Star-forming galaxies are not the main source of IceCube neutrinos

Thursday, November 5, 2015 - 11:00am

IceCube data are stubbornly showing us only a glimpse of the extreme universe at a time. Ever since the discovery of a flux of TeV-PeV astrophysical neutrinos, scientists have been studying various potential sources of these neutrinos, and little by little, learning details about the composition of the cosmic neutrino flux. This search has already led to strong constraints on the contribution of gamma-ray bursts (GRBs) and subclasses of active galactic nuclei (AGN) to the IceCube signal.

Star-forming galaxies (SFGs), which include galaxies like the Milky Way and also starburst galaxies that produce more supernovas and cosmic rays, are also expected to contribute to the observed neutrino flux. However, a new study published yesterday on arXiv shows that SFGs alone are not able to explain the high rate of astrophysical neutrinos. This time the exclusion of a source class could be good news for those seeking to find the first individual neutrino source, since the detection of individual SFGs would be a challenge for IceCube and other neutrino telescopes.

galaxy
NGC 253 is one of the nearby starburst galaxies detected in GeV and TeV gamma rays. Image: Hubble Legacy Archive, ESA, NASA; Processing and additional imaging - Robert Gendler

Cosmic rays can interact with gas in the interstellar medium to produce pions, which then decay into both energetic gamma rays and neutrinos. This process is thought to be responsible for most of the high-energy gamma-ray emission observed in SFGs. Although the neutrino flux from individual SFGs is below IceCube’s detection sensitivity, the cumulative emission of all SFGs has been a leading contender to explain the measured flux.

A recent study by the Fermi Large Area Telescope Collaboration has provided new insight on the composition of the extragalactic gamma-ray background. Blazars account for most of the gamma-ray emission, leaving only a small residual for other source classes, including SFGs. Researchers at WIPAC, working in collaboration with scientists at SLAC and Clemson University, have used these results to set an upper limit on the corresponding neutrino emission from SFGs. “Using this multimessenger approach, if SFGs were entirely responsible for the IceCube neutrinos, their associated gamma-ray emission would exceed the non-blazar part of the gamma-ray background,” explains Keith Bechtol, a John Bahcall fellow at WIPAC and one of the authors of this paper.

data graph
The gamma-ray (red lines) and per-flavor neutrino (black lines) contributions of the hadronic emission are displayed in this image. These fluxes are normalized to the best-fit on non-blazar emission, which would be responsible for 14% of gamma-ray emission, including SFGs. The predicted neutrino flux from non-blazar sources is much smaller than the observed IceCube signal.

An ongoing study by the IceCube Collaboration has also set an upper limit on the contribution of neutrino emission from blazars, which would not be larger than about 20%. “We now have constraints on three of the most widely considered neutrino sources: GRBs, blazars, and SFGs. Considering that neutrinos and gamma rays are emitted together, it is interesting that the sources of IceCube neutrinos are not more conspicuous in the gamma-ray sky,” says Markus Ahlers, who is also a John Bahcall fellow at WIPAC and a co-author of this work.

Galactic neutrino emission could bring some understanding to the picture, although so far there is no evidence that the neutrino flux in IceCube has a strong galactic component. Another possibility is that the neutrinos are emitted in environments where gamma rays are absorbed, or that multiple distinct source classes have roughly comparable contributions. We will learn more from ongoing observations with IceCube. According to Justin Vandenbroucke, an assistant professor of physics at UW–Madison and the third WIPAC author of this paper, “If star-forming galaxies had been the main source class, we might never confirm it because the diffuse flux would be produced by a huge number of faint sources. Ruling them out brings us one step closer to identifying individual astrophysical neutrino sources with IceCube or a possible upgraded detector.”

+ info “Evidence against star-forming galaxies as the dominant source of IceCube neutrinos,” K. Bechtol, M. Ahlers, M. Di Mauro, M. Ajello and J.Vandenbroucke. arxiv.org/abs/1511.00688

+ info “Analysis of the cumulative neutrino flux from Fermi-LAT blazar populations using 3 years of IceCube data,” Thorsten Glüsenkamp, for the IceCube Collaboration. Proceedings for the RICAP-14 conference in Noto, Sicily, Sept. 30th - Oct. 3rd 2014, arxiv.org/abs/1502.03104