Star forming rates between z = 0.25 and z = 1.2 from the CADIS emission line survey

Abstract

The emission line survey within the Calar Alto Deep Imaging Survey (CADIS) detects emission line galaxies by a scan with an imaging Fabry-Perot interferometer. It covers 5 fields of >100□′ each in three wavelengths windows centered on λ ≃ 700, 820, and 920 nm, and reaches to a typical limiting line flux of 3 × 10-20 W m-2. This is the deepest emission line survey covering a field of several 100 □′. Galaxies between z = 0.25 and z = 1.4 are detected by prominent emission lines (from Hα to [O II]372.7) falling into the FP scans. Additional observations with a dozen medium band filters allow to establish the line identification and thus the redshift of the galaxies to better than σz = 0.001. On the basis of a total of more than 400 emission line galaxies detected in Hα (92 galaxies), [O III]500.7 (124 galaxies), or [O II]372.7 (222 galaxies) we measure the instantaneous star formation rate (SFR) in the range 0.24 < z < 1.21. With this purely emission line selected sample we are able to reach much fainter emission line galaxies than previous, continuum-selected samples. Thus completeness corrections are much less important. Although the relative [O III] emission line strength depends on excitation and metallicity and shows strong variation, the mean line ratios yield SFR[O III] values consistent with the SFR evolution. Our results substantiates the indications from previous studies (based on small galaxy samples) that the SFR decreases by a factor of ∼20 between z = 1.2 and today. In fact, for a Ωm = 0.3, Ωλ = 0.7 cosmology, we find an exponential decline ρ̇SFR ∝ exp(-tlookback/2.6 Gyr). This decrease of the SFR with time follows an exponential law which is compatible with the decreasing galaxy merger rate as expected from model calculations. The inferred SF density is in perfect agreement with that deduced from the FIR emission of optically selected galaxies which is explained by a large overlap between both populations. We show that self-consistent extinction corrections of both our emission lines and the UV continua lead to consistent results for the SF density.