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Title: PROPERTIES OF NEARBY STARBURST GALAXIES BASED ON THEIR DIFFUSE GAMMA-RAY EMISSION

Abstract

The physical relationship between the far-infrared and radio fluxes of star-forming galaxies has yet to be definitively determined. The favored interpretation, the 'calorimeter model', requires that supernova generated cosmic-ray (CR) electrons cool rapidly via synchrotron radiation. However, this cooling should steepen their radio spectra beyond what is observed, and so enhanced ionization losses at low energies from high gas densities are also required. Further, evaluating the minimum energy magnetic field strength with the traditional scaling of the synchrotron flux may underestimate the true value in massive starbursts if their magnetic energy density is comparable to the hydrostatic pressure of their disks. Gamma-ray spectra of starburst galaxies, combined with radio data, provide a less ambiguous estimate of these physical properties in starburst nuclei. While the radio flux is most sensitive to the magnetic field, the GeV gamma-ray spectrum normalization depends primarily on gas density. To this end, spectra above 100 MeV were constructed for two nearby starburst galaxies, NGC 253 and M82, using Fermi data. Their nuclear radio and far-infrared spectra from the literature are compared to new models of the steady-state CR distributions expected from starburst galaxies. Models with high magnetic fields, favoring galaxy calorimetry, are overall better fits tomore » the observations. These solutions also imply relatively high densities and CR ionization rates, consistent with molecular cloud studies.« less

Authors:
;  [1]
  1. Department of Earth and Physical Sciences, York College, City University of New York, 94-20 Guy R. Brewer Blvd., Jamaica, NY 11451 (United States)
Publication Date:
OSTI Identifier:
22039086
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 755; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASTRONOMY; ASTROPHYSICS; CALORIMETRY; COSMIC ELECTRONS; COSMIC PHOTONS; DENSITY; ENERGY DENSITY; GALAXIES; GAMMA RADIATION; GAMMA SPECTRA; GEV RANGE; INFRARED SPECTRA; IONIZATION; MAGNETIC FIELDS; MATHEMATICAL SOLUTIONS; MEV RANGE; PHOTON EMISSION; STARS; STEADY-STATE CONDITIONS; SYNCHROTRON RADIATION

