arXiv subjects

James Wadsley

Publications and source records attributed to James Wadsley.

At least 19 recordsLinked to original sources

arXiv2026

The Simulated Oxygen Shortage (SOS): Mapping the Missing OVI in Simulated Dwarf Galaxies to Subgrid Physics

Observations reveal extended \OVI\, reservoirs in the circumgalactic medium (CGM) of dwarf galaxies, yet current simulations systematically underpredict \OVI\, column densities. Utilizing two suites run with different simulation codes, the \MM\, simulations (Marvelous Massive Dwarfs and Marvel-ous Dwarfs) and the publicly available FIRE-2 simulations, we explore the role of subgrid models and the resulting CGM phase in shaping \OVI\, production. By comparing observationally derived \OVI\, masses to the mass of oxygen produced over the galaxies' star formation history, we find evidence for an underproduction of oxygen for low-mass simulated galaxies. Despite clear differences in feedback implementation, CGM structure, and metal mixing, we find that \OVI\, in both suites generally self-selects cool/warm ($\rm log\, T\,/K \sim 4.5$), diffuse ($\rm log\,n_{gas}\,/cm^{-3} \sim -5.0 $), and moderately metal-enriched ($\rm log\, Z/Z_{\odot} \sim -1 $) material at large radii from the galaxy. We show that neither the choice of ultraviolet background nor plausible variations in CGM thermal structure can close the gap with observations. Taken together, our results point to a possible underproduction of oxygen in low-mass galaxies. Feedback prescriptions contribute via insufficient metal transport to large radii. Hence, the \OVI\, deficit may motivate an investigation of current modeling choices for supernova yields, star formation, and feedback in low-metallicity environments.

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arXiv2026

Fast, Stable, and Physical: Hyperbolic, Magnetic Field-Aligned Diffusion in SPH

In this paper, we introduce the first implementation of magnetic field-aligned hyperbolic diffusion for standard smoothed particle (magneto-)hydrodynamics (SPH), and its linear-exact gradient extension (LESPH). Hyperbolic diffusion differs from traditional parabolic methods by incorporating the physical characteristic speed of diffusing particles and is computationally faster. This work extends it to encompass field-aligned diffusion, linear-exact gradients, and linear reconstruction to limit dissipation. Several standard test problems are presented: a diffusing slab, diffusion around a ring, a Gaussian pulse, and the magneto-thermal instability (MTI). The MTI only grows for for LESPH with reconstruction, and not for SPH. Both LESPH and SPH remain stable while fully aligning diffusion to magnetic fields. LESPH is more accurate and converges faster in the L1 error norm. SPH and LESPH both see improvements when using when also using linear reconstruction. These methods apply to other diffusive transport such as cosmic rays, viscosity, or magnetic resistivity.

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arXiv2025

The Formation of Dwarf Galaxy Disks

Dwarf galaxies are dark matter-dominated systems that are sensitive to feedback and display a diversity of baryonic morphologies. This makes them excellent probes for understanding dark matter and galaxy evolution. This work investigates the physical processes that influence the sizes of isolated dwarf galaxies using high-resolution cosmological zoom-in simulations of $39$ dwarf galaxies drawn from the Marvelous Massive Dwarfs simulation suite ($7.5 < \log(M_{\star}/M_{\odot}) < 9.1$). Our simulations show that dwarf galaxies initially form as compact galaxies ($R_e < 2$ kpc). However, several of these galaxies ($54\%$) experience periods of gradual size growth at relatively stable sSFR, allowing them to become extended galaxies. We find that the growth of rotation-supported stellar disks is the primary means by which isolated dwarfs become extended in size. These stellar disks are formed by mergers with high orbital angular momentum satellites on high angular momentum (spiraling-in) orbits, which spin up the gas surrounding the central galaxy and contribute $\approx 30 \%$ of the cold gas mass at $z=0$. For these systems, star formation in the angular momentum supported gas and the gradual build up of stars in the disk result in secular size growth.

