18 research outputs found
Does the HCN/CO ratio trace the star-forming fraction of gas? I. A comparison with analytical models of star formation
We use archival ALMA observations of the HCN and CO transitions, in
addition to the radio continuum at 93 GHz, to assess the relationship between
dense gas, star formation, and gas dynamics in ten, nearby (U)LIRGs and
late-type galaxy centers. We frame our results in the context of turbulent and
gravoturbulent models of star formation to assess if the HCN/CO ratio tracks
the gravitationally-bound, star-forming gas in molecular clouds
() at sub-kpc scales in nearby galaxies. We confirm that the
HCN/CO ratio is a tracer of gas above cm,
but the sub-kpc variations in HCN/CO do not universally track
. We find strong evidence for the use of varying star
formation density threshold models, which are able to reproduce trends observed
in and that fixed threshold models
cannot. Composite lognormal and powerlaw models outperform pure lognormal
models in reproducing the observed trends, even when using a fixed powerlaw
slope. The ability of the composite models to better reproduce star formation
properties of the gas provides additional indirect evidence that the star
formation efficiency per free-fall time is proportional to the fraction of
gravitationally-bound gas.Comment: 23 pages, 11 figures, 1 appendix, accepted for publication in Ap
A nearly constant CN/HCN line ratio in nearby galaxies: CN as a new tracer of dense gas
We investigate the relationship between CN N = 1 - 0 and HCN J = 1 - 0
emission on scales from 30 pc to 400 pc using ALMA archival data, for which CN
is often observed simultaneously with the CO J = 1 - 0 line. In a sample of 9
nearby galaxies ranging from ultra-luminous infrared galaxies to normal spiral
galaxies, we measure a remarkably constant CN/HCN line intensity ratio of 0.86
0.07 (standard deviation of 0.20). This relatively constant CN/HCN line
ratio is rather unexpected, as models of photon dominated regions have
suggested that HCN emission traces shielded regions with high column densities
while CN should trace dense gas exposed to high ultraviolet radiation fields.
We find that the CN/HCN line ratio shows no significant correlation with
molecular gas surface density, but shows a mild trend (increase of ~ 1.3 per
dex) with both star formation rate surface density and star formation
efficiency (the inverse of the molecular gas depletion time). Some starburst
and active galactic nuclei show small enhancements in their CN/HCN ratio, while
other nuclei show no significant difference from their surrounding disks. The
nearly constant CN/HCN line ratio implies that CN, like HCN, can be used as a
tracer of dense gas mass and dense gas fraction in nearby galaxies.Comment: Accepted to MNRAS; 21 pages, 12 figure
Hydrodynamic simulations of the disc of gas around supermassive black holes (HDGAS) – I. Molecular gas dynamics
We present hydrodynamic simulations of the interstellar medium (ISM) within the circumnuclear disc (CND) of a typical active galactic nucleus (AGN)-dominated galaxy influenced by mechanical feedback from an AGN. The simulations are coupled with the CHIMES non-equilibrium chemistry network to treat the radiative-cooling and AGN-heating. A focus is placed on the central 100 pc scale where AGN outflows are coupled to the ISM and constrained by observational Seyfert-2 galaxies. AGN-feedback models are implemented with different wind-velocity and mass-loading factors. We post-process the simulation snapshots with a radiative-transfer code to obtain the molecular emission lines. We find that the inclusion of an AGN promotes the formation of CO in clumpy and dense regions surrounding supermassive black holes (SMBHs). The CO(1-0) intensity maps (<6 Myr) in the CND seem to match well with observations of NGC 1068 with a best match for a model with 5000 km s-1 wind-velocity and a high mass-loading factor. We attempt to discern between competing explanations for the apparent counter-rotating gas disc in the NGC 1068 through an analysis of kinematic maps of the CO line emission. We suggest that mechanical AGN-feedback could explain the alignment-stability of position-angle across the different CND radii around the SMBH through momentum and energy loading of the wind. It is the wind-velocity that drives the disc out of alignment on a 100 pc scale for a long period of time. The position-velocity diagrams are in broad agreement with the predicted Keplerian rotation-curve in the model without AGN, but the AGN models exhibit a larger degree of scatter, in better agreement with NGC 1068 observations
Hydrodynamic simulations of the Disk of Gas Around Supermassive black holes (HDGAS) -I; Molecular Gas Dynamics
We present hydrodynamic simulations of the interstellar medium (ISM) within
the circumnuclear disk (CND) of a typical AGN-dominated galaxy influenced by
mechanical feedback from an active galactic nucleus(AGN). The simulations are
coupled with the CHIMES non-equilibrium chemistry network to treat the
radiative-cooling and AGN-heating. A focus is placed on the central 100 pc
scale where AGN outflows are coupled to the ISM and constrained by
observational Seyfert-2 galaxies. AGN-feedback models are implemented with
different wind-velocity and mass-loading factors. We post-process the
simulation snapshots with a radiative-transfer code to obtain the molecular
emission lines. We find that the inclusion of an AGN promotes the formation of
CO in clumpy and dense regions surrounding supermassive-blackholes (SMBH). The
CO(1-0) intensity maps (6 Myr) in the CND seem to match well with
observations of NGC 1068 with a best match for a model with 5000
wind-velocity and a high mass-loading factor. We attempt to discern between
competing explanations for the apparent counter-rotating gas disk in the NGC
1068 through an analysis of kinematic maps of the CO line emission. We suggest
that mechanical AGN-feedback could explain the alignment-stability of
position-angle across the different CND radii around the SMBH through momentum
and energy loading of the wind. It is the wind-velocity that drives the disk
