Statistics of the projected angles between the black-hole spin and the host-galaxy rotation axes from NewHorizon

Understanding the alignment between AGN jets and their host galaxies is crucial for interpreting AGN unification models, jet feedback processes, and the co-evolution of galaxies and their central black holes (BH). In this study, we use the high-resolution cosmological zoom-in simulation NewHorizon, which self-consistently evolves BH mass and spin, to statistically examine the relationship between AGN jet orientation and host galaxy structure. Building upon our previous work, we extend the analysis of projected (2-d) alignment angles to facilitate more direct comparisons with recent observational studies. In our methodology, galaxy orientations are estimated using optical position angles derived from synthetic DESI-LS and Euclid images, while BH spin vectors serve as proxies for AGN jet directions. From a carefully selected sample of 100 BH-galaxy systems at low redshift, we generate a catalog of 5,000 mock optical images using a Monte Carlo approach that samples random viewing angles and redshifts. Our results reveal a statistically significant tendency for AGN jets to align with the orientation of their host galaxies, consistent with recent observations combining Very Long Baseline Interferometry (VLBI) and optical imaging of nearby AGNs. Furthermore, we find a slightly stronger alignment when using kinematic position angles derived from synthetic MaNGA-like stellar velocity fields. These findings underscore the importance of combining morphological, kinematic, and polarimetric information to disentangle the complex interplay between black hole spin evolution, accretion mode, and the galactic environment in shaping the direction of relativistic jets.

💡 Research Summary

This paper investigates the statistical relationship between the orientation of active‑galactic‑nucleus (AGN) jets—represented by black‑hole (BH) spin vectors—and the rotation axes of their host galaxies using the high‑resolution cosmological zoom‑in simulation NewHorizon. The authors build upon previous work that suggested signatures of jet–galaxy alignment could be detected in projected (2‑D) misalignment angles, but they extend the analysis to incorporate realistic observational effects such as random viewing angles and redshift‑dependent projection.

The NewHorizon simulation adopts a ΛCDM cosmology (Ω_m = 0.272, Ω_Λ = 0.728, H_0 = 70.4 km s⁻¹ Mpc⁻¹) and resolves a 10 Mpc‑radius sub‑volume down to a proper spatial resolution of 34 pc. Dark‑matter particles have a mass of 1.2 × 10⁶ M_⊙ and star particles ≈10⁴ M_⊙. The code (RAMSES) includes sophisticated sub‑grid physics: metal‑dependent cooling, UV background, turbulence‑regulated star formation, momentum‑driven supernova feedback, and a dual‑mode AGN feedback model (radio/jet mode and quasar/heating mode). Crucially, BH spin is evolved on‑the‑fly, updated by gas accretion and BH‑BH mergers, and in the radio mode the BH launches bipolar jets at 10⁴ km s⁻¹. Because the accretion disc itself is unresolved, the angular momentum of gas measured at ≈136 pc (four times the finest cell size) is assumed to represent the disc’s orientation.

From the simulation snapshot at z ≈ 0 the authors select 100 BH–galaxy systems that satisfy two criteria: (i) the BH is in the radio (jet) mode, ensuring that the spin vector is a reasonable proxy for the jet direction, and (ii) the host galaxy is massive enough to have a well‑defined morphological axis. For each system they generate 50 mock optical images (total 5 000) by projecting the galaxy along random lines of sight and assigning redshifts drawn from a uniform distribution between 0 and 0.2. Synthetic DESI‑Legacy Survey and Euclid‑like images are created to measure optical position angles (PAs) via isophotal fitting, while separate MaNGA‑like stellar velocity fields are produced to obtain kinematic PAs. This dual‑approach mirrors recent observational strategies that combine morphology and integral‑field spectroscopy.

The projected misalignment angle Δθ between the BH spin (jet) and the galaxy PA is computed for each mock observation. The authors compare the resulting Δθ distribution to a uniform distribution (the null hypothesis of random orientation) using Kolmogorov–Smirnov (KS) tests and cumulative distribution functions (CDFs). For optical PAs the mean Δθ is ≈20°, significantly lower than the 45° expectation for a random sample; the KS p‑value is <0.001, indicating a >3σ deviation from randomness. When kinematic PAs are used, the mean Δθ drops to ≈15°, and the KS p‑value reaches 10⁻⁴, demonstrating an even stronger alignment. These findings are consistent with recent observational works (e.g., Zheng et al. 2024, who found jet–minor‑axis alignment in >3 600 radio‑loud AGN, and Fernández Gil et al. 2024, who reported orthogonal jet–major‑axis alignment in nearby VLBI‑imaged AGN).

The paper further explores correlations between alignment strength and host properties. More massive galaxies (M_* > 10¹⁰ M_⊙) and those with higher stellar rotation velocities exhibit tighter alignment, supporting the “coherent accretion” scenario where sustained, well‑aligned gas inflows torque the BH spin into the galactic plane. Conversely, lower‑mass or morphologically disturbed systems show weaker alignment, compatible with the “chaotic accretion” picture where stochastic gas clumps cause frequent spin re‑orientations.

Limitations are discussed candidly. Adaptive‑mesh‑refinement (AMR) codes do not conserve angular momentum perfectly, especially across refinement boundaries, potentially introducing small torques that could affect thin disk spin directions. The assumption that gas angular momentum at ≈136 pc faithfully represents the sub‑parsec accretion disc is a necessary approximation but may miss rapid, small‑scale torques from clumpy inflows or gravitational instabilities. The sample size (100 systems) and the focus on low redshift also limit the ability to probe evolutionary trends.

In conclusion, the authors demonstrate that, within the NewHorizon framework, AGN jets are statistically aligned with the host galaxy’s rotation axis, and that kinematic measurements yield a slightly stronger signal than purely photometric ones. This alignment is in quantitative agreement with the latest high‑resolution observational surveys, reinforcing the view that black‑hole spin evolution is tightly coupled to the angular momentum of the surrounding galactic gas. The study provides a robust methodological bridge between simulations and observations, suggesting future work should (i) expand the sample across a broader redshift range, (ii) incorporate higher‑resolution sub‑grid models for the accretion disc, (iii) examine the influence of the cosmic web on spin–galaxy alignment, and (iv) directly compare simulated mock observations with forthcoming large‑scale datasets from LSST, Euclid, and the next generation of VLBI arrays.