No evidence for a central IMBH in M15
  Intermediate mass black holes (IMBHs) with expected masses M_BH ~ 10^4 M_sun
are thought to bridge the gap between stellar mass black holes (M_BH ~ 3 - 100
M_sun) and supermassive black holes found at the centre of galaxies (M_BH >
10^6 M_sun). Until today, no IMBH has been confirmed observationally. The most
promising objects to host an IMBH as their central mass are globular clusters.
Here, we present high sensitivity multi-epoch 1.6 GHz very long baseline
interferometry observations of the globular cluster M15 that has been suggested
to host an IMBH. Assuming the IMBH to be accreting matter from its surrounding
we expect to detect it as a point source moving with the global motion of the
cluster. However, we do not detect any such object within a radius of 6000 AU
of the cluster centre in any of the five observations spread over more than one
year. This rules out any variability of the putative IMBH on the time scale of
one to two months. To get the most stringent upper limit for the flux density
of the putative IMBH we concatenate the data of all five epochs. In this data
we measure a 3{\sigma} upper flux limit of 10 {\mu}Jy for a central source. We
employ the fundamental plane of black hole activity to estimate the mass of the
central IMBH candidate. Based on previous X-ray observations of M15 our
measurements indicate a 3{\sigma} upper mass limit of ~500 M_sun.
