Rotating Ionized Gas Ring around the Galactic Center IRS13E3
  We detected a compact ionized gas associated physically with IRS13E3, an
Intermediate Mass Black Hole (IMBH) candidate in the Galactic Center, in the
continuum emission at 232 GHz and H30$\alpha$ recombination line using ALMA
Cy.5 observation (2017.1.00503.S, P.I. M.Tsuboi). The continuum emission image
shows that IRS13E3 is surrounded by an oval-like structure. The angular size is
$0".093\pm0".006\times 0".061\pm0".004$ ( $1.14\times10^{16}$ cm $\times
0.74\times10^{16}$ cm). The structure is also identified in the H30$\alpha$
recombination line. This is seen as an inclined linear feature in the
position-velocity diagram, which is usually a defining characteristic of a
rotating gas ring around a large mass. The gas ring has a rotating velocity of
$V_\mathrm{rot}\simeq230$ km s$^{-1}$ and an orbit radius of
$r\simeq6\times10^{15}$ cm. From these orbit parameters, the enclosed mass is
estimated to be $M_{\mathrm{IMBH}}\simeq2.4\times10^4$ $M_\odot$. The mass is
within the astrometric upper limit mass of the object adjacent to Sgr
A$^{\ast}$. Considering IRS13E3 has an X-ray counterpart, the large enclosed
mass would be supporting evidence that IRS13E3 is an IMBH. Even if a dense
cluster corresponds to IRS13E3, the cluster would collapse into an IMBH within
$\tau<10^7$ years due to the very high mass density of $\rho
\gtrsim8\times10^{11} M_\odot pc^{-3}$. Because the orbital period is estimated
to be as short as $T=2\pi r/V_\mathrm{rot}\sim 50-100$ yr, the morphology of
the observed ionized gas ring is expected to be changed in the next several
decades. The mean electron temperature and density of the ionized gas are
$\bar{T}_{\mathrm e}=6800\pm700$ K and $\bar{n}_{\mathrm e}=6\times10^5$
cm$^{-3}$, respectively. Then the mass of the ionized gas is estimated to be
$M_{\mathrm{gas}}=4\times10^{-4} M_\odot$.
