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 H30α 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 ± 0{^''_.}006 × 0{^''_.}061 ± 0{^''_.}004 (1.14 × 10¹⁶ cm × 0.74 × 10¹⁶ cm). The structure is also identified in the H30α 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_rot ≃ 230 km s^-1 and an orbit radius of r ≃ 6 × 10¹⁵ cm. From these orbit parameters, the enclosed mass is estimated to be M_{IMBH}∼eq 2.4× 10⁴ M_{⊙}. The mass is within the astrometric upper limit mass of the object adjacent to Sgr A*. 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 τ < 10⁷ yr due to the very high mass density of ρ ≳ 8× 10^{11} M_{⊙}pc^-3. Because the orbital period is estimated to be as short as T = 2πr/V_rot ∼ 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}_e=6800± 700K and \bar{n}_e=6× 10⁵cm^-3, respectively. Then the mass of the ionized gas is estimated to be M_{gas}=4× 10^{-4} M_{⊙}.