Interactions of the Galactic bar and spiral arm in NGC 3627

Abstract

Aims. To gain insight into the expected gas dynamics at the interface of the Galactic bar and spiral arms in our own Milky Way galaxy, we examine as an extragalactic counterpart the evidence for multiple distinct velocity components in the cold, dense molecular gas populating a comparable region at the end of the bar in the nearby galaxy NGC 3627. Methods. We assemble a high resolution view of molecular gas kinematics traced by CO(2-1) emission and extract line-of-sight velocity profiles from regions of high and low gas velocity dispersion. Results. The high velocity dispersions arise with often double-peaked or multiple line-profiles. We compare the centroids of the different velocity components to expectations based on orbital dynamics in the presence of bar and spiral potential perturbations. A model of the region as the interface of two gas-populated orbits families supporting the bar and the independently rotating spiral arms provides an overall good match to the data. An extent of the bar to the corotation radius of the galaxy is favored. Conclusions. Using NGC3627 as an extragalactic example, we expect situations like this to favor strong star formation events such as observed in our own Milky Way since gas can pile up at the crossings between the orbit families. The relative motions of the material following these orbits is likely even more important for the build up of high density in the region. The surface densities in NGC3627 are also so high that shear at the bar end is unlikely to significantly weaken the star formation activity. We speculate that scenarios in which the bar and spiral rotate at two different pattern speeds may be the most favorable for intense star formation at such interfaces.

The Gas–Star Formation Cycle in Nearby Star-forming Galaxies. I. Assessment of Multi-scale Variations

“…CO emission remains visible in this gap, while Hα emission appears to be largely absent. In NGC 3627 particular, the phenomenon is harder to identify because of the brightness of star-forming complexes at the bar ends (e.g., see Beuther et al 2017). A similar depression in star formation has been identified along the spiral arms in NGC 5194 and attributed to streaming motions along the spiral arms (not a stellar bar) by Meidt et al (2013).…”

Star Formation Efficiency per Free-fall Time in nearby Galaxies

“…Following Meidt et al (2013), this may reflect strong gas flows in M51 that act to stabilize the gas and suppress star formation. Strong gas flows were also observed in NGC 3627 (Beuther et al 2017), where ff is low (≈ 0.6%). Figure 2 shows overall results for the whole sample, but we also observe strong galaxy-to-galaxy variations in ff .…”

The lifecycle of molecular clouds in nearby star-forming disc galaxies

“…As a consequence, it is not surprising that the data point at the tip of the bar in NGC3627 seems to be an outlier in Figure 5. In NGC3627, Beuther et al (2017) suggest that the interaction between the end of the bar and the spiral arms might induce strong star formation events. The particularly short cloud lifetime (t CO = 3.8 +2.6 −0.8 Myr) measured in the second bin of NGC3627 (as well as a short t fb = 0.8 +0.7 −0.4 Myr) is due to the fact that two very bright regions both in Hα and in CO dominate this bin.…”