---
title: Auriga Galaxy Formation Model
url: https://www.emergentmind.com/topics/auriga-galaxy-formation-model
type: topic
---

# Auriga Galaxy Formation Model

Searching arXiv for recent and core Auriga papers to ground the article.
The **Auriga galaxy formation model** is a cosmological galaxy formation framework implemented in the moving-mesh magnetohydrodynamics code **Arepo** and used primarily for zoom-in simulations of Milky Way–mass haloes, with later extensions to dwarf galaxies and star-cluster modelling. In the literature surveyed here, it appears as a full-physics model coupling gravity, ideal MHD, radiative cooling, a multiphase interstellar medium, stochastic star formation, stellar evolution, supernova-driven winds, supermassive black holes, and AGN feedback, all within a $\Lambda$CDM cosmology [2401.08750]. The model is defined not only by its numerical platform but also by a distinctive subgrid treatment of baryonic physics, especially its **kinetic, isotropic wind** feedback and its regulation of circumgalactic-medium cooling, which have been shown to shape stellar masses, disc structure, bars, spiral arms, satellites, and baryon cycling in Milky Way analogues [2603.21837].

## 1. Definition and numerical framework

Auriga is a suite of cosmological zoom-in simulations of Milky Way–mass haloes run with **Arepo**, a **moving-mesh MHD** code [2603.21837]. In the broader augmented project, the same galaxy formation model is applied to **40 Milky Way-mass halos** and **26 dwarf galaxy-mass halos** in a Planck 2013 $\Lambda$CDM cosmology, with initial redshift $z_{\rm init}=127$ and parent initial conditions derived from the EAGLE dark-matter-only volume [2401.08750]. The target Milky Way–mass systems span
\[
0.5 < M_{200} / [10^{12}\,{\rm M}_\odot] < 2,
\]
while additional suites extend to lower halo masses [2401.08750].

At the standard Milky Way resolution level, Auriga typically uses baryonic mass resolution of order $5\times 10^4\,M_\odot$ and dark-matter particle mass of order $3\times 10^5$–$4\times 10^5\,M_\odot$, with higher-resolution variants improving mass resolution by a factor of 8 [2502.14972]. The model includes ideal MHD with a uniform comoving seed field of strength $10^{-14}\,\mathrm{G}$ at $z=127$, which is subsequently amplified by structure formation and disc dynamics [2502.14972]. Magnetic fields are evolved self-consistently and produce large-scale ordered azimuthal configurations in discs by $z=0$, while the global galaxy properties remain only weakly affected because equipartition is reached relatively late [1701.07028].

The published data release emphasizes that Auriga follows the coupled evolution of dark matter, gas, stars, supermassive black holes, and magnetic fields, and that the framework has been shown to produce numerically well-converged galaxy properties for Milky Way–mass systems [2401.08750]. A plausible implication is that Auriga is intended as a unified galaxy formation model rather than a family of mass-specific prescriptions.

## 2. Baryonic subgrid physics

Auriga adopts a comprehensive galaxy formation model descended from the Vogelsberger–Marinacci–Grand lineage and includes primordial and metal-line cooling, a UV background, a subgrid multiphase ISM, stochastic star formation, stellar evolution and enrichment, kinetic stellar feedback, and AGN feedback [2401.08750]. In the comparative Milky Way analogue study, the most explicit distinguishing feature is the **kinetic wind model of Vogelsberger et al. (2013)** [2603.21837].

In this scheme, stellar feedback is implemented by creating **“wind particles”** that are launched **kinematically** and **isotropically** [2603.21837]. These particles are **temporarily decoupled** from the dense ISM, travel into lower-density gas, and later **recouple** once they have moved sufficiently far or reached a low-density region, depositing **mass, metals, and both thermal and kinetic energy** back into the gas [2603.21837]. The resulting feedback is non-local: energy avoids immediate radiative loss in dense star-forming gas and is thermalized in the outer ISM or CGM. In comparison studies against EAGLE-like or APOSTLE-like models, this is contrasted directly with **thermal** supernova feedback that heats dense gas in situ to a fixed temperature [2603.21837].

