A study of PAHs in the universe : organic molecules tracing astrophysical structures, grain growth, and distribution
No Thumbnail Available
Authors
Meeting name
Sponsors
Date
Journal Title
Format
Thesis
Subject
Abstract
Polycyclic Aromatic Hydrocarbons (PAHs) are a widespread and chemically significant component of the interstellar medium, recognized by their distinct infrared emission bands at 3.3, 6.2, 7.7, 8.6, and 11.3 μm. These emission features—originally known as the Unidentified Infrared Emission (UIE) bands—are sensitive to the physical and chemical conditions of their environments, making PAHs powerful tracers of local radiation fields, grain growth, and molecular evolution. This dissertation presents a comprehensive study of PAHs and aliphatic hydrocarbons across a wide range of astrophysical systems, from circumstellar envelopes to high-redshift galaxies, using data from Spitzer, the Very Large Telescope (VLT), and the James Webb Space Telescope (JWST). Through a combination of spectral modeling and spatially resolved analysis, I characterize the size, ionization state, and chemical composition of organic molecules and evaluate their diagnostic potential in tracing grain growth, radiation environments, and the structure of their host systems. The first part of this dissertation focuses on the presence of PAHs in the UV-poor environments surrounding cool carbon-rich stars. Using archival data from the Spitzer Space Telescope, I investigate seven such sources: IRAS Z02229+6208, IRAS 20000+3239, IRAS 22272+5435, IRAS 22574+6609, IRAS 23304+6147, W Orionis, and IRAS 13416–624. Despite the relatively low effective temperatures of these stars (Teff < 6000 K), strong PAH emission bands are observed in their circumstellar envelopes. I model the vibrational excitation of PAHs under these UV-deficient conditions and demonstrate that visible photons are sufficient to excite PAHs to levels consistent with the observed UIE features. For each source, I derive the characteristic PAH sizes, charge fractions, and total PAH mass, and discuss the implications for PAH formation, survivability, and chemical processing in these evolved environments. In the second part, I transition to the protoplanetary disk (PPD) phase of star formation, where PAHs play a role in disk chemistry and structure. Using high spatial resolution VISIR-NEAR longslit spectroscopic observations from the VLT, I analyze the PAH emission bands at 8.6 and 11.3 μm in the disks surrounding two Herbig Ae stars: HD 97048 and HD 169142. These bands, extending out to ∼100–200AU from the central stars, offer insight into the spatial distribution and evolution of PAH properties within the disks. By modeling these features, I derive radial profiles of PAH size, ionization state, and mass, revealing how local disk conditions—such as gas temperature, electron density, and radiation intensity—affect the charge state and composition of PAHs. This analysis underscores the utility of PAHs as diagnostics of disk structure and evolutionary stage. The third component of this work examines merging galaxies with prominent starburst and AGN activity, using JWST/MIRI integral field spectroscopy. I focus on NGC 3256 and VV 114, both nearby luminous infrared galaxies (LIRGs) characterized by merging cores and active star-forming arms. In each system, I select multiple regions with a mix of physical and spatial characteristics and use the spatially resolved 6.2, 7.7, and 11.3 μm PAH bands to characterize the ionization fraction and representative size of the PAH population. Despite the intense radiation fields expected near an AGN, we find surprisingly small PAH molecules surviving within 100 pc of the nuclei. Additionally, I estimate the energy density parameter U for each region, bounded below by the local mass and above by the requirement for single-photon heating, to trace the evolution of the local radiation field. These results suggest that even in AGN-dominated environments, PAHs persist and can be used to trace energetic feedback. Expanding the spectral coverage to shorter wavelengths, I employ JWST/NIRSpec integral field spectroscopy to probe the 3.3 and 3.4 μm bands in NGC 3256 and VV 114 across 64 regions each, with 20-40 pc apertures. These features are associated with aromatic and aliphatic hydrocarbons, respectively, and provide critical insight into grain chemistry and ice interactions. Both galaxies exhibit clear aliphatic emission bands closer to their AGN centers than previously anticipated, along with enhanced 3.4-3.6 μm emission features that had been too weak to detect prior to JWST. The presence of strong water ice extinction across these systems points to heavily obscured cores. We quantify the aliphatic-to-aromatic ratios across the spatial regions and conclude that the AGN influence extends to ∼180 pc in both galaxies, reshaping our understanding of where complex organic molecules can persist in such extreme environments. Finally, I turn to a high-redshift system, J0749+2255 at z = 2.17, which hosts two merging AGN cores and active star formation. Using rest-frame JWST/MIRI data targeting the 3.3 and 3.4 μm features, I analyze 42 spatial regions to study the distribution and abundance of aliphatic hydrocarbons in the early universe. This work confirms, for the first time, the widespread presence of aliphatic compounds at high redshift, marking J0749+2255 as the most distant galaxy with confirmed aliphatic emission to date. I derive regional aliphatic fractions and discuss the implications for dust processing and organic grain evolution in the early stages of galaxy assembly. Together, these studies demonstrate that PAHs and aliphatic hydrocarbons serve as sensitive tracers of the local radiation environment, grain evolution, and feedback processes across a vast range of astrophysical contexts—from cool stars and protoplanetary disks to starburst galaxies and the high-redshift universe. With the unparalleled spatial and spectral resolution of JWST, combined with legacy observations from Spitzer and the VLT, this dissertation reveals the resilience, variability, and diagnostic power of organic molecules in shaping and tracing the universe’s most dynamic environments.
Table of Contents
DOI
PubMed ID
Degree
Ph. D.