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Azadeh Fattahi

Publications and source records attributed to Azadeh Fattahi.

At least 19 recordsLinked to original sources
arXiv2026

Slow stellar halo rotation as a signature of disc flips and GES-like mergers

The stellar halo of the Milky Way (MW) exhibits a weak net prograde rotational signal ($v_{\phi} \lesssim 25$ km~s$^{-1}$), yet its origin remains unexplained. To investigate this, we use the Auriga cosmological simulation suite of MW-mass haloes to probe the rotation of stellar haloes over the redshift range $z_{\mathrm{red}}=0-2.5$. The rotational signal is found to persist over this redshift range. We show that satellite progenitors of the stellar halo exhibit a non-uniform distribution of infall directions, with a tendency to align with the host disc, albeit with significant scatter. Thus, we attribute the net rotation to the anisotropic accretion of satellites. Haloes that host \textit{Gaia}-Enceladus-Sausage (GES)-like substructures exhibit consistently slower rotational velocity, likely due to the head-on, more radial trajectory of the dominant progenitor in such systems. Haloes whose stellar discs have reorientated by $\geq 90^{\circ}$ also exhibit slower rotation, likely because the longer dynamical timescales of haloes prevent them from quickly adjusting to disc reorientations. We also report a correlation between the rotation of stellar and dark matter haloes, suggesting a possible common origin driven by anisotropic accretion. We therefore suggest that the MW experienced a disc flip and has a slowly rotating dark matter halo.

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arXiv2026

Columba: isolated dwarf galaxy populations in diverse cosmological environments simulated with a cold interstellar medium

We introduce a suite of LambdaCDM cosmological, hydrodynamical simulations that track the evolution of a large population of dwarf galaxies. The suite comprises zoom-in simulations of 25 spherical, under-dense regions of r=5cMpc, selected to span $\approx1.5$ dex in mean enclosed density, covering voids to filamentary structures, whilst excluding haloes of Milky Way-mass or larger. The simulations achieve a mass resolution of $\sim 10^5$ solar masses with a galaxy formation model including cold, dense interstellar gas and whose subgrid stellar feedback efficiency reproduces the z=0 galaxy stellar mass function. We investigate the impact of the cosmic environment on dwarf galaxy formation and evolution. We find that the 5 cMpc environment influences the normalisation of the halo and galaxy mass functions, but does not significantly affect the stellar mass - halo mass (SMHM) relation and halo occupation fraction for galaxies with $M_{\star}=10^6-10^9$ solar masses. Instead, host halo concentration, estimated from DM-only counterparts, is more important: both the fraction of haloes hosting a resolved galaxy and the scatter about the SMHM relation correlate positively with concentration. Owing to halo assembly bias, concentration also influences galaxy formation times, such that at fixed halo mass more concentrated haloes host galaxies that are both older and more massive. The offset from the mean SMHM relation also anti-correlates with $t_{90}$, the time at which 90 percent of a galaxy's stellar mass has assembled. These correlations between halo properties and galaxy star formation histories present testable predictions for forthcoming observational surveys.

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arXiv2026

Large Scale Structure and Environmental Effects on Dwarf Galaxy Growth

Dwarf galaxies serve as key models for understanding galaxy assembly in the early universe, with their final properties influenced by environmental factors. Using the dark matter-only simulation "Copernicus Complexio" (COCO) and the semi-analytic model GALFORM, we examine the stellar mass assembly of dwarf galaxies across different cosmic web regions, defined by the NEXUS+/CACTUS algorithm. We identify significant variations in stellar mass assembly based on final mass, with the largest dwarf galaxies assembling, on average, 50% of their mass 7.7 Gyrs later than the smallest ones. Central galaxies also differ in their assembly from satellites of comparable final mass, forming 50% of their mass 2.5 Gyrs later. The location within the cosmic web further influences assembly, with satellite galaxies showing greater differences than centrals. Satellites in the densest regions assemble their mass 1.5 Gyrs earlier than those in the least dense regions, compared to 0.69 Gyrs for central galaxies. This disparity arises from varying infall times, with satellites in dense environments infalling 5.2 Gyrs earlier than those in voids. Additionally, we investigate the impact of reionisation parameters, specifically the timing ($z_{cut}$) and filtering scale ($v_{cut}$) of reionisation. The stellar-to-halo-mass relation shows a power law break between $10^8~\mathrm{M}_\odot < M_{200} < 10^{10}~\mathrm{M}_\odot$, with earlier $z_{cut}$ or higher $v_{cut}$ leading to more star formation suppression in lower-mass haloes. The halo occupation fraction is also affected, with later $z_{cut}$ or lower $v_{cut}$ resulting in fewer lower-mass haloes being occupied at $z=0$. Our investigation provides a valuable theoretical framework for interpreting upcoming observational data in this mass regime.

