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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 galaxies at ? ∼ 2 with the MOSFIRE instrument; however, there are notable differences in survey construction and key results (e.g., the magnitude of the offset from the local star-forming sequence on the [N II] BPT diagram). Using consistent spectral-energy-distribution (SED) modeling for both surveys reveals that the MOSDEF ? ∼ 2 targeted sample has a higher median stellar mass, lower star-formation rate (SFR) and specific SFR, and redder U−V color compared to KBSS. However, the subsets of the surveys that have been analyzed in previous work with high S/N spectra and multiple emission lines detected are strikingly similar. Aside from stellar population age, all sample properties investigated agree within the median uncertainties. Additionally, applying uniform stellar Balmer absorption correction and emission-line fitting techniques for both samples results in the same offset on the [N II] BPT diagram. I find that the previously believed differences in key results between the two surveys can be attributed toutilizing different SED and emission-line fitting techniques.

Third, I analyze the completeness of the MOSDEF ? ∼ 2 survey. Specifically, I use SED modeling and composite spectra created from spectral stacking to test if the subset of the MOSDEF ? ∼ 2 star-forming galaxies with high S/N spectra is representative of the complete sample of star-forming galaxies. I find that the host galaxy and emission-line properties (most notably offset from the local SDSS sequence on the [N II] BPT diagram) are very similar, indicating that the smaller spectroscopic samples are representative of the full catalog of star-forming galaxies. Additionally, comparing galaxy properties obtained through SED modeling reveals that the ? ∼ 2 sample observed by MOSDEF is representative of all ? ∼ 2 galaxies that met the selection criteria for the MOSDEF survey. Taken together, these results reveal that the emission-line trends established using high S/N ? ∼ 2 detection samples of star-forming galaxies in MOSDEF studies to date are representative of the rest-optical-magnitude-limited star-forming galaxy population at ? ∼ 2.

Fourth, I focus on two actively merging galaxies in the MOSDEF survey at ? = 1.89. We model the SEDs of the merging galaxies to find that they are both massive with low SFRs and similar stellar population ages. Additionally, the star formation in both galaxies began and peaked within a few hundred Myr of each other, suggesting that their bursts of star formation may be connected. For one of these galaxies, GOODS-S 43114, S�rsic profile fitting and a relatively low velocity dispersion estimate indicates that it is a face-on disk; therefore, it will likely undergo a large structural change as it evolves into a massive, slowly-rotating elliptical galaxy in the present day.

Finally, as an earlier part of research I used far-IR ??? ??ℎ??/PACS spectra to investigate the profiles of six OH doublets for a large sample of 178 local galaxies. I assembled ancillary data to probe AGN luminosity, radiation field hardness, dust temperature, and dust obscuration, and find correlations between the EW(OH) and these galaxy observables. Additionally, I comment on how the origin of emission for these OH doublets, whether from radiative pumping by infrared photons or from collisional excitation, may influence these relationships.

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