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arXiv2025

Dwarf diversity in $\Lambda$CDM with baryons

Observed rotation curves of dwarf galaxies exhibit significant diversity at fixed halo mass, challenging galaxy formation within the Cold Dark Matter (CDM) model. Previous cosmological galaxy formation simulations with baryonic physics fail to reproduce the full diversity of rotation curves, suggesting either that there is a flaw in baryonic feedback models, or that an alternative to CDM must be invoked. In this work, we use the Marvelous Massive Dwarf zoom-in simulations, a suite of high-resolution dwarf simulations with $M_{200}~\sim 10^{10}-10^{11}$ ${\rm M}_{\odot}$ and $M_{*}\sim 10^{7}-10^{9}$ ${\rm M}_{\odot}$, designed to target the mass range where galaxy rotation curve diversity is maximized, i.e., between $V_{\rm max} \sim 70-100~ {\rm km/s}$. We add to this a set of low-mass galaxies from the Marvel Dwarf Zoom Volumes to extend the galaxy mass range to lower values. Our fiducial star formation and feedback models produce simulated dwarfs with a broader range of rotation curve shapes, similar to observations. These are the first simulations that can both create dark matter cores via baryonic feedback, reproducing the slower rising rotation curves, while also allowing for compact galaxies and steeply rising rotation curves. Our simulated dwarfs also reproduce the observed size$-M_*$ relation, including scatter, producing both extended and compact dwarfs for the first time in simulated field dwarfs. We explore star formation and feedback models and conclude that previous simulations may have had feedback that was too strong to produce compact dwarfs.

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arXiv2025

Optimized smoothing kernels for SPH

We present a set of new smoothing kernels for smoothed particle hydrodynamics (SPH) that improve the convergence of the method without any additional computational cost. These kernels are generated through a linear combination of other SPH kernels, combined with an optimization strategy to minimize the error in the Gresho-Chan vortex test case. To facilitate the different choices in gradient operators for SPH in the literature, we perform this optimization for both geometric density average force SPH (GDSPH) and linear-corrected gradient SPH (ISPH). In addition to the Gresho-Chan vortex, we also perform simulations of the hydrostatic glass, Kelvin-Helmholtz instability, and the Sod shocktube case. At low neighbour numbers (< 128), there is a significant improvement across the different tests, with the greatest impact shown for GDSPH. Apart from the popular Wendland kernels, we also explore other positive-definite kernels in this paper, which include the "missing" Wendland kernels, Wu kernels, and Buhmann kernels. In addition, we also present a method for producing arbitrary non-biased initial conditions in SPH. This method uses the SPH momentum equation together with an artificial pressure, combined with a global and local relaxation stage to minimize local and global errors.

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arXiv2025

Star Formation and Magnetic Field Amplification due to Galactic Spirals

We use global MHD galaxy simulations to investigate the effects of spiral arms on the evolution of magnetic fields and star formation within a self-regulated interstellar medium (ISM). The same galaxy is simulated twice: once with self-consistent stellar spiral arms and once more with the stellar spirals suppressed via a novel numerical approach, using the Ramses AMR code. Spiral arms continually promote star formation, with 2.6 times higher rates in the spiral galaxy. The higher rate is due to high gas columns gathered along the spiral arms, rather than increasing the star formation efficiency at a given gas column. In both cases, the magnetic field is initially amplified via a small-scale dynamo driven by turbulence due to supernova feedback. Only the spiral galaxy exhibits late-time, consistent field growth due to a large-scale dynamo (e-folding time $\sim600$ Myr). This results in volume-averaged field strengths of $\sim 1$ $\mu$G after 1 Gyr of evolution. The mean-fields tend to align themselves with the spiral arms and are coherent up to 10 kpc scales. We demonstrate a novel large-scale dynamo mechanism, whereby spiral-driven radial flows enable the mean-field amplification.

