Low star formation efficiency due to turbulent adiabatic compression in the Taffy bridge

The Taffy system (UGC 12914/15) consists of two massive spiral galaxies that had a head-on collision about 20 Myr ago. It represents an ideal laboratory for studying the reaction of the interstellar medium (ISM) to a high-speed (∼1000 km s−1) gas-gas collision. New sensitive, high-resolution (2.7″ or ∼800 pc) CO(1−0) observations of the Taffy system with the IRAM Plateau de Bure Interferometer (PdBI) are presented. The total CO luminosity of the Taffy system detected with the PdBI is LCO, tot = 4.8 × 109 K km s−1 pc2, 60% of the CO luminosity found with the IRAM 30 m telescope. About 25% of the total interferometric CO luminosity stems from the bridge region. Assuming a Galactic N(H2)/ICO conversion factor for the galactic disks and a third of this value for the bridge gas, about 10% of the molecular gas mass is located in the bridge region. The giant H II region close to UGC 12915 is located at the northern edge of the high-surface-brightness giant molecular cloud association (GMA), which has the highest velocity dispersion among the bridge GMAs. The bridge GMAs are clearly not virialized because of their high velocity dispersion. Three dynamical models are presented and while no single model reproduces all of the observed features, they are all present in at least one of the models. Most of the bridge gas detected in CO does not form stars. We suggest that turbulent adiabatic compression is responsible for the exceptionally high velocity dispersion of the molecular ISM and the suppression of star formation in the Taffy bridge. In this scenario the turbulent velocity dispersion of the largest eddies and turbulent substructures or clouds increase such that giant molecular clouds are no longer in global virial equilibrium. The increase in the virial parameter leads to a decrease in the star formation efficiency. The suppression of star formation caused by turbulent adiabatic compression was implemented in the dynamical simulations and decreased the star formation rate in the bridge region by ∼90%. Most of the low-surface-density, CO-emitting gas will disperse without forming stars but some of the high-density gas will probably collapse and form dense star clusters, such as the luminous H II region close to UGC 12915. We suggest that globular clusters and super star clusters formed and still form through the gravitational collapse of gas previously compressed by turbulent adiabatic compression during galaxy interactions

Interacting galaxies hiding into one, revealed by MaNGA

Interacting galaxies represent a fundamental tool for investigating the underlying mechanisms that drive galaxy evolution. In order to identify merging systems, high-resolution spectroscopic data are required, especially when the morphology does not show clear galaxy pairs. Here, we present a merging galaxy, MaNGA 1-114955, in which we highlighted the superimposition of two distinct rotating discs along the line of sight. We suggest that we are observing a pre-coalescence stage of a merger. Our results demonstrate how a galaxy can hide another one and the relevance of a multi-component approach for studying ambiguous systems

SIGNALS: I. Survey description

SIGNALS, the Star formation, Ionized Gas, and Nebular Abundances Legacy Survey, is a large observing programme designed to investigate massive star formation and H II regions in a sample of local extended galaxies. The programme will use the imaging Fourier transform spectrograph SITELLE at the Canada–France–Hawaii Telescope. Over 355 h (54.7 nights) have been allocated beginning in fall 2018 for eight consecutive semesters. Once completed, SIGNALS will provide a statistically reliable laboratory to investigate massive star formation, including over 50 000 resolved H II regions: the largest, most complete, and homogeneous data base of spectroscopically and spatially resolved extragalactic H II regions ever assembled. For each field observed, three datacubes covering the spectral bands of the filters SN1 (363–386 nm), SN2 (482–513 nm), and SN3 (647–685 nm) are gathered. The spectral resolution selected for each spectral band is 1000, 1000, and 5000, respectively. As defined, the project sample will facilitate the study of small-scale nebular physics and many other phenomena linked to star formation at a mean spatial resolution of ∼20 pc. This survey also has considerable legacy value for additional topics, including planetary nebulae, diffuse ionized gas, and supernova remnants. The purpose of this paper is to present a general outlook of the survey, notably the observing strategy, galaxy sample, and science requirements