arXiv subjects

Andrew Wetzel

Publications and source records attributed to Andrew Wetzel.

At least 19 recordsLinked to original sources

arXiv2026

Modeling CO Line Luminosity with FIRE simulations: II. Comparison With Observations, CO Luminosity Functions and Redshift Evolution

We present predictions for the CO(1-0) and CO(2-1) luminosities of 765, 850, 887 and 915 galaxies at z=0, 1, 2 and 3, respectively, drawn from the Feedback in Realistic Environments (FIRE-2) simulations and post-processed with SKIRT radiative transfer and CLOUDY line emission calculations. We compare our CO line luminosity predictions with existing observations. At z=0, our predictions reproduce the observed scatter and mean relation between star formation rate (SFR) and CO(1-0) luminosity for both massive star-forming galaxies and low-metallicity dwarf galaxies. We also reproduce the observed superlinear power-law slope between CO(1-0) luminosity and metallicity. At z=1, our CO(2-1) predictions scatter to much lower luminosities than observed, which likely reflects the limited sensitivity of current surveys. At fixed SFR, our CO(1-0) luminosities decrease by roughly an order of magnitude per unit increase in redshift. This decline is likely driven by the decrease in metallicity with increasing redshift, since FIRE-2 galaxies with comparable SFR and metallicity produce similar CO(1-0) luminosities regardless of redshift. At fixed stellar mass, the predicted luminosities show no redshift dependence. These results show that our FIRE-2 predictions reproduce the observed scaling relations between CO(1-0) luminosity and galaxy properties wherever the observed galaxies are robustly detected. This gives us confidence in our predictions for the regime where no observational data exist, and suggests that our luminosity predictions can be used to build predictive models for CO line intensity mapping experiments.

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arXiv2026

"Helium Enrichment Law": The Relation Between Helium and Metal Enrichment in Stars in FIRE-3 Simulations of Milky Way-mass Galaxies

Helium abundance is a key input to stellar models that are used to infer accurate ages, masses, and compositions for individually resolved stars in the Milky Way (MW), underpinning Galactic archaeology. Since spectroscopically measuring He abundances is difficult, stellar models often assume a fixed "chemical enrichment law" dY/dZ (the slope linearly relating a star's helium mass fraction Y to its metal mass fraction Z). If incorrect, this assumption can introduce significant systematics in inferring stellar properties. Using the FIRE-3 cosmological baryonic zoom-in simulations, we investigate helium versus metal enrichment in the stars of MW-mass galaxies. Given the nucleosynthetic yields in FIRE-3, overall metal production is dominated by core-collapse supernovae (CCSN) at all progenitor metallicities. He production is dominated by CCSN for progenitor metallicities of Z <= 0.1 Zsun, but by stellar winds for progenitors near Zsun. Our simulations produce a range of dY/dZ slopes slopes with best-fit values of ~0.75-2.1 depending on the age of the stars considered at present day; on average consistent with the range of values from MW observations but with no single "universal" enrichment law. The dY/dZ slope also shows galaxy-to-galaxy variance in our simulations, and to leading order increases over cosmic time within the same galaxy, based on the peculiarities of the formation histories of each MW-mass system. Despite choosing a fixed set of nucleosynthetic yields for stellar feedback processes in FIRE-3, we predict variance in dY/dZ for stars in MW-mass galaxies, and observations of MW stars must be interpreted in this context.

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arXiv2026

From protogalaxy through thick and thin: Why did the Milky Way evolve in three kinematic phases?

