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Philip F. Hopkins

Publications and source records attributed to Philip F. Hopkins.

At least 109 records · Page 6Linked to original sources
arXiv2024

Synchrotron Emission on FIRE: Equipartition Estimators of Magnetic Fields in Simulated Galaxies with Spectrally-Resolved Cosmic Rays

Synchrotron emission is one of few observable tracers of galactic magnetic fields (\textbf{B}) and cosmic rays (CRs). Much of our understanding of \textbf{B} in galaxies comes from utilizing synchrotron observations in conjunction with several simplifying assumptions of equipartition models, however it remains unclear how well these assumptions hold, and what \textbf{B} these estimates physically represent. Using FIRE simulations which self consistently evolve CR proton, electron, and positron spectra from MeV to TeV energies, we present the first synthetic synchrotron emission predictions from simulated L$_{*}$ galaxies with "live" spectrally-resolved CR-MHD. We find that synchrotron emission can be dominated by relatively cool and dense gas, resulting in equipartition estimates of \textbf{B} with fiducial assumptions underestimating the "true" \textbf{B} in the gas that contributes the most emission by factors of 2-3 due to small volume filling factors. Motivated by our results, we present an analytic framework that expands upon equipartition models for estimating \textbf{B} in a multi-phase medium. Comparing our spectrally-resolved synchrotron predictions to simpler spectral assumptions used in galaxy simulations with CRs, we find that spectral evolution can be crucial for accurate synchrotron calculations towards galactic centers, where loss terms are large.

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arXiv2023

Observational Signatures of AGN Feedback in the Morphology and the Ionization States of Milky Way-like Galaxies

We make an in-depth analysis of different AGN jet models' signatures, inducing quiescence in galaxies with a halo mass of $10^{12} M_\odot$. Three jet models, including cosmic ray-dominant, hot thermal, and precessing kinetic jets, are studied at two energy flux levels each, compared to a jet-free, stellar feedback-only simulation. We examine the distribution of Mg II, O VI, and O VIII ions, alongside gas temperature and density profiles. Low-energy ions, like Mg II, concentrate in the ISM, while higher energy ions, e.g., O VIII, prevail at the AGN jet cocoon's edge. High-energy flux jets display an isotropic ion distribution with lower overall density. High-energy thermal or cosmic ray jets pressurize at smaller radii, significantly suppressing core density. The cosmic ray jet provides extra pressure support, extending cool and warm gas distribution. A break in the ion-to-mass ratio slope in O VI and O VIII is demonstrated in the ISM-to-CGM transition (between 10-30 kpc), growing smoothly towards the CGM at greater distances.

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arXiv2023

Constraining the H2 column densities in the diffuse interstellar medium using dust extinction and HI data

Carbon monoxide (CO) is a poor tracer of H$_{2}$ in the diffuse interstellar medium (ISM), where most of the carbon is not incorporated into CO molecules unlike the situation at higher extinctions. We present a novel, indirect method to constrain H$_{2}$ column densities ($N_{H_{2}}$) without employing CO observations. We show that previously-recognized nonlinearities in the relation between the extinction, $A_{V} ({H}_{2})$, derived from dust emission and the HI column density ($N_{HI}$) are due to the presence of molecular gas. We employ archival $N_{H_{2}}$ data, obtained from the UV spectra of stars, and calculate $A_{V} ({H}_{2})$ towards these sight lines using 3D extinction maps. We derive an empirical relation between $A_{V} ({H}_{2})$ and $N_{H_{2}}$, which we use to constrain $N_{H_{2}}$ in the diffuse ISM. We construct a $N_{H_{2}}$ map of our Galaxy and compare it to the CO integrated intensity ($W_{CO}$) distribution. We find that the average ratio ($X_{CO}$) between $N_{H_{2}}$ and $W_{CO}$ is approximately equal to $2 \times 10^{20}$ cm$^{-2}$ (K km s$^{-1}$)$^{-1}$, consistent with previous estimates. However, we find that the $X_{CO}$ factor varies by orders of magnitude on arcminute scales between the outer and the central portions of molecular clouds. For regions with $N_{H_{2}} \gtrsim 10^{20}$ cm$^{-2}$, we estimate that the average H$_{2}$ fractional abundance, $f_{{H}_{2}}$ = $2N_{H_{2}}$/(2$N_{H_{2}}$ + $N_{HI}$), is 0.25. Multiple (distinct) largely atomic clouds are likely found along high-extinction sightlines ($A_{V} \geq 1$ mag), hence limiting $f_{{H}_{2}}$ in these directions. More than $50 \%$ of the lines of sight with $N_{H_{2}} \geq 10^{20}$ cm$^{-2}$ are untraceable by CO with a $J$ = 1-0 sensitivity limit $W_{CO} = 1$ K km s$^{-1}$.

