Starburst activity in a ROSAT narrow emission-line galaxy

Gunn, Katherine F.; McHardy, I. M.; Almaini, O.; Shanks, T.; Sumner, T. J.; Muxlow, T. W. B.; Efstathiou, A.; Jones, L. R.; Croom, S. M.; Manners, J. C.; Newsam, A. M.; Mason, K. O.; Serjeant, S. B. G. and Rowan-Robinson, M. (2001). Starburst activity in a ROSAT narrow emission-line galaxy. Monthly Notices of the Royal Astronomical Society, 324(2) pp. 305–312.



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. 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-infrared 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 error box, 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 1 per cent and, for the observed far-infrared and radio emission, we would expect 10 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.

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