* Astronomy

Title: Fermi Discovery of Gamma-Ray Emission from NGC 1275
Authors: The Fermi/LAT Collaboration: A. A. Abdo, et al, H.D. Aller, M.F. Aller, K.I. Kellermann, Y.Y. Kovalev, Y.A. Kovalev, M. L. Lister, A. B. Pushkarev

We report the discovery of high-energy (E>100 MeV) gamma-ray emission from NGC 1275, a giant elliptical galaxy lying at the center of the Perseus cluster of galaxies, based on observations made with the Large Area Telescope (LAT) of the Fermi Gamma ray Space Telescope. The positional center of the gamma-ray source is only ~3' away from the NGC 1275 nucleus, well within the 95% LAT error circle of ~5'.The spatial distribution of gamma-ray photons is consistent with a point source. The average flux and power-law photon index measured with the LAT from 2008 August 4 to 2008 December 5 are F_gamma = (2.10 ±0.23)x 10^{-7} ph (>100 MeV) cm^{-2} s^{-1} and Gamma = 2.17 ±0.05, respectively. The measurements are statistically consistent with constant flux during the four-month LAT observing period. Previous EGRET observations gave an upper limit of F_gamma < 3.72x 10 ^{-8} ph (>100 MeV) cm^{-2} s^{-1} to the gamma-ray flux from NGC 1275. This indicates that the source is variable on timescales of years to decades, and therefore restricts the fraction of emission that can be produced in extended regions of the galaxy cluster. Contemporaneous and historical radio observations are also reported. The broadband spectrum of NGC 1275 is modelled with a simple one-zone synchrotron/synchrotron self-Compton model and a model with a decelerating jet flow.

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Title: Magnetic support of the optical emission line filaments in NGC 1275
Authors: A.C. Fabian (1), R.M. Johnstone (1), J.S. Sanders (1), C.J. Conselice (2), C.S. Crawford (1), J.S. Gallagher III (3), E. Zweibel (3) ((1) Institute of Astronomy, Cambridge, (2) University of Nottingham, (3) University of Wisconsin)

The giant elliptical galaxy NGC 1275, at the centre of the Perseus cluster, is surrounded by a well-known giant nebulosity of emission-line filaments, which are plausibly about >10^8 yr old. The filaments are dragged out from the centre of the galaxy by the radio bubbles rising buoyantly in the hot intracluster gas before later falling back. They act as dramatic markers of the feedback process by which energy is transferred from the central massive black hole to the surrounding gas. The mechanism by which the filaments are stabilised against tidal shear and dissipation into the surrounding 4x10^7 K gas has been unclear. Here we report new observations that resolve thread-like structures in the filaments. Some threads extend over 6 kpc, yet are only 70 pc wide. We conclude that magnetic fields in the threads, in pressure balance with the surrounding gas, stabilize the filaments, so allowing a large mass of cold gas to accumulate and delay star formation.

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Each of these fuzzy blobs is a galaxy, together making up the Perseus Cluster, one of the closest clusters of galaxies. The cluster is seen through a foreground of faint stars in our own Milky Way Galaxy.


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Credit Ken Crawford

Near the cluster centre, roughly 250 million light-years away, is the cluster's dominant galaxy NGC 1275, seen here just left of picture centre. A prodigious source of x-rays and radio emission, NGC 1275 accretes matter as gas and galaxies fall into it. The Perseus Cluster of Galaxies is part of the Pisces-Perseus supercluster spanning over 15 degrees and containing over 1,000 galaxies. At the distance of NGC 1275, this view covers about 1.5 million light-years.