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arXiv2023

Does God play dice with star clusters?

When a detailed model of a stellar population is unavailable, it is most common to assume that stellar masses are independently and identically distributed according to some distribution: the universal initial mass function (IMF). However, stellar masses resulting from causal, long-ranged physics cannot be truly random and independent, and the IMF may vary with environment. To compare stochastic sampling with a physical model, we run a suite of 100 STARFORGE radiation magnetohydrodynamics simulations of low-mass star cluster formation in $2000M_\odot$ clouds that form $\sim 200$ stars each on average. The stacked IMF from the simulated clouds has a sharp truncation at $\sim 28 M_\odot$, well below the typically-assumed maximum stellar mass $M_{\rm up} \sim 100-150M_\odot$ and the total cluster mass. The sequence of star formation is not totally random: massive stars tend to start accreting sooner and finish later than the average star. However, final cluster properties such as maximum stellar mass and total luminosity have a similar amount of cloud-to-cloud scatter to random sampling. Therefore stochastic sampling does not generally model the stellar demographics of a star cluster as it is forming, but may describe the end result fairly well, if the correct IMF -- and its environment-dependent upper cutoff -- are known.

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arXiv2023

$\rm [C_{II}]$ 158 $\rm μm$ emission as an indicator of galaxy star formation rate

Observations of local star-forming galaxies (SFGs) show a tight correlation between their singly ionized carbon line luminosity ($L_{\rm [C_{II}]}$) and star formation rate (SFR), suggesting that $L_{\rm [C_{II}]}$ may be a useful SFR tracer for galaxies. Some other galaxy populations, however, are found to have lower $L_{\rm [C_{II}]}{}/{}\rm SFR$ than the local SFGs, including the infrared-luminous, starburst galaxies at low and high redshifts, as well as some moderately star-forming galaxies at the epoch of re-ionization (EoR). The origin of this `$\rm [C_{II}]$ deficit' is unclear. In this work, we study the $L_{\rm [C_{II}]}$-SFR relation of galaxies using a sample of $z=0-8$ galaxies with $M_*\approx10^7-5\times10^{11}\,M_\odot$ extracted from cosmological volume and zoom-in simulations from the Feedback in Realistic Environments (FIRE) project. We find a simple analytic expression for $L_{\rm [C_{II}]}$/SFR of galaxies in terms of the following parameters: mass fraction of $\rm [C_{II}]$-emitting gas ($f_{\rm [C_{II}]}$), gas metallicity ($Z_{\rm gas}$), gas density ($n_{\rm gas}$) and gas depletion time ($t_{\rm dep}{}={}M_{\rm gas}{}/{}\rm SFR$). We find two distinct physical regimes, where $t_{\rm dep}$ ($Z_{\rm gas}$) is the main driver of the $\rm [C_{II}]$ deficit in $\rm H_2$-rich ($\rm H_2$-poor) galaxies. The observed $\rm [C_{II}]$ deficit of IR-luminous galaxies and early EoR galaxies, corresponding to the two different regimes, is due to short gas depletion time and low gas metallicity, respectively. Our result indicates that $\rm [C_{II}]$ deficit is a common phenomenon of galaxies, and caution needs to be taken when applying a constant $L_{\rm [C_{II}]}$-to-SFR conversion factor derived from local SFGs to estimate cosmic SFR density at high redshifts and interpret data from upcoming $\rm [C_{II}]$ line intensity mapping experiments.

