The contribution of main PM pollution sources and their geographic origin in three urban sites of the Danube macro-region (Zagreb, Budapest and Sofia) were determined by combining receptor and ...Lagrangian models. The source contribution estimates were obtained with the Positive Matrix Factorization (PMF) receptor model and the results were further examined using local wind data and backward trajectories obtained with FLEXPART. Potential Source Contribution Function (PSCF) analysis was applied to identify the geographical source areas for the PM sources subject to long-range transport. Gas-to-particle transformation processes and primary emissions from biomass burning are the most important contributors to PM in the studied sites followed by re-suspension of soil (crustal material) and traffic. These four sources can be considered typical of the Danube macro-region because they were identified in all the studied locations. Long-range transport was observed of: a) sulphate-enriched aged aerosols, deriving from SO2 emissions in combustion processes in the Balkans and Eastern Europe and b) dust from the Saharan and Karakum deserts. The study highlights that PM pollution in the studied urban areas of the Danube macro-region is the result of both local sources and long-range transport from both EU and no-EU areas.
•The Danube Region encompasses one of Europe's air pollution “hot spots”.•Causes of PM pollution were studied in the pilot cities: Zagreb, Budapest and Sofia.•PM sources and their geographical origin were estimated using PMF and FLEXPART.•Main PM sources in the Danube are aged aerosols, biomass burning, soil and traffic.•Long-range transport of PM from non-EU areas is a key issue in the Danube Region.
Display omitted
We identify stellar structures in the PHANGS sample of 74 nearby galaxies and construct morphological masks of sub-galactic environments based on
Spitzer
3.6
μ
m images. At the simplest level, we ...distinguish five environments: centres, bars, spiral arms, interarm regions, and discs without strong spirals. Slightly more sophisticated masks include rings and lenses, which are publicly released but not explicitly used in this paper. We examine trends with environment in the molecular gas content, star formation rate, and depletion time using PHANGS–ALMA CO(2–1) intensity maps and tracers of star formation. The interarm regions and discs without strong spirals clearly dominate in area, whereas molecular gas and star formation are quite evenly distributed among the five basic environments. We reproduce the molecular Kennicutt–Schmidt relation with a slope compatible with unity within the uncertainties and without significant slope differences among environments. In contrast to what has been suggested by early studies, we find that bars are not always deserts devoid of gas and star formation, but instead they show large diversity. Similarly, spiral arms do not account for most of the gas and star formation in disc galaxies, and they do not have shorter depletion times than the interarm regions. Spiral arms accumulate gas and star formation, without systematically boosting the star formation efficiency. Centres harbour remarkably high surface densities and on average shorter depletion times than other environments. Centres of barred galaxies show higher surface densities and wider distributions compared to the outer disc; yet, depletion times are similar to unbarred galaxies, suggesting highly intermittent periods of star formation when bars episodically drive gas inflow, without enhancing the central star formation efficiency permanently. In conclusion, we provide quantitative evidence that stellar structures in galaxies strongly affect the organisation of molecular gas and star formation, but their impact on star formation efficiency is more subtle.
ABSTRACT
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 H ii regions identified from the PHANGS-MUSE survey of 19 nearby (<20 Mpc) star-forming, main-sequence spiral galaxies log(M⋆/M⊙) = 9.4–11. These optical spectroscopic maps are essential to constrain the physical properties of the H ii regions, which we use to investigate their internal pressure terms. We estimate the photoionized gas (Ptherm), direct radiation (Prad), and mechanical wind pressure (Pwind), which we compare to the confining pressure of their host environment (Pde). The H ii regions remain unresolved within our ∼50–100 pc resolution observations, so we place upper (Pmax) and lower (Pmin) limits on each of the pressures by using a minimum (i.e. clumpy structure) and maximum (i.e. smooth structure) size, respectively. We find that the Pmax measurements are broadly similar, and for Pmin the Ptherm is mildly dominant. We find that the majority of H ii regions are overpressured, Ptot/Pde = (Ptherm + Pwind + Prad)/Pde > 1, and expanding, yet there is a small sample of compact H ii regions with Ptot,max/Pde < 1 (∼1 per cent of the sample). These mostly reside in galaxy centres (Rgal < 1 kpc), or, specifically, environments of high gas surface density; log(Σgas/M⊙ pc−2) ∼ 2.5 (measured on kpc-scales). Lastly, we compare to a sample of literature measurements for Ptherm and Prad to investigate how dominant pressure term transitions over around 5 dex in spatial dynamic range and 10 dex in pressure.
