The AGORA High-resolution Galaxy Simulations Comparison Project. VIII: Disk Formation and Evolution of Simulated Milky Way Mass Galaxy Progenitors at $1<z<5$

TL;DR

The paper compares eight cosmological zoom-in simulations of Milky Way–mass progenitors (the AGORA CosmoRun suite) to understand how subgrid stellar feedback shapes disk formation and evolution across 1 < z < 5. By applying a consistent disk-identification scheme, mock observations, and morphology metrics, it reveals two main evolutionary tracks: strong wet compaction producing compact, rotation-dominated cores and weaker compaction leading to more extended disks, with dust attenuation reconciling some size and shape differences in mock observations. The results underscore a strong sensitivity of early disk assembly to feedback prescriptions, including delayed cooling and outflow strengths, and show that mergers largely drive initial gas inflows while subsequent evolution depends on the feedback model. The study also highlights code-dependent variations in compaction timing and emphasizes careful interpretation of observables due to dust and methodological differences, informing future modeling and comparison efforts.

Abstract

We investigate how differences in the stellar feedback produce disks with different morphologies in Milky Way-like progenitors over 1 $\leq z \leq 5$, using eight state-of-the-art cosmological hydrodynamics simulation codes in the \textit{AGORA} project. In three of the participating codes, a distinct, rotation-dominated inner core emerges with a formation timescale of $\lesssim 300$ Myr, largely driven by a major merger event, while two other codes exhibit similar signs of wet compaction -- gaseous shrinkage into a compact starburst phase -- at earlier epochs. The remaining three codes show only weak evidence of wet compaction. Consequently, we divide the simulated galaxies into two groups: those with strong compaction signatures and those with weaker ones. Galaxies in these two groups differ in size, stellar age gradients, and disk-to-total mass ratios. Specifically, codes with strong wet compaction build their outer disks in an inside-out fashion, leading to negative age gradients, whereas codes with weaker compaction feature flat or positive age gradients caused primarily by outward stellar migration. Although the stellar half-mass radii of these two groups diverge at $z \sim 3$, the inclusion of dust extinction brings their sizes and shapes in mock observations closer to each other and to observed galaxies. We attribute the observed morphological differences primarily to variations in the stellar feedback implementations -- such as delayed cooling timescales, and feedback strengths -- that regulate both the onset and duration of compaction. Overall, our results suggest that disk assembly at high redshifts is highly sensitive to the details of the stellar feedback prescriptions in simulations.

Figures (32)

• Figure 1: Face-on and edge-on views of stellar surface density at redshift $z=2.8$. From top to bottom, each set of rows represents the total, thin disk, thick disk, and spheroid stellar component. The mass fractions of each component are shown in the upper right corner of each panel, and the circularity threshold between the thin and thick disk, $\epsilon_{\text{thre}}$, is displayed below the mass fraction of the thin disk component. The black dashed circles in the top panels represent $3 \times r_{1/2}$. The results at $z=1$ are presented in Appendix \ref{['sec:z1']}. For more information, see Section \ref{['sec:disk_id']}.
• Figure 2: Mock JWST NIRCam RGB images (R: F356W, G: F200W, B: F115W) at $z=2.8$. Upper panels and lower panels represent the face-on and edge-on views, the same viewing angles with Figure \ref{['fig:disk_decomp_z28']}. For more information, see Section \ref{['sec:mock']}.
• Figure 3: Same as Figure \ref{['fig:mock_z28']} but with different broadband filters (R: F200W, G: F115W, B: F070W) at $z = 1$. Note that six out of eight codes seen in Figure \ref{['fig:mock_z28']} reach redshift $z = 1$, while Ramses and Gizmo end their simulations at $z=2$ (see Sections \ref{['sec:suite']} and \ref{['sec:mock']}).
• Figure 4: Evolution of disk properties of the target galaxy in each code. The panels display the disk-to-total ratio (D/T), the half-mass radius ($r_{1/2}$), the stellar surface density within a $1 \,{\rm kpc}$ radius ($\Sigma_{{\star}, 1 \,{\rm kpc}}$), and the concentration ($C_{82}$; see Section \ref{['sec:morphology']}). The gray-shaded regions indicate mergers at $z \sim 4.5$ and $z \sim 2.2$, during which D/T may not be well-defined. The D/T of the target galaxies exceeds 0.5 before the merger event at $z \sim 2.2$, roughly coinciding with the increases in $\Sigma_{{\star}, 1 \,{\rm kpc}}$ and $C_{82}$, and the decrease in $r_{1/2}$. For more information, see Section \ref{['sec:disk_growth']}.
• Figure 5: Evolution of the half-mass radius ($r_{1/2}$, left panel), the size of the star-forming disk ($r_{\rm SFR}$, middle panel), and the concentration ($C_{82}$, right panel) as functions of stellar mass of the target galaxy in each code. The thick black dashed line in the left panel represents the evolution of galaxies undergoing wet compaction in the Vela3 simulations 2023MNRAS.522.4515L. In both $r_{1/2}$ and $r_{\rm SFR}$, Enzo, Ramses, and Gear exhibit a steady increase in size with stellar mass, while the other five codes show a contraction phase followed by an expansion phase. Differences in $C_{82}$ between these two code groups become apparent at $M_{\star} \gtrsim 10^{9.5} \,{\rm M}_\odot$. The dashed portion in each line indicates the period that is heavily influenced by the major merger at $z \sim 4.5$ (denoted by a circle marker). For more information, see Section \ref{['sec:merger']}.