863 research outputs found
13CO 1-0 imaging of the Medusa merger, NGC4194
Studying molecular gas properties in merging galaxies gives important clues
to the onset and evolution of interaction-triggered starbursts. The CO/13CO 1-0
line intensity ratio can be used as a tracer of how dynamics and star formation
processes impact the gas properties. The Medusa (NGC~4194) merger is
particularly interesting to study since its LFIR/LCO ratio rivals that of
ultraluminous galaxies (ULIRGs), despite the comparatively modest luminosity,
indicating an exceptionally high star formation efficiency (SFE) in the Medusa
merger.
Interferometric OVRO observations of CO and 13CO 1-0 in the Medusa show the
CO/13CO intensity ratio increases from normal, quiescent values (7-10) in the
outer parts (r>2 kpc) of the galaxy to high (16 to >40) values in the central
(r<1 kpc) starburst region. In the centre there is an east-west gradient where
the line ratio changes by more than a factor of three over 5" (945 pc). The
integrated 13CO emission peaks in the north-western starburst region while the
central CO emission is strongly associated with the prominent crossing
dust-lane. We discuss the central east-west gradient in the context of gas
properties in the starburst and the central dust lane. We suggest that the
central gradient is mainly caused by diffuse gas in the dust lane. In this
scenario, the actual molecular mass distribution is better traced by the 13CO
1-0 emission than the CO. The possibilities of temperature and abundance
gradients are also discussed. We compare the central gas properties of the
Medusa to those of other minor mergers and suggest that the extreme and
transient phase of the Medusa star formation activity has similar traits to
those of high-redshift galaxies.Comment: 7 pages, 4 figures, accepted for publication in Astronomy and
Astrophysic
Star-formation in the central kpc of the starburst/LINER galaxy NGC1614
A high angular resolution, multi-wavelength study of the LINER galaxy NGC1614
has been carried out. OVRO CO 1-0 observations are presented together with
extensive multi-frequency radio continuum and HI absorption observations with
the VLA and MERLIN. Toward the center of NGC1614, we have detected a ring of
radio continuum emission with a radius of 300 pc. This ring is coincident with
previous radio and Paschen-alpha observations. The dynamical mass of the ring
based on HI absorption is 3.1 x 10E9 Msun. The peak of the integrated CO 1-0
emission is shifted by 1" to the north-west of the ring center and a
significant fraction of the CO emission is associated with a crossing dust
lane. An upper limit to the molecular gas mass in the ring region is 1.7 x 10E9
Msun. Inside the ring, there is a north to south elongated 1.4GHz radio
continuum feature with a nuclear peak. This peak is also seen in the 5GHz radio
continuum and in the CO. We suggest that the R=300 pc star forming ring
represents the radius of a dynamical resonance - as an alternative to the
scenario that the starburst is propagating outwards from the center into a
molecular ring. The ring-like appearance probably part of a spiral structure.
Substantial amounts of molecular gas have passed the radius of the ring and
reached the nuclear region. The nuclear peak seen in 5GHz radio continuum and
CO is likely related to previous star formation, where all molecular gas was
not consumed. The LINER-like optical spectrum observed in NGC1614 may be due to
nuclear starburst activity, and not to an Active Galactic Nucleus (AGN).
Although the presence of an AGN cannot be excluded.Comment: Accepted by Astronomy and Astrophysics, 12 pages, 10 figure
Molecular clouds in the centers of galaxies: Constraints from HCN and CO-13 line emission
We have searched for HCN J=1-0 line emission in the centers of 12 galaxies and have detected it in 10 of them. We have obtained complementary data on J=1-0 and 2-1 transitions of CO-12 and CO-13 in these systems. The ratio of integrated intensities, I(CO 1-0)/I(HCN 1-0) = 25 +/- 11 for this sample. We find that HCN emission of this strength can be produced under conditions of subthermal excitation. In combination with the line ratios in CO and CO-13, HCN puts constraints on the mean conditions of molecular clouds and on the mix of cloud types within the projected beam
A survey of HC_3N in extragalactic sources: Is HC_3N a tracer of activity in ULIRGs?
Context. HC_3N is a molecule that is mainly associated with Galactic star-forming regions, but it has also been detected in extragalactic environments.
Aims. To present the first extragalactic survey of HC_3N, when combining earlier data from the literature with six new single-dish detections, and to compare HC_3N with other molecular tracers (HCN, HNC), as well as other properties (silicate absorption strength, IR flux density ratios, C_(II) flux, and megamaser activity).
