The gamma-ray / infrared luminosity correlation of star-forming galaxies

P. Kornecki ... [et al.].

Serie: Trabajos publicados del IAR ; no. 1542

Resumen: Context. Near a dozen star-forming galaxies have been detected in gamma-rays by the Fermi observatory in the last decade. A remarkable propertyof this sample is the quasi-linear relation between the gamma-rayluminosity and the star formation rate, obtained assuming that thelatter is well traced by the infra-red luminosity of the galaxies. Thenon-linearity of this relation has not been fully explained yet. Aims.We aim at determining the biases derived from the use of the infraredluminosity as a proxy for the star formation rate, and shed light ontothe more fundamental relation between the latter and the gamma-rayluminosity. We expect to quantify and explain some trends observed inthis relation. Methods. We compile from the literature a near-homogeneous set of distances, ultraviolet, optical, infra-red, andgamma-ray fluxes for all known gamma-ray emitting star-forming galaxies.From these data we compute the infra-red and gamma-ray luminosities, andstar formation rates. We determine the best-fitting relation between thelatter two, and describe the trend using simple, population-orientedmodels for cosmic-ray transport and cooling. Results. We find that thegamma-ray luminosity-star formation rate relation obtained from infra-red luminosities is biased to shallower slopes. The actual relation issteeper than previous estimates, having a power-law index of 1.35 +-0.05, in contrast to 1.23 +- 0.06. Conclusions. The unbiased gamma-rayluminosity-star formation rate relation can be explained at high starformation rates by assuming that the cosmic ray cooling region iskiloparsec-sized, and pervaded by mild to fast winds. Combined withprevious results about the scaling of wind velocity with star formationrate, our work provides support to advection as the dominant cosmic rayescape mechanism in low-star formation rate galaxies.