arXiv subjects

Stacy S. McGaugh

Publications and source records attributed to Stacy S. McGaugh.

At least 19 recordsLinked to original sources

arXiv2026

Testing dark matter models and modified gravity theories with spatially resolved HI observations

Understanding the nature of dark matter (DM) is one of the most pressing questions of modern physics. The flat rotation curves of galaxies, especially the extended HI rotation curves from radio observations at 21 cm, played a key historical role to establish the DM problem and continue to be a major tool to test different galaxy formation models, particle DM models, and modified gravity theories. The SKA observatory will revolutionize the study of HI rotation curves thanks to its unprecedented angular resolution, sensitivity, and survey speed. In combination with optical and near-infrared ancillary data, HI surveys with SKA-Mid AA4 will allow us to (1) obtain HI data at spatial resolutions of 1"-2", probing the inner galaxy dynamics with unprecedented details (e.g., the cusp-core problem); (2) move from representative samples of a few hundreds of objects to statistical samples of tens of thousands of objects, allowing us to study the dynamical scaling laws of galaxies and their DM properties with unprecedented statistical power across different cosmic environments; (3) move from galaxies at z~0 to galaxies at z~1, probing DM halos at earlier cosmic epochs and exploring the redshift evolution of the dynamical laws across about 8 Gyrs. The massive SKA data, however, will pose important technical challenges for the derivation of HI rotation curves with automated kinematic software. Developing such kinematic software in the SKA era will be crucial to reach major scientific goals, such as discriminating between different solutions to the DM problem.

astro-ph.GA↗

arXiv2025

The odd primordial halo of the Milky Way implied by Gaia. A shallow core, but a steep decline

Primordial dark matter halos are well understood from cold dark matter-only simulations. Since they can contract significantly as baryons settle into their centers, direct comparisons with observed galaxies are complicated. We present an approach to reversing the halo contraction by numerically calculating the halo response to baryonic infall and iterating the initial condition. This allowed us to derive spherically averaged primordial dark matter halos for observed galaxies. We applied this approach to the Milky Way and found that the latest Gaia measurements for the rotation velocities imply an odd primordial Galactic halo: Its concentration and total mass differ by more than 3$σ$ from the predictions, and the density profile presents an inner core that is too shallow and an outer decline that is too steep to be compatible with the cold dark matter paradigm.

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arXiv2024

The empirical laws of galaxy dynamics: from gas kinematics to weak lensing

Galaxies obey a set of strict dynamical laws, which imply a close coupling between the visible matter (stars and gas) and the observed dynamics (set by dark matter in the standard cosmological context). Here we review recent results from weak gravitational lensing, which allows studying the empirical laws of galaxy dynamics out to exceedingly large radii in both late-type galaxies (LTGs) and early-type galaxies (ETGs). We focus on three laws: (1) the circular velocity curves of both LTGs and ETGs remain indefinitely flat out to several hundreds of kpc; (2) the same baryonic Tully-Fisher relation is followed by LTGs and ETGs; (3) the same radial acceleration relation (RAR) is followed by LTGs and ETGs. Combining galaxy data with Solar System data, the RAR covers about 16 orders of magnitude in the Newtonian baryonic acceleration. Remarkably, these empirical facts were predicted a priori by MOND.

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arXiv2024

Accelerated Structure Formation: the Early Emergence of Massive Galaxies and Clusters of Galaxies

Galaxies in the early universe appear to have grown too big too fast, assembling into massive, monolithic objects more rapidly than anticipated in the hierarchical $Λ$CDM structure formation paradigm. The available photometric data are consistent with there being a population of massive galaxies that form early ($z \gtrsim 10$) and quench rapidly over a short ($\lesssim 1$ Gyr) timescale, consistent with the traditional picture for the evolution of giant elliptical galaxies. Similarly, kinematic observations as a function of redshift show that massive spirals and their scaling relations were in place at early times. Explaining the early emergence of massive galaxies requires either an extremely efficient conversion of baryons into stars at $z>10$ or a more rapid assembly of baryons than anticipated in $Λ$CDM. The latter possibility was explicitly predicted in advance by MOND. We discuss some further predictions of MOND, such as the early emergence of clusters of galaxies and early reionization.

