A large amount of diffuse molecular gases in the bar of the strongly barred galaxy NGC 1300: cause of the low star formation efficiency

Abstract

In many barred galaxies, star formation efficiency (SFE) in the bar is lower than those in the arm and bar-end, and its cause has still not been clear. Focusing on the strongly barred galaxy NGC 1300, we investigate the possibility that the presence of a large amount of diffuse molecular gas, which would not contribute to the SF, makes the SFE low in appearance. We examine the relation between the SFE and the diffuse molecular gas fraction (fdif), which is derived using the 12CO(1–0) flux obtained from the interferometer of ALMA 12-m array, which has no sensitivity on diffuse (extended; full width at half-maximum ⪆700 pc) molecular gases due to the lack of ACA, and the total 12CO(1–0) flux obtained from Nobeyama 45-m single-dish telescope. We find that the SFE decreases with increasing fdif. The fdif and SFE are 0.74−0.91 and $0.06\!-\!0.16 ~\rm Gyr^{-1}$ in the bar regions, and 0.28−0.65 and $0.23\!-\!0.96 ~\rm Gyr^{-1}$ in the arm and bar-end regions, respectively. This result supports the idea that the presence of a large amount of diffuse molecular gas makes the SFE low. The suppression of the SFE in the bar has also been seen even when we exclude the diffuse molecular gas components. This suggests that the low SFE appears to be caused not only by a large amount of diffuse molecular gases but also by other mechanisms such as fast cloud–cloud collisions.

Molecular gas and star formation within 12 strong galactic bars observed with IRAM-30 m

“…Here, SFEs in the central regions are not systematically different than those in outer parts of the bar and at the beginning of the arms or rings. Maeda et al (2020) argue that the presence of a large amount of diffuse molecular gas across bars makes the SFE low in appearance. However, they conclude that the bar SFE remains low even when diffuse gas is excluded, and thus other mechanisms might explain this trend.…”

“…Bigiel et al (2008) find average values of Σ mol for disks of local non-barred spiral galaxies of ∼10-15 M pc −2 . At the bar region, Σ mol is typically in the range ≈20-90 M pc −2 (Reynaud & Downes 1998;Regan et al 1999;Kuno et al 2007;Muraoka et al 2016;Maeda et al 2018Maeda et al , 2020Yajima et al 2019), under the assumption of a common α CO = 3.2 M (K km s −1 pc −2 ) −1 , as used here. Our use of a bigger sample reveals a somewhat larger range: Σ mol varies by ≈1.5−2 orders of magnitude between the probed galaxies.…”

“…A lower CO-to-H 2 conversion factor in the bar region than in the rest of the disk has been suggested in previous observational work (e.g., the case of NGC 3627, studied by Morokuma-Matsui et al 2015), but remains a matter of debate (see also Watanabe et al 2011). Sorai et al (2012) argues that SFEs in the bar of Maffei 2 can be underestimated by a factor of 0.5-0.8 by assuming a constant α CO , which is linked to the presence of diffuse, non-bound, and non-optically thick molecular gas (see also the analysis of NGC 1300 by Maeda et al 2020). We cannot entirely exclude this possibility based on our data, and we thus acknowledge that part of the dispersion of SFE along bars might be a consequence of the α CO uncertainty.…”

“…Combining data from the Nobeyama 45m and ALMA 12m (with no sensitivity to diffuse gas), Maeda et al (2020) study the star formation activity in NGC 1300 and report a lower SFE at the bar region than in the bar-end or spiral arms (see their Fig. A.2).…”

Galaxy Zoo: 3D – crowdsourced bar, spiral, and foreground star masks for MaNGA target galaxies

“…Since the strong correlation between optical colour (as an indicator of quenching) and bar fractions was first noted (Masters et al 2011) there has been significant interest in the role bars may play in the quenching of star formation in disc galaxies (e.g. Masters et al 2012;Gavazzi et al 2015;Khoperskov et al 2018;Kruk et al 2018;George et al 2019;Maeda et al 2020). Even before this, the role of bars in radial mixing (and corresponding impact on radial gradients) had been much debated (see Zurita et al 2021a, b for a discussion of the history of this topic).…”