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Centro de Astrofísica da Universidade do Porto

On the estimation of intensity for low S/N ratio narrow emission lines

C. Rola, D. Pelat

Abstract
The aim of this paper is to investigate the errors involved by the intensity measure of low signal-to-noise ratio narrow emission lines. We also studied the errors associated with line intensity ratios. The spectral data acquisition was simulated by a Monte-Carlo program. These data were tested for the presence of a line and the lines detected were measured by a chi2 fitting program. This procedure generated a sample of intensities for a given signal-to-noise ratio. From these samples, we have established that the 'measured' intensities are strongly biased towards overestimated values and their distribution is well represented by a log-normal distribution. If this phenomenon is not taken into account, erroneous astrophysical conclusions may be derived. In addition, we determined that a line ratio follows also a log-normal distribution. Finally we provide some charts able to give the error bars attached to a line intensity measurement and also to a line ratio. These charts are given in terms of signal-to-noise ratio, and can be easily converted into line intensities if the energy of the noise is known.

Astronomy and Astrophysics
Volume 287, Page 676
July 1994

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