Abstract. Simulations of structure formation in the Universe predict accretion shock waves at the boundaries of the large-scale structures as sheets, filaments, and clusters of galaxies. If magnetic fields are present at these shocks, particle acceleration should take place, and could contribute to the observed cosmic rays of high energies. When the radio plasma of an old invisible radio lobe is dragged into such a shock wave, the relativistic electron population will be reaccelerated and the plasma becomes radio-luminous again. Such tracers of the accretion shock waves are observed at the boundaries of some clusters of galaxies: the so-called cluster radio relics, which are large regions of diffuse radio emission, without any parent galaxy nearby. The observed properties of the cluster radio relics are naturally explained by accretion shock waves. Radio relics therefore give the first evidence for the existence of accretion shocks of the large-scale structure formation and they allow investigations of the shock properties. Shock Waves of the Large-Scale Structure FormationThe large-scale structure of the Universe, seen in the structured galaxy distribution, is still forming. Matter is flowing out of the cosmic voids onto sheets and filaments. Within the filaments the matter flows to the density cusps frequently located at the intersection points of filaments: the clusters of galaxies. Whenever the flow passes from one structure into another, its velocity suddenly changes and several Mpc sized shock waves occur. At these shock waves the kinetic energy of the gravitationally accelerated gas is

We present interferometric H i 21 cm line observations of the extended gas around the irregular galaxy NGC 4449 covering 67Ј on the sky at a resolution of ∼1Ј. The main star-forming body of NGC 4449 is relatively normal for a Magellanic irregular galaxy, but the galaxy is unusual in that it has two counterrotating gas systems and H i that extends to 6 times the Holmberg radius. Our new, detailed H i maps of this extended gas show that most of the extended H i is located in large, highly structured, extended clouds and very long streamers. We compare NGC 4449 with other systems in the context of possible models for the origin of these structures, the most likely of which involves an interaction with another galaxy. Thus, NGC 4449 no longer fits the standard picture of an irregular galaxy quietly evolving in isolation.

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Abstract. NGC 4449 is an active star-forming dwarf galaxy of Magellanic type. From radio observations, van Woerden et al. (1975) found an extended HI-halo around NGC 4449 which is at least a factor of 10 larger than the optical diameter D25 ≈ 5.6 kpc. Recently, Hunter et al. (1998) discerned details in the HI-halo: a disc-like feature around the center of NGC 4449 and a lopsided arm structure. We combined several N-body methods in order to investigate the interaction scenario between NGC 4449 and DDO 125, a close companion in projected space. In a first step fast restricted N-body models are used to confine a region in parameter space reproducing the main observational features. In a second step a genetic algorithm is applied for a uniqueness test of our preferred parameter set. We show that our genetic algorithm reliably recovers orbital parameters, provided that the data are sufficiently accurate, i.e. all the key features are included. In the third step the results of the restricted N-body models are compared with self-consistent N-body simulations. In the case of NGC 4449, the applicability of the simple restricted N-body calculations is demonstrated. Additionally, it is shown that the HI gas can be modeled here by a purely stellar dynamical approach. In a series of simulations, we demonstrate that the observed features of the extended HI disc can be explained by a gravitational interaction between NGC 4449 and DDO 125. According to these calculations the closest approach between both galaxies happened ∼4 − 6 10 8 yr ago at a minimum distance of ∼25 kpc on a parabolic or slightly elliptic orbit. In the case of an encounter scenario, the dynamical mass of DDO 125 should not be smaller than 10% of NGC 4449's mass. Before the encounter, the observed HI gas was arranged in a disc with a radius of 35-40 kpc around the center of NGC 4449. It had the same orientation as the central ellipsoidal HI structure. The origin of this disc is still unclear, but it might have been caused by a previous interaction.

Sensitive images of low-level, megaparsec-sized radio cocoons offer new opportunities to probe large-scale intergalactic gas flows outside clusters of galaxies. New radio images of high surface brightness sensitivity at strategically chosen wavelengths of the giant radio galaxy NGC 315 reveal significant asymmetries and particularities in the morphology, radio spectrum, and polarization of the ejected radio plasma. We argue that the combination of these signatures provides a sensitive probe of an environmental shock wave. Analysis of optical redshifts in NGC 315's vicinity confirms its location to be near, or at, a site of large-scale flow collisions in the 100 Mpc sized Pisces-Perseus supercluster region. NGC 315 resides at the intersection of several galaxy filaments, and its radio plasma serves there as a "weather station," probing the flow of the elusive and previously invisible intergalactic medium gas. If our interpretation is correct, this is the first indication for a shock wave in flows caused by the cosmological large-scale structure formation, which is located in a filament of galaxies. The possibility that the putative shock wave is a source of gamma rays and ultrahigh-energy cosmic rays is briefly discussed.