ABSTRACT
It is still poorly constrained how the densest phase of the interstellar medium varies across galactic environment. A large observing time is required to recover significant emission from ...dense molecular gas at high spatial resolution, and to cover a large dynamic range of extragalactic disc environments. We present new NOrthern Extended Millimeter Array (NOEMA) observations of a range of high critical density molecular tracers (HCN, HNC, HCO+) and CO isotopologues (13CO, C18O) towards the nearby (11.3 Mpc) strongly barred galaxy NGC 3627. These observations represent the current highest angular resolution (1.85 arcsec; 100 pc) map of dense gas tracers across a disc of a nearby spiral galaxy, which we use here to assess the properties of the dense molecular gas, and their variation as a function of galactocentric radius, molecular gas, and star formation. We find that the HCN(1–0)/CO(2–1) integrated intensity ratio does not correlate with the amount of recent star formation. Instead, the HCN(1–0)/CO(2–1) ratio depends on the galactic environment, with differences between the galaxy centre, bar, and bar-end regions. The dense gas in the central 600 pc appears to produce stars less efficiently despite containing a higher fraction of dense molecular gas than the bar ends where the star formation is enhanced. In assessing the dynamics of the dense gas, we find the HCN(1–0) and HCO+(1–0) emission lines showing multiple components towards regions in the bar ends that correspond to previously identified features in CO emission. These features are cospatial with peaks of Hα emission, which highlights that the complex dynamics of this bar-end region could be linked to local enhancements in the star formation.
Abstract
Infrared dark clouds (IRDCs) are thought to be potential hosts of the elusive early phases of high-mass star formation. Here, we conduct an in-depth kinematic analysis of one such IRDC, ...G034.43+00.24 (Cloud F), using high sensitivity and high spectral resolution IRAM-30m N2H+ (1–0) and C18O (1–0) observations. To disentangle the complex velocity structure within this cloud, we use Gaussian decomposition and hierarchical clustering algorithms. We find that four distinct coherent velocity components are present within Cloud F. The properties of these components are compared to those found in a similar IRDC, G035.39-00.33 (Cloud H). We find that the components in both clouds have high densities (inferred by their identification in N2H+), trans-to-supersonic non-thermal velocity dispersions with Mach numbers of ∼1.5–4, a separation in velocity of ∼3 km s−1, and a mean red-shift of ∼0.3 km s−1 between the N2H+ (dense gas) and C18O emission (envelope gas). The latter of these could suggest that these clouds share a common formation scenario. We investigate the kinematics of the larger-scale Cloud F structures, using lower-density-tracing 13CO(1–0) observations. A good correspondence is found between the components identified in the IRAM-30m observations and the most prominent component in the 13CO data. We find that the IRDC Cloud F is only a small part of a much larger structure, which appears to be an inter-arm filament of the Milky Way.
We present SOFIA/FIFI-LS observations of the C ii 158 m cooling line across the nearby spiral galaxy NGC 6946. We combine these with UV, IR, CO, and H i data to compare C ii emission to dust ...properties, star formation rate (SFR), H2, and H i at 560 pc scales via stacking by environment (spiral arms, interarm, and center), radial profiles, and individual, beam-sized measurements. We attribute 73% of the C ii luminosity to arms, and 19% and 8% to the center and interarm region, respectively. C ii/TIR, C ii/CO, and C ii/PAH radial profiles are largely constant, but rise at large radii ( 8 kpc) and drop in the center ("C ii deficit"). This increase at large radii and the observed decline with the 70 m/100 m dust color are likely driven by radiation field hardness. We find a near proportional C ii-SFR scaling relation for beam-sized regions, though the exact scaling depends on methodology. C ii also becomes increasingly luminous relative to CO at low SFR (interarm or large radii), likely indicating more efficient photodissociation of CO and emphasizing the importance of C ii as an H2 and SFR tracer in such regimes. Finally, based on the observed C ii and CO radial profiles and different models, we find CO to increase with radius, in line with the observed metallicity gradient. The low CO (galaxy average 2 M pc−2 (K km s−1)−1) and low C ii/CO ratios (∼400 on average) imply little CO-dark gas across NGC 6946, in contrast to estimates in the Milky Way.
Abstract
In this paper we provide a comprehensive description of the internal dynamics of G0.253+0.016 (a.k.a. ‘the Brick’); one of the most massive and dense molecular clouds in the Galaxy to lack ...signatures of widespread star formation. As a potential host to a future generation of high-mass stars, understanding largely quiescent molecular clouds like G0.253+0.016 is of critical importance. In this paper, we reanalyse Atacama Large Millimeter Array cycle 0 HNCO J = 4(0, 4) − 3(0, 3) data at 3 mm, using two new pieces of software that we make available to the community. First, scousepy, a Python implementation of the spectral line fitting algorithm scouse. Secondly, acorns (Agglomerative Clustering for ORganising Nested Structures), a hierarchical n-dimensional clustering algorithm designed for use with discrete spectroscopic data. Together, these tools provide an unbiased measurement of the line-of-sight velocity dispersion in this cloud, $\sigma _{v_{\mathrm{ los}}, {\rm 1D}}=4.4\pm 2.1$ km s−1, which is somewhat larger than predicted by velocity dispersion-size relations for the central molecular zone (CMZ). The dispersion of centroid velocities in the plane of the sky are comparable, yielding $\sigma _{v_{\mathrm{ los}}, {\rm 1D}}/\sigma _{v_{\mathrm{ pos}}, {\rm 1D}}\sim 1.2\pm 0.3$. This isotropy may indicate that the line-of-sight extent of the cloud is approximately equivalent to that in the plane of the sky. Combining our kinematic decomposition with radiative transfer modelling, we conclude that G0.253+0.016 is not a single, coherent, and centrally condensed molecular cloud; ‘the Brick’ is not a brick. Instead, G0.253+0.016 is a dynamically complex and hierarchically structured molecular cloud whose morphology is consistent with the influence of the orbital dynamics and shear in the CMZ.