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arXiv2026

Implications of a contracted dark matter halo for the Milky Way's inferred virial mass

We investigate how reliably the global properties of Milky Way-mass dark matter haloes can be recovered from dynamical data over a limited radial range, particularly $\lesssim 30~\mathrm{kpc}$ where observations are most sensitive but baryonic processes modify the halo structure. Using the ARTEMIS simulations, which produce varying degrees of baryon-induced contraction, we fit dark matter profiles over restricted radial ranges using commonly adopted parametric models. Assuming negligible observational uncertainties allows the systematic errors from these choices to be isolated. When fits are confined to inner radii, an NFW profile underestimates the virial mass by a factor of $\approx 2$ on average ($\approx 4$ for some systems), and the concentration by a factor of $\approx 2$. Einasto and generalised-NFW models provide excellent local fits but retain similar global biases. In contrast, the contracted halo prescription from Cautun et al. (2020) yields stable extrapolations and recovers unbiased halo mass estimates over all radii. The inferred mass improves systematically with increasing radial coverage, and tracers beyond $\gtrsim 50~\mathrm{kpc}$ largely eliminate the mean bias for all models. The local dark matter density at the Solar radius is recovered to within $\lesssim 5\%$ for all profiles other than NFW. These biases are sufficient to reconcile recent low Milky Way mass estimates derived from inner rotation-curve analyses with the canonical $\approx 10^{12}~\mathrm{M}_\odot$. We additionally find a halo-to-halo scatter of $\gtrsim 0.1$ dex ($\approx 25\%$) persists even under idealised conditions, setting a likely lower limit for the precision of halo mass estimates.

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arXiv2026

Stirring Things Up: Bar-induced substructures in the stellar halo of a cosmological Milky Way analogue

The stellar halo of the Milky Way contains the remnants of past accretion events, which could be detectable as substructures in the classical integrals of motion space, such as energy and angular momentum (E-Lz). However, our galaxy also contains a non-axisymmetric stellar bar, which traps stars in resonant orbits, leading to substructures in phase-space. Using a high-resolution magneto-hydrodynamic cosmological zoom-in simulation of a Milky Way analogue, we explore the connection between the bar and the accreted stellar halo. We find that the bar induces prominent substructures, or "ridges", in E-Lz, caused by the resonances. The most pronounced of these is caused by the corotation and the retrograde 1:1 resonances, with weaker ridges visible due to the prograde 1:1 and outer Lindblad resonance. The ridges are present across much of the stellar halo, with variations in radius due to the morphology of different orbital families. We explore the scattering of orbits at the resonances, finding that stars trapped at the 1:1 retrograde resonance become more circularised and have more negative angular momentum. Additionally, stars can move between the corotation and retrograde 1:1 families, thus alternating between prograde and retrograde motion. Due to these scatterings and the pre-existing metallicity gradients in the accreted population, the bar-induced substructures have distinct metallicities compared to stars in the surrounding phase-space. Our results suggest the need for caution when searching the Milky Way stellar halo for accreted substructures in both integral of motions and chemical spaces, since these can be induced by internal perturbations.

