The potential influence of far-infrared emission lines on the selection of high-redshift galaxies

TL;DR

This paper explores how bright far-infrared emission lines, like [CII], can significantly affect the detection and property estimation of high-redshift galaxies in broad-band surveys, potentially causing biases.

Contribution

It quantifies the impact of emission line contamination on broad-band flux measurements and highlights its implications for high-redshift galaxy identification and property inference.

Findings

Line emission can boost broad-band flux by over 20-40%.

Line contamination may bias redshift and luminosity estimates.

High-redshift, low-metallicity galaxies may have even stronger line effects.

Abstract

We investigate whether strong molecular and atomic emission lines at far-infrared wavelengths can influence the identification and derived properties of galaxies selected from broad-band, far-infrared or submillimetre observations. Several of these lines, e.g. [CII]158um, have been found to be very bright in some high-redshift galaxies, with fluxes of >0.1-1% of the total far-infrared luminosity, and may be even brighter in certain populations at high redshifts. At redshifts where these lines fall in instrument pass-bands they can significantly increase the broad-band flux measurements. We estimate that the contributions from line emission could boost the apparent broad-band flux by >20-40% in the Herschel and SCUBA-2 bands. Combined with the steep source counts in the submillimetre and far-infrared bands, line contamination has potentially significant consequences for the properties of sources detected in flux-limited continuum surveys, biasing the derived redshift distributions and bolometric luminosities. Indeed, it is possible that some z>4 sources found in 850-um surveys are being identified in part due to line contamination from strong [CII] emission. These biases may be even stronger for less-luminous and lower-metallicity populations at high redshifts which are observable with ALMA and which may have even stronger line-to-continuum ratios.