Interpreting the Si II and C II line spectra from the COS Legacy Spectroscopic SurveY using a virtual galaxy from a high-resolution radiation-hydrodynamic simulation

TL;DR

This study uses high-resolution radiation-hydrodynamic simulations to generate mock spectra of low-ionization metal lines, helping interpret real galaxy observations and revealing how different spectral features originate from various ISM regions.

Contribution

It demonstrates that a single simulated galaxy can produce diverse spectra comparable to real observations, improving interpretation of low-ionization line profiles in galaxies.

Findings

Mock spectra accurately replicate observations of 38 galaxies.

Aperture losses significantly affect fluorescent emission detection.

Spectral features originate from distinct ISM regions and vary with viewing angle.

Abstract

Observations of low-ionization state (LIS) metal lines provide crucial insights into the interstellar medium of galaxies, yet, disentangling the physical processes responsible for the emerging line profiles is difficult. This work investigates how mock spectra generated using a single galaxy in a radiation-hydrodynamical simulation can help us interpret observations of a real galaxy. We create 22,500 C II and Si II spectra from the virtual galaxy at different times and through multiple lines of sight and compare them with the 45 observations of low-redshift star-forming galaxies from the COS Legacy Spectroscopic SurveY (CLASSY). We find that the mock profiles provide accurate replicates to the observations of 38 galaxies with a broad range of stellar masses ($10^6$ to $10^9$ $M_\odot$) and metallicities (0.02 to 0.55 $Z_\odot$). Additionally, we highlight that aperture losses explain the weakness of the fluorescent emission in several CLASSY spectra and must be accounted for when comparing simulations to observations. Overall, we show that the evolution of a single simulated galaxy can produce a large diversity of spectra whose properties are representative of galaxies of comparable or smaller masses. Building upon these results, we explore the origin of the continuum, residual flux, and fluorescent emission in the simulation. We find that these different spectral features all emerge from distinct regions in the galaxy's ISM, and their characteristics can vary as a function of the viewing angle. While these outcomes challenge simplified interpretations of down-the-barrel spectra, our results indicate that high-resolution simulations provide an optimal framework to interpret these observations.