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arXiv2026

Local Group analogues in a cosmological context -- I. Relating velocity structure to the cosmic web

Our Local Group, dominated in mass by the Milky Way (MW) and M31, provides a unique laboratory for testing $\Lambda$CDM cosmology on small scales owing to its proximity. However, its connection to the surrounding large-scale environment, which is essential for interpreting its properties, is inadequately understood. In this work, we explore the connection between Local Group analogues (LGAs) and their surrounding large-scale environments using the ABACUSSUMMIT simulation suite, highlighting the key role of the coupling energy of the MW-M31 orbit, $E_{\rm coupling}$. We find that LGAs with high $E_{\rm coupling}$ preferentially reside in denser regions, whereas those with low $E_{\rm coupling}$ tend to occupy low-density environments. Furthermore, LGAs with low $E_{\rm coupling}$ exhibit strong alignment with cosmic filaments, manifested as a pronounced polar anisotropy in the distribution of tracer haloes. By contrast, LGAs with high $E_{\rm coupling}$ show a weaker polar anisotropy but an enhanced azimuthal anisotropy, with large-scale tracer haloes preferentially lying in the plane spanned by the halo pair and the orbital spin vector. Within this framework, our Local Group is characterised by typical $E_{\rm coupling}$ residing in a relatively under-dense environment, yet it remains consistent with the 95\% range of analogue systems identified in the simulation.

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arXiv2025

LYRA ultra-faints: The emergence of faint dwarf galaxies in the presence of an early Lyman-Werner background

We present a suite of zoom-in cosmological hydrodynamical simulations of dwarf galaxies using the LYRA galaxy formation model with an extremely high mass resolution of $4\, \mathrm{M_{\odot}}$, evolved to $z=0$. The suite contains 65 haloes selected from Local Group like environments, spanning $M_{\mathrm{200c}}=10^7$ to $5\times10^9\, \mathrm{M_{\odot}}$. The sample includes small ultra-faints with $M_\ast\sim100\, \mathrm{M_{\odot}}$ through to classical dwarfs with $M_\ast \sim 5\times10^6 \mathrm{M_{\odot}}$, as well as haloes that remain dark to the present day. We explore two prescriptions for the high-redshift ($z>7$) Lyman-Werner background (LWB), differing in intensity and redshift evolution. Star formation begins early ($z\gtrsim8$) in progenitors with $M_{\mathrm{200c}}\sim10^5$-$10^6 \mathrm{M_{\odot}}$, where molecular hydrogen enables warm moderate-density gas to efficiently cool. The LWB strongly influences the $z=0$ halo occupation fraction, shifting the dark-to-luminous transition from $M_{\mathrm{200c}}\sim10^7 \mathrm{M_{\odot}}$ (weaker LWB) to $M_{\mathrm{200c}}\sim10^8 \mathrm{M_{\odot}}$ (stronger LWB). Galaxies with $M_\ast\gtrsim10^5 \mathrm{M_{\odot}}$ are mostly insensitive to the LWB choice, whereas lower mass systems respond strongly, producing markedly different stellar mass-halo mass (SMHM) relations. The weaker LWB yields a very shallow SMHM slope with nearly constant scatter, while the stronger LWB introduces a pronounced break at $M_{\mathrm{200c}}\sim10^9 \mathrm{M_{\odot}}$, where haloes of similar mass host galaxies with $M_\ast\sim10^3$ to $10^5 \mathrm{M_{\odot}}$ or remain dark. Both models produce a minimum stellar mass floor at $M_\ast\sim10^3 \mathrm{M_{\odot}}$, originating from galaxies that undergo a single burst of star formation at high redshift before self-quenching from their first supernovae.

