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ABSTRACT Theoretical arguments and observations suggest that in massive haloes ($$>10^{12}\, {\rm M}_\odot$$), the circumgalactic medium (CGM) is dominated by a ‘hot’ phase with gas temperature near the virial temperature ($$T\approx T_{\rm vir}$$) and a quasi-hydrostatic pressure profile. Lower-mass haloes are however unlikely to be filled with a similar quasi-static hot phase, due to rapid radiative cooling. Using the FIRE (Feedback In Realistic Environment) cosmological zoom simulations, we demonstrate that the hot phase is indeed subdominant at inner radii ($$\lesssim 0.3 R_{\rm vir}$$) of $$\lesssim 10^{12}\, {\rm M}_\odot$$ haloes, and the inner CGM is instead filled with $$T\ll T_{\rm vir}$$ gas originating in outflows and inflows, with a turbulent velocity comparable to the halo virial velocity. The turbulent velocity thus exceeds the mass-weighted sound speed in the inner CGM, and the turbulence is supersonic. UV absorption features from such CGM trace the wide lognormal density distributions of the predominantly cool and turbulent volume-filling phase, in contrast with tracing localized cool ‘clouds’ embedded in a hot medium. We predict equivalent widths of $$W_\lambda \sim 2\lambda v_{\rm c}/c\sim 1$$Å for a broad range of strong UV and EUV transitions (Mg ii, C ii, C iv, Si ii–iv, O iii–v) in sightlines through inner CGM dominated by turbulent pressure of $$\lesssim L^\star$$ galaxies at redshifts $$0\le z\lesssim 2$$, where $$\lambda$$ is the transition wavelength, $$v_{\rm c}$$ is the circular velocity, and c is the speed of light. Comparison of our predictions with observational constraints suggests that star forming $$\lesssim$$ $$L^\star$$ and dwarf galaxies are generally dominated by turbulent pressure in their inner CGM, rather than by thermal pressure. The inner CGM surrounding these galaxies is thus qualitatively distinct from that around quenched galaxies and massive discs such as the Milky-Way and M31, in which thermal pressure likely dominates. 

ABSTRACT The dark matter (DM) distribution in dwarf galaxies provides crucial insights into both structure formation and the particle nature of DM. GraphNPE (Graph Neural Posterior Estimator), first introduced in Nguyen et al. (2023), is a novel simulation-based inference framework that combines graph neural networks and normalizing flows to infer the DM density profile from line-of-sight stellar velocities. Here, we apply GraphNPE to satellite dwarf galaxies in the FIRE-2 Latte simulation suite of Milky Way-mass haloes, testing it against both Cold and Self-Interacting DM scenarios. Our method demonstrates superior precision compared to conventional Jeans-based approaches, recovering DM density profiles to within the 95 per cent confidence level even in systems with as few as 30 tracers. Moreover, we present the first evaluation of mass modelling methods in constraining two key parameters from realistic simulations: the peak circular velocity, $$V_\mathrm{max}$$, and the peak virial mass, $$M_\mathrm{200m}^\mathrm{peak}$$. Using only line-of-sight velocities, GraphNPE can reliably recover both $$V_\mathrm{max}$$ and $$M_\mathrm{200m}^\mathrm{peak}$$ within our quoted uncertainties, including those experiencing tidal effects ($$\gtrsim 63~{{\rm per\ cent}}$$ of systems are recovered within our 68 per cent confidence intervals and $$\gtrsim 92~{{\rm per\ cent}}$$ within our 95 per cent confidence intervals). The method achieves $$10-20~{{\rm per\ cent}}$$ accuracy in $$V_\mathrm{max}$$ recovery, while $$M_\mathrm{200m}^\mathrm{peak}$$ is recovered to $$0.1-0.4 \, \mathrm{dex}$$ accuracy. This work establishes GraphNPE as a robust tool for inferring DM density profiles in dwarf galaxies, offering promising avenues for constraining DM models. The framework’s potential extends beyond this study, as it can be adapted to non-spherical and disequilibrium models, showcasing the broader utility of simulation-based inference and graph-based learning in astrophysics. 

