X-ray absorption of cold gas: Simulating interstellar molecular clouds in the laboratory.

Area of Science:

• Astrophysics
• Laboratory Astrophysics
• X-ray Spectroscopy

Background:

• Galactic and extragalactic X-rays are absorbed by molecules and atoms in giant molecular clouds (GMCs).
• This absorption provides insights into GMC composition and physical conditions.
• Astrophysical X-ray spectra often lack clear molecular signatures.

Purpose of the Study:

• To mimic astrophysical X-ray absorption phenomena in a laboratory setting.
• To develop a technique for identifying molecular signatures in X-ray spectra.
• To investigate the X-ray absorption properties of molecular gases relevant to GMCs.

Main Methods:

• Utilizing a laboratory Z-pinch X-ray source to generate a soft X-ray pseudocontinuum.
• Impacting neutral molecular gas injected from a windowless planar gas-puff.
• Recording absorbed spectra using an X-ray spectrometer with resolving power (λ/Δλ∼420).

Main Results:

• Successfully resolved molecular X-ray absorption lines from atomic lines.
• Measured K-shell spectra of CO2, N2, and O2 at GMC-relevant column densities (10^16–10^18 cm⁻²).
• Observed an increase in excited state populations contributing to absorption edges with increasing gas density.

Conclusions:

• The laboratory technique effectively mimics cosmic X-ray absorption by molecular gas.
• Distinct molecular X-ray absorption lines can be resolved, supporting the search for molecular signatures in astrophysical spectra.
• Experimental results provide valuable data for interpreting X-ray observations of giant molecular clouds.