The CO emission in the Taffy Galaxies (UGC 12914/5) at 60pc resolution-I: The battle for star formation in the turbulent Taffy Bridge

TL;DR

This study uses high-resolution ALMA observations to analyze molecular gas in the turbulent Taffy Galaxies' bridge, revealing suppressed star formation due to turbulence and transient cloud structures, with star formation occurring only in localized hotspots.

Contribution

First detailed high-resolution ALMA study of CO emission in the Taffy Galaxies' turbulent bridge, linking turbulence to star formation suppression and transient molecular structures.

Findings

Most molecular filaments lack star formation and fall below the Kennicutt-Schmidt law.

High velocity dispersion regions are gravitationally unbound and short-lived.

Star formation occurs only in localized hotspots with lower turbulence.

Abstract

We present ALMA observations at a spatial resolution of 0.2 arcsec (60 pc) of CO emission from the Taffy galaxies (UGC 12914/5). The observations are compared with narrow-band Pa$\alpha$, mid-IR, radio continuum and X-ray imaging, plus optical spectroscopy. The galaxies have undergone a recent head-on collision, creating a massive gaseous bridge which is known to be highly turbulent. The bridge contains a complex web of narrow molecular filaments and clumps. The majority of the filaments are devoid of star formation, and fall significantly below the Kennicutt-Schmidt relationship for normal galaxies, especially for the numerous regions undetected in Pa$\alpha$ emission. Within the loosely connected filaments and clumps of gas we find regions of high velocity dispersion which appear gravitationally unbound for a wide range of likely values of $X_{\rm CO}$. Like the "Firecracker" region in the Antennae system, they would require extremely high external dynamical or thermal pressure to stop them dissipating rapidly on short crossing timescales of 2-5~Myrs. We suggest that the clouds may be transient structures within a highly turbulent multi-phase medium which is strongly suppressing star formation. Despite the overall turbulence in the system, stars seem to have formed in compact hotspots within a kpc-sized extragalactic HII region, where the molecular gas has a lower velocity dispersion than elsewhere, and shows evidence for a collision with an ionized gas cloud. Like the shocked gas in the Stephan's Quintet group, the conditions in the Taffy bridge shows how difficult it is to form stars within a turbulent, multi-phase, gas.