Confronting the Diversity Problem: The Limits of Galaxy Rotation Curves as a Tool to Understand Dark Matter Profiles

The Astrophysical Journal · 2026-03-18

Abstract While galaxy rotation curves (RCs) provide one of the most powerful methods for measuring dark matter profiles in the inner regions of rotation-supported galaxies, at the dwarf scale there are factors that can complicate this analysis. Given the expectation of a universal profile in dark-matter-only simulations, the diversity of the observed RCs has become an often-discussed issue in Lambda cold dark matter cosmology on galactic scales. We analyze a suite of Feedback in Realistic Environments simulations of 10 10 –10 12 M ⊙ halos with standard cold dark matter and compare the true circular velocity to RC reconstructions. We find that, for galaxies with well-ordered gaseous disks, the measured RC may deviate from the true circular velocity by at most ∼10% within the radius of the disk. However, nonequilibrium behaviors, noncircular motions, and nonthermal and nonkinetic stresses may cause much larger discrepancies, of ∼50% or more. Most RC reconstructions underestimate the true circular velocity, while some reconstructions transiently overestimate it in the central few kiloparsecs, due to dynamical phenomena. We further demonstrate that the features that contribute to these failures are not always visibly obvious in H I observations. If such dwarf galaxies are included in galaxy catalogs, they may give rise to the appearance of “artificial” RC diversity that does not reflect the true variation in underlying dark matter profiles.

<tt>Bloodhound</tt> unleashed: particle-based substructure tracking for cosmological simulations

Monthly Notices of the Royal Astronomical Society · 2026-02-13

ABSTRACT Modern studies of galaxy formation rely heavily on numerical simulations, which in turn require tools to identify and track self-bound structures in stars and dark matter. In this paper, we present Bloodhound, a new halo tracking algorithm optimized to track and characterize substructure in cosmological simulations, a regime that is crucial for studies of the nature of dark matter but where standard methods often have difficulties. Using simulations of Milky Way-mass haloes, we demonstrate that Bloodhound extends subhalo tracking by $3{\text{-}}4\, \mathrm{Gyr}$ on average, and significantly longer for subhaloes with small pericentres, relative to the widely used Rockstar $+$ consistent-trees halo tracking pipeline. We also show that Bloodhound provides continuous tracking, mitigating an issue for the standard technique where subhaloes can be lost and then found again – but assigned to a new merger tree – after several snapshots. This improved tracking leads to a substantially larger number of surviving subhaloes in the inner regions of dark matter haloes, which has several implications for studies of the Milky Way’s satellite galaxy system and its use for constraining properties of dark matter. For example, within the radius where current surveys are complete to ultra-faint galaxies ($D_{\rm MW} \lesssim 50$ kpc), Bloodhound finds more than twice as many subhaloes above the atomic cooling scale relative to the standard tracking method. Our results underscore the importance of robust subhalo tracking techniques in advancing our understanding of galaxy formation and cosmological models.

Pickles on FIRE: The 3D Shape Evolution of Simulated Milky Way-Mass Galaxies

Monthly Notices of the Royal Astronomical Society · 2026-05-21 · 1 citations

Abstract We use reduced-mass eigentensors to quantify the 3D ellipsoidal shape evolution of thirteen Milky Way-mass galaxies simulated using zoom simulations with FIRE-2 physics; all but one form disks at z = 0. We find that all of our Milky Way progenitors go through phases when they are elongated. They often oscillate between spheroidal and elongated shapes in the early Universe over billion-year timescales, with ~25 − 45% of the population having elongated luminosity-weighted shapes at any given time at z = 0.5 − 8.5. In contrast, all stellar populations in our z = 0 Milky Way analogs are symmetric about their minor axes at z = 0, even though the old and intermediate-age stellar populations were often arranged in the shape of elongated pickles or triaxial spheroids at the time they formed meaning these populations changed shape significantly over time. During their transient elongated phases, our galaxies have anisotropic velocity dispersion ellipsoids directed along their spatial major axis; however, their shapes do not correlate with their dark matter fraction nor with the shapes and orientations of their underlying dark matter halos. We find that when treated as a population, the fraction of our galaxy progenitors that are elongated at z &amp;gt; 0.5 is roughly consistent with what is observed for systems of the same mass and redshift. Our results suggest that observed elongated galaxies seen in the early Universe with JWST and HST are not stable structures, but rather transitory phases that are nevertheless statistically common. Some of these observed objects may evolve into Milky Way-like galaxies at z = 0.

