Shashank Dattathri

Despite its remarkable success on large scales, the standard Lambda cold dark matter ($\Lambda$CDM) paradigm faces persistent small-scale challenges that have motivated alternative models for the dark sector. Self-interacting dark matter (SIDM) offers a compelling possibility, in which dark matter particles can scatter off each other. Stellar bars are a ubiquitous feature of disc galaxies across cosmic time. Bars are dynamically coupled to their host galaxy’s dark matter halo, and therefore their properties provide a powerful probe of the nature and distribution of dark matter. In this paper, we use idealized, high-resolution N-body simulations and analytic calculations based on kinetic theory to study bar formation and evolution in disc galaxies embedded in SIDM haloes. We find that compared with collisionless CDM, SIDM produces bars that form earlier and grow to larger amplitudes, even for modest self-interaction cross-sections. In several cases, discs that remain stable in CDM, including kinematically hot and dark-matter-dominated discs, develop strong bars in SIDM. This accelerated bar growth occurs because self-interactions broaden the bar–halo resonances and increase angular momentum transfer from the stellar disc to the halo. We explicitly show that this phenomenon is not related to core formation in SIDM haloes. At late times, gravothermal core collapse can raise the central dark matter density enough to weaken or dissolve the bar. These results suggest that the abundance, strength, and redshift evolution of barred galaxies offer a promising observational route to constraining dark matter self-interactions, particularly in light of the growing sample of high-redshift bars revealed by JWST.

The distribution of systems in the black hole (BH) mass–stellar mass ( M _BH – M _⋆ ) plane encodes both the integrated growth of galaxies and their central BHs, and the processes that shape their evolution. Using the ASTRID and TNG300 cosmological simulations, we track massive BHs from cosmic noon ( z = 2) to z = 0.5. TNG300 repositions BHs to the centers of their host galaxies at every time step, while ASTRID instead advances them under the resolved gravitational forces plus a subgrid dynamical friction model, allowing BHs to wander off-center when orbital decay is inefficient. We follow the BHs rather than their original host galaxies, thereby capturing central BHs, BHs in satellites, and wandering BHs. We find that central BHs in both simulations evolve along a tight, nearly redshift-invariant M _BH – M _⋆ relation that is broadly consistent with local empirical constraints. Departures from this relation trace distinct evolutionary channels in which mergers play a key role. Major galaxy mergers drive BH–BH coalescence that dominates the growth of the most massive central BHs. These BHs subsequently quench their hosts through active galactic nucleus (AGN) kinetic feedback. Minor mergers tidally strip satellites to lower M _⋆ at nearly fixed M _BH , producing weakly accreting, overmassive central BHs in gas-poor systems. In ASTRID, satellite accretion and inefficient dynamical friction generate wandering BHs that are undermassive relative to their new hosts and experience minimal accretion or merger-driven growth. A BH’s location in the M _BH – M _⋆ and specific BH accretion rate-specific star formation rate planes is, therefore, a fossil record of its dynamical, accretion, merger, and feedback history.

We present new observations from JWST that reveal a striking kiloparsec-wide cavity in the stellar distribution of the central galaxy in the cluster A402. Supporting data from the Hubble Space Telescope (HST) allow us to rule out extinction due to dust as an explanation, and instead, suggest that this is a localized depression in the stellar density field corresponding to ∼2 × 10 ⁹ M _⊙ in missing stars within a volume of 0.5 kpc ³ . On larger scales, both the JWST and HST data show evidence for a 2.2 kpc flattened core in the stellar distribution (on which the smaller-scale cavity is superimposed), which implies the presence of a central ultramassive black hole with M _BH ∼ 5 × 10 ¹⁰ M _⊙ . We report evidence for a mid-IR-bright point source at one edge of the cavity, suggesting that this black hole is actively accreting. MUSE spectroscopy reveals that this source is a low-ionization nuclear emission-line region active galactic nucleus (AGN) and that there is a second candidate AGN on the opposite side of the cavity with a relative velocity of 370 km s ⁻¹ —if real, this implies the presence of a kiloparsec-separation dual AGN with a total binary mass of 6 ± 2 × 10 ¹⁰ M _⊙ , which would make this the most massive binary black hole system discovered to date. We propose that this unique stellar cavity is the result of a short-lived dynamical interaction between at least one supermassive black hole and the background stellar density field, caused either by three-body scattering during binary hardening or the induction of a dipole instability in the stellar density field.

