Arcsecond Imaging of CO Emission in the Nucleus of Arp 220

Abstract

We report high-resolution (1A) imaging of CO (2È1) and dust continuum emission in the ultraluminous galaxy Arp 220. The CO (1È0) line was also imaged at 2A resolution for comparison. Both data sets recover essentially all of the observed single-dish line emission. Our aperture synthesis maps reveal for the Ðrst time, multiple components in the dense gas : peaks corresponding to each of the double nuclei (separated by at P.A. \ 101¡) seen in the near infrared and radio continuum and a more extended 0A .95 disklike structure at P.A. \ 53¡, similar to the dust lane seen in optical images. Approximately two-thirds of the total CO emission (and presumably the mass) coincides with the compact double nucleus H 2 region. The ISM associated with these nuclear sources is most apparent in the 1.3 mm dust continuum emission, but the brightest CO (2È1) emission is also correlated with the near infrared nuclei and exhibits a radial velocity di †erence of 250È300 km s~1 between the two nuclei. The latter is in excellent agreement with published near-infrared recombination line measurements. The observed velocity di †erence between the two nuclei is probably much less than their orbital velocity because the nuclei do not lie along the kinematic major axis of the inner disk. The elongated disk feature exhibits a monotonic velocity gradient parallel to the major axis of the CO intensity distribution with the highest receding velocities in the southwest and the highest approach velocities in the northeast. From the major/minor axis ratio (0.66), we infer that the disk is moderately inclined to the line of sight (i \ 40È50¡).Detailed modeling of the CO line proÐles using a Doppler image-deconvolution technique, analogous to Doppler radar imaging, yields a best-Ðt CO emissivity distribution and rotation curve which are mutually consistent in the sense that if the total mass distribution follows the CO emissivity, then it yields the derived rotation curve. The implied CO-toconversion ratio is 0.45 times the Galactic value H 2 if the bulk of the mass resides in the molecular gas, rather than stars. This value is also consistent with that expected based on the likely molecular density and temperature in the nuclear disk of Arp 220. The total molecular gas mass for Arp 220 is D9 ] 109 with an uncertainty of D30% based on the line M _ proÐle modeling. The peak gas surface density is D5.8 ] 104 pc~2 at 130 pc radius, while the two M _ stellar nuclei are at D235 pc radius and at position angle midway between the major and minor axes of the gaseous disk. From the proÐle modeling we derive an intrinsic velocity dispersion in the disk of 90^20 km s~1 and thus a disk thickness (FWHM) of only 16 pc, assuming the disk is in hydrostatic equilibrium. With 5.4 ] 109 of molecular gas concentrated in the very thin disk associated with the M _ twin nuclei, the mean density will be cm~3 (^30%), a value which is consistent with the n H2^2 ] 104 strong molecular emission from high dipole moment molecules such as HCN and HCO`. From the high bright...

Evolution of star formation and gas

“…The CO(2-1) line emission exhibits two peaks separated by 0.9 arcsec, and a larger inclined disk of molecular gas (Scoville et al 1997;Downes & Solomon 1998;Sakamoto et al 1999;Tacconi et al 1999). At 0.5 arcsec resolution (Sakamoto et al 1999), the CO and 1.3 mm continuum imaging shown in Fig.…”

“…The nucleus of Arp 220 has a total molecular gas mass M H 2 ∼ 9 × 10 9 M within 300 pc (Scoville et al 1997;Downes & Solomon 1998;Sakamoto et al 1999;Tacconi et al 1999). This H 2 mass within 300 pc is 3-4 times that of the entire Milky Way (most of which is in the ring at 4-8 kpc).…”

“…Millimetre-wave imaging provides a unique capability to probe such starburst nuclei in dusty LIRG/ULIRGs since the dust is optically thin at long wavelengths. Virtually all of the ULIRGs observed so far have massive concentrations of molecular gas in the central few kiloparsec (Scoville et al 1997;Downes & Solomon 1998;Bryant & Scoville 1999;Tacconi et al 1999).…”

“…The high brightness temperature of the observed CO emission (Scoville et al 1997) requires that the gas distribution be continuous, i.e., filled nuclear disks, not a cloudy medium. The mass density within the disks implies a visual extinction A V ∼ 2000 mag perpendicular to the disks!…”

“…We assume a central point mass (black hole) and a uniform gas surface density disk extending to 100 pc radius. The turbulent velocity dispersion within the disk is taken to be 50 km/s (based on Arp 220; Scoville et al 1997). The disk thickness as a function of radius for hydrostatic equilibrium is shown in the right panel and the mean gas density in the left panel (solid red line).…”