APOSTLE vs. AURIGA Simulations: How Subgrid Models Shape Milky Way Analogs

Abstract

Despite significant progress in cosmological simulations of galaxy formation, the role of subgrid physics in shaping the detailed properties of galaxies remains incompletely understood. In this work, we analyze two sets of zoom-in simulations that share identical initial conditions but adopt distinct implementations of baryonic physics, enabling a controlled comparison of their predictions. We examine the stellar properties, morphological structures, and satellite populations of the simulated galaxies at $z=0$. We find that AURIGA galaxies systematically exhibit higher stellar masses and surface densities than their APOSTLE counterparts. These differences are primarily driven by variations in the efficiency of gas cooling from the circumgalactic medium (CGM) into the star-forming gas. Both simulations form well-defined disk galaxies; however, AURIGA systems generally display higher disk-to-total mass ratios, earlier disk formation, and more prominent dynamical structures such as bars and spiral arms. Nevertheless, strongly disk-dominated systems are present in both simulations, although they do not arise in the same host haloes. The vertical disk structure in both simulations is well described by a sech density profile, with scale heights below ~ 1 kpc in the inner regions. The satellite populations also differ, with AURIGA producing systematically more massive satellites, including a ~ 0.3 dex increase in the most massive system, while the number of satellites above $10^6 M_{\odot}$ remains comparable in most halo pairs. Both simulations reproduce similar satellite stellar mass--metallicity relations, albeit ~ 0.25 dex higher than observation. This comparative study therefore provides useful benchmarks for future efforts to better constrain galaxy formation models.

Figures (13)

• Figure 1: Mass evolution histories of galaxies. Four representative pairs are shown (as labeled in the upper-left corners of panels), with Apostle and Auriga simulations indicated by red and blue curves respectively. Stellar (solid), gas (dashed), and dark matter (dotted) components are displayed separately.
• Figure 2: Time evolution of the three factors entering the stellar mass decomposition for the Auriga and Apostle simulations. The solid lines show the cooling efficiency from the CGM to the ISM ($f_{\rm ISM}$) and the cold baryon retention fraction ($f_{\rm re}$), while the dot-dashed lines indicate the stellar mass fraction ($f_{*}$). The two simulations show significant differences in $f_{\rm ISM}$ but very similar values of $f_{\rm re}$.
• Figure 3: The face-on and edge-on projected stellar surface density of galaxies at $z=0$. Top and bottom 2 rows are for counterparts in Apostle and Auriga simulations respectively. The projected box has dimensions of $50 \times 50 \times 20$ kpc along x, y, z directions.
• Figure 4: Stellar surface density profiles at $z=0$. Each panel display results of galaxy counterparts (with name marked at top left) in the Apostle (red curves) and Auriga (blue curves) simulations. Black solid lines indicate the best-fit single Sérsic profiles, whose parameters (total stellar mass $\lg M_*$, effective radius $R_e$, and Sérsic index $n$) are listed in the top-right corner of each panel (color-coded to match the simulations). The stellar mass values in parentheses of $\lg M_*$ represent the actual summed masses of stellar particles within the analysis region $R \le$ 20 kpc and $|z| \le$ 5 kpc.
• Figure 5: Profiles of (mass-weighted) stellar age and metallicity for galaxies at $z=0$. Color coding is the same as that in Fig. \ref{['fig:sdens']}.