Abstract
Abstract Observational studies are increasingly finding evidence against major mergers being the dominant mechanism responsible for triggering an active galactic nucleus (AGN). After studying the connection between major mergers and AGNs with the highest Eddington ratios at z = 2, we here expand our analysis to , exploring the same AGN parameter space. Using ESO VLT/FORS2 , , and color images, we examine the morphologies of 17 galaxies hosting AGNs with Eddington ratios , and 25 mass- and redshift-matched control galaxies. To match the appearance of the two samples, we add synthetic point sources to the inactive comparison galaxies. The combined sample of AGN and inactive galaxies was independently ranked by 19 experts with respect to the degree of morphological distortion. We combine the resulting individual rankings into multiple overall rankings, from which we derive the respective major merger fractions of the two samples. With a best estimate of f m,agn = 0.41 ± 0.12 for the AGN host galaxies and f m,ina = 0.08 ± 0.06 for the inactive galaxies, our results imply that our AGN host galaxies have a significantly higher merger rate, regardless of the observed wavelength or applied methodology. We conclude that although major mergers are an essential mechanism to trigger local high Eddington ratio AGNs at , the origin of of this specific AGN subpopulation still remains unclear.
Highlights
An ever-growing number of empirical studies are finding that the properties of the black holes (BHs) at the center of galaxies are closely correlated with the properties of the host galaxy, i.e., BH mass, bulge velocity dispersion and mass, stellar host mass, velocity dispersion, or luminosity (e.g., Marconi & Hunt 2003; Häring & Rix 2004; Jahnke et al 2009; Bennert et al 2010, 2011; Beifiori et al 2012; Graham & Scott 2013; McConnell & Ma 2013; Davis et al 2018, 2019; de Nicola et al 2019; Sahu et al 2019; Ding et al 2020; Shankar et al 2020)
We examine 17 galaxies hosting active galactic nucleus (AGN) with ledd > 0.3 at z < 0.2 and 25 inactive control galaxies and compare the relative difference of the respective merger fractions in order to conclude whether major mergers play a dominant role
Since we selected a total of 11 AGNs from the HES catalog and in each case three from the Sloan Digital Sky Survey (SDSS) and PG catalogs, we conclude that the parent catalogs from which the AGN host galaxies are drawn from are not introducing any bias with respect to morphological classification
Summary
An ever-growing number of empirical studies are finding that the properties of the black holes (BHs) at the center of galaxies are closely correlated with the properties of the host galaxy, i.e., BH mass, bulge velocity dispersion and mass, stellar host mass, velocity dispersion, or luminosity (e.g., Marconi & Hunt 2003; Häring & Rix 2004; Jahnke et al 2009; Bennert et al 2010, 2011; Beifiori et al 2012; Graham & Scott 2013; McConnell & Ma 2013; Davis et al 2018, 2019; de Nicola et al 2019; Sahu et al 2019; Ding et al 2020; Shankar et al 2020). Like in almost all the aforementioned studies that reject major mergers as the dominant triggering mechanism of AGNs, we compare a specific sample of AGN host galaxies to a sample of inactive comparison galaxies, matched in redshift, stellar mass, observed wavelength, depth, and signal-to-noise ratio (S/N). As the confidence of a detected difference in merger fractions can only increase for smaller values of fm,ina and to ensure we achieve this desired level of confidence, we use this, when compared to literature results (e.g., Lotz et al 2011; Man et al 2016; Mundy et al 2017), rather large value for fm,ina We expect this fiducial fraction to be an upper limit of the real merger rate for inactive galaxies in our mass and redshift range. With our final sample sizes we can conclude whether or not AGN host galaxies show a significant enhancement in merger rates, indicating a causal dependence of our population of AGNs on major mergers
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