A new sky subtraction technique for low surface brightness data
Low-surface brightness (mu_B > 23 mag/arcsec^2) outer regions of galaxies contain crucially important information for understanding the properties of their extended discs and dark matter haloes. Analysis of absorption line spectra at such surface brightness levels is often hampered by systematic errors of the sky subtraction. Here we present a new approach to the sky subtraction for long-slit spectra based on the controlled reconstruction of the night sky spectrum in the Fourier space using twilight or arc-line frames as references. It can be easily adopted for FLAMINGOS-type multi-slit data. Compared to existing sky subtraction algorithms, our technique is taking into account variations of the spectral line spread along the slit thus qualitatively improving the sky subtraction quality for extended targets. We performed a number of tests at various spectral resolutions and wavelength regions and demonstrated the stability of our technique. As an example, we show how the stellar metallicity and stellar velocity dispersion profiles in the outer disc of the spiral galaxy NGC5440 are affected by the sky subtraction quality. Our technique is used in the survey of early-type galaxies carried out at the Russian 6-m telescope, and it strongly increases the scientific potential of large amounts of long-slit data for nearby galaxies available in major data archives.
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