astro-ph.GA↗

arXiv2024

Satellite group infall into the Milky Way: exploring the Crater-Leo case with new HST proper motions

Context. Within $Λ$ Cold Dark Matter ($Λ$CDM) simulations, Milky Way-like galaxies accrete some of their satellite galaxies in groups of 3-5 members rather than individually. It was also suggested that this might be the reason behind the origin of satellite planes. Objects accreted in groups are expected to share similar specific total energy and angular momentum, and also identical orbital planes and directions. Aims. Looking at observations of Milky Way satellites, the dwarf galaxies Leo II, IV, V, Crater II, and the star cluster Crater 1 were proposed to be a vestige of group infall. The suggested "Crater-Leo group" shows a coherent distance gradient and all these objects align along a great circle on the sky. We use proper motion data to investigate whether the phase-space distribution of the members of the proposed group are indeed consistent with group infall. Methods. To further investigate this possibility, we use Gaia Data Release 3 (DR3) and new Hubble Space Telescope (HST) proper motions, $(μ_{α*}, μ_δ) = (-0.1921 \pm 0.0514, -0.0686 \pm 0.0523)$ mas yr$^{-1}$ for Leo IV and $(μ_{α*}, μ_δ) = (0.1186 \pm 0.1943, -0.1183 \pm 0.1704)$ mas yr$^{-1}$ for Leo V, to derive accurate orbital properties for the proposed group objects. In addition, we explore other possible members of this putative association. Results. Leo II, Leo IV, and Crater 1 show orbital properties consistent with those we predict from assuming group infall. However, our results suggest that Crater II was not accreted with the rest of the objects. If confirmed with increasingly accurate proper motions in the future, the Crater-Leo objects appear to constitute the first identified case of a cosmologically expected, typical group infall event, as opposed to the highly hierarchical Magellanic Cloud system.

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arXiv2024

BST1047+1156: A (Failing) Ultradiffuse Tidal Dwarf in the Leo I Group

We use deep Hubble Space Telescope imaging to study the resolved stellar populations in BST1047+1156, a gas-rich, ultradiffuse dwarf galaxy found in the intragroup environment of the Leo I galaxy group. While our imaging reaches approximately two magnitudes below the tip of the red giant branch at the Leo I distance of 11 Mpc, we find no evidence for an old red giant sequence that would signal an extended star formation history for the object. Instead, we clearly detect the red and blue helium burning sequences of its stellar populations, as well as the fainter blue main sequence, all indicative of a recent burst of star formation having taken place over the past 50--250 Myr. Comparing to isochrones for young metal-poor stellar populations, we infer this post-starburst population to be moderately metal poor, with metallicity [M/H] in the range -1 to -1.5. The combination of a young, moderately metal-poor post starburst population and no old stars motivates a scenario in which BST1047 was recently formed during a weak burst of star formation in gas that was tidally stripped from the outskirts of the neighboring massive spiral M96. BST1047's extremely diffuse nature, lack of ongoing star formation, and disturbed HI morphology all argue that it is a transitory object, a "failing tidal dwarf" in the process of being disrupted by interactions within the Leo I group. Finally, in the environment surrounding BST1047, our imaging also reveals the old, metal-poor ([M/H]=-1.3 +/- 0.2) stellar halo of M96 at a projected radius of 50 kpc.

