PhD Defense Public Talk

Emergence, Evolution, and Impact of Stellar Bars in the Last 12 Billion Years

Stellar bars are considered to play a crucial role in the secular evolution of disk galaxies and in the redistribution of gas and angular momentum. In the local universe, stellar bars are common in massive disk galaxies and are well studied. However, at z ≥ 1, the abundance, properties, and impact of stellar bars are poorly known due in part to the limitations of earlier-generation instruments (e.g., Hubble Space Telescope (HST)). With the advent of the James Webb Space Telescope (JWST), we, for the first time, are able to explore stellar bars out to z ∼ 4, corresponding to a time when the Universe was only 11% of its current age. In this thesis, I presented some of the first pioneering explorations of stellar bars in the JWST era, including the abundance, properties, and impact of stellar bars on galaxy evolution at z ∼ 0.5–4.

First, using JWST NIRCam imaging from the CEERS survey, I identify and quantitatively characterize bars in the rest-frame near-infrared at z > 1, demonstrating quantitatively, for the first time, that well-developed bars already exist at z > 2. Second, I perform the first exploration of high-redshift bars at z ∼ 0.5–4, using a mass-complete sample of 1770 galaxies and refining current bar identification methods by employing both ellipse-fitting and visual classification in multiple NIRCam bands to robustly measure bar abundance and properties, and account for systematic effects and observational biases. For bars with projected semi-major axis > 1.5 kpc that can be robustly identified, we find that the observed bar fraction rises from low values (≲10%) at z ∼ 2–4 to ∼30% at z ∼ 0.5, and the average projected bar length and projected bar strength rise over time. Comparison with state-of-the-art cosmological simulations reveals that our observed bar fraction is consistent with the bar fraction predicted by TNG50 simulations for large bars with semi-major axis > 1.5 kpc at z ∼ 0.5–4 and with the bar fraction from Auriga simulations out to z ∼ 1.5. At z ≥ 1.5, many barred galaxies show nearby neighbors, suggesting bars may be tidally triggered. The results highlight the early emergence of stellar bars and the increasing importance of bar-driven secular processes over time. Finally, I performed a pilot spatially resolved SED-fitting study of five barred galaxies at z > 1, combining HST and JWST imaging to map stellar populations and star formation on kiloparsec scales across disks, bars, and central regions. I find the five galaxies show star formation signatures consistent with a bar-driven gas inflow phase in bar-driven scenarios of secular evolution, whereby bar-driven gas inflows cause gas to accumulate to high densities in the CN region and can lead to high SFR and the building of disky bulges.

Together, these results demonstrate the early emergence and growing impact of stellar bars beyond the local universe, and provide important pioneering steps toward future work with larger samples and more comprehensive, multi-wavelength datasets, as well as with next-generation facilities, to further investigate the nature of bars at early cosmic times.