Contrasting Normal and Extreme Environments: A Multiwavelength Census of Star Clusters and Star Forming Regions in Nearby Galaxies

Author: ORCID icon
Linden, Sean, Astronomy - Graduate School of Arts and Sciences, University of Virginia
Evans, Aaron, University of Virginia

This thesis is a study of star formation in a sample of luminous infrared galaxies (LIRGs) and normal star-forming galaxies in the local (z < 0.1) Universe. The study makes use of Hubble Space Telescope (HST), Spitzer Space Telescope and Jansky Very Large Array (VLA) data. The analysis of these data sets has allowed us to make fundamental conclusions about the nature and (in the case of star clusters) fate of star formation in a broad range of star-forming environments. The results are presented in four Chapters. In Chapter 2 I will discuss my work in identifying and characterizing the UV-bright population of super star clusters (SSCs) in a large sample of 22 LIRGs in the Great Observatories All-Sky LIRG Survey (GOALS) with HST. A major result of this work is the discovery that, relative to the normal star-forming galaxies studied with the Legacy Extragalactic UV Survey (LEGUS), the survival rate and maximum mass of SSCs is affected by the active merging-environments of U/LIRGs. In particular, the large number of $10^{6} M_{\odot}$ young clusters identified in the sample demonstrates that LIRGs are capable of producing more high-mass clusters than what is observed to date in any lower luminosity star-forming galaxy in the local Universe. In Chapters 3 and 4 I will present results from two large VLA 33 GHz, 15 GHz, and 3 GHz imaging campaigns of star-forming regions in 50 normal star-forming galaxies, taken from the SINGS/KINGFISH legacy survey as part of the Star Formation in Radio Survey (SFRS), and 68 LIRGs taken as part of GOALS. We have measured flux densities, spectral indices, star-formation rates (SFRs), and ages for nearly 400 individual star-forming regions across a combined galaxy sample which spans nearly 4 decades in stellar and molecular gas mass. Overall, we find that extranuclear regions identified in our LIRG survey have radio spectral indices and thermal fractions consistent with circumnuclear star-forming regions found in the SFRS, and that on 10-100 pc scales radio emission from individual star-forming regions in both normal and extreme galaxies is dominated (> 90\%) by free-free emission, making it one of the most direct and universal probes of the ionizing photon production rate from massive star-forming regions, free from the complications of spatially varying dust extinction. When we place all regions on the star-forming main sequence of galaxies (SFMS), defined here by the SFRS galaxy sample, we find that the star formation rates of extranuclear star-forming regions in LIRGs are not consistent with their host galaxies' globally averaged values, and have a considerably shallower SFR-$M_{*}$ slope. Finally, in Chapter 5 I will present results of a HST WFC3 NUV and ACS/WFC optical study into the cluster populations of a sample of 5 LIRGs in GOALS. The filter selection and the depth of the WFC3 NUV images provide an improved age estimate and wider field of view, respectively, over our prior (Chapter 2) study. This study has yielded strong evidence that SSCs are being rapidly destroyed in luminous galaxy mergers at a rate that exceeds the cluster destruction process occurring in nearby normal galaxies at all galactocentric radii. Further, we show that the overall magnitude of this disruption is location-dependent: clusters found in the inner-regions of LIRGs show greater disruption rates relative to SSCs identified in their outer-disks. Thus, not only does rapid cluster disruption appear to be ubiquitous in LIRGs at all galactocentric radii, the magnitude of the differences between inner- and outer-disk cluster disruption are amplified relative to observations of nearby normal star-forming galaxies.

PHD (Doctor of Philosophy)
Luminous Infrared Galaxies, Galaxy Interactions and Mergers, Spiral Galaxies, Star Formation and Star-Forming Regions, Star Clusters, Radio Continuum Observations, HII Regions
Issued Date: