Tidal Dwarf Galaxies as Laboratories for Understanding the Origins of Star Formation

: (Alternative Format Thesis)

Abstract

Understanding how gas transforms into stars and how this process is influenced by local and global environment represents one of the biggest challenges in modern astrophysics. Tidal dwarf galaxies (TDGs) are unique objects that form in the debris of galaxy interactions. Since the material that forms them is recycled from the interaction, they have near-solar metallicities, making CO a reliable tracer of molecular gas. These systems are expected to have low dark matter content since tidal forces predominantly expel baryonic material from the parent galaxy discs. Combined with their typically high gas fractions and strong tidal forces, these properties make TDGs ideal nearby laboratories for testing star formation theories in extreme dynamical environments.

This thesis takes a two-pronged approach to investigate star formation in TDGs using radio interferometric observations. First, new VLA radio continuum observations of a single TDG were analysed to trace the star formation history of the system by distinguishing thermal and non-thermal emission components. Second, newly proposed and obtained ALMA CO(1-0) observations were used to investigate the molecular gas distribution across multiple spatial scales and its connection to star formation in a sample of four TDGs.

The study of radio continuum emission in TDGs remains a relatively new field, with only two confirmed detections prior to this work. We extended this sample by reducing and analysing VLA radio continuum observations in the 1-2 GHz band of the Arp 94 interacting system, where TDG J1023+1952 is located. We constrained the emission from the TDG and generated an in-band spectral index map of the system. This map reveals the expected behaviour of combined thermal and synchrotron radio emission throughout most of the system. We also discovered an unexpected flat-spectrum region (spectral index ∼ −0.4) at the intersection of a faint tidal tail, suggesting potential re-acceleration of cosmic ray electrons.

We obtained new ALMA CO(1-0) observations using 12-m + 7-m + TP (Total Power) arrays for TDG SQ B, and 7-m + TP array observations for three additional TDGs (NGC 5291N, 5291S, and 7252NW) that had existing 12-m array high-resolution data. By combining multiple ALMA array configurations, we measured the diffuse molecular gas fraction in this four-TDG sample by comparing emission filtered out by the 12-m array with that recovered by the 7-m and TP arrays. We measured small diffuse molecular gas fractions in all four galaxies (< 35%, down to ∼ 0% in two of them). All four TDGs in our sample lie above the integrated Kennicutt-Schmidt relation, in the starburst regime, and including diffuse molecular gas does not alter this conclusion (unlike what has been reported for TDG J1023+1952). Additionally, new high-resolution sub-kpc (∼ 350 pc) observations from the 12-m array for SQ B, combined with ancillary data, allowed us to probe resolved scaling relations between molecular gas mass (M_mol), star formation rate (SFR), and stellar mass (M_⋆). These results indicate that SQ B is a starburst system, where star formation is primarily regulated by molecular gas, and where gas and stars appear to have evolved mostly independently.

Overall, this Thesis provides new insights into TDG systems through radio interferometric observations and expands the sample of TDGs with radio continuum detections and comprehensive molecular gas studies, advancing our understanding of star formation in these unique galactic environments

Date of Award	18 Feb 2026
Original language	English
Awarding Institution	University of Bath
Sponsors	Instituto de Astrofísica de Canarias (IAC) & Science and Technology Facilities Council
Supervisor	Philippe Blondel (Supervisor), Cathryn Mitchell (Supervisor), Ute Lisenfeld (Supervisor), Miguel Querejeta (Supervisor) & Carole Mundell (Supervisor)