An ASCA Observation of M51 (NGC 5194): Iron K Emission from an Obscured Active Galactic Nucleus

We present an ASCA observation of the nearby spiral galaxy M51 (NGC 5194). We detected hard X-ray emission with a photon index of ~1.4 and a luminosity of LX ~ 1.1 × 1040 ergs s-1 in the 2-10 keV band (assuming a distance of 9.6 Mpc). A strong fluorescent iron K line (equivalent width ~900 eV) was detected at 6.4 keV in the X-ray spectra. Such an intense iron line is characteristic of Seyfert 2 galaxies and strongly suggests the presence of a heavily obscured active galactic nucleus (AGN). However, the X-ray image is extended even at energies above 2 keV. From the strong iron line and the extended hard X-ray image we speculate that the AGN is obscured by matter with a hydrogen column density more than several times 1023 cm-2 and that the observed 2-10 keV X-ray flux is not dominated by emission from the AGN but rather by other components, such as low-mass X-ray binaries, which typically dominate the X-ray emission of normal spiral galaxies. Emission lines from O K, Ne K, Fe L, Mg K and Si K were detected in the soft energy spectra, which indicate the presence of hot gas. The soft component is well represented by a Raymond-Smith thermal plasma model (with kT ~ 0.4 keV), which suggests a lower iron abundance (<0.1 solar) than other elements (~0.1-0.4 solar), or by a two temperature (kT ~ 0.3 keV and kT ~ 0.8 keV) model with ~0.1 solar abundance, which is reminiscent of the X-ray-emitting gas in starburst galaxies.

[1]  K. Makishima,et al.  Detection of Excess Hard X-Ray Emission from the Optical Jet Galaxy NGC 1097 , 1996 .

[2]  Andrew C. Fabian,et al.  X-ray reflection from cold matter in Active Galactic Nuclei and X-ray binaries , 1991 .

[3]  K. Kohno,et al.  Aperture Synthesis CO and HCN Observations of M51: Dense Molecular Disk around a Low-Luminosity Active Galactic Nucleus , 1996 .

[4]  Carl Heiles,et al.  The bell laboratories H I survey , 1992 .

[5]  M. Matsuoka,et al.  A reflection-dominated X-ray spectrum discovered by ASCA in the Circinus galaxy , 1996 .

[6]  W. Forman,et al.  X-ray properties of bright far-infrared galaxies , 1992 .

[7]  A. Kinney,et al.  RECENT RESULTS FROM THE HUBBLE SPACE TELESCOPE , 1993 .

[8]  K. O. Mason,et al.  The Physics of Accretion onto Compact Objects , 1986 .

[9]  Alexei V. Filippenko,et al.  A Search for “Dwarf'' Seyfert Nuclei. III. Spectroscopic Parameters and Properties of the Host Galaxies , 1997, astro-ph/9704107.

[10]  R. Mushotzky,et al.  The Complex X-Ray Spectra of M82 and NGC 253 , 1997 .

[11]  Yasushi Fukazawa,et al.  The reflection-dominated X-ray spectrum of NGC 6552 , 1994 .

[12]  Barham W. Smith,et al.  Soft X-ray spectrum of a hot plasma. , 1977 .

[13]  Y. Kohmura,et al.  X-Ray Properties of the Nucleus of M81 , 1996 .

[14]  Giorgio G. C. Palumbo,et al.  An X-ray study of M51 (NGC 5194) and its compagnion (NGC 5195). , 1985 .

[15]  A. Sandage,et al.  Steps toward the Hubble constant. V. The Hubble constant from nearby galaxies and the regularity of the local velocity field. , 1975 .

[16]  M. Lehnert,et al.  The Complex Broadband X-Ray Spectrum of the Starburst Galaxy M82 , 1996, astro-ph/9611223.

[17]  Hideyo Kunieda,et al.  X-Ray Evidence for Seyfert Activity Buried in the Infrared Galaxy NGC 4945 , 1993 .

[18]  A. Filippenko,et al.  A search for «dwarf» Seyfert 1 nuclei. I: The initial data and results , 1985 .

[19]  P. Serlemitsos,et al.  The Nature of the Low-Luminosity Active Galactic Nuclei in NGC 3147 , 1996 .

[20]  Giuseppina Fabbiano,et al.  X Rays From Normal Galaxies , 1989 .

[21]  Y. Kohmura,et al.  In-Orbit Performance of the Gas Imaging Spectrometer onboard ASCA , 1996 .

[22]  Barry E. Burke,et al.  An abuttable CCD imager for visible and X-ray focal plane arrays , 1989, IEEE International Solid-State Circuits Conference, 1989 ISSCC. Digest of Technical Papers.

[23]  Andrew M. Read,et al.  ROSAT PSPC observations of nearby spiral galaxies — I. The data , 1997 .

[24]  H. Kaneda,et al.  The Gas Imaging Spectrometer on Board ASCA , 1996 .

[25]  G. Palumbo,et al.  X-Ray Spectrum of the Spiral Galaxy M51 (NGC 5194) , 1990 .

[26]  Yoshitaka Ishisaki,et al.  Detection of strong Fe-K lines from the spiral galaxies NGC 1365 and NGC 1386 , 1997 .

[27]  An Atlas of Hubble Space Telescope Ultraviolet Images of Nearby Galaxies , 1996, astro-ph/9605170.

[28]  Kathryn A. Flanagan,et al.  ROSAT observations of the luminous X-ray sources in M51 , 1995 .

[29]  J. R. Stauffer,et al.  A nuclear spectroscopic survey of disk galaxies. II - Galaxies with emission lines not excited by stellar photoionization , 1982 .

[30]  George K. Miley,et al.  On the nature and implications of starburst-driven galactic superwinds , 1990 .