Implications of the correlation between radio and far-infrared emission for spiral galaxies

Abstract

Summary A theory is proposed for the correlation between radio continuum (RC) and thermal far-infrared (FIR) radiation for normal spiral galaxies. In this theory, we assume global energy outputs of the FIR and cosmic rays to be proportional to the star formation rate, and adopt energy equipartition between cosmic rays and the interstellar magnetic field so as to keep dynamical stability and steady star formation in the galaxies. It is shown that the tight correlation between the RC and FIR emission results from a steady star formation process in the galaxies rather than both of them being simply from the same source. An important feature of the model is that, in the majority of galaxies, most of the cosmic ray electrons responsible for the radio emission at the observed frequencies escape. An approximate solution to the problem of electron propagation with a free escape boundary condition is derived and the emissivity of synchrotron radio emission is calculated. Direct evidence for the theory is the prediction of a non-unity slope for the correlation between RC and FIR, and this is observed. From small and less luminous galaxies to large and more luminous ones, we predict that there is a turning point above which most of the electrons will be trapped in the galaxies rather than escaping, and correspondingly the slope of the correlation will become unity. From a sample of data, we find this trend.

Radio Properties of Infrared‐Selected Galaxies in theIRAS2 Jy Sample

“…Dust heating by low mass stars ("cirrus" emission) may contribute only to the observed FIR emission (Helou 1986;Lonsdale-Persson & Helou 1987;Cox et al 1988;Devereux & Eales 1989). Alternatively, the radio luminosity of less luminous (and generally less massive) galaxies may be low if cosmic-ray loss by diffusion is important (Klein et al 1984;Chi & Wolfendale 1990). Because this sample is FIR selected, there is a potential bias towards sources with higher FIR/radio flux density ratios (see Condon & Broderick 1986).…”

Estimating Star Formation Rates from Infrared and Radio Luminosities: The Origin of the Radio‐Infrared Correlation

“…Dwarf galaxies seem to have a lower non-thermal to thermal emission ratio than normal spiral galaxies (Klein, Wielebinski & Thuan 1984;Klein 1991;Klein et al 1991;Price & Duric 1992), although estimating the balance of thermal and non-thermal radio emission is painfully difficult, and can be uncertain for even well-studied galaxies at a factor of five level (Condon 1992). This difference between dwarf and larger galaxies is often interpreted as a higher efficiency of cosmic ray confinement in physically larger (or more massive) galaxies (e.g., Klein, Wielebinski & Thuan 1984;Chi & Wolfendale 1990;Price & Duric 1992). For interesting discussions about the relative balance of non-thermal and thermal emission see Condon (1992) and Niklas et al (1997).…”

“…In contrast, it has been suggested for nearly 20 years that the non-thermal synchrotron emission of low-luminosity galaxies can be significantly suppressed (Klein, Wielebinski & Thuan 1984;Klein 1991;Klein et al 1991;Price & Duric 1992, although the thermal contribution can be very challenging to reliably estimate; Condon 1992). This can be explained in a number of ways, as the physics which links the SF rate with non-thermal emission is complex, and involves the cosmic ray production rate, galaxy magnetic field strength, and galaxy size to name just a few of the many variables (Chi & Wolfendale 1990;Helou & Bicay 1993;Lisenfeld et al 1996). For example, Chi & Wolfendale (1990) discuss a model in which the non-thermal emission from low-luminosity galaxies is strongly suppressed, because most of the cosmic-ray electrons escape from the galaxy due to their small sizes (although the size of the effect that they predict is a factor of 3-5 in excess of the trend allowed by these observations).…”

“…This can be explained in a number of ways, as the physics which links the SF rate with non-thermal emission is complex, and involves the cosmic ray production rate, galaxy magnetic field strength, and galaxy size to name just a few of the many variables (Chi & Wolfendale 1990;Helou & Bicay 1993;Lisenfeld et al 1996). For example, Chi & Wolfendale (1990) discuss a model in which the non-thermal emission from low-luminosity galaxies is strongly suppressed, because most of the cosmic-ray electrons escape from the galaxy due to their small sizes (although the size of the effect that they predict is a factor of 3-5 in excess of the trend allowed by these observations).…”

The Physics of the Far-Infrared-Radio Correlation. I. Calorimetry, Conspiracy, and Implications

“…We also do not include free-free emission, which can flatten the spectral slope, especially in low surface density galaxies. In ULIRGs like Arp 220, the observed α is typically ∼ 0.5 (Clemens et al 2008), but radio emission in these galaxies may suffer free-free absorption which flattens the spectrum; the unabsorbed synchrotron spectrum α may be as high as 0.7 (Condon et al 1991). To some extent, a small to moderate difference in α from its observed value can be adjusted by altering p, since decreasing p by 0.1 generally decreases α by 0.05, and p often is not well constrained in the considered range 2 − 2.6.…”