Radio synchrotron spectra of star-forming galaxies

Abstract

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 M 82). 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 α nth = 0.59 ± 0.20 (S ν ∝ ν −α ), 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 α 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) Hα 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.

Nature of Faint Radio Sources in GOODS-North and GOODS-South Fields. I. Spectral Index and Radio–FIR Correlation

“…The broad radio spectral index distributions for the GS and GN fields shown in Figure 4 sug-gests the existence of both steep spectrum (α = 0.5−1.0) and flatter or inverted spectrum (α < 0.5) sources at S 5GHz < 150 µJy, supporting the conclusions of the more recent analyses indicating that the faint µJy radio population consist of both SFGs and radio-quiet AGN (Padovani et al 2009;Bonzini et al 2013;Rudnick & Owen 2014). A detailed study of a small sample of 14 local SFGs by Klein et al (2018) has shown that there is also some scatter in the observed radio spectral index in the GHz range due to a varying degree of free-free emission and opacity effects. What our study further indicates is that a larger sample with higher quality radio spectral index measurements are needed to characterize the relative contribution by these two populations.…”

Quantifying the thermal Sunyaev–Zel’dovich effect and excess millimetre emission in quasar environments

“…Synchrotron emission can also exhibit an exponential cut-off at high frequencies. Curved spectral shapes that decline toward increasing radio frequencies have been observed in the spectra of many star forming galaxies (e.g., Williams & Bower 2010;Marvil et al 2015;Klein et al 2018). One explanation of the decline is that there is an exponential cutoff in the energy spectrum of relativistic electrons at a specific Lorentz factor (Kardashev 1962;Jaffe & Perola 1973).…”

“…The frequency ν b at which the spectrum declines depends on the conditions of the magnetic field that determined the acceleration of the particles, their energy loss, and their escape rates (Schlickeiser 1984). Klein et al (2018) found cutoff frequencies ranging from 4.7-12.4 GHz. Based on these results, we test a synchrotron model with an exponential cutoff frequency of 10 GHz.…”

“…The marginalized coupling fraction from the SZ amplitude is (4.1 ± 0.9)τ −1 8 per cent. Only half of star-forming galaxies show the exponential cutoff in their synchrotron spectra (Klein et al 2018), and even less is known about the synchrotron cutoff of radio-quiet quasars. If the synchrotron exponential cutoff frequency is as high as 40 GHz, the SZ amplitude is still measured at the 3.6σ level.…”

CMB foreground measurements through broad-band radio spectro-polarimetry: prospects of the SKA-MPG telescope

“…(2) to model the total intensity radio continuum emission below 10 GHz in our Galaxy, i.e., as a sum of synchrotron emission (T B,syn ) and an optically thin free-free emission (T B,ff ) -so far only used in the characterization of other galaxies' radio spectra (see, e.g., Paladino et al 2009;Tabatabaei et al 2017;Klein et al 2018). Since sharp spectral steepening is somewhat unlikely in galactic ISM, we assume a fixed ν c ≫ 10 GHz, which leads to setting the exponential-term in Eq.…”