The observations of our target, USNO-A0600-15865535, were obtained at the ESO Very Large Telescope (VLT) at Cerro Paranal, Chile on the nights of May 6th and May 7th, 2016. The target was observed as a part of our program “Mapping the Cool Circumgalactic Medium with Calcium II” (097.A-0552, PI: Peek) that uses the FLAMES/GIRAFFE spectrograph, a fiber-fed multiobject spectrograph mounted on the Nasmyth focus of UT2 \cite{Pasquini_2003}. USNO-A0600-15865535 is a bright (\(g= 14.2\)) very blue (\(g-r = -0.21\)) source, unresolved in Pan-STARRS imaging \citep{Magnier_2013, Schlafly_2012,Tonry_2012}. Through a combination of overoptimism and clerical error, it was targeted as a quasar candidate behind the circumgalactic medium of M83.
The GIRAFFE High Resolution mode was used in the H395.8 setup (HR02), which gives access to the 385.4 to 404.9nm wavelength range in the near UV at a spectral resolving power \(\lambda / \delta \lambda\) of 22,700. A total of 4.5 h of integration were obtained, split in 3 exposures of 90 min. Given that most of the program targets were faint, the non-standard 50 khz,1×1, high gain readout mode was used \cite{2008Msngr.133...17M}. Corresponding calibration frames were obtained within 24h of the science observations. The raw scientific data were bias- and dark-subtracted, flat-fielded and wavelength-calibrated using the instrument’s pipeline v12.14.2 under the esorex environment. The source spectra were extracted in the SUM mode. Finally the individual exposures were summed up to improve the signal-to-noise ratio of the extracted data.
In addition to the new data taken with FLAMES, we also used archival HI data from the GASS survey \cite{Kalberla_2010}. GASS is a survey of Galactic HI taken with the Parkes antenna, with a beam size of 16\(^\prime\), a spectral resolution of 1 km/s, and an rms noise of 57 mK. These data allow us to map out the structure of Complex WD on the sky, and compare the spectrum of the HI emission to Ca II H & K absorption along the line of sight. We present in Figure \ref{fig:WD} an image of Complex WD from GASS, along with the positions of clouds found in the \citet{1991A&A...250..509W} catalog, and the line-of-sight toward USNO-A0600-15865535.