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

arXiv (Cornell University) · 2026-04-29

HI-rich starless halos, should they exist, hold great promise for elucidating properties of dark matter halos. This Letter examines the properties of HI-rich failed halos at redshift zero across state-of-the-art cosmological simulations (FIREbox, NIVARIA-LG and Recal-EAGLE). First we compare two numerical analogs with Cloud-9, purported to be the first discovery of a starless HI-rich halo. We argue that differences may be driven by environmental factors, and/or the treatment of gas self-shielding -- which might further limit existing analytic schemes aimed at inferring dark matter halo information from 21 cm HI observations. We also find that the failed halo samples in the three simulations span different regions of the HI-gas-halo mass ($M_{\rm HI}-M_{\rm gas}-M_{\rm 200}$) plane. FIREbox objects occupy a very narrow regime, while NIVARIA-LG extends to a wider range of $M_{\rm 200}$ values - and achieves higher $M_{\rm HI}$ and $M_{\rm gas}$ values. Recal-EAGLE $M_{\rm HI}$ values are similar to FIREbox, albeit with lower gas and halo masses. Lastly, we predict that more HI-rich starless halos can be discovered by exploring the HI-poor regime in the local universe, rather than HI-rich populations at high redshift. Overall, we advocate for the allocation of resources to detect and characterize other HI-rich (and HI-poor) failed halos in the local universe, plus dedicated follow-up spectroscopic observations that scrutinize claims to the absence of a faint stellar component, and that assess their isolation status in detail.

Igniting Galaxy Formation in the Postreionization Universe

The Astrophysical Journal · 2026-01-22 · 1 citations

Abstract It is widely believed that the ultraviolet background produced during the epoch of reionization conspires against the formation of low-mass galaxies. Indeed, this mechanism is often invoked as part of the solution to the so-called “missing satellites problem.” In this paper we employ FIREbox , a large-volume cosmological simulation based on the Feedback In Realistic Environments physics model, to characterize the mechanisms governing galaxy ignition in the postreionization era. By carefully matching recently ignited halos (with stellar ages below 100 Myr at the time of selection) to halos that failed to form any stars, we conclude that the presence of cold dense gas and halo concentration helps incite the process of galaxy formation. Concretely, we find that 100% of recently ignited halos experience cold dense gas enhancements relative to their matched failed counterparts. Likewise, approximately 83% display enhancements in both cold dense gas and Navarro–Frenk–White concentration ( c NFW ), while the remaining ∼17% exhibit enhanced cold dense gas content and suppressed c NFW values. Lastly, our simulation suggests that galaxy ignition can occur as late as z = 2, potentially allowing us to observationally catch this process “in the act” in the foreseeable future.

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

ArXiv.org · 2026-04-29

HI-rich starless halos, should they exist, hold great promise for elucidating properties of dark matter halos. This Letter examines the properties of HI-rich failed halos at redshift zero across state-of-the-art cosmological simulations (FIREbox, NIVARIA-LG and Recal-EAGLE). First we compare two numerical analogs with Cloud-9, purported to be the first discovery of a starless HI-rich halo. We argue that differences may be driven by environmental factors, and/or the treatment of gas self-shielding -- which might further limit existing analytic schemes aimed at inferring dark matter halo information from 21 cm HI observations. We also find that the failed halo samples in the three simulations span different regions of the HI-gas-halo mass ($M_{\rm HI}-M_{\rm gas}-M_{\rm 200}$) plane. FIREbox objects occupy a very narrow regime, while NIVARIA-LG extends to a wider range of $M_{\rm 200}$ values - and achieves higher $M_{\rm HI}$ and $M_{\rm gas}$ values. Recal-EAGLE $M_{\rm HI}$ values are similar to FIREbox, albeit with lower gas and halo masses. Lastly, we predict that more HI-rich starless halos can be discovered by exploring the HI-poor regime in the local universe, rather than HI-rich populations at high redshift. Overall, we advocate for the allocation of resources to detect and characterize other HI-rich (and HI-poor) failed halos in the local universe, plus dedicated follow-up spectroscopic observations that scrutinize claims to the absence of a faint stellar component, and that assess their isolation status in detail.

The Hubble Space Telescope Survey of M31 Satellite Galaxies IV. Survey Overview and Lifetime Star Formation Histories

ArXiv.org · 2025-01-22

From $&gt;1000$ orbits of HST imaging, we present deep homogeneous resolved star color-magnitude diagrams that reach the oldest main sequence turnoff and uniformly measured star formation histories (SFHs) of 36 dwarf galaxies ($-6 \ge M_V \ge -17$) associated with the M31 halo, and for 10 additional fields in M31, M33, and the Giant Stellar Stream. From our SFHs we find: i) the median stellar age and quenching epoch of M31 satellites correlate with galaxy luminosity and galactocentric distance. Satellite luminosity and present-day distance from M31 predict the satellite quenching epoch to within $1.8$ Gyr at all epochs. This tight relationship highlights the fundamental connection between satellite halo mass, environmental history, and star formation duration. ii) There is no difference between the median SFH of galaxies on and off the great plane of Andromeda satellites. iii) $\sim50$\% of our M31 satellites show prominent ancient star formation ($&gt;12$ Gyr ago) followed by delayed quenching ($8-10$ Gyr ago), which is not commonly observed among the MW satellites. iv) A comparison with TNG50 and FIRE-2 simulated satellite dwarfs around M31-like hosts show that some of these trends (dependence of SFH on satellite luminosity) are reproduced in the simulations while others (dependence of SFH on galactocentric distance, presence of the delayed-quenching population) are weaker or absent. We provide all photometric catalogs and SFHs as High-Level Science Products on MAST.

