Starburst activity in a ROSAT narrow emission‐line galaxy

We present multiwaveband photometric and optical spectropolarimetric observations of the R = 15.9 narrow emission line galaxy R117 A which lies on the edge of the error circle of the ROSAT X-ray source R117 (from M c Hardy et al. 1998). The overall spectral energy distribution of the galaxy is well modelled by a combination of a normal spiral galaxy and a moderate-strength burst of star formation. The far infrared and radio emission is extended along the major axis of the galaxy, indicating an extended starburst. On positional grounds, the galaxy is a good candidate for the identification of R117 and the observed X-ray flux is very close to what would be expected from a starburst of the observed far infra-red and radio fluxes. Although an obscured high redshift QSO cannot be entirely ruled out as contributing some fraction of the X-ray flux, we find no candidates to K = 20.8 within the X-ray errorbox and so conclude that R117 A is responsible for a large fraction, if not all, of the X-ray emission from R117. Searches for indicators of an obscured AGN in R117 A have so far proven negative; deep spectropolarimetric observations show no signs of broad lines to a limit of one per cent and, for the observed far infra-red and radio emission, we would expect a ten times greater X-ray flux if the overall emission were powered by an AGN. We therefore conclude that the X-ray emission from R117 is dominated by starburst emission from the galaxy R117 A.

[1]  Ralf Siebenmorgen,et al.  Massive star formation in galaxies: radiative transfer models of the UV to millimetre emission of starburst galaxies , 2000 .

[2]  G. C. Perola,et al.  Spectroscopic identification of ten faint hard X-ray sources discovered by Chandra , 2000 .

[3]  D. Elbaz,et al.  The European Large Area ISO Survey — II. Mid-infrared extragalactic source counts , 2000 .

[4]  R. McMahon,et al.  The Cambridge-Cambridge ROSAT Serendipity Survey - I.X-ray-luminous galaxies , 1995 .

[5]  Donald E. Osterbrock,et al.  Spectral Classification of Emission-Line Galaxies , 1987 .

[6]  A. Cimatti,et al.  On the dust extinction in high-z galaxies and the case of extremely red objects , 1997, astro-ph/9708197.

[7]  C. Surace,et al.  The European Large Area ISO Survey – III. 90-μm extragalactic source counts , 2000 .

[8]  J. Hough,et al.  Polarimetry and modelling of narrow-line active galaxies , 1996 .

[9]  N. Roche,et al.  A deep ROSAT survey — XI. Enhanced X-ray emission from faint galaxies , 1996 .

[10]  J. Hough,et al.  An optical and near-infrared polarization survey of nearby starburst and interacting galaxies , 1991 .

[11]  UK,et al.  The X-ray evolution of merging galaxies , 1998, astro-ph/9801317.

[12]  B. Peterson,et al.  Galaxy number counts. - I. Photographic observations to B=23.5 mag. , 1991 .

[13]  D. Osterbrock,et al.  Astrophysics of Gaseous Nebulae and Active Galactic Nuclei , 1989 .

[14]  James J. Condon,et al.  Radio Emission from Normal Galaxies , 1992 .

[15]  P. Padovani,et al.  Star-forming galaxies and the X-ray background , 1990 .

[16]  S. Anderson,et al.  A compilation of active and normal galaxies observed in both infrared and X-rays , 1992 .

[17]  E. al.,et al.  The European Large Area ISO Survey - I. Goals, definition and observations , 2000, astro-ph/0003263.

[18]  Richard E. Griffiths,et al.  A deep ROSAT survey — V. The extragalactic populations at faint fluxes , 1996 .

[19]  N. Roche,et al.  A deep ROSAT survey - VII. A faint galaxy contribution to the X-ray background , 1995 .

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

[21]  A. C. Fabian,et al.  The unified Seyfert scheme and the origin of the cosmic X-ray background , 1994 .

[22]  D. S. Mathewson,et al.  Wavelength dependence of interstellar polarization and ratio of total to selective extinction. , 1975 .

[23]  Isabella M. Gioia,et al.  The Einstein Observatory Extended Medium-Sensitivity Survey. II - The optical identifications , 1991 .

[24]  Hilo,et al.  SCUBA: A Common - user submillimetre camera operating on the James Clerk Maxwell telescope , 1998, astro-ph/9809122.

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

[26]  M. Rowan-Robinson,et al.  The origin of the cosmic soft X-ray background: optical identification of an extremely deep ROSAT survey , 1998 .

[27]  Rob Ivison,et al.  A hyperluminous galaxy at z = 2.8 found in a deep submillimetre survey , 1997, astro-ph/9712161.

[28]  C. L. Carilli,et al.  The Radio-to-Submillimeter Spectral Index as a Redshift Indicator , 1998, astro-ph/9812251.