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.
Aims. We aim to use the high spatial resolution of the Atacama Large Millimeter/submillimeter Array (ALMA) to map the flow pattern of molecular gas near DG Tauri and its disk, a young stellar object ...driving a jet and a molecular outflow. Methods. We use observations from ALMA in the J = 2−1 transition of 12CO, 13CO, and C18O to study the Keplerian disk of DG Tauri and outflows that may be related to the disk and the jet. Results. We find a new wind component flowing radially at a steep angle (≈25° from the vertical) above the disk with a velocity of ≈3.1 km s−1. It continues the trend of decreasing velocity for increasing distance from the jet axis (“onion-like velocity structure”). Conclusions. The new component is located close to the protostellar disk surface and may be related to photoevaporative winds.
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.
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 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.
Context.
Stellar feedback is one of the fundamental factors regulating the evolution of galaxies. However, we still do not have access to strong observational constraints on the relative importance ...of the different feedback mechanisms (e.g. radiation, ionised gas pressure, stellar winds) in driving H
II
region evolution and molecular cloud disruption. To quantify and compare the different feedback mechanisms, the size of an H
II
region is crucial, whereas samples of well-resolved H
II
regions are scarce.
Aims.
We constrain the relative importance of the various feedback mechanisms from young massive star populations by resolving H
II
regions across the disk of the nearby star-forming galaxy NGC 1672.
Methods.
We combined measurements of ionised gas nebular lines obtained by PHANGS-MUSE, with high-resolution (PSF
FWHM
∼ 0.1″; ∼10 pc) imaging from
Hubble
Space Telescope (HST) in both the narrow-band H
α
and broad-band (NUV,
U
,
B
,
V
,
I
) filters. We identified a sample of 40 isolated, compact H
II
regions in the HST H
α
image. We measured the sizes of these H
II
regions, which were previously unresolved in seeing-limited ground-based observations. In addition, we identified the ionisation source(s) for each H
II
region from catalogues produced as part of the PHANGS-HST survey. In doing so, we were able to link young stellar populations with the properties of their surrounding H
II
regions.
Results.
The HST observations allowed us to resolve all 40 regions, with radii between 5 and 40 pc. The H
II
regions investigated here are mildly dominated by thermal or wind pressure, yet their elevation above the radiation pressure is within the expected uncertainty range. We see that radiation pressure provides a substantially higher contribution to the total pressure than previously found in the literature over similar size scales. In general, we find higher pressures within more compact H
II
regions, which is driven by the inherent size scaling relations of each pressure term, albeit with significant scatter introduced by the variation in the stellar population properties (e.g. luminosity, mass, age, metallicity).
Conclusions.
For nearby galaxies, the combination of MUSE/VLT observations with stellar population and resolved H
α
observations from HST provides a promising approach that could yield the statistics required to map out how the importance of different stellar feedback mechanisms evolve over the lifetime of a H
II
region.
Nitrogen hydrides such as NH
3
and N
2
H
+
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 N
2
H
+
(1-0) across different environments of an external spiral galaxy, NGC 6946. We find a strong correlation (
r
> 0.98,
p
< 0.01) between the HCN (1-0) and N
2
H
+
(1-0) intensities across the inner ∼8 kpc of the galaxy, at kiloparsec scales. This correlation is equally strong between the ratios N
2
H
+
(1-0)/CO (1-0) and HCN (1-0)/CO (1-0), tracers of dense gas fractions (
f
dense
). We measure an average intensity ratio of N
2
H
+
(1-0)/HCN (1-0) = 0.15 ± 0.02 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
f
dense
and the efficiency of dense gas (SFE
dense
) would not change if we employed N
2
H
+
(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 N
2
H
+
(1-0) over those regions. Our results suggest that, even if HCN (1-0) and N
2
H
+
(1-0) trace different density regimes within molecular clouds, subcloud differences average out at kiloparsec scales, yielding the two tracers proportional to each other.
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