ABSTRACT
Infrared dark clouds (IRDCs) are potential hosts of the elusive early phases of high mass star formation (HMSF). Here, we conduct an in-depth analysis of the fragmentation properties of a ...sample of 10 IRDCs, which have been highlighted as some of the best candidates to study HMSF within the Milky Way. To do so, we have obtained a set of large mosaics covering these IRDCs with Atacama Large Millimeter/submillimeter Array (ALMA) at Band 3 (or 3 mm). These observations have a high angular resolution (∼3 arcsec; ∼0.05 pc), and high continuum and spectral line sensitivity (∼0.15 mJy beam−1 and ∼0.2 K per 0.1 km s−1 channel at the N2H+ (1 − 0) transition). From the dust continuum emission, we identify 96 cores ranging from low to high mass (M = 3.4−50.9 M⊙) that are gravitationally bound (αvir = 0.3−1.3) and which would require magnetic field strengths of B = 0.3−1.0 mG to be in virial equilibrium. We combine these results with a homogenized catalogue of literature cores to recover the hierarchical structure within these clouds over four orders of magnitude in spatial scale (0.01–10 pc). Using supplementary observations at an even higher angular resolution, we find that the smallest fragments (<0.02 pc) within this hierarchy do not currently have the mass and/or the density required to form high-mass stars. None the less, the new ALMA observations presented in this paper have facilitated the identification of 19 (6 quiescent and 13 star-forming) cores that retain >16 M⊙ without further fragmentation. These high-mass cores contain trans-sonic non-thermal motions, are kinematically sub-virial, and require moderate magnetic field strengths for support against collapse. The identification of these potential sites of HMSF represents a key step in allowing us to test the predictions from high-mass star and cluster formation theories.
Recent studies from our laboratory demonstrate that TNF-α signaling contributes to the regulation of chondrocyte apoptosis and a lack of TNF-α signaling leads to a persistence of cartilaginous callus ...and delayed resorption of mineralized cartilage. This study examines how delays in the endochondral repair process affect the expression of specific mediators of proteolytic cartilage turnover and vascularization. Simple closed fractures were produced in wild type and TNF-α receptor (p55
−/−/p75
−/−)-deficient mice. Using ribonuclease protection assay (RPA) and microarray analysis, the expression of multiple mRNAs for various angiogenic factors and the metalloproteinase gene family were measured in fracture calluses. The direct actions of TNFα on the expression of specific angiogenic factors and metalloproteinases (MMPs) was examined in both cultured callus cells and articular chondrocytes to compare the effects of TNF-α in growth cartilage versus articular cartilage. MMPs 2, 9, 13, and 14 were quantitatively the most prevalent metalloproteases and all showed peaks in expression during the chondrogenic period. In the absence of TNF-α signaling, the expression of all of these mRNAs was reduced. The angiopoietin families of vascular regulators and their receptors were expressed at much higher levels than the VEGFs and their receptors and while the angiopoietins showed diminished or delayed expression in the absence of TNF-α signaling, VEGF and its receptors remained unaltered. The expression of vascular endothelial growth inhibitor (VEGI or TNFSF15) showed a near absence in its expression in the TNF-α receptor-deficient mice. In vitro assessment of cultured fracture callus cells in comparison to primary articular chondrocytes showed that TNF-α treatment specifically induced the expression of MMP9, MMP14, VEGI, and Angiopoietin 2. These results suggest that TNF-α signaling in chondrocytes controls vascularization of cartilage through the regulation of angiopoietin and VEGI factors which play counterbalancing roles in the induction of growth arrest, or apoptosis in endothelial cells. Furthermore, TNF-α appears to regulate, in part, the expression of two key proteolytic enzymes, MMP 9 and MMP14 that are known to be crucial to the progression of vascularization and turnover of mineralized cartilage. Thus, TNF-α signaling in healing fractures appears to coordinate the expression of specific regulators of endothelial cell survival and metalloproteolytic enzymes and is essential in the transition and progression of the endochondral phase of fracture repair.