Context.
Massive stars form within dense clumps inside giant molecular clouds (GMCs). Finding appropriate chemical tracers of the dense gas (
n
(H
2
) > several 10
4
cm
−3
or
A
V
> 8 mag) and linking ...their line luminosity with the star formation rate is of critical importance.
Aims.
Our aim is to determine the origin and physical conditions of the HCN-emitting gas and study their relation to those of other molecules.
Methods.
In the context of the IRAM 30m ORION-B large program, we present 5 deg
2
(~250 pc
2
) HCN, HNC, HCO
+
, and CO
J
=1–0 maps of the Orion B GMC, complemented with existing wide-field C
I
492 GHz maps, as well as new pointed observations of rotationally excited HCN, HNC, H
13
CN, and HN
13
C lines. We compare the observed HCN line intensities with radiative transfer models including line overlap effects and electron excitation. Furthermore, we study the HCN/HNC isomeric abundance ratio with updated photochemical models.
Results.
We spectroscopically resolve the HCN
J
= 1–0 hyperfine structure (HFS) components (and partially resolved
J
= 2−1 and 3−2 components). We detect anomalous HFS line intensity (and line width) ratios almost everywhere in the cloud. About 70% of the total HCN
J
= 1−0 luminosity,
L
′(HCN
J
= 1−0) = 110 K km s
−1
pc
−2
, arises from
A
V
< 8 mag. The HCN/CO
J
= 1−0 line intensity ratio, widely used as a tracer of the dense gas fraction, shows a bimodal behavior with an inflection point at
A
V
< 3 mag typical of translucent gas and illuminated cloud edges. We find that most of the HCN
J
= 1−0 emission arises from extended gas with
n
(H
2
) < 10
4
cm
−3
, and even lower density gas if the ionization fraction is χ
e
≥ 10
−5
and electron excitation dominates. This result contrasts with the prevailing view of HCN
J
= 1−0 emission as a tracer of dense gas and explains the low-
A
V
branch of the HCN/CO
J
= 1−0 intensity ratio distribution. Indeed, the highest HCN/CO ratios (~ 0.1) at
A
V
< 3 mag correspond to regions of high C
I
492 GHz/CO
J
= 1−0 intensity ratios (>1) characteristic of low-density photodissociation regions. The low surface brightness (≲ 1 K km s
−1
) and extended HCN and HCO
+
J
= 1−0 emission scale with
I
FIR
– a proxy of the stellar far-ultraviolet (FUV) radiation field – in a similar way. Together with CO
J
= 1−0, these lines respond to increasing
I
FIR
up to
G
0
≃ 20. On the other hand, the bright HCN
J
= 1−0 emission (> 6 K km s
−1
) from dense gas in star-forming clumps weakly responds to
I
FIR
once the FUV field becomes too intense (
G
0
> 1500). In contrast, HNC
J
= 1−0 and C
I
492 GHz lines weakly respond to
I
FIR
for all
G
0
. The different power law scalings (produced by different chemistries, densities, and line excitation regimes) in a single but spatially resolved GMC resemble the variety of Kennicutt-Schmidt law indexes found in galaxy averages.
Conclusions.
Given the widespread and extended nature of the C
I
492 GHz emission, as well as its spatial correlation with that of HCO
+
, HCN, and
13
CO
J
= 1−0 lines (in this order), we argue that the edges of GMCs are porous to FUV radiation from nearby massive stars. Enhanced FUV radiation favors the formation and excitation of HCN on large scales, not only in dense star-forming clumps, and it leads to a relatively low value of the dense gas mass to total luminosity ratio,
α
(HCN) = 29
M
⊙
/(K km s
−1
pc
2
) in Orion B. As a corollary for extragalactic studies, we conclude that high HCN/CO
J
= 1−0 line intensity ratios do not always imply the presence of dense gas, which may be better traced by HNC than by HCN.
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.