Auriga’s ISM follows the Springel & Hernquist two-phase model in which gas above a threshold density enters a pressurized star-forming phase [2502.14972]. One paper states explicitly that when gas reaches densities
\[
n > 0.11\ \mathrm{cm^{-3}},
\]
it enters a subgrid two-phase ISM model with hot and cold phases in pressure equilibrium and an effective equation of state [2502.14972]. Another Auriga overview gives the star-formation threshold as
\[
n_{\rm H,\,th} = 0.13\ \mathrm{cm}^{-3},
\]
with star formation realized stochastically and calibrated to a Kennicutt–Schmidt-like relation [2401.08750]. In the context of the comparison paper, the ISM is operationally defined as gas that has at least once satisfied the star formation criterion, i.e. gas with $\mathrm{SFR}>0$ at least once [2603.21837].

A useful decomposition of stellar mass growth employed in the Milky Way analogue comparison is
\[
M_* = f_{\rm ISM}\, f_{\rm re}\, f_*\, M_{\rm acc},
\]
where $M_{\rm acc}$ is the total gas mass ever accreted into the halo, $f_{\rm ISM}$ is the fraction of accreted gas that ever cools into the star-forming ISM, $f_{\rm re}$ is the retained cold-baryon fraction, and $f_*$ is the efficiency of converting cold baryons into stars [2603.21837]. This decomposition is central to how Auriga’s behavior is interpreted: the main differences from comparison models arise not inside the star-forming ISM, but in the amount of gas that cools from the CGM into the ISM.

## 3. Circumgalactic regulation and the baryon cycle

A recurring conclusion across Auriga studies is that the model produces a CGM that is comparatively favorable to cooling, recycling, and long-lived baryon retention. In the APOSTLE–Auriga comparison, the dominant systematic difference is that **Auriga has a much higher efficiency of cooling and condensation from the CGM into the ISM**, quantified by
\[
f_{\rm ISM}^{\rm Auriga} \approx 1.4 \, f_{\rm ISM}^{\rm Apostle}
\]
at $z=0$ [2603.21837]. By contrast, the total amount of gas ever accreted into the halo differs by at most $\sim 15\%$ between the matched runs, and the cold baryon retention factor satisfies
\[
f_{\rm re} \approx 0.4
\]
in both models at $z=0$ [2603.21837]. This implies that Auriga’s higher stellar masses are driven primarily by enhanced CGM$\rightarrow$ISM supply rather than by a more efficient star-formation law once gas is cold.

The Local Group baryon-cycle comparison with EAGLE sharpens this picture. There, Auriga predicts that the Milky Way is **almost baryonically closed**, whereas EAGLE predicts that only half of the expected baryons reside within the halo [2106.08618]. At $z=0$, within $R_{200}$, the primary Auriga haloes in that study have
\[
f_{\rm b} \simeq 0.86-0.99,
\]
while the matched EAGLE haloes have
\[
f_{\rm b} \simeq 0.53-0.82
\]
[2106.08618]. The paper attributes this to differences in the energy injection method from supernovae to gas: EAGLE’s thermal heating drives halo-wide hot outflows at high redshift that both eject baryons and impede fresh accretion, whereas in Auriga gas accretion is almost unaffected by feedback [2106.08618].

The same study shows that, among baryons initially associated with the halo’s Lagrangian region, Auriga has very little permanently **impeded gas**—gas that never enters the halo—whereas EAGLE has a substantial permanently prevented component [2106.08618]. Auriga’s missing baryons are predominantly temporarily **ejected** and then recycled. This supports the description of Auriga as a **fountain-dominated** baryon cycle rather than a strongly preventive feedback model.