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arXiv2025

Co-evolution of baryons and dark matter halos of LYRA dwarf galaxies

We use the extremely high-resolution ($m_{\rm bary}=4\rm{M}_\odot$) LYRA cosmological galaxy formation simulations of six dwarf galaxies with $M_{\rm 200c}\sim10^9\rm{M}_\odot$ at $z=0$ to investigate their stellar assembly histories. Based on the age of stars in these galaxies at $z=0$, $40-100\%$ of their stellar mass was formed by the time of reionization, when star formation (SF) abruptly shuts down. Depending on their halo mass evolution, some of the dwarfs reignite SF post-reionization (rejuvenators), while others remain quenched for the rest of cosmic time (reionization relics). However, the stellar mass of relics can still grow by more than $50\%$ through mergers post-reionisation. We find clear correlations between metallicity distributions of the galaxies and the fraction of stars formed post-reionization ($f_{\rm post-reio}^\star$) such that relics have lower median $\rm [Fe/H]$ with a more prominent low metallicity tail. Moreover, the shape of the galaxies at $z=0$ correlates with their $f_{\rm post-reio}^\star$, with rejuvenators showing more spherical stellar distribution than relics. This difference arises only post-reionization when rejuvenators become rounder with more SF activity. Similarly, the shape of dark matter (DM) halos in the inner regions display more spherical distributions in rejuvenators than in relics. The shape evolution shows that DM haloes in all galaxy formation simulations become rounder in comparison to their collision-less, DM-only counterparts. However, DM haloes of rejuvenators evolve more significantly. We do not find any correlation between SF activity and formation of shallow DM density cores in these galaxies. These predictions can be tested using upcoming observational data. In particular, our results indicate that the scatter in the mass-metallicity relation in the low mass regime is correlated with SF histories and the shape of galaxies.

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arXiv2025

The lives and deaths of faint satellite galaxies around M31

We present predictions for proper motions, infall times and times of first pericentric passage for 39 of M31's satellite galaxies. We estimate these by sampling satellite orbits from cosmological N-body simulations matched on mass, distance and velocity along the line of sight, in addition to properties of the host system. Our predictions are probabilistic based on repeated sampling from the uncertainty distributions of all quantities involved. We use these constraints on the satellites' orbital histories in conjunction with their published star formation histories to investigate the dominant environmental mechanisms for quenching satellites of M31-like hosts. Around half of the satellites appear to have quenched before their first pericentric passage around M31. Only the most massive satellites (with stellar masses > 10^8 M_sun) are able to maintain star formation for up to billions of years after infall. The majority of faint satellites, with stellar masses < 10^8 M_sun , were likely quenched before entering the M31 system. We compare our results for M31 against predictions for the Milky Way's satellites from the literature; M31's has a more active recent accretion history with more recently quenched satellites than the Milky Way.

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arXiv2025

Self-interacting dark matter in the center of a Local Group dwarf galaxy and its satellites

We present a detailed comparison of a Local Group dwarf galaxy analogue evolved in two cosmological models: the standard $\Lambda$CDM and a self-interacting dark matter (SIDM) model with a velocity-dependent cross-section. Both simulations are run with the high-resolution, hydrodynamical LYRA galaxy formation model, allowing us to explore the global and substructure properties of the dwarf in a consistent context. While the overall halo growth, final mass, and subhalo mass functions remain largely unchanged across models, SIDM produces a central dark matter core extending to $\sim$1 kpc, which does not significantly vary with the inclusion of baryons. Baryonic properties, however, differ notably. The SIDM model leads to a 25% reduction in stellar mass and retains more gas within the stellar half-mass radius due to a prolonged quiescent phase in star formation. The stellar distribution is less centrally concentrated, and a population of in-situ star clusters form at late times. Substructure analysis reveals fewer luminous satellites and more stellar-only systems in SIDM, driven in part by tidal stripping that affects the dark matter more than the stars. A subset of satellites undergoes tidal-triggered core collapse after infall, enhancing the diversity of SIDM satellite rotation curves. These differences offer potential observational signatures of SIDM in low-mass galaxies.

