Dušan Kereš

We introduce a new, discretized grain size evolution model, incorporated into the gizmo code and coupled with fire-3 stellar feedback and ISM physics, to investigate variations in dust abundance, chemical composition, and grain sizes observed in the Local Group. This model tracks the size evolution of specific dust species, and includes stellar production of dust, dust growth through gas-phase metal accretion, dust destruction by sputtering, SNe shocks, and astration, grain–grain collisional shattering and coagulation, and turbulent dust diffusion. Using idealized galaxy simulations, we test the dependence of MW dust properties on variations in each dust process and find that our model uniquely predicts a bimodal grain size distribution. This bimodality is due to our simulation’s ability to resolve each dust process and where they occur in the ISM, unlike other works. We find that Local Group dust abundances are determined by dust growth and destruction, with little dependence on coagulation or shattering, explaining why models that do not include these processes can match abundance observations. We also find that variations in Local Group extinction curve slopes are determined by coagulation, with inefficient coagulation leading to steeper slopes. However, inefficient coagulation also results in stronger extinction curve bumps, which are not observed. We also do not predict a population of very small (${\lt }1$ nm) carbonaceous grains, required for MIR emission features, due to their rapid growth by accretion. These results highlight the possible necessity of ‘top-down’ PAH formation from preexisting grains as a means to inhibit carbonaceous dust growth.

We employ the high-redshift suite of FIRE-2 cosmological hydrodynamic zoom-in simulations to investigate the evolution of gas-phase metallicity radial gradients in galaxies in the epoch of reionization (EOR). Our sample consists of 22 galaxies spanning the redshift range z ∼ 10–5. We find that galaxies at z ∼ 10 exhibit a median metallicity gradient of −0.15 dex kpc ⁻¹ with substantial scatter, which gradually flattens to −0.1 dex kpc ⁻¹ at z ∼ 6, accompanied by a reduction in scatter. In the EOR, metallicity gradients correlate positively with stellar mass: more massive galaxies display flatter gradients with smaller scatter, broadly consistent with recent JWST observations. At fixed stellar mass, galaxies with a higher star formation rate (SFR) exhibit steeper negative gradients, while the specific SFR shows a strong anticorrelation with gradient slope. Because EOR galaxies in FIRE-2 generally lack significant rotational support, we adopt the ratio of the peak-to-peak velocity shear to twice the velocity dispersion (Δ v /2 σ ) as a proxy for the strength of gas flows. We find a strong positive correlation between metallicity gradients and Δ v /2 σ : galaxies with a lower Δ v /2 σ (i.e., weaker gas flows) tend to exhibit steeper negative gradients. Furthermore, galaxies with a steeper gradient display higher central SFR surface densities, suggesting localized star formation with inefficient interstellar medium mixing that drives inside-out chemical enrichment in galaxy evolution in the early Universe.

Many state-of-the-art galaxy simulations featuring traditional feedback modes have significant challenges producing enough extended soft X-ray (∼0.5−2 keV) emission at R ∼ 0.5 R _vir −1 R _vir observed around galaxies with stellar masses M _* ≲ 10 ¹¹ M _⊙ , without violating galaxy mass function constraints. Moreover, thermal Sunyaev–Zel’dovich (tSZ) measurements probing the thermal pressure of similar galaxies indicate that it is orders of magnitude lower than predictions from simple halo hydrodynamics and many hydrodynamical simulations. We demonstrate that these constraints can be met congruously with a large nonthermal pressure contribution in the form of cosmic rays (CRs) from supernovae and/or active galactic nuclei (AGN), which lowers the tSZ signal, while CR leptons produce plentiful soft X-rays via inverse Compton scattering of the cosmic microwave background. The combination of these two observations is far more constraining on the pressure budget of galactic halos than either alone—if these novel tSZ and X-ray observations are borne out by future studies, then taken together they reveal the strongest evidence for CR support in halos to date . Conversely, it is very difficult to produce the extended X-rays via traditional thermal emission without increasing the overall thermal pressure and thus tSZ signal in tandem, making these tensions even worse. Finally, tSZ and X-rays together unlock a novel observational method to constrain halo CR pressure relative to thermal pressure, with implications for CR transport parameters and AGN feedback energetics across various galaxy mass scales. Taking the currently observed constraints at M _halo ∼ 10 ¹² M _⊙ imply that the halo CR pressure must at least be equal to the gas thermal pressure.