APOGEE and Gaia data suggest the Milky Way's kinematic structure evolved through three distinct phases: a disordered protogalaxy, which ``spun up'' into a thick stellar disk, and then ``cooled down'' to a final cold, thin stellar disk. We use a suite of FIRE-2 cosmological zoom-in simulations of Milky Way-mass galaxies to demonstrate that the same three phases arise in simulations, and we study their physical origin. In our simulations, the early protogalaxy phase occurs when the rate of cool gas ($T \leq 10^4$ K) converting into stars is low, star formation is bursty, and the baryonic mass ``sloshes'' within the host potential with respect to the center of mass. The gas begins to spin coherently after sloshing ends, followed by the spin-up of young stars. The central potential is least concentrated just prior to gas spin-up. This second, thick disk phase coincides with the highest rate of cool gas converting into stars, though star formation remains bursty. The final transition to the thin disk phase occurs when the inner circumgalactic medium virializes, and is associated with steady star formation and intermediate consumption rates of cool gas converting into stars. Mergers do not appear to play a defining role in driving transitions between the three phases. The condition for thick disk formation appears to be minimal: a stable center of mass motion. Thin disk formation requires more: gas must accrete slowly enough for its angular momentum to mix and become coherent prior to joining the galaxy.

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arXiv2026

Modeling CO Line Luminosity Using FIRE Simulations: I. Methodology and Validation of Molecular Gas and CO Abundance

We present a post-processing pipeline that predicts CO line luminosities, molecular hydrogen masses, and CO abundances for galaxies in the Feedback in Realistic Environments (FIRE-2) simulations, as a first step toward CO response functions for line intensity mapping mocks. SKIRT gives the radiation field reaching each gas cell. CLOUDY, tabulated on a five-parameter grid (metallicity, density, turbulent velocity, radiation field, cloud scale) and interpolated, gives $L'_{\rm CO(1-0)}$ and the $\rm H_2$ and CO masses. In this first of three papers we validate the pipeline against observations. For Milky Way (MW) molecular clouds, the predicted $L'_{\rm CO(1-0)}$ rises sharply at $A_V\sim1$ and matches the observed values above that column. For a uniform $3\,Z_\odot$ the summed luminosity is within 35\% of the observed total. A factor of three increase in metallicity changes the total $L'_{\rm CO(1-0)}$ by an order of magnitude. Along MW sight lines, the predicted $N_{\rm CO}/N_{\rm H_2}$ reproduces the observed upturn within 0.3 dex in $N_{\rm H_2}$ but falls below the data at higher columns. At $z=0$ our molecular gas masses agree with xCOLD GASS for ${\rm SFR}\gtrsim0.3\,M_\odot\,yr^{-1}$ and are converged across our simulation resolutions. At $z\gtrsim1$ observed gas masses exceed ours at fixed stellar mass, though selection effects and conversion-factor assumptions limit this comparison. CO luminosities are compared with extragalactic observations in Paper II. Capturing the metallicity-dependent dust shielding of CO from photodissociation is essential for modeling CO emission in low-mass and high-redshift galaxies.

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arXiv2026

The Impact of Simulation Resolution on Dwarf Galaxy Stellar Stream Populations in FIRE

Cosmological simulations allow us to study galaxy-progenitor stream populations, allowing comparisons with observations to constrain galaxy formation physics. Previous work found that present-day galaxy-progenitor streams in FIRE have larger pericenters than observed in the MW, which might indicate that they tidally disrupt at larger distances, where the Galactic tides are weaker, than in the MW. One possible explanation is that they tidally disrupt too quickly because of limited resolution. At lower resolution, low-mass dwarf galaxies can be puffier and more susceptible to disruption. We build and compare dwarf-galaxy stream catalogs in two FIRE-2 cosmological zoom-in simulations with identical initial conditions (m12i) but different particle mass resolution (7070 $M_\odot$ and 880 $M_\odot$). The higher-resolution run yields a larger dwarf galaxy stream population (22 streams versus 14) using the same cutoff of 100 star particles, primarily because it resolves lower-mass streams (down to $\sim10^{5}$ $M_\odot$). Both simulations form streams at galactocentric radii < 50 kpc, but the higher-resolution Triple Latte simulation produces more streams at these small radii. We find that dwarf galaxies in the higher-resolution simulation begin disrupting at smaller pericenters than in the lower-resolution simulation. By cross-matching stream candidates, we identify counterparts for $\sim$89$\%$ of the systems in the lower-resolution simulation. For matched systems that remain on similar orbits, the higher-resolution counterparts remain intact for longer. The orbital properties of streams in the higher-resolution simulation are broadly consistent with the MW dwarf-galaxy stream population, in contrast to the lower-resolution simulation.

