U. Klein

New 325 MHz observations of H II galaxies - The mechanisms that shape the unusual radio spectra

Original article can be found at: http://adsabs.harvard.edu/abs/ Copyright American Astronomical Society. DOI: 10.1086/172780 [Full text of this article is not available in the UHRA]

X-RAY EMISSION FROM AN EXPANDING SUPERGIANT SHELL IN IC 2574

We present a multiwavelength study of a supergiant shell within the violent interstellar medium of the nearby dwarf galaxy IC 2574, which is a member of the M81 group of galaxies. Neutral hydrogen (H i) observations obtained with the Very Large Array (VLA) reveal a prominent expanding supergiant H i shell in the northeast quadrant of IC 2574 which is thought to be produced by the combined effects of stellar winds and supernova explosions. It measures roughly pc in size and is expanding at »25 km s 21 . The H i data suggest 1000 # 500 an age of » yr; the energy input must have been of order ergs. Massive star-forming 6 53 1.4 # 10 (2.6 5 1) # 10 regions, as traced by Ha emission, are situated predominantly on the rim of this H i shell. This supports the view that the accumulated H i on the rim has reached densities that are high enough for secondary star formation to commence. VLA radio continuum observations at cm show that these star-forming regions are the main l 5 6 sources of radio continuum emission in this galaxy. This emission is mainly thermal in origin. Soft X-ray emission from within the H i hole is detected by a pointed ROSAT PSPC observation. The emission is resolved, coinciding in size and orientation with the H i shell. These spatial properties suggest that the emission is generated by an X-ray‐emitting plasma located within the H i shell, although a contribution from X-ray binaries cannot be completely ruled out. The X-ray luminosity within the 0.11‐2.4 keV energy range is 38

Magnetic fields and star formation in low-mass Magellanic-type and peculiar galaxies

We investigate how magnetic properties of Magellanic-type and perturbed objects are related to star-forming activity, galactic type, and mass. We present radio and magnetic properties of 5 Magellanic-type and 2 peculiar low-mass galaxies observed at 4.85 and/or 8.35 GHz with the Effelsberg 100-m telescope. The sample is extended to 17 objects by including 5 Magellanic-type galaxies and 5 dwarf ones. The radio emission of low-mass galaxies at 4.85/8.35 GHz is closely connected with their optical discs. The strengths of total magnetic field are within 5-9 \mu G, while the ordered fields reach 1-2 \mu G. The magnetic field strengths are well correlated with the surface density of SFR and manifest a power-law relation with an exponent of 0.25 extending a similar relation found for dwarf galaxies. The production of magnetic energy per supernova event is similar for all the various galaxies. It constitutes about 3% of the individual SN energy release. We show that the total magnetic field energy in galaxies is almost linearly related to the galactic gas mass, which indicates equipartition of the magnetic energy and the turbulent kinetic energy of the ISM. The Magellanic-type galaxies fit very well with the radio-infrared relation constructed for surface brightness of galaxies of various types, including bright spirals and interacting objects. We found that the typical far-infrared relation based on luminosity of galaxies is tighter and steeper but more likely to inherit a partial correlation from a tendency that larger objects are also more luminous. The thermal fractions, radio spectral indices, and magnetic field strengths of the Magellanic-type galaxies are between the values determined for spirals and dwarf galaxies. The confirmed magnetic field-star formation and radio-infrared relations for low-mass galaxies point to similar physical processes that must be at work in all galaxies.

Radio synchrotron spectra of star-forming galaxies

The radio continuum spectra of 14 star-forming galaxies are investigated by fitting nonthermal (synchrotron) and thermal (free-free) radiation laws. The underlying radio continuum measurements cover a frequency range of ~325 MHz to 24.5 GHz (32 GHz in case of M82). It turns out that most of these synchrotron spectra are not simple power-laws, but are best represented by a low-frequency spectrum with a mean slope alpha_nth = 0.59 +/- 0.20 (S_nu ~ nu^-alpha), and by a break or an exponential decline in the frequency range of 1 - 12 GHz. Simple power-laws or mildly curved synchrotron spectra lead to unrealistically low thermal flux densities, and/or to strong deviations from the expected optically thin free-free spectra with slope alpha_th = 0.10 in the fits. The break or cutoff energies are in the range of 1.5 - 7 GeV. We briefly discuss the possible origin of such a cutoff or break. If the low-frequency spectra obtained here reflect the injection spectrum of cosmic-ray electrons, they comply with the mean spectral index of Galactic supernova remnants. A comparison of the fitted thermal flux densities with the (foreground-corrected) Halpha fluxes yields the extinction, which increases with metallicity. The fraction of thermal emission is higher than believed hitherto, especially at high frequencies, and is highest in the dwarf galaxies of our sample, which we interpret in terms of a lack of containment in these low-mass systems, or a time effect caused by a very young starburst.

The Magnetic-Field Structure and Dynamics of NGC 253

Total-power and polarization observations of NGC 253 at 10.7 GHz have been performed with the 100-m MPIfR radio telescope. The observed arm/interarm polarization contrasts are discussed in the context of possible field configurations in spiral arms.