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arXiv2025

From order to chaos: the blurred out metallicity gradient of the Gaia-Enceladus/Sausage progenitor

The powerful combination of Gaia with other Milky Way large survey data has ushered in a deeper understanding of the assembly history of our Galaxy, which is marked by the accretion of Gaia-Enceladus/Sausage (GES). As a step towards reconstructing this significant merger, we examine the existence and destruction of its stellar metallicity gradient. We investigate 8 GES-like progenitors from the Auriga simulations and find that all have negative metallicity gradients at infall with a range of -0.09 to -0.03 dex/kpc against radius and -1.99 to -0.41 dex/$\rm 10^{-5} km^{2}s^{-2}$ against the stellar orbital energy. These gradients get blurred and become shallower when measured at $z=0$ in the Milky Way-like host. The percentage change in the radial metallicity gradient is consistently high (78-98\%), while the percentage change in the energy space varies much more (9-91\%). We also find that the most massive progenitors show the smallest changes in their energy metallicity gradients. At the same present-day galactocentric radius, lower metallicity stars originate from the outskirts of the GES progenitor. Similarly, at fixed metallicity, stars at higher galactocentric radii tend to originate from the GES outskirts. We find that the GES stellar mass, total mass, infall time, and the present-day Milky Way total mass are correlated with the percentage change in metallicity gradient, both in radius and in energy space. It is therefore vital to constrain these properties further to pin down the infall metallicity gradient of the GES progenitor and understand the onset of such ordered chemistry at cosmic noon.

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arXiv2025

Tiny galaxies and dark substructures: exploring the "dark" subhaloes in TNG50

Dark matter haloes and subhaloes that host no luminous counterpart are predicted within our current understanding of galaxy formation within a $\Lambda$CDM paradigm. Observational tests, such as gravitational lensing, have made potential detections of such objects around individual galaxies as well as in galaxy groups and clusters. The question of whether or not a dim counterpart might reside in these objects remains an open question. We investigate this point using the TNG50-1 simulation of the IllustrisTNG project. Under the assumption of TNG50's galaxy formation model, we do not find haloes or subhaloes above a total mass of $\rm 10^{9.7} \ M_{\odot}$ that are entirely dark. However, under realistic effective surface brightness cuts of $\rm \leq 29 \ mag \ arcsec^{-2}$, the inference of the most massive dark subhalo in galaxy groups and clusters becomes $\rm M_{DM} \gtrsim 2 \times 10^{10} \ M_{\odot}$. Concentrating on galaxy groups and clusters, we find that dark subhaloes are ubiquitous with more massive dark subhaloes tending to preferentially reside further from the centers of clusters. We find that subhaloes in the mass range of $\rm 4.5 \times 10^{7} \leq M_{DM}/M_{\odot} \leq 2.1\times 10^{8}$ tend to be the most likely to reside in the strong lensing regions of galaxy groups and clusters, and argue that future dark subhalo searches should investigate this mass range.

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arXiv2024

Auriga Streams I: disrupting satellites surrounding Milky Way-mass haloes at multiple resolutions

In a hierarchically formed Universe, galaxies accrete smaller systems that tidally disrupt as they evolve in the host's potential. We present a complete catalogue of disrupting galaxies accreted onto Milky Way-mass haloes from the Auriga suite of cosmological magnetohydrodynamic zoom-in simulations. We classify accretion events as intact satellites, stellar streams, or phase-mixed systems based on automated criteria calibrated to a visually classified sample, and match accretions to their counterparts in haloes re-simulated at higher resolution. Most satellites with a bound progenitor at the present day have lost substantial amounts of stellar mass -- 67 per cent have $f_\text{bound} < 0.97$ (our threshold of lost stellar mass to no longer be considered intact), while 53 per cent satisfy a more stringent $f_\text{bound} < 0.8$. Streams typically outnumber intact systems, contribute a smaller fraction of overall accreted stars, and are substantial contributors at intermediate distances from the host centre ($\sim$0.1 to $\sim$0.7$R_\text{200m}$, or $\sim$35 to $\sim$250 kpc for the Milky Way). We also identify accretion events that disrupt to form streams around massive intact satellites instead of the main host. Streams are more likely than intact or phase-mixed systems to have experienced preprocessing, suggesting this mechanism is important for setting disruption rates around Milky Way-mass haloes. All of these results are preserved across different simulation resolutions, though we do find some hints that satellites disrupt more readily at lower resolution. The Auriga haloes suggest that disrupting satellites surrounding Milky Way-mass galaxies are the norm and that a wealth of tidal features waits to be uncovered in upcoming surveys.

