Studying Outflows with Synthetic Absorption Line Spectra from High Resolution Simulations

Abstract: Down-the-barrel absorption line spectra are a powerful probe of the properties of multiphase galactic outflows. In this work, we demonstrate a method to generate realistic synthetic spectra using high resolution ($Δx=5-20 \ \mathrm{pc}$), 20-kiloparsec-scale outflow simulations from the next generation of the CGOLS project. Aiming to mimic modern observational surveys of nearby star-forming galaxies, we generate a suite of UV absorption lines, from which we measure common observables such as equivalent widths and velocity statistics, and compare them to observed values. The velocity statistics from the simulated data agree well with high-resolution, far-UV HST treasury data, but the equivalent widths (EWs) display more variability. Our fiducial EWs are systematically lower than the empirical predictions by $\sim50-97\%$, particularly in the lines with higher ionization potentials, though these values increase with inclination angle, star formation rate, and time. We find a strong correlation between the measured outflow velocity and ionization potential, reminiscent of the known correlation between velocity and temperature from the simulations. We also study the effect of simulation resolution on our statistics, finding the EWs to be much more sensitive to the simulation resolution than the velocities. Overall, we find that we can generate realistic synthetic spectra from high-resolution hydrodynamic simulations that are qualitatively similar to observations. In future work, we will further improve the physical realism of our synthetic spectra by implementing a more accurate ionizing background spectrum, with the ultimate goal of improving constraints on derived outflow properties from observations.