Active Galactic Nuclei and the Correlated Properties of Neighboring Galaxies

When a galaxy’s central supermassive black hole is actively accreting matter, it becomes an active galactic nucleus (AGN). The probability that a galaxy hosts an AGN is known to be correlated with the color of its stellar population. In a plot of r band absolute magnitude versus u-r color, bright galaxies tend to lie either in the red sequence, with little recent star formation, or in the blue sequence, with ongoing star formation occurring. However, AGN are most prevalent in the less densely populated “green valley” region between the blue and red sequences. This is generally thought to be the result of AGN feedback quenching star formation in their host galaxy, with the resulting migration of the galaxy from the blue sequence to the red sequence. However, the probability of a galaxy hosting an AGN can also be correlated with the properties of neighboring galaxies as well as the properties of the host galaxy itself. For example, tidal interactions with neighboring galaxies are known to lead to the activation of the galactic nucleus and the formation of new stars in one or both of the interacting galaxies, though it is not clear how these seemingly conflicting affects impact each other. In our study, we performed an analysis of target galaxies collected from the spectroscopic portion of the Sloan Digital Sky Survey (SDSS) Legacy Survey. For each galaxy, we identified neighboring galaxies lying within a specified projected distance r_p of the target galaxy. We then searched for properties of the neighboring galaxies that correlate with the probability P_agn that the target galaxy harbors an AGN. Results of this analysis indicate that P_agn is inversely correlated with the number of neighboring galaxies within r_p = 0.5 megaparsecs. Although this result is affected by projection effects, we show it is statistically implausible that the projection effects are the sole cause of this correlation. We are currently searching for statistical correlations of P_agn with the colors of the neighboring galaxies and with the prevalence of AGN in the neighboring galaxies.