This paper briefly describes the capabilities of a new generation large telescope such as the Square Kilometer Array (SKA) for imaging HI in a one degree field out to z ≍ 3

The redshifted 1420 MHz emission from the HI in unresolved Lyman-α clouds (DLA) at high z will appear as a background radiation in low frequency radio observations. We calculate the brightness temperature and the angular two-point correlation function of the fluctuations of this radiation. This can be an important probe of the large-scale structure at high z.

Low Surface Brightness galaxies (LSBs) may well constitute an important component of the Universe and a good fraction of baryons may even be locked up in their gravitational potential. Existence of such massive objects was predicted from an investigation of gravitational lens systems more than a decade ago (Narasimha & Chitre, 1989). There are more compelling reasons for believing their universality based on the of the analysis of the images around the cusp caustics in some of the well-studied lenses. A complete sample of the mass, size and rotation velocity of LSBs in our neighbourhood, as well as in a limited volume at a larger redshift will be important for estimation of power spectrum as well as evolution of structures in the Universe. The GMRT type instrument is ideally suited for detection and study of the LSBs. We expect to detect 21 cm emission from around 2 to 10 LSBs per beam of GMRT dish at the level of 5 mJy flux with a survey covering 30 Mhz bandwidth, if their line width is of the order of 30 km/sec.

We investigate the possibility of detecting damped Lyman-α systems at high redshifts using GMRT if these clouds are magnified by intervening cluster lenses. We show that if the HI mass of the damped clouds ≥ 2 × 1010 M⊙ then they can be detected with a probability ≥ 0.2 using GMRT.

A search for radio recombination lines near 20 cm at z=0.193 and z=0.886 towards the gravitational lens system PKS1830-211 has yielded upper limits of |τL| ≤ 5 × 10−5 and ≤ 5 × 10−4 at the two redshifts respectively. Based on the non-detections, we derive upper limits to the emission measure of the ionized gas in the absorbing systems. We also present continuum flux density measurements over the frequency range 0.3—45 GHz made at a single epoch.

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 first neutral atomic hydrogen (HI) gas images of QSO host galaxies. The results come from our ongoing survey of all twenty-three QSOs at redshifts z < 0.07 visible from the VLA. Our images show tidal interactions in all of the QSO host galaxies detected in HI, even when no such interactions are clearly visible in the optical. The results are not consistent with the suggestion by Sanders et al. (1988a, 1988b) that ultraluminous infrared galaxies, the vast majority of which are later-stage mergers, are the parent population of a significant fraction of radio-quiet QSOs. Instead, our results suggest that QSOs reside in a wide variety of interacting environments.

This project was inspired by optical and IR imaging suggesting that, while most QSO hosts at high z are too faint to detect, radio-loud quasars are hosted by exceptionally luminous galaxies. Thus, a search was undertaken for HI absorption against the continuum emission of strong radio emitters whose redshifts (z ∼ 2.3) bring their HI line into the 430-MHz Arecibo passband2. The aim was to detect, or set useful limits, on the HI in the hosts, say in dust lanes, introduced by mergers, or situated in obscuring circumnuclear tori. At Arecibo there is a choice of, a) the 430-MHz line-feed system, or b) the new 430-MHz Gregorian-dome feed. In practice, most of the data were taken with the Gregorian system, although some line-feed data were also acquired. This represents the first Arecibo line spectroscopy using the 430-MHz Gregorian receiver.

Recent VLBA and VLA observations indicate the possibility of a circumnuclear disc or torus of HI in detected in absorption in NGC 3894. Strong absorption lines are detected toward both the aproaching and receding jets. Absorption profiles integrated over six regions across the source are shown. At lease two velocity components are present in all lines of sight except toward the southeast. The parameters are obtained from these fits(Peck and Taylor,1998,Ap.J.,502,L23). We use the bending model to explain the four prominent absorption features.

