Star formation in M33: multiwavelength signatures across the disk

TL;DR

This study uses multiwavelength observations to analyze dust and star formation across M33's disk, revealing the contributions of different sources to infrared emission and estimating the galaxy's recent star formation rate.

Contribution

It provides a detailed multiwavelength analysis of M33's dust and star formation, highlighting the roles of various IR sources and estimating the galaxy's star formation rate over the last 100 million years.

Findings

Discrete IR sources contribute 20-40% of IR emission depending on radius.

Diffuse IR emission is mainly from small grains and evolved AGB stars.

Star formation rate in M33 is approximately 0.45 solar masses per year.

Abstract

Aims. We use different tracers, such as Ha, ultraviolet (UV), and infrared (IR) emissions at various wavelengths, to study the dust and star-formation (SF) conditions throughout the disk of M33. Methods. We derive the radial distribution of dust, of the old and young stellar population using Spitzer and GALEX data, complemented by ground-based optical data and available surveys of atomic and molecular gas. We separate the contribution of discrete sources to the IR brightness from the diffuse emission. Results. At 8 and 24 um, discrete sources account for 40% of the IR emission in the innermost 3 kpc, and for 20% further out. We find that stochastic emission from very small grains in the diffuse interstellar medium accounts for only 10% of the diffuse 24 um emission, and that dusty circumstellar shells of unresolved, evolved AGB stars (carbon stars) are a viable alternative. The 8 um profile suggests that PAH emission declines faster with radius than the dust continuum. In annular regions, 0.24 kpc wide, we find a mean extinction value for stellar continuum AV = 0.25 mag with a weak dependence on radius, consistent with the shallow metallicity gradient observed. Dust opacity derived from the 160 um emission decreases instead by a factor 10 from the center to edge of the SF disk. Conclusions. Using extinction corrected UV and Ha maps we find the global SF rate in M33, over the last 100 Myr, to be 0.45 +- 0.10 M yr-1. FIR (far-IR) and TIR (total-IR) luminosities can trace SF even though a high conversion factor is required to recover the effective rate. If carbon stars are powering the diffuse 24 um emission in M33 this can trace star formation 1 Gyr ago and provide a more complete view of the SF history of the galaxy. [abridged]