- ID:
- ivo://CDS.VizieR/J/ApJ/747/63
- Title:
- 7mm VLBA observations of BL Lac object OJ287
- Short Name:
- J/ApJ/747/63
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the results from an ultra-high-resolution 7 mm Very Long Baseline Array study of the relativistic jet in the BL Lacertae object OJ287 from 1995 to 2011 containing 136 total intensity images. Analysis of the image sequence reveals a sharp jet-position-angle swing by >100{deg} during [2004,2006], as viewed in the plane of the sky, which we interpret as the crossing of the jet from one side of the line of sight to the other during a softer- and longer-term swing of the inner jet. Modulating such long-term swing, our images also show for the first time a prominent erratic wobbling behavior of the innermost ~0.4 mas of the jet with fluctuations in position angle of up to ~40{deg} over timescales ~2 yr. This is accompanied by highly superluminal motions along non-radial trajectories, which reflect the remarkable non-ballistic nature of the jet plasma on these scales. The erratic nature and short timescales of the observed behavior rule out scenarios such as binary black hole systems, accretion disk precession, and interaction with the ambient medium as possible origins of the phenomenon on the scales probed by our observations, although such processes may cause longer-term modulation of the jet direction. We propose that variable asymmetric injection of the jet flow, perhaps related to turbulence in the accretion disk, coupled with hydrodynamic instabilities leads to the non-ballistic dynamics that causes the observed non-periodic changes in the direction of the inner jet.
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- ID:
- ivo://CDS.VizieR/J/ApJ/727/L36
- Title:
- 1.3mm VLBI observations of Sagittarius A*
- Short Name:
- J/ApJ/727/L36
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Sagittarius A*, the ~4x10^6^M_{sun}_ black hole candidate at the Galactic center, can be studied on Schwarzschild radius scales with (sub)millimeter wavelength very long baseline interferometry (VLBI). We report on 1.3mm wavelength observations of Sgr A* using a VLBI array consisting of the JCMT on Mauna Kea, the Arizona Radio Observatory's Submillimeter Telescope on Mt. Graham in Arizona, and two telescopes of the CARMA array at Cedar Flat in California. Both Sgr A* and the quasar calibrator 1924-292 were observed over three consecutive nights, and both sources were clearly detected on all baselines. For the first time, we are able to extract 1.3mm VLBI interferometer phase information on Sgr A* through measurement of closure phase on the triangle of baselines. On the third night of observing, the correlated flux density of Sgr A* on all VLBI baselines increased relative to the first two nights, providing strong evidence for time-variable change on scales of a few Schwarzschild radii.
- ID:
- ivo://CDS.VizieR/J/ApJ/849/L36
- Title:
- mm-wave size study of ALMA submm galaxies
- Short Name:
- J/ApJ/849/L36
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report the study of the far-infrared (IR) sizes of submillimeter galaxies (SMGs) in relation to their dust-obscured star formation rate (SFR) and active galactic nuclei (AGN) presence, determined using mid-IR photometry. We determined the millimeter-wave ({lambda}_obs_=1100um) sizes of 69 Atacama Large Millimeter/submillimeter Array (ALMA)-identified SMGs, selected with >=10{sigma} confidence on ALMA images (F_1100um_=1.7-7.4mJy). We found that all of the SMGs are located above an avoidance region in the size-flux plane, as expected by the Eddington limit for star formation. In order to understand what drives the different millimeter-wave sizes in SMGs, we investigated the relation between millimeter-wave size and AGN fraction for 25 of our SMGs at z=1-3. We found that the SMGs for which the mid-IR emission is dominated by star formation or AGN have extended millimeter-sizes, with respective median R_c,e_=1.6_-0.21_^+0.34^ and 1.5_-0.24_^+0.93^kpc. Instead, the SMGs for which the mid-IR emission corresponds to star-forming/AGN composites have more compact millimeter-wave sizes, with median R_c,e_=1.0_-0.20_^+0.20^kpc. The relation between millimeter-wave size and AGN fraction suggests that this size may be related to the evolutionary stage of the SMG. The very compact sizes for composite star-forming/AGN systems could be explained by supermassive black holes growing rapidly during the SMG coalescing, star-formation phase.
