# Peak star formation efficiency and no missing baryons in massive spirals

## TL;DR

This study finds that massive spiral galaxies have very high star formation efficiencies, nearly converting all their baryons into stars, challenging the traditional view of low efficiency and missing baryons in such galaxies.

## Contribution

It provides observational evidence that massive spirals can reach near 100% star formation efficiency, contradicting previous models of declining efficiency at high masses.

## Key findings

- Massive spirals have star formation efficiencies of 30-100%.
- No evidence of efficiency decline at high galaxy masses.
- Massive spirals contain virtually no missing baryons.

## Abstract

It is commonly believed that galaxies use, throughout the Hubble time, a very small fraction of the baryons associated to their dark matter halos to form stars. This so-called low "star formation efficiency" $f_\star\equiv M_\star/f_{\rm b} M_{\rm halo}$, where $f_{\rm b}\equiv\Omega_{\rm b}/\Omega_{\rm c}$ is the cosmological baryon fraction, is expected to reach its peak at nearly $L^\ast$ (at efficiency $\approx 20\%$) and decline steeply at lower and higher masses. We have tested this using a sample of nearby star-forming galaxies, from dwarfs ($M_\star\simeq 10^7 M_\odot$) to high-mass spirals ($M_\star\simeq 10^{11} M_\odot$) with HI rotation curves and 3.6$\mu$m photometry. We fit the observed rotation curves with a Bayesian approach by varying three parameters, stellar mass-to-light ratio, halo concentration and mass. We found two surprising results: 1) the star formation efficiency is a monotonically increasing function of $M_\star$ with no sign of a decline at high masses, and 2) the most massive spirals ($M_\star\simeq 1-3 \times 10^{11} M_\odot$) have $f_\star\approx 0.3-1$, i.e. they have turned nearly all the baryons associated to their haloes into stars. These results imply that the most efficient galaxies at forming stars are massive spirals (not $L^\ast$ galaxies), they reach nearly 100% efficiency and thus, once both their cold and hot gas is considered into the baryon budget, they have virtually no missing baryons. Moreover, there is no evidence of mass quenching of the star formation occurring in galaxies up to halo masses of $M_{\rm halo}\approx {\rm a\, few}\times 10^{12} M_\odot$.