# Interstellar medium (`data/ism/`) — sources and terms

`data/ism/ism_points.bin`, `data/ism/molecular_clouds.csv` and
`data/ism/dust_clouds.csv` draw the Milky Way's dust, gas and plasma on the holo
map (`scripts/galaxy_ism.gd`; docs/GALAXY.md, "Interstellar medium"). All three
are generated by `tools/build_ism.py`, which downloads every source below
(cached in `tools/.ism_cache/`, ~1.8 GB) and can be rerun to reproduce them.
Downloads: 2026-09-28.

The files are reductions of published data and of published models. Expanse
distributes them under **Creative Commons Attribution 4.0 (CC BY 4.0)** —
https://creativecommons.org/licenses/by/4.0/ — the licence of the Edenhofer et
al. data release. Please keep the citations below.

**VizieR / CDS.** This research has made use of the VizieR catalogue access tool
and the CDS archive, Strasbourg, France (DOI 10.26093/cds/vizier; Ochsenbein,
Bauer & Marcout 2000, A&AS 143, 23). Terms:
https://cds.unistra.fr/vizier-org/licences_vizier.html — use with
acknowledgement of CDS and of each catalogue's paper, as done here.

## Measured data

| What | Source | Used as |
|---|---|---|
| 3D dust within 1.25 kpc | **Edenhofer, G., Zucker, C., Frank, P., Saydjari, A. K., Speagle, J. S., Finkbeiner, D. & Enßlin, T. 2024**, *A parsec-scale Galactic 3D dust map out to 1.25 kpc from the Sun*, A&A 685, A82. Data: Zenodo, DOI 10.5281/zenodo.10658339 (v1.0.2, CC BY 4.0), `mean_and_std_healpix.fits`, HDU `MEAN` (posterior mean; the standard deviation is not used) | 110,000 dust puffs (tag 0); the Local Bubble's wall |
| 3D dust to ~5 kpc in the plane | **Vergely, J.-L., Lallement, R. & Cox, N. L. J. 2022**, *Three-dimensional extinction maps: Inverting inter-calibrated extinction catalogs*, A&A 664, A174 — CDS J/A+A/664/A174, `fits/explore_cube_density_values_025pc_v2.fits` (10 pc steps, 6 × 6 × 0.8 kpc) and `_050pc_v2.fits` (20 pc steps, 10 × 10 × 0.8 kpc); maps produced for the EXPLORE project (https://explore-platform.eu) | 90,000 dust puffs (tags 1, 2) |
| 8,107 molecular clouds | **Miville-Deschênes, M.-A., Murray, N. & Lee, E. J. 2017**, *Physical properties of molecular clouds for the entire Milky Way disk*, ApJ 834, 57 — CDS J/ApJ/834/57, `table1.dat`; from the CO survey of **Dame, T. M., Hartmann, D. & Thaddeus, P. 2001**, ApJ 547, 792 | `molecular_clouds.csv`: every cloud, the paper's near / far choice (`INF`) for distance, radius and mass |
| Named local clouds | **Zucker, C., Speagle, J. S., Schlafly, E. F., Green, G. M., Finkbeiner, D. P., Goodman, A. & Alves, J. 2020**, *A compendium of distances to molecular clouds in the Star Formation Handbook*, A&A 633, A51 — CDS J/A+A/633/A51, `handbook.dat` (326 sightlines) | `dust_clouds.csv`: one row per cloud name (mean direction, median distance, the sightlines' 16–84 % span) |

## Models (published fits, drawn as samples — not observations)

| What | Source | Used as |
|---|---|---|
| Neutral hydrogen disc | **Kalberla, P. M. W. & Dedes, L. 2008**, *Global properties of the H I distribution in the outer Milky Way*, A&A 487, 951 (surface density, mid-plane density, flaring); **Kalberla, P. M. W. & Kerp, J. 2009**, ARA&A 47, 27 (review); warp shape after **Levine, E. S., Blitz, L. & Heiles, C. 2006**, *The vertical structure of the outer Milky Way H I disk*, ApJ 643, 881 and **Kalberla, P. M. W. et al. 2007**, A&A 469, 511 (amplitudes illustrative) | 70,000 gas points (tag 0) |
| Free electrons (warm ionized medium) | **Yao, J. M., Manchester, R. N. & Wang, N. 2017**, *A new electron-density model for estimation of pulsar and FRB distances*, ApJ 835, 29 (YMW16). Parameters as published (Table 2; `ymw16par.txt` / `spiral.txt` of the release). The density functions are our numpy re-implementation of the paper's equations: no code from the GPL-licensed YMW16 program is included | 80,000 gas points (tags 1–7) |
| eROSITA bubbles | **Predehl, P. et al. 2020**, *Detection of large-scale X-ray bubbles in the Milky Way halo*, Nature 588, 227 (≈ 14 kpc above and below the Galactic Centre) | shell shape in `GalaxyISM.BUBBLES` (illustrative spheroids) |
| Fermi bubbles | **Su, M., Slatyer, T. R. & Finkbeiner, D. P. 2010**, *Giant gamma-ray bubbles from Fermi-LAT*, ApJ 724, 1044 (±50° in latitude, 40° wide); spheroids as adopted by YMW16 | shell shape in `GalaxyISM.BUBBLES` |
| Local Bubble | **Zucker, C. et al. 2022**, *Star formation near the Sun is driven by expansion of the Local Bubble*, Nature 601, 334; wall-finding after **Pelgrims, V., Ferrière, K., Boulanger, F., Lallement, R. & Montier, L. 2020**, A&A 636, A17 | wall radii in `ism_points.bin`, inferred from the Edenhofer map |

## Reduction in one paragraph

Each dust map cell holds a dust "mass" m = ρ·V (A_V·pc²; Edenhofer's E is
converted with A_V = 2.8 E, which the build checks against Vergely: the mean
ratio over 300–1,000 pc is 1.04). 200,000 puffs are drawn with probability ∝
ρ^0.75·V, each carrying m/(N p) of its cell's dust and a Gaussian size from the
volume it stands for, clamped to the map's resolution; each is stored as its
peak A_V. The gas models are sampled by rejection from their densities. All
positions are stored as float16 light-years in the holo map's frame. Details:
docs/GALAXY.md and the header of `tools/build_ism.py`.