Variability, Multi-wavelength Properties, and Environments of Active Galactic Nuclei

Abstract

In the early twentieth century, Albert Einstein predicted the existence of black holes with his theory of general relativity. Over the past century, astronomers discovered many evidences that prove Einstein right. One of the most extreme cases of black holes is the supermassive black holes (SMBHs) found at the centers of most galaxies, with masses ranging from millions to billions of solar masses. Some of them are known to be actively consuming surrounding gas and dust, emitting a tremendous amount of energy during this process. These are called active galactic nuclei (AGNs), and these are some of the most energetic phenomena in the universe. AGNs are fascinating objects to study, as they are related to a number of areas in astronomy such as galaxy evolution and black hole physics. One of prominent features of AGNs is variability; their brightness is always changing over time. Observations show that the AGN variability is stochastic, or a "red noise", which means that the brightness difference increases as a function of time lag. This relation follows an approximate power law relation. However, past a certain time lag threshold, the power law index shifts to a smaller value. This point is called the break frequency, and this parameter is known to be correlated with the SMBH mass, suggesting that variability is closely related to the fundamental nature of AGNs. To better understand the nature of AGNs, the AGN variability was analyzed in a few different ways in this dissertation. First, variability characteristics of a sample of galaxies, computed using the data from All-Sky Automated Survey for SuperNovae (ASAS-SN), were examined to distinguish AGNs from quiescent galaxies. By constraining the AGN fraction among galaxies, the timescale of AGN activities compared to galaxies’ age can be estimated. Then the multi-wavelength and environmental properties of the variability selected AGNs are analyzed. Second, the AGN break frequencies in optical power spectral densities (PSDs) are measured and compared to X-ray breaks. Current AGN models suggest that different regions around the AGN emit light at different wavelengths—shorter wavelength from near the central SMBH and longer wavelength from outer parts. If there is a correlation between optical and X-ray break frequencies, it can provide clues on the structure of AGNs and interactions between different regions. To achieve this goal, the optical break frequencies were measured using the data from ASAS-SN and Transiting Exoplanet Survey Satellite (TESS) on the sample of AGNs with X-ray break frequencies previously measured. With these methods, this study can contribute to the understanding of AGNs in both temporal and spatial terms. The variability selection method showed that approximately 3% of galaxies are AGNs, and about 80% of them are classified as low-luminosity AGNs (LLAGNs). Because a portion of luminous AGNs are excluded from the parent galaxy sample, the true AGN fraction may be greater. LLAGNs are a subcategory of AGNs that display signs of activity, but not very energetic. This dissertation shows that LLAGNs most likely reside in denser environments compared to luminous AGNs, suggesting they may be triggered by different mechanisms compared to luminous AGNs. This result provides clues on timescales of different stages and evolution of AGN activities. Upon multi-wavelength analysis, the variability selected AGNs are often not classified as AGNs by traditional methods, meaning that this method provides a useful tool for discovering a larger population of AGNs, especially in the era of big data, as this procedure is automated and versatile so it can be used to any sets of data. From the optical PSDs, a set of break frequencies is measured that is consistent with previous studies. In addition, a new set of break frequencies that is 1--2 orders of magnitude greater is also discovered. The high-frequency optical breaks displayed approximately 1-to-1 correlation with X-ray break frequencies, but with an offset in order of a few days. This can be interpreted as the X-ray variability driving the optical variability with a time delay, as it takes time for light to travel from one region to another. The size estimate from this time delay is consistent with measurements from previous studies, suggesting that this method introduces a new way to analyze the AGN structure.