VHE image of the M82 region.

M82 - TeV gamma-ray emission from a new class of source: starburst galaxies

Reference:  V. A. Acciari et al. (The VERITAS Collaboration), Nature, volume 472, 770-772, 2009.

Full text version, supplemental information

ArXiv version: ArXiV:0911.0873

Contact person: Wystan Benbow

M 82 is the prototype small starburst galaxy and is widely predicted to be the brightest starburst galaxy in gamma rays.  This article reports the discovery of hard spectrum (photon index of 2.5), >700 GeV gamma-ray emission from M 82 during an intense (~140 h) observation campaign carried out over two years. M 82 is among the weakest VHE sources ever detected (~0.9% Crab) and is only one of two starburst galaxies known to emit gamma-rays. It is also one of the few extragalactic gamma-ray sources not associated with an active galactic nucleus.

The active regions of starburst galaxies have exceptionally high rates of star formation. The cosmic rays produced in the formation, life, and death of their massive stars  are expected to eventually produce diffuse gamma-ray emission via their interactions with interstellar gas and radiation. From the VERITAS data we determine a cosmic-ray density of 250 eV cm^-3 in the starburst core of M 82, or about 500 times the average Galactic density. This result strongly supports that cosmic-ray acceleration is tied to star formation activity, and that supernovae and massive-star winds are the dominant accelerators.

Figures from paper (click to get full size image): 

Figure 1 (shown above): VHE image of the M82 region. The sky map shows the measured excess (colour scale) of γ-ray-like events above the estimated background from a region centred on M82. Each pixel contains the excess in a circular region of radius 0.1°. The map is oversampled; neighbouring pixels are thus correlated. The background for each point is estimated using an annulus centred on its position (the ring method28). The spatial distribution of the observed excess is consistent with that expected from a point-like source located near the core of M82. The white circle represents the VERITAS point spread function (68% containment) for individual γ-rays. The uncertainty in the source localization is much smaller. The black star denotes the location of the core of M82. The coordinates are for the J2000 epoch.

Figure 2: Gamma-ray flux compared with a theoretical prediction. The differential energy spectrum (EdN/dE, where E denotes energy and N is the number of photons) of M82 observed using VERITAS between ~0.9 TeV and ~5 TeV. The data are given by open diamonds with 1σ statistical error bars, and can be fitted (χ2 = 0.1 with 1 d.f.) with a power-law function (thick grey line: dN/dE ≈ E-Γ, where E is measured in teraelectronvolts and Γ = 2.5 ± 0.6stat ± 0.2syst). The VERITAS flux upper limit (99% confidence level29) shown at ~6.6 TeV is above the extrapolation of the fitted power-law function at these energies. The thin lines represent a recent model3 for the γ-ray emission from M82. The thin solid line is the total emission predicted and the dashed lines represent components of this emission that result from the interactions of cosmic-ray ions with interstellar matter (decay of neutral pions (π0)), from radiation from cosmic-ray electrons through inverse Compton scattering (IC) and from Bremsstrahlung radiation (Bremss). The IC and π0 decay components are the dominant contributions of cosmic-ray electrons and ions, respectively. Notably, the spectral slopes of these dominant components are markedly different.

Figures from supplemental information (click to get full size image):

Supplementary Figure 1: The distribution of the square of the difference between the reconstructed arrival direction of an event and the direction of M 82. The points represent the measurement and the shaded region represents the estimated background. The vertical line shows the size of the circular integration region used in this analysis. The statistical errors correspond to one standard deviation (σ).

Supplementary Figure 2: The time-averaged flux observed from M 82 during each monthly observation period. The integral flux above 700 GeV is calculated assuming a photon index of Γ = 2.5. The fit of a constant value to the monthly flux is shown as a horizontal dashed line. Only the statistical errors (1σ) are shown. The horizontal bars on each point reflect the actual range of dates for each monthly exposure.