What Boosts Star Formation In Disk Galaxies? (Cosmology)

Jing Wang and colleagues investigated the impact of spiral structure on star formation using a sample of 2226 nearby bright disk galaxies. They found that, spiral arms help boost the star formation efficiency in disk galaxies and stronger arms are associated with higher specific star formation rate (SFR). Their study recently appeared in Arxiv.

Galaxy main sequence (or galactic main sequence or star formation main sequence or SF main sequence) is a term for the relationship between a galaxy’s star formation (SFR) and its stellar mass or you can say the relationship between SFR and stellar mass in star forming disk galaxies, displays a degree of regularity, which is often expressed as ‘Galaxy main sequence’.

Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies. They may play an intertwined role with gas content and gas compression, and hence with the overall star formation process. Studying the interdependence of spiral structure on global star formation sheds light not only on the star formation process but also on how spiral arms evolve in disk galaxies.

“Recently, Yu & Ho analyze the spiral structure of a large, comprehensive sample of nearby galaxies. We use their catalog of spiral arm strength to study the impact of spiral structure on the global star formation process of disk galaxies.”

Examining the relationship between spiral arms, star formation rate (SFR), and stellar mass, they found that arm strength correlates well with the variation of SFR as a function of stellar mass. Arms are stronger above the star-forming galaxy main sequence (MS) and weaker below it: arm strength increases with higher log (SFR/SFRMS), where SFRMS is the SFR along the MS. Likewise, stronger arms are associated with higher specific SFR.

“This finding is confirmed for the full sample of 4378 disk galaxies using the u − r color index, showing that the position of spiral galaxies on the blue cloud depends systematically on their arm strength.”

They also found that, the dependence of log (SFR/SFRMS) on arm strength is independent of other galaxy structural parameters, such as bar strength, global stellar mass concentration (related to bulge dominance), and stellar mass surface density.

Figure 1: Systematic bias of spiral arm strength (log sarm) as a function of signal-to-noise ratio (SNR). The solid curve and shaded area mark the mean and scatter for a given SNR. © Jing Wang et al.

In addition, for the subset of galaxies with cold gas measurements, it has been found that spiral arm strength positively correlates with H I and H2 mass fraction, even after removing the mutual dependence of these quantities on log (SFR/SFRMS).

“We argue that the sensitivity of arm strength to gas content is in line with the notion that spiral arms are maintained by dynamical cooling provided by gas damping.”

Moreover, they showed that stronger arms lead to higher log (SFR/SFRMS) for a given gas fraction. This results in a trend of increasing arm strength with shorter gas depletion time, independent of stellar mass surface density. These correlations suggested that spiral arms enhance star formation efficiency, and that the relationship between spiral arms and star formation is driven only in part by gas fraction. Spiral arms and gas content play an intertwined role in disk galaxies.

“On the one hand, dissipation by gas damping maintains spiral structure, and on the other hand, spiral structure helps boost the star formation efficiency of the cold gas reservoir. This accounts for systematic variation of spiral arm strength on the star-forming MS.”

Reference: Si-Yue Yu, Luis C. Ho and Jing Wang, “Spiral Structure Boosts Star Formation In Disk Galaxies”, Arxiv, pp. 1-11, 2021. arXiv:2106.09715

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