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Register a new userHST/WFC3 Observations of an Off-Nuclear Superbubble in Arp 220
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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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We present an imaging and spectral analysis of the nuclear region of the ULIRG merger Arp 220, using deep textit{Chandra}-ACIS observations summing up to (sim 300mbox{ ks}). Narrow-band imaging with sub-pixel resolution of the innermost nuclear region reveals two distinct Fe-K emitting sources, coincident with the infrared and radio nuclear clusters. These sources are separated by 1 ((sim 380) pc). The X-ray emission is extended and elongated in the eastern nucleus, like the disk emission observed in millimeter radio images, suggesting starburst dominance in this region. We estimate Fe-K equivalent width (gtrsim 1) keV for both sources, and observed 2-10 keV luminosities (sim 2times{10}^{40}mbox{ erg}mbox{ s}^{-1}) (W) and (sim 3 times {10}^{40}mbox{ erg}mbox{ s}^{-1}) (E). In the 6-7 keV band the emission from these regions is dominated by the 6.7 keV Fe textsc{xxv} line, suggesting contribution from collisionally ionized gas. The thermal energy content of this gas is consistent with kinetic energy injection in the interstellar medium by Type II SNe. However, nuclear winds from hidden AGN ((varvsim 2000 mbox{ km}mbox{ s}^{-1})) cannot be excluded. The (3sigma) upper limits on the neutral Fe-K(alpha) flux of the nuclear regions correspond to intrinsic AGN 2-10 keV luminosities (< 1times {10}^{42}mbox{ erg}mbox{ s}^{-1}) (W) and (< 0.4times {10}^{42}mbox{ erg}mbox{ s}^{-1}) (E). For typical AGN SEDs the bolometric luminosities are (< 3times {10}^{43}mbox{ erg}mbox{ s}^{-1}) (W) and (< 8times {10}^{43}mbox{ erg}mbox{ s}^{-1}) (E), and black hole masses (<1times{10}^5 M_{astrosun}) (W) and (< 5times{10}^5 M_{astrosun}) (E) for Eddington limited AGNs with a standard 10% efficiency.
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 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.
We analyze 3 mm emission of the ultraluminous infrared galaxy Arp 220 for spatially-resolved structure and spectral properties of the merger nuclei. ALMA archival data at ~0.05 resolution are used for extensive visibility fitting and deep imaging of continuum emission. The data are fitted well with two concentric components for each nucleus, such as two Gaussians or one Gaussian plus one exponential disk. The larger components in individual nuclei are similar in shape and extent, ~100-150 pc, to the cm-wave emission due to supernovae. They are therefore identified with the known starburst nuclear disks. The smaller components in both nuclei have about a few 10 pc sizes and peak brightness temperatures (Tb) more than twice higher than in previous single-Gaussian fitting. They correspond to the dust emission that we find centrally concentrated in both nuclei by subtracting the plasma emission measured at 33 GHz. The dust emission in the western nucleus is found to have a peak Tb ~ 530 K and a full width at half maximum of about 20 pc. This component is estimated to have a bolometric luminosity on the order of 10^{12.5} Lsun and a 20 pc-scale luminosity surface density 10^{15.5} Lsun/kpc^2. A luminous AGN is a plausible energy source for these high values while other explanations remain to be explored. Our continuum image also reveals a third structural component of the western nucleus --- a pair of faint spurs perpendicular to the disk major axis. We attribute it to a bipolar outflow from the highly inclined (i ~ 60 deg) western nuclear disk.
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