Auriga’s CGM itself is highly diverse. In a sample of isolated Milky Way–mass haloes, the covering fractions of common ions span broad ranges and correlate with stellar mass, AGN luminosity, and disc fraction [1907.04336]. The paper finds that the covering fractions of hydrogen and metals positively correlate with stellar mass, that the covering fractions of H I, C IV, and Si II anticorrelate with AGN luminosity due to ionization effects, and that the covering fractions of H I, C IV, and Si II positively correlate with disc fraction because outflows populate the CGM with cool and dense gas [1907.04336]. This suggests that the Auriga model links CGM phase structure to the morphology and feedback state of the central disc, not only to halo mass.

## 4. Milky Way analogues: masses, discs, and morphology

When applied to Milky Way–mass zoom-ins, Auriga forms well-defined disc galaxies with flat rotation curves, realistic H I discs, and structural scaling relations that compare well with observations [2502.14972]. In the matched comparison against APOSTLE, Auriga galaxies have halo masses
\[
M_{200} \sim 10^{12} M_\odot
\]
with counterpart differences below 0.1 dex, but stellar masses that are systematically larger by roughly $0.3$–$0.45$ dex [2603.21837]. The paper summarizes this as a systematic $\sim 0.4$ dex higher stellar mass in Auriga galaxies at fixed halo mass [2603.21837].

These galaxies also have higher stellar surface densities at all radii and are often slightly more compact, with somewhat smaller effective radii and larger Sérsic indices in matched pairs [2603.21837]. The stellar surface density is fit with
\[
\Sigma(R) = \Sigma_e \exp\{ -b_n [(R/R_e)^{1/n} - 1] \},
\]
and the photometric disc component with
\[
\Sigma_{\rm disk}(R) = \Sigma_0 \exp(-R/R_s)
\]
[2603.21837].

Auriga discs are generally more prominent than their matched APOSTLE counterparts. Using a kinematic decomposition based on orbital circularity
\[
\epsilon \equiv j_z / j_{\rm circ}(E),
\]
with $\epsilon > 0.7$ defining the kinematic disc, all Auriga galaxies in that comparison have kinematic
\[
D/T \gtrsim 0.4,
\]
and three of four matched Auriga systems have higher disk-to-total ratios than their APOSTLE twins [2603.21837]. Auriga discs are also systematically older at a given radius, indicating **earlier disc formation** [2603.21837].

Bars and spiral arms are especially important morphological outcomes of the Auriga model. The comparative study reports that Auriga discs show **more prominent bars and spiral arms**, and that these structures are associated with younger, more metal-rich stars in the age and metallicity maps [2603.21837]. The dedicated bar study expands this substantially: in 39 Milky Way–mass discs, bars are identified through the $m=2$ Fourier mode
\[
A_2(R) = \frac{\sqrt{a_2^2(R)+b_2^2(R)}}{a_0(R)},
\]
with barred galaxies satisfying $A_2(z=0)\ge 0.25$ and visual confirmation [2406.09453]. In that sample, the bar fraction at $z=0$ is approximately $60\%$, decreasing with redshift and plateauing around $\sim 20\%$ at $z\sim 3$ [2406.09453].

Barred Auriga galaxies tend to be more baryon-dominated, to assemble their stellar mass earlier, and to have lower Toomre $Q$ at the epoch of bar formation [2406.09453]. A central result is that barred galaxies are more baryon-dominated at all redshifts, and galaxies that are baryon-dominated but remain unbarred have higher ex-situ bulge fractions [2406.09453]. This connects bar formation in Auriga to the detailed balance between in-situ disc growth and merger-built bulges.

Spiral structure in Auriga is also diverse. In the high-resolution Auriga Superstars extension, the pattern-speed profiles show that several classical spiral theories are realized in different systems or even in the same system at different times: large-scale kinematic density waves, manifold spirals, dynamic co-rotating spirals, and overlapping modes [2602.15108]. The same galaxy may show qualitative evolution of its spiral pattern-speed profile on sub-gigayear timescales [2602.15108]. This indicates that Auriga does not impose a single spiral mechanism; rather, spiral structure emerges from the interaction of cosmological perturbations, bars, and disc dynamics.

