Reverberation mapping of narrow-line Seyfert 1 galaxy I Zwicky 1: black
  hole mass
  We report results of the first reverberation mapping campaign of I Zwicky 1
during $2014$-$2016$, which showed unambiguous reverberations of the broad
H$\beta$ line emission to the varying optical continuum. From analysis using
several methods, we obtain a reverberation lag 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 luminosity and $c$ is the speed
of light. By decomposing {\it Hubble Space Telescope} images, we find that the
stellar mass of the bulge of its host galaxy is $\log (M_{\rm bulge}/M_{\odot})
= \rm 10.92\pm 0.07$. This leads to a black hole to bulge mass ratio of $\sim
10^{-4}$, which is significantly smaller than that of classical bulges and
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.
