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We present Submillimeter Array (SMA) molecular line observations in two 2 GHz-wide bands centered at 217.5 and 227.5 GHz, toward the massive star forming region W51 North. We identified 84 molecular line transitions from 17 species and their isotopologues. The molecular gas distribution of these lines mainly peaks in the continuum position of W51 North, and has a small tail extending to the west, probably associated with W51 d2. In addition to the commonly detected nitrogen and oxygen-bearing species, we detected a large amount of transitions of the Acetone (CH$_3$COCH$_3$) and Methyl Formate (CH$_3$OCHO), which may suggest that these molecules are present in an early evolutionary stage of the massive stars. We also found that W51 North is an ethanol-rich source. There is no obvious difference in the molecular gas distributions between the oxygen-bearing and nitrogen-bearing molecules. Under the assumption of Local Thermodynamic Equilibrium (LTE), with the XCLASS tool, the molecular column densities, and rotation temperatures are estimated. We found that the oxygen-bearing molecules have considerable higher column densities and fractional abundances than the nitrogen-bearing molecules. The rotation temperatures range from 100 to 200 K, suggesting that the molecular emission could be originated from a warm environment. Finally, based on the gas distributions, fractional abundances and the rotation temperatures, we conclude that CH$_3$OH, C$_2$H$_5$OH, CH$_3$COCH$_3$ and CH$_3$CH$_2$CN might be synthesized on the grain surface, while gas phase chemistry is responsible for the production of CH$_3$OCH$_3$, CH$_3$OCHO and CH$_2$CHCN.
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We present deep radio images of the inner 50 kpc of Centaurus A, taken with the Karl G. Jansky Very Large Array (VLA) at 90cm. We focus on the Transition Regions between the inner galaxy - including the active nucleus, inner radio lobes, and star-forming disk - and the outer radio lobes. We detect previously unknown extended emission around the Inner Lobes, including radio emission from the star-forming disk. We find that the radio-loud part of the North Transition Region, known as the North Middle Lobe, is significantly overpressured relative to the surrounding ISM. We see no evidence for a collimated flow from the Active Galactic Nucleus (AGN) through this region. Our images show that the structure identified by Morganti et al. (1999) as a possible large-scale jet appears to be part of a narrow ridge of emission within the broader, diffuse, radio-loud region. This knotty radio ridge is coincident with other striking phenomena: compact X-ray knots, ionized gas filaments, and streams of young stars. Several short-lived phenomena in the North Transition Region, as well as the frequent re-energization required by the Outer Lobes, suggest that energy must be flowing through both Transition Regioins at the present epoch. We suggest that the energy flow is in the form of a galactic wind.
We present the first high-angular resolution (up to 0.7, ~5000 AU) polarization and thermal dust continuum images toward the massive star-forming region W51 North. The observations were carried out with the Submillimeter Array (SMA) in both the subcompact (SMA-SubC) and extended (SMA-Ext) configurations at a wavelength of 870 micron. W51 North is resolved into four cores (SMA1 to SMA4) in the 870 micron continuum image. The associated dust polarization exhibits more complex structures than seen at lower angular resolution. We analyze the inferred morphologies of the plane-of-sky magnetic field (B_bot) in the SMA1 to SMA4 cores and in the envelope using the SMA-Ext and SMA-SubC data. These results are compared with the B_bot archive images obtained from the CSO and JCMT. A correlation between dust intensity gradient position angles (phi_{nabla I}) and magnetic field position angles (phi_B) is found in the CSO, JCMT and both SMA data sets. This correlation is further analyzed quantitatively. A systematically tighter correlation between phi_{nabla I} and phi_B is found in the cores, whereas the correlation decreases in outside-core regions. Magnetic field-to-gravity force ratio (Sigma_B) maps are derived using the newly developed polarization - intensity gradient method by Koch, Tang & Ho 2012. We find that the force ratios tend to be small (Sigma_B <= 0.5) in the cores in all 4 data sets. In regions outside of the cores, the ratios increase or the field is even dominating gravity (Sigma_B > 1). This possibly provides a physical explanation of the tightening correlation between phi_{nabla I} and phi_B in the cores: the more the B field lines are dragged and aligned by gravity, the tighter the correlation is. Finally, we propose a schematic scenario for the magnetic field in W51 North to interpret the four polarization observations at different physical scales.
