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.
The PHANGS–MUSE nebular catalogue Groves, B; Kreckel, K; Santoro, F ...
Monthly notices of the Royal Astronomical Society,
02/2023, Letnik:
520, Številka:
4
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
Ionized nebulae provide critical insights into the conditions of the interstellar medium (ISM). Their bright emission lines enable the measurement of physical properties, such as the ...gas-phase metallicity, across galaxy discs and in distant galaxies. The PHANGS–MUSE survey has produced optical spectroscopic coverage of the central star-forming discs of 19 nearby main-sequence galaxies. Here, we use the $\rm {H}\,\alpha$ morphology from this data to identify 30 790 distinct nebulae, finding thousands of nebulae per galaxy. For each nebula, we extract emission line fluxes and, using diagnostic line ratios, identify the dominant excitation mechanism. A total of 23 244 nebulae (75 per cent) are classified as H ii regions. The dust attenuation of every nebulae is characterized via the Balmer decrement and we use existing environmental masks to identify their large-scale galactic environment (centre, bar, arm, interarm, and disc). Using strong-line prescriptions, we measure the gas-phase oxygen abundances (metallicity) and ionization parameter for all H ii regions. With this new catalogue, we measure the radial metallicity gradients and explore second-order metallicity variations within each galaxy. By quantifying the global scatter in metallicity per galaxy, we find a weak negative correlation with global star formation rate and stronger negative correlation with global gas velocity dispersion (in both ionized and molecular gas). With this paper we release the full catalogue of strong line fluxes and derived properties, providing a rich data base for a broad variety of ISM studies.
ABSTRACT
We present spatially resolved Echelle spectroscopy of an intervening Mg ii–Fe ii–Mg i absorption-line system detected at zabs = 0.73379 towards the giant gravitational arc PSZ1 ...G311.65–18.48. The absorbing gas is associated with an inclined disc-like star-forming galaxy, whose major axis is aligned with the two arc-segments reported here. We probe in absorption the galaxy’s extended disc continuously, at ≈3 kpc sampling, from its inner region out to 15× the optical radius. We detect strong ($W_0^{2796}\gt 0.3$Å) coherent absorption along 13 independent positions at impact parameters D = 0–29 kpc on one side of the galaxy, and no absorption at D = 28–57 kpc on the opposite side (all de-lensed distances at zabs). We show that (1) the gas distribution is anisotropic; (2) $W_0^{2796}$, $W_0^{2600}$, $W_0^{2852}$, and the ratio $W_0^{2600}\!/W_0^{2796}$, all anticorrelate with D; (3) the $W_0^{2796}$–D relation is not cuspy and exhibits significantly less scatter than the quasar-absorber statistics; (4) the absorbing gas is co-rotating with the galaxy out to D ≲ 20 kpc, resembling a ‘flat’ rotation curve, but at D ≳ 20 kpc velocities decline below the expectations from a 3D disc-model extrapolated from the nebular O ii emission. These signatures constitute unambiguous evidence for rotating extra-planar diffuse gas, possibly also undergoing enriched accretion at its edge. Arguably, we are witnessing some of the long-sought processes of the baryon cycle in a single distant galaxy expected to be representative of such phenomena.
Aims.
The complexity of star formation at the physical scale of molecular clouds is not yet fully understood. We investigate the mechanisms regulating the formation of stars in different environments ...within nearby star-forming galaxies from the Physics at High Angular resolution in Nearby GalaxieS (PHANGS) sample.
Methods.
Integral field spectroscopic data and radio-interferometric observations of 18 galaxies were combined to explore the existence of the resolved star formation main sequence (Σ
stellar
versus Σ
SFR
), resolved Kennicutt–Schmidt relation (Σ
mol. gas
versus Σ
SFR
), and resolved molecular gas main sequence (Σ
stellar
versus Σ
mol. gas
), and we derived their slope and scatter at spatial resolutions from 100 pc to 1 kpc (under various assumptions).
Results.
All three relations were recovered at the highest spatial resolution (100 pc). Furthermore, significant variations in these scaling relations were observed across different galactic environments. The exclusion of non-detections has a systematic impact on the inferred slope as a function of the spatial scale. Finally, the scatter of the Σ
mol. gas + stellar
versus Σ
SFR
correlation is smaller than that of the resolved star formation main sequence, but higher than that found for the resolved Kennicutt–Schmidt relation.
Conclusions.
The resolved molecular gas main sequence has the tightest relation at a spatial scale of 100 pc (scatter of 0.34 dex), followed by the resolved Kennicutt–Schmidt relation (0.41 dex) and then the resolved star formation main sequence (0.51 dex). This is consistent with expectations from the timescales involved in the evolutionary cycle of molecular clouds. Surprisingly, the resolved Kennicutt–Schmidt relation shows the least variation across galaxies and environments, suggesting a tight link between molecular gas and subsequent star formation. The scatter of the three relations decreases at lower spatial resolutions, with the resolved Kennicutt–Schmidt relation being the tightest (0.27 dex) at a spatial scale of 1 kpc. Variation in the slope of the resolved star formation main sequence among galaxies is partially due to different detection fractions of Σ
SFR
with respect to Σ
stellar
.
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.
