A KMOS survey of the nuclear disk of the Milky Way I: Survey design and metallicities

Abstract: In the central few degrees of the bulge of the Milky Way there is a flattened structure of gas, dust and stars (the central molecular zone) similar to nuclear disks in other galaxies. Due to extreme foreground extinction we possess only sparse information about the (mostly old) stellar population of the nuclear disc. Here we present our KMOS spectroscopic survey of the stars in the nuclear disk reaching the old populations. In order to obtain an unbiased data set, we sample stars in the full extinction range along each line-of-sight. We also observe reference fields in neighboring regions of the Galactic bulge. Here we describe the design and execution of the survey and present first results. We obtain spectra and five spectral indices of 3113 stars with a median S/N of 67 and measure radial velocities for 3051 stars. Of those, 2735 sources have sufficient S/N to estimate temperatures and metallicities from indices. We derive metallicities using the CO 2-0 and Na I K-band spectral features, where we derive our own empirical calibration using metallicities obtained with higher resolution observations. We use 183 giant stars for calibration spanning in metallicity from -2.5 to 0.6 dex and covering temperatures of up to 5500 K. The derived index based metallicities deviate from the calibration values with a scatter of 0.32 dex. The internal uncertainty of our metallicities is likely smaller. We use these metallicity measurements together with the CO index to derive effective temperatures using literature relations. We publish the catalog here. Our data set complements Galactic surveys such as Gaia and APOGEE for the inner 200 pc radius of the Milky Way which is not readily accessible by those surveys due to extinction. We will use the derived properties in future papers for further analysis of the nuclear disk.