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arXiv2024

Apostle--Auriga: Effects of stellar feedback subgrid models on the evolution of angular momentum in disc galaxies

Utilizing the Apostle--Auriga simulations, which start from the same zoom-in initial conditions of Local Group-like systems but run with different galaxy formation subgrid models and hydrodynamic solvers, we study the impact of stellar feedback models on the evolution of angular momentum in disc galaxies. At $z = 0$, Auriga disc galaxies tend to exhibit higher specific angular momenta compared to their cross-matched Apostle counterparts. By tracing the evolution history of the Lagrangian mass tracers of the in-situ star particles in the $z = 0$ galaxies, we find that the specific angular momentum distributions of the gas tracers from the two simulations at the halo accretion time are relatively similar. The present-day angular momentum difference is mainly driven by the physical processes occurring inside dark matter haloes, especially galactic fountains. Due to the different subgrid implementations of stellar feedback processes, Auriga galaxies contain a high fraction of gas that has gone through recycled fountain (${\sim} 65$ per cent) which could acquire angular momentum through mixing with the high angular momentum circumgalactic medium (CGM). In Apostle, however, the fraction of gas that has undergone the recycled fountain process is significantly lower (down to ${\sim} 20$ per cent for Milky Way-sized galaxies) and the angular momentum acquisition from the CGM is marginal. As a result, the present-day Auriga galaxies overall have higher specific angular momenta.

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arXiv2024

The accreted stellar haloes of Milky Way-mass galaxies as a probe of the nature of the dark matter

Galactic stellar haloes are largely composed of the remnants of galaxies accreted during the assembly of their host galaxies, and hence their properties reflect the mass spectrum and post-accretion evolution of their satellites. As the nature of dark matter (DM) can affect both, we explore how the properties of the accreted stellar component vary across cold (CDM), warm (WDM) and self-interacting (SIDM) models. We do this by studying accreted stellar populations around eight MW-mass haloes using cosmological hydrodynamical simulations based on the EAGLE galaxy formation model, in which we find that the accreted stellar mass remains similar across models. Contrary to WDM, which only presents minor differences relative to CDM, the distribution of accreted stars in SIDM changes significantly within $0.05R_{200}$ ($10\,\mathrm{kpc}$). The central density reduces to $\langle \rho^{\mathrm{SIDM}}_{\mathrm{exsitu}} / \rho^{\mathrm{CDM}}_{\mathrm{exsitu}} \rangle = 0.3$ and has a shallower radial dependence, with logarithmic density slopes of $\langle \alpha_{\mathrm{SIDM}} \rangle = -1.4$ vs $\langle \alpha_{\mathrm{CDM}} \rangle = -1.7$. Additionally, stars are on more tangential orbits than their CDM counterparts, with a change in the velocity anisotropy of $\langle \Delta \beta \rangle = - 0.2$. Finally, SIDM stellar haloes have the largest number and prominence of overdensities in radius vs radial velocity space. This is due to a combination of shorter stellar halo progenitor merging timescales and shallower host potentials, with the former resulting in less time for dynamical friction and radialisation to operate. In summary, we show that the phase-space structure of galactic stellar haloes encode key information that can be used to distinguish and rule out different DM models.

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arXiv2024

ARTEMIS emulator: exploring the effect of cosmology and galaxy formation physics on Milky Way-mass haloes and their satellites