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arXiv2023

Scaling K2 VII: Evidence for a high occurrence rate of hot sub-Neptunes at intermediate ages

The NASA K2 mission obtained high precision time-series photometry for four young clusters, including the near-twin 600-800 Myr-old Praesepe and Hyades clusters. Hot sub-Neptunes are highly prone to mass-loss mechanisms, given their proximity to the the host star and the weakly bound gaseous envelopes, and analyzing this population at young ages can provide strong constraints on planetary evolution models. Using our automated transit detection pipeline, we recover 15 planet candidates across the two clusters, including 10 previously confirmed planets. We find a hot sub-Neptune occurrence rate of 79-107% for GKM stars in the Praesepe cluster. This is 2.5-3.5 sigma higher than the occurrence rate of 16.54+1.00-0.98% for the same planets orbiting the ~3-9 Gyr-old GKM field stars observed by K2, even after accounting for the slightly super-solar metallicity ([Fe/H]~0.2 dex) of the Praesepe cluster. We examine the effect of adding ~100 targets from the Hyades cluster, and extending the planet parameter space under examination, and find similarly high occurrence rates in both cases. The high occurrence rate of young, hot sub-Neptunes could indicate either that these planets are undergoing atmospheric evolution as they age, or that planetary systems that formed when the Galaxy was much younger are substantially different than from today. Under the assumption of the atmospheric mass-loss scenario, a significantly higher occurrence rate of these planets at the intermediate ages of Praesepe and Hyades appears more consistent with the core-powered mass loss scenario sculpting the hot sub-Neptune population, compared to the photoevaporation scenario.

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arXiv2023

Disk settling and dynamical heating: histories of Milky Way-mass stellar disks across cosmic time in the FIRE simulations

We study the kinematics of stars both at their formation and today within 14 Milky Way (MW)-mass galaxies from the FIRE-2 cosmological zoom-in simulations. We quantify the relative importance of cosmological disk settling and post-formation dynamical heating. We identify three eras: a Pre-Disk Era (typically >8 Gyr ago), when stars formed on dispersion-dominated orbits; an Early-Disk Era (~ 8 - 4 Gyr ago), when stars started to form on rotation-dominated orbits but with high velocity dispersion, sigma_form; and a Late-Disk Era (< 4 Gyr ago), when stars formed with low sigma_form. sigma_form increased with time during the Pre-Disk Era, peaking ~ 8 Gyr ago, then decreased throughout the Early-Disk Era as the disk settled and remained low throughout the Late-Disk Era. By contrast, the velocity dispersion measured today, sigma_now, increases monotonically with age because of stronger post-formation heating for Pre-Disk stars. Importantly, most of sigma_now was in place at formation, not added post-formation, for stars younger than ~ 10 Gyr. We compare the evolution of the three velocity components: at all times, sigma_R,form > sigma_phi,form > sigma_Z,form. Post-formation heating primarily increased sigma_R at ages < 4 Gyr but acted nearly isotropically for older stars. The kinematics of young stars in FIRE-2 broadly agree with the range observed across the MW, M31, M33, and PHANGS-MUSE galaxies. The lookback time that the disk began to settle correlates with its dynamical state today: earlier-settling galaxies currently form colder disks. Including stellar cosmic-ray feedback does not significantly change disk rotational support at fixed stellar mass.