We present deep GALEX images of NGC 5128, the parent galaxy of Centaurus A. We detect a striking weather ribbon of Far-UV and H$alpha$ emission, which extends more than 35 kpc northeast of the galaxy. The ribbon is associated with a knotty ridge of radio/X-ray emission, and is an extension of the previously known string of optical emission-line filaments. Many phenomena in the region are too short-lived to have survived transit out from the inner galaxy; something must be driving them locally. We also detect Far-UV emission from the galaxys central dust lane. Combining this with previous radio and Far-IR measurements, we infer an active starburst in the central galaxy, which is currently forming stars at $sim 2 M_{sun}$yr$^{-1}$, and has been doing so for 50-100Myr. If the wind from this starburst is enhanced by energy and mass driven out from the AGN, the powerful augmented wind can be the driver needed for the northern weather system. We argue that both the diverse weather system, and the enhanced radio emission in the same region, result from the winds encounter with cool gas left by one of the recent merger/encounter events in the history of NGC 5128.
We have studied four complex organic molecules (COMs), methyl formate ($CH_3OCHO$), dimethyl ether ($CH_3OCH_3$), formamide ($NH_2CHO$), and ethyl cyanide ($C_2H_5CN$), towards a large sample of 39 high-mass star-forming regions representing different evolutionary stages, from early to evolved phases. We aim to identify potential correlations between the molecules and to trace their evolutionary sequence through the star formation process. We analysed spectra obtained at 3, 2, and 0.9 mm with the IRAM-30m telescope. We derived the main physical parameters for each species by fitting the molecular lines. We compared them and evaluated their evolution, also taking several other interstellar environments into account. We report detections in 20 sources, revealing a clear dust absorption effect on column densities. Derived abundances are ~$10^{-10}-10^{-7}$ for $CH_3OCHO$ and $CH_3OCH_3$, ~$10^{-12}-10^{-10}$ for $NH_2CHO$, and ~$10^{-11}-10^{-9}$ for $C_2H_5CN$. The abundances of $CH_3OCHO$, $CH_3OCH_3$, and $C_2H_5CN$ are very strongly correlated (r>0.92) across ~4 orders of magnitude. $CH_3OCHO$ and $CH_3OCH_3$ show the strongest correlations in most parameters, and a nearly constant ratio (~1) over a remarkable ~9 orders of magnitude in luminosity for a wide variety of sources: pre-stellar to evolved cores, low- to high-mass objects, shocks, Galactic clouds, and comets. This indicates that COMs chemistry is likely early developed and then preserved through evolved phases. Moreover, the molecular abundances clearly increase with evolution. We consider $CH_3OCHO$ and $CH_3OCH_3$ to be most likely chemically linked: they could e.g. share a common precursor, or be formed one from the other. We propose a general scenario for all COMs, involving a formation in the cold, earliest phases of star formation and a following increasing desorption with the progressive heating of the evolving core.
We observed the W51 high-mass star-forming complex with ALMAs longest-baseline configurations, achieving an angular resolution of $sim$20 milliarcseconds, corresponding to a linear resolution of $sim$100 au at $D_{mathrm{W51}}=5.4$ kpc. The observed region contains three high-mass protostars in which the dust continuum emission at 1.3 mm is optically-thick up to a radius $lesssim$1000 au and has brightness temperatures $gtrsim$200 K. The high luminosity ($gtrsim10^4$ L$_{odot}$) in the absence of free-free emission suggests the presence of massive stars ($Mgtrsim20$ M$_{odot}$) at the earliest stages of their formation. Our continuum images reveal remarkably complex and filamentary structures arising from compact cores. Molecular emission shows no clear signs of rotation nor infall on scales from 150 to 2000 au: we do not detect disks. The central sources drive young ($sim$100 years), fast ($sim 100$ km s$^{-1}$), powerful ($dot{M}>10^{-4}$ M$_{odot} yr^{-1}$), collimated outflows. These outflows provide indirect evidence of accretion disks on scales $rlesssim$100--500 au (depending on the object). The active outflows are connected to fossil flows that have different orientations on larger spatial scales, implying that the orientations of these small disks change over time. These results together support a variant of an accretion model for high-mass star formation in which massive protostars do not form a large, stable Keplerian disk during their early stages, but instead they accrete material from multiple massive flows with different angular momentum vectors. This scenario therefore contrasts with the simplified classic paradigm of a stable disk+jet system, which is the standard model for low-mass star formation, and provides an experimental confirmation of a multi-directional and unsteady accretion model for massive star formation.
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