The Synthetic Absorption Line Spectral Almanac (SALSA)

Dylan Nelson, Celine Peroux, Philipp Richter, Matthew M. Pieri, Sebastian Lopez, Rongmon Bordoloi, Siwei Zou, Joseph N. Burchett, Rebecca L. Davies, Rahul Ramesh, Matthew C. Smith, Sanchayeeta Borthakur, Christopher W. Churchill

First listed 2025-10-22 | Last updated 2025-10-22

Abstract

We create the first large-scale mock spectroscopic survey of gas absorption sightlines traversing the interstellar medium (ISM), circumgalactic medium (CGM), and intergalactic medium (IGM) surrounding galaxies of virtual Universes. That is, we create mock, or synthetic, absorption spectra by drawing lines-of-sight through cosmological hydrodynamical simulations, using a new mesh-free Voronoi ray-tracing algorithm. The result is the Synthetic Absorption Line Spectral Almanac (SALSA), which is publicly released on a feature-rich online science platform (www.tng-project.org/spectra). It spans a range of ions, transitions, instruments, observational characteristics, assumptions, redshifts, and simulations. These include, but are not limited to: (ions) HI, OI, CI, MgI, MgII, FeII, SiII, CaII, ZnII, SiIII, SiIV, NV, CII, CIV, OVI; (instruments) SDSS-BOSS, KECK-HIRES, UVES, COS, DESI, 4MOST, WEAVE, XSHOOTER; (model choices) with/without dust depletion, noise, quasar continua, foregrounds; (redshift) from z=0 to z~6; (ancillary data) integrated equivalent widths, column densities, distances and properties of nearby galaxies; (simulations) IllustrisTNG including TNG50, TNG-Cluster, EAGLE, and SIMBA. This scope is not fixed, and will grow and evolve with community interest and requests over time -- suggestions are welcome. The resulting dataset is generic and broadly applicable, enabling diverse science goals such as: (i) studies of the underlying physical gas structures giving rise to particular absorption signatures, (ii) galaxy-absorber and halo-absorber correlations, (iii) virtual surveys and survey strategy optimization, (iv) stacking experiments and the identification of faint absorption features, (v) assessment of data reduction methods and completeness calculations, (vi) inference of physical properties from observables, and (vii) apples-to-apples comparisons between simulations and data.

Short digest

SALSA introduces a first-of-its-kind, large-scale library of synthetic absorption-line spectra through the ISM, CGM, and IGM, generated with a mesh-free Voronoi ray-tracing algorithm applied to cosmological hydrodynamical simulations. The public platform spans ions from HI/MgII to CIV/OVI/NeVIII, multiple instruments (SDSS-BOSS, KECK-HIRES, UVES, COS, DESI, 4MOST, WEAVE, XSHOOTER), redshifts 0–~6, and simulations including IllustrisTNG (TNG50, TNG-Cluster), EAGLE, and SIMBA, with ancillary equivalent widths, column densities, and galaxy–absorber metadata. Example spectra show how multi-component kinematics in CIV and NeVIII emerge at high resolution, enabling apples-to-apples comparisons, stacking tests, and survey optimization. By tying absorber profiles to underlying 3D gas structure, SALSA becomes a versatile community resource for interpreting quasar absorption and galaxy–halo correlations.

Key figures to inspect

• Figure 1: Use the NeVIII column-density map and single sightline crossing a galaxy’s virial radius to connect bubble-like CGM morphology from feedback to the resulting NeVIII 780/770 Å and CIV 1548/1550 Å absorption profiles—i.e., how 3D structure imprints velocity components.
• Figure 2: Inspect the ion–redshift–instrument coverage grid to plan virtual surveys—identify which transitions fall in-band for a chosen spectrograph at each snapshot, and what catalog combinations (ion/line/z) are downloadable.
• Figure 3: Browse the 121-spectrum CIV gallery ordered by equivalent width to gauge the diversity of IGM→CGM→ISM absorbers and how profile shape and blending evolve with increasing EW; useful for stacking and completeness experiments.
• Figure 4: Compare the same CIV systems observed as SDSS-BOSS vs KECK-HIRES to see how low-resolution blending hides narrow velocity components and alters inferred EW/column structure, illustrating instrument-driven systematics.