Abstract The observed prevalence of galaxies exhibiting bursty star formation histories (SFHs) atz≳ 6 has created new challenges and opportunities for understanding their formation pathways. The degenerate effects of the efficiency and burstiness of star formation on the observed UV luminosity function are separable by galaxy clustering. However, quantifying the timescales of burstiness requires more than just the continuum UV measurements. Here we develop a flexible semi-analytic framework for modeling both the amplitude of star formation rate (SFR) variations and their temporal correlation, from which the luminosity function and clustering can be derived for SFR indicators tracing different characteristic timescales (e.g., UV continuum and Hα luminosities). Based on this framework, we study the prospect of using galaxy summary statistics to distinguish models where SFR fluctuations are prescribed by different power spectral density (PSD) forms. Using the Fisher matrix approach, we forecast the constraints on parameters in our PSD-based model that can be extracted from mock JWST observations of the UV and Hαluminosity functions and clustering bias factors atz∼ 6. If potential confusion due to e.g., dust attenuation and stellar population effects can be properly quantified, these results imply the possibility of probing the burstiness of high-zgalaxies with one-point and two-point statistics and highlight the benefits of combining long-term and short-term SFR tracers. Our flexible framework can be readily extended to characterize the SFH of high-redshift galaxies with a wider range of observational diagnostics. 

ABSTRACT We study the morphology of hundreds of simulated central galaxies in the stellar mass range $$M_\star =$$ 107.5–1011  $$\rm M_\odot$$ from the FIREbox cosmological volume. We demonstrate that FIREbox is able to predict a wide variety of morphologies, spanning from disc-dominated objects to spheroidal galaxies supported by stellar velocity dispersion. However, the simulations predict a strong relation between morphology (degree of rotational support) and stellar mass: galaxies comparable to the Milky Way are often disc-dominated while the presence of stellar discs mostly vanishes for dwarfs with $$M_\star < 10^9 ~$$\rm M_\odot$$. This defines a ‘morphology transition’ regime for galaxies with $$10^9 < M_\star /\rm {M_\odot }< 10^{10}$$ in which discs become increasingly common, but below which discs are rare. We show that burstiness in the star formation history and the deepening of the gravitational potential strongly correlate in our simulations with this transition regime, with discs forming in objects with lower levels of burstiness in the last $$\sim 6$$ Gyr and haloes with mass $$\sim 10^{11} ~ \rm {{\rm M}_{\odot }}$$ and above. While observations support a transition towards thicker discs in the regime of dwarfs, our results are in partial disagreement with observations of at least some largely rotationally supported gas discs in dwarfs with $$M_\star < 10^9$$\rm M_\odot$$. This study highlights dwarf morphology as a fundamental benchmark for testing future galaxy formation models. 

ABSTRACT Galactic outflows strongly influence galactic evolution and have been detected in a range of observations. Hydrodynamic simulations can help interpret these by connecting direct observables to the physical conditions of the outflowing gas. Here we use simulations of isolated disc galaxies ranging from dwarf mass ($$M_{200} = 10^{10}\, \mathrm{M}_{\odot }$$) to Milky Way mass ($$M_{200} = 10^{12}\, \mathrm{M}_{\odot }$$), based on the FIRE-2 subgrid models to investigate multiphase galactic outflows. We use the chimes non-equilibrium chemistry module to create synthetic spectra of common outflow tracers ([C ii]$$_{158\, \mu\rm m}$$, $$\mathrm{CO}_{J(1-0)}$$, H$$\alpha$$ and $$[\mathrm{O}{\small III}]_{5007\, \rm{\mathring{\rm A}}}$$). Using our synthetic spectra we measure the mass outflow rate, kinetic power and momentum flux using observational techniques. In [C ii]$$_{158\, \mu\rm m}$$ we measure outflow rates of $$10^{-4}$$ to 1 $$\mathrm{\, {\rm M}_{\odot }\, \rm yr^{-1}}$$ across an SFR range of $$10^{-3}$$ to 1 $$\text{M}_{\odot }\text{yr}^{-1}$$, which is in reasonable agreement with observations. The significant discrepancy is in $$\mathrm{CO}_{J(1-0)}$$, with the simulations lying $$\approx 1$$ dex below the observational sample. We test observational assumptions used to derive outflow properties from synthetic spectra. We find the greatest uncertainty lies in measurements of electron density, as estimates using the SII doublet can overestimate the actual electron density by up to 2 dex, which changes mass outflow rates by up to 4 dex. We also find that molecular outflows are especially sensitive to the conversion factor between CO luminosity and H2 mass, with outflow rates changing by up to 4 dex in our least massive galaxy. Comparing the outflow properties derived from the synthetic spectra to those derived directly from the simulation, we find that [C ii]$$_{158\, \mu\rm m}$$ probes outflows at greater distances from the disc, whilst we find that molecular gas does not survive at large distances within outflows within our modestly star-forming disc galaxies simulated in this work. 

