The Host Galaxy and Rapidly Evolving Broad-line Region in the Changing-look Active Galactic Nucleus 1ES 1927+654

Changing-look active galactic nuclei (AGNs) present an important laboratory to understand the origin and physical properties of the broad-line region (BLR). We investigate follow-up optical spectroscopy spanning 1/4500 days after the outburst of the changing-look AGN 1ES 1927+654. The emission lines displayed dramatic, systematic variations in intensity, velocity width, velocity shift, and symmetry. Analysis of optical spectra and multiband images indicates that the host galaxy contains a pseudobulge and a total stellar mass of 3.56-0.35+0.38×109M⊙ . Enhanced continuum radiation from the outburst produced an accretion disk wind, which condensed into BLR clouds in the region above and below the temporary eccentric disk. Broad Balmer lines emerged 1/4100 days after the outburst, together with an unexpected, additional component of narrow-line emission. The newly formed BLR clouds then traveled along a similar eccentric orbit (e ≈ 0.6). The Balmer decrement of the BLR increased by a factor of 1/44-5 as a result of secular changes in cloud density. The drop in density at late times allowed the production of He i and He ii emission. The mass of the black hole cannot be derived from the broad emission lines because the BLR is not virialized. Instead, we use the stellar properties of the host galaxy to estimate MBH=1.38-0.66+1.25×106M⊙ . The nucleus reached near or above its Eddington limit during the peak of the outburst. We discuss the nature of the changing-look AGN 1ES 1927+654 in the context of other tidal disruption events.