An "InCLOSE" View of the Circumgalactic Medium of z~2 Star-Forming Galaxies

Citation

Nunez-Cravin, Evan Haze (2025) An "InCLOSE" View of the Circumgalactic Medium of z~2 Star-Forming Galaxies. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/0tj7-d742. https://resolver.caltech.edu/CaltechTHESIS:05282025-211817948

Abstract

This thesis focuses on using diffuse gas to investigate the galactic chemical evolution and circumgalactic medium of galaxies near the peak of cosmic star-formation rate density z ~ 2. There are many fundamental questions that remain unanswered about these processes due to the lack of large observational samples including what the typical yields of massive stars are, the interplay between diffuse circumgalactic and dense interstellar gas in terms of kinematic complexity, metal content, stellar mass, and star formation rate, and the evolution of circumgalactic gas over cosmic time. The common thread between each investigation is the use of QSO absorption line objects (QSO absorbers) to probe diffuse gas that otherwise would be unseen due to its diffusivity.

The chemical evolution of galaxies requires accounting for all sources of nucleosynthesis. During the earliest stages of galactic chemical evolution, the metal yields from core-collapse supernovae (CCSNe) are very important but acquiring empirical constraints is difficult because they cannot be easily disentangled from objects that currently exist because they have been enriched by some fraction of CCSNe and late time nucleosynthetic processes e.g., Type Ia SN. To address this, I used the metal abundances of very-metal poor (VMP; [Fe/H] <-2) Damped Lyman Alpha Absorbers (DLAs; QSO absorbers with high H I column density comparable to the interstellar medium, log(N_HI/cm⁻² > 20.3) to place empirical constraints on the yields of low-metallicity CCSNe. I found that this approach is comparable to, and sometimes superior to, using abundances from the atmospheres of metal poor stars because of the model-independent nature of measuring abundances from dense, cold gas provided by the DLA.

It has been known in the literature that DLAs and other QSO absorbers have a variety of origins so I began an observational campaign to find galaxies associated with QSO absorbers that would allow detailed analysis of the circumgalactic medium (CGM) of z ~ 2 galaxies. This has historically been challenging at all z, but especially at z ~ 2 where, before this thesis, there were only nine galaxies with their inner CGM analyzed (within a projected distance of 100 kpc) and with characterized nebular emission and stellar population properties.

To this end, I am leading a survey that builds on the Keck Baryonic Structure Survey (KBSS) that aims to find close-in galaxy-QSO pairs to directly connect the Inner CGM of QSO Line Of Sight Emitting (InCLOSE) galaxies with their ISM. KBSS-InCLOSE relies on new observations that I have conducted using the twin Keck telescopes on Mauna Kea in Hawaii. I use the new optical integral field unit (rest-FUV at z ~ 2.3) called KCWI to discover new "InCLOSE" galaxies; obtain follow Keck/MOSFIRE near infrared (NIR) spectroscopy to confirm their redshifts and infer nebular properties including star-formation rate; use ground- and space-based optical and NIR images to infer stellar mass and age; and finally use high-resolution optical spectra of the KBSS QSOs to perform detailed analysis of CGM gas seen as absorption in the QSO spectra. The novelty of KBSS-InCLOSE goes beyond its large size (55 galaxies currently); the NIR spectra and images allow for the direct determination of galaxy properties, including stellar mass, which are rarely included in similar high-z surveys.

The first results from KBSS-InCLOSE showcased the tools and techniques required to remove the bright QSOs from the datacubes, images, and spectra to reveal new faint, close-in galaxy-QSO pairs. Particular focus was payed on the processing of the IFU data because it serves as the main driving instrument for the survey since it provides both images and spectra of each galaxy in the field. By analyzing their CGM absorption, I showed that a M=M_*=10¹⁰ M_☉ z=2.43 galaxy exhibited strong, multiphase, kinematically complex, and gravitationally unbound metals in its CGM. This has been seen before in previous studies and may suggest that a consensus picture of the CGM of z ~ 2.3, M_* galaxies is emerging.

In KBSS-InCLOSE II, I focused on the first low-mass galaxies examined in the sample. I showed that the galaxies were star-forming, at the same redshift, and had sizes and masses consistent with dwarf galaxies, and found preliminary insights into the low-mass CGM that would make it distinct from both the massive $z\sim2$ CGM and low-mass local CGM suggesting that there may be strong evolution of the CGM across both stellar mass and redshift.

In Chapter \ref{chapter5_InCLOSEIII} I preview work that is yet to be completed, KBSS-InCLOSE III, where I examined the entire sample showing that the z ~ 2 CGM often shows strong of metal absorption, is likely clumpy, and multiphase, and that future IFU follow-up is necessary to find more galaxies, and NIR spectroscopic follow-up is necessary to secure redshifts to mitigate mismatches between galaxy's and absorbers.

Altogether, this thesis has laid fundamental groundwork towards expanding our understanding of the galaxy-scale baryon cycle of z ~ 2 star-forming galaxies by building the largest z ~ 2 close-in galaxy-QSO pair observational dataset thus far. It provides the data required to perform the most detailed examination of the connection between galaxies and their CGM during the peak epoch for galaxy formation.

Thesis Files