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arXiv2025

Marvelous Metals: Surveying the Circumgalactic Medium of Simulated Dwarf Galaxies

Dwarf galaxies are uniquely sensitive to energetic feedback processes and are known to experience substantial mass and metal loss from their disk. Here, we investigate the circumgalactic medium (CGM) of 64 isolated dwarf galaxies ($6.0<$log(M$_*/M_{\odot}$)$<9.5$) at $z=0$ from the Marvel-ous Dwarfs and Marvelous Massive Dwarfs hydrodynamic simulations. Our galaxies produce column densities broadly consistent with current observations. We investigate these column densities in the context of mass and metal retention rates and the physical properties of the CGM. We find $48\pm11\%$ of all baryons within $R_{200c}$ reside in the CGM, with $\sim70\%$ of CGM mass existing in a warm gas phase, $10^{4.5} 40\%$ towards lower masses. Our findings highlight the CGM (primarily its warm component) as a key reservoir of mass and metals for dwarf galaxies across stellar masses and underscore its importance in understanding the baryon cycle in the low-mass regime. Finally, we provide individual galaxy properties of our full sample and quantify the fraction of ultraviolet observable mass to support future observational programs, particularly those aimed at performing a metal budget around dwarf galaxies.

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arXiv2025

Predictions for Detecting a Turndown in the Baryonic Tully Fisher Relation

The baryonic Tully Fisher relation (bTFR) provides an empirical connection between baryonic mass and dynamical mass (measured by the maximum rotation velocity) for galaxies. Due to the impact of baryonic feedback in the shallower potential wells of dwarf galaxies, the bTFR is predicted to turn down at low masses from the extrapolated power-law relation at high masses. The low-mass end of the bTFR is poorly constrained due to small samples and difficulty in connecting the galaxy's gas kinematics to its dark matter halo. Simulations can help us understand this connection and interpret observations. We measure the bTFR with 75 dwarf galaxies from the Marvel-ous and Marvelous Massive Dwarfs hydrodynamic simulations. Our sample has M$_\star = 10^6-10^9$ M$_\odot$, and is mostly gas dominated. We compare five velocity methods: V$_\mathrm{out,circ}$ (spatially resolved mass-enclosed), V$_\mathrm{out,mid}$ (spatially resolved midplane gravitational potential), and unresolved HI linewidths at different percentages of the peak flux (W$_\mathrm{10}$, W$_\mathrm{20}$, and W$_\mathrm{50}$). We find an intrinsic turndown in the bTFR for maximum halo speeds $\lesssim 50$ km s$^{-1}$, or total baryonic mass M$_\mathrm{bary}\lesssim 10^{8.5}$ M$_\odot$. We find that observing HI in lower-mass galaxies to the conventional surface density limit of 1 M$_\odot$ pc$^{-2}$ is not enough to detect a turndown in the bTFR; none of the HI velocity methods, spatially resolved or unresolved, recover the turndown, and we find bTFR slopes consistent with observations of higher-mass galaxies. However, we predict that the turndown can be recovered by resolved rotation curves if the HI limit is $\lesssim 0.08$ M$_\odot$ pc$^{-2}$, which is within the sensitivity of current HI surveys like FEASTS and MHONGOOSE.

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arXiv2025

A MARVEL-ous study of how well galaxy shapes reflect Dark Matter halo shapes in Cold Dark Matter Simulations

