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

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, for example, [C ii] 158 μm, have been found to be very bright in some galaxies, with fluxes of ≳0.1–1 per cent 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 passbands, 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 per cent 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-μm surveys are being identified in part due to the line contamination from strong [C ii] emission. These biases may be even stronger for less-luminous and lower metallicity populations at high redshifts, which are observable with the ALMA and which may have even stronger line-to-continuum ratios.