Noam I. Libeskind

There are indications that the Large Magellanic Cloud (LMC) hosts a central supermassive black hole (SMBH), yet no direct detection exists: one reason is that its present-day location need not necessarily coincide with the photometric or kinematic centers. Here we frame a dynamical targeting problem and predict the present-day sky location of the LMC SMBH. We model the recent LMC–Small Magellanic Cloud (SMC) interaction, which induces a bar-disk offset, and integrate SMBH orbits within a realistic, time-dependent LMC–SMC–Milky Way (MW) potential. We show that the MW tidal field at the present epoch exceeds that of the SMC by at least an order of magnitude, setting a preferred direction for coherent, vertical low-amplitude displacements, while the disk asymmetry drives in-plane offsets. Projecting onto the observed LMC frame, the mean of our predicted SMBH probability distribution is located at ( α , δ ) = ( 80 . ″ 23 , − 69 . ″ 55 ) g, with a 1 σ confidence ellipse having semimajor and semiminor axes of ( a , b ) = ( 1 . ″ 34 , 0 . ″ 56 ) , respectively. This mean is ∼6′ north of the adopted dynamical center, while the mode coincides with the dynamical center and lies well within the 1 σ confidence region. Furthermore, all literature estimates of the LMC center considered here fall within the 2 σ confidence region. This provides a concrete, testable target for ongoing and upcoming spectroscopic surveys, transforming the search for the LMC SMBH from an unconstrained problem into a focused observational strategy.

Numerical simulations are a key tool for deciphering the dynamics of gravitation. Yet, they fail to spatially reproduce the Universe we observe, limiting the comparison between observations and simulations to a statistical level. This is highly problematic for rare, faint, or well-studied nearby objects that are observed in a single environment. The computational cost of recovering this environment in random simulations is prohibitive for most realistic applications. We present ̋amletPM, a method that enables the constraining of initial conditions for cosmological simulations so as to produce evolved numerical universes that can be directly compared to observations of the local Universe. Such simulations are known as constrained simulations. Our method implements the field-level forward modeling of the early-time density field from sparse and noisy measurements of late-time peculiar velocities. The dynamics are integrated with a coarse particle-mesh gravity solver, probing the mildly nonlinear regime. We applied our code to the Cosmicflows-4 compilation of peculiar velocities up to z ). The resulting constrained simulations were analyzed under the framework of the local Universe model as well as an amended version that captures systematic spatial shifts in the scattering of simulated counterparts. (160,h^ -1 ̊m Mpc h^ -1 Mpc ^3 box are presented. These spatially reproduce the observed large-scale structure in the constrained volume. Special attention is given to twelve prominent nearby galaxy clusters, whose simulated counterparts are matched on criteria of mass and separation. We provide a mass estimate constrained by the dynamical environment for each cluster. Field-level forward modeling of the initial conditions can be applied to actual observations so as to produce highly constrained cosmological simulations. In its current form, this method already overtakes in quality the pipeline currently in use in the peculiar-velocity community, although systematic biases still need to be addressed. Furthermore, improving the model can be done easily thanks to the inherent flexibility of the Bayesian approach.

Context. Dark galaxies are small, dark matter-dominated haloes whose gas remains in hydrostatic and thermal equilibrium and has never formed stars. These systems are of particular interest because they represent a strong prediction of the Lambda cold dark matter (ΛCDM) model. As of today, only a few dark galaxy candidates have been detected so far, the most intriguing of which being Cloud-9. Aims. Using several state-of-the-art hydrodynamical simulations, we aim to predict the abundance of dark galaxies within our Local Group, characterise their physical properties, and provide guidance for their potential observational detection. Methods. We analyse Local Group simulations with constrained initial conditions, each run with different codes, implementing different baryonic physics, feedback prescriptions, and employing two distinct values of star-formation density threshold, n _th = 0.13 and 10 cm ⁻³ , to select samples of dark and bright galaxies harboured in haloes of similar mass. Results. We demonstrate that dark galaxies exist in all such simulations, though their number is larger in simulations that use a higher, more realistic n _th . These galaxies, whose gas remains diffuse and never forms stars, predominantly inhabit less-concentrated, higher-spin dark matter haloes than their luminous counterparts. Dark galaxies are typically found in low-density regions at the outskirts of the Local Group. Their formation and evolution across redshift indicate that both the dark matter and gas densities in the surroundings of dark galaxies were consistently lower than those found around bright galaxies, making them less susceptible to interactions, mergers, or gas inflows. We estimate that up to eight dark galaxies should be detectable in H I emission within 2.5 Mpc of the Milky Way, with the FAST radio telescope, accounting for its sky coverage and minimum H I mass and column density. Conclusions. Current hydrodynamical simulations of galaxies, combined with upcoming H I surveys, will offer a direct and powerful test of ΛCDM through their ability to predict and measure properties of dark galaxies within and beyond the Local Group.

