Search

Our principal aim is to compare global star formation rates between cluster galaxies and field galaxies in order to clarify environmental influence on star formation. We use an objective prism technique to survey over 200 Zwicky catalogue (CGCG) galaxies within ∼ 2°.5 of Abell 1367 for Hα emission. After a brief discussion of the survey characteristics, we consider first the dependence of Hα detection on Hubble type, galaxy disturbance and the presence of a bar. As expected, we rarely detect early-type galaxies and consequently restrict further discussion to spirals (type S0/a and later), of which we detect ∼ 35 per cent in Hα. We find that an extremely valuable distinction to make is between galaxies with diffuse Hα and galaxies with compact Hα. There is a very significant tendency for galaxies with compact Hα emission to be disturbed, and there may be a weak tendency for them to be barred. Neither of these tendencies is found for galaxies with diffuse Hα emission. We infer that compact emission results from tidally induced star formation, while diffuse emission results from more normal disc star formation. After considering field contamination, we adopt as a ‘predominantly cluster’ sample the spiral population inside 0.5 rA; a ‘predominantly field’ sample outside 0.5 rA; and a ‘pure field’ sample outside 1.5 rA. We consistently find a much larger fraction of spirals detected with compact Hα in the cluster sample compared to the field samples (e.g. 38 versus 0 per cent detected in cluster and ‘pure field’ samples, χ2 significance 3.6 σ). This increased fraction detected in the cluster is found for early-, mid- and late-type spirals separately. No such cluster/field differences are found for galaxies with diffuse Hα emission. We conclude that tidal perturbations are more common in the cluster than in the field, leading to a higher incidence of compact tidally triggered star formation. By combining information on galaxy disturbance, galaxy companions, and the location of galaxies within the cluster, we have tried to identify the origin of the tidal perturbations. We find strong evidence that near-neighbour interaction plays a significant role in triggering star formation. However, we also find candidate objects near the cluster core which may be perturbed by the overall cluster tidal field, and candidate objects which may be influenced by a higher speed ‘harassment’ interaction between galaxies.

We present the results of the multi-wavelength campaigns on 3C 273 in 1993-1995. During the observations in late 1993 this quasar showed an increase of its flux for energies greater equal 100 MeV from about 2.1 x 10^-7 photons/cm^2/s to approximately 5.6 x 10^-7 photons/cm^2/s during a radio outburst at 14.5, 22 and 37 GHz. However, no one-to-one correlation of the gamma-ray radiation with any frequency could be found. The photon spectral index of the high energy spectrum changed from (3.20 +- 0.54) to (2.20 +- 0.22) in the sense that the spectrum flattened when the gamma-ray flux increased. Fits of the three most prominent models (synchrotron self-Comptonization, external inverse Comptonization and the proton initiated cascade model) for the explanation of the high gamma-ray emission of active galactic nuclei were performed to the multi-wavelength spectrum of 3C 273. All three models are able to represent the basic features of the multi-wavelength spectrum. Although there are some differences the data are still not decisive enough to discriminate between the models., 63 pages including 9 eps figures and 9 tables, LateX, uses aaspp4.sty and tighten.sty, accepted for publication in ApJ

The blazar 3C 279, one of the brightest identified extragalactic objects in the gamma-ray sky, underwent a flare of a factor 10 amplitude in gamma-rays towards the end of a 3-week pointing by CGRO, in 1996 January-February. The flare peak represents the highest gamma-ray intensity ever recorded for this object. During the high state, extremely rapid gamma-ray variability was seen. Coordinated multifrequency observations were carried out with RXTE, ASCA, ROSAT and IUE and from many ground-based observatories, covering most accessible wavelengths. The well-sampled, simultaneous RXTE light curve shows an outburst of lower amplitude (factor of ~3) well correlated with the gamma-ray flare without any apparent lag. The optical-UV light curves, which are not well sampled during the high energy flare, exhibit more modest variations (factor of ~2) and a lower degree of correlation. The flux at millimetric wavelengths was near an historical maximum during the gamma-ray flare peak, with suggestion of a correlated decay. We present simultaneous spectral energy distributions of 3C 279 prior to and near to the flare peak. The gamma-rays vary by more than the square of the observed IR-optical flux change, which poses some problems for specific blazar emission models. The synchrotron-self Compton model would require that the largest synchrotron variability occurred in the mostly unobserved sub-mm/far-infrared region. Alternatively, a large variation in the external photon field could occur over a time scale of few days. This occurs naturally in the ``mirror'' model, wherein the flaring region in the jet photoionizes nearby broad-emission-line clouds, which in turn provide soft photons that are Comptonized to gamma-ray energies., Comment: 18 pages, Latex, 2 tables, 3 postscript figures, to appear in The Astrophysical Journal