We present new H
I
observations of the nearby massive spiral galaxy M 83 taken with the JVLA at 21″ angular resolution (≈500 pc) of an extended (∼1.5 deg
2
) ten-point mosaic combined with GBT ...single-dish data. We study the super-extended H
I
disk of M 83 (∼50 kpc in radius), in particular disk kinematics, rotation, and the turbulent nature of the atomic interstellar medium. We define distinct regions in the outer disk (
r
gal
> central optical disk), including a ring, a southern area, a southern arm and a northern arm. We examine H
I
gas surface density, velocity dispersion, and noncircular motions in the outskirts, which we compare to the inner optical disk. We find an increase of velocity dispersion (
σ
v
) toward the pronounced H
I
ring, indicative of more turbulent H
I
gas. Additionally, we report over a large galactocentric radius range (until
r
gal
∼ 50 kpc) where
σ
v
is slightly larger than thermal component (i.e., > 8 km s
−1
). We find that a higher star-formation rate (as traced by far UV emission) is not necessarily always associated with a higher H
I
velocity dispersion, suggesting that radial transport could be a dominant driver for the enhanced velocity dispersion. Furthermore, we find a possible branch that connects the extended H
I
disk to the dwarf irregular galaxy UGCA 365 and 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
r
gal
∼ 2 kpc, inflowing gas at
r
gal
∼ 5.5 kpc, and outflowing gas at
r
gal
∼ 14 kpc. We caution that mass flow rates are highly sensitive to the assumed kinematic disk parameters, in particular to inclination.
The complex physical, kinematic, and chemical properties of galaxy centres make them interesting environments to examine with molecular line emission. We present new 2 − 4″ (∼75 − 150 pc at 7.7 Mpc) ...observations at 2 and 3 mm covering the central 50″ (∼1.9 kpc) of the nearby double-barred spiral galaxy NGC 6946 obtained with the IRAM Plateau de Bure Interferometer. We detect spectral lines from ten molecules: CO, HCN, HCO
+
, HNC, CS, HC
3
N, N
2
H
+
, C
2
H, CH
3
OH, and H
2
CO. We complemented these with published 1 mm CO observations and 33 GHz continuum observations to explore the star formation rate surface density Σ
SFR
on 150 pc scales. In this paper, we analyse regions associated with the inner bar of NGC 6946 – the nuclear region (NUC), the northern (NBE), and southern inner bar end (SBE) and we focus on short-spacing corrected bulk (CO) and dense gas tracers (HCN, HCO
+
, and HNC). We find that HCO
+
correlates best with Σ
SFR
, but the dense gas fraction (
f
dense
) and star formation efficiency of the dense gas (SFE
dense
) fits show different behaviours than expected from large-scale disc observations. The SBE has a higher Σ
SFR
,
f
dense
, and shocked gas fraction than the NBE. We examine line ratio diagnostics and find a higher CO(2−1)/CO(1−0) ratio towards NBE than for the NUC. Moreover, comparison with existing extragalactic datasets suggests that using the HCN/HNC ratio to probe kinetic temperatures is not suitable on kiloparsec and sub-kiloparsec scales in extragalactic regions. Lastly, our study shows that the HCO
+
/HCN ratio might not be a unique indicator to diagnose AGN activity in galaxies.
We present new HCN and HCO
+
(
J
= 3–2) images of the nearby star-forming galaxies (SFGs) NGC 3351, NGC 3627, and NGC 4321. The observations, obtained with the Morita ALMA Compact Array, have a ...spatial resolution of ∼290–440 pc and resolve the inner
R
gal
≲ 0.6–1 kpc of the targets, as well as the southern bar end of NGC 3627. We complement this data set with publicly available images of lower excitation lines of HCN, HCO
+
, and CO and analyse the behaviour of a representative set of line ratios: HCN(3–2)/HCN(1–0), HCN(3–2)/HCO
+
(3–2), HCN(1–0)/CO(2–1), and HCN(3–2)/CO(2–1). Most of these ratios peak at the galaxy centres and decrease outwards. We compare the HCN and HCO
+
observations with a grid of one-phase, non-local thermodynamic equilibrium (non-LTE) radiative transfer models and find them compatible with models that predict subthermally excited and optically thick lines. We study the systematic variations of the line ratios across the targets as a function of the stellar surface density (Σ
star
), the intensity-weighted CO(2–1) (⟨
I
CO
⟩), and the star formation rate surface density (Σ
SFR
). We find no apparent correlation with Σ
SFR
, but positive correlations with the other two parameters, which are stronger in the case of ⟨
I
CO
⟩. The HCN/CO–⟨
I
CO
⟩ relations show ≲0.3 dex galaxy-to-galaxy offsets, with HCN(3–2)/CO(2–1)–⟨
I
CO
⟩ being ∼2 times steeper than HCN(1–0)/CO(2–1). In contrast, the HCN(3–2)/HCN(1–0)–⟨
I
CO
⟩ relation exhibits a tighter alignment between galaxies. We conclude that the overall behaviour of the line ratios cannot be ascribed to variations in a single excitation parameter (e.g., density or temperature).