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arXiv2023

Measuring galaxy cluster mass profiles into the low acceleration regime with galaxy kinematics

We probe the dynamical mass profiles of 10 galaxy clusters from the HIghest X-ray FLUx Galaxy Cluster Sample (HIFLUGCS) using galaxy kinematics. We numerically solve the spherical Jeans equation, and parameterize the dynamical mass profile and the galaxy velocity anisotropy profile using two general functions to ensure that our results are not biased towards any specific model. The mass-velocity anisotropy degeneracy is ameliorated by using two "virial shape parameters" that depend on the fourth moment of velocity distribution. The resulting velocity anisotropy estimates consistently show a nearly isotropic distribution in the inner regions, with an increasing radial anisotropy towards large radii. We compare our derived dynamical masses with those calculated from X-ray gas data assuming hydrostatic equilibrium, finding that massive and rich relaxed clusters generally present consistent mass measurements, while unrelaxed or low-richness clusters have systematically larger total mass than hydrostatic mass by an average of 50\%. This might help alleviate current tensions in the measurement of $σ_8$, but it also leads to cluster baryon fractions below the cosmic value. Finally, our approach probes accelerations as low as $10^{-11}$ m s$^{-2}$, comparable to the outskirts of individual late-type galaxies. We confirm that galaxy clusters deviate from the radial acceleration relation defined by galaxies.

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arXiv2023

Cold gas disks in main-sequence galaxies at cosmic noon: Low turbulence, flat rotation curves, and disk-halo degeneracy

We study the dynamics of cold molecular gas in two main-sequence galaxies at cosmic noon (zC-488879 at $z\simeq1.47$ and zC-400569 at $z\simeq2.24$) using new high-resolution ALMA observations of multiple $^{12}$CO transitions. For zC-400569 we also re-analyze high-quality H$α$ data from the SINS/zC-SINF survey. We find that (1) Both galaxies have regularly rotating CO disks and their rotation curves are flat out to $\sim$8 kpc contrary to previous results pointing to outer declines in the rotation speed $V_{\rm rot}$; (2) The intrinsic velocity dispersions are low ($σ_{\rm CO}\lesssim15$ km/s for CO and $σ_{\rm Hα}\lesssim37$ km/s for H$α$) and imply $V_{\rm rot}/σ_{\rm CO}\gtrsim17-22$ yielding no significant pressure support; (3) Mass models using HST images display a severe disk-halo degeneracy: models with inner baryon dominance and models with "cuspy" dark matter halos can fit the rotation curves equally well due to the uncertainties on stellar and gas masses; (4) Milgromian dynamics (MOND) can successfully fit the rotation curves with the same acceleration scale $a_0$ measured at $z\simeq0$. The question of the amount and distribution of dark matter in high-$z$ galaxies remains unsettled due to the limited spatial extent of the available kinematic data; we discuss the suitability of various emission lines to trace extended rotation curves at high $z$. Nevertheless, the properties of these two high-$z$ galaxies (high $V_{\rm rot}/σ_{\rm V}$ ratios, inner rotation curve shapes, bulge-to-total mass ratios) are remarkably similar to those of massive spirals at $z\simeq0$, suggesting weak dynamical evolution over more than 10 Gyr of the Universe's lifetime.

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arXiv2023

Estimating the Oblateness Of Dark Matter Halos Using Neutral Hydrogen Velocity Dispersion

We derive the oblateness parameter q of the dark matter halo of a sample of gas rich, face-on disk galaxies. We have assumed that the halos are triaxial in shape but their axes in the disk plane (a and b) are equal, so that q=c/a measures the halo flattening. We have used the HI velocity dispersion, derived from the stacked HI emission lines and the disk surface density to determine the disk potential and the halo shape at the R25 and 1.5R25 radii. We have applied our model to 20 nearby galaxies, of which 6 are large disk galaxies with M(stellar)>10^10 solar mass, 8 have moderate stellar masses and 6 are low surface brightness dwarf galaxies. Our most important result is that gas rich galaxies that have M(gas)/M(baryons)>0.5 have oblate halos (q < 0.55), whereas stellar dominated galaxies have a range of q values, ranging from 0.21+-0.07 in NGC4190 to 1.27+-0.61 in NGC5194. Our results also suggest a positive correlation between the stellar mass and the halo oblateness q, which indicates that galaxies with massive stellar disks have a higher probability of having halos that are spherical or slightly prolate, whereas low mass galaxies have oblate halos (q < 0.55).

