The magnetic field of the irregular galaxy NGC 4214.
KEPLEY A.A., ZWEIBEL E.G., WILCOTS E.M., JOHNSON K.E. and ROBISHAW T.
Abstract (from CDS):
We examine the magnetic field in NGC 4214, a nearby irregular galaxy, using multi-wavelength radio continuum polarization data from the Very Large Array. We find that the global radio continuum spectrum shows signs that free-free absorption and/or synchrotron losses may be important. The 3 cm radio continuum morphology is similar to that of the Hα while the 20 cm emission is more diffuse. We estimate that 50% of the radio continuum emission in the center of the galaxy is thermal. Our estimate of the magnetic field strength is 30±9.5 µG in the center and 10±3 µG at the edges. We find that the hot gas, magnetic, and the gravitational pressures are all the same order of magnitude. Inside the central star-forming regions, we find that the thermal and turbulent pressures of the H II regions dominate the pressure balance. We do not detect any significant polarization on size scales greater than 200 pc. We place an upper limit of 8 µG on the uniform field strength in this galaxy. We suggest that the diffuse synchrotron region, seen to the north of the main body of emission at 20 cm, is elongated due to a uniform magnetic field with a maximum field strength of 7.6 µG. We find that, while the shear in NGC 4214 is comparable to that of the Milky Way, the supernova rate is half that of the Milky Way and suggest that the star formation episode in NGC 4214 needs additional time to build up enough turbulence to drive an α-ω dynamo.
galaxies: individual: NGC 4214 - galaxies: irregular - galaxies: ISM - galaxies: magnetic fields - polarization