## 5. Vertical structure, thick discs, and satellite systems

Auriga produces thin stellar discs in Milky Way–mass haloes. For the kinematic disc component, the vertical density profile is well described by
\[
\rho(z) = \rho_0 \, \text{sech}^2\left(\frac{z}{2 h_z}\right),
\]
with scale heights that increase with radius and remain below $\sim 1$ kpc inside $\sim 10$ kpc in both Auriga and APOSTLE [2603.21837]. A model-independent thickness measure,
\[
E|z| = \frac{\sum m_i |z_i|}{\sum m_i},
\]
agrees with the fitted $\mathrm{sech}^2$ model, validating that description [2603.21837]. This is consistent with the broader conclusion that Auriga forms thin, rotationally supported stellar discs despite its energetic feedback and cosmological merger histories.

The thick-disc formation study adds a chemically resolved perspective. Using 24 spiral galaxies from the Auriga zoom-in sample, it finds that thick discs are older, more metal-poor, and more [Mg/Fe]-enhanced than thin discs, but also internally complex, with contributions from in-situ formation, accreted gas, and ex-situ stars [2301.10666]. Across the sample, thick-disk regions host **7–61%** of their stellar mass in accreted stars, with a median accreted fraction of about **28%** [2301.10666]. Thick disks thus emerge in Auriga as composite structures produced by the interplay between internal enrichment and external gas and stellar accretion [2301.10666].

The model’s treatment of satellites also has characteristic consequences. In the APOSTLE–Auriga comparison, satellites within 400 kpc and with
\[
M_* \ge 10^6 M_\odot
\]
are systematically more massive in Auriga, with the most massive satellite in each halo typically about **0.3 dex** more massive than in the corresponding APOSTLE halo [2603.21837]. Yet in three of four halo pairs, the total number of satellites above $10^6 M_\odot$ is similar between the models [2603.21837]. This reflects two countervailing effects explicitly discussed in the paper: a more massive central galaxy in Auriga produces stronger tidal fields that can destroy more subhaloes, but Auriga dwarfs form more stars at fixed halo mass [2603.21837].

The satellite stellar mass–metallicity relation in both simulations lies about **0.25 dex** above the observed relation of Local Group dwarfs [2603.21837]. Auriga’s very low-mass satellites appear less extreme than APOSTLE’s, which the authors link indirectly to more efficient metal mixing in the Arepo-based framework [2603.21837]. Both models also lack a significant population of faint, blue, low-mass star-forming satellites, though the discrepancy is more pronounced in APOSTLE [2603.21837].

## 6. Extensions, robustness, and interpretive uses

Auriga has increasingly been used not only as a forward galaxy-formation model but also as an inference backbone. In **GalactiKit**, the Auriga cosmological MHD simulations define the forward mapping between progenitor merger properties at infall and the chemo-dynamical properties of their debris at $z=0$ [2502.14972]. Using simulation-based inference with Masked Autoregressive Flows, the study shows that the combined use of $E$, $L$, [Fe/H], and [$\alpha$/Fe] from stellar debris can recover infall times to about $\sim 2$ Gyr and stellar masses to about $\sim 0.5$ dex [2502.14972]. This indicates that Auriga’s merger histories, enrichment histories, and dynamical evolution encode a sufficiently informative mapping to support Galactic archaeology applications.

The model’s numerical robustness has also been quantified directly. A study of seven realizations of the same Milky Way–like halo, differing only in the random numbers used by stochastic star formation, wind launching, and AGN bubble placement, finds that global galaxy properties at $z=0$—including stellar mass, star formation history, bulge and disc masses, and disc radius and height—change by less than **10%** between realizations [2507.13440]. By contrast, the present-day star formation rate can vary by a factor of two, and detailed internal morphology such as bar strength can differ [2507.13440]. This establishes that the Auriga model is globally robust at fixed resolution, while remaining sensitive in chaotic fine-grained details.

Resolution changes matter more than stochastic variability. The same robustness study shows that lowering mass resolution by a factor of 8 reduces stellar mass by **21.1%**, lowers outer halo stellar mass by **39.4%**, makes discs **28.6%** thicker, and lowers total stellar angular momentum by **33.6%**, all shifts larger than the intrinsic run-to-run scatter [2507.13440]. This is important for interpreting cross-resolution comparisons: in Auriga, numerical resolution induces systematic changes that are larger than stochastic differences [2507.13440].