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arXiv2026

Old Disks Die Hard: How Does AGN Feedback Suppress Disk Formation in Milky Way Mass Galaxies?

The connection between a galaxy's visual shape and its star formation rate is one of the oldest established trends in galaxy evolution, yet the origin of this trend remains unsettled. We analyze two Milky-Way mass halos simulated with FIRE-2 galaxy formation physics, each run without AGN and with up to three implementations of AGN feedback. We use them to study how and why AGN star formation suppression affects morphological evolution over cosmic time. Both runs without AGN feedback produce prominent, thin, star-forming spiral disks at $z=0$. Each of the AGN runs has less late-time star formation, and ends up with a higher spheroid fraction and a lower thin-disk mass fraction. In two instances, the AGN runs produce quenched lenticular galaxies with no cool gas at $z=0$. The main reason for these morphological differences is that AGN feedback becomes effective {\em just after} the onset of disk formation, at ``spin up," in every run. This time-differentiated impact suppresses the formation of disk stars, especially thin-disk stars, preferentially because thin-disk formation occurs late. The spheroidal components are assembled early in all runs, before AGN feedback is effective, and are therefore similar in mass and size across runs.

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arXiv2026

Hot or Cold? Radial Redistribution of Stars in FIRE Simulations of Milky Way-Mass Galaxies and the Asymmetry of Inward versus Outward Migrators

Stars can radially redistribute (migrate) within galactic disks. The degree to which this occurs as dynamically `cold' (preserves orbital eccentricity) or `hot' (increases eccentricity) remains debated. Many models presume that radial redistribution occurs primarily via cold torquing, resulting in changes in angular momentum without dynamical heating. We test the net dynamical heating associated with redistribution over stellar lifetimes using the FIRE cosmological zoom-in simulations of 12 Milky Way-mass galaxies. We select star particles today that underwent significant changes in orbital angular momentum, j_phi, since birth. We investigate net changes in their orbital eccentricity, e, and we quantify the `cold-torqued' fraction of star particles with |Delta j_phi/j_phi,birth| > 0.2 that preserved eccentricity (|Delta e| < 0.1) since birth. The direction of radial redistribution is most critical: outward-migrating stars experienced smaller net changes in eccentricity, whereas inward-migrating stars almost always heat since birth. For stars born on near-circular orbits (e_birth < 0.2), the cold-torqued fraction decreases rapidly with age today and is generally < 50% at ages >~2 Gyr. Stars born on moderately eccentric orbits (e_birth ~ 0.4) are the most likely to preserve their birth eccentricity. However, the cold-torqued fraction is higher in earlier-forming and/or dynamically-colder disks. Significantly, we identify a population of stars that dynamically `cooled', decreasing in eccentricity since birth: this is the primary way that stars end up on near-circular orbits today. Overall, a star's migration direction, its e_birth, and its age primarily determine whether it was dynamically heated, cooled, or unchanged. In general, radial redistribution in FIRE is typically not cold between birth and today.

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arXiv2026

The Star Formation History and Evolution of the Ultra-Diffuse M81 Satellite F8D1

We present deep HST imaging of one of the nearest ultra-diffuse galaxies (UDGs) outside of the Local Group: F8D1, a satellite of M81 known to be tidally disrupting. UDGs are an enigmatic and diverse population, with evolutionary pathways ranging from tidal processing to bursty feedback and high initial angular momentum. To determine F8D1's evolutionary drivers, we resolve stars in F8D1's central $\sim$1 kpc and in a parallel field $\sim$6 kpc along its major axis to deep photometric limits, reaching below the Red Clump. We also image eight shallower fields along F8D1's major and minor axes. We calculate the star formation history (SFH) in the two deep fields, finding that while currently quiescent, both regions experienced a substantial burst $\sim$2 Gyr ago and a smaller burst $\sim$500 Myr ago, which likely formed F8D1's nuclear star cluster. In the shallow fields, using the ratio of evolved Asymptotic Giant Branch and Red Giant Branch stars out to $\sim$13 kpc along F8D1's known stellar stream, we confirm that F8D1 was globally star-forming until at least $\sim$2 Gyr ago. We estimate a total progenitor stellar mass, including the stream, of $\sim$1.3$\times$10$^8 M_{\odot}$, with an average [M/H] $\sim$ $-$0.8. We compare F8D1's properties to those of Local Group galaxies with similar initial stellar mass. We find that F8D1 is consistent with a progenitor star-forming galaxy similar to NGC 6822 that is in the midst of a transition to a Sagittarius-like system. Notably, this evolutionary sequence can be accomplished through tidal processing alone in galaxies that have experienced sufficiently bursty feedback to have created cored profiles.

