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Project note · July 10, 2026

July 2026 Update: Absorption Lines and Tracers

Two new analysis operators extend THOR's ray-tracing workflows: mock absorption-line spectra from sightlines and field-aligned tracing along curved trajectories.

Absorption sightlines

The new sightline operator records every gas cell intersected by a ray and integrates its optical depth τ(λ). Peculiar velocity shifts each cell's contribution, while temperature sets its line width, producing a velocity-resolved absorption spectrum.

The sightline-absorption cookbook follows the pipeline from per-cell output to a mock observation with a QSO continuum, an instrument line-spread function, and photon noise, including a comparison with Trident. Any ion with a Cloudy-tabulated density can be integrated, with multiple metal lines handled together. Below, one CGM sightline shows the same gas in O VI, C IV, and Si IV.

A single sightline through the AGORA z = 4 GIZMO CGM, in three ions. The panel is the halo's total-hydrogen column density (N_H projection along +x); the marker is one off-centre sightline through the CGM (impact parameter b ≈ 12 kpc), and the inset shows that sightline's transmitted flux e^−τ in three stacked panels — O VI, C IV and Si IV (±500 km/s about each line) — the same gas seen in three ions, each with its own kinematic structure. A single sightline through the AGORA z = 4 GIZMO CGM, in three ions. The panel is the halo's total-hydrogen column density (N_H projection along +x); the marker is one off-centre sightline through the CGM (impact parameter b ≈ 12 kpc), and the inset shows that sightline's transmitted flux e^−τ in three stacked panels — O VI, C IV and Si IV (±500 km/s about each line) — the same gas seen in three ions, each with its own kinematic structure.

THOR synthesizes millions of absorption-line sightlines in minutes on a single GPU:

HI Lyα sightline benchmark, sightlines-per-second throughput: THOR on GPU and CPU versus Trident HI Lyα sightline benchmark, sightlines-per-second throughput: THOR on GPU and CPU versus Trident

The absorption-lines comparison documents the benchmark setup and accuracy cross-check.

Tracing properties along rays

The tracer operator records requested fields along a trajectory and groups samples into segments. Segments may close at every cell boundary or through a configurable rule, with statistics such as path length and field means stored for each.

Trajectories can be redirected at each cell crossing, allowing rays to follow local magnetic or velocity fields. Magnetic coherence lengths are measured by closing a segment once the field direction changes beyond a threshold. This method is applied in Magnetogenesis by galactic processes (Ramesh, Garaldi & Byrohl 2026) to compare magnetic topology across cosmological seeding models.

Field-aligned rays tracing the magnetic field of the TNG50-4 box (periodic), colored by the local field strength |B|. At the end, straight stretches fade out and each trajectory keeps only its most tangled contiguous stretch.

The coherence-length cookbook covers straight and field-aligned rays, velocity and magnetic fields, and pooled statistics from random ray sources.

Also in this release

This release also adds distributed ray tracing across MPI ranks, device-side Voronoi construction, and multi-observer peeling on HEALPix grids. The Apptainer images now combine CUDA, ROCm, CPU, and MPI support in a single build.