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Register a new userThe influence of the cluster environment on the large-scale radio continuum emission of 8 Virgo cluster spirals
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B. Vollmer
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The influence of the environment on the polarized and total power radio continuum emission of cluster spiral galaxies is investigated. We present deep scaled array VLA 20 and 6 cm observations including polarization of 8 Virgo spiral galaxies. These data are combined with existing optical, HI, and Halpha data. Ram pressure compression leads to sharp edges of the total power distribution at one side of the galactic disk. These edges coincide with HI edges. In edge-on galaxies the extraplanar radio emission can extend further than the HI emission. In the same galaxies asymmetric gradients in the degree of polarization give additional information on the ram pressure wind direction. The local total power emission is not sensitive to the effects of ram pressure. The radio continuum spectrum might flatten in the compressed region only for very strong ram pressure. This implies that neither the local star formation rate nor the turbulent small-scale magnetic field are significantly affected by ram pressure. Ram pressure compression occurs mainly on large scales (>=1 kpc) and is primarily detectable in polarized radio continuum emission.
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The Virgo cluster of galaxies provides excellent conditions for studying interactions of galaxies with the cluster environment. Both the high-velocity tidal interactions and effects of ram pressure stripping by the intracluster gas can be investigated in detail. We extend our systematic search for possible anomalies in the magnetic field structures of Virgo cluster spirals in order to characterize a variety of effects and attribute them to different disturbing agents. Six angularly large Virgo cluster spiral galaxies (NGC4192, NGC4302, NGC4303, NGC4321, NGC4388, and NGC4535) were targets of a sensitive total power and polarization study using the 100-m radio telescope in Effelsberg at 4.85GHz and 8.35GHz (except for NGC4388 observed only at 4.85GHz, and NGC4535 observed only at 8.35GHz). Magnetic field structures distorted to various extent are found in all galaxies. Three galaxies (NGC4302, NGC4303, and NGC4321) show some signs of possible tidal interactions, while NGC4388 and NGC4535 have very likely experienced strong ram-pressure and shearing effects, respectively, visible as distortions and asymmetries of polarized intensity distributions. As in our previous study, even strongly perturbed galaxies closely follow the radio-far-infrared correlation. In NGC4303 and NGC4321, we observe symmetric spiral patterns of the magnetic field and in NGC4535 an asymmetric pattern. Magnetic fields allow us to trace even weak interactions that are difficult to detect with other observations. Our results show that the degree of distortions of a galaxy is not a simple function of the distance to the cluster center but reflects also the history of its interactions. The angle between the velocity vector and the rotation vector of a galaxy may be a general parameter that describes the level of distortions of galactic magnetic fields.
Deep scaled array VLA 20 and 6cm observations including polarization of 19 Virgo spirals are presented. This sample contains 6 galaxies with a global minimum of 20cm polarized emission at the receding side of the galactic disk and quadrupolar type large-scale magnetic fields. In the new sample no additional case of a ram-pressure stripped spiral galaxy with an asymmetric ridge of polarized radio continuum emission was found. In the absence of a close companion, a truncated HI disk, together with a ridge of polarized radio continuum emission at the outer edge of the HI disk, is a signpost of ram pressure stripping. 6 out of the 19 observed galaxies display asymmetric 6cm polarized emission distributions. Three galaxies belong to tidally interacting pairs, two galaxies host huge accreting HI envelopes, and one galaxy had a recent minor merger. Tidal interactions and accreting gas envelopes can lead to compression and shear motions which enhance the polarized radio continuum emission. In addition, galaxies with low average star formation rate per unit area have a low average degree of polarization. Shear or compression motions can enhance the degree of polarization. The average degree of polarization of tidally interacting galaxies is generally lower than expected for a given rotation velocity and star formation activity. This low average degree of polarization is at least partly due to the absence of polarized emission from the thin disk. Ram pressure stripping can decrease whereas tidal interactions most frequently decreases the average degree of polarization of Virgo spiral galaxies. We found that moderate active ram pressure stripping has no influence on the spectral index, but enhances the global radio continuum emission with respect to the FIR emission, while an accreting gas envelope can but not necessarily enhances the radio continuum emission with respect to the FIR emission.