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arXiv2026

Beyond Cloud-9: The case for discovering more HI-rich failed halos

HI-rich starless halos, should they exist, hold great promise for elucidating dark matter halo structure. Yet realizing this potential demands reliable theoretical predictions for their properties and abundances. Indeed, the recent identification of Cloud-9 as a strong HI-rich starless halo candidate in the nearby universe makes such predictions timely. This Letter examines HI-rich ($M_\mathrm{HI} \geq 10^6\,M_\odot$) starless (isolated/central) halos at $z=0$ across three cosmological simulations: FIREbox, Recal-EAGLE and NIVARIA-LG. All three successfully produce such objects, with $M_\mathrm{HI}$ extending up to $\sim\!1$--$2$ dex above Cloud-9, but with number densities that vary by a factor of $\sim30$. These populations span different regions of the $M_\mathrm{HI}$--$M_\mathrm{gas}$--$M_{200}$ space: NIVARIA-LG produces objects with higher $M_\mathrm{HI}$ and $M_\mathrm{gas}$ values ($\gtrsim 10^7$ and $\gtrsim 10^8\,M_\odot$), while FIREbox predicts they lie within extremely narrow ranges of $M_\mathrm{gas} \sim(1.1$--$1.6)\times10^8\,M_\odot$ and $M_{200} \sim(7.8$--$8.6)\times10^9\,M_\odot$. Recal-EAGLE and NIVARIA-LG exhibit a strong $M_\mathrm{gas}-M_{200}$ correlation, with similar slopes but different normalizations. The simulations predict numerical Cloud-9 analogs -- though similarities in the shapes of their HI column-density profiles may be driven by FAST's modest beam; halving it already reveals differences. Collectively, these inter-simulation discrepancies make a compelling case for discoveries beyond Cloud-9: a statistical sample of well-resolved HI-rich starless halos is needed to discriminate amongst competing predictions.

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arXiv2026

The quenched fraction of satellites around simulated Milky Way-mass galaxies

We compare satellite quenched fractions across three cosmological simulation suites (FIREbox, the FIRE-2 zoom-ins, and IllustrisTNG50) and observational datasets from SAGA, ELVES, and the combined satellite population of the Milky Way and M31. To enable consistent comparisons, we select Milky Way-mass hosts with $M_{\rm halo} = 10^{11.9}$ - $10^{12.2} \, M_{\odot}$ and satellites with stellar masses of $10^7$ - $10^{10} \, M_{\odot}$, applying nearly uniform radial selections and a common quenching definition. All three simulations reproduce the strong observed trend that lower-mass satellites are more likely to be quenched, closely matching the stellar mass dependence seen in SAGA, ELVES, and the Milky Way and M31 system. This agreement indicates that the mass dependence of satellite quenching is a robust outcome of contemporary galaxy formation models. Radial trends, however, show greater diversity. SAGA and ELVES exhibit gently declining quenched fractions with projected distance, consistent with stronger quenching at small radii. TNG50 most closely matches this behavior, while FIREbox remains broadly consistent with a weak radial trend within uncertainties. The FIRE-2 zoom-ins show suppressed quenched fractions at small projected distances, driven primarily by their paired MW-M31 analogs. We show that this discrepancy is not explained by host environment alone, but instead reflects atypical satellite populations in the paired systems, where star-forming and quenched satellites occupy distinct spatial distributions. Overall, our results demonstrate that stellar mass-quenched fraction trends are robust across simulations and observations, while radial trends are more sensitive to the detailed properties and distributions of satellite populations

