  The subsurface structure of a solar sunspot is important in the stability of
the sunspots and the energy transport therein. Two subsurface structure models
have been proposed, the monolithic and cluster models, but no clear
observational evidence supporting a particular model has been found so far. To
obtain clues about the subsurface structure of sunspots, we analyzed umbral
flashes in merging sunspots registered by IRIS Mg II 2796 Angstrom slit-jaw
images (SJIs). Umbral flashes are regarded as an observational manifestation of
magnetohydrodynamic (MHD) shock waves originating from convection cells below
the photosphere. By tracking the motion of individual umbral flashes, we
determined the position of the convection cells that are the oscillation
centers located below the umbra. We found that the oscillation centers are
preferentially located at dark nuclei in the umbral cores rather than in bright
regions such as light bridges or umbral dots. Moreover, the oscillation centers
tend to deviate from the convergent interface of the merging sunspots where
vigorous convection is expected to occur. We also found that the inferred
depths of the convection cells have no noticeable regional dependence. These
results suggest that the subsurface of the umbra is an environment where
convection can occur more easily than the convergent interface, and hence
support the cluster model. For more concrete results, further studies based on
umbral velocity oscillations in the lower atmosphere are required.
