45 research outputs found
Magnetocaloric properties of (RE)GaO (RE=Tb,Gd,Nd,Dy)
We report the characteristic magnetic properties of several members of the
rare earth garnet family, GdGaO (GGG), DyGaO
(DGG), TbGaO (TGG), and NdGaO (NGG), and compare
their relative potential utility for magnetocaloric cooling, including their
minimal adiabatic demagnetisation refrigeration (ADR) temperatures and relative
cooling parameters. A main objective of this work was to find potential
improvements over the magnetocaloric properties of GGG for use in low
temperature ADR cryostats. Using Tb and Dy in the RE-site offers
in principle higher saturation magnetisation and Nd gives a lower de
Gennes factor and therefore potentially low transition temperature. Our results
show that DyGaO yields an optimal relative cooling parameter
() at low applied fields and a low transition temperature, which would
allow for the design of more efficient ADR cryostats.Comment: 10 pages, 10 figures, submitted to Physical Review Applie
Fuelling the nuclear ring of NGC 1097
Galactic bars can drive cold gas inflows towards the centres of galaxies. The
gas transport happens primarily through the so-called bar ``dust lanes'', which
connect the galactic disc at kpc scales to the nuclear rings at hundreds of pc
scales much like two gigantic galactic rivers. Once in the ring, the gas can
fuel star formation activity, galactic outflows, and central supermassive black
holes. Measuring the mass inflow rates is therefore important to understanding
the mass/energy budget and evolution of galactic nuclei. In this work, we use
CO datacubes from the PHANGS-ALMA survey and a simple geometrical method to
measure the bar-driven mass inflow rate onto the nuclear ring of the barred
galaxy NGC~1097. The method assumes that the gas velocity in the bar lanes is
parallel to the lanes in the frame co-rotating with the bar, and allows one to
derive the inflow rates from sufficiently sensitive and resolved
position-position-velocity diagrams if the bar pattern speed and galaxy
orientations are known. We find an inflow rate of averaged over a time span of 40 Myr, which varies by a
factor of a few over timescales of 10 Myr. Most of the inflow appears to
be consumed by star formation in the ring which is currently occurring at a
rate of -, suggesting that the
inflow is causally controlling the star formation rate in the ring as a
function of time.Comment: Accepted in MNRA
A constant NH(1-0)-to-HCN(1-0) ratio on kiloparsec scales
Nitrogen hydrides such as NH and NH are widely used by Galactic
observers to trace the cold dense regions of the interstellar medium. In
external galaxies, because of limited sensitivity, HCN has become the most
common tracer of dense gas over large parts of galaxies. We provide the first
systematic measurements of NH(1-0) across different environments of an
external spiral galaxy, NGC6946. We find a strong correlation ()
between the HCN(1-0) and NH(1-0) intensities across the inner
of the galaxy, at kiloparsec scales. This correlation is
equally strong between the ratios NH(1-0)/CO(1-0) and HCN(1-0)/CO(1-0),
tracers of dense gas fractions (). We measure an average
intensity ratio of NH(1-0)/HCN(1-0) over our set of five
IRAM-30m pointings. These trends are further supported by existing measurements
for Galactic and extragalactic sources. This narrow distribution in the average
ratio suggests that the observed systematic trends found in kiloparsec-scale
extragalactic studies of and the efficiency of dense gas
(SFE) would not change if we employed NH(1-0) as a
more direct tracer of dense gas. At kiloparsec scales our results indicate that
the HCN(1-0) emission can be used to predict the expected NH(1-0) over
those regions. Our results suggest that, even if HCN(1-0) and NH(1-0)
trace different density regimes within molecular clouds, subcloud differences
average out at kiloparsec scales, yielding the two tracers proportional to each
other.Comment: Accepted for publication in Astronomy & Astrophysic
Calibrating mid-infrared emission as a tracer of obscured star formation on HII-region scales in the era of JWST
Measurements of the star formation activity on cloud scales are fundamental
to uncovering the physics of the molecular cloud, star formation, and stellar
feedback cycle in galaxies. Infrared (IR) emission from small dust grains and
polycyclic aromatic hydrocarbons (PAHs) are widely used to trace the obscured
component of star formation. However, the relation between these emission
features and dust attenuation is complicated by the combined effects of dust