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arXiv2022

Deep Narrowband Photometry of the M101 Group: Strong-Line Abundances of 720 HII Regions

We present deep, narrowband imaging of the nearby spiral galaxy M101 and its satellites to analyze the oxygen abundances of their HII regions. Using CWRU's Burrell Schmidt telescope, we add to the narrowband dataset of the M101 Group, consisting of H$α$, H$β$, and [OIII] emission lines, the blue [OII]$λ$3727 emission line for the first time. This allows for complete spatial coverage of the oxygen abundance of the entire M101 Group. We used the strong-line ratio $R_{23}$ to estimate oxygen abundances for the HII regions in our sample, utilizing three different calibration techniques to provide a baseline estimate of the oxygen abundances. This results in ~650 HII regions for M101, 10 HII regions for NGC 5477, and ~60 HII regions for NGC 5474, the largest sample for this Group to date. M101 shows a strong abundance gradient while the satellite galaxies present little or no gradient. There is some evidence for a flattening of the gradient in M101 beyond $R \sim 14 \text{ kpc}$. Additionally, M101 shows signs of azimuthal abundance variations to the west and southwest. The radial and azimuthal abundance variations in M101 are likely explained by an interaction it had with its most massive satellite NGC 5474 ~300 Myr ago combined with internal dynamical effects such as corotation.

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arXiv2022

Testing Modified Gravity Theories with Numerical Solutions of the External Field Effect in Rotationally Supported Galaxies

The strong equivalence principle is violated by gravity theories of Milgromian dynamics (MOND) through the action of the external field effect. We test two different Lagrangian theories AQUAL and QUMOND based on their numerical solutions of the external field effect, by comparing two independent estimates of the mean external field strength of the nearby universe: a theory-deduced value from fitting the outer rotation curves of 114 galaxies and an empirical value from the large-scale distribution of cosmic baryons. The AQUAL-deduced external field strength from rotation curves agrees with that from the large-scale cosmic environment, while QUMOND-deduced value is somewhat higher. This suggests that AQUAL is likely to be preferred over QUMOND as an effective non-relativistic limit of a potential relativistic modified gravity theory.

astro-ph.GA↗

arXiv2022

Incorporating baryon-driven contraction of dark matter halos in rotation curve fits

The condensation of baryons within a dark matter (DM) halo during galaxy formation should result in some contraction of the halo as the combined system settles into equilibrium. We quantify this effect on the cuspy primordial halos predicted by DM-only simulations for the baryon distributions observed in the galaxies of the SPARC database. We find that the DM halos of high surface brightness galaxies (with $Σ_{\rm eff}\gtrsim100$ $L_\odot$ pc$^{-2}$ at 3.6 $μ$m) experience strong contraction. Halos become more cuspy as a result of compression: the inner DM density slope increases with the baryonic surface mass density. We iteratively fit rotation curves to find the balance between initial halo parameters (constrained by abundance matching), compression, and stellar mass-to-light ratio. The resulting fits often require lower stellar masses than expected for stellar populations, particularly in galaxies with bulges: stellar mass must be reduced to make room for the DM it compresses. This trade off between dark and luminous mass is reminiscent of the cusp-core problem in dwarf galaxies, but occurs in more massive systems: the present-epoch DM halos cannot follow from cuspy primordial halos unless (1) the stellar mass-to-light ratios are systematically smaller than expected from standard stellar population synthesis models, and/or (2) there is a net outward mass redistribution from the initial cusp, even in massive galaxies widely considered to be immune from such effects.

astro-ph.GA↗

arXiv2022

The Effect of Adiabatic Compression on Dark Matter Halos and the Radial Acceleration Relation

We use a semi-empirical model to investigate the radial acceleration relation (RAR) in a cold dark matter (CDM) framework. Specifically, we build 80 model galaxies covering the same parameter space as the observed galaxies in the SPARC database, assigning them to dark matter halos using abundance matching and halo mass-concentration relations. We consider several abundance matching relations, finding some to be a better match to the kinematic data than others. We compute the unavoidable gravitational interactions between baryons and their dark matter halos, leading to an overall compression of the original NFW halos. Before halo compression, high-mass galaxies approximately lie on the observed RAR whereas low-mass galaxies display up-bending "hooks" at small radii due to DM cusps, making them deviate systematically from the observed relation. After halo compression, the initial NFW halos become more concentrated at small radii, making larger contributions to rotation curves. This increases the total accelerations, moving all model galaxies away from the observed relation. These systematic deviations suggest that the CDM model with abundance matching alone cannot explain the observed RAR. Further effects (e.g., feedback) would need to counteract the compression with precisely the right amount of halo expansion, even in high mass galaxies with deep potential wells where such effects are generally predicted to be negligible.

