Properties of giant molecular clouds in a strongly barred galaxy NGC 1300

Recent studies show that star formation efficiency (SFE) depends on galactic-scale environments. However, what controls the SFE within galaxies is still unclear. To clear the SF mechanism, it is important to study on extreme environments as a pilot study. Thus, we focus on a prototype strongly barred galaxy NGC 1300 which hosts extreme environments; the SFE of total molecular gas in the bar is 5 times smaller than that in the arm. In ALMA Cycle 5, we carried out 12CO(1−0) observations toward the western bar and arm regions of NGC 1300 with the 12-m array at a high angular resolution of ~40 pc to investigate the impact of the galactic environment on the properties of GMCs. We detected the GMC-like gas clouds associated with the dark lanes. We used the CPROPS package to identify GMCs and measured their physical properties. The parameters of CPROPS are the same as those used in the previous GMC study on a typical spiral galaxy M51 (Colombo et al. 2014). Typical values of basic properties of the GMCs are radius of 45 pc, velocity dispersion of 5 km/s, molecular gas mass of 6 × 10^5 Msun and virial parameter of 1.5, and these values are not significantly different in the bar, arm and bar end regions based on two-sided K-S test. This result suggests GMC properties themselves are unlikely to be the cause for the low SF efficiency in the bar. To further study on the cause of low SFE in bar regions, we compared the CO(1-0) data with Nobeyama 45-m single dish telescope and the12-m array data. We find that the recovery fraction of the molecular gases, or flux derived from the 12-m array/ flux derived from Nobeyama 45-m, tends to be smaller in the bar region (~0.4) than arm region (~0.9). Due to the lack of ACA (7m array + Total Power) data, the 12-m array data has no sensitivity on a large- scale structure; 50 % (10 %) flux for a Gaussian component with a FWHM of 500 pc (1 kpc) cannot be recovered based on the CASA simulation. Therefore, the tendency of the recovery fraction suggests that a large amount of extended molecular gas exists in the bar which would make the SFE low.