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

Abstract

\n Roman Space Telescope will survey about 17 million emission-line galaxies over a range of redshifts. Its main targets are Hα emission-line galaxies at low redshifts (z < 2) and [O\xa0iii] emission-line galaxies at high redshifts (z > 2). 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 Hβ and [O\xa0iii] lines as the two lines are close in wavelength and hence the photometric information may not be sufficient to separate them reliably. Misidentifying Hβ emitter as [O\xa0iii] 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. Our results show a systematic shift in the recovered isotropic BAO positions that depends on the percentage of interlopers (${{\\ \\rm per\\ cent}}{\\rm H}\\beta$) in the sample. The systematic shift can be as large as $0.1-0.3{{\\ \\rm per\\ cent}} \\, {\\rm x}\\, {{\\ \\rm per\\ cent}}{\\rm H}\\beta$ for analysis performed at redshifts z = 1.3 − 1.9.