Central densities of dark matter haloes in <scp>fire-2</scp> simulations of low-mass galaxies with cold dark matter and self-interacting dark matter

Monthly Notices of the Royal Astronomical Society · 2025-09-13 · 4 citations

ABSTRACT We investigate the central density structure of dark matter haloes in cold dark matter (CDM) and self-interacting dark matter (SIDM) models using simulations that are part of the Feedback In Realistic Environments (fire) project. For simulated haloes of dwarf galaxy scale ($M_{\rm halo}(z=0)\approx 10^{10}\, \mathrm{ M}_\odot$), we study the central structure in both dissipationless simulations and simulations with full fire-2 galaxy formation physics. As has been demonstrated extensively in recent years, both baryonic feedback and self-interactions can convert central cusps into cores, with the former process doing so in a manner that depends sensitively on stellar mass at fixed $M_{\rm halo}$. Whether the two processes (baryonic feedback and self-interactions) are distinguishable, however, remains an open question. Here we demonstrate that, compared to feedback-induced cores, SIDM-induced cores transition more quickly from the central region of constant density to the falling density at larger radial scales. This result holds true even when including identical galaxy formation modelling in SIDM simulations as is used in CDM simulations, since self-interactions dominate over galaxy formation physics in establishing the central structure of SIDM haloes in this mass regime. The change in density profile slope as a function of radius therefore holds the potential to discriminate between self-interactions and galaxy formation physics as the driver of core formation in dwarf galaxies.

Effects of galactic environment on size and dark matter content in low-mass galaxies

arXiv (Cornell University) · 2025-01-07 · 2 citations

We utilize the cosmological volume simulation, FIREbox, to investigate how a galaxy's environment influences its size and dark matter content. Our study focuses on approximately 1,200 galaxies (886 central and 332 satellite halos) in the low-mass regime, with stellar masses between $10^6$ to $10^9$ $M_{\odot}$. We analyze the size-mass relation ($r_{50} - M_{\star}$), inner dark matter mass-stellar mass ($M^{50}_{\rm DM} - M_{\star}$) relation, and the halo mass-stellar mass ($M_{\rm halo} - M_{\star}$) relation. At fixed stellar mass, we find the galaxies experiencing stronger tidal influences, indicated by higher Perturbation Indices (PI $&gt;$ 1) are generally larger and have lower masses relative to their counterparts with lower Perturbation Indices (PI $&lt;$ 1). Applying a Random Forest regression model, we show that both the environment (PI) and halo mass ($M_{rm halo}$) are significant predictors of a galaxy's relative size and dark matter content. Notably, because $M_{\rm halo}$ is also strongly affected by the environment, our findings indicate that environmental conditions not only influence galactic sizes and relative inner dark matter content directly, but also indirectly through their impact on halo mass. Our results highlight a critical interplay between environmental factors and halo mass in shaping galaxy properties, affirming the environment as a fundamental driver in galaxy formation and evolution.

Supplemental data for The Shape of FIREbox Galaxies and a Potential Tension with Low-Mass Disks

Zenodo (CERN European Organization for Nuclear Research) · 2025-12-20

Data behind the plots as seen in "The Shape of FIREbox Galaxies and a Potential Tension with Low-Mass Disks." The FIREboxShapes_ files store q-values (the projected axis ratio derived from Sersic fits) and other relevant Sersic fitting parameters we use in producing the 2D shape histograms of randomly oriented galaxies. ugrBands: corresponds to Sersic fits of mock images made in the SDSS u-, g-, and r-bands across all mass ranges (Figure 4, 5, A2). MassHigh: corresponds to stellar-mass-surface-density Sersic fits of galaxies in the high mass range (Figure 4). rbandHighNoDust: corresponds to Sersic fits of mock images made in the SDSS r-band without accounting for dust (Figure 4). The 3DShapes_ files store the axis lengths derived from the weighted-tensors as well as the axis lengths derived from Sersic fits of mock images of galaxies rotated to their face-on and edge-on orientations. ASTensor: axes lengths measured from the All Stars tensor (Figure 2, 3) YSTensor: axes lengths measured from the Young Stars tensor (Figure 2, 3) LuminosityTensor: axes lengths measured from the Luminosity tensor (Figure 2, 3) MockRBand: "mock" axes lengths measured from the face-on and edge-on Sersic fits of mock images (Figure 2, 3, A1) GAMA_rband: data from GAMA DR3 used to produce the black solid line in Figure 4. ShapesFraction: fraction of galaxies in each shape classification (Spheroidal, Elongated, Disky) used in Figure 4.