The Role of Spiral Arms in Galaxies II: Similarities Amid Diversity

TL;DR

This work addresses whether spiral arms primarily trigger star formation or simply gather material by examining arm versus interarm regions in six nearby galaxies using multi-wavelength SED fitting. The authors derive SFR and M⋆ from MAGPHYS fits and combine them with CO-based gas masses to compute sSFR and SFE at ~1.5 kpc scales, finding a tight SFE–sSFR correlation across environments. Across the sample, arm regions exhibit higher SFR and M⋆ but show only modest differences in sSFR and SFE, with overall arm/interarm ratios around $1.8$ (sSFR) and $1.5$ (SFE); NGC 1097 and NGC 628 are notable outliers. These results support the gatherer scenario, where spiral arms collect material rather than dramatically boosting star formation efficiency, and demonstrate that resolved SED-based analyses are effective for testing spiral-structure effects.

Abstract

The role of spiral arms in galaxies -- whether they enhance star formation efficiency or primarily act as material gatherers -- remains an open question. Observational studies have yielded ambiguous results, in part due to the choice of star formation rate (SFR) tracers and their inherent limitations. These limitations are addressed here by applying multi-wavelength spectral energy distribution (SED) fitting to individual arm and interarm regions. We expand on our previous study of two galaxies to include six diverse galaxies, spanning over an order of magnitude in total stellar mass and factors of several in total SFR, for which spiral arms have been mapped. We find that the specific star formation rate (sSFR = SFR/M$_{star}$) can be used as a proxy for the star formation efficiency (SFE=SFR/M$_{gas}$), since the two quantities are directly proportional to each other in our regions. In our analysis of both tracers (sSFR and SFE) no significant difference is found the between arm and interarm regions, except for one galaxy (NGC 1097), supporting the gatherers scenario.

Figures (14)

• Figure 1: Parameter maps for NGC 1097. From upper to bottom rows: SFR; stellar mass; and specific star-formation rates (sSFR). In each row, the three panels from left to right are: best-fit values of all spaxels with $S/N \geq 3$ in IRAC 3.6 $\mu$m band (including include bulge spaxels); arm spaxels; and interarm spaxels, with the bulge spaxels excluded.
• Figure 2: SFE vs sSFR in all galaxies. Left: all spaxels in six galaxies. Middle: all arm spaxels in six galaxies. Right: all interarm spaxels in six galaxies. The straight lines are the linear fit of log(sSFR/yr$^{-1}$) and log(SFE/yr$^{-1}$).
• Figure 3: sSFR distributions for arm and interarm spaxels across six galaxies. The median values are -0.681 (arm) and -0.937 (interarm). The median ratio is 1.800. The black dotted lines are the gaussian fit of all arm/interarm spaxels. The solid lines are the gaussian fit of the right side of the distribution, centered on the median values, using the same color coding of the two distributions.
• Figure 4: SFR and M$_\textup{star}$ distributions for arm and interarm spaxels across six galaxies. Left: Normalized SFR, with median log values of -0.636 (arm) and -1.218 (interarm), yielding a median ratio of 3.823. Right: Normalized M$_\textup{star}$, with median log values of -0.061 (arm) and -0.424 (interarm), corresponding to a median ratio of 2.304.
• Figure 5: The molecular gas mass M$_\textup{gas}$ and star-formation efficiency (SFE) across six galaxies after correcting for inclinations. Left: The median of M$_\textup{gas}$ in arm and interarm regions are --0.124 and -0.301, respectively, the median ratio is 1.505. Right: the SFE median in arm and interarm regions are -0.560 and -0.745, corresponding to a median ratio of 1.533, consistent with the gatherer scenario.