We present a 3D shape analysis of both dark matter (DM) and stellar matter (SM) in simulated dwarf galaxies to determine whether stellar shape traces DM shape. Using 80 central and satellite galaxies from three simulation suites (Marvelous Massive Dwarfs, Marvelous Dwarfs, and DC Justice League) spanning stellar masses of $10^6$--$10^{10}$ $M_\odot$, we measure 3D shapes through the moment of inertia tensor at two times the effective radius to derive axis ratios ($C/A$, $B/A$) and triaxiality. We find that stellar shape does indeed follow DM halo shape for our dwarf galaxies. However, the presence of a stellar disk in more massive dwarfs ($M_* \gtrsim 10^{7.5}$ $M_\odot$) pulls the distribution of stellar $C/A$ ratios to lower values, while in lower mass galaxies the gravitational potential remains predominantly shaped by DM. Similarly, stellar triaxiality generally tracks dark matter halo triaxiality, with this relationship being particularly strong for non-disky galaxies though weaker in disky systems. This correlation is reinforced by strong alignment between SM and DM axes, particularly in disk galaxies. Further, we find no detectable difference in either SM or DM shape comparing two different SNe feedback implementations, demonstrating that shape measurements may be robust to different implementations of baryonic feedback in dwarf galaxies. We also observe that a dwarf galaxy's shape is largely unperturbed by recent mergers (with merger ratios $>4$). This comprehensive study demonstrates that stellar shape measurements can serve as a reliable tool for inferring DM shapes in dwarf galaxies.

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arXiv2024

Intermediate Mass Ratio Inspirals in Milky Way Galaxies

A consequence of a non-zero occupation fraction of massive black holes (MBHs) in dwarf galaxies is that these MBHs can become residents of larger galaxy halos via hierarchical merging and tidal stripping. Depending on the parameters of their orbits and original hosts, some of these MBHs will merge with the central supermassive black hole in the larger galaxy. We examine four cosmological zoom-in simulations of Milky Way-like galaxies to study the demographics of the black hole mergers which originate from dwarf galaxies. Approximately half of these mergers have mass ratios less than 0.04, which we categorize as intermediate mass ratio inspirals, or IMRIs. Inspiral durations range from 0.5 - 8 Gyr, depending on the compactness of the dwarf galaxy. Approximately half of the inspirals may become more circular with time, while the eccentricity of the remainder does not evolve. Overall, IMRIs in Milky Way-like galaxies are a significant class of black hole merger that can be detected by LISA, and must be prioritized for waveform modeling.

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arXiv2024

Testable predictions of outside-in age gradients in dwarf galaxies of all types

We use a sample of 73 simulated satellite and central dwarf galaxies spanning a stellar mass range of $10^{5.3}-10^{9.1} M_\odot$ to investigate the origin of their stellar age gradients. We find that dwarf galaxies often form their stars "inside-out," i.e., the stars form at successively larger radii over time. However, the oldest stars get reshuffled beyond the star forming radius by fluctuations in the gravitational potential well caused by stellar feedback (the same mechanisms that cause dwarfs to form dark matter cores). The result is that many dwarfs appear to have an "outside-in" age gradient at $z=0$, with younger stellar populations more centrally concentrated. However, for the reshuffled galaxies with the most extended star formation, young stars can form out to the large radii to which the old stars have been reshuffled, erasing the age gradient. We find that major mergers do not play a significant role in setting the age gradients of dwarfs. We find similar age gradient trends in satellites and field dwarfs, suggesting environment plays only a minor role, if any. Finally, we find that the age gradient trends are imprinted on the galaxies at later times, suggesting that the stellar reshuffling dominates after the galaxies have formed 50% of their stellar mass. The later reshuffling is at odds with results from the FIRE-2 simulations. Hence, age gradients offer a test of current star formation and feedback models that can be probed via observations of resolved stellar populations.

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arXiv2024

Bursting with Feedback: The Relationship between Feedback Model and Bursty Star Formation Histories in Dwarf Galaxies

We use high-resolution cosmological simulations to compare the effect of bursty star formation histories on dwarf galaxy structure for two different subgrid supernovae (SNe) feedback models in dwarf galaxies with stellar masses from $5000 <$ M$_*$/M$_\odot$ $< 10^{9}$. Our simulations are run using two distinct supernova feedback models: superbubble and blastwave. We show that both models are capable of producing galaxies that are cored and reproduce observed scaling relations for metallicity, luminosity, mass, and size. We show that continuous bursty star formation and the resulting stellar feedback are able to sustain dark matter cores in the higher dwarf galaxy mass regime, while the majority of ultra-faint and classical dwarfs retain cuspy central dark matter density profiles. We find that both subgrid SN models are able to create bursty star formation histories. We find that effective core formation peaks at M$_*$/M$_\odot$ $\simeq 5 \times 10^{-3}$ for both feedback models. Galaxies simulated with superbubble feedback peak at lower mean burstiness values relative to blastwave feedback, indicating that core formation in the superbubble sample may be less motivated by the burstiness of star formation.