We present our investigation of H I -rich Faint Dwarf Galaxies in Simulations (HIDES), specifically using the hestia and Auriga simulations in this work. We select galaxies that are faint ( M _g > −10) and contain sufficient H I ( M _HI > 10 ⁵ M _⊙ ), and identify 89 such objects, one of which is completely starless. Their demographics generally converge across simulations of different resolution, with median masses within the virial radius of M ₂₀₀ ∼ 10 ^9.5 M _⊙ , M _gas ∼ 10 ^7.4 M _⊙ , M _HI ∼ 10 ^6.5 M _⊙ , and M _* ∼ 10 ^5.6 M _⊙ ; low gas metallicity; little or no current star formation; a mean stellar age of ∼11 Gyr; a broadly Navarro–Frenk–White-shaped dark matter profile; and some of them can survive in dense environments as close as ∼300 kpc from a Milky Way–mass neighbor. We find a large scatter in their H I density profiles and M _HI − M _* relation, which cannot be fully modelled by their current halo mass or concentration as in the literature, but can be attributed to ram pressure stripping in dense environments, past mergers, or stellar feedback. In particular, close encounters with massive halos and dense environments can reshape the H I content, which may explain the asymmetric H I map of an intriguing observed analogue, Cloud-9. An empirical fit to their number density profile extended to 3.7 Mpc in constrained local volume simulations, n = 0.25 d MW / 1 Mpc − 1.4 Mpc − 3 , is also provided to aid observational forecasts. We conclude that both the mass assembly history and environmental history play a crucial role in the formation and subsequent diversity of these galaxies.

Context. As tracers of the underlying mass distributions, the peculiar velocities of galaxies are valuable probes of the Universe, allowing us to measure the Hubble constant or to map the large-scale structure and its dynamics. The catalogs of peculiar velocities, however, are noisy, scarce, and prone to various interpretation biases. Aims. We measured the radial and bulk flow directly from the largest available sample of peculiar velocities and did not impose a cosmological prior on the velocity field. Furthermore, a minimum assumption on the shape of the radial flow at large distances enabled us to estimate the local Hubble constant. Methods. To address these issues, we analyzed the Cosmicflows-4 catalog (CF4), the most extensive catalog of galaxy peculiar velocities, reaching a redshift z = 0.1. Specifically, we constructed a forward-modeling approach assuming only a flat Universe, which reconstructs the radial and bulk flows of the velocity field directly from measurements of peculiar velocities. Our method was tested on a series of 64 simulated catalogs that mimicked the complex selection function of CF4 in space and in magnitude. Based on our mock data, we propose a simulation-based correction method that we applied to the CF4 data. Results. Our method recovers the radial flow and the direction and magnitude of the bulk flow throughout the covered volume without bias. The incompleteness of the data leads to a systematic amplification of the underlying flow, however, which in turn leads to a systematic overestimation of the amplitude of the bulk flow. By estimating the (cosmic) variance of the density field at large distances from the Lambda Cold Dark Matter (ΛCDM) model, we were able to extract a value of 75.9 ± 1 (stat) km/s/Mpc from the radial inflow. With regard to the bulk flow, a 3 σ tension is found with ΛCDM on the supergalactic X direction and on the magnitude of the bulk flow around 140 Mpc/ h and 240 Mpc/ h . In summary, our work confirms the existing tension on the Hubble constant measured locally and a significant tension in the local bulk flow with ΛCDM predictions. More work is needed to handle all the possible sources of systematic errors inherent to the treating of composite catalogs such as CF4.

