arXiv subjects

Shashank Dattathri

Publications and source records attributed to Shashank Dattathri.

13 recordsLinked to original sources

arXiv2026

Tracing black hole and galaxy growth across environments since cosmic noon

The distribution of systems in the black hole (BH) mass-stellar mass ($M_\mathrm{BH}-M_\star$) plane encodes both the integrated growth of galaxies and their central black holes, 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 $M_\mathrm{BH}-M_\star$ 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 $M_\star$ at nearly fixed $M_\mathrm{BH}$, 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 $M_\mathrm{BH}-M_\star$ and specific BH accretion rate-specific star formation rate planes is therefore a fossil record of its dynamical, accretion, merger, and feedback history.

astro-ph.GA↗

arXiv2026

Self-interacting dark matter promotes bar formation in disk galaxies

Despite its remarkable success on large scales, the standard $\Lambda$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 disk 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 disk galaxies embedded in SIDM halos. 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, disks that remain stable in CDM, including kinematically hot and dark-matter-dominated disks, develop strong bars in SIDM. This accelerated bar growth occurs because self-interactions broaden the bar-halo resonances and enhance angular momentum transfer from the stellar disk to the halo. We explicitly show that this phenomenon is not related to core formation in SIDM halos. 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.

astro-ph.GA↗

arXiv2026

A Kiloparsec-Scale Stellar Cavity in the Center of Abell402-BCG May be Caused by Dynamic Interactions with an Ultramassive Black Hole

We present new observations from JWST NIRCam that reveal a striking kpc-wide cavity in the stellar distribution of the central galaxy in the cluster Abell402. Supporting data from 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 ~2x10^9 Msun in missing stars within a volume of 0.5kpc^3. On larger scales, both the JWST and HST data show evidence for a 2.2kpc flattened core in the stellar distribution (on which the smaller-scale cavity is superimposed), which implies the presence of a central ultra-massive black hole with M_BH = 6 +/- 4 x10^10 Msun. 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 reveal that this source is a LINER AGN and that there is a second candidate AGN on the opposite side of the cavity with a relative velocity of 370km/s -- if real, this implies the presence of a kpc-separation dual AGN with a total binary mass of 6 +/- 2 x10^10 Msun, 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.

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arXiv2025

JWST Observations of the Double Nucleus in NGC 4486B: Possible Evidence for a Recent Binary SMBH Merger and Recoil

A recent study of the compact elliptical galaxy NGC 4486B using JWST-NIRSpec IFU kinematics confirmed a supermassive black hole (SMBH) of mass $M_{BH}=3.6\pm0.7\times10^8$ (~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 offset in the mean stellar line-of-sight velocity near the SMBH. We examine the origin of the 12 pc double nucleus and these asymmetries and show that the observations favor an SMBH surrounded by an eccentric nuclear disk (END). END formation models require the SMBH to experience a gravitational wave (GW) recoil following a binary SMBH merger. Our orbit-superposition models contain ~50% retrograde stars at the edge of the nuclear region, in striking agreement with END-formation simulations. We infer a pre-merger mass ratio q>0.15 and a recoil kick of ~340 km/s. 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 pre-merger SMBH binary-but neither reproduces the observed offsets, favoring the GW-kick scenario. Our direct N-body simulations further show that a prograde SMBH binary in a rotating host can stall in a corotation resonance, delaying coalescence. Thus, although NGC 4486B is an old, relaxed galaxy near the Virgo cluster center, its SMBH appears to have merged only recently, making its nucleus a rare nearby laboratory for studying post-merger SMBH dynamics.

astro-ph.GA↗

arXiv2025

Dynamics in the Cores of Self-Interacting Dark Matter Halos: Reduced Stalling and Accelerated Core Collapse

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.

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arXiv2025

Not all cores are equal: Phase-space origins of dynamical friction, stalling and buoyancy

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.

astro-ph.GA↗

arXiv2025

The Dipole Instability in Gravitational $N$-body Systems: A Natural Explanation for Lopsidedness and Off-Centered Nuclei in Galaxies

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 ${\rm d} f/{\rm d} E > 0$, 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 non-linear, 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 towards 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.

astro-ph.GA↗

arXiv2024

Orbital Support and Evolution of CX/OX Structures in Boxy/Peanut Bars

Barred galaxies exhibit boxy/peanut or X-shapes (BP/X) protruding from their disks in edge-on views. Two types of BP/X morphologies exist depending on whether the X-wings meet at the center (CX) or are off-centered (OX). Orbital studies indicate that various orbital types can generate X-shaped structures. Here, we provide a classification approach that identifies the specific orbit families responsible for generating OX and CX-shaped structures. Applying this approach to three different N-body bar models, we show that both OX and CX structures are associated with the x1 orbit family, but OX-supporting orbits possess higher angular momentum (closer to x1 orbits) than orbits in CX structures. Consequently, as the bar slows down the contribution of higher angular momentum OX-supporting orbits decreases and that of lower angular momentum orbits increases resulting in an evolution of the morphology from OX to CX. If the bar does not slow down, the shape of the BP/X structure and the fractions of OX/CX supporting orbits remain substantially unchanged. Bars that do not undergo buckling but that do slow down initially show the OX structure and are dominated by high angular momentum orbits, transitioning to a CX morphology. Bars that buckle exhibit a combination of both OX and CX supporting orbits immediately after the buckling, but become more CX dominated as their pattern speed decreases. This study demonstrates that the evolution of BP/X morphology and orbit populations strongly depends on the evolution of the bar angular momentum.

astro-ph.GA↗

arXiv2024

The redshift evolution of the $M_{\rm BH}-M_*$ scaling relation: new insights from cosmological simulations and semi-analytic models