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arXiv2026

Resolving galaxy formation in the early Universe with BonFIRE and CampFIRE

The abundance and rapid growth of galaxies at cosmic dawn revealed by the James Webb Space Telescope challenges models of galaxy formation, motivating new simulations to uncover the processes driving early galaxy assembly. We present the first results from BonFIRE ($L\approx40$ cMpc, $m_{\rm baryon}\approx5\times10^4~\rm{M}_{\odot}$) and CampFIRE ($L\approx5$ cMpc, at both $m_{\rm baryon}\approx800~\rm{M}_{\odot}$ and $\approx6\times10^3~\rm{M}_{\odot}$), a suite of cosmological hydrodynamic simulations of early galaxy formation ($z\gtrsim6$) from the Feedback In Realistic Environments (FIRE) project, using the FIRE-3 model. We use a resampling procedure to combine the large statistics of BonFIRE with the higher resolution of CampFIRE and robustly predict galaxy properties over a wide dynamic range ($M_{\star}\sim10^4-10^{10}~\rm{M}_{\odot}$). Galaxy formation in this suite emerges through clustered, bursty star formation, with halo-scale star formation efficiencies reaching $10-30\%$ in high-mass halos. A subset of low-mass halos also have surprisingly high efficiencies of $\gtrsim1\%$ and host ultra-compact galaxies with narrow age spreads. We predict galaxy UV luminosity functions at $9\lesssim~z\lesssim25$ in broad agreement with observations at $M_{\rm UV}\gtrsim-19$, with a faint-end turnover at $M_{\rm UV}\approx-14$, but we slightly overpredict the abundance of brighter galaxies. We find that UV luminosity variability in early galaxies is strongly mass-dependent, with halo-to-halo scatter dominating at low masses and contributing comparably to rapid temporal burstiness at $M_{\rm halo}\gtrsim10^{10}~\rm{M}_{\odot}$. We also present first results from a simple Pop~III model with a top-heavy IMF, demonstrating broad agreement with independent Pop~III predictions and observational constraints.

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arXiv2026

Hubble Astrometry for the Local Group and Beyond in the 2030s

Hubble's long, stable astrometric baseline creates a rare opportunity for discovery in the Local Group and beyond. Many nearby galaxies, streams, and star clusters already have archival first-epoch imaging in hand, so future HST observations over the next decade can turn those data into precise proper motions. For many Milky Way satellites, existing measurements already constrain orbital motion at a useful level, but HST still offers a path to full 3D kinematics, internal motions, and more distant systems where current data remain insufficient. That opens the window to dynamical studies inaccessible through line-of-sight velocities alone, revealing orbital histories, internal kinematics, environmental processing, and the dark-matter structure of nearby galaxies. This white paper identifies HST astrometry as an opportunity to capitalize on archival baselines by completing long-baseline measurements where first epochs already exist, establishing new first epochs where critical gaps remain, and assembling a legacy sample for future JWST, Roman, and HWO-era follow-up. The result will be a transformative dataset for the Local Group and Local Volume, driving discovery now while laying the groundwork for the next generation of dynamical studies for resolved stellar populations.

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arXiv2026

No Stream Left Unscathed: The imprint of a host galaxy

Stellar streams from disrupted globular clusters are excellent probes of dark matter (DM) subhalos. Observed Milky Way streams display a remarkable diversity of features: spurs, gaps, kinks, cocoons, and density variations, many attributed to subhalo encounters. But how much of this diversity arises from the host itself? We simulate $\sim$15,000 globular cluster streams across four Milky Way-mass halos from the FIRE-2 cosmological simulations, evolved in basis function expansion potentials capturing the evolving disk, halo, and large-scale structure while excluding small-scale perturbers such as DM subhalos and giant molecular clouds. We find that roughly three quarters of streams develop complex features from the host potential, such as spurs, kinks, and cocoon-like envelopes. Even the smoothest streams exhibit 10--25\% width variation along their track and host overdensities and gaps at scales of ${\sim}2^\circ$, squarely in the $1^\circ$--$5^\circ$ range predicted for subhalo-induced gaps. Pericentric distance is the primary predictor of stream morphology, with ${\sim}15$ kpc separating smooth from disturbed streams and circular orbits beyond $\sim$20 kpc producing the smoothest streams. Only $\sim$70 out of $\sim$15,000 streams are free of detectable wiggles in the track at any scale. Analogs to observed features, such as the GD-1 spur and the ATLAS--Aliqa Uma kink, emerge even without the presence of subhalos. As next-generation surveys (LSST, Euclid, and Roman) resolve stream structure across hundreds of streams, the baseline established here, streams evolved without small-scale perturbers, becomes essential for extracting DM substructure constraints.