ABSTRACT The circumgalactic medium (CGM) in $$\gtrsim 10^{12}\ \mathrm{M}_{\odot }$$ haloes is dominated by a hot phase ($$T \gtrsim 10^{6}$$ K). While many models exist for the hot gas structure, there is as yet no consensus. We compare cooling flow models, in which the hot CGM flows inwards due to radiative cooling, to the CGM of $$\sim 10^{12}{\,\rm to\,}10^{13}\ \mathrm{M}_{\odot }$$ haloes in galaxy formation simulations from the Feedback in Realistic Environments (FIRE) project at $$z\sim 0$$. The simulations include realistic cosmological evolution and feedback from stars but neglect AGN feedback. At both mass scales, CGM inflows are typically dominated by the hot phase rather than by the ‘precipitation’ of cold gas. Despite being highly idealized, we find that cooling flows describe $$\sim 10^{13}\ \mathrm{M}_{\odot }$$ haloes very well, with median agreement in the density and temperature profiles of $$\sim 20{{\ \rm per\ cent}}$$ and $$\sim 10{{\ \rm per\ cent}}$$, respectively. This indicates that stellar feedback has little impact on CGM scales in those haloes. For $$\sim 10^{12}\ \mathrm{M}_{\odot }$$ haloes, the thermodynamic profiles are also accurately reproduced in the outer CGM. For some of these lower-mass haloes, cooling flows significantly overpredict the hot gas density in the inner CGM. This could be due to multidimensional angular momentum effects not well captured by our one-dimensional cooling flow models and/or to the larger cold gas fractions in these regions. Turbulence, which contributes $$\sim 10{\!-\!}40{{\ \rm per\ cent}}$$ of the total pressure, must be included to accurately reproduce the temperature profiles. Overall, cooling flows predict entropy profiles in better agreement with the FIRE simulations than other idealized models in the literature. 

Abstract 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 ( $\log \left(\mathrm{sSFR}\right[{\mathrm{yr}}^{-1}\left]\right)\gtrsim -9.2$) 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. 

