Spectrophotometry of planetary nebulae in the bulge of M31

We introduce crowded-field integral field (3D) spectrophotometry as a useful technique for the study of resolved stellar populations in nearby galaxies. The spectroscopy of individual extragalactic stars, which is now feasible with efficient instruments and large telescopes, is confronted with the observational challenge of accurately subtracting the bright, spatially and wavelength-dependent nonuniform background of the underlying galaxy. As a methodological test, we present a pilot study with selected extragalactic planetary nebulae (XPNe) in the bulge of M31, demonstrating how 3D spectroscopy is able to improve the limited accuracy of background subtraction that one would normally obtain with classical slit spectroscopy. It is shown that because of the absence of slit effects, 3D spectroscopy is a most suitable technique for spectrophometry. We present spectra and line intensities for five XPNe in M31, obtained with the MPFS instrument at the Russian 6 m Bolshoi Teleskop Azimutal'nij, INTEGRAL at the William Herschel Telescope, and PMAS at the Calar Alto 3.5 m telescope. The results for two of our targets, for which data are available in the literature, are compared with previously published emission-line intensities. The three remaining PNe have been observed spectroscopically for the first time. One object is shown to be a previously misidentified supernova remnant. Our monochromatic Halpha maps are compared with direct Fabry-Perot and narrowband filter images of the bulge of M31, verifying the presence of filamentary emission of the interstellar medium in the vicinity of our objects. We present an example of a flux-calibrated and continuum-subtracted filament spectrum and demonstrate how the interstellar medium component introduces systematic errors in the measurement of faint diagnostic PN emission lines when conventional observing techniques are employed. It is shown how these errors can be eliminated with 3D spectroscopy, using the full two-dimensional spatial information and point-spread function (PSF) fitting techniques. Using 3D spectra of bright standard stars, we demonstrate that the PSF is sampled with high accuracy, providing a centroiding precision at the milliarcsecond level. Crowded-field 3D spectrophotometry and the use of PSF fitting techniques is suggested as the method of choice for a number of similar observational problems, including luminous stars in nearby galaxies, supernovae, QSO host galaxies, gravitationally lensed QSOs, and others.