Centaurus A (Cen A) and M 83 form one of the most massive galaxy pairs in the nearby Universe. Although their observed heliocentric velocities suggest motion that is not obviously indicative of mutual attraction, this work presents evidence that Cen A and M 83 are in fact gravitationally attracted toward each other, exhibiting a dynamical interaction analogous to the binary-like motion of the Milky Way and Andromeda in the Local Group (LG). Using the timing argument, calibrated with analog galaxy pairs from the AbacusSummit simulation, we estimated the total mass of the Cen A/M83 system under the assumption that the line-of-sight velocity is dominated by motion toward the system’s barycenter. This yields a total mass of (6.36 ± 1.30)⋅10 ¹² M _⊙ . The inferred mass agrees well with independent estimates based on virial mass measurements and K -band luminosity-to-mass ratios. Together, the consistent bound signature and robust mass determination highlight the Cen A/M83 system as a compelling nearby analog to the LG. A further discussion of NGC 4945 as a main perturber (similar to the Large Magellanic Cloud for the LG) of Cen A is also presented.

We present a study of the gas kinematics within the H ESTIA project, a state-of-the-art set of simulations of the Local Group, with a particular focus on the velocity patterns of different ions and the large-scale motion of gas and galaxies towards the Local Group’s barycentre. Using two of the H ESTIA high-resolution runs, we examined the distribution and velocities of H I , C IV , Si III , O VI , O VII , and O VIII and their imprints on sightlines observed from the Sun’s location in different reference frames. To mimic observational strategies, we assessed the contribution of rotating disc gas, assuming simple kinematic and geometrical considerations. Our results indicate that local absorption features in observed sightlines most likely trace material in the circumgalactic medium of the Milky Way. Some sightlines, however, show that intragroup material could be more easily observed towards the barycentre, which defines a preferred direction in the sky. In particular, H I , Si III , and C IV roughly trace cold gas inside Milky Way and Andromeda haloes, as most of their mass flux occurs within the virial region of each galaxy, while oxygen high ions mostly trace hot halo and intragroup gas, with comparable mass fluxes in the Local Group outskirts and the circumgalactic medium of the two main galaxies. Additionally, we find that pressures traced by different ionic species outside the Milky Way’s halo show systematically higher values towards the barycentre direction in contrast to its antipode in the sky. Kinematic imprints of the global motion towards the barycentre can be seen at larger distances for all ionic species as the Milky Way rams into material in the direction of Andromeda, with gas towards the anti-barycentre lagging behind.

We identify and investigate a preinfall analog of the Large and Small Magellanic Clouds (LMCs, SMCs) in the High-Resolution Environmental Simulations of the Immediate Area suite of constrained cosmological simulations. The system, dynamically isolated from the Local Group, evolves over ∼6 Gyr and forms a multiphase warm coronal halo and a neutral gas stream via repeated tidal interactions, ∼150 kpc in length. The LMC analog’s corona forms self-consistently through virial accretion and inhibits the survival of clumpy neutral structures beyond ∼600 Myr. The SMC analog remains bound through to z = 0, and the pair also exhibits bridge-like and leading-arm features. These results suggest that while most of the ionized stream is formed by the LMC coronal gas, the neutral gas stream, bridge, and leading arm components of the Magellanic System can arise from dwarf–dwarf interactions prior to infall, while the survival and ionization of these features likely require additional environmental processing. Furthermore, we identify a stellar component out of phase to the neutral component of the stream, implying that if the Magellanic stellar stream exists, it may not be spatially coexistent to the dominant H i stream. This system offers a valuable preinfall reference point for interpreting the Magellanic System and identifying analogs beyond the Local Group.