A recent study of the compact elliptical galaxy NGC 4486B using JWST Near-Infrared Spectrograph integral-field-unit-mode kinematics confirmed a supermassive black hole (SMBH) of mass M BH = 3.6 ± 0.7 × 1 0 8 (∼8% of the stellar mass). In addition to its double nucleus, the nuclear kinematics show pronounced asymmetries: a velocity dispersion peak displaced by 6 pc from the galaxy center and a ∼16 km s ⁻¹ offset in the mean stellar line-of-sight velocity near the SMBH. We examine the origin of the 12 pc double nucleus and show that the observations favor an SMBH surrounded by an eccentric nuclear disk (END). One proposed formation pathway for ENDs involves a gravitational-wave (GW) recoil of the SMBH following a binary merger. Our orbit superposition models contain ∼50% retrograde stars at the edge of the nuclear region, in agreement with END formation simulations. We infer a premerger mass ratio q > 0.15 and a recoil kick of ∼340 km s ⁻¹ . Our N -body simulations show that with such a kick, the SMBH returns to the center within ∼30 Myr. Its flat central core is also consistent with earlier “binary black hole scouring.” We test two alternative mechanisms—buoyancy-driven oscillations and a premerger SMBH binary—but neither reproduces the observed offsets, favoring the GW kick scenario. Our N -body simulations also show that a prograde SMBH binary in a rotating host can become trapped in a corotation resonance, delaying coalescence. Although NGC 4486B is an old galaxy near the Virgo Cluster center, its SMBH appears to have merged only recently, making its nucleus a rare nearby laboratory for postmerger SMBH dynamics.

Self-interacting dark matter (SIDM) is an intriguing alternative to the standard cold dark matter (CDM) paradigm, which predicts that dark matter halos typically have large, isothermal cores. Numerical simulations have shown that dynamical friction ceases to operate in cores of (roughly) constant density, a phenomenon known as core stalling. In addition, such cores often are unstable to a dipole instability that gives rise to dynamical buoyancy, causing massive central objects to move outward. Thus far, these manifestations of core dynamics have only been demonstrated in collisionless systems. In this paper, we use idealized N-body simulations to study whether core stalling and dynamical buoyancy operate in SIDM halos. We find that if the self-interactions are sufficiently strong, neither core stalling nor buoyancy are present, and a massive perturber will quickly sink all the way to the center of its host. In doing so, it gravitationally contracts the core, catalyzing a strongly accelerated core collapse. The reason why core dynamics are so different in SIDM halos is that self-interactions drive the halo’s distribution function to a featureless exponential, removing any inflections or plateaus that are responsible for the dipole instability and core stalling in the case of CDM. We discuss implications of our finding for constraining the nature of dark matter by using observations of massive objects such as supermassive black holes (SMBHs), globular clusters, and nuclear star clusters in the central regions of galaxies.

Dynamical friction governs the orbital decay of massive perturbers within galaxies and dark matter halos, yet its standard Chandrasekhar formulation fails in systems with cores of (roughly) constant density, where inspiral can halt or even reverse, phenomena known respectively as core stalling and dynamical buoyancy. Although these effects have been observed in simulations, the conditions under which they arise remain unclear. Using high-resolution N-body simulations and analytic insights from kinetic theory, we systematically explore the physical origin of these effects. We demonstrate that the overall distribution function (DF) of the host, not just its central density gradient, determines the efficiency and direction of dynamical friction. Core stalling arises when the perturber encounters a plateau in the DF, either pre-existing or dynamically created through its own inspiral, while buoyancy emerges in systems whose DFs possess an inflection that drives an unstable dipole mode. We show that double power-law density profiles with rapid outer-to-inner slope transitions naturally produce such DF features, which is why structurally similar cores can yield radically different dynamical outcomes. Our results provide a unified framework linking the phase-space structure of galaxies to the fate of embedded massive objects, with direct implications for off-center AGN, the dynamics of nuclear star clusters, and the stalled coalescence of black holes in dwarf galaxies and massive ellipticals.

