Reverberation mapping of narrow-line Seyfert 1 galaxy I Zwicky 1: black hole mass

We report results of the first reverberation mappingcampaign of I Zwicky 1 during $2014$-$2016$, which showed unambiguous reverberationsof the broad H$\beta$ line emission to the varying optical continuum. From analysis using several methods, we obtain a reverberationlag of $\tau_{\rm H\beta}=37.2^{+4.5}_{-4.9}\,$ days. Taking a virial factor of $f_{_{\rm BLR}}=1$, we find a black hole mass of $M_{\bullet}=9.30_{-1.38}^{+1.26}\times 10^6 M_{\odot}$ from the mean spectra. The accretion rate is estimated to be $203.9_{-65.8}^{+61.0}\,L_{\rm Edd}c^{-2}$, suggesting a super-Eddington accretor, where $L_{\rm Edd}$ is the Eddington luminosityand $c$ is the speedof light. By decomposing {\it Hubble Space Telescope} images, we find that the stellar massof the bulgeof its host galaxy is $\log (M_{\rm bulge}/M_{\odot}) = \rm 10.92\pm 0.07$. This leads to a black hole to bulgemass ratio of $\sim 10^{-4}$, which is significantly smaller than that of classical bulgesand elliptical galaxies. After subtracting the host contamination from the observed luminosity, we find that I Zw 1 follows the empirical $R_{\rm BLR}\propto L_{5100}^{1/2}$ relation.