Converting From 3.6 and 4.5 Micron Fluxes to Stellar Mass

TL;DR

This study develops an empirical conversion from infrared fluxes at 3.6 and 4.5 microns to stellar mass in the Large Magellanic Cloud, accounting for contamination and uncertainties, and discusses implications for the initial mass function.

Contribution

We present a new empirical calibration linking infrared fluxes to stellar mass, with an analysis of contamination effects and uncertainty estimates, applicable to unresolved galaxies.

Findings

The conversion formula accurately estimates stellar mass with about 0.3*sqrt(N/10^6) fractional uncertainty.

Contamination from young stars and hot dust affects measurements but is minimally reducible with available data.

Results favor a bottom-heavy initial mass function like Salpeter, disfavoring lighter IMFs.

Abstract

We use high spatial resolution maps of stellar mass and infrared flux of the Large Magellanic Cloud (LMC) to calibrate a conversion between 3.6 and 4.5 micron fluxes and stellar mass, M_* = 10^{5.65} * F_{3.6}^{2.85} * F_{4.5}^{-1.85} * (D/0.05)^2 M_solar, where fluxes are in Jy and D is the luminosity distance to the source in Mpc, and to provide an approximate empirical estimate of the fractional internal uncertainty in M_* of 0.3*sqrt{N/10^6}, where N is the number of stars in the region. We find evidence that young stars and hot dust contaminate the measurements, but attempts to remove this contamination using data that is far superior than what is generally available for unresolved galaxies resulted in marginal gains in accuracy. The scatter among mass estimates for regions in the LMC is comparable to that found by previous investigators when modeling composite populations, and so we conclude that our simple conversion is as precise as possible for the data and models currently available. Our results allow for a reasonably bottom-heavy initial mass function, such as Salpeter or heavier, and moderately disfavor lighter versions such as a diet-Salpeter or Chabrier initial mass function.