Environmental Conditions of the Extremes of Star Formation: From Quiescent Clouds to Super Star Clusters

Author: ORCID icon orcid.org/0000-0001-9338-2594
Finn, Molly, Astronomy - Graduate School of Arts and Sciences, University of Virginia
Johnson, Kelsey, Astronomy, University of Virginia
Indebetouw, Remy, Astronomy, University of Virginia

Galaxies form stars in a wide variety of cluster configurations, from loose open clusters that are prevalent in the Milky Way and quickly dissociate, to compact globular clusters that have survived as bound systems for billions of years. To understand the full range of star formation throughout cosmic time, we need to observe extreme environments with a wide variety of star-forming abilities and we need to observe the cold, star-forming molecular gas directly. This dissertation presents work focused on understanding the physical conditions of the molecular gas in a variety of environments and developing new methods for analyzing molecular tracers.

In Chapter 2, I present observations of a molecular cloud identified in the merging Antennae galaxies with the potential to form a globular cluster, nicknamed the “Firecracker.” Since star formation has not yet begun at an appreciable level in this region, this cloud provides an example of what the birth environment of a globular cluster may have looked like before stars form and disrupt the natal physical conditions. I demonstrate that the Firecracker would require an extremely high external pressure to remain bound, which is remarkably consistent with theoretical expectations.

In Chapter 3, I present a comparative study of two galaxies from the LEGUS sample: barred spiral NGC 1313 and flocculent spiral NGC 7793. These two galaxies have similar masses, metallicities, and star formation rates, but NGC 1313 is forming significantly more massive star clusters than NGC 7793. I find surprisingly small differences between the molecular cloud populations in the two galaxies, but there are much larger variations in cloud properties between different regions within each galaxy, especially for NGC 1313. The massive cluster formation of NGC 1313 may be driven by its greater variation in environments, allowing more clouds with the necessary conditions to arise.

In Chapter 4, I present a comparison of four different regions in the LMC: The regions 30 Dor, N159, and N113 are actively forming massive stars, while the quiescent Molecular Ridge is forming almost no massive stars, despite its large reservoir of molecular gas. I find that the Ridge has significantly lower kinetic energy at a given size scale and also lower surface densities than the other regions, resulting in higher virial parameters. This suggests that the Ridge is not forming massive stars as actively as the other regions because it has less dense gas and not because collapse is suppressed by excess kinetic energy.

In Chapter 5, I present a multi-line non-LTE fitting tool to create pixel-by-pixel maps of kinetic temperature, volume density, and column density in the LMC’s Molecular Ridge. The fitted volume density is strongly correlated with the YSOs while no other easily observed metric could match this correlation. This indicates that the fitted volume density is uniquely able to capture the relevant star-forming ability of the cloud.

Finally, in Chapter 6 I depart from star formation to discuss Dark Skies, Bright Kids, an astronomy outreach program that focuses on enriching science education in under-served elementary schools. I find that over the course of our programs, students become more confident in their science abilities, especially female students. I also find that on days that students report being creative and asking questions, they were more likely to say they felt like a scientist and were interested in the day’s topic, suggesting that creativity can be just as important as doing experiments for generating interest in and a sense of participating in science.

PHD (Doctor of Philosophy)
star formation, molecular gas, ALMA
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