The Synthetic Absorption Line Spectral Almanac (SALSA)

TL;DR

This work presents SALSA, a public library of synthetic absorption spectra generated by mesh-free Voronoi ray-tracing through cosmological hydrodynamical simulations to probe the ISM, CGM, and IGM across $0\le z\le6$. It details an end-to-end pipeline—ionization modeling with Cloudy under a metagalactic UV background, optional dust depletion, Voronoi-based sightline generation, Voigt-profile deposition, instrument-specific realism, and structured data outputs—producing extensive catalogs for dozens of ions and instruments. The SALSA catalogs enable apples-to-apples comparisons between simulations and data, forward-modeling approaches, and survey-design studies, with public release and ongoing expansion to include more simulations, ions, redshifts, and observational realism. By bridging theoretical gas distributions with observable absorption features, SALSA provides a robust platform for interpreting CGM/IGM gas physics and guiding future observational campaigns.

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.

Figures (12)

• Figure 1: Schematic overview of the creation of synthetic absorption spectra from a cosmological hydrodynamical simulation. The main view shows projected NeVIII column density at $z=0.7$ across the TNG50 volume. A sightline traverses this simulated volume (dashed white line) and intersects the projected virial radius (white circle) of a galaxy with $M_\star = 10^{10.3}$ M$_{\odot}$ and $M_{\rm halo} = 10^{11.9}$ M$_{\odot}$, as shown in the lower-right inset. The circumgalactic medium of this halo shows a complex and multi-scale morphology, including bubble-like features driven by galactic feedback. The resulting density and kinematic structure is imprinted in observable spectroscopic signatures such as the NeVIII 780, 770Å doublet (mock COS-G130M; orange, inspired by the CASBaH survey of burchett19) or the CIV 1548, 1550Å doublet (mock HIRES-B14, at $z=2$; blue).
• Figure 2: Example of ion and redshift coverage for available mock spectra catalogs, shown here for the TNG50-1 simulation. Each filled marker represents a possible dataset: the combination of a particular ion (colors; upper left legend) and transition and the resulting observed wavelength of a full TNG snapshot. The lower right legend shows currently available instruments, although we show only one marker for clarity. For each transition we show the evolution of the observed-frame wavelength as a function of redshift (lines). An online, interactive version of this table is available to actually select and download datasets of interest (www.tng-project.org/spectra).
• Figure 3: Gallery of mock absorption spectra, focusing on the CIV 1548, 1550Å doublet. For a SDSS-BOSS spectral configuration, we randomly select 121 sightlines drawn through the $z=2$ volume of TNG50 that have non-zero CIV equivalent width. These are ordered in ascending EW, from the upper left to the lower right. They pass through a variety of CIV bearing gas structures, from the IGM, to the CGM and ISM of galaxies, all of which imprint a diverse variety of observable absorption signatures in these synthetic spectra.
• Figure 4: Ten mock absorption spectra from TNG50 at $z=2$, selected randomly such that they are evenly spaced in total CIV equivalent width from $\sim 0.3$Å to $\sim 4.6$Å. The left versus right panels compare the same sightlines, and same absorption features, as observed with two different instruments: SDSS-BOSS (left) and KECK-HIRES (right). Complex, multi-component structure is evident at sufficiently high velocity resolution.
• Figure 5: 2D stacked gallery of absorption spectra focusing on the CIV 1548+1550 doublet, where color shows relative flux. From the 1M sample of TNG50 at $z=2$ we include 182,744 spectra with $\rm{EW_{CIV}} > 0.1$Å. These are sorted in descending equivalent width, from top to bottom, where each row shows an individual spectrum. The large statistics of the mock spectra catalogs enables use cases that require abundant statistics and good sampling of the diversity of gas structures giving rise to absorption throughout the diffuse Universe.