 0.25-arcsec-resolution observations allow elongated radio structures tens of pc in size to be resolved, and enable radio components smaller than 3.5 arcsec to be isolated from diffuse galactic disc emission. When combined with previous data, matched radio observations covering 90 per cent of the sample have been collected, and these represent the largest subarcsecond–resolution radio imaging survey of a homogeneously selected sample of Seyfert galaxies to date.
We use our observations to identify five radio-loud AGN in the sample. The nature of the radio emission from Seyfert nuclei will be discussed in subsequent papers.     

Gradients of excitation in the nuclei of starburst galaxies

This article forms a part of a wider study of the nuclear and circum-nuclear zones of moderately active galaxies. The use of long-slit spectroscopy at high spectral and spatial resolution has enabled us to measure gradients of gaseous excitation and also velocity fields in a sample of four bright nuclear starburst galaxies selected from the sample of Balzano (1983). We find variations not only in the intensity but also in the quality of the emission spectra (line ratios and line widths) of the interstellar gas between regions separated by relatively short distances. We stress the need for studies with at least the present degree of angular and spectral resolution if physical sense is to be made of the interstellar excitation regimes in external galaxies, as well as to investigate a possible evolutionary link between nuclei of intermediate activity.     

Identification of newly discovered radio-emitting X-ray sources: results from spectroscopy

We present optical identifications for a sample of 20 previously unknown X-ray/radio sources that are present both in the source catalogue of ROSAT PSPC pointed observations ( ROSAT SRC) and in the NRAO VLA Sky Survey (NVSS). The optical spectroscopy was carried out with the 2.1-m telescope at San Pedro Martir (Mexico) during 1995 April and September. We have identified 15 active galactic nuclei [including 12 broad-emission-line (FWHM >1000 km s⁻¹) objects, one bona fide BL Lac, one BL Lac candidate and one narrow-line (FWHM < 1000 km s⁻¹) radio galaxy] and five radio galaxies. We derive the X-ray fluxes and luminosities by analysing the PSPC exposures, and show the radio morphology from the NVSS maps.  We find that the correlation between the monochromatic X-ray luminosity at 2 keV and the core radio luminosity at 5 GHz for the radio galaxies in our sample follows that found for the 3CR radio galaxies, suggesting a possible nuclear origin for the X-ray emission in these sources. This correlation is weaker in the case of broad-line objects, indicating the presence of another (unbeamed) mechanism for the X-ray emission only weakly related to the radio emission.     

Low Frequency Radio Emission and Star Formation in the Nuclei of Infrared Galaxies

Results of radio observations of luminous and ultraluminous infrared galaxies are discussed. The declining or flattening of the radio spectra of compact and (in some cases) extended sources found using interplanetary scintillation observations at 102 MHz is accounted for by absorption in thermal plasma, in agreement with the observed brightness of optical emission lines. It is argued that a region of violent star formation occupies the area of a molecular disc 1 kpc in size. The discrepancy between the linear sizes of the compact radio sources and the star forming regions is discussed.     

Infrared–red cores in nearby elliptical galaxies

We present the Spitzer Space Telescope Infrared Array Camera observations for a sample of local elliptical galaxies to study later stages of active galactic nucleus (AGN) activity. A sample of 36 elliptical galaxies is selected from the Palomar spectroscopic survey. We detect nuclear non-stellar infrared emission in nine of them. There is unambiguous evidence of circumnuclear dust in these nine galaxies in their optical images. We also find a remarkable correlation between the infrared excess emission and the nuclear radio/X-ray emission, suggesting that infrared excess emission is tightly related to nuclear activity. The possible origin of infrared excess emission from hot dust heated by the central AGN is supported by the spectral indices of the infrared excess emission.     