heating from old stellar populations and an uncertain dust geometry with
respect to heating sources. We use images obtained with NIRCam and MIRI as part
of the PHANGS--JWST survey to calibrate dust emission at 21, and the
emission in the PAH-tracing bands at 3.3, 7.7, 10, and 11.3 as
tracers of obscured star formation. We analyse 20000 optically selected
HII regions across 19 nearby star-forming galaxies, and benchmark their IR
emission against dust attenuation measured from the Balmer decrement. We model
the extinction-corrected H flux as the sum of the observed H
emission and a term proportional to the IR emission, with as the
proportionality coefficient. A constant leads to extinction-corrected
H estimates which agree with those obtained with the Balmer decrement
with a scatter of 0.1 dex for all bands considered. Among these bands,
21 emission is demonstrated to be the best tracer of dust
attenuation. The PAH-tracing bands underestimate the correction for bright HII
regions, since in these environments the ratio of PAH-tracing bands to 21 decreases, signalling destruction of the PAH molecules. For fainter HII
regions all bands suffer from an increasing contamination from the diffuse
infrared background.Comment: accepted for publication in A&
The Physical Drivers and Observational Tracers of CO-to-H2 Conversion Factor Variations in Nearby Barred Galaxy Centers
The CO-to-H conversion factor (\alpha_\rm{CO}) is central to measuring
the amount and properties of molecular gas. It is known to vary with
environmental conditions, and previous studies have revealed lower
\alpha_\rm{CO} in the centers of some barred galaxies on kpc scales. To
unveil the physical drivers of such variations, we obtained ALMA Band 3, 6, and
7 observations toward the inner 2 kpc of NGC 3627 and NGC 4321 tracing
CO, CO, and CO lines on 100 pc scales. Our multi-line
modeling and Bayesian likelihood analysis of these datasets reveal variations
of molecular gas density, temperature, optical depth, and velocity dispersion,
which are among the key drivers of \alpha_\rm{CO}. The central 300 pc nuclei
in both galaxies show strong enhancement of temperature T_\rm{k}>100 K and
density n_\rm{H_2}>10^3 cm. Assuming a CO-to-H abundance of
, we derive 4-15 times lower \alpha_\rm{CO} than the Galactic
value across our maps, which agrees well with previous kpc-scale measurements.
Combining the results with our previous work on NGC 3351, we find a strong
correlation of \alpha_\rm{CO} with low-J CO optical depths
(\tau_\rm{CO}), as well as an anti-correlation with T_\rm{k}. The
\tau_\rm{CO} correlation explains most of the \alpha_\rm{CO} variation in
the three galaxy centers, whereas changes in T_\rm{k} influence
\alpha_\rm{CO} to second order. Overall, the observed line width and
CO/CO 2-1 line ratio correlate with \tau_\rm{CO} variation in
these centers, and thus they are useful observational indicators for
\alpha_\rm{CO} variation. We also test current simulation-based
\alpha_\rm{CO} prescriptions and find a systematic overprediction, which
likely originates from the mismatch of gas conditions between our data and the
simulations.Comment: Accepted for publication in ApJ; 30 pages of main text + 3 appendice
The ALMOND Survey: Molecular cloud properties and gas density tracers across 25 nearby spiral galaxies with ALMA
We use new HCN(1-0) data from the ALMOND (ACA Large-sample Mapping Of Nearby
galaxies in Dense gas) survey to trace the kpc-scale molecular gas density
structure and CO(2-1) data from PHANGS-ALMA to trace the bulk molecular gas
across 25 nearby, star-forming galaxies. At 2.1 kpc scale, we measure the
density-sensitive HCN/CO line ratio and the SFR/HCN ratio to trace the star
formation efficiency in the denser molecular medium. At 150 pc scale, we
measure structural and dynamical properties of the molecular gas via CO(2-1)
line emission, which is linked to the lower resolution data using an
intensity-weighted averaging method. We find positive correlations (negative)
of HCN/CO (SFR/HCN) with the surface density, the velocity dispersion and the
internal turbulent pressure of the molecular gas. These observed correlations
agree with expected trends from turbulent models of star formation, which
consider a single free-fall time gravitational collapse. Our results show that
the kpc-scale HCN/CO line ratio is a powerful tool to trace the 150 pc scale
average density distribution of the molecular clouds. Lastly, we find
systematic variations of the SFR/HCN ratio with cloud-scale molecular gas
properties, which are incompatible with a universal star formation efficiency.
