Continuum removal in H\alpha\ extragalactic measurements

TL;DR

This paper identifies significant errors in extragalactic H-alpha measurements caused by continuum subtraction issues, especially colour effects, and proposes a method to reduce these errors for more accurate star formation rate estimates.

Contribution

It provides a detailed analysis of continuum subtraction errors in H-alpha measurements and introduces a simple colour-based correction method to improve accuracy.

Findings

Errors can cause up to 40% underestimation in H-alpha equivalent width.

Colour effects can bias star formation rate estimates in surveys.

A single colour measurement can significantly reduce continuum subtraction errors.

Abstract

We point out an important source of error in measurements of extragalactic H-alpha emission and suggest ways to reduce it. The H-alpha line, used for estimating star formation rates, is commonly measured by imaging in a narrow band and a wide band, both which include the line. The image analysis relies on the accurate removal of the underlying continuum. We discuss in detail the derivation of the emission line's equivalent width and flux for extragalactic extended sources, and the required photometric calibrations. We describe commonly used continuum-subtraction procedures, and discuss the uncertainties that they introduce. Specifically, we analyse errors introduced by colour effects. We show that the errors in the measured H-alpha equivalent width induced by colour effects can lead to underestimates as large as 40% and overestimates as large as 10%, depending on the underlying galaxy's stellar population and the continuum-subtraction procedure used. We also show that these errors may lead to biases in results of surveys, and to the underestimation of the cosmic star formation rate at low redshifts (the low z points in the Madau plot). We suggest a method to significantly reduce these errors using a single colour measurement.