Methods. We present mm IRAM 30 m, OSO 20 m, and SEST observations of HC_3N rotational lines (mainly the J = 10–9 transition) and of the J = 1–0 transitions of HCN and HNC. Our combined HC_3N data account for 13 galaxies (excluding the upper limits reported for the non-detections), while we have HCN and HNC data for more than 20 galaxies.
Results. A preliminary definition “HC_3N-luminous galaxy” is made based upon the HC_3N/HCN ratio. Most (~80%) HC_3N-luminous galaxies seem to be deeply obscured galaxies and (U)LIRGs. A majority (~60% or more) of the HC3N-luminous galaxies in the sample present OH mega- or strong kilomaser activity. A possible explanation is that both HC_3N and OH megamasers need warm dust for their excitation. Alternatively, the dust that excites the OH megamaser offers protection against UV destruction of HC_3N. A high silicate absorption strength is also found in several of the HC_3N-luminous objects, which may help the HC3N to survive. Finally, we find that a high HC_3N/HCN ratio is related to a high dust temperature and a low C_(II) flux
The influence of flow discharge variations on the morphodynamics of a diffluence-confluence unit on a large river: Impacts of discharge variation on a diffluence-confluence unit
© 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. Bifurcations are key geomorphological nodes in anabranching and braided fluvial channels, controlling local bed morphology, the routing of sediment and water, and ultimately defining the stability of their associated diffluence–confluence unit. Recently, numerical modelling of bifurcations has focused on the relationship between flow conditions and the partitioning of sediment between the bifurcate channels. Herein, we report on field observations spanning September 2013 to July 2014 of the three-dimensional flow structure, bed morphological change and partitioning of both flow discharge and suspended sediment through a large diffluence–confluence unit on the Mekong River, Cambodia, across a range of flow stages (from 13 500 to 27 000 m 3 s −1 ). Analysis of discharge and sediment load throughout the diffluence–confluence unit reveals that during the highest flows (Q = 27 000 m 3 s −1 ), the downstream island complex is a net sink of sediment (losing 2600 ± 2000 kg s −1 between the diffluence and confluence), whereas during the rising limb (Q = 19 500 m 3 s −1 ) and falling limb flows (Q = 13 500 m 3 s −1 ) the sediment balance is in quasi-equilibrium. We show that the discharge asymmetry of the bifurcation varies with discharge and highlight that the influence of upstream curvature-induced water surface slope and bed morphological change may be first-order controls on bifurcation configuration. Comparison of our field data to existing bifurcation stability diagrams reveals that during lower (rising and falling limb) flow the bifurcation may be classified as unstable, yet transitions to a stable condition at high flows. However, over the long term (1959–2013) aerial imagery reveals the diffluence–confluence unit to be fairly stable. We propose, therefore, that the long-term stability of the bifurcation, as well as the larger channel planform and morphology of the diffluence–confluence unit, may be controlled by the dominant sediment transport regime of the system. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd
COLA. III. Radio Detection of Active Galactic Nucleus in Compact Moderate Luminosity Infrared Galaxies
We present results from 4.8 GHz Very Large Array (VLA) and global very long baseline interferometry (VLBI) observations of the northern half of the moderate FIR luminosity (median L_(IR) = 10^(11.01) L_☉) COLA sample of star-forming galaxies. VLBI sources are detected in a high fraction (20/90) of the galaxies observed. The radio luminosities of these cores (~10^(21) W Hz^(–1)) are too large to be explained by radio supernovae or supernova remnants and we argue that they are instead powered by active galactic nuclei (AGNs). These sub-parsec scale radio cores are preferentially detected toward galaxies whose VLA maps show bright 100-500 parsec scale nuclear radio components. Since these latter structures tightly follow the FIR to radio-continuum correlation for star formation, we conclude that the AGN-powered VLBI sources are associated with compact nuclear starburst environments. The implications for possible starburst-AGN connections are discussed. The detected VLBI sources have a relatively narrow range of radio luminosity consistent with models in which intense compact Eddington-limited starbursts regulate the gas supply onto a central supermassive black hole. The high incidence of AGN radio cores in compact starbursts suggests little or no delay between the starburst phase and the onset of AGN activity
Gas and Dust in the Taffy Galaxies: Ugc12914/15
We present a comprehensive study of the dust and gas properties in the
after-head-on-collision UGC12914/15 galaxy system using multi-transition CO
data and SCUBA sub-mm continuum images at both 450 and 850m. CO(3-2) line
emission was detected in the disks of UGC 12914 and UGC 12915 as well as in a
bridge connecting the two galaxies. Dust emission at 450m was detected for
the first time in the two galactic disks and in the connecting bridge. Using an
LVG excitation analysis model we have obtained good estimates of the physical
parameters in different regions of this system and the amount of molecular gas
was found to be 3-4 times lower than that estimated by other investigators
using the standard Galactic CO-to-H2 conversion factor. Comparing with the dust
mass derived from the SCUBA data, we found that the gas-to-dust ratio was
comparable to the Galactic value in the two galaxy disks but a factor of ~3
higher in the bridge. The physical condition of the molecular gas in the bridge
is comparable to that in the diffuse clouds in our Galaxy. Our result is
consistent with the scenario that the bridge molecular gas originated from the
disk molecular clouds and has been drawn out of the galactic disks due to
direct cloud-cloud collision.