- ID:
- ivo://CDS.VizieR/J/A+A/615/A125
- Title:
- Molecular cloud in Corona Australis
- Short Name:
- J/A+A/615/A125
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a catalogue of prestellar and starless cores within the Corona Australis molecular cloud using photometric data from the Herschel Space Observatory. At a distance of d~130pc, Corona Australis is one of the closest star-forming regions. Herschel has taken multi-wavelength data of Corona Australis with both the Spectral and Photometric Imaging Receiver (SPIRE; Griffin et al. 2010A&A...518L...3G) and the Photodetector Array Camera and Spectrometer (PACS; Poglitsch et al. 2010A&A...518L...2P) photometric cameras in a parallel mode with wavelengths in the range 70um to 500um. A complete sample of starless and prestellar cores and embedded protostars is identified. Other results from the Herschel Gould Belt Survey have shown spatial correlation between the distribution of dense cores and the filamentary structure within the molecular clouds. We go further and show correlations between the properties of these cores and their spatial distribution within the clouds, with a particular focus on the mass distribution of the dense cores with respect to their filamentary proximity. We find that only lower-mass starless cores form away from filaments, while all of the higher-mass prestellar cores form in close proximity to or directly on the filamentary structure. This result supports the paradigm that prestellar cores mostly form on filaments. We analyse the mass distribution across the molecular cloud, finding evidence that the region around the Coronet appears to be at a more dynamically advanced evolutionary stage in comparison to the rest of the clumps within the cloud.
- ID:
- ivo://CDS.VizieR/J/ApJ/835/278
- Title:
- Molecular clouds in the dwarf galaxy NGC6822
- Short Name:
- J/ApJ/835/278
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the Atacama Large Millimeter/submillimeter Array survey of CO(2-1) emission from the 1/5 solar metallicity, Local Group dwarf galaxy NGC 6822. We achieve high (0.9"~2pc) spatial resolution while covering a large area: four 250pcx250pc regions that encompass ~2/3 of NGC 6822's star formation. In these regions, we resolve ~150 compact CO clumps that have small radii (~2-3pc), narrow line width (~1km/s), and low filling factor across the galaxy. This is consistent with other recent studies of low-metallicity galaxies, but here shown with a 15x larger sample. At parsec scales, CO emission correlates with 8{mu}m emission better than with 24{mu}m emission and anticorrelates with H{alpha}, so that polycyclic aromatic hydrocarbon emission may be an effective tracer of molecular gas at low metallicity. The properties of the CO clumps resemble those of similar-size structures in Galactic clouds except of slightly lower surface brightness and with CO-to-H_2_ ratio ~1-2x the Galactic value. The clumps exist inside larger atomic-molecular complexes with masses typical for giant molecular clouds. Using dust to trace H_2_ for the entire complex, we find the CO-to-H_2_ ratio to be ~20-25x the Galactic value, but with strong dependence on spatial scale and variations between complexes that may track their evolutionary state. The H_2_-to-HI ratio is low globally and only mildly above unity within the complexes. The ratio of star formation rate to H_2_ is ~3-5x higher in the complexes than in massive disk galaxies, but after accounting for the bias from targeting star-forming regions, we conclude that the global molecular gas depletion time may be as long as in massive disk galaxies.
- ID:
- ivo://CDS.VizieR/J/ApJ/901/L8
- Title:
- Molecular gas properties of 70 PHANGS-ALMA galaxies
- Short Name:
- J/ApJ/901/L8
- Date:
- 23 Feb 2022 00:10:50
- Publisher:
- CDS
- Description:
- Using the PHANGS-ALMA CO(2-1) survey, we characterize molecular gas properties on ~100pc scales across 102,778 independent sightlines in 70 nearby galaxies. This yields the best synthetic view of molecular gas properties on cloud scales across the local star-forming galaxy population obtained to date. Consistent with previous studies, we observe a wide range of molecular gas surface densities (3.4dex), velocity dispersions (1.7dex), and turbulent pressures (6.5dex) across the galaxies in our sample. Under simplifying assumptions about subresolution gas structure, the inferred virial parameters suggest that the kinetic energy of the molecular gas typically exceeds its self-gravitational binding energy at ~100pc scales by a modest factor (1.3 on average). We find that the cloud-scale surface density, velocity dispersion, and turbulent pressure (1) increase toward the inner parts of galaxies, (2) are exceptionally high in the centers of barred galaxies (where the gas also appears less gravitationally bound), and (3) are moderately higher in spiral arms than in inter-arm regions. The galaxy-wide averages of these gas properties also correlate with the integrated stellar mass, star formation rate, and offset from the star-forming main sequence of the host galaxies. These correlations persist even when we exclude regions with extraordinary gas properties in galaxy centers, which contribute significantly to the inter-galaxy variations. Our results provide key empirical constraints on the physical link between molecular cloud populations and their galactic environment.
- ID:
- ivo://CDS.VizieR/J/A+A/496/153
- Title:
- Molecular hydrogen flows along Ori A cloud
- Short Name:
- J/A+A/496/153
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- A census of molecular hydrogen flows across the entire Orion A giant molecular cloud is sought. With this paper we aim to associate each flow with its progenitor and associated molecular core, so that the characteristics of the outflows and outflow sources can be established. We present wide-field near-infrared images of Orion A, obtained with the Wide Field Camera, WFCAM, on the United Kingdom Infrared Telescope. Broad-band K and narrow-band H_2_ 1-0S(1) images of a contiguous ~8 square degree region are compared to mid-IR photometry from the Spitzer Space Telescope and (sub)millimetre dust-continuum maps obtained with the MAMBO and SCUBA bolometer arrays. Using previously-published H_2_ images, we also measured proper motions for H_2_ features in 33 outflows, and use these data to help associate flows with existing sources and/or dust cores.