We explore the stability of isotropic, spherical, self-gravitating systems with a double power-law density profile. Systems with rapid transitions between the inner and outer slopes are shown to have an inflection in their isotropic distribution function (DF), where d f / d E > 0, thereby violating Antonov’s stability criterion. Using high-resolution N -body simulations, we show that the resulting instability causes the growth of a rotating dipole (or l = 1) mode. The inflection feature in the DF responds to the mode by promoting its growth, driving the instability. The growth of the dipole results in a torque that dislodges the original cusp from its central location and sets it in motion throughout the central region. Once the mode goes nonlinear, it saturates, together with the cusp, into a long-lived soliton (the l = 1 equivalent of a bar in a disk galaxy), which maintains its sloshing motion through the center of the halo along a slowly precessing, elliptical orbit. Concurrently, the soliton traps increasingly more particles into libration, and the exchange of energy and angular momentum with these trapped particles works toward eroding the bump in the distribution function. We point out similarities between the dipole mode and the bump-on-tail instability in electrostatic plasmas, and highlight a potential connection with core stalling and dynamical buoyancy in systems with a cored density profile. Finally, we discuss the astrophysical implications in terms of lopsidedness and off-center nuclei in galaxies.

We study the coevolution of black holes (BHs) and their host galaxies in the ASTRID and TNG300 cosmological simulations and the DARK SAGE semianalytic model (SAM), focusing on the evolution of the BH mass–stellar mass (M _BH–M _*) relation. Due to differences in the adopted subgrid modeling of BH seeding, dynamics, and feedback, the models differ in their predicted redshift evolution of the M _BH–M _* relation. We find that it is the interplay between the star formation rate (SFR) and the black hole accretion rate (BHAR) that drives the evolution of the mean relation. We define a quantity R , the ratio between the specific BHAR and SFR (i.e., R ≡ sBHAR/sSFR), and demonstrate that it is R that governs the evolution of individual sources in the M _BH–M _* plane. The efficiency of BH growth versus stellar mass growth in the sSFR–sBHAR plane reflects the partitioning of gas between fueling star formation versus BH accretion. This partitioning depends on the implementation of BH dynamics and the nature of how black hole feedback quenches galaxies. In the cosmological simulations (ASTRID and TNG300), the BHAR and SFR are intrinsically linked, resulting in a tight M _BH–M _* correlation, while the DARK SAGE SAM produces a significantly larger scatter. We discuss these results in the context of recently discovered overmassive BHs and massive quenched galaxies at high redshift by the James Webb Space Telescope.

We present a new method to infer the 3D luminosity distributions of edge-on barred galaxies with boxy-peanut/X (BP/X) shaped structures from their 2D surface brightness distributions. Our method relies on forward modelling of newly introduced parametric 3D density distributions for the BP/X bar, disc and other components using an existing image fitting software package (imfit). We validate our method using an N-body simulation of a barred disc galaxy with a moderately strong BP/X shape. For fixed orientation angles, the derived 3D BP/X-shaped density distribution is shown to yield a gravitational potential that is accurate to at least 5 per cent and forces that are accurate to at least 15 per cent, with average errors being |$\sim 1.5~{{\ \rm per\ cent}}$| for both. When additional quantities of interest, such as the orientation of the bar to the line of sight, its pattern speed, and the stellar mass-to-light ratio are unknown they can be recovered to high accuracy by providing the parametric density distribution to the Schwarzschild modelling code FORSTAND. We also explore the ability of our models to recover the mass of the central supermassive black hole. This method is the first to be able to accurately recover both the orientation of the bar to the line of sight and its pattern speed when the disc is perfectly edge-on.