X-ray observations of low-power radio galaxies from the B2 catalogue

We present an analysis of X-ray data, taken with ROSAT , for a well-defined sample of low-power radio galaxies from the Bologna B2 catalogue. Where possible, the HRI has been used in order to take advantage of the high spatial resolution provided by this instrument. A variety of models are fitted to radial profiles in order to separate the resolved and unresolved X-ray emission from the galaxies. We demonstrate a strong, approximately linear, correlation between the luminosities of the unresolved X-ray components and the 5-GHz luminosities of the radio cores in this sample. This suggests a physical relationship between the soft X-ray emission of radio galaxies and the jet-generated radio core emission. We infer a nuclear jet-related origin for at least some of the X-ray emission.     

Infrared velocities of carbon stars

We present CS(J = 1 0) observations of the high density gas in a sample of eleven star forming regions with molecular or optical outflows. The sources of this sample cover a wide range of physical sizes. All these sources had been previously mapped in NH₃ (1,1) with similar angular resolution. In all the sources of this sample CS emission was detected, indicating a global correlation between the emissions traced by the CS and the NH₃ molecules. However, the detailed characteristics of these two emissions (e.g., the extent and the location of the emission peak) show, in general, significant differences in the sources which are well resolved by the beam. As a general trend, the emission traced by the NH₃ molecule appears as compact clumps which engulf an outflow activity center. In contrast, the emission traced by the CS molecule, usually more extended, appears as a background which connects different outflow activity centers associated with each NH₃ clump.     

Giant radio sources

We present multifrequency Very Large Array (VLA) observations of two giant quasars, 0437−244 and 1025−229, from the Molonglo Complete Sample. These sources have well-defined FR II radio structure, possible one-sided jets, no significant depolarization between 1365 and 4935 MHz and low rotation measure (|RM|<20 rad m⁻²). The giant sources are defined to be those with overall projected size 1 Mpc. We have compiled a sample of about 50 known giant radio sources from the literature, and have compared some of their properties with a complete sample of 3CR radio sources of smaller sizes to investigate the evolution of giant sources, and test their consistency with the unified scheme for radio galaxies and quasars. We find an inverse correlation between the degree of core prominence and total radio luminosity, and show that the giant radio sources have similar core strengths to smaller sources of similar total luminosity. Hence their large sizes are unlikely to be caused by stronger nuclear activity. The degree of collinearity of the giant sources is also similar to that of the sample of smaller sources. The luminosity–size diagram shows that the giant sources are less luminous than our sample of smaller sized 3CR sources, consistent with evolutionary scenarios in which the giants have evolved from the smaller sources, losing energy as they expand to these large dimensions. For the smaller sources, radiative losses resulting from synchrotron radiation are more significant while for the giant sources the equipartition magnetic fields are smaller and inverse Compton loss owing to microwave background radiation is the dominant process. The radio properties of the giant radio galaxies and quasars are consistent with the unified scheme.     

Radio structure of extragalactic X-ray sources

The radio properties ofUhuru X-ray sources with fairly certain extragalactic identifications are described briefly. Radio to X-ray flux ratios are low for rich clusters of galaxies and high for double radio sources. There is some evidence from the Abell 426 (Perseus) and Abell 1367 clusters that a radio galaxy in a rich cluster may be the centre of extended X-ray emission. Nuclei of galaxies have an enormous range in X-ray luminosity; the known range is from 10³⁰ W for our galaxy to 3×10³⁸ W for 3C 273. Unidentified X-ray sources at high galactic latitudes may include new classes of objects with very low radio to X-ray flux ratios or hard X-ray emission.     

High-resolution radio observations of Seyfert galaxies in the extended 12-μm sample – II. The properties of compact radio components

We discuss the properties of compact nuclear radio components in Seyfert galaxies from the extended 12-μm AGN sample of Rush et al. Our main results can be summarized as follows.
Type 1 and type 2 Seyferts produce compact radio components which are indistinguishable in strength and aspect, indicating that their central engines are alike, as proposed by the unification model. Infrared IRAS fluxes are more closely correlated with low-resolution radio fluxes than high-resolution radio fluxes, suggesting that they are dominated by kiloparsec-scale, extranuclear emission regions; extranuclear emission may be stronger in type 2 Seyferts. Early-type Seyfert galaxies tend to have stronger nuclear radio emission than late-type Seyfert galaxies. V-shaped extended emission-line regions, indicative of 'ionization cones', are usually found in sources with large, collimated radio outflows. Hidden broad lines are most likely to be found in sources with powerful nuclear radio sources. Type 1 and type 2 Seyferts selected by their IRAS 12-μm flux densities have well-matched properties.     

