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2007A&A...474..415C - Astronomy and Astrophysics, volume 474, 415-429 (2007/11-1)

Magnetic fields and gas in the cluster-influenced spiral galaxy NGC 4254. I. Radio and X-rays observations.

CHYZY K.T., EHLE M. and BECK R.

Abstract (from CDS):

Radio observations can show how cluster galaxies are affected by various environmental factors, that perturb their morphology, as well as modify properties of the interstellar medium (ISM), especially the characteristics of its magnetic field. We made high-resolution and high-sensitivity radio-polarimetric VLA observations of NGC4254 at three frequencies (8.46, 4.86, and 1.43GHz). The interferometric data were extended with single-dish (100-m Effelsberg) observations. Next we performed sensitive XMM-Newton observations in X-rays and UV light to investigate the hot gas component and its possible interaction with the hot cluster medium. For a complete picture of the interplay between various gas phases, we also used optical, HI, and infrared (Spitzer) data. The distribution of total radio intensity at 8.46GHz and 4.86GHz reveals a global asymmetry with a more diffuse and almost two times larger extension to the north than to the south. The radio-polarized intensity is even more asymmetric, showing a strange bright ridge in the southern disk edge, displaced to the downstream side of the local density wave. Magnetic arms can also be seen in other disk portions, mostly (but not always) avoiding nearby optical spiral arms. Spatially-resolved emission of hot X-ray gas from the whole galactic disk, with its soft component closely tracing star-forming regions, is detected. Various gas components of a thermal origin show strong wavelet crosscorrelations (rw≥0.8), but the polarized intensity anticorrelates (rw=-0.4) with the thermal and X-ray emission. The slope of the local radio nonthermal-infrared relation is <1, thus smaller than for the radio thermal-infrared one (≥1). Using the radio thermal emission-based star-formation rate (SFR), we find higher extinction in more Hα luminous star-forming regions with a power-law slope of 0.83. The galaxy's estimated mean SFR of 0.026M/yr/kpc2 is three times larger than in other spirals with a similar Hubble type. NGC4254 seems to belong to the class of young'' Virgo cluster members, which recently experienced a gravitational encounter at the cluster's periphery, which in turn perturbed its spiral arms by tidal forces and triggered a burst of star formation that still maintains strong radio and infrared emissions. Tidal forces could also shear the magnetic field in the southern disk portion and lead to the observed polarized ridge, although magnetic field compression by weak ram pressure forces of the cluster gas cannot be excluded. The case of NGC4254 shows that the polarized signal provides additional information on the MHD processes acting on magnetized plasma during the galaxy's evolution, which cannot be obtained from any other ISM component.

Journal keyword(s): galaxies: general - galaxies: ISM - galaxies: magnetic fields - galaxies: interactions - radio continuum: galaxies - ISM: magnetic fields