One of the textbook characteristics of an active galactic nucleus (AGN) is a spectrum showing prominent emission lines. However, in the late 1990s astronomers were starting to find galaxies that very definitely harbored an AGN (judging by their strong X-ray emission), but somehow lacked conspicuous emission lines. Many scenarios have been put forward to explain these so called "optically dull AGN": absorption of lines by dust, stellar contamination of spectroscopy, or even some unusual modes of gas accretion onto the supermassive black hole. However, no scenario was unambiguously supported by the observations.
In a paper A New Physical Picture for Active Galactic Nuclei Lacking Optical Emission Lines published in the Astrophysical Journal, recent IU graduate Chris Agostino and collaborators assembled the largest sample (~500) so far of low-redshift AGN that were detected in X-rays, but also had optical spectra available. The analysis revealed that there exists a very wide range of emission line strengths, and that the optically dull AGN are just the tail of this distribution, rather than a separate population. Furthermore, a correlation was found between the absence of emission lines and low levels of star formation, and therefore less gas, in a galaxy. This finding led the authors to a new picture in which the amount of gas in the central few kiloparsec is what ultimately governs how luminous the AGN emission lines get, rather than just how powerful the AGN is intrinsically. The gas is still needed to fuel these optically dull AGN, but it is apparently limited to just the immediate vicinity of the black hole.
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