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arXiv2024

Regulating star formation in a magnetized disk galaxy

We use high-resolution MHD simulations of isolated disk galaxies to investigate the co-evolution of magnetic fields with a self-regulated, star-forming interstellar medium (ISM). The simulations are conducted using the Ramses AMR code on the standard Agora initial condition, with gas cooling, star formation and feedback. We run galaxies with a variety of initial magnetic field strengths. The fields grow rapidly and achieve approximate saturation within 500 Myr, but at different levels. The galaxies reach a quasi-steady state, with slowly declining star formation due to both gas consumption and increases in the field strength at intermediate ISM densities. We connect this behaviour to differences in the gas properties and overall structure of the galaxies. In particular, strong fields limit feedback bubbles. Different cases support the ISM using varying combinations of magnetic pressure, turbulence and thermal energy. Magnetic support is closely linked to stellar feedback in the case of initially weak fields but not for initially strong fields. The spatial distribution of these supports is also different in each case, and this is reflected in the stability of the gas disk. We relate this back to the overall distribution of star formation in each case. We conclude that a weak initial field can grow to produce a realistic model of a local disk galaxy, but starting with typical field strengths will not.

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arXiv2024

Filamentary Hierarchies and Superbubbles: Galactic Multiscale MHD Simulations of GMC to Star Cluster Formation

There is now abundant observational evidence that star formation is a highly dynamical process that connects filament hierarchies and supernova feedback from galaxy scale kpc filaments and superbubbles, to giant molecular clouds (GMCs) on 100 pc scales and star clusters (1 pc). Here we present galactic multi-scale MHD simulations that track the formation of structure from galactic down to sub pc scales in a magnetized, Milky Way like galaxy undergoing supernova driven feedback processes. We do this by adopting a novel zoom-in technique that follows the evolution of typical 3-kpc sub regions without cutting out the surrounding galactic environment, allowing us to reach 0.28 pc resolution in the individual zoom-in regions. We find a wide range of morphologies and hierarchical structure including superbubbles, turbulence, kpc atomic gas filaments hosting multiple GMC condensations that are often associated with superbubble compression; down to smaller scale filamentary GMCs and star cluster regions within them. Gas accretion and compression ultimately drive filaments over a critical, scale - dependent, line mass leading to gravitational instabilities that produce GMCs and clusters. In quieter regions, galactic shear can produce filamentary GMCs within flattened, rotating disk-like structures on 100 pc scales. Strikingly, our simulations demonstrate the formation of helical magnetic fields associated with the formation of these disk like structures.

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arXiv2023

Planetesimal formation via the streaming instability with multiple grain sizes

Kilometre-sized planetesimals form from pebbles of a range of sizes. We present the first simulations of the streaming instability that begin with a realistic, peaked size distribution, as expected from grain growth predictions. Our 3D numerical simulations directly form planetesimals via the gravitational collapse of pebble clouds. Models with multiple grain sizes show spatially distinct dust populations. The smallest grains in the size distribution do not participate in the formation of filaments or the planetesimals that are formed by the remaining 80% of the dust mass. This implies a size cutoff for pebbles incorporated into asteroids and comets. Disc observations cannot resolve this dust clumping. However, we show that clumping, combined with optical depth effects, can cause significant underestimates of the dust mass, with 20%-80% more dust being present even at moderate optical depths if the streaming instability is active.

