An uncertainty principle for star formation -- III. The characteristic emission time-scales of star formation rate tracers

TL;DR

This paper calibrates the absolute emission time-scales of star formation rate tracers like Hα and UV filters using simulations and stellar population models, enabling more accurate star formation studies.

Contribution

It provides calibrated reference time-scales for SFR tracers across different filters and metallicities, facilitating observational application of a new statistical method for star formation analysis.

Findings

Hα time-scale is about 4.3 Myr with continuum subtraction.

UV filters have time-scales of 17-33 Myr, increasing with wavelength.

Time-scales decrease at higher metallicities and lower star formation surface densities.

Abstract

We recently presented a new statistical method to constrain the physics of star formation and feedback on the cloud scale by reconstructing the underlying evolutionary timeline. However, by itself this new method only recovers the relative durations of different evolutionary phases. To enable observational applications, it therefore requires knowledge of an absolute 'reference time-scale' to convert relative time-scales into absolute values. The logical choice for this reference time-scale is the duration over which the star formation rate (SFR) tracer is visible because it can be characterised using stellar population synthesis (SPS) models. In this paper, we calibrate this reference time-scale using synthetic emission maps of several SFR tracers, generated by combining the output from a hydrodynamical disc galaxy simulation with the SPS model SLUG2. We apply our statistical method to obtain self-consistent measurements of each tracer's reference time-scale. These include H${\alpha}$ and 12 ultraviolet (UV) filters (from GALEX, Swift, and HST), which cover a wavelength range 150-350 nm. At solar metallicity, the measured reference time-scales of H${\alpha}$ are ${4.32^{+0.09}_{-0.23}}$ Myr with continuum subtraction, and 6-16 Myr without, where the time-scale increases with filter width. For the UV filters we find 17-33 Myr, nearly monotonically increasing with wavelength. The characteristic time-scale decreases towards higher metallicities, as well as to lower star formation rate surface densities, owing to stellar initial mass function sampling effects. We provide fitting functions for the reference time-scale as a function of metallicity, filter width, or wavelength, to enable observational applications of our statistical method across a wide variety of galaxies.