Figuring Out Gas & Galaxies in Enzo (FOGGIE). III. The Mocky Way: Investigating Biases in Observing the Milky Way's Circumgalactic Medium

The circumgalactic medium (CGM) of the Milky Way is mostly obscured by nearby gas in position-velocity space due to the location and motion of observers inside the Galaxy. While most studies on the Milky Way's CGM to date focus on easier-to-detect high-velocity gas ($|v_{\rm LSR}|\gtrsim100$ km s$^{-1}$), it only partially represents the entire CGM, resulting in substantial biases. Here we investigate four observational biases inherent in the studies of the Milky Way's CGM with mock observations on a Milky-Way analog from the FOGGIE simulations. With a mock observer placed inside the simulated galaxy at off-center locations consistent with that of the Sun, we find that: (i) high-velocity-based studies could miss $\sim$2/3 of the Milky Way's CGM mass; (ii) once we correct velocities from the local standard of rest to the galaxy's rest frame, gas infall rate ($\dot{M}$) for cold gas observable in HI 21cm emission is reduced from -0.25 M$_\odot$ to -0.05 M$_\odot$, while $\dot{M}$ for gases at warmer phases are less affected; (iii) OVI and NV are likely to be good UV tracers of the Milky Way's outer CGM ($r\gtrsim$15 kpc), whereas CIV maybe less sensitive; (iv) because of the clumpiness and radial distribution of CGM gas, the scatter in ionic column densities is a factor of 2 higher when the CGM is observed from the inside-out versus from external views. Our work highlights that observations of the Milky Way's CGM, especially those using HI emission and UV absorption lines, are highly biased because we reside inside the Galaxy. We demonstrate that these biases can be quantified and calibrated through synthetic observations with simulated Milky-Way analogs.