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arXiv2023

Unraveling Jet Quenching Criteria Across L* Galaxies and Massive Cluster Ellipticals

In the absence of supplementary heat, the radiative cooling of halo gas around massive galaxies (Milky Way mass and above) leads to an excess of cold gas or stars beyond observed levels. AGN jet-induced heating is likely essential, but the specific properties of the jets remain unclear. Our previous work (Su et al. 2021) concludes from simulations of a halo with $10^{14} M_\odot$ that a successful jet model should have an energy flux comparable to the free-fall energy flux at the cooling radius and should inflate a sufficiently wide cocoon with a long enough cooling time. In this paper, we investigate three jet modes with constant fluxes satisfying the criteria, including high-temperature thermal jets, cosmic ray (CR)-dominant jets, and widely precessing kinetic jets in $10^{12}-10^{15}\,{\rm M}_{\odot}$ halos using high-resolution, non-cosmological MHD simulations with the FIRE-2 (Feedback In Realistic Environments) stellar feedback model, conduction, and viscosity. We find that scaling the jet energy according to the free-fall energy at the cooling radius can successfully suppress the cooling flows and quench galaxies without obviously violating observational constraints. We investigate an alternative scaling method in which we adjust the energy flux based on the total cooling rate within the cooling radius. However, we observe that the strong interstellar medium (ISM) cooling dominates the total cooling rate in this scaling approach, resulting in a jet flux that exceeds the amount needed to suppress the cooling flows. With the same energy flux, the CR-dominant jet is most effective in suppressing the cooling flow across all the surveyed halo masses due to the enhanced CR pressure support. We confirm that the criteria for a successful jet model, which we proposed in Su et al. (2021), work across a much wider range, encompassing halo masses of $10^{12}-10^{15} {\rm M_\odot}$.

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arXiv2023

TRINITY IV: Predictions for Supermassive Black Holes at $z \gtrsim 7$

We present predictions for the high-redshift halo-galaxy-supermassive black hole (SMBH) connection from the TRINITY model. Constrained by a comprehensive compilation of galaxy ($0\leq z \leq 10$) and SMBH datasets ($0\leq z \leq 6.5$), TRINITY finds: 1) The number of SMBHs with $M_\bullet > 10^9 M_\odot$ in the observable Universe increases by six orders of magnitude from $z\sim10$ to $z\sim2$, and by another factor of $\sim 3$ from $z\sim2$ to $z=0$; 2) The $M_\bullet > 10^9/10^{10} M_\odot$ SMBHs at $z\sim 6$ live in haloes with $\sim (2-3)/(3-5) \times 10^{12} M_\odot$; 3) the new JWST AGNs at $7\lesssim z \lesssim 11$ are broadly consistent with the median SMBH mass-galaxy mass relation for AGNs from TRINITY; 4) Seeds from runaway mergers in nuclear star clusters are viable progenitors for the SMBHs in GN-z11 ($z=10.6$) and CEERS_1019 ($z=8.7$); 5) $z=6-10$ quasar luminosity functions from wide area surveys by, e.g., Roman and Euclid, will reduce uncertainties in the $z=6-10$ SMBH mass-galaxy mass relation by up to $\sim 0.5$ dex.

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arXiv2023

The opacity limit

The opacity limit is an important concept in star formation: isothermal collapse cannot proceed without limit, because eventually cooling radiation is trapped and the temperature rises quasi-adiabatically, setting a minimum Jeans mass $M_{\rm J}^{\rm min}$. Various works have considered this scenario and derived expressions for $M_{\rm J}^{\rm min}$, generally $\sim 10^{-3}-10^{-2}M_\odot$ in normal star-forming conditions, but with conflicting results about the scaling with ambient conditions and material properties. We derive expressions for the thermal evolution of dust-cooled collapsing gas clumps in various limiting cases, given a general ambient radiation field ($u_{\rm rad}$, $T_{\rm rad}$) and a general power-law dust opacity law $σ_{\rm d} = A_{\rm d} T^β$. By accounting for temperature evolution self-consistently we rule out a previously-proposed regime in which the adiabatic transition occurs while the core is still optically-thin. If the radiation field is weak or dust opacity is small, $M_{\rm J}^{\rm min}$ is insensitive to dust properties/abundance ($\sim A_{\rm d}^{-\frac{1}{11}}-A_{\rm d}^{-\frac{1}{15}}$), but if the radiation field is strong and dust is abundant it scales $\propto A_{\rm d}^{1/3}$. This could make the IMF less bottom-heavy in dust-rich and/or radiation-dense environments, e.g. galactic centers, starburst galaxies, massive high-$z$ galaxies, and proto-star clusters that are already luminous.

