![]() Therefore a remarkable abundance uniformity is found despite the different excitations found throughout the nebula. The elemental abundances computed from the ‘direct method’ are typical of high-metallicity disc H ii regions, with a mean value of 12+log(O/H) = 8.65, comparable to what has been found in this galaxy by other authors for regions at similar galactocentric distance. The temperatures of and were estimated in the four cases. The electron temperature was measured in knots A, B, C and in the integrated PPak field, and was found to be around 8000 K. For all the knots the electron density has been found to be very similar and below 100 cm−3. We selected four luminous knots to perform a detailed integrated spectroscopic analysis of these structures and of the whole PPak field of view (FOV). The configuration was chosen to cover the whole spectrum from 3600 up to 10 000 Å, allowing the measurement of the near-infrared lines. This technique provides detailed maps of the region in different emission lines, yielding spatially resolved information about the physical properties of the gas. Integral field spectroscopy obtained with the Potsdam Multi-Aperture Spectrophotometer Fiber Package (PPak) and the 3.5-m telescope at the Calar Alto Observatory has been used to study an outer H ii region complex in the well-studied galaxy NGC 6946. Optical spectra of HCG 54c and d show HII region features and are interpreted as tidal debris of the collision undergoing active star formation. Based on the analysis of our new data we propose that HCG 54 is the remnant of a merger in an advanced stage, whose main body is what has been called HCG 54a, while HCG 54b marks the location of a strong starburst induced by the interaction, as evidenced by the Wolf Rayet stellar population that we detect. Both the HI distribution and kinematics are consistent with a recent history of a deeply penetrating interaction or a merger. This system is embedded in an HI cloud 12 kpc in diameter, and a 20 kpc HI tidal tail emerges from its western edge. These observed features are similar to those found in the numerical simulations of tidal interactions involving two equal mass disk galaxies. Several ripples or shells whose center is located near HCG 54a and b are also detected. Our optical images have revealed a blue elongated structure associated with HCG 54a embedded in a rounder and redder stellar envelope. Both optical and radio data indicate clearly that HCG~54 is a product of a recent merger involving at least two galaxies. We present here a study of HCG~54 based on deep optical images and spectroscopy as well as high spatial and spectral resolution HI observations obtained at the VLA. ![]() HCG 54 has been classified as a compact galaxy group by Hickson, but its nature is uncertain because of its unusual properties.
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