We study the co-evolution of black holes (BHs) and their host galaxies in the ASTRID and Illustris-TNG300 cosmological simulations and the Dark Sage Semi-Analytic Model (SAM), focusing on the evolution of the BH mass - stellar mass ($M_{\rm BH}-M_*$) relation. Due to differences in the adopted sub-grid modeling of BH seeding, dynamics, and feedback, the models differ in their predicted redshift evolution of the $M_{\rm BH}-M_*$ relation. We find that it is the interplay between the star formation rate (SFR) and the black hole accretion rate (BHAR) which drives the evolution of the mean relation. We define a quantity $\mathcal{R}$, the ratio between the specific BHAR and SFR (i.e. $\mathcal{R} \equiv\ $sBHAR/sSFR), and demonstrate that it is $\mathcal{R}$ that governs the evolution of individual sources in the $M_{\rm BH}-M_*$ 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 AGN feedback quenches galaxies. In the cosmological simulations (ASTRID and Illustris-TNG300), the BHAR and SFR are intrinsically linked, resulting in a tight $M_{\rm BH}-M_*$ correlation, while the Dark Sage SAM produces a significantly larger scatter. We discuss these results in the context of recently discovered over-massive BHs and massive quenched galaxies at high redshift by the James Webb Space Telescope.

astro-ph.GA↗

arXiv2023

Redshifted iron emission and absorption lines in the Chandra X-ray spectrum of Centaurus A

Cen A hosts the closest active galactic nucleus to the Milky Way, which makes it an ideal target for investigating the dynamical processes in the vicinity of accreting supermassive black holes. In this paper, we present 14 Chandra HETGS spectra of the nucleus of Cen A that were observed throughout 2022. We compared them with each other, and contrasted them against the two previous Chandra HETGS spectra from 2001. This enabled an investigation into the spectral changes occurring on timescales of months and 21 years. All Chandra spectra could be well fitted by an absorbed power law with a strong and narrow Fe K$α$ line, a leaked power law feature at low energies, and Si and S K$α$ lines that could not be associated with the central engine. The flux of the continuum varied by a factor of $2.74\pm0.05$ over the course of the observations, whereas the Fe line only varied by $18.8\pm8.8\%$. The photon index increased over 21 years, and the Hydrogen column density varied significantly within a few months as well. The Fe K$α$ line was found at a lower energy than expected from the Cen A redshift, amounting to an excess velocity of $326^{+84}_{-94}~\mathrm{km}~\mathrm{s}^{-1}$ relative to Cen A. We investigated warped accretion disks, bulk motion, and outflows as possible explanations of this shift. The spectra also featured ionized absorption lines from Fe XXV and Fe XXVI, describing a variable inflow.

astro-ph.HE↗

arXiv2023

Early Growing Supermassive Black Holes Strengthen Bars and Boxy/Peanut Bulges

Using N-body simulations we explore the effects of growing a supermassive black hole (SMBH) prior to or during the formation of a stellar bar. Keeping the final mass and growth rate of the SMBH fixed, we show that if it is introduced before or while the bar is still growing, the SMBH does not cause a decrease in bar amplitude. Rather, in most cases, it is strengthened. In addition early growing SMBHs always either decreases the buckling amplitude, delay buckling, or both. This weakening of buckling is caused by an increase in the disk vertical velocity dispersion at radii well beyond the nominal black hole sphere-of-influence. While we find considerable stochasticity and sensitivity to initial conditions, the only case where the SMBH causes a decrease in bar amplitude is when it is introduced after the bar has attained a steady state. In this case we confirm previous findings that the decrease in bar strength is a result of scattering of bar-supporting orbits with small pericenter radii. By heating the inner disk both radially and vertically, an early growing SMBH increases the fraction of stars that can be captured by the Inner Lindblad Resonance (ILR) and the vertical ILR, thereby strengthening both the bar and the boxy peanut shaped bulge. Using orbital frequency analysis of star particles, we show that when an SMBH is introduced early and the bar forms around it, the bar is populated by different families of regular bar-supporting orbits than when the bar forms without an SMBH.

astro-ph.GA↗

arXiv2023

Deprojection and stellar dynamical modelling of boxy/peanut bars in edge-on discs

We present a new method to infer the 3D dimensional 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 modeling 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% and forces that are accurate to at least 15%, with average errors being ~1.5% 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 even when the disc is perfectly edge-on.

astro-ph.GA↗

arXiv2022

Cosmological evolution of gas and supermassive black holes in idealized isolated halos

We study the evolution of baryonic gas in cosmologically growing dark matter halos. To accurately model both the inner and outer regions of the halos, we use a dark matter density profile that transitions smoothly from the NFW profile within the virial radius to a more realistic flat profile far beyond the halo. We construct a dark matter gravitational potential consistent with this density profile, and we use a "cosmological" potential that accounts for gas evolution consistent with Hubble expansion at large radii. Gas is initialized with a density $\approx$ 0.2 times the dark matter density, consistent with the universal baryon fraction $ρ_{\rm g}/(ρ_{\rm g}+ρ_{\rm DM}) \approx 0.17$. We study the formation of the virial shock and evolution of the baryon fraction, including the effects of radiative cooling and AGN jet feedback. The feedback is powered by the accretion of cold gas onto a central supermassive black hole (SMBH). The cores of the halo exhibit heating and cooling cycles, whose strength and duration depend on the feedback efficiency and the halo mass. The central SMBH initially grows exponentially with time in the early quasar phase, but the growth slows down at later times. The baryon fraction in the core decreases with increasing feedback efficiency and decreasing halo mass. While the halo outskirts evolve self-similarly, the core density is non-evolving, in agreement with cluster observations. We analyze the correlations between the properties of the gas and the central SMBH, and explore the existence of a fundamental plane.

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