Analyzing the Rest-frame Optical Emission-line and Host Galaxy Properties of ? ∼ 2 Star-forming Galaxies in the MOSDEF Survey

The study of galaxies across multiple epochs is essential for understanding the evolution of the universe. One key time period to study is ? ∼ 2, when star formation activity in the universe is at its peak. Comparing local galaxies to those in this more active time period is a critical way to learn about galaxy evolution by examining the differences and/or similarities in galaxy properties. In this thesis, I study the rest-frame optical emission-line and host galaxy properties of star-forming galaxies at ? ∼ 2 in the MOSFIRE Deep Evolution Field (MOSDEF) survey to better understand the evolution of galaxies over the past 10 Gyr of our universe’s history.First, I investigate correlations between the emission-line properties and the physical and chemical properties of ? ∼ 2 star-forming galaxies in the MOSDEF survey. It is necessary to understand the known offset of ? > 1 galaxies on the [O III]?5008/H? vs. [N II]?6585/H? ([N II] “BPT”) diagram compared to their local counterparts because strong rest-optical emission-lines are commonly used to infer a variety of galaxy properties (e.g. gas-phase oxygen abundance). To investigate the physical driver of this shift, I defined two populations of ? ∼ 2 MOSDEF galaxies on the [N II] BPT diagram, one on and one off (i.e., offset from) the local sequence. I find that these two groups remain separated on the [O III]?5008/H? vs. [S II]??6718,6733/H? ([S II] BPT) diagram and the [O III]??4960,5008/[O II]??3727,3730 vs. ([O III]??4960,5008+[O II]??3727,3730)/H? (O 32 vs. R 23 ) diagram, which suggests that star-forming regions in the more offset galaxies are characterized by harder ionizing spectra at fixed nebular oxygen abundance. Such a phenomenon may be tied to ?-enhancement and massive stars that are chemically “young.”Second, I compare the ? ∼ 2 MOSDEF survey with the ? ∼ 2 portion of the Keck Baryonic Structure Survey (KBSS) that has been observed with MOSFIRE. Like MOSDEF, KBSS studies a large sample of star-forming ga