Because of its proximity the Virgo Cluster is an excellent target for studying interactions of galaxies with the cluster environment. Both the high-velocity tidal interactions and effects of ram pressure stripping by the intracluster gas can be investigated. Optical and/or ion{H}{i} observations do not always show effects of weak interactions between galaxies and their encounters with the cluster medium. For this reason we searched for possible anomalies in the magnetic field structure in Virgo Cluster spirals which could be attributed to perturbations in their gas distribution and kinematics. Five angularly large Virgo Cluster spiral galaxies (NGC 4501, NGC 4438, NGC 4535, NGC 4548 and NGC 4654) were the targets for a sensitive total power and polarization study using the 100-m radio telescope in Effelsberg at 4.85 GHz. For two objects polarization data at higher frequencies have been obtained allowing Faraday rotation analysis. Distorted magnetic field structures were identified in all galaxies. Interaction-induced magnetized outflows were found in NGC 4438 (due to nuclear activity) and NGC 4654 (a combination of tidal tails and ram pressure effects). Almost all objects (except the anaemic NGC 4548) exhibit distortions in polarized radio continuum attributable to influence of the ambient gas. For some galaxies they agree with observations of other species, but sometimes (NGC 4535) the magnetic field is the only tracer of the interaction with the cluster environment. The cluster environment clearly affects the evolution of the galaxies due to ram pressure and tidal effects. Magnetic fields provide a very long-lasting memory of past interactions. Therefore, they are a good tracer of weak interactions which are difficult to detect by other observations.
The influence of the environment on gas surface density and star formation efficiency of cluster spiral galaxies is investigated. We extend previous work on radial profiles by a pixel-to pixel analysis looking for asymmetries due to environmental interactions. The star formation rate is derived from GALEX UV and Spitzer total infrared data. As in field galaxies, the star formation rate for most Virgo galaxies is approximately proportional to the molecular gas mass. Except for NGC 4438, the cluster environment does not affect the star formation efficiency with respect to the molecular gas. Gas truncation is not associated with major changes in the total gas surface density distribution of the inner disk of Virgo spiral galaxies. In three galaxies, possible increases in the molecular fraction and the star formation efficiency with respect to the total gas, of factors of 1.5 to 2, are observed on the windward side of the galactic disk. A significant increase of the star formation efficiency with respect to the molecular gas content on the windward side of ram pressure-stripped galaxies is not observed. The ram-pressure stripped extraplanar gas of 3 highly inclined spiral galaxies shows a depressed star formation efficiency with respect to the total gas, and one of them (NGC 4438) shows a depressed rate even with respect to the molecular gas. The interpretation is that stripped gas loses the gravitational confinement and associated pressure of the galactic disk, and the gas flow is diverging, so the gas density decreases and the star formation rate drops. However, the stripped extraplanar gas in one highly inclined galaxy (NGC 4569) shows a normal star formation efficiency with respect to the total gas. We propose this galaxy is different because it is observed long after peak pressure, and its extraplanar gas is now in a converging flow as it resettles back into the disk.
The intracluster light (ICL) is a faint diffuse stellar component in clusters made of stars not bound to individual galaxies. We have carried out a large scale study of this component in the nearby Virgo cluster. The diffuse light is traced using planetary nebulae (PNe). The PNe are detected in the on-band image due to their strong emission in the [OIII] 5007 line, but disappear in the off-band image. The contribution of Ly-alpha emitters at z=3.14 are corrected statistically using blank field surveys. We have surveyed a total area of 3.3 square degrees in the Virgo cluster with eleven fields located at different radial distances. Those fields located at smaller radii than 80 arcmin from the cluster center contain most of the detected diffuse light. In this central region of the cluster, the ICL has a surface brightness in the range 28.8 - 30 mag per sqarsec in the B band, it is not uniformly distributed, and represents about 7% of the total galaxy light in this area. At distances larger than 80 arcmin the ICL is confined to single fields and individual sub-structures, e.g. in the Virgo sub-clump B, the M60/M59 group. For several fields at 2 and 3 degrees from the Virgo cluster center we set only upper limits. These results indicate that the ICL is not homogeneously distributed in the Virgo core, and it is concentrated in the high density regions of the Virgo cluster, e.g. the cluster core and other sub-structures. Outside these regions, the ICL is confined within areas of 100 kpc in size, where tidal effects may be at work. These observational results link the formation of the ICL with the formation history of the most luminous cluster galaxies.
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