At the occasion of the 5th anniversary of the H.E.S.S. Source of the Month, H.E.S.S. presents

A Novel Type of Source: the Starburst Galaxy NGC 253

October 2009

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The starburst galaxy NGC 253 viewed with the 2.2 m telescope at La Silla Observatory (source: ESO). Close-ups of the core region of NGC 253 are provided by Hubble, see e.g. here and here.

Starburst galaxies are characterized by a high rate of formation of massive stars in localized regions, which explode - at the end of their life cycle - as supernovae and hence give rise to a high flux of cosmic rays, accelerated in the supernova shocks. Relatively nearby starburst galaxies include NGC 253 at a distance of about 10 million light years, in the southern hemisphere (see top image), and M82 in the north. The high amount of energy deposited in the starburst regions at the cores of NGC 253 and M82 creates a "wind" of hot gas emerging from the centers of these galaxies, very nicely visible in  M82 but also seen in NGC 253 through the X-ray emission of the hot gas (Fig. 1). The starburst region features high gas density, feeding star formation. The high intensity of cosmic rays, coupled with the large amount of target gas, has resulted in predictions that these objects should be detectable in very high energy gamma rays created in collisions of cosmic rays with gas particles, despite the Megaparsec distances of these objects (e.g. Paglione et al. (1996), Domingo-Santamaria & Torres (2005), Rephaeli et al. (2009); also Thompson et al. (2006)). In fact, the estimates predict that a significant fraction of the cosmic-ray energy is being converted into gamma rays.

In deep observations of NGC 253, accumulating 119 h of data and using advanced image analysis techniques, H.E.S.S. has now succeeded to establish gamma-ray emission from the core of NGC 253 (Fig. 2); comparison with the optical image of NGC 253 shows clearly that the gamma rays come from the location of the central starburst region. This is the first non-AGN extragalactic object detected in very high energy gamma rays, i.e. the first extragalactic object where gamma ray emission is powered by supernova explosions, rather than by a accretion of mass onto a supermassive black hole. The observed flux level is well in the range of the theoretical estimates based on the rate of supernovae in NGC 253 (Fig. 3).

With a flux corresponding to 0.3% of the flux from the Crab nebula (Fig. 3), NGC 253 represents the faintest very high energy gamma ray source detected thus far. The dynamic range of almost four orders of magnitude between the faintest (NGC 253) and the strongest sources observed so far (AGN outbursts exceeding 10 times the Crab flux, see e.g. SOM 2007/09), illustrates nicely how rapidly instrumental capabilities are advancing.

Finally, we note that in parallel with the report of the discovery of NGC 253 by H.E.S.S., the VERITAS team announced the detection of the northern starbust galaxy M82 at the International Cosmic Ray Conference in Lodz, 2009.

Reference: F. Acero et al., "Detection of Gamma Rays from a Starburst Galaxy ", Science Express, Sept. 24, 2009 and arXiv:0909.4651

Fig. 1: Rosat X-ray image of NGC 253 (red) superimposed onto the optical image (blue). The X-ray emission traces the "wind" of hot gas driven out of the region of star formation at the center of NGC 253. Clicking on the image shows a version where in addition the very high energy gamma ray emission is superimposed in green.
Fig. 2: Very high energy gamma-ray image of NGC 253. The black star shows the optical center of NGC 253, and the white contours illustrate the shape of the galaxy, demonstrating that the gamma rays originate in the nucleus and not in the disk. The inlay shows the point spread function of the H.E.S.S. instrument; on the scale of the H.E.S.S. resolution (about 5', corresponding to 4 kpc at the distance of NGC 253), the source appears pointlike.
Fig. 3: The observed flux of gamma rays above 220 GeV from NGC 253 (red point) is compared to theoretical estimates. The solid line corresponds to the prediction by Paglione et al. (1996). The dashed line shows the model of Domingo-Santamaria & Torres (2005). The gray-shaded band denotes the estimate by Rephaeli et al. (2009).