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arXiv2026

Radial redistribution of stellar orbits in FIRE simulations of Milky-Way-mass galaxies

A central question in galactic dynamics and galactic archeology is: how much do the orbits of stars redistribute (migrate) after birth? We use the FIRE-2 cosmological zoom-in simulations of 11 Milky Way-mass galaxies to quantify the change in the orbital specific angular momentum, j_phi, orbital radius, R_orbit, and azimuthal velocity, v_phi, of stars from birth to today. We examine the dependences on stellar age, present-day R_orbit, and birth R_orbit, characterizing both the median (net) change, Delta R_orbit, and its scatter, sigma(Delta R_orbit). We comprehensively compare five ways of measuring orbital radius; we find generally consistent trends, but only when measuring radius today and radial redistribution self-consistently. Stars selected by their birth R_orbit typically decreased in R_orbit, j_phi, and v_phi since birth. The trend for stars at a given R_orbit today depends on age: those younger than ~5 Gyr generally decreased in R_orbit, j_phi, and v_phi since birth, while those older generally increased in R_orbit, j_phi, and v_phi since birth. sigma(Delta R_orbit), a standard metric of radial redistribution, increases with stellar age only up to ~ 3 Gyr; it saturates at sigma(Delta R_orbit) ~2 kpc for older stars. This saturation contradicts a common expectation of a monotonic increase with age. Our results broadly agree with recent observational inferences of Delta R_orbit and sigma(Delta R_orbit) in the Milky Way. Across our FIRE-2 sample, the timing of disk formation does not correlate with sigma(Delta R_orbit), but it correlates with (net) Delta R_orbit.

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arXiv2026

Galaxy Metallicity Gradients in the Reionization Epoch from the FIRE-2 Simulations

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 $z \sim 10-5$. We find that galaxies at $z\sim10$ exhibit a median metallicity gradient of $-0.15\,\mathrm{dex\cdot kpc^{-1}}$ with substantial scatter, which gradually flatten to $-0.1\,\mathrm{dex\cdot kpc^{-1}}$ at $z\sim6$, 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 higher star formation rates (SFRs) exhibit steeper negative gradients, while sSFR shows a strong anti-correlation with gradient slope. Because EoR galaxies in FIRE-2 generally lack significant rotational support, we adopt the ratio of peak-to-peak velocity shear to twice the velocity dispersion ($Δv/2σ$) as a proxy for the strength of gas flows. We find a strong positive correlation between metallicity gradients and $Δv/2σ$: galaxies with lower $Δv/2σ$ (i.e., weaker gas flows) tend to exhibit steeper negative gradients. Furthermore, galaxies with steeper gradients 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.

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arXiv2026

Set the Night on FIRE: Building an Empirical Local Dark Matter Velocity Distribution

The majority of terrestrial direct detection experiments for Dark Matter (DM) rely on the Standard Halo Model (SHM), which assumes the local DM velocity distribution follows a Maxwell-Boltzmann distribution. However, galaxy mergers can deposit DM that remains kinematically clustered today, inducing deviations from the smooth SHM prediction. Previous studies have suggested that the local stellar velocity distribution may serve as a tracer for DM populations originating from the same progenitor systems. In this work, we systematically investigate how merger mass and accretion time affect the correlation between local stellar and DM velocity distributions in Milky Way-like galaxies from the FIRE-2 simulations. We find a strong correlation between traceable DM components and their stellar counterparts, with the tightest correspondence arising from lower-mass mergers accreted at earlier cosmic times. For the remaining DM that lacks an identifiable stellar counterpart, which dominate the full DM fraction, we find that its velocity distribution is well described by a component-wise generalized Gaussian. Combining these two ingredients, we reconstruct the full local DM velocity distribution. This framework captures merger-induced features-such as co-rotation of accreted material with the galactic disk-that are entirely absent in the SHM. Finally, we propagate uncertainties through the reconstruction and show that they are dominated by the stellar mass-halo mass relation, which is unlikely to improve substantially in the near term. We therefore argue that this framework approaches the current limit of our ability to characterize the local DM velocity distribution.

