Very Large Telescope Spectroscopy of Ultra-faint Dwarf Galaxies. I. Boötes I, Leo IV, and Leo V

We perform consistent reductions and measurements for three ultra-faint dwarf galaxies (UFDs):Bo ̈otes I, Leo IV and Leo V. Using the public archival data from the GIRAFFE spectrograph on theVery Large Telescope (VLT), we locate new members and provide refined measurements of physicalparameters for these dwarf galaxies. We identify nine new Leo IV members and four new Leo Vmembers, and perform a comparative analysis of previously discovered members. Additionally, weidentify one new binary star in both Leo IV and Leo V. After removing binary stars, we recalculatethe velocity dispersions of Bo ̈otes I and Leo IV to be 5.1+0.7−0.8and 3.4+1.3−0.9km s−1, respectively; Wedo not resolve the Leo V velocity dispersion. We identify a weak velocity gradient in Leo V thatis∼4×smaller than the previously calculated gradient and that has a corresponding position anglewhich differs from the literature value by∼120 deg. Combining the VLT data with previous literature,we re-analyze the Bo ̈otes I metallicity distribution function and find that a model including infall ofpristine gas while Bo ̈otes I was forming stars best fits the data. Our analysis of Leo IV, Leo V andother UFDs will enhance our understanding of these enigmatic stellar populations and contribute tofuture dark matter studies. This is the first in a series of papers examining thirteen UDFs observedwith VLT/GIRAFFE between 2009 and 2017. Similar analyses of the remaining ten UFDs will bepresented in forthcoming papers