# Galaxy Zoo: star-formation versus spiral arm number

## TL;DR

This study examines how spiral arm number in low-redshift disc galaxies relates to star-formation and gas properties, revealing that two-armed spirals are more efficient at converting gas into stars and have less obscured star formation than many-armed galaxies.

## Contribution

It provides new insights into the relationship between spiral arm structure and star-formation, using visual measurements and multi-wavelength data to analyze differences in star-formation efficiency and obscuration.

## Key findings

- Two-armed spirals convert gas to stars more efficiently.
- Two-armed galaxies have a higher fraction of unobscured star formation.
- Many-armed galaxies are offset below the IRX-$eta$ relation.

## Abstract

Spiral arms are common features in low-redshift disc galaxies, and are prominent sites of star-formation and dust obscuration. However, spiral structure can take many forms: from galaxies displaying two strong `grand design' arms, to those with many `flocculent' arms. We investigate how these different arm types are related to a galaxy's star-formation and gas properties by making use of visual spiral arm number measurements from Galaxy Zoo 2. We combine UV and mid-IR photometry from GALEX and WISE to measure the rates and relative fractions of obscured and unobscured star formation in a sample of low-redshift SDSS spirals. Total star formation rate has little dependence on spiral arm multiplicity, but two-armed spirals convert their gas to stars more efficiently. We find significant differences in the fraction of obscured star-formation: an additional $\sim 10$ per cent of star-formation in two-armed galaxies is identified via mid-IR dust emission, compared to that in many-armed galaxies. The latter are also significantly offset below the IRX-$\beta$ relation for low-redshift star-forming galaxies. We present several explanations for these differences versus arm number: variations in the spatial distribution, sizes or clearing timescales of star-forming regions (i.e., molecular clouds), or contrasting recent star-formation histories.