γ-ray/infrared luminosity correlation of star-forming galaxies

Abstract

Context. Nearly a dozen star-forming galaxies have been detected in γ-rays by the Fermi observatory in the last decade. A remarkable property of this sample is the quasi-linear relation between the γ-ray luminosity and the star formation rate, which was obtained assuming that the latter is well traced by the infrared luminosity of the galaxies. The non-linearity of this relation has not been fully explained yet. Aims. We aim to determine the biases derived from the use of the infrared luminosity as a proxy for the star formation rate and to shed light on the more fundamental relation between the latter and the γ-ray luminosity. We expect to quantify and explain some trends observed in this relation. Methods. We compiled a near-homogeneous set of distances, ultraviolet, optical, infrared, and γ-ray fluxes from the literature for all known γ-ray emitting, star-forming galaxies. From these data, we computed the infrared and γ-ray luminosities, and star formation rates. We determined the best-fitting relation between the latter two, and we describe the trend using simple, population-orientated models for cosmic-ray transport and cooling. Results. We find that the γ-ray luminosity–star formation rate relation obtained from infrared luminosities is biased to shallower slopes. The actual relation is steeper than previous estimates, having a power-law index of 1.35 ± 0.05, in contrast to 1.23 ± 0.06. Conclusions. The unbiased γ-ray luminosity–star formation rate relation can be explained at high star formation rates by assuming that the cosmic-ray cooling region is kiloparsec-sized and pervaded by mild to fast winds. Combined with previous results about the scaling of wind velocity with star formation rate, our work provides support to advection as the dominant cosmic-ray escape mechanism in galaxies with low star formation rates.

Star-forming galaxies dominate the diffuse, isotropic γ-ray background

“…In Figure 2 we show the resulting distribution of galaxies in the L FIR -L γ -plane, along with a power law fit to the data (blue dotted line), L γ /erg s −1 = 10 27.69 (L FIR /L ) 1.19 . For comparison, the solid green line shows a fit to the 14 nearby SFGs detected in γ-rays by Fermi LAT [33]. Our model prediction shows excellent agreement with the observed relation, and we note that both the model and the observed correlation differ noticeably from the calorimetric limit obtained by simply setting f cal (E) = 1 in Equation 2 (red dashed line in the figure).…”

“…Similarly, we calculate f cal as a function of energy directly, rather than relying on an empirical FIR-γ correlation, and our calorimetry fractions are on average larger than those implicitly assumed in earlier works. This is because many of the lower estimates for the contribution from SFGs to the γ-ray background rely on a FIR-γ relation derived from early Fermi detections of < 10 individually-resolved SFGs [3] that yields somewhat lower γ-ray luminosities than more recent fits using a larger (but still small) sample of SFGs [33], and with which our model agrees ( Figure 2). Thus the reason we find that SFGs can produce the full background, whereas earlier models could not, is that our model predicts γ-ray emission that is both somewhat brighter and has a more complex spectral shape than the values adopted in earlier work.…”

“…[3]. 10 17 10 10 15 10 14 10 13 10 12 10 11 10 10 10 9 10 8 10 [33] and model-predicted source counts for observed CANDELS galaxies at z > 0.1, respectively; error bars show 90% confidence intervals or upper limits. The three brightest non-empty Fermi bins each contain only a single SFG, which we have labelled.…”

A multi-wavelength view of the cosmic ray confinement in star-forming galaxies

“…In this section we summarise the key aspects of the model, while we refer the reader to [13] and [14] for a more detailed description. The existence of the -SFR [6] and radio -SFR [8] relations suggests that a model based on the energy balance between the CR injection, escape and loss mechanisms can describe properly the global emission of SFGs.…”

“…For the minimum energies we adopt min,p = 1.2 GeV and min,e = 1 MeV. We adopt 0, / 0, = 50 [13,14,17,21]. We normalise the accelerated proton distribution assuming that they inherit a fraction CR = 10% of the supernova energy ( SN = 10 51 erg), so that = R SN SN SN .…”