Abstract We report statistically significant detection of Hi21 cm emission from intermediate-redshift (z ≈ 0.2–0.6) galaxies. By leveraging multisightline galaxy survey data from the Cosmic Ultraviolet Baryon Survey and deep radio observations from the MeerKAT Absorption Line Survey, we have established a sample of ≈6000 spectroscopically identified galaxies in 11 distinct fields to constrain the neutral gas content at intermediate redshifts. The galaxies sample a broad range in stellar mass, from $\log M_{\mathrm{star}}/M_{\odot }\approx 8$to $\log M_{\mathrm{star}}/M_{\odot }\approx 11$, with a median of ${〈\log M_{\mathrm{star}}/M_{\odot }〉}_{\mathrm{med}}\approx 10$and a wide range in redshift fromz ≈ 0.24 toz ≈ 0.63 with a median of 〈z〉_med = 0.44. While no individual galaxies show detectable Hiemission, the emission line signal is detected in the stacked spectra of all subsamples at greater than 4σsignificance. The observed total Hi21 cm line flux translates to a Himass,M_{H I}≈10¹⁰M_⊙. We find a high Hi-to-stellar-mass ratio ofM_HI/M_star ≈ 6 for low-mass galaxies with $〈\log M_{\mathrm{star}}/M_{\odot }〉\approx 9.3$(>3.7σ). For galaxies with $〈\log M_{\mathrm{star}}/M_{\odot }〉\approx 10.6$, we findM_HI/M_star ≈ 0.3 (>4.7σ). In addition, the redshift evolution of Himass, 〈M_{H I}〉, in both low- and high-mass field galaxies, inferred from the stacked emission-line signal, aligns well with the expectation from the cosmic star formation history. This suggests that the overall decline in the cosmic star formation activity across the general galaxy population may be connected to a decreasing supply of neutral hydrogen. Finally, our analysis has revealed significant 21 cm signals at distances greater than 75 kpc from these intermediate-redshift galaxies, indicating a substantial reservoir of Higas in their extended surroundings. 

ABSTRACT Galaxy formation is a complex problem that connects large-scale cosmology with small-scale astrophysics over cosmic time-scales. Hydrodynamical simulations are the most principled approach to model galaxy formation, but have large computational costs. Recently, emulation techniques based on convolutional neural networks (CNNs) have been proposed to predict baryonic properties directly from dark matter simulations. The advantage of these emulators is their ability to capture relevant correlations, but at a fraction of the computational cost compared to simulations. However, training basic CNNs over large redshift ranges is challenging, due to the increasing non-linear interplay between dark matter and baryons paired with the memory inefficiency of CNNs. This work introduces EMBER-2, an improved version of the EMBER (EMulating Baryonic EnRichment) framework, to simultaneously emulate multiple baryon channels including gas density, velocity, temperature, and H i density over a large redshift range, from $z=6$ to $z=0$. EMBER-2 incorporates a context-based styling network paired with Modulated Convolutions for fast, accurate, and memory efficient emulation capable of interpolating the entire redshift range with a single CNN. Although EMBER-2 uses fewer than 1/6 the number of trainable parameters than the previous version, the model improves in every tested summary metric including gas mass conservation and cross-correlation coefficients. The EMBER-2 framework builds the foundation to produce mock catalogues of field level data and derived summary statistics that can directly be incorporated in future analysis pipelines. We release the source code at the official website https://maurbe.github.io/ember2/. 

ABSTRACT Fuelling star formation in large, discy galaxies requires a continuous supply of gas accreting into star-forming regions. Previously, we characterized this accretion in four Milky Way mass galaxies ($$M_{\rm halo}\sim 10^{12}{\rm M}_{\odot }$$) in the FIRE-2 cosmological zoom-in simulations. At $$z\sim 0$$, we found that gas within the inner circumgalactic medium (iCGM) approaches the disc with comparable angular momentum (AM) to the disc edge, joining in the outer half of the gaseous disc. Within the disc, gas moves inwards at velocities of $$\sim$$1–5 km s$$^{-1}$$ while fully rotationally supported. In this study, we analyse the torques that drive these flows. In all cases studied, we find that the torques in discs enable gas accreted near the disc edge to transport inwards and fuel star formation in the central few kpc. The primary sources of torque come from gravity, hydrodynamical forces, and the sub-grid $$P \mathrm{ d}V$$ work done by supernova (SN) remnants interacting with gas on $$\lesssim$$10 pc scales. These SNe remnant interactions induce negative torques within the inner disc and positive torques in the outer disc. The gas–gas gravitational, hydro, and ‘feedback’ torques transfer AM outwards to where accreting gas joins the disc, playing an important role in driving inflows and regulating disc structure. Gravitational torques from stars and dark matter provide an AM sink within the innermost regions of the disc and iCGM, respectively. Feedback torques are dominant within the disc, while gravitational and hydrodynamical torques have similar significance depending on the system/region. Torques from viscous shearing, magnetic forces, stellar winds, and radiative transfer are less significant.