Line confusion in spectroscopic surveys and its possible effects: Shifts in Baryon Acoustic Oscillations position

TL;DR

This paper investigates how line confusion in spectroscopic surveys, especially misidentifying emission lines like Hβ and [O III], can cause shifts in galaxy positions that affect the measurement of the BAO scale.

Contribution

It introduces a systematic effect of line confusion on BAO measurements and quantifies its impact using simulations, highlighting a potential source of bias in cosmological surveys.

Findings

Line confusion can cause ~90 Mpc/h shifts in galaxy positions.

Misidentification may broaden and shift the BAO peak.

Simulation results quantify the potential bias in BAO measurements.

Abstract

Roman Space Telescope will survey about 17 million emission-line galaxies over a range of redshifts. Its main targets are H$\alpha$ emission-line galaxies at low redshifts and [O III] emission-line galaxies at high redshifts. The Roman Space Telescope will estimate the redshift these galaxies with single line identification. This suggests that other emission-line galaxies may be misidentified as the main targets. In particular, it is hard to distinguish between the H$\beta$ and [O III] lines as the two lines are close in wavelength and hence the photometric information may not be sufficient to separate them reliably. Misidentifying H$\beta$ emitter as [O III] emitter will cause a shift in the inferred radial position of the galaxy by approximately 90 Mpc/h. This length scale is similar to the Baryon Acoustic Oscillation (BAO) scale and could shift and broaden the BAO peak, possibly introduce errors in determining the BAO peak position. We qualitatively describe the effect of this new systematic and further quantify it with a lightcone simulation with emission-line galaxies.