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arXiv2021

Testing the Strong Equivalence Principle. II. Relating the External Field Effect in Galaxy Rotation Curves to the Large-Scale Structure of the Universe

Theories of modified gravity generically violate the strong equivalence principle, so that the internal dynamics of a self-gravitating system in free fall depends on the strength of the external gravitational field (the external field effect). We fit rotation curves (RCs) from the SPARC database with a model inspired by Milgromian dynamics (MOND), which relates the outer shape of a RC to the external Newtonian field from the large-scale baryonic matter distribution through a dimensionless parameter $e_{\rm N}$. We obtain a $>4σ$ statistical detection of the external field effect (i.e. $e_{\rm N}>0$ on average), confirming previous results. We then locate the SPARC galaxies in the cosmic web of the nearby Universe and find a striking contrast in the fitted $e_{\rm N}$ {values} for galaxies in underdense versus overdense regions. Galaxies in an underdense region between 22 and 45 Mpc from the celestial axis in the northern sky have RC fits consistent with $e_{\rm N}\simeq0$, while those in overdense regions adjacent to the CfA2 great wall and the Perseus-Pisces supercluster return $e_{\rm N}$ that are a factor of two larger than the median for SPARC galaxies. We also calculate independent estimates of $e_{\rm N}$ from galaxy survey data and find that they agree with the $e_{\rm N}$ inferred from the RCs within the uncertainties, the chief uncertainty being the spatial distribution of baryons not contained in galaxies or clusters.

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arXiv2021

Mass-Velocity Dispersion Relation in MaNGA Brightest Cluster Galaxies

We investigate a kinematic scaling relation between the baryonic mass and the flat velocity dispersion, i.e. mass-velocity dispersion relation (MVDR), from the brightest cluster galaxies (BCGs) to the galaxy clusters. In our studies, the baryonic mass of BCGs is mainly estimated by photometry. The velocity dispersion profiles are explored with the integrated field unit (IFU) by Mapping Nearby Galaxies at Apache Point Observatory (MaNGA). For the first time, we reveal two significant results with 54 MaNGA BCGs: (1) the flat velocity dispersion profiles; (2) a tight empirical relation on the BCG-cluster scale together with cluster samples, i.e., MVDR, $\log(M_\mathrm{bar}/M_\odot)=4.1^{+0.1}_{-0.1}\log(σ_{\mathrm{los}}/\mathrm{km}\,\mathrm{s}^{-1})+1.6^{+0.3}_{-0.3}$, with a tiny lognormal intrinsic scatter of $10^{+2}_{-1}\%$. This slope is identical to the acceleration relation in galaxy clusters, which is reminiscent of the spiral galaxies, albeit at a larger characteristic acceleration scale. The residuals of the MVDR represent a Gaussian distribution, displaying no correlations with four properties: baryonic mass, scale length, surface density, and redshift. Notably, the MVDR on the BCG-cluster scale provides a strict test, which disfavors the general prediction of the slope of three in the dark matter model.

astro-ph.GA↗

arXiv2021

Comparing the Inner and Outer Star Forming Complexes in the Nearby Spiral Galaxies NGC 628, NGC 5457 and NGC 6946 using UVIT Observations