Morphological properties of radio elliptical galaxies

A CCD imaging survey of elliptical galaxies which present radio jets is described. The photometric and morphological properties of these galaxies and their nearby companions are described by Fourier analysis of the intensity variations along a set of elliptical contours fitted to the intensity distribution. A large fraction of the sample shows evidence for interaction with close companions. By interpolation between the best-fit ellipses an elliptical galaxy model of the underlying stellar distribution is constructed and by subtraction of these models interesting morphological features such as boxy isophotes, dust lanes and emission line regions are revealed. The data analysis technique developed for this study is described. The morphological and photometric properties of the objects can be explained by gravitational interaction of galaxies in close encounters. Five objects of a complete sample of low luminosity radio galaxies are briefly discussed.     

Continuum radiation from active galactic nuclei

Summary Active galactic nuclei (AGN) can be divided into two broad classes, where the emitted continuum power is dominated either by thermal emission (radio-quiet AGN), or by nonthermal emission (blazars). Emission in the 0.01–1 m range is the primary contributor to the bolometric luminosity and is probably produced through thermal emission from an accretion disk, modified by electron scattering and general relativistic effects. The 1–1000 m continuum, the second most important contributor to the power, is generally dominated by thermal emission from dust with a range of temperatures from 40 K to 1000–2000 K. The dust is probably reemitting 0.01–0.3 m continuum emission, previously absorbed in an obscuring cone (or torus) or an extended disk. The 1–10 keV X-ray emission is rapidly variable and originates in a small region. This emission may be produced through Compton scattering by hot thermal electrons surrounding an accretion disk, although the observations are far from being definitive. The weak radio emission, which is due to the nonthermal synchrotron process, is usually elongated in the shape of jets and lobes (a core may be present too), and is morphologically distinct from the radio emission of starburst galaxies.In the blazar class, the radio through ultraviolet emission is decidedly non-thermal, and apparently is produced through the synchrotron process in an inhomogeneous plasma. The plasma probably is moving outward at relativistic velocities within a jet in which the Lorentz factor of bulk motion (typically 2–6) increases outward. This is inferred from observations indicating that the opening angle becomes progressively larger from the radio to the optical to the X-ray emitting regions. Shocks propagating along the jet may be responsible for much of the flux variability. In sources where the X-ray continuum is not a continuation of the optical-ultraviolet synchrotron emission, some objects show variability consistent with Compton scattering by relativistic electron in a large region (in BL Lacertae), while other objects produce their X-ray emission in a compact region, possibly suggesting pair production.When orientation effects are included, all AGN may be decomposed into a radio-quiet AGN, a blazar, or a combination of the two. Radio-quiet AGN appear to have an obscuring cone or torus containing the broad emission line clouds and an ionizing source. Most likely, the (non-relativistic) directional effects of this obscuring region give rise to the difference between Seyfert 1 and 2 galaxies or narrow and broad line radio galaxies. For different orientations of the nonthermal jet, relativistic Doppler boosting can produce BL Lacertae objects or FR I radio galaxies, or at higher jet luminosities, flat-spectrum high-polarization quasars or FR II radio galaxies.     