Using the FIRE-2 cosmological zoom-in simulations, we investigate the temporal evolution of gas-phase metallicity radial gradients of Milky Way–mass progenitors in the redshift range of 0.4 < z < 3. We pay special attention to the occurrence of positive (i.e., inverted) metallicity gradients—where metallicity increases with galactocentric radius. This trend, contrary to the more commonly observed negative radial gradients, has been frequently seen in recent spatially resolved grism observations. The rate of occurrence of positive gradients in FIRE-2 is about ∼7% for 0.4 < z < 3 and ∼13% at higher redshifts (1.5 < z < 3), broadly consistent with observations. Moreover, we investigate the correlations among galaxy metallicity gradient, stellar mass, star formation rate (SFR), and degree of rotational support. Metallicity gradients show a strong correlation with both sSFR and the rotational-to-dispersion velocity ratio (v _c /σ), implying that starbursts and kinematic morphology of galaxies play significant roles in shaping these gradients. The FIRE-2 simulations indicate that galaxies with high sSFR ( log ( sSFR [ yr − 1 ] ) ≳ − 9.2 ) and weak rotational support (v _c /σ ≲ 1) are more likely—by ∼15%—to develop positive metallicity gradients. This trend is attributed to galaxy-scale gas flows driven by stellar feedback, which effectively redistribute metals within the interstellar medium. Our results support the important role of stellar feedback in governing the chemo-structural evolution and disk formation of Milky Way–mass galaxies at the cosmic noon epoch.

The far-infrared–radio correlation (FRC) is one of the most promising empirical constraints on the role of cosmic rays (CRs) and magnetic fields in galaxy formation and evolution. While many theories have been proposed in order to explain the emergence and maintenance of the FRC across a gamut of galaxy properties and redshift, the nonlinear physics at play remain unexplored in full complexity and within a cosmological context. We present the first reproduction of the z ∼ 0 FRC using detailed synthetic observations of state-of-the-art cosmological zoom-in simulations from the Feedback in Realistic Environments (FIRE-3) suite with explicitly evolved CR proton and electron (CRe) spectra, for three models for CR transport and multichannel active galactic nucleus (AGN) feedback. In doing so, we generally verify the predictions of “calorimeter” theories at high FIR luminosities (L _{60 μm} ≳ 10^9.5 L _⊙) and at low FIR luminosities (L _{60 μm} ≲ 10^9.5 L _⊙), the so-called “conspiracy” of increasing UV radiation escape in tandem with increasing CRe escape, and find that the global FRC is insensitive to orders-of-magnitude locally variable CR transport coefficients. Importantly, the indirect effect of AGN feedback on emergent observables highlights novel interpretations of outliers in the FRC. In particular, we find that in many cases “radio-excess” objects can be better understood as “IR-dim” objects with longer-lived radio contributions at low z from Type Ia supernovae and intermittent black hole accretion in quenching galaxies, though this is sensitive to the interplay of CR transport and AGN feedback physics. This creates characteristic evolutionary tracks leading to the z = 0 FRC, which shape the subsequent late-time behavior of each model.

Simulations and observations suggest that galaxy interactions may enhance the star formation rate (SFR) in merging galaxies. One proposed mechanism is the torque exerted on the gas and stars in the larger galaxy by the smaller galaxy. We analyze the interaction torques and star formation activity on six galaxies from the FIRE-2 simulation suite with masses comparable to the Milky Way galaxy at redshift z = 0. We trace the halos from z = 3.6 to z = 0, calculating the torque exerted by the nearby galaxies on the gas in the central galaxy. We calculate the correlation between the torque and the SFR across the simulations for various mass ratios. For near-equal-stellar-mass-ratio interactions in the galaxy sample, occurring between z = 1.2−3.6, there is a positive and statistically significant correlation between the torque from nearby galaxies on the gas of the central galaxies and the SFR. For all other samples, no statistically significant correlation is found between the torque and the SFR. Our analysis shows that some, but not all, major interactions cause starbursts in the simulated Milky Way-mass galaxies, and that most starbursts are not caused by galaxy interactions. The transition from “bursty” at high redshift (z ≳ 1) to “steady” star formation state at later times is independent of the interaction history of the galaxies, and most of the interactions do not leave significant imprints on the overall trend of the star formation history of the galaxies.