Luminous infrared galaxies. III. Multiple merger, extended massive star formation, galactic wind, and nuclear inflow in NGC 3256

Abstract

We report detailed evidence for multiple merger, extended massive star formation, galactic wind, and circular/noncircular motions in the luminous infrared galaxy NGC 3256, based on observations of highresolution imaging (Hubble Space Telescope, ESO NTT), and extensive spectroscopic data (more than 1000 spectra, collected at Estación Astrof•ísica de Bosque Alegre, Complejo Astronómico el Leoncito, Cerro Tololo InterAmerican Observatory, and IUE observatories). We find in a detailed morphological study (resolution ~15 pc) that the extended massive star formation process detected previously in NGC 3256 shows extended triple asymmetrical spiral arms (r~5 kpc), emanating from three di†fferent nuclei. The main optical nucleus shows a small spiral disk (r~500 pc), which is a continuation of the external one and reaches the very nucleus. The core shows blue elongated structure (50 pc x 25 pc) and harbors a blue stellar cluster candidate (r~8 pc). We discuss this complex morphology in the framework of an extended massive star formation driven by a multiple merger process (models of Hernquist et al. and Taniguchi et al.). We study the kinematics of this system and present a detailed Hα velocity field for the central region (40" x 40"; rmax~30"~5 kpc), with a spatial resolution of 1" and errors of ±15 km s-ˡ. The color and isovelocity maps show mainly (1) a kinematic center of circular motion with "spider" shape, located between the main optical nucleus and the close (5") mid-IR nucleus and (2) noncircular motions in the external parts. We obtained three "sinusoidal rotation curves" (from the Hα velocity field) around position angle (P.A.) ~55º, ~90º, and ~130º. In the main optical nucleus we found a clear "outflow component" associated with galactic winds plus an "inflow radial motion". The outflow component was also detected in the central and external regions (r≤5−6 kpc). The main axis of the inflow region (P.A.~80º) is practically perpendicular to the ouflow axis (at P.A.~160º). We analyze in detail the physical conditions in the giant H II regions located in the asymmetric spiral arms, the two main optical nuclei, and the outflow component (using long-slit spectroscopy, plus standard models of photoionization, shocks, and starbursts). We present four detailed emission-line ratios (N II/Hα, S II/Hα, S II/S II), and FWHM (Hα) maps for the central region (30" x 30"; rmax~22"~4 kpc), with a spatial resolution of 1". In the central region (r~5−6 kpc) we detected that the nuclear starburst and the extended giant H II regions (in the spiral arms) have very similar properties, i.e., high metallicity and low-ionization spectra, with Teff=35,000 K, solar abundance, a range of Te~6000−7000 K, and Ne~100−1000 cm-³. The nuclear and extended outflow shows properties typical of galactic wind/ shocks, associated with the nuclear starburst. We suggest that the interaction between dynamical e†ects, the galactic wind (outflow), low-energy cosmic rays, and the molecular+ionized gas (probably in the inflow phase) could be the possible mechanism that generate the "similar extended properties in the massive star formation, at a scale of 5−6 kpc!" We have also studied the presence of the close merger/interacting systems NGC 3256C (at~150 kpc, ΔV = − 100 km s-ˡ) and the possible association between the NGC 3256 and 3263 groups of galaxies. In conclusion, these results suggest that NGC 3256 is the product of a multiple merger, which generated an extended massive star formation process with an associated galactic wind plus a nuclear inflow. Therefore, NGC 3256 is another example in which the relation between mergers and extreme starburst (and the powerful galactic wind, "multiple" Type II supernova explosions) play an important role in the evolution of galaxies (the hypothesis of Rieke et al., Joseph et al., Terlevich et al., Heckman et al., and L•ípari et al.).