KROSS: mapping the Hα emission across the star formation sequence at z ≈ 1

Abstract

We present first results from the KMOS (K-band Multi-Object Spectrograph) Redshift One Spectroscopic Survey, an ongoing large kinematical survey of a thousand, z ∼ 1 star-forming galaxies, with VLT KMOS. Out of the targeted galaxies (∼500 so far), we detect and spatially resolve Hα emission in ∼90 and 77 per cent of the sample, respectively. Based on the integrated Hα flux measurements and the spatially resolved maps, we derive a median star formation rate (SFR) of ∼7.0 M⊙ yr−1 and a median physical size of 〈r′1/2r1/2′〉 = 5.1 kpc. We combine the inferred SFRs and effective radii measurements to derive the star formation surface densities (ΣSFR) and present a ‘resolved’ version of the star formation main sequence (MS) that appears to hold at subgalactic scales, with similar slope and scatter as the one inferred from galaxy-integrated properties. Our data also yield a trend between ΣSFR and Δ(sSFR) (distance from the MS) suggesting that galaxies with higher sSFR are characterized by denser star formation activity. Similarly, we find evidence for an anticorrelation between the gas phase metallicity (Z) and the Δ(sSFR), suggesting a 0.2 dex variation in the metal content of galaxies within the MS and significantly lower metallicities for galaxies above it. The origin of the observed trends between ΣSFR–Δ(sSFR) and Z–Δ(sSFR) could be driven by an interplay between variations of the gas fraction or the star formation efficiency of the galaxies along and off the MS. To address this, follow-up observations of our sample that will allow gas mass estimates are necessary.