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arXiv2026

Rise of the forsaken relics: connecting present-day stellar streams and phase-mixed galaxies to the Epoch of Reionization

The `near-far' approach to studying reionization leverages the star formation histories of the Milky Way (MW) or Local Group (LG) galaxies, derived from resolved photometry, to infer the low-mass/faint-end of the stellar mass functions (SMFs) or the ultraviolet luminosity functions (UVLFs) of high-redshift galaxies ($z \gtrsim 6$), beyond the current JWST detection limits ($M_{\mathrm{UV}} \gtrsim -15$). Previous works considered only intact low-mass galaxies in the MW and LG, neglecting disrupted galaxies such as stellar streams and phase-mixed objects. Using the FIRE-2 simulations, we show that these disrupted galaxies contribute up to $\sim50\%$ of the total stellar mass budget of the proto-MW/LG at $z =6-9$. Including all the progenitors of these disrupted galaxies improves the normalization of the recovered SMFs/UVLFs by factors of $\sim2-3$ and reduces the halo-to-halo variation in the slope by $\sim20-40\%$. This enables robust constraints down to at least the resolution limit of the simulations, near $M_\star$ $\sim$ $10^{5}$ $M_\odot$ or $M_{\mathrm{UV}} \sim -10$ at $z \gtrsim 6$. We also show that `fossil record' reconstructions - which assume each present-day system descends from a single reionization-era progenitor - are sensitive to the stellar mass/UV magnitude thresholds, which introduces bias in the inferred low-mass/faint-end slopes. Additionally, we demonstrate that neglecting disrupted systems underestimates the contribution of galaxies with $M_{\mathrm{UV}} \lesssim -15$ to the reionization-era UV luminosity density. Finally, we estimate that a significant fraction ($\sim50\%$) of streams with $M_\star$ $\gtrsim$ $10^{6}$ $M_\odot$ at $z=0$ should be detectable from upcoming Rubin Observatory and Roman Space Telescope observations.

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arXiv2026

The origin of strong $α$-element bimodalities in FIRE simulations of Milky Way-mass galaxies

One of the Milky Way's characteristic features is a strongly bimodal distribution of $α$-process elements, such as Mg, at fixed [Fe/H] in stellar abundances. We examine patterns in [Mg/Fe] versus [Fe/H] in FIRE-2 simulations of Milky Way-mass galaxies. Out of 16 galaxies, 4 are capable of producing a strongly bimodal distribution. In all four galaxies, the high-$α$ population corresponds to an older, radially-compact, thick disk, and the low-$α$ population corresponds to a younger, radially-extended, thin disk, similar to the MW.The transition from high- to low-$α$ took $0.3-1.2\Gyr$ and began $5.5-6.5\Gyr$ ago. [Mg/Fe] decreased at relatively fixed [Fe/H], both in the galaxy overall and at fixed radii: Fe enrichment nearly balanced gas accretion (and therefore dilution), but Mg enrichment was weaker. Importantly, this transition occurred during a period of relatively low gas fraction ($5-15\%$), immediately after a rapid decline in star formation (halving within a few hundred Myr), which caused an increase in Fe-producing white-dwarf supernovae relative to Mg-producing core-collapse supernovae. Only one case coincided with a major merger coalescence. We find similar trends in measuring stars by their current radius and by their birth radius, therefore, radial redistribution did not play a dominant role in the formation of a bimodality or its spatial dependence today. Overall, in FIRE-2, strong $α$-element bimodalities are relatively uncommon ($\sim25\%$), often not associated with a major merger, and arise primarily from a rapid decline in star formation during relatively low gas fraction.

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