Auriga has also been extended with subgrid globular-cluster formation and evolution. The AuriGLOBES model implements star-cluster formation in **tidally compressive, high-pressure gas** and includes enhanced mass loss from compact object remnants heating [2606.30746]. In that extension, the resulting globular-cluster populations reproduce the empirical GC system mass–halo mass relation within a $2\sigma$ scatter and require both formation in compressive tides and enhanced remnant-driven mass loss to transform an initial Schechter mass function into the observed globular-cluster mass function [2606.30746]. This constitutes a substantial generalization of Auriga from galaxy formation alone to coupled galaxy–cluster population modelling.

A notable counterpoint is provided by the older globular-cluster-candidate analysis, which used old star particles as a proxy for GC formation sites. That study concluded that Auriga’s old stellar population is too metal-rich and too radially extended to be reconciled with the observed GC systems of the Milky Way and M31 under simple cluster formation and destruction assumptions [1909.02630]. The AuriGLOBES extension can be read as a response to that limitation: rather than relying on old field stars as GC proxies, it introduces explicit cluster-formation physics within the same host galaxy model.

## 7. Physical interpretation and limitations

Across these studies, the Auriga galaxy formation model is consistently interpreted as a **kinetic-wind, moving-mesh implementation** whose distinctive behavior lies in how it regulates the CGM and therefore the supply of star-forming gas [2603.21837]. Its characteristic chain of causation is: kinetic, isotropic winds plus Arepo hydrodynamics produce weaker preventive feedback and more recycling; this raises the fraction of accreted gas that cools into the ISM; that in turn yields higher stellar masses, denser discs, earlier disc formation, stronger bars and spiral arms, and more massive satellites [2603.21837].

Several strengths recur in the literature. Auriga produces thin disc galaxies in $10^{12}\,M_\odot$ haloes, early and dynamically rich discs with bars and spirals, plausible satellite counts above $10^6 M_\odot$, realistic H I discs and star-forming main-sequence behavior, and broadly successful magnetic, kinematic, and structural properties across a wide mass range [2603.21837]. The public data release emphasizes that the model compares well with the Tully–Fisher relation, the star-forming main sequence, and H I gas fraction and disc thickness, and that gas discs build rotation and settle into increasing $V/\sigma$ in rough agreement with some H$\alpha$ observations [2401.08750].

The limitations are equally clear. In matched Milky Way–mass comparisons, Auriga overproduces stellar mass relative to APOSTLE by about $\sim 0.4$ dex at fixed halo mass and yields high stellar surface densities and central concentrations [2603.21837]. Satellite metallicities are too high by about $\sim 0.25$ dex relative to Local Group dwarfs [2603.21837]. The globular-cluster proxy study found the old stellar component too metal-rich and too radially extended for realistic Milky Way/M31 GC systems [1909.02630]. The CGM study indicates that Auriga’s AGN luminosities at $z=0$ may be too high relative to many observed $L^\star$ galaxies, which can over-ionize low ions and elevate O VI columns [1907.04336]. The robustness study shows that detailed morphology is seed-sensitive in marginally unstable systems, and that resolution dependence is systematic and non-negligible [2507.13440].

A broader interpretive point follows from the controlled comparison papers. Because Auriga and its comparators can be run on identical initial conditions, differences in final galaxies can be causally tied to the **subgrid model**, especially the feedback implementation [2603.21837]. This makes Auriga an important reference model for understanding how Milky Way analogues depend on CGM cooling, wind recycling, and baryonic angular-momentum regulation. The accumulated evidence suggests that Auriga’s most consequential modeling choice is not a radical star-formation law inside the ISM, but its comparatively permissive treatment of how gas moves from halo scales into the star-forming disc [2603.21837].

Source: https://www.emergentmind.com/topics/auriga-galaxy-formation-model