How Density Variations Impact Emission Lines in CLASSY Spectra

As recent discoveries suggest, the average nebular density of star-forming galaxies appears to increase with redshift (e.g., Sanders et al. 2016; Kaasinen et al. 2017). The COS Legacy Archive Spectroscopic SurveY (CLASSY) is a treasury survey that builds on the Hubble Space Telescope (HST) archive to construct the first high-quality, high-resolution far-ultraviolet (far-UV) spectral database of nearby star-forming galaxies. One of CLASSY’s main objectives is to improve the diagnostic power of the far-UV lines that are our best probes for understanding high redshift galaxies. In doing this, CLASSY will create a powerful atlas of spectra for interpreting the rest-frame Far-UV spectra of the most distant galaxies that the upcoming James Webb Space Telescope and extremely large telescopes will observe. The CLASSY sample consists of 45 nearby star-forming galaxies that have similar properties seen at high redshifts, including a subsample of high-density galaxies classified by their optical [SII] abundances. To better understand this sample, we measure densities across multiple ionization zones, using 45 detections of the optical [SII] line ratio, 21 detections of the optical [ArIV] line ratio, and 27 detections of the far-UV CIII] line ratio. We explore variations in density with ionization state. These results will be important for understanding the role density inhomogeneities play in the strong emissions lines we will observe for distant galaxies with JWST and beyond.