We present the new ARTEMIS Emulator suite of high resolution (baryon mass of $2.23 \times 10^{4}$ $h^{-1}$M$_{\odot}$) zoom-in simulations of Milky Way mass systems. Here, three haloes from the original ARTEMIS sample have been rerun multiple times, systematically varying parameters for the stellar feedback model, the density threshold for star formation, the reionisation redshift and the assumed warm dark matter (WDM) particle mass (assuming a thermal relic). From these simulations emulators are trained for a wide range of statistics that allow for fast predictions at combinations of parameters not originally sampled, running in $\sim 1$ms (a factor of $\sim 10^{11}$ faster than the simulations). In this paper we explore the dependence of the central haloes' stellar mass on the varied parameters, finding the stellar feedback parameters to be the most important. When constraining the parameters to match the present-day stellar mass halo mass relation inferred from abundance matching we find that there is a strong degeneracy in the stellar feedback parameters, corresponding to a freedom in formation time of the stellar component for a fixed halo assembly history. We additionally explore the dependence of the satellite stellar mass function, where it is found that variations in stellar feedback, the reionisation redshift and the WDM mass all have a significant effect. The presented emulators are a powerful tool which allows for fundamentally new ways of analysing and interpreting cosmological hydrodynamic simulations. Crucially, allowing their free (subgrid) parameters to be varied and marginalised, leading to more robust constraints and predictions.

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arXiv2024

Pre-supernova stellar feedback in nearby starburst dwarf galaxies

Stellar feedback in dwarf galaxies remains, to date, poorly explored, yet is crucial to understanding galaxy evolution in the early Universe. In particular, pre-supernova feedback has recently been found to play a significant role in regulating and disrupting star formation in larger spiral galaxies, but it remains uncertain if it also plays this role in dwarfs. We study the ionised gas properties and stellar content of individual star-forming regions across three nearby, low-metallicity, dwarf starburst galaxies (J0921, KKH046, and Leo P) to investigate how massive stars influence their surroundings and how this influence changes as a function of environment. To achieve this, we extracted integrated spectra of 30 HII regions from archival VLT/MUSE integral field spectroscopic observations of these three dwarf starburst galaxies. We fitted the HII regions' main emission lines with Gaussian profiles to derive their oxygen abundances, electron densities, and luminosities, and we used the Stochastically Lighting Up Galaxies (SLUG) code to derive the stellar mass, age, and bolometric luminosity of the stellar populations driving the HII regions. We then quantified two pre-supernova stellar feedback mechanisms, namely the direct radiation pressure and photoionisation feedback, and explored how feedback strength varies with HII region properties. Our findings suggest that stellar feedback has less of an impact on evolved regions, with both the pressure of the ionised gas and the direct radiation pressure decreasing as a function of HII region size. We also find that these stellar feedback mechanisms are dependent on the metallicity of the HII regions. These findings extend results from stellar feedback studies of more massive star-forming galaxies to the low-mass, low-metallicity regime.

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arXiv2023

Magnetic field amplification in cosmological zoom simulations from dwarf galaxies to galaxy groups

Magnetic fields are ubiquitous in the Universe. Recently, cosmological simulations of galaxies have successfully begun to incorporate magnetic fields and their evolution in galaxies and their haloes. However, so far they have mostly focused on Milky Way-like galaxies. Here we analyse a sample of high resolution cosmological zoom simulations of disc galaxies in haloes with mass $M_\mathrm{200c}$ from $10^{10}\,\mathrm{M}_\odot$ to $10^{13}\,\mathrm{M}_\odot$, simulated with the Auriga galaxy formation model. We show that with sufficient numerical resolution the magnetic field amplification and saturation is converged. The magnetic field strength reaches equipartition with turbulent energy density for galaxies in haloes with $M_\mathrm{200c}\gtrsim 10^{11.5}\,\mathrm{M_\odot}$. For galaxies in less massive haloes, the magnetic field strength saturates at a fraction of equipartition that decreases with decreasing halo mass. For our lowest mass haloes, the magnetic field saturates significantly below $10\%$ of equipartition. We quantify the resolution we need to obtain converged magnetic field strengths and discuss our resolution requirements also in the context of the IllustrisTNG cosmological box simulations. We show that, at $z=0$, rotation-dominated galaxies in our sample exhibit for the most part an ordered large scale magnetic field, with fewer field reversals in more massive galaxies. Finally, we compare the magnetic fields in our cosmological galaxies at $z=0$ with simulations of isolated galaxies in a collapsing halo setup. Our results pave the way for detailed studies of cosmic rays and other physical processes in similar cosmological galaxy simulations that crucially depend on the strength and structure of magnetic fields.

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