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Register a new userChemically Distinct Nuclei and Outflowing Shocked Molecular Gas in Arp 220
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We present the results of interferometric spectral line observations of Arp 220 at 3.5mm and 1.2mm from the Plateau de Bure Interferometer (PdBI), imaging the two nuclear disks in H$^{13}$CN$(1 - 0)$ and $(3 - 2)$, H$^{13}$CO$^+(1 - 0)$ and $(3 - 2)$, and HN$^{13}$C$(3 - 2)$ as well as SiO$(2 - 1)$ and $(6 - 5)$, HC$^{15}$N$(3 - 2)$, and SO$(6_6 - 5_5)$. The gas traced by SiO$(6 - 5)$ has a complex and extended kinematic signature including a prominent P Cygni profile, almost identical to previous observations of HCO$^+(3 - 2)$. Spatial offsets $0.1$ north and south of the continuum centre in the emission and absorption of the SiO$(6 - 5)$ P Cygni profile in the western nucleus (WN) imply a bipolar outflow, delineating the northern and southern edges of its disk and suggesting a disk radius of $sim40$ pc, consistent with that found by ALMA observations of Arp 220. We address the blending of SiO$(6 - 5)$ and H$^{13}$CO$^+(3 - 2)$ by considering two limiting cases with regards to the H$^{13}$CO$^+$ emission throughout our analysis. Large velocity gradient (LVG) modelling is used to constrain the physical conditions of the gas and to infer abundance ratios in the two nuclei. Our most conservative lower limit on the [H$^{13}$CN]/[H$^{13}$CO$^+$] abundance ratio is 11 in the WN, cf. 0.10 in the eastern nucleus (EN). Comparing these ratios to the literature we argue on chemical grounds for an energetically significant AGN in the WN driving either X-ray or shock chemistry, and a dominant starburst in the EN.
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Aims. We present new IRAM Plateau de Bure Interferometer observations of Arp 220 in HCN, HCO$^{+}$, HN$^{13}$C J=1-0, C$_{2}$H N=1-0, SiO J = 2-1, HNCO J$_{k,k}$ = 5$_{0,4}$ - 4$_{0,4}$, CH$_{3}$CN(6-5), CS J=2-1 and 5-4 and $^{13}$CO J=1-0 and 2-1 and of NGC 6240 in HCN, HCO$^{+}$ J = 1-0 and C$_{2}$H N = 1-0. In addition, we present Atacama Large Millimeter/submillmeter Array science verification observations of Arp 220 in CS J = 4-3 and CH$_{3}$CN(10-9). Various lines are used to analyse the physical conditions of the molecular gas including the [$^{12}$CO]/[$^{13}$CO] and [$^{12}$CO]/[C$^{18}$O] abundance ratios. These observations will be made available to the public. Methods. We create brightness temperature line ratio maps to present the different physical conditions across Arp 220 and NGC 6240. In addition, we use the radiative transfer code RADEX and a Monte Carlo Markov Chain likelihood code to model the $^{12}$CO, $^{13}$CO and C$^{18}$O lines of Arp 220 at ~2 (~700 pc) scales, where the $^{12}$CO and C$^{18}$O measurements were obtained from literature. Results. Line ratios of optically thick lines such as $^{12}$CO show smoothly varying ratios while the line ratios of optically thin lines such as $^{13}$CO show a east-west gradient across Arp 220. The HCN/HCO$^{+}$ line ratio differs between Arp 220 and NGC 6240, where Arp 220 has line ratios above 2 and NGC 6240 below 1. The radiative transfer analysis solution is consistent with a warm (~40 K), moderately dense (~10$^{3.4}$ cm$^{-3}$) molecular gas component averaged over the two nuclei. We find [$^{12}$CO]/[$^{13}$CO] and [$^{12}$CO]/[C$^{18}$O] abundance ratios of ~90 for both. The abundance enhancement of C$^{18}$O can be explained by stellar nucleosynthesis enrichment of the interstellar medium.
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.
Understanding the heating and cooling mechanisms in nearby (Ultra) luminous infrared galaxies can give us insight into the driving mechanisms in their more distant counterparts. Molecular emission lines play a crucial role in cooling excited gas, and recently, with Herschel Space Observatory we have been able to observe the rich molecular spectrum. CO is the most abundant and one of the brightest molecules in the Herschel wavelength range. CO transitions are observed with Herschel, and together, these lines trace the excitation of CO. We study Arp 299, a colliding galaxy group, with one component harboring an AGN and two more undergoing intense star formation. For Arp 299 A, we present PACS spectrometer observations of high-J CO lines up to J=20-19 and JCMT observations of $^{13}$CO and HCN to discern between UV heating and alternative heating mechanisms. There is an immediately noticeable difference in the spectra of Arp 299 A and Arp 299 B+C, with source A having brighter high-J CO transitions. This is reflected in their respective spectral energy line distributions. We find that photon-dominated regions (PDRs) are unlikely to heat all the gas since a very extreme PDR is necessary to fit the high-J CO lines. In addition, this extreme PDR does not fit the HCN observations, and the dust spectral energy distribution shows that there is not enough hot dust to match the amount expected from such an extreme PDR. Therefore, we determine that the high-J CO and HCN transitions are heated by an additional mechanism, namely cosmic ray heating, mechanical heating, or X-ray heating. We find that mechanical heating, in combination with UV heating, is the only mechanism that fits all molecular transitions. We also constrain the molecular gas mass of Arp 299 A to 3e9 Msun and find that we need 4% of the total heating to be mechanical heating, with the rest UV heating.
We analyse new observations with the International Low Frequency Array (LOFAR) telescope, and archival data from the Multi-Element Radio Linked Interferometer Network (MERLIN) and the Karl G. Jansky Very Large Array (VLA). We model the spatially resolved radio spectrum of Arp 220 from 150 MHz to 33 GHz. We present an image of Arp 220 at 150 MHz with resolution $0.65times0.35$, sensitivity 0.15 mJy beam$^{-1}$, and integrated flux density $394pm59$ mJy. More than 80% of the detected flux comes from extended ($6approx$2.2 kpc) steep spectrum ($alpha=-0.7$) emission, likely from star formation in the molecular disk surrounding the two nuclei. We find elongated features extending $0.3$ (110 pc) and $0.9$ (330 pc) from the eastern and western nucleus respectively, which we interpret as evidence for outflows. The extent of radio emission requires acceleration of cosmic rays far outside the nuclei. We find that a simple three component model can explain most of the observed radio spectrum of the galaxy. When accounting for absorption at 1.4 GHz, Arp 220 follows the FIR/radio correlation with $q=2.36$, and we estimate a star formation rate of 220 M$_odottext{yr}^{-1}$. We derive thermal fractions at 1 GHz of less than 1% for the nuclei, which indicates that a major part of the UV-photons are absorbed by dust. International LOFAR observations shows great promise to detect steep spectrum outflows and probe regions of thermal absorption. However, in LIRGs the emission detected at 150 MHz does not necessarily come from the main regions of star formation. This implies that high spatial resolution is crucial for accurate estimates of star formation rates for such galaxies at 150 MHz.
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.
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