Figuring Out Gas & Galaxies in Enzo (FOGGIE). III. The Mocky Way: Investigating Biases in Observing the Milky Way's Circumgalactic Medium (2001.07736v2)

Abstract: The circumgalactic medium (CGM) of the Milky Way is mostly obscured by nearby gas in position-velocity space because we reside inside the Galaxy. Substantial biases exist in most studies on the Milky Way's CGM that focus on easier-to-detect high-velocity gas. With mock observations on a Milky-Way analog from the FOGGIE simulation, we investigate four observational biases related to the Milky Way's CGM. First, QSO absorption-line studies probe a limited amount of the CGM mass: only 35% of the mass is at high Galactic latitudes $|b|>20$ degrees, of which only half is moving at $|v_{\rm LSR}|\gtrsim100$ km s$^{-1}$. Second, the inflow rate ($\dot{M}$) of the cold gas observable in HI 21cm is reduced by a factor of $\sim10$ as we switch from the local standard of rest to the galaxy's rest frame; meanwhile $\dot{M}$ of the cool and warm gas does not change significantly. Third, OVI and NV are promising ions to probe the Milky Way's outer CGM ($r\gtrsim$15 kpc), but CIV may be less sensitive. Lastly, the scatter in ion column density is a factor of 2 higher if the CGM is observed from inside-out than from external views because of the gas radial density profile. Our work highlights that observations of the Milky Way's CGM, especially those using HI 21cm and QSO absorption lines, are highly biased. We demonstrate that these biases can be quantified and calibrated through synthetic observations with simulated Milky-Way analogs.

• The paper reveals that current techniques detect only 18% of the Milky Way's CGM mass due to high-latitude bias.
• It demonstrates that converting to the galactic rest frame lowers cold gas inflow estimates by tenfold compared to LSR measures.
• Synthetic observations identify ions like O VI and Ne VIII as effective tracers for probing the outer CGM beyond 15 kpc.

An Analysis of Observational Biases in Studying the Milky Way's Circumgalactic Medium

The paper "Figuring Out Gas Galaxies in Enzo (FOGGIE). III. The Mocky Way: Investigating Biases in Observing the Milky Way's Circumgalactic Medium" by Zheng and colleagues addresses the complex issue of biases that arise when studying the circumgalactic medium (CGM) of our own galaxy, the Milky Way. These biases are primarily due to our position within the galaxy, complicating observations and interpretations of the CGM's properties.

Key Findings

1. Observational Scope Limitations: The research highlights that a significant portion of the Milky Way's CGM mass is obscured in observations. Due to current observational techniques relying heavily on quasar absorption lines at high galactic latitudes and high velocities to escape contamination from the Milky Way's interstellar medium (ISM), only 18% of the CGM mass is accounted for. This figure is stark, showing that existing studies likely overlook approximately 82% of the CGM's mass.
2. Rest Frame Considerations: The paper explores velocity biases stemming from our movement with the Sun. The mock observations reveal that while infall rates of the Milky Way's high-velocity clouds (HVCs)—detected in neutral hydrogen—are overestimated, this bias largely does not affect ionized gas inflow rates significantly. The researchers find that converting measurements to the galactic rest frame reduces inflow rate estimates of cold gas by tenfold, which is 0.004 compared to 0.03 M⊙ yr⁻¹ in the local standard of rest (LSR).
3. Probing Outer CGM: Through synthetic observations, the paper evaluates which ions are effective for probing the Milky Way's outer CGM beyond 15 kpc. The findings suggest ions like O VI and Ne VIII are sensitive indicators, demonstrating their potential utility in future observations.
4. Inside-Out vs. External Observational Biases: The comparison reveals that when viewing the Milky Way’s CGM from within, measurements exhibit significantly greater scatter—up to 1.5 dex for O VI column densities—compared to external observations of other galaxies, where the scatter is limited to approximately 0.7 dex. This discrepancy underscores the inherent biases when directly comparing the Milky Way to other galaxies.

Implications

The implications of this paper are profound. They underscore the need for a recalibration of our understanding of the Milky Way’s CGM and draw attention to the limitations of current observational methods. The biases identified imply that current models of the CGM's role in galaxy evolution and star formation might need adjustment. Additionally, these findings push for increased resolution in simulations and potential adoption of non-thermal feedback mechanisms to better replicate observed column densities of various ions.

Future Directions

To build upon this work, future simulations should aim to increase resolution, particularly vertically through the disk to improve gravitational potential accuracy and corresponding gas distributions. Incorporating non-thermal feedback mechanisms could provide more accurate models of ion distribution and interaction within the CGM. Furthermore, enhanced synthetic spectroscopy that accounts for instrumental effects and biases will serve to build more robust interpretations of empirical data. These refined approaches will be crucial to bridging the gap between simulated and observed data, leading toward more comprehensive models of CGM physics and their implications for galaxy evolution.

In summary, Zheng et al.'s paper delivers a critical examination of observational biases in the paper of the Milky Way's CGM and emphasizes the necessity for recalibrated observational techniques and advanced simulation parameters to faithfully characterize this complex and vital galactic component.