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arXiv2021

VERTICO: The Virgo Environment Traced In CO Survey

We present the Virgo Environment Traced in CO (VERTICO) survey, a new effort to map $^{12}$CO($2-1$), $^{13}$CO($2-1$), and C$^{18}$O($2-1$) in 51 Virgo Cluster galaxies with the Atacama Compact Array, part of the Atacama Large Millimeter/submillimeter Array (ALMA). The primary motivation of VERTICO is to understand the physical mechanisms that perturb molecular gas disks, and therefore star formation and galaxy evolution, in dense environments. This first paper contains an overview of VERTICO's design and sample selection, $^{12}$CO($2-1$) observations, and data reduction procedures. We characterize global $^{12}$CO($2-1$) fluxes and molecular gas masses for the 49 detected VERTICO galaxies, provide upper limits for the two non-detections, and produce resolved $^{12}$CO($2-1$) data products (median resolution $= 8^{\prime\prime} \approx 640~{\rm pc}$). Azimuthally averaged $^{12}$CO($2-1$) radial intensity profiles are presented along with derived molecular gas radii. We demonstrate the scientific power of VERTICO by comparing the molecular gas size--mass scaling relation for our galaxies with a control sample of field galaxies, highlighting the strong effect that radius definition has on this correlation. We discuss the drivers of the form and scatter in the size--mass relation and highlight areas for future work. VERTICO is an ideal resource for studying the fate of molecular gas in cluster galaxies and the physics of environment-driven processes that perturb the star formation cycle. Upon public release, the survey will provide a homogeneous legacy dataset for studying galaxy evolution in our closest cluster.

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arXiv2020

Streaming instability on different scales. I. Planetesimal mass distribution variability

We present numerical simulations of dust clumping and planetesimal formation initiated by the streaming instability with self-gravity. We examine the variability in the planetesimal formation process by employing simulation domains with large radial and azimuthal extents and a novel approach of re-running otherwise identical simulations with different random initializations of the dust density field.We find that the planetesimal mass distribution and the total mass of dust that is converted to planetesimals can vary substantially between individual small simulations and within the domains of larger simulations. Our results show that the non-linear nature of the developed streaming instability introduces substantial variability in the planetesimal formation process that has not been previously considered and suggests larger scale dynamics may affect the process.

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arXiv2020

The baryon cycle of Seven Dwarfs with superbubble feedback

We present results from a high-resolution, cosmological, $Λ$CDM simulation of a group of field dwarf galaxies with the "superbubble" model for clustered SN feedback, accounting for thermal conduction and cold gas evaporation. The initial conditions and the galaxy formation physics, other than SN feedback, are the same as in Shen et al. (2014). The simulated luminous galaxies have blue colors, low star formation efficiencies and metallicities, and high cold gas content, reproducing the observed scaling relations of dwarfs in the Local Volume. Bursty star formation histories and superbubble-driven outflows lead to the formation of kpc-size DM cores when stellar masses reaches $M_{*} > 10^6$ $M_{\odot}$, similar to previous findings. However, the superbubble model appears more effective in destroying DM cusps than the previously adopted "blastwave" model, reflecting a higher coupling efficiency of SN energy with the ISM. On larger scale, superbubble-driven outflows have a more moderate impact: galaxies have higher gas content, more extended stellar disks, and a smaller metal-enriched region in the CGM. The two halos with $M_{vir} \sim 10^9$ $M_{\odot}$, which formed ultra-faint dwarf galaxies in Shen et al. (2014), remain dark due to the different impact of metal-enriched galactic winds from two nearby luminous galaxies. The column density distributions of H I, Si II, C IV and O VI are in agreement with recent observations of CGM around isolated dwarfs. While H I is ubiquitous with a covering fraction of unity within the CGM, Si II and C IV are less extended. O VI is more extended, but its mass is only 11% of the total CGM oxygen budget, as the diffuse CGM is highly ionised by the UVB. Superbubble feedback produces C IV and O VI an order of magnitude higher column densities than those with blastwave feedback. The CGM and DM cores are most sensitive probes of feedback mechanisms.

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