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arXiv2023

What Causes The Formation of Disks and End of Bursty Star Formation?

As they grow, galaxies can transition from irregular/spheroidal with 'bursty' star formation histories (SFHs), to disky with smooth SFHs. But even in simulations, the direct physical cause of such transitions remains unclear. We therefore explore this in a large suite of numerical experiments re-running portions of cosmological simulations with widely varied physics, further validated with existing FIRE simulations. We show that gas supply, cooling/thermodynamics, star formation model, Toomre scale, galaxy dynamical times, and feedback properties do not have a direct causal effect on these transitions. Rather, both the formation of disks and cessation of bursty star formation are driven by the gravitational potential, but in different ways. Disk formation is promoted when the mass profile becomes sufficiently centrally-concentrated in shape (relative to circularization radii): we show that this provides a well-defined dynamical center, ceases to support the global 'breathing modes' which can persist indefinitely in less-concentrated profiles and efficiently destroy disks, promotes orbit mixing to form a coherent angular momentum, and stabilizes the disk. Smooth SF is promoted by the potential or escape velocity (not circular velocity) becoming sufficiently large at the radii of star formation that cool, mass-loaded (momentum-conserving) outflows are trapped/confined near the galaxy, as opposed to escaping after bursts. We discuss the detailed physics, how these conditions arise in cosmological contexts, their relation to other correlated phenomena (e.g. inner halo virialization, vertical disk 'settling'), and observations.

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arXiv2023

Novel Conservative Methods for Adaptive Force Softening in Collisionless and Multi-Species N-Body Simulations

Modeling self-gravity of collisionless fluids (e.g. ensembles of dark matter, stars, black holes, dust, planetary bodies) in simulations is challenging and requires some force softening. It is often desirable to allow softenings to evolve adaptively, in any high-dynamic range simulation, but this poses unique challenges of consistency, conservation, and accuracy, especially in multi-physics simulations where species with different softening laws may interact. We therefore derive a generalized form of the energy-and-momentum conserving gravitational equations of motion, applicable to arbitrary rules used to determine the force softening, together with consistent associated timestep criteria, interaction terms between species with different softening laws, and arbitrary maximum/minimum softenings. We also derive new methods to maintain better accuracy and conservation when symmetrizing forces between particles. We review and extend previously-discussed adaptive softening schemes based on the local neighbor particle density, and present several new schemes for scaling the softening with properties of the gravitational field, i.e. the potential or acceleration or tidal tensor. We show that the tidal softening scheme not only represents a physically-motivated, translation and Galilean invariant and equivalence-principle respecting (and therefore conservative) method, but imposes negligible timestep or other computational penalties, ensures that pairwise two-body scattering is small compared to smooth background forces, and can resolve outstanding challenges in properly capturing tidal disruption of substructures (minimizing artificial destruction) while also avoiding excessive N-body heating. We make all of this public in the GIZMO code.