- ID:
- ivo://CDS.VizieR/J/A+A/570/A51
- Title:
- Molecular lines in 4 IRCS clumps
- Short Name:
- J/A+A/570/A51
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The earliest phases of high-mass star formation are poorly understood. Aims. Our goal is to determine the physical conditions and kinematic structure of massive starforming cloud clumps. We analyse H_2_O 557GHz line profiles observed with HIFI toward four positions in two infrared-dark cloud clumps. By comparison with ground-based C_17_O, N_2_H^+^, CH_3_OH, and NH_3_ line observations, we constrain the volume density and kinetic temperature of the gas and estimate the column density and abundance of H_2_O and N_2_H^+^. The observed water lines are complex with emission and absorption components. The absorption is redshifted and consistent with a cold envelope, while the emission is interpreted as resulting from proto-stellar outflows. The gas density in the clumps is ~10^7^cm^-3^. The o-H_2_O outflow column density is 0.3-3.0x10^14^cm^-2^. The o-H_2_O absorption column density is between 1.5x10^14^ and 2.6x10^15^cm^-2^ with cold o-H_2_O abundances between 1.5x10^-9^ and 3.1x10^-8^. All clumps have high gas densities (~10^7^cm^-3^) and display infalling gas. Three of the four clumps have outflows. The clumps form an evolutionary sequence as probed by H_2_O N_2_H+, NH_3_, and CH_3_OH. We find that G28-MM is the most evolved, followed by G11-MM and then G28-NH3. The least evolved clump is G11-NH3 which shows no signposts of starformation; G11-NH3 is a high-mass pre-stellar core.
- ID:
- ivo://CDS.VizieR/J/ApJ/780/85
- Title:
- Molecular line study of infrared dark clouds
- Short Name:
- J/ApJ/780/85
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- It is currently assumed that infrared dark clouds (IRDCs) represent the earliest evolutionary stages of high-mass stars (>8M_{sun}_). Submillimeter and millimeter-wave studies performed over the past 15yr show that IRDCs possess a broad variety of properties, and hence a wide range of problems and questions that can be tackled. In this paper, we report an investigation of the molecular composition and chemical processes in two groups of IRDCs. Using the Mopra, APEX, and IRAM radio telescopes over the last four years, we have collected molecular line data for CO, H_2_CO, HNCO, CH_3_CCH, CH_3_OH, CH_3_CHO, CH_3_OCHO, and CH_3_OCH_3_. For all of these species we estimated molecular abundances. We then undertook chemical modeling studies, concentrating on the source IRDC028.34+0.06, and compared observed and modeled abundances. This comparison showed that to reproduce observed abundances of complex organic molecules, a zero-dimensional gas-grain model with constant physical conditions is not sufficient. We achieved greater success with the use of a warm-up model, in which warm-up from 10K to 30K occurs following a cold phase.
- ID:
- ivo://CDS.VizieR/J/ApJ/851/76
- Title:
- Molecular transitions toward NGC5128 with ALMA
- Short Name:
- J/ApJ/851/76
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Centaurus A, with its gas-rich elliptical host galaxy, NGC 5128, is the nearest radio galaxy at a distance of 3.8Mpc. Its proximity allows us to study the interaction among an active galactic nucleus, radio jets, and molecular gas in great detail. We present ALMA observations of low-J transitions of three CO isotopologues, HCN, HCO+, HNC, CN, and CCH toward the inner projected 500pc of NGC 5128. Our observations resolve physical sizes down to 40pc. By observing multiple chemical probes, we determine the physical and chemical conditions of the nuclear interstellar medium of NGC 5128. This region contains molecular arms associated with the dust lanes and a circumnuclear disk (CND) interior to the molecular arms. The CND is approximately 400pc by 200pc and appears to be chemically distinct from the molecular arms. It is dominated by dense gas tracers while the molecular arms are dominated by ^12^CO and its rare isotopologues. The CND has a higher temperature, elevated CN/HCN and HCN/HNC intensity ratios, and much weaker ^13^CO and C^18^O emission than the molecular arms. This suggests an influence from the AGN on the CND molecular gas. There is also absorption against the AGN with a low velocity complex near the systemic velocity and a high velocity complex shifted by about 60km/s. We find similar chemical properties between the CND in emission and both the low and high velocity absorption complexes, implying that both likely originate from the CND. If the HV complex does originate in the CND, then that gas would correspond to gas falling toward the supermassive black hole.