The Magnetic Field of the Irregular Galaxy NGC 4214

TL;DR

This study investigates the magnetic field structure and strength in the irregular galaxy NGC 4214 using radio polarization data, revealing the dominance of thermal and turbulent pressures in star-forming regions and setting limits on the galaxy's magnetic field uniformity.

Contribution

First detailed magnetic field measurement in NGC 4214, combining multi-wavelength radio data with pressure analysis to understand magnetic and gas dynamics in irregular galaxies.

Findings

Magnetic field strength is approximately 30 μG in the center and 10 μG at the edges.

No significant polarization detected on scales >200 pc, with an upper limit of 8 μG on uniform field.

Star formation activity influences magnetic field development, with implications for dynamo processes.

Abstract

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 3cm radio continuum morphology is similar to that of the Halpha, while the 20cm 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 $30\pm 9.5$ \uG\ in the center and $10\pm3$ \uG\ 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 HII 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 \uG\ 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 20cm, is elongated due to a uniform magnetic field with a maximum field strength of 7.6 \uG. 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 $\alpha-\omega$ dynamo.