Could Nearby Star-forming Galaxies Light Up the Pointlike Neutrino Sky?

Abstract

Star-forming and starburst galaxies, which are well-known cosmic-ray reservoirs, are expected to emit gamma-rays and neutrinos predominantly via hadronic collisions. In this Letter, we analyze the 10 yr Fermi-LAT spectral energy distributions of 13 nearby galaxies by means of a physical model which accounts for high-energy proton transport in starburst nuclei and includes the contribution of primary and secondary electrons. In particular, we test the hypothesis that the observed gamma-ray fluxes are mostly due to star-forming activity, in agreement with the available star formation rates coming from IR and UV observations. Through this observation-based approach, we determine the most likely neutrino counterpart from star-forming and starburst galaxies and quantitatively assess the ability of current and upcoming neutrino telescopes to detect them as pointlike sources. Remarkably, we find that the cores of the Small Magellanic Cloud and the Circinus galaxy are potentially observable by KM3NeT/ARCA with 6 yr of observation. Moreover, most of the nearby galaxies are likely to be just a factor of a few below the KM3NeT and IceCube-Gen2 pointlike sensitivities. After investigating the prospects for detection of gamma-rays above TeV energies from these sources, we conclude that the joint observations of high-energy neutrinos and gamma-rays with upcoming telescopes will be an objective test for our emission model and may provide compelling evidence of star-forming activity as a tracer of neutrino production.

Observable Signatures of Cosmic Rays Transport in Starburst Galaxies on Gamma-ray and Neutrino Observations

“…We determine the neutrino production rate by assuming that pions always carry 17% of their parent proton energy (Kelner et al 2006). For each galaxy, the density n ISM is obtained by means of the same scaling relation with the SFR reported by Ambrosone et al (2021a), in agreement with the Kennicutt relation (Kennicutt 1998;Kennicutt & Evans 2012;Kennicutt & De Los Reyes 2021). Such a relation connects the surface density of SFR, Σ SFR , and the gas surface density, Σ gas .…”

“…In particular, we consider both an advective model adopted, e.g., by Peretti et al (2020), and the model provided by Krumholz et al (2020), which considers CR transport in the nucleus dominated by diffusion on self-induced turbulence. Following the analysis described in (Ambrosone et al 2021a), we compare the gamma-ray predictions of the two CR transport models with 10-year Fermi-LAT data (Ajello et al 2020) (Ambrosone et al 2021a), for each source we require the star formation rate (SFR) to be in agreement with the one derived from infrared and ultraviolet observations (Kornecki et al 2020). We show that current data are slightly better accommodated by the model developed by Peretti et al (2019), suggesting advection to be important for high-energy CR transport in SBNs.…”

“…Gamma-rays are principally emitted in hadronic processes through neutral pion decays (π 0 → γγ). Nonetheless, we also take into account the gamma-ray emission from bremsstrahlung and Inverse Compton scatterings of primary electrons as well as secondary ones (see Peretti et al (2019) and Ambrosone et al (2021a) for details). The gamma-ray emission from Inverse Compton scatterings depends on the density of background photons acting as a target.…”

“…We study the gamma-ray spectral energy distributions (SEDs) of 13 galaxies observed by Fermi-LAT in 10 years of observations (Ajello et al 2020) (Ambrosone et al 2021a), we determine the most-likely values for the two free parameters of the two models: the star formation rate Ṁ * and the spectral index Γ of injected protons and electrons. We adopt a Bayesian approach, using as a posterior distribution…”

“…A major limitation comes from the fact that only 13 SFGs are observed through gamma rays and just a few of them are observed at energies greater than 10 GeV (Ajello et al 2020;Kornecki et al 2020). This points out the importance for future gamma-ray experiments (Acharya et al 2018;Albert et al 2019;Hinton 2021) as well as possible neutrino point-like observations (Adrian-Martinez et al 2016;Aiello et al 2019;Aartsen et al 2020;Ambrosone et al 2021a) to better constrain the characteristics and the CR transport scenario for the nuclei of SBGs.…”