We present a far-UV (FUV) study of the star-forming complexes (SFCs) in three nearby galaxies using the Ultraviolet Imaging Telescope (UVIT). The galaxies are close to face-on and show significant outer disk star formation. Two of them are isolated (NGC 628, NGC 6946), and one is interacting with distant companions (NGC 5457). We compared the properties of the SFCs inside and outside the optical radius (R$_{25}$). We estimated the sizes, star formation rates (SFRs), metallicities, and the Toomre Q parameter of the SFCs. We find that the outer disk SFCs are at least ten times smaller in area than those in the inner disk. The SFR per unit area ($Σ_{SFR}$) in both regions have similar mean values, but the outer SFCs have a much smaller range of $Σ_{SFR}$. They are also metal-poor compared to the inner disk SFCs. The FUV emission is well correlated with the neutral hydrogen gas (\HI) distribution and is detected within and near several \HI~holes. Our estimation of the Q parameter in the outer disks of the two isolated galaxies suggests that their outer disks are stable (Q$>$1). However, their FUV images indicate that there is ongoing star formation in these regions. This suggests that there may be some non-luminous mass or dark matter in their outer disks, which increases the disk surface density and supports the formation of local gravitational instabilities. In the interacting galaxy, NGC 5457, the baryonic surface density is sufficient (Q$<$1) to trigger local disk instabilities in the outer disk.

astro-ph.GA↗

arXiv2021

Testing the Strong Equivalence Principle: Detection of the External Field Effect in Rotationally Supported Galaxies

The strong equivalence principle (SEP) distinguishes General Relativity from other viable theories of gravity. The SEP demands that the internal dynamics of a self-gravitating system under free-fall in an external gravitational field should not depend on the external field strength. We test the SEP by investigating the external field effect (EFE) in Milgromian dynamics (MOND), proposed as an alternative to dark matter in interpreting galactic kinematics. We report a detection of this EFE using galaxies from the Spitzer Photometry and Accurate Rotation Curves (SPARC) sample together with estimates of the large-scale external gravitational field from an all-sky galaxy catalog. Our detection is threefold: (1) the EFE is individually detected at $8σ$ to $11σ$ in "golden" galaxies subjected to exceptionally strong external fields, while it is not detected in exceptionally isolated galaxies, (2) the EFE is statistically detected at more than $4σ$ from a blind test of 153 SPARC rotating galaxies, giving a mean value of the external field consistent with an independent estimate from the galaxies' environments, and (3) we detect a systematic downward trend in the weak gravity part of the radial acceleration relation at the right acceleration predicted by the EFE of the MOND modified gravity. Tidal effects from neighboring galaxies in the $Λ$CDM context are not strong enough to explain these phenomena. They are not predicted by existing $Λ$CDM models of galaxy formation and evolution, adding a new small-scale challenge to the $Λ$CDM paradigm. Our results point to a breakdown of the SEP, supporting modified gravity theories beyond General Relativity.

astro-ph.GA↗

arXiv2021

Mass-Velocity Dispersion Relation in HIFLUGCS Galaxy Clusters

We investigate the mass-velocity dispersion relation (MVDR) in 29 galaxy clusters in the HIghest X-ray FLUx Galaxy Cluster Sample (HIFLUGCS). We measure the spatially resolved, line-of-sight velocity dispersion profiles of these clusters, which we find to be mostly flat at large radii, reminiscent of the rotation curves of galaxies. We discover a tight empirical relation between the baryonic mass $M_\mathrm{bar}$ and the flat velocity dispersion $σ$ of the member galaxies, i.e. MVDR, $\log(M_\mathrm{bar}/M_\odot)=4.1^{+0.4}_{-0.4}\,\log(σ/\mathrm{km}\,\mathrm{s}^{-1})+1.6^{+1.0}_{-1.3}$, with the lognormal intrinsic scatter of $12^{+3}_{-3}\%$. The residuals of the MVDR are uncorrelated with other cluster properties like temperature, cluster radius, baryonic mass surface density, and redshift. These characteristics are reminiscent of the MVDR for individual galaxies, albeit about ten times larger characteristic acceleration scale. The cluster baryon fraction falls short of the cosmic value, exposing a problem: the discrepancy increases systematically for clusters of lower mass and lower baryonic acceleration.

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