Hydrogen 21cm Line Lab

An observation-led browser instrument for exploring Galactic neutral atomic hydrogen (H I) at the 1420.40575177 MHz hyperfine line. The console starts from real survey products and keeps a forward Galactic rotation model available as an explicitly labelled comparison overlay.

Observational Products

The default display is not a decorative face-on galaxy. An observer inside the Milky Way cannot directly photograph a top-down H I disk: such maps require distance and rotation assumptions. This laboratory therefore opens with measured sky and spectral products.

Display	Product Used	Processing in This Repository
H I sky panel	Official HI4PI all-sky public visualisation	Rendered as the observed Galactic sky projection with selectable longitude marker
Spectrum panel	Leiden/Argentine/Bonn (LAB) survey profiles at b = 0.00 deg, 0.60 deg FWHM	Profiles downloaded at 5-degree longitude spacing and interpolated between adjacent observed sightlines
Longitude-velocity panel	The same LAB profiles	Native LSR spectral samples linearly resampled to a common -300 to +300 km s^-1 display grid at 1 km s^-1 intervals

Bundled reduced assets:

data/observations/
  hi4pi_allsky.jpg
  hi4pi_allsky_metadata.json
  lab_plane_profiles.json

The source and processing metadata remain embedded in the JSON products. Credit for the HI4PI visualisation belongs to the HI4PI Collaboration; LAB spectra are supplied through the AIfA EU-HOU LAB extraction service.

Peak Interpretation: What the Spectrum Actually Shows

data/observations/lab_plane_profiles.json bundles real LAB survey spectra at 73 Galactic longitudes (l = -180 to +180 deg in 5-degree steps), not a single fixed sightline -- dragging the longitude slider or clicking the sky panel loads a genuinely different observed brightness-temperature profile each time.

A real HI spectrum along the Galactic plane is rarely a single smooth peak: distinct velocity components correspond to different gas clouds and spiral-arm crossings along the line of sight, each Doppler-shifted by its own circular-rotation velocity. The PEAK INTERPRETATION panel now detects these components automatically (a local-maximum finder with a noise floor and an 8 km/s merge radius to avoid double-counting a single blended hump) and converts each one's observed LSR velocity into an implied Galactocentric radius by inverting the flat-rotation-curve relation:

v_r(R, l) = V0 (R0/R - 1) sin(l)   =>   R = R0 / (1 + v_r / (V0 sin l))

This is the standard kinematic-distance relation used to read spiral structure directly off an HI longitude-velocity diagram (Binney & Merrifield, 1998, Galactic Astronomy, section 9.1). Each detected peak is then classified by its implied radius relative to the solar circle:

• R < 0.35 R0 -- inner Galaxy / bar-influenced region (the flat-rotation-curve distance is unreliable here; real inner-Galaxy kinematics are non-circular).
• 0.35-0.85 R0 -- inner disk, typically tangent-point gas near a spiral-arm crossing.
• 0.85-1.15 R0 -- solar neighbourhood, local (Orion) spur gas.
• 1.15-1.8 R0 -- outer disk, likely Perseus or an outer-arm crossing.
• > 1.8 R0, or |sin(l)| too close to zero -- increasingly unreliable or kinematically undefined (a purely radial line of sight carries no orbital-velocity information).

At the default l = 32 deg sightline, for example, the real LAB spectrum shows multiple resolved components spanning roughly R ~ 5.6 to 11.1 kpc -- direct observational evidence that the line of sight crosses several distinct spiral-arm segments, not one single cloud. This is exactly the kind of qualitative-to-quantitative reasoning a real HI survey paper walks through when reading a longitude-velocity diagram, now automated and attached to every real sightline in the dataset.

Caveat stated deliberately: this inversion assumes a flat rotation curve and pure circular motion. Real gas has non-circular streaming (especially near the bar and spiral shocks), so the implied radii are order-of-magnitude kinematic distances, not survey-grade parallax measurements -- the classification labels above say so explicitly rather than presenting a false precision.

Simulation Overlay

Switch on SIMULATION OVERLAY to compare the measured LAB spectrum with a controlled forward model. Amber curves and the model-components table are computed results, never observational data. Adjustable parameters expose the path length, velocity dispersion, rotation curve, Solar radius and circular speed, and smooth H I emissivity assumptions.

For rest frequency nu_0, the non-relativistic Doppler conversion is:

nu = nu_0 (1 - v_r / c)

For circular planar rotation, line-of-sight radial velocity relative to the local standard of rest is modelled as:

v_r = [v(R) R_0 / R - V_0] sin(l)

The synthetic brightness-temperature overlay is produced by integrating emissivity cells along the sightline and broadening each radial velocity contribution with a Gaussian cloud dispersion. It is suitable for testing how kinematic assumptions resemble or disagree with survey spectra, not for claiming a unique Galactic reconstruction.

Architecture

This is a zero-build application designed for a local HTTP server.

index.html
assets/
  css/style.css                 Mission-control layout and display system
  js/app.js                     Canvas renderer and interaction layer
  js/physicsWorker.js           LAB loading, interpolation and optional forward model
data/observations/              Bundled real-data products and metadata
tools/fetch_lab_observations.py Reproducible public-data acquisition pipeline

All spectral interpolation, l-v image preparation and forward modelling run in physicsWorker.js. The main thread handles Canvas rendering and controls only, keeping interaction smooth even when model settings change rapidly.

Run

From this directory:

python -m http.server 8080

Open http://localhost:8080/. Web Workers and JSON loading require HTTP rather than opening index.html directly from the filesystem.

Rebuild Observational Assets

The compact browser dataset can be regenerated from the public services:

python tools/fetch_lab_observations.py

This downloads LAB brightness-temperature profiles for Galactic longitudes -180 deg through +180 deg in 5 deg increments at b = 0 deg, and retrieves the official HI4PI public image. The build script records the resampling operation in the JSON provenance block.

Scientific Limits

• The LAB slice samples the Galactic plane at 5-degree longitude intervals for responsive teaching and comparison; it is not the complete LAB data cube.
• Spectral interpolation between adjacent LAB positions is a visual exploration aid.
• The HI4PI panel is a real all-sky projection from the Solar viewpoint, not a face-on disk.
• The optional forward model omits radiative-transfer opacity corrections, non-circular flows, distance ambiguity resolution and cloud-by-cloud structure.

References

Binney, J. and Tremaine, S., 2008. Galactic dynamics. 2nd ed. Princeton: Princeton University Press.

Dickey, J.M. and Lockman, F.J., 1990. H I in the Galaxy. Annual Review of Astronomy and Astrophysics, 28(1), pp.215-261.

HI4PI Collaboration, 2016. HI4PI: A full-sky H I survey based on EBHIS and GASS. Astronomy & Astrophysics, 594, A116. https://doi.org/10.1051/0004-6361/201629178.

Kalberla, P.M.W., Burton, W.B., Hartmann, D., Arnal, E.M., Bajaja, E., Morras, R. and Poppel, W.G.L., 2005. The Leiden/Argentine/Bonn (LAB) Survey of Galactic H I. Astronomy & Astrophysics, 440, pp.775-782.