The electron density structure and kinematics of the supernova remnant N 49

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Context. Supernova remnants commonly display strong internal inhomogeneities in physical properties; however, most of the observational studies found in the literature are limited to integrated data for extragalactic objects or data from very specific parts of the galactic objects. Thus, important information may be lost.Aims. We studied the spatial variation of the electron density and the kinematics of N 49, the brightest supernova remnant in the Large Magellanic Cloud, based on spatial resolve observations sampling the whole object.Methods. We acquired long-slit spectrophotometric data of high signal-to-noise from twelve locations equally spaced in declination. We extracted 1D spectra in the range 5950 to 6750 A from 312 different regions each 2.3″ × 2″ in angular size. The electron density was derived from the line ratio [S ii] λ 6717/λ 6731. The radial velocity and velocity dispersion were obtained from the Hα profile.Results. The map of the electron density presents a strong gradient with the density increasing from west to east. The densest areas, with N e ≥ 2000 cm-3 , are found on the east border of the nebula, near but not exactly coinciding with the brightest areas in the optical. The dense area at the south-east border is on the edge of the bright ridge of filaments associated with the peak of the emission from the nebula at different frequencies. However, the dense zone at the north-east borders is in an area of low brightness. From the Hα total flux, we estimate a mass of 207 ± 66 M ⊙ for N 49. The maps of the radial velocities of the blue and red shifted components of Hα and of the velocity dispersion at 3σ showed a rough radial symmetry that can be interpreted as the projection effect of a expanding spherical shell. However, the kinematic centre of symmetry is far from the centre of the X-ray or radio images, although it is near the centre of the optical image of N 49.Conclusions. The detected gradient in density confirmed a previous inference based on the decrease in the brightness ratio between the X-ray and radio emission from east and west and is consistent with the presence of a molecular cloud on the south-east border. We were able to fit the radial profile of the near side velocity using a self-similar solution of a blast wave travelling through a non-homogeneous medium characterized by a power-law density distribution.