THE PHYSICS OF THE FAR-INFRARED–RADIO CORRELATION. I. CALORIMETRY, CONSPIRACY, AND IMPLICATIONS

By B. Lacki, T. Thompson, E. Quataert. 2009

152 citations recorded by the source corpus.

The far-infrared (FIR) and radio luminosities of star-forming galaxies are linearly correlated over a very wide range in star formation rate, from normal spirals like the Milky Way to the most intense starbursts. Using one-zone models of cosmic ray (CR) injection, cooling, and escape in star-forming galaxies, we attempt to reproduce the observed FIR–radio correlation (FRC) over its entire span. The normalization and linearity of the FRC, together with constraints on the CR population in the Milky Way, have strong implications for the CR and magnetic energy densities in star-forming galaxies. We show that for consistency with the FRC, ∼2% of the kinetic energy from supernova explosions must go into high-energy primary CR electrons and that ∼10%–20% must go into high-energy primary CR protons. Secondary electrons and positrons are likely comparable to or dominate primary electrons in dense starburst galaxies. We discuss the implications of our models for the magnetic field strengths of starbursts, the detectability of starbursts by Fermi, and CR feedback. Overall, our models indicate that both CR protons and electrons escape from low surface density galaxies, but lose most of their energy before escaping dense starbursts. The FRC is caused by a combination of the efficient cooling of CR electrons (calorimetry) in starbursts and a conspiracy of several factors. For lower surface density galaxies, the decreasing radio emission caused by CR escape is balanced by the decreasing FIR emission caused by the low effective UV dust opacity. In starbursts, bremsstrahlung, ionization, and inverse Compton cooling decrease the radio emission, but they are countered by secondary electrons/positrons and the dependence of synchrotron frequency on energy, both of which increase the radio emission. Our conclusions hold for a broad range of variations in our fiducial model, such as those including winds, different magnetic field strengths, and different diffusive escape times.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/fb319e5b3091a490a79949737f5591f8e0527dab

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Citations in the corpus

DIRECT NUMERICAL SIMULATION OF RADIATION PRESSURE-DRIVEN TURBULENCE AND WINDS IN STAR CLUSTERS AND GALACTIC DISKS (2012, background): “…based on observations of synchotron emission suggest that the magnetic energy density is sub-equipartition compared to the turbulent or gravitational energies (Thompson et al. 2009; Lacki et al. 2010; Batejat et al. 2011), which suggests that magnetic effects are unlikely to be important there.”
GeV OBSERVATIONS OF STAR-FORMING GALAXIES WITH THE FERMI LARGE AREA TELESCOPE (2012, methodology): “…that the majority of starburst emission at energies> 0.1 GeV is produced by interactions of CR nuclei in the interstellar medium, assuming primary CR proton to electron ratios similar to values found in the Milky Way (e.g. Paglione et al. 1996; Torres 2004; Persic et al. 2008; Lacki et al. 2010).”
The physics of galactic winds driven by active galactic nuclei (2012, methodology): “The efficiency factor ξ ≈ 1% is constrained for SNR shocks based on modeling the far infrared-radio correlation (e.g., Lacki et al. 2010).”
Dusty Star Forming Galaxies at High Redshift (2014, background): “Bremsstrahlung radiation and ionization can flatten the spectrum, but would meanwhile break the FIR/radio correlation, so cosmic ray proton cooling is needed to make up for radio emission lost due to Bremsstrahlung and ionization (Thompson et al., 2007; Lacki et al., 2010; Lacki and Thompson, 2010).”
The far-infrared/radio correlation as probed by Herschel (2010, background): “Prevailing theory (e.g. Lacki et al. 2010) suggests that variations in the farIR/radio flux ratio should be virtually unavoidable and that the FIRRC thus arises due to a mysterious combination of effects involving bremsstrahlung, inverse Compton cooling, ionisation and the relative fractions of…”
Feedback-regulated star formation in molecular clouds and galactic discs (2013, background): “Cosmic ray models predict and γ−ray observations indicate that cosmic rays are also dynamically unimportant for the gas surface densities considered here (Lacki et al. 2010, 2011). c© 0000 RAS, MNRAS 000, 000–000 Table 1.”
SIMULATIONS OF DISK GALAXIES WITH COSMIC RAY DRIVEN GALACTIC WINDS (2013, background): “A tight link between CRs and star formation is evidenced by the correlation between a galaxy’s infra-red luminosity, closely related to its SFR, and the luminosity of its radio halo (e.g. Helou et al. 1985; Lacki et al. 2010).”