Abstract
Abstract We present results of 12CO(J = 1–0) imaging observations of NGC 4567/4568, a galaxy pair in a close encounter, with the Atacama Large Millimeter/Submillimeter Array (ALMA). For the first time, we find clear evidence of a molecular collision front with a velocity dispersion that is 16.8 ± 1.4 km s−1 at the overlapping region, owing to high spatial and velocity resolution. By integrating over the velocity width that corresponds to the molecular collision front, we find a long filamentary structure with a size of 1800 pc × 350 pc at the collision front. This filamentary molecular structure spatially coincides with a dark lane seen in the R-band image. We find four molecular clouds in the filament, each with a radius of 30 pc and mass of 106 M ⊙; the radii matching a typical value for giant molecular clouds (GMCs) and the masses corresponding to those between GMCs and giant molecular associations (GMAs). All four clouds are gravitationally bound. The molecular filamentary structure and its physical conditions are similar to the structure expected via numerical simulation. The filament could be a progenitor of super star clusters.
Highlights
Gravitational interactions of galaxies play an important role in the evolution of galaxies
It is known that the activation of star formation follows the progression of the interaction: while the star formation rate (SFR) in interacting galaxies during the early stage is only a few times higher than in field galaxies, the SFR becomes 10–100 times higher than field galaxies during the late stage of the interaction (Kennicutt et al 1987; Teyssier et al 2010)
Ultra-luminous infrared galaxies (ULIRGs: LIR > 1012 L ), most of which are thought to be in the late stage of the interaction (Clements et al 1996), show bursts of star formation
Summary
Gravitational interactions of galaxies (collisions and mergers) play an important role in the evolution of galaxies. A close galaxy-galaxy interaction event greatly alters the distribution and kinematics of stars and gas, resulting in the formation of elliptical galaxies (Toomre et al 1977). During this phenomenon, significant enhancement of star formation activity is observed (Bushouse 1986). High-resolution numerical simulations revealed that multiple nuclei in ULIRGs can be made by multiple major mergers, and a single merger, and produce massive and compact star clusters (Matsui et al 2012) In spite of these findings from observations and simulations, the detailed mechanisms of active star formation in interacting galaxies are still unclear, for example, how off-centre starbursts occur.
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