Stellar Populations of Tidal Tails

Author:

Rodruck, Michael

Graduate Program:

Astronomy and Astrophysics

Degree:

Doctor of Philosophy

Document Type:

Dissertation

Date of Defense:

July 28, 2021

Committee Members:

Robin Ciardullo, Committee Member & Major Field Represnt
Jane Charlton, Chair & Dissertation Advisor
Yuexing Li, Committee Member & Major Field Represnt
Stephane Coutu, Committee Member & Related Areas Repres
Caryl Gronwall, Committee Member & Major Field Represnt
Randy McEntaffer (he, him), Professor in Charge/Director of Graduate Studies
Jane Charlton, Chair & Major Field Represnt

Keywords:

Interacting galaxies
star formation
star clusters

Abstract:

Galaxy interactions are known to provoke star formation in galactic disks and across tidal tails and other tidally induced features. While previous models have predicted only 10% of the total star formation rate (SFR) to occur in tidal features, recent simulations with explicit stellar feedback have increased this estimate to 20 ~ 50% Observations of the Tadpole galaxy confirm this prediction, as 30% of the system’s SFR is occurring in tidal tail star clusters. More specifically, star clusters themselves can host up to 70% of stellar formation in regions of high gas density. If we want to understand the stellar populations that galaxy interactions produce, we must therefore understand their star clusters. However, many clusters are short-lived, subjected to internal (gas expulsion via winds and supernovae) and external (tidal shocks) forces which disrupt them and spill their contents into the tidal debris. Thus, while the majority of star clusters will be destroyed, evidence of their existence is found in the diffuse light. My dissertation studies these stellar populations: bright, compact sources (star clusters), and faint, diffuse light. Old stars from the parent galaxies and young stars formed after the interaction live within the diffuse tidal debris, both contributing to the total luminosity. If we want to look at the stars formed in the interaction, we need a method for disentangling the two. I show how these two populations can be identified in the cumulative tidal tail light, using imaging with the broadband Sloan ugri filters. This technique allows for determination of the total contribution of luminosity and mass from the young and old stellar components. I use the merging system NGC 3256 and its twin tidal tails as a pilot study for this analysis. I find that both tails are dominated in mass by an old population, formed in the host galaxy, with contributions by a young population, formed during the interaction. I also show that the colors of the diffuse light in the Western tidal tail are similar to the colors of its star clusters, suggesting they have a common origin. While ground-based imaging is suitable for deep photometry, star cluster analysis requires high-resolution imaging. I use the Hubble Space Telescope to obtain images in broadband UBVI filters for a sample of 12 tidal tails, in 7 merging systems. I compare these data to stellar evolutionary models to determine ages and masses for star clusters within the tail, finding that many objects show evidence of emission lines, indicating very young ages. Additionally, I use far-ultraviolet (UV) data from GALEX and Swift to obtain star formation rates within the tails. When compared to the total cluster formation rate, I find that the efficiency of cluster formation follows the star formation rate density in these diffuse regions. Finally, I discuss future studies of tidal tails, and how my work on the diffuse light and star clusters will tie together with 21 cm observations of neutral hydrogen (HI) to create a holistic view of the requirements of star formation in tidal tails.