Overall, these findings show that mean molecular gas density, molecular cloud
properties and star formation are closely linked in a coherent way, and
observations of density-sensitive molecular gas tracers are a useful tool to
analyse these variations, linking molecular gas physics to stellar output
across galaxy discs.Comment: 48 pages, 40 figure
Kinematic analysis of the super-extended HI disk of the nearby spiral galaxy M 83
Funding: CE, FB, AB, IB, JdB and JP acknowledge funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No.726384/Empire). TGW acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 694343). JMDK gratefully acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme via the ERC Starting Grant MUSTANG (grant agreement number 714907). SCOG acknowledges funding from the European Research Council via the ERC Synergy Grant “ECOGAL – Understanding our Galactic ecosystem: From the disk of the Milky Way to the formation sites of stars and planets” (project ID 855130). WJGdB received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 882793 ‘MeerGas’).We present new HI observations of the nearby massive spiral galaxy M83, taken with the VLA at 21″ angular resolution (≈500 pc) of an extended (1.5 deg2) 10-point mosaic combined with GBT single dish data. We study the super-extended HI disk of M83 (∼50 kpc in radius), in particular disc kinematics, rotation and the turbulent nature of the atomic interstellar medium. We define distinct regions in the outer disk (rgal > central optical disk), including ring, southern area, and southern and northern arm. We examine HI gas surface density, velocity dispersion and non-circular motions in the outskirts, which we compare to the inner optical disk. We find an increase of velocity dispersion (σv) towards the pronounced HI ring, indicative of more turbulent HI gas. Additionally, we report over a large galactocentric radius range (until rgal ∼ 50 kpc) that σv is slightly larger than thermal (i.e. > 8 km s-1). We find that a higher star formation rate (as traced by FUV emission) is not always necessarily associated with a higher HI velocity dispersion, suggesting that radial transport could be a dominant driver for the enhanced velocity dispersion. We further find a possible branch that connects the extended HI disk to the dwarf irregular galaxy UGCA365, that deviates from the general direction of the northern arm. Lastly, we compare mass flow rate profiles (based on 2D and 3D tilted ring models) and find evidence for outflowing gas at rgal ∼ 2 kpc, inflowing gas at rgal ~ 5.5 kpc and outflowing gas at rgal ~ 14 kpc. We caution that mass flow rates are highly sensitive to the assumed kinematic disk parameters, in particular, to the inclination.Publisher PDFPeer reviewe
Comparing the pre-SNe feedback and environmental pressures for 6000 HII regions across 19 nearby spiral galaxies
The feedback from young stars (i.e. pre-supernova) is thought to play a crucial role in molecular cloud destruction. In this paper, we assess the feedback mechanisms acting within a sample of 5810 HII regions identified from the PHANGS-MUSE survey of 19 nearby (, and expanding, yet there is a small sample of compact HII regions with (1% of the sample). These mostly reside in galaxy centres (kpc), or, specifically, environments of high gas surface density; log()2.5 (measured on kpc-scales). Lastly, we compare to a sample of literature measurements for and to investigate how dominant pressure term transitions over around 5dex in spatial dynamic range and 10 dex in pressure
Highway increases concentrations of toxic metals in giant panda habitat
The Qinling panda subspecies (Ailuropoda melanoleuca qinlingensis) is highly endangered with fewer than 350 individuals inhabiting the Qinling Mountains. Previous studies have indicated that giant pandas are exposed to heavy metals, and a possible source is vehicle emission. The concentrations of Cu, Zn, Mn, Pb, Cr, Ni, Cd, Hg, and As in soil samples collected from sites along a major highway bisecting the panda's habitat were analyzed to investigate whether the highway was an important source of metal contamination. There were 11 sites along a 30-km stretch of the 108th National Highway, and at each site, soil samples were taken at four distances from the highway (0, 50, 100, and 300 m) and at three soil depths (0, 5, 10 cm). Concentrations of all metals except As exceeded background levels, and concentrations of Cu, Zn, Mn, Pb, and Cd decreased significantly with increasing distance from the highway. Geo-accumulation index indicated that topsoil next to the highway was moderately contaminated with Pb and Zn, whereas topsoil up to 300 m away from the highway was extremely contaminated with Cd. The potential ecological risk index demonstrated that this area was in a high degree of ecological hazards, which were also due to serious Cd contamination. And, the hazard quotient indicated that Cd, Pb, and Mn especially Cd could pose the health risk to giant pandas. Multivariate analyses demonstrated that the highway was the main source of Cd, Pb, and Zn and also put some influence on Mn. The study has confirmed that traffic does contaminate roadside soils and poses a potential threat to the health of pandas. This should not be ignored when the conservation and management of pandas is considered