Context. Massive stars form within dense clumps inside giant molecular clouds (GMCs). Finding appropriate chemical tracers of the dense gas (n(H2) > several 104 cm−3 or AV > 8 mag) and linking their ...line luminosity with the star formation rate is of critical importance. Aims. Our aim is to determine the origin and physical conditions of the HCN-emitting gas and study their relation to those of other molecules. Methods. In the context of the IRAM 30m ORION-B large program, we present 5 deg2 (~250 pc2) HCN, HNC, HCO+, and CO J =1–0 maps of the Orion B GMC, complemented with existing wide-field C I 492 GHz maps, as well as new pointed observations of rotationally excited HCN, HNC, H13CN, and HN13C lines. We compare the observed HCN line intensities with radiative transfer models including line overlap effects and electron excitation. Furthermore, we study the HCN/HNC isomeric abundance ratio with updated photochemical models. Results. We spectroscopically resolve the HCN J = 1–0 hyperfine structure (HFS) components (and partially resolved J = 2−1 and 3−2 components). We detect anomalous HFS line intensity (and line width) ratios almost everywhere in the cloud. About 70% of the total HCN J = 1−0 luminosity, L′(HCN J = 1−0) = 110 K km s−1 pc−2, arises from AV < 8 mag. The HCN/CO J = 1−0 line intensity ratio, widely used as a tracer of the dense gas fraction, shows a bimodal behavior with an inflection point at AV < 3 mag typical of translucent gas and illuminated cloud edges. We find that most of the HCN J = 1−0 emission arises from extended gas with n(H2) < 104 cm−3, and even lower density gas if the ionization fraction is χe ≥ 10−5 and electron excitation dominates. This result contrasts with the prevailing view of HCN J = 1−0 emission as a tracer of dense gas and explains the low-AV branch of the HCN/CO J = 1−0 intensity ratio distribution. Indeed, the highest HCN/CO ratios (~ 0.1) at AV < 3 mag correspond to regions of high C I 492 GHz/CO J = 1−0 intensity ratios (>1) characteristic of low-density photodissociation regions. The low surface brightness (≲ 1 K km s−1) and extended HCN and HCO+ J = 1−0 emission scale with IFIR – a proxy of the stellar far-ultraviolet (FUV) radiation field – in a similar way. Together with CO J = 1−0, these lines respond to increasing IFIR up to G0 ≃ 20. On the other hand, the bright HCN J = 1−0 emission (> 6 K km s−1) from dense gas in star-forming clumps weakly responds to IFIR once the FUV field becomes too intense (G0 > 1500). In contrast, HNC J = 1−0 and C I 492 GHz lines weakly respond to IFIR for all G0. The different power law scalings (produced by different chemistries, densities, and line excitation regimes) in a single but spatially resolved GMC resemble the variety of Kennicutt-Schmidt law indexes found in galaxy averages. Conclusions. Given the widespread and extended nature of the C I 492 GHz emission, as well as its spatial correlation with that of HCO+, HCN, and 13CO J = 1−0 lines (in this order), we argue that the edges of GMCs are porous to FUV radiation from nearby massive stars. Enhanced FUV radiation favors the formation and excitation of HCN on large scales, not only in dense star-forming clumps, and it leads to a relatively low value of the dense gas mass to total luminosity ratio, α (HCN) = 29 M⊙/(K km s−1pc2) in Orion B. As a corollary for extragalactic studies, we conclude that high HCN/CO J = 1−0 line intensity ratios do not always imply the presence of dense gas, which may be better traced by HNC than by HCN.
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