ALMA Resolves the Nuclear Disks of Arp 220

Abstract

We present 90 mas (37 pc) resolution ALMA imaging of Arp 220 in the CO (1-0) line and continuum at λ = 2.6 mm. The internal gas distribution and kinematics of both galactic nuclei are well resolved for the first time. In the west nucleus, the major gas and dust emission extends out to 0.”2 radius (74 pc); the central resolution element shows a strong peak in the dust emission but a factor of 3 dip in the CO line emission. In this nucleus, the dust is apparently optically thick (τ_(2.6 mm) ~ 1) at λ = 2.6 mm with a dust brightness temperature of ~147 K. The column of interstellar matter at this nucleus is N_(H2) ⩾ 2 x 10^(26)cm^(−2), corresponding to ~900 gr cm^(−2). The east nucleus is more elongated with radial extent 0.”3 or ~111 pc. The derived kinematics of the nuclear disks provide a good fit to the line profiles, yielding the emissivity distributions, the rotation curves, and velocity dispersions. In the west nucleus, there is evidence of a central Keplerian component requiring a central mass of 8 × 10^8 M⊙. The intrinsic widths of the emission lines are Δν(FWHM) = 250 (west) and 120 (east) km s^(−1). Given the very short dissipation timescales for turbulence (≾10^5 years), we suggest that the line widths may be due to semicoherent motions within the nuclear disks. The symmetry of the nuclear disk structures is impressive, implying the merger timescale is significantly longer than the rotation period of the disks.