High resolution radio continuum survey of M33: I. The radio maps

TL;DR

This study presents high-resolution radio continuum maps of M33, revealing detailed magnetic field structures, spectral index variations, and star formation regions, enhancing understanding of galactic magnetic fields and cosmic ray distributions.

Contribution

First high-resolution radio maps of M33 combining Effelsberg and VLA data, analyzing magnetic fields, spectral indices, and star formation distribution with new detailed insights.

Findings

Magnetic field exhibits well-ordered spiral structure.

Spectral index increases faster beyond 4 kpc radius.

Star forming regions mainly within 4 kpc, with possible cosmic ray correlation.

Abstract

We study the exponential scale length of total radio emission, the spectral index distribution, and the linear radio polarization in the Scd galaxy M33. Observations were carried out using the 3.6 cm dual channel and the 6.2 cm four channel receivers of the 100--m Effelsberg telescope along with the L-band VLA D--array at 20 cm. High spatial resolution and sensitivity in both total and linearly polarized radio continuum emission from M33 were achieved. We found considerable extended emission, not only from the main arms I S and I N, but also from the weaker arms. The large--scale magnetic field exhibits well--ordered spiral structure with almost the same orientation as that of the optical spiral arms, however, it does not show a clear structural correlation or anti--correlation with the optical arms. There is a north-south asymmetry in polarization that is frequency-dependent. We found that the ring mean spectral index versus radius increases faster beyond $R$ = 4 kpc. At each wavelength, the exponential scale length is larger inside than outside $R$ = 4 kpc. From the larger scales lengths at $R$ $<$ 4 kpc, we conclude that star forming regions are mainly spread over the region $R$ $<$ 4 kpc without a dominant nuclear concentration. Furthermore, at $R$ $<$ 4 kpc, a spatial correlation between cosmic rays and star forming regions may exist. From the behaviour of the mean spectral indices obtained from different pairs of the radio continuum data at 3.6, 6.2, and 20 cm, we confirm that a decrease in the thermal fraction causes an increase in the spectral index. The frequency-dependent asymmetry in the polarization hints to an asymmetry in Faraday depolarization.