HST and UKIRT imaging observations of z∼ 1 6C radio galaxies – II. Galaxy morphologies and the alignment effect

Powerful radio galaxies often display enhanced optical/ultraviolet emission regions, elongated and aligned with the radio jet axis. The aim of this series of papers is to investigate separately the effects of radio power and redshift on the alignment effect, together with other radio galaxy properties. In this second paper, we present a deeper analysis of the morphological properties of these systems, including both the host galaxies and their surrounding aligned emission.
The host galaxies of our 6C subsample are well described as de Vaucouleurs ellipticals, with typical scale sizes of  ∼10 kpc  . This is comparable to the host galaxies of low- z radio sources of similar powers, and also the more powerful 3CR sources at the same redshift. The contribution of nuclear point source emission is also comparable, regardless of radio power.
The 6C alignment effect is remarkably similar to that seen around more powerful 3CR sources at the same redshift in terms of extent and degree of alignment with the radio source axis, although it is generally less luminous. The bright, knotty features observed in the case of the z ∼ 1 3CR sources are far less frequent in our 6C subsample; neither do we observe such strong evidence for evolution in the strength of the alignment effect with radio source size/age. However, we do find a very strong link between the most extreme alignment effects and emission-line region properties indicative of shocks, regardless of source size/age or power. In general, the 6C alignment effect is still considerably stronger than that seen around lower redshift galaxies of similar radio powers. Cosmic epoch is clearly just as important a factor as radio power: although aligned emission is observed on smaller scales at lower redshifts, the processes which produce the most extreme features simply no longer occur, suggesting considerable evolution in the properties of the extended haloes surrounding the radio source.     

Confirming a population of hot-dust dominated, star-forming, ultraluminous galaxies at high redshift

We identify eight   z > 1  radio sources undetected at 850 μm but robustly detected at 70 μm, confirming that they represent ultraluminous infrared galaxies (ULIRGs) with hotter dust temperatures  (〈 T _d〉= 52 ± 10 K)  than submillimetre galaxies (SMGs) at similar luminosities and redshifts. These galaxies share many properties with SMGs: ultraviolet spectra consistent with starbursts, high stellar masses and radio luminosities. We can attribute their radio emission to star formation since high-resolution Multi-Element Radio Linked Interferometer Network (MERLIN) radio maps show extended emission regions (with characteristic radii of 2–3 kpc), which are unlikely to be generated by active galactic nucleus (AGN) activity. These observations provide the first direct confirmation of hot, dusty ULIRGs which are missed by current submillimetre surveys. They have significant implications for future observations from the Herschel Space Observatory and Submillimetre Common-User Bolometer Array 2 (SCUBA2), which will select high-redshift luminous galaxies with less selection biases.     

A search for molecular gas in restarting radio galaxies

Several radio galaxies are known that show radio morphological signatures that are best interpreted as restarting of nuclear activity after a period of quiescence. The conditions surrounding the phenomenon of nuclear recurrence are not understood. In this paper we have attempted to address this question by examining the nuclear fuelling characteristics in a sample of restarting radio galaxies. We have examined the detection rate for molecular gas in a representative sample of nine restarting radio galaxies, for seven of which we present new upper limits to the molecular gas mass derived from CO line observations we made with the IRAM 30-m telescope. We derive a low CO detection rate for the relatively young restarted radio galaxies suggesting that the cessation of the nuclear activity and its subsequent restarting may be a result of instabilities in the fuelling process rather than a case of depletion of fuel followed by a recent fuel acquisition. It appears that abundant molecular gas content at the level of few  10⁸–10⁹ M_⊙  does not necessarily accompany the nuclear restarting phenomenon. For comparison we also discuss the molecular gas properties of five normal giant radio galaxies, three of which we observed using Swedish-ESO Millimetre Telescope (SEST). Despite obvious signs of interactions and nuclear dust discs none of them has been found to host significant quantities of molecular gas.     

Activity in colliding galaxies

A substantial body of evidence now exists that indicates that there is a connection between the observed activity in galaxies and the apparent involvement of these active galaxies in tidal interactions with other galaxies. To investigate this phenomenon, we are studying the morphologies and environments of various interacting galaxies showing evidence of activity. In particular, we present here some preliminary results from our investigations of the claimed interaction-activity connection for a sample of colliding radio galaxies endowed with a bent radio jet morphology. We find that the timing of the jet activity is consistent with a tidal-triggering hypothesis. Our results therefore lend further support to the idea that the galactic activity observed in some interactive galaxies is in fact collision-induced.