Two-dimensional kinematical and ionization structure of the warm gas in the nuclear regions of Arp 220

Integral field optical spectroscopy using the INTEGRAL system has been used to characterize the kinematic and ionization properties of the warm gas wi thin 2 kpc of the dust-enshrouded nucleus of Arp 220. Owing to the large in ternal extinction toward the nuclei, the brightest stellar and line-emittin g regions observed at optical wavelengths do not coincide with the dust-ens hrouded near-infrared and radio nuclei of Arp 220 but are located northwest of the nucleus at a distance of about 750 pc. Moreover, although the conti nuum and the line-emitting gas share similar distributions, their emission peaks are displaced, with the H alpha emission peak located at about 300 pc southwest of the optical stellar continuum emission peak. A line decomposi tion analysis has been performed in the complex and high spatially variable emission-line profiles. Three different kinematically distinct and extende d gaseous components have been identified in the ionized gas. One narrow co mponent (R) indicates rotation, while the other two components (O and B) ar e well interpreted by the presence of a biconical outflow. Specifically, th e rotational component R traces quiescent gas located in a nuclear disk wit h the spin axis along the southeast-northwest direction (P.A. 135 degrees). This component of ionized gas seems to be coupled with the 100 kpc disklik e H I gas and the 1 kpc molecular disk detected in CO (Scoville et al.). Th e inclination-corrected rotational velocities imply a dynamical mass (M-dyn ) of 2 x 10(10) M-. within a radius of 1.5 kpc. This relatively high value indicates a large mass concentration in the nuclear region of Arp 220, as S coville et al. already inferred by the presence of 5 x 10(9) M-. of molecul ar gas in a nuclear, 0.5 kpc disk. One of the outflow components, O, has pe ak-to-peak velocities of 1000 km s(-1). The broad component B, with an aver age width of 815 km s(-1), is detected at about 600 pc northwest of the dus t-enshrouded nucleus and is blueshifted by 300 km s(-1) with respect to the system velocity. The two-dimensional distribution and kinematics of the co mponents are consistent with a bipolar cone geometry with an opening angle of about 90 degrees and are perpendicular to the nuclear disk of gas, as ex pected in the starburst-driven galactic wind scenario proposed by Heckman, Armus, & Miley. In most of the observed regions, the ionization status of t he different gas components is consistent with a shock-heated LINER-like or Seyfert 2 nebula as judged by the [N II]/H alpha and [S II]/H alpha emissi on-line ratios. Although the [O III] and Hb lines are undetected in most of the regions, the brightest zone located northwest of the nucleus could be classified as a Seyfert 2 nebula based on the additional constraint given b y the measured lower limit (greater than or equal to5) for the [O III]/H be ta ratio. There is no evidence of excitation gradients along the symmetry a xis of the outflow nor of a biconical ionization structure, which suggests that the ionizing field is homogeneous and less collimated than the gas out flow. However, there are four clearly identified extranuclear regions where the [N II]/H alpha ratio decreases by a factor of 5 and is close to the ty pical values of H II regions. None of these regions are in spatial agreemen t with the star clusters found in the infrared by Scoville et al., confirmi ng that they must be relatively old globular clusters.