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Register a new userExtreme dust disks in Arp 220 as revealed by ALMA
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We present new images of Arp 220 from the Atacama Large Millimeter/submillimeter Array with the highest combination of frequency (691 GHz) and resolution ($0.36 times 0.20^{primeprime}$) ever obtained for this prototypical ultraluminous infrared galaxy. The western nucleus is revealed to contain warm (200 K) dust that is optically thick ($tau_{434mu m} = 5.3$), while the eastern nucleus is cooler (80 K) and somewhat less opaque ($tau_{434mu m} = 1.7$). We derive full-width half-maximum diameters of $ 76 times le 70$ pc and $123 times 79$ pc for the western and eastern nucleus, respectively. The two nuclei combined account for ($83 ^{+65}_{-38}$ (calibration) $^{+0}_{-34}$ (systematic))% of the total infrared luminosity of Arp 220. The luminosity surface density of the western nucleus ($ log(sigma T^4) = 14.3pm 0.2 ^{+0}_{-0.7}$ in units of L$_odot$ kpc$^{-2}$) appears sufficiently high to require the presence of an AGN or a hot starburst, although the exact value depends sensitively on the brightness distribution adopted for the source. Although the role of any central AGN remains open, the inferred mean gas column densities of $0.6-1.8 times 10^{25}$ cm$^{-2}$ mean that any AGN in Arp 220 must be Compton-thick.
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We report ALMA Band 7 (350 GHz) imaging at 0.4 - 0.6arcsec resolution and Band 9 (696 GHz) at ~0.25arcsec resolution of the luminous IR galaxies Arp 220 and NGC 6240. The long wavelength dust continuum is used to estimate ISM masses for Arp 220 East, West and NGC 6240 of 1.9, 4.2 and 1.6x10^9 msun within radii of 69, 65 and 190 pc. The HCN emission was modeled to derive the emissivity distribution as a function of radius and the kinematics of each nuclear disk, yielding dynamical masses consistent with the masses and sizes derived from the dust emission. In Arp 220, the major dust and gas concentrations are at radii less than 50 pc in both counter-rotating nuclear disks. The thickness of the disks in Arp 220estimated from the velocity dispersion and rotation velocities are 10-20 pc and the mean gas densities are n_H2 ~10^5 cm^-3 at R < 50 pc. We develop an analytic treatment for the molecular excitation (including photon trapping), yielding volume densities for both the HCN and CS emission with n_H2 ~2x10^5 cm^-3. The agreement of the mean density from the total mass and size with that required for excitation suggests that the volume is essentially filled with dense gas, i.e. it is not cloudy or like swiss cheese.
We present the first very-long-baseline interferometry (VLBI) detections of Zeeman splitting in another galaxy. We used Arecibo Observatory, the Green Bank Telescope, and the Very Long Baseline Array to perform dual-polarization observations of OH maser lines in the merging galaxy Arp 220. We measured magnetic fields of $sim$1-5 mG associated with three roughly parsec-sized clouds in the nuclear regions of Arp 220. Our measured magnetic fields have comparable strengths and the same direction as features at the same velocity identified in previous Zeeman observations with Arecibo alone. The agreement between single dish and VLBI results provides critical validation of previous Zeeman splitting observations of OH megamasers that used a single large dish. The measured magnetic field strengths indicate that magnetic energy densities are comparable to gravitational energy in OH maser clouds. We also compare our total intensity results to previously published VLBI observations of OH megamasers in Arp 220. We find evidence for changes in both structure and amplitude of the OH maser lines that are most easily explained by variability intrinsic to the masing region, rather than variability produced by interstellar scintillation. Our results demonstrate the potential for using high-sensitivity VLBI to study magnetic fields on small spatial scales in extragalactic systems.
We present the first spatially and spectrally resolved image of the molecular outflow in the western nucleus of Arp,220. The outflow, seen in HCN~(1--0) by ALMA, is compact and collimated, with extension $lesssim$ 120,pc. Bipolar morphology emerges along the minor axis of the disk, with redshifted and blueshifted components reaching maximum inclination-corrected velocity of $sim,pm$,840,km,s$^{-1}$. The outflow is also seen in CO and continuum emission, the latter implying that it carries significant dust. We estimate a total mass in the outflow of $geqslant$,10$^{6}$,M$_{odot}$, a dynamical time of $sim$,10$^{5}$,yr, and mass outflow rates of $geqslant55$,M$_{odot}$,yr$^{-1}$ and $geqslant,15$,M$_{odot}$,yr$^{-1}$ for the northern and southern lobes, respectively. Possible driving mechanisms include supernovae energy and momentum transfer, radiation pressure feedback, and a central AGN. The latter could explain the collimated morphology of the HCN outflow, however we need more complex theoretical models, including contribution from supernovae and AGN, to pinpoint the driving mechanism of this outflow.
We present new, high angular resolution (~0.22) MERLIN observations of neutral hydrogen (HI) absorption and 21-cm radio continuum emission across the central ~900 parsecs of the ultraluminous infrared galaxy, Arp220. Spatially resolved HI absorption is detected against the morphologically complex and extended 21-cm radio continuum emission, consistent with two counterrotating disks of neutral hydrogen, with a small bridge of gas connecting the two. We propose a merger model in which the two nuclei represent the galaxy cores which have survived the initial encounter and are now in the final stages of merging, similar to conclusions drawn from previous CO studies (Sakamoto, Scoville & Yun 1999). However, we suggest that instead of being coplanar with the main CO disk (in which the eastern nucleus is embedded), the western nucleus lies above it and, as suggested by bridge of HI connecting the two nuclei, will soon complete its final merger with the main disk. We suggest that the collection of radio supernovae (RSN) detected in VLBA studies in the more compact western nucleus represent the second burst of star formation associated with this final merger stage and that free-free absorption due to ionised gas in the bulge-like component can account for the observed RSN distribution. (Abridged)
We present a high spatial resolution optical and infrared study of the circumnuclear region in Arp 220, a late-stage galaxy merger. Narrowband imaging using HST/WFC3 has resolved the previously observed peak in H$alpha$+[NII] emission into a bubble-shaped feature. This feature measures 1.6 in diameter, or 600 pc, and is only 1 northwest of the western nucleus. The bubble is aligned with the western nucleus and the large-scale outflow axis seen in X-rays. We explore several possibilities for the bubble origin, including a jet or outflow from a hidden active galactic nucleus (AGN), outflows from high levels of star formation within the few hundred pc nuclear gas disk, or an ultraluminous X-ray source. An obscured AGN or high levels of star formation within the inner $sim$100 pc of the nuclei are favored based on the alignment of the bubble and energetics arguments.
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