Infrared Dark Clouds (IRDCs) are cold, dense regions that are usually found within Giant Molecular Clouds. Ongoing star formation within IRDCs is typically still deeply embedded within the ...surrounding molecular gas. Characterizing the properties of relatively quiescent IRDCs may therefore help us to understand the earliest phases of the star formation process. Studies of local molecular clouds have revealed that deuterated species are enhanced in the earliest phases of star formation. In this paper, we test this towards IRDC G035.39−00.33. We present an 80 arcsec by 140 arcsec map of the J = 2 → 1 transition of N2D+, obtained with the Institut de Radioastronomie Millimétrique 30 m telescope telescope. We find that N2D+ is widespread throughout G035.39−00.33. Complementary observations of N2H+ (1 − 0) are used to estimate the deuterium fraction, D
$_\mathrm{frac}^\mathrm{N_2H^+}$
≡ N(N2D+)/N(N2H+). We report a mean D
$_\mathrm{frac}^\mathrm{N_2H^+}$
= 0.04 ± 0.01, with a maximum of D
$_\mathrm{frac}^\mathrm{N_2H^+}$
= 0.09 ± 0.02. The mean deuterium fraction is ∼3 orders of magnitude greater than the interstellar D/H ratio. High angular resolution observations are required to exclude beam dilution effects of compact deuterated cores. Using chemical modelling, we find that the average observed values of D
$_\mathrm{frac}^\mathrm{N_2H^+}$
are in agreement with an equilibrium deuterium fraction, given the general properties of the cloud. This implies that the IRDC is at least ∼3 Myr old, which is ∼8 times longer than the mean free-fall time of the observed deuterated region.
Abstract Non-invasive characterization of fracture callus structure and composition may facilitate development of surrogate measures of the regain of mechanical function. As such, quantitative ...computed tomography- (CT-) based analyses of fracture calluses could enable more reliable clinical assessments of bone healing. Although previous studies have used CT to quantify and predict fracture healing, it is unclear which of the many CT-derived metrics of callus structure and composition are the most predictive of callus mechanical properties. The goal of this study was to identify the changes in fracture callus structure and composition that occur over time and that are most closely related to the regain of mechanical function. Micro-computed tomography (μCT) imaging and torsion testing were performed on murine fracture calluses ( n = 188) at multiple post-fracture timepoints and under different experimental conditions that alter fracture healing. Total callus volume (TV), mineralized callus volume (BV), callus mineralized volume fraction (BV/TV), bone mineral content (BMC), tissue mineral density (TMD), standard deviation of mineral density ( σTMD ), effective polar moment of inertia ( J eff ), torsional strength, and torsional rigidity were quantified. Multivariate statistical analyses, including multivariate analysis of variance, principal components analysis, and stepwise regression were used to identify differences in callus structure and composition among experimental groups and to determine which of the μCT outcome measures were the strongest predictors of mechanical properties. Although calluses varied greatly in the absolute and relative amounts of mineralized tissue (BV, BMC, and BV/TV), differences among timepoints were most strongly associated with changes in tissue mineral density. Torsional strength and rigidity were dependent on mineral density as well as the amount of mineralized tissue: TMD, BV, and σTMD explained 62% of the variation in torsional strength ( p < 0.001); and TMD, BMC, BV/TV, and σTMD explained 70% of the variation in torsional rigidity ( p < 0.001). These results indicate that fracture callus mechanical properties can be predicted by several μCT-derived measures of callus structure and composition. These findings form the basis for developing non-invasive assessments of fracture healing and for identifying biological and biomechanical mechanisms that lead to impaired or enhanced healing.
We use integral field spectroscopy from the PHANGS–MUSE survey, which resolves the ionised interstellar medium structure at ∼50 pc resolution in 19 nearby spiral galaxies, to study the origin of the ...diffuse ionised gas (DIG). We examine the physical conditions of the diffuse gas by first removing morphologically defined H
II
regions and then binning the low-surface-brightness areas to achieve significant detections of the key nebular lines in the DIG. A simple model for the leakage and propagation of ionising radiation from H
II
regions is able to reproduce the observed distribution of H
α
in the DIG. This model infers a typical mean free path for the ionising radiation of 1.9 kpc for photons propagating within the disc plane. Leaking radiation from H
II
regions also explains the observed decrease in line ratios of low-ionisation species (S
II
/H
α
, N
II
/H
α
, and O
I
/H
α
) with increasing H
α
surface brightness (Σ
H
α
). Emission from hot low-mass evolved stars, however, is required to explain: (1) the enhanced low-ionisation line ratios observed in the central regions of some of the galaxies in our sample; (2) the observed trends of a flat or decreasing O
III
/H
β
with Σ
H
α
; and (3) the offset of some DIG regions from the typical locus of H
II
regions in the Baldwin–Phillips–Terlevich (BPT) diagram, extending into the area of low-ionisation (nuclear) emission-line regions (LINERs). Hot low-mass evolved stars make a small contribution to the energy budget of the DIG (2% of the galaxy-integrated H
α
emission), but their harder spectra make them fundamental contributors to O
III
emission. The DIG might result from a superposition of two components, an energetically dominant contribution from young stars and a more diffuse background of harder ionising photons from old stars. This unified framework bridges observations of the Milky Way DIG with LI(N)ER-like emission observed in nearby galaxy bulges.
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