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