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arXiv2023

A Simple Sub-Grid Model For Cosmic Ray Effects on Galactic Scales

Many recent numerical studies have argued that cosmic rays (CRs) from supernovae (SNe) or active galactic nuclei (AGN) could play a crucial role in galaxy formation, in particular by establishing a CR-pressure dominated circum-galactic medium (CGM). But explicit CR-magneto-hydrodynamics (CR-MHD) remains computationally expensive, and it is not clear whether those results can be applied to simulations that do not explicitly treat magnetic fields or resolved ISM phase structure. We therefore present an intentionally extremely-simplified sub-grid model for CRs, which attempts to capture the key qualitative behaviors of greatest interest for those interested in simulations or semi-analytic models including some approximate CR effects on galactic (>kpc) scales, while imposing negligible computational overhead. The model is numerically akin to some recently-developed sub-grid models for radiative feedback, and allows for a simple constant parameterization of the CR diffusivity and/or streaming speed; it allows for an arbitrary distribution of sources (proportional to black hole accretion rates or star-particle SNe rates or gas/galaxy star formation rates), and interpolates between the limits where CRs escape the galaxies with negligible losses and those where CRs lose most of their energy catastrophically before escape (relevant in e.g. starburst galaxies). The numerical equations are solved trivially alongside gravity in most codes. We compare this to explicit CR-MHD simulations and discuss where the (many) sub-grid approximations break down, and what drives the major sources of uncertainty.

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arXiv2023

Local positive feedback in the overall negative: the impact of quasar winds on star formation in the FIRE cosmological simulations

Negative feedback from accreting supermassive black holes is regarded as a key ingredient in suppressing star formation and quenching massive galaxies. However, several models and observations suggest that black hole feedback may have a positive effect, triggering star formation by compressing interstellar medium gas to higher densities. We investigate the dual role of black hole feedback using cosmological hydrodynamic simulations from the Feedback In Realistic Environments (FIRE) project, including a novel implementation of hyper-refined accretion-disc winds. Focusing on a massive, star-forming galaxy at $z \sim 2$ ($M_{\rm halo} \sim 10^{12.5} \, {\rm M}_{\odot}$), we show that strong quasar winds with kinetic power $\sim$10$^{46}$ erg/s acting for $>$20$\,$Myr drive the formation of a central gas cavity and can dramatically reduce the star formation rate surface density across the galaxy disc. The suppression of star formation is primarily driven by reducing the amount of gas that can become star-forming, compared to directly evacuating the pre-existing star-forming gas reservoir (preventive feedback dominates over ejective feedback). Despite the global negative impact of quasar winds, we identify several plausible signatures of local positive feedback, including: (1) spatial anti-correlation of wind-dominated regions and star-forming clumps, (2) higher local star formation efficiency in compressed gas near the edge of the cavity, and (3) increased local contribution of outflowing material to star formation. Stars forming under the presence of quasar winds tend to do so at larger radial distances. Our results suggest that positive and negative AGN feedback can coexist in galaxies, but local positive triggering of star formation plays a minor role in global galaxy growth.

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arXiv2023

Born this way: thin disc, thick disc, and isotropic spheroid formation in FIRE-2 Milky-Way-mass galaxy simulations

We investigate the formation of Milky-Way-mass galaxies using FIRE-2 LCDM cosmological zoom-in simulations by studying the orbital evolution of stars formed in the main progenitor of the galaxy, from birth to the present day. We classify in situ stars as isotropic spheroid, thick-disc, and thin-disc according to their orbital circularities and show that these components are assembled in a time-ordered sequence from early to late times, respectively. All simulated galaxies experience an early phase of bursty star formation that transitions to a late-time steady phase. This transition coincides with the time that the inner CGM virializes. During the early bursty phase, galaxies have irregular morphologies and new stars are born on radial orbits; these stars evolve into an isotropic spheroidal population today. The bulk of thick-disc stars form at intermediate times, during a clumpy-disc ``spin-up'' phase, slightly later than the peak of spheroid formation. At late times, once the CGM virializes and star formation ``cools down," stars are born on circular orbits within a narrow plane. Those stars mostly inhabit thin discs today. Broadly speaking, stars with disc-like or spheroid-like orbits today were born that way. Mergers onto discs and secular processes do affect kinematics in our simulations, but play only secondary roles in populating thick-disc and in situ spheroid populations at z=0. The age distributions of spheroid, thick disc, and thin disc populations scale self-similarly with the steady-phase transition time, which suggests that morphological age dating can be linked to the CGM virialization time in galaxies.

