Mainly on the Plane: Observing the Extended, Ionized Disks of Milky Way Analogs in IllustrisTNG

Abstract

This paper explores the extent to which the circumgalactic medium (CGM) of Milky Way-like galaxies is located in an extended, ionized, disklike structure. To test this hypothesis, we analyze the spatial and kinematic distributions of different ion species within a sample of MW-like systems in IllustrisTNG. We model commonly observed ions (HI, MgII, SiIV, CIV and OVI) and calculate (1) their angular momentum misalignment from the star-forming disk ($θ$) and (2) the fraction of absorption consistent with galaxy rotation ($f_\mathrm{EWcorot}$). We find that 63% of MgII, 45% of SiIV, 38% of CIV, and 35% of OVI mass along the major axis have kinematics aligned with the galaxy angular momentum axis. We extend this to a mock absorption line survey and quantify $f_\mathrm{EWcorot}$. We find that $f_\mathrm{EWcorot}$(MgII) $\sim80\%$ and $f_\mathrm{EWcorot}$(OVI) $\sim60\%$ at $\sim0.5\ \mathrm{R_{200c}}$, in agreement with recent observational work. We find that in the typical MW analog, there is evidence of cool-warm material in an extended, corotating structure, regardless of whether the angular momentum or observational definition is used. Hence, we expect that the typical MW CGM, especially in the low ions, should be mainly on the plane.

Figures (5)

• Figure 1: The $\mathrm{M_\ast \hbox{-} SFR}$ selection of TNG-50 MW analogs adopted from Pillepich2023. The MW selection region (black box) is bounded by star-formation rate and mass estimates of the MW, as defined in Pillepich2023. The present-day TNG50 star-forming main-sequence (purple) is included for reference.
• Figure 2: For a single MW analog in our sample (Subhalo ID: 497557), the edge-on gas column density map of four distinct components: 1. the aligned cool ($10^4\ \mathrm{K} \leq \mathrm{T} < 10^5\ \mathrm{K}$) gas (far left), 2. the non-aligned cool gas (middle left), 3. the aligned warm ($10^5\ \mathrm{K} \leq \mathrm{T} < 10^6\ \mathrm{K}$) component (middle right), and 4. the non-aligned warm gas (far right). In each image, the inner circle is 0.20 $\mathrm{R_{200c}}$ and the outer circle is $\mathrm{R_{200c}}$.
• Figure 3: The median normalized cumulative distribution of the angular momentum misalignment angle in the TNG50 MW analog sample for all gas (solid black), cool gas (solid blue), and warm gas (solid red). We include all gas within each subhalo. The shaded regions are bounded by the 16th percentile and 84th percentile of their respective distribution. This is compared to the distribution in the idealized disk galaxy (§\ref{['subsec:agora']}) (solid purple), where nearly all of the gas is aligned. We also include the distribution of the individual MW analog in Figure \ref{['fig:warmcool_image']} (Subhalo ID: 497557) along with the distribution from random motion (§\ref{['subsec:define misalignment angle']}).
• Figure 4: The median column density profile for H$\textsc{i}$, Mg$\textsc{ii}$, Si$\textsc{iv}$, C$\textsc{iv}$, and O$\textsc{vi}$ (top to bottom). The total ion column density (blue) and corotating ion column density (red) are shown for the major (dashed) and minor (dotted) axes. The radial column density profile is also calculated as a function of inclination angle (light-to-dark blue and red).
• Figure 5: For each ion, the $i=0^\circ$ (left) and $i=90^\circ$ (right) median fraction of the total column density from angular momentum aligned gas. Each image is 1 $\mathrm{R_{200c}} \times 1 \mathrm{R_{200c}}$. In the edge-on view, it is clear that each ion is aligned in a disklike structure, even for warmer gas (e.g., O vi).