out of alignment on a 100 pc scale for a long period of time. The
position-velocity diagrams are in broad agreement with the predicted Keplerian
rotation-curve in the model without-AGN, but the AGN models exhibit a larger
degree of scatter, in better agreement with NGC 1068 observations.Comment: 16 pages, 13 figures. Accepted for publication in MNRA
The MALATANG Survey : The L GAS-L IR Correlation on Sub-kiloparsec Scale in Six Nearby Star-forming Galaxies as Traced by HCN J = 4 → 3 and HCO + J = 4 → 3
This is an author-created, un-copyedited version of an article published in The Astrophysical Journal. The Version of Record is available online at https://doi.org/10.3847/1538-4357/aac512.We present HCN J = 4→3 and HCO+ J = 4→3 maps of six nearby star-forming galaxies, NGC 253, NGC 1068, IC 342, M82, M83, and NGC 6946, obtained with the James Clerk Maxwell Telescope as part of the MALATANG survey. All galaxies were mapped in the central 2×2 region at 14 (FWHM) resolution (corresponding to linear scales of ∼0.2-1.0 kpc). The LIR-Ldense relation, where the dense gas is traced by the HCN J = 4→3 and the HCO+ J = 4→3 emission, measured in our sample of spatially resolved galaxies is found to follow the linear correlation established globally in galaxies within the scatter. We find that the luminosity ratio, LIR/Ldense, shows systematic variations with LIR within individual spatially resolved galaxies, whereas the galaxy-integrated ratios vary little. A rising trend is also found between LIR/Ldense ratio and the warm-dust temperature gauged by the 70 μm/100 μm flux ratio. We find that the luminosity ratios of IR/HCN (4-3) and IR/HCO+ (4-3), which can be taken as a proxy for the star formation efficiency (SFE) in the dense molecular gas (SFE dense), appear to be nearly independent of the dense gas fraction ( f dense) for our sample of galaxies. The SFE of the total molecular gas (SFEmol) is found to increase substantially with f dense when combining our data with those on local (ultra)luminous infrared galaxies and high-z quasars. The mean LHCN(4-3) LHCO+(4-3) line ratio measured for the six targeted galaxies is 0.9±0.6. No significant correlation is found for the L'HCN(4-3) L'HCO+(4-3) ratio with the star formation rate as traced by L IR, nor with the warm-dust temperature, for the different populations of galaxies.Peer reviewe
Molecular Outflows Identified in the FCRAO CO Survey of the Taurus Molecular Cloud
The 100 square degree FCRAO CO survey of the Taurus molecular cloud provides
an excellent opportunity to undertake an unbiased survey of a large, nearby,
molecular cloud complex for molecular outflow activity. Our study provides
information on the extent, energetics and frequency of outflows in this region,
which are then used to assess the impact of outflows on the parent molecular
cloud. The search identified 20 outflows in the Taurus region, 8 of which were
previously unknown. Both CO and CO data cubes from the Taurus
molecular map were used, and dynamical properties of the outflows are derived.
Even for previously known outflows, our large-scale maps indicate that many of
the outflows are much larger than previously suspected, with eight of the flows
(40%) being more than a parsec long. The mass, momentum and kinetic energy from
the 20 outflows are compared to the repository of turbulent energy in Taurus.
Comparing the energy deposition rate from outflows to the dissipation rate of
turbulence, we conclude that outflows by themselves cannot sustain the observed
turbulence seen in the entire cloud. However, when the impact of outflows is
studied in selected regions of Taurus, it is seen that locally, outflows can
provide a significant source of turbulence and feedback. Five of the eight
newly discovered outflows have no known associated stellar source, indicating
that they may be embedded Class 0 sources. In Taurus, 30% of Class I sources
and 12% of Flat spectrum sources from the Spitzer YSO catalogue have outflows,
while 75% of known Class 0 objects have outflows. Overall, the paucity of
outflows in Taurus compared to the embedded population of Class I and Flat
Spectrum YSOs indicate that molecular outflows are a short-lived stage marking
the youngest phase of protostellar life.Comment: Accepted for publication in MNRAS; 28 pages, 60 figures, 5 tables.
For full abstract, see pape
Aquatic-terrestrial transitions of feeding systems in vertebrates : a mechanical perspective
Transitions to terrestrial environments confront ancestrally aquatic animals with several mechanical and physiological problems owing to the different physical properties of water and air. As aquatic feeders generally make use of flows of water relative to the head to capture, transport and swallow food, it follows that morphological and behavioral changes were inevitably needed for the aquatic animals to successfully perform these functions on land. Here, we summarize the mechanical requirements of successful aquatic-to-terrestrial transitions in food capture, transport and swallowing by vertebrates and review how different taxa managed to fulfill these requirements. Amphibious ray-finned fishes show a variety of strategies to stably lift the anterior trunk, as well as to grab ground-based food with their jaws. However, they still need to return to the water for the intra-oral transport and swallowing process. Using the same mechanical perspective, the potential capabilities of some of the earliest tetrapods to perform terrestrial feeding are evaluated. Within tetrapods, the appearance of a mobile neck and a muscular and movable tongue can safely be regarded as key factors in the colonization of land away from amphibious habitats. Comparative studies on taxa including salamanders, which change from aquatic feeders as larvae to terrestrial feeders as adults, illustrate remodeling patterns in the hyobranchial system that can be linked to its drastic change in function during feeding. Yet, the precise evolutionary history in form and function of the hyolingual system leading to the origin(s) of a muscular and adhesive tongue remains unknown