Citation Formats

Paglione, Timothy A. D., and Abrahams, Ryan D., E-mail: paglione@york.cuny.edu. PROPERTIES OF NEARBY STARBURST GALAXIES BASED ON THEIR DIFFUSE GAMMA-RAY EMISSION. United States: N. p., 2012. Web. doi:10.1088/0004-637X/755/2/106.
Paglione, Timothy A. D., & Abrahams, Ryan D., E-mail: paglione@york.cuny.edu. PROPERTIES OF NEARBY STARBURST GALAXIES BASED ON THEIR DIFFUSE GAMMA-RAY EMISSION. United States. doi:10.1088/0004-637X/755/2/106.
Paglione, Timothy A. D., and Abrahams, Ryan D., E-mail: paglione@york.cuny.edu. 2012. "PROPERTIES OF NEARBY STARBURST GALAXIES BASED ON THEIR DIFFUSE GAMMA-RAY EMISSION". United States. doi:10.1088/0004-637X/755/2/106.
@article{osti_22039086,
title = {PROPERTIES OF NEARBY STARBURST GALAXIES BASED ON THEIR DIFFUSE GAMMA-RAY EMISSION},
author = {Paglione, Timothy A. D. and Abrahams, Ryan D., E-mail: paglione@york.cuny.edu},
abstractNote = {The physical relationship between the far-infrared and radio fluxes of star-forming galaxies has yet to be definitively determined. The favored interpretation, the 'calorimeter model', requires that supernova generated cosmic-ray (CR) electrons cool rapidly via synchrotron radiation. However, this cooling should steepen their radio spectra beyond what is observed, and so enhanced ionization losses at low energies from high gas densities are also required. Further, evaluating the minimum energy magnetic field strength with the traditional scaling of the synchrotron flux may underestimate the true value in massive starbursts if their magnetic energy density is comparable to the hydrostatic pressure of their disks. Gamma-ray spectra of starburst galaxies, combined with radio data, provide a less ambiguous estimate of these physical properties in starburst nuclei. While the radio flux is most sensitive to the magnetic field, the GeV gamma-ray spectrum normalization depends primarily on gas density. To this end, spectra above 100 MeV were constructed for two nearby starburst galaxies, NGC 253 and M82, using Fermi data. Their nuclear radio and far-infrared spectra from the literature are compared to new models of the steady-state CR distributions expected from starburst galaxies. Models with high magnetic fields, favoring galaxy calorimetry, are overall better fits to the observations. These solutions also imply relatively high densities and CR ionization rates, consistent with molecular cloud studies.},
doi = {10.1088/0004-637X/755/2/106},
journal = {Astrophysical Journal},
number = 2,
volume = 755,
place = {United States},
year = 2012,
month = 8
}
  • In a previous work, we showed that the observed far-infrared/submillimeter/millimeter line spectra of a starburst galaxy (M82) can be successfully modeled in terms of the evolutionary scheme of an ensemble of giant molecular clouds (GMCs) and shells, and such studies can usefully constrain the age(s) or star formation history of a starburst galaxy. In this paper, we present a preliminary study of using the template of an ensemble of evolving GMCs/shells we developed for M82. We apply the model to represent various stages of starburst evolution in a well-known sample of nearby luminous infrared galaxies. In this way, we attemptmore » to interpret the relationship between the degree of molecular excitation and ratio of far-infrared (FIR) to {sup 12}CO (or simply CO) luminosity to possibly reflect different stages of the evolution of star-forming activity within their nuclear regions.« less
  • The origin of the diffuse hard X-ray (2-10 keV) emission from starburst galaxies is a long-standing problem. We suggest that synchrotron emission of 10-100 TeV electrons and positrons (e {sup {+-}}) can contribute to this emission, because starbursts have strong magnetic fields. We consider three sources of e {sup {+-}} at these energies: (1) primary electrons directly accelerated by supernova remnants, (2) pionic secondary e {sup {+-}} created by inelastic collisions between cosmic ray (CR) protons and gas nuclei in the dense interstellar medium of starbursts, and (3) pair e {sup {+-}} produced between the interactions between 10 and 100more » TeV {gamma}-rays and the intense far-infrared (FIR) radiation fields of starbursts. We create one-zone steady-state models of the CR population in the Galactic center (R {<=} 112 pc), NGC 253, M82, and Arp 220's nuclei, assuming a power-law injection spectrum for electrons and protons. We consider different injection spectral slopes, magnetic field strengths, CR acceleration efficiencies, and diffusive escape times, and include advective escape, radiative cooling processes, and secondary and pair e {sup {+-}}. We compare these models to extant radio and GeV and TeV {gamma}-ray data for these starbursts, and calculate the diffuse synchrotron X-ray and inverse Compton (IC) luminosities of these starbursts in the models which satisfy multiwavelength constraints. If the primary electron spectrum extends to {approx}PeV energies and has a proton/electron injection ratio similar to the Galactic value, we find that synchrotron emission contributes 2%-20% of their unresolved, diffuse hard X-ray emission. However, there is great uncertainty in this conclusion because of the limited information on the CR electron spectrum at these high energies. IC emission is likewise a minority of the unresolved X-ray emission in these starbursts, from 0.1% in the Galactic center to 10% in Arp 220's nuclei, with the main uncertainty being the starbursts' magnetic field. We also model generic starbursts, including submillimeter galaxies, in the context of the FIR-X-ray relation, finding that anywhere between 0% and 16% of the total hard X-ray emission is synchrotron for different parameters, and up to 2% in the densest starbursts assuming an E {sup -2.2} injection spectrum and a diffusive escape time of 10 Myr (E/3 GeV){sup -1/2} (h/100 pc). Neutrino observations by IceCube and TeV {gamma}-ray data from HESS, VERITAS, and CTA can further constrain the synchrotron X-ray emission of starbursts. Our models do not constrain the possibility of hard, second components of primary e {sup {+-}} from sources like pulsars in starbursts, which could enhance the synchrotron X-ray emission further.« less
  • One attractive scenario for the excess of sub-PeV/PeV neutrinos recently reported by IceCube is that they are produced by cosmic rays in starburst galaxies colliding with the dense interstellar medium. These proton-proton (pp) collisions also produce high-energy gamma rays, which finally contribute to the diffuse high-energy gamma-ray background. We calculate the diffuse gamma-ray flux with a semi-analytic approach and consider that the very high energy gamma rays will be absorbed in the galaxies and converted into electron-positron pairs, which then lose almost all of their energy through synchrotron radiation in the strong magnetic fields in the starburst region. Since themore » synchrotron emission goes into energies below GeV, this synchrotron loss reduces the diffuse high-energy gamma-ray flux by a factor of about two, thus leaving more room for other sources to contribute to the gamma-ray background. For an E{sub ν}{sup −2} neutrino spectrum, we find that the diffuse gamma-ray flux contributes about 20% of the observed diffuse gamma-ray background in the 100 GeV range. However, for a steeper neutrino spectrum, this synchrotron loss effect is less important, since the energy fraction in absorbed gamma rays becomes lower.« less
  • Star-forming galaxies have been predicted to contribute considerably to the diffuse gamma-ray background as they are guaranteed reservoirs of cosmic rays. Assuming that the hadronic interactions responsible for high-energy gamma rays also produce high-energy neutrinos and that O(100) PeV cosmic rays can be produced and confined in starburst galaxies, we here discuss the possibility that star-forming galaxies are also the main sources of the high-energy neutrinos observed by the IceCube experiment. First, we compute the diffuse gamma-ray background from star-forming galaxies, adopting the latest Herschel PEP/HerMES luminosity function and relying on the correlation between the gamma-ray and infrared luminosities reportedmore » by Fermi observations. Then we derive the expected intensity of the diffuse high-energy neutrinos from star-forming galaxies including normal and starburst galaxies. Our results indicate that starbursts, including those with active galactic nuclei and galaxy mergers, could be the main sources of the high-energy neutrinos observed by the IceCube experiment. We find that assuming a cosmic-ray spectral index of 2.1–2.2 for all starburst-like galaxies, our predictions can be consistent with both the Fermi and IceCube data, but larger indices readily fail to explain the observed diffuse neutrino flux. Taking the starburst high-energy spectral index as free parameter, and extrapolating from GeV to PeV energies, we find that the spectra harder than E{sup -2.15} are likely to be excluded by the IceCube data, which can be more constraining than the Fermi data for this population.« less
  • Here, we report the detection of high-energy γ-ray emission from two starburst galaxies using data obtained with the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. Furthermore, we detected a steady point-like emission above 200 MeV at significance levels of 6.8σ and 4.8σ, respectively, from sources positionally coincident with locations of the starburst galaxies M82 and NGC 253. The total fluxes of the sources are consistent with γ-ray emission originating from the interaction of cosmic rays with local interstellar gas and radiation fields and constitute evidence for a link between massive star formation and γ-ray emission in star-formingmore » galaxies.« less