Our data indicate that the global star formation efficiency (SFE) in UGC
12915 is comparable to that of normal spiral galaxies, and the SFE is 40% lower
in UGC 12914 than in UGC 12915. Little star formation activity was found in the
bridge except in an HII region adjacent to the disk of UGC 12915.Comment: Accepted by AJ. 45 pages, 10 figures (Fig 1-5 and Fig 7 in gif
format
Major impact from a minor merger - The extraordinary hot molecular gas flow in the Eye of the NGC 4194 Medusa galaxy
Minor mergers are important processes contributing significantly to how
galaxies evolve across the age of the Universe. Their impact on supermassive
black hole growth and star formation is profound. The detailed study of dense
molecular gas in galaxies provides an important test of the validity of the
relation between star formation rate and HCN luminosity on different galactic
scales. We use observations of HCN, HCO+1-0 and CO3-2 to study the dense gas
properties in the Medusa merger. We calculate the brightness temperature ratios
and use them in conjunction with a non-LTE radiative line transfer model. The
HCN and HCO+1-0, and CO3-2 emission do not occupy the same structures as the
less dense gas associated with the lower-J CO emission. The only emission from
dense gas is detected in a 200pc region within the "Eye of the Medusa". No HCN
or HCO+ is detected for the extended starburst. The CO3-2/2-1 brightness
temperature ratio inside "the Eye" is ~2.5 - the highest ratio found so far.
The line ratios reveal an extreme, fragmented molecular cloud population inside
"the Eye" with large temperatures (>300K) and high gas densities (>10^4 cm^-3).
"The Eye" is found at an interface between a large-scale minor axis inflow and
the Medusa central region. The extreme conditions inside "the Eye" may be the
result of the radiative and mechanical feedback from a deeply embedded, young,
massive super star cluster, formed due to the gas pile-up at the intersection.
Alternatively, shocks from the inflowing gas may be strong enough to shock and
fragment the gas. For both scenarios, however, it appears that the HCN and HCO+
dense gas tracers are not probing star formation, but instead a post-starburst
and/or shocked ISM that is too hot and fragmented to form new stars. Thus,
caution is advised in linking the detection of emission from dense gas tracers
to evidence of ongoing or imminent star formation.Comment: 10 pages, 5 figures, 2 tables, accepted for publication in A&
Extreme flood-driven fluvial bank erosion and sediment loads: direct process measurements using integrated Mobile Laser Scanning (MLS) and hydro-acoustic techniques: Direct measurement of flood-driven erosion using MLS and MBES
Copyright © 2016 John Wiley & Sons, Ltd. This methods paper details the first attempt at monitoring bank erosion, flow and suspended sediment at a site during flooding on the Mekong River induced by the passage of tropical cyclones. We deployed integrated mobile laser scanning (MLS) and multibeam echo sounding (MBES), alongside acoustic Doppler current profiling (aDcp), to directly measure changes in river bank and bed at high (~0.05 m) spatial resolution, in conjunction with measurements of flow and suspended sediment dynamics. We outline the methodological steps used to collect and process this complex point cloud data, and detail the procedures used to process and calibrate the aDcp flow and sediment flux data. A comparison with conventional remote sensing methods of estimating bank erosion, using aerial images and Landsat imagery, reveals that traditional techniques are error prone at the high temporal resolutions required to quantify the patterns and volumes of bank erosion induced by the passage of individual flood events. Our analysis reveals the importance of cyclone-driven flood events in causing high rates of erosion and suspended sediment transport, with a c. twofold increase in bank erosion volumes and a fourfold increase in suspended sediment volumes in the cyclone-affected wet season. Copyright © 2016 John Wiley & Sons, Ltd
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