The number of known diffuse radio sources in clusters of galaxies has grown in recent years, making it possible to derive statistical properties of these sources and of the hosting clusters. We show that diffuse sources are associated with X-ray luminous clusters which have undergone recent merger processes. The radio and X-ray structures are often similar, and correlations are found between radio and X-ray parameters. This is indication of a link between the diffuse relativistic and thermal plasma in clusters of galaxies.

A short review on theoretical implications of non-thermal emission (radio, extreme ultraviolet, high energy X-ray) from the intra-cluster medium is given. The origin of cluster radio halos and cluster radio relics is discussed within the framework of a network of processes producing a non-thermal electron population. Emphasis is given to the role of old, remnant, presently invisible relativistic plasma released by former radio galaxies.

New sensitive VLA observations confirm the existence of halo and relic sources in 6 Abell clusters where a diffuse emission was found in the NVSS.

We find evidence that the frequency of clusters with halos and relics is larger in clusters with high X-ray luminosity. The evidence that the occurrence of a halo source is larger at high redshifts is marginal.

To investigate the rate of occurrence and possible triggering mechanisms for diffuse radio sources, we have carried out a statistical study of a volume-limited sample of 92 clusters of galaxies.

We describe the highest-resolution radio observations yet made of relic radio sources. These relics have extremely steep (α ≤ −2.5) spectra and are located in four southern Abell clusters (A 13, A 85, A133, A 4038). VLA images at 1.4 GHz and ∼4″ resolution show a remarkable variety of fine structure like arcs, wisps, plumes, and loops. Integrated polarization fractions range from 2.3% to 16.8%. Deep red CCD images do not reveal any optical object with a brightness and/or position typical of a radio host galaxy. The X-ray centroids in ROSAT images tend to be displaced from the cluster potential wells in the direction of the relics in A85, A133 and A4038, suggesting that some of the X-rays may be inverse Compton or excess thermal emission from the relic itself.

A3667 is a remarkable cluster of galaxies possessing the two largest and brightest radio relics yet seen. It provides an important opportunity to examine the environments associated with relic production.

Discovery of a diffuse ultra-steep spectrum radio source of size ∼ 0.3 Mpc, possibly a radio halo, in a cluster of galaxies at z = 0.13 is reported hilighting the presence of a giant radio spiral within the halo.

We present preliminary results of a multifrequency and multiresolution study carried out with the Australia Telescope Compact Array for nine of the ten extended radio galaxies located in the merging cluster complexes A3558 and A3528, at the centre of the Shapley Concentration. We found that 5 out of the 9 extended radio galaxies are active radio galaxies, i.e. they have a clear active radio nucleus coincident with the central region of the associated optical galaxy, radio jets and extended lobes; the remaining four lack an obvious radio nucleus, have a very diffuse and amorphous morphology and exhibit peculiar spectral properties. We call these radio sources as remnants and propose that they are (a) either radio galaxies where the nuclear activity has ceased; or (b) regions where pre-existing electrons have been reaccelerated as consequence of shocks due to cluster mergers.

We present results obtained from our 21cm VLA survey of spirals in Coma. The goal of this HI imaging of galaxies is twofold: first, study the interaction between individual galaxies and the cluster environment, looking for a connection with the starburst and post starburst phenomena. Second, to investigate the dynamical state of the cluster, analyzing the gas content of different groups of galaxies.

We have imaged in HI a ∼ 25′ field of view in the Coma Southwest region using the Giant Meterwave Radio Telescope. No HI was detected to a 3σ limit of 3 × 108M⊙ in the surveyed volume. Except for NGC 4839, no radio continuum emission was detected from the galaxies in the Coma Southwest to a 3σ limit of 3 × 1020 W Hz−1 of spectral luminosity at 1.4 GHz.

Extended radio emission and its relation to parent galaxy properties is briefly reviewed. Our current understanding of the relation between absolute radio and optical luminosity, radio morphology and linear size is discussed. The impact of radio jets on dense cluster cores is discussed using M87 as an example. Finally, the relation of AGN's to star-bursting galaxies at high redshift is considered.