High Energy Processes in Active Galaxies

Abstract

This thesis has as an objective the study of very energetic phenomena in the central regions of active galaxies. Intense star formation is triggered by mergers of galaxies, and the funneling of large quantities of gas toward the inner regions is thought to also cause accretion onto the central black hole. Whether the two processes necessarily take place together as a step of galaxy evolution is still debated.

Regardless of the origin of their connection, star formation and strong AGN activity often coexist in the nuclei of galaxies. Even after star formation is no longer ongoing, the nuclear starburst in a merger results in the formation of galactic bulges. Therefore, stellar densities in the inner kiloparsecs of a galaxy are typically high. As stars, or star forming regions, are close to the AGN, interaction between the two is expected to be frequent, and it can result in a multitude of phenomena worthy of study.

The thesis thus focuses on high energy processes taking place in regions where AGN activity and stellar presence is likely important. In particular, the goal of the thesis is to study the emission produced by these phenomena, in the range of X-rays to gamma-rays, whether to disentangle their individual contributions or to detect emission resulting from their interaction. The thesis intends to provide data to continue the study of the starburst-AGN connection, and new models to explain the production of gamma rays in other galaxies.

Part I focuses on the X-ray analysis of a local sample of LIRGs. The Great Observatories All-Sky LIRG survey (GOALS) is a project to study the brightest infrared galaxies at z < 0.1, in all possible wavelengths. In this part we analyze data for 63 LIRGs in the lower luminosity range of GOALS. We provide X-ray images, fluxes, luminosities and radial profiles for 84 individual galaxies, spanning a wide range of activity types and merger stages. We also provide information on AGN presence using X-ray and IR diagnostics, and compare the results to those derived for the brightest GOALS galaxies. The data provided in this part will help shine light into the importance of AGN presence in LIRGs, in the context of galaxy evolution.

Parts II and III focus on the interaction of AGN jets with stellar populations. When the jet of an AGN impacts a stellar wind, or a blob of material, a shock is formed in which particles can be accelerated up to relativistic energies. We estimate, for different scenarios, the dynamical evolution of the obstacles in the jet and gamma-ray emission generated by the accelerated particles. Part II focuses on the presence of whole populations of stars with high mass-loss rates interacting with the jet, whether because they are inside it, or because they expel layers of accumulated shocked material at the moment they penetrate it. Part III deals with single obstacles, in particular the remnant of a supernova explosion that takes place within the jet. In both parts II and III we include dynamical simulations that illustrate the process and how it affects the jet.

Part IV focuses on the interaction of AGN jets with the environment at large scales, in this case considering them as possible sources of reionization at z=6. We estimate the number of jetted sources at the epoch of reionization using rest-UV detections of AGN. We then correct the observed density of sources for effects of obscuration using X-ray data. Jets at high redshift interact strongly with the CMB, upscattering its photons up to ionizing wavelengths through IC, and possibly ionizing the ambient medium significantly.

Esta tesis tiene como objetivo el estudio de fenómenos altamente energéticos en las regiones centrales de las galaxias activas. La formación intensa de estrellas se produce tras una fusión de galaxias, y este mismo fenómeno puede resultar en la acreción de material sobre el agujero negro supermasivo central (AGN). Incluso cuando el episodio de formación de estrellas ha terminado, este ha dado lugar a un bulbo galáctico; por lo que las densidades estelares en las zonas centrales de las galaxias son típicamente elevadas. Por lo tanto, es de esperar que las interacciones entre estrellas y AGN sean un fenómeno frecuente, que puede dar lugar a una gran cantidad de fenómenos interesantes.

En particular, la tesis tiene como objetivo estudiar la emisión producida por estos fenómenos, en el rango de rayos X a rayos gamma, ya sea para distinguir las contribuciones individuales de cada uno o para estudiar la emisión generada en su interacción. La tesis pretende proveer datos para continuar el estudio de la relación entre formación estelar y actividad AGN, y nuevos modelos para explicar la producción de rayos gamma en otras galaxias.

La primera parte de la tesis es un estudio en rayos X de galaxias lumínicas en el infrarojo. Se analiza una muestra local, proporcionando imágenes, flujos, luminosidades y perfiles radiales en rayos X de 84 galaxias individuales. También se proporciona información sobre la presencia de AGN en ellas, usando datos en rayos X e infrarojo.

La parte central de la tesis se centra en el estudio de la interacción entre jets de AGN y estrellas, ya sea porque estas se encuentran dentro de él, porque le inyectan material al penetrarlo, o porque explotan como supernovas en su interior. Se estudia la evolución dinámica de estos fenómenos, y los rayos gamma producidos.

La última parte estima la contribución de los jets de AGN a la reionización del universo, puesto que su interacción con los fotones del fondo cósmico de microondas puede resultar en radiación ionizante a través del mecanismo de Compton inverso.