Low Ionization Nuclear Emission Line Regions: The "Missing Link" in the Active Galactic Nucleus Population

Abstract

The horizon of the universe, once thought to extend only to the disk of the MilkyWay, is now known to embrace a host of diverse galaxies, from active galaxies suchas quasars and Seyfert galaxies to normal galaxies such as our own. The recentdiscovery that virtually all local galaxies harbor massive nuclear black holes, hasprovided convincing evidence that active galactic nuclei (AGNs) and normal galaxiesare indeed connected. The nature of this connection and the evolutionary historyconnecting them, however, continues to be elusive.Low Ionization Nuclear Emission-line Regions (LINERs) are the dominantpopulation of 'active' galaxies in our local universe and may indeed be the missingpiece to the evolutionary puzzle. LINERs are defined by optical line ratiosuncharacteristic of photoionization by normal main sequence stars. While classicalAGNs represent at most a few percent of the galaxy population, LINERs constituteas much as 50% of the total local extragalactic population. However, despite severaldecades of intense research, the ionization mechanism responsible for the unusualLINER spectrum remains a mystery. What is the ionization mechanism responsiblefor the empirical line ratios characteristic of LINER galaxies? How do LINERs fitinto the overall evolution of galaxies as we know it? Are LINERs a subclass of AGN?What is the evolutionary connection, if any, between galaxies with heavy starburstactivity and AGNs?The majority of LINERs are dust enshrouded and therefore very luminous in thefar-infrared. The far-infrared (far-IR) luminosity to the luminosity in the optical Bband(center = 4400A), the so-called IR-brightness ratio, can be used as a gauge ofthe amount of dust in host galaxy. LINERs span a wide range of LFIR/LB ratios,tending predominantly toward the IR-bright end. However, the majority of researchto-date has been based on optically selected samples which are partial toward IR-faintLINERs. This bias toward IR-faint galaxies could have important consequences onstatistical analyses which examine the fraction of LINERs hosting AGNs. In orderfor an accurate picture of LINERs to emerge, IR-bright as well as IR-faint galaxiesmust be studied. What fraction of IR-bright LINERs are AGNs?In light of the open questions regarding these remarkable objects, the centralgoal of this dissertation is to carry out a systematic multi-wavelength X-ray imagingand Infrared spectroscopic survey of nearby LINERs spanning a wide range of IRbrightnessratios in order to 1) characterize the dominant energy source responsible fortheir optical line ratios, 2) compare the AGN detection rate in our infrared selectedsample with the optically selected samples, 3) determine the luminosities, spectralcharacteristics and accretion properties of the AGN-LINERs and compare them withthe standard active galaxies, and finally, 4) relate the host galaxies properties to theproperties of the central source in an attempt to constrain the role of LINERs ingalaxy evolution and formation models.