The Schmidt-Kennicutt Law of Matched-Age Star Forming Regions; Pa-alpha Observations of the Early-Phase Interacting Galaxy Taffy I

TL;DR

This study investigates the Schmidt-Kennicutt law in the early-phase merger galaxy Taffy I, revealing a tight correlation between molecular gas and star formation rates in regions of similar age, supporting the idea that evolutionary stage affects star formation dispersion.

Contribution

It provides the first detailed Pa-alpha observations of Taffy I, demonstrating a tight star formation law among regions of similar age and offering evidence that evolutionary stage influences star formation variability.

Findings

Most star forming regions are ~7 Myr old.

Star formation efficiencies are comparable to starburst galaxies.

A tight, linear Schmidt-Kennicutt law is observed among regions of similar age.

Abstract

In order to test a recent hypothesis that the dispersion in the Schmidt-Kennicutt law arises from variations in the evolutionary stage of star forming molecular clouds, we compared molecular gas and recent star formation in an early-phase merger galaxy pair, Taffy I (UGC\ 12915/UGC\ 12914, VV\ 254) which went through a direct collision 20 Myr ago and whose star forming regions are expected to have similar ages. Narrow-band Pa-alpha image is obtained using the ANIR near-infrared camera on the mini-TAO 1m telescope. The image enables us to derive accurate star formation rates within the galaxy directly. The total star formation rate, 22.2 M_sun/yr, was found to be much higher than previous estimates. Ages of individual star forming blobs estimated from equivalent widths indicate that most star forming regions are ~7 Myr old, except for a giant HII region at the bridge which is much younger. Comparison between star formation rates and molecular gas masses for the regions with the same age exhibits a surprisingly tight correlation, a slope of unity, and star formation efficiencies comparable to those of starburst galaxies. These results suggest that Taffy I has just evolved into a starburst system after the collision, and the star forming sites are at a similar stage in their evolution from natal molecular clouds except for the bridge region. The tight Schmidt-Kennicutt law supports the scenario that dispersion in the star formation law is in large part due to differences in evolutionary stage of star forming regions.