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arXiv2023

A New Discrete Dynamical Friction Estimator Based on $N$-body Simulations

A longstanding problem in galactic simulations is to resolve the dynamical friction (DF) force acting on massive black hole particles when their masses are comparable to or less than the background simulation particles. Many sub-grid models based on the traditional Chandrasekhar DF formula have been proposed, yet they suffer from fundamental ambiguities in the definition of some terms in Chandrasekhar's formula when applied to real galaxies, as well as difficulty in evaluating continuous quantities from (spatially) discrete simulation data. In this work we present a new sub-grid dynamical friction estimator based on the discrete nature of $N$-body simulations, which also avoids the ambiguously-defined quantities in Chandrasekhar's formula. We test our estimator in the GIZMO code and find that it agrees well with high-resolution simulations where DF is fully captured, with negligible additional computational cost. We also compare it with a Chandrasekhar estimator and discuss its applications in real galactic simulations.

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arXiv2023

FIREbox: Simulating galaxies at high dynamic range in a cosmological volume

We introduce a suite of cosmological volume simulations to study the evolution of galaxies as part of the Feedback in Realistic Environments project. FIREbox, the principal simulation of the present suite, provides a representative sample of galaxies (~1000 galaxies with Mstar > 10^8 Msun at z=0) at a resolution (~20 pc, m_b ~ 6x10^4 Msun) comparable to state-of-the-art galaxy zoom-in simulations. FIREbox captures the multiphase nature of the interstellar medium in a fully cosmological setting (L=22.1 Mpc) thanks to its exceptionally high dynamic range (~10^6) and the inclusion of multi-channel stellar feedback. Here, we focus on validating the simulation predictions by comparing to observational data. We find that simulated galaxies with Mstar < 10^{10.5-11} Msun have star formation rates, gas masses, and metallicities in broad agreement with observations. These galaxy scaling relations extend to low masses (Mstar ~ 10^7 Msun) and follow a (broken) power-law relationship. Also reproduced are the evolution of the cosmic HI density and the HI column density distribution at z~0-5. At low z, FIREbox predicts a peak in the stellar-mass--halo-mass relation, but also a higher abundance of massive galaxies and a higher cosmic star formation rate density than observed, showing that stellar feedback alone is insufficient to reproduce the properties of massive galaxies at late times. Given its high resolution and sample size, FIREbox offers a baseline prediction of galaxy formation theory in a $Λ$CDM Universe while also highlighting modeling challenges to be addressed in next-generation galaxy simulations.

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arXiv2023

Simulating Atomic Dark Matter in Milky Way Analogues

Dark sector theories naturally lead to multi-component scenarios for dark matter where a sub-component can dissipate energy through self-interactions, allowing it to efficiently cool inside galaxies. We present the first cosmological hydrodynamical simulations of Milky Way analogues where the majority of dark matter is collisionless Cold Dark Matter (CDM), but a sub-component (6%) is strongly dissipative minimal Atomic Dark Matter (ADM). The simulations, implemented in GIZMO and utilizing FIRE-2 galaxy formation physics to model the standard baryonic sector, demonstrate that the addition of even a small fraction of dissipative dark matter can significantly impact galactic evolution despite being consistent with current cosmological constraints. We show that ADM gas with roughly Standard-Model-like masses and couplings can cool to form a rotating "dark disk" with angular momentum closely aligned with the visible stellar disk. The morphology of the disk depends sensitively on the parameters of the ADM model, which affect the cooling rates in the dark sector. The majority of the ADM gas gravitationally collapses into dark "clumps" (regions of black hole or mirror star formation), which form a prominent bulge and a rotating thick disk in the central galaxy. These clumps form early and quickly sink to the inner ~kpc of the galaxy, affecting the galaxy's star-formation history and present-day baryonic and CDM distributions.

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