We quantify the stellar rotation of galaxies by computing the $ R $ parameter, a proxy for the stellar angular momentum in a sample of 106 intermediate-redshift galaxies (0.1 $<$ z $<$ 0.8). The ...sample is located in the CANDELS/GOODS-S and CANDELS/COSMOS regions, and it was observed by various MUSE surveys. We created spatially resolved stellar velocity and velocity dispersion maps using a full-spectrum fitting technique, covering spatially sim 2$R_ e $ for the galaxies. The sample spans stellar masses from sim 10$^ $ M$_ odot $ to 10$^ $ M$_ odot $ with star formation rates (SFRs) from log$_ $(SFR) approx -3 M$_ odot $yr$^ $ to approx 1.7 M$_ odot $yr$^ $ over a range of 6 Gyr in cosmic time. We studied how the atmospheric seeing, introduced by the instrumental point spread function (PSF), affects the measured spin parameter, and we applied corrections when pertinent. Through the analysis of the $ R diagram, we note that the fraction of round and massive galaxies increases with redshift. We did not measure any galaxy with $ R $ < 0.1 in the sample, and we found only one potential (but uncertain) low-mass slow rotator at z $ more similar to the z=0 low-mass slow rotators characterized by counter-rotation than to massive ellipticals. Moreover, we do not see an evident evolution or trend in the stellar angular momentum with redshift. We characterized the galaxy environment using two different indicators: a local estimator based on the Voronoi tesselation method, and a global estimator derived by the use of the friends-of-friends (FoF) algorithm. We find no correlation between the environment and $ R $ given that we are not probing dense regions or massive galaxy structures. We also analysed the kinematic maps of the sample finding that about 40<!PCT!> of the galaxies are consistent with being regular rotators (RRs), having rotating stellar discs with flat velocity dispersion maps, while $ have complex velocity maps and can be identified as non-regular rotators in spite of their $ R $ values. For the remaining galaxies the classification is uncertain. As we lack galaxies with $ R $ < 0.1 in the sample, we are not able to identify when galaxies lose their angular momentum and become slow rotators within the surveyed environments, area, and redshift range.
Analyzing resolved stellar populations across the disk of a galaxy can provide unique insights into how that galaxy assembled its stellar mass over its lifetime. Previous work at ∼1 kpc resolution ...has already revealed common features in the mass buildup (e.g., inside-out growth of galaxies). However, even at approximate kpc scales, the stellar populations are blurred between the different galactic morphological structures such as spiral arms, bars and bulges. Here we present a detailed analysis of the spatially resolved star formation histories (SFHs) of 19 PHANGS-MUSE galaxies, at a spatial resolution of ∼100 pc. We show that our sample of local galaxies exhibits predominantly negative radial gradients of stellar age and metallicity, consistent with previous findings, and a radial structure that is primarily consistent with local star formation, and indicative of inside-out formation. In barred galaxies, we find flatter metallicity gradients along the semi-major axis of the bar than along the semi-minor axis, as is expected from the radial mixing of material along the bar during infall. In general, the derived assembly histories of the galaxies in our sample tell a consistent story of inside-out growth, where low-mass galaxies assembled the majority of their stellar mass later in cosmic history than high-mass galaxies (also known as “downsizing”). We also show how stellar populations of different ages exhibit different kinematics. Specifically, we find that younger stellar populations have lower velocity dispersions than older stellar populations at similar galactocentric distances, which we interpret as an imprint of the progressive dynamical heating of stellar populations as they age. Finally, we explore how the time-averaged star formation rate evolves with time, and how it varies across galactic disks. This analysis reveals a wide variation of the SFHs of galaxy centers and additionally shows that structural features become less pronounced with age.
Aims.
There exists some consensus that the stellar mass surface density (Σ
⋆
) and molecular gas mass surface density (Σ
mol
) are the main quantities responsible for locally setting the star ...formation rate. This regulation is inferred from locally resolved scaling relations between these two quantities and the star formation rate surface density (Σ
SFR
), which have been extensively studied in a wide variety of works. However, the universality of these relations is debated. Here, we probe the interplay between these three quantities across different galactic environments at a spatial resolution of 150 pc.
Methods.
We performed a hierarchical Bayesian linear regression to find the best set of parameters
C
⋆
,
C
mol
, and
C
norm
that describe the star-forming plane conformed by Σ
⋆
, Σ
mol
, and Σ
SFR
, such that logΣ
SFR
=
C
⋆
logΣ
⋆
+
C
mol
logΣ
mol
+
C
norm
. We also explored variations in the determined parameters across galactic environments, focusing our analysis on the
C
⋆
and
C
mol
slopes.
Results.
We find signs of variations in the posterior distributions of
C
⋆
and
C
mol
across different galactic environments. The dependence of Σ
SFR
on Σ
⋆
spans a wide range of slopes, with negative and positive values, while the dependence of Σ
SFR
on Σ
mol
is always positive. Bars show the most negative value of
C
⋆
(−0.41), which is a sign of longer depletion times, while spiral arms show the highest
C
⋆
among all environments (0.45). Variations in
C
mol
also exist, although they are more subtle than those found for
C
⋆
.
Conclusions.
We conclude that systematic variations in the interplay of Σ
⋆
, Σ
mol
, and Σ
SFR
across different galactic environments exist at a spatial resolution of 150 pc, and we interpret these variations to be produced by an additional mechanism regulating the formation of stars that is not captured by either Σ
⋆
or Σ
mol
. Studying environmental variations in single galaxies, we find that these variations correlate with changes in the star formation efficiency across environments, which could be linked to the dynamical state of the gas that prevents it from collapsing and forming stars, or to changes in the molecular gas fraction.
Context.
Star formation and stellar feedback are interlinked processes that redistribute energy, turbulence, and material throughout galaxies. Because young and massive stars form in spatially ...clustered environments, they create pockets of expanding gas termed superbubbles, which retain information about the physical processes that drive them. As these processes play a critical role in shaping galaxy discs and regulating the baryon cycle, measuring the properties of superbubbles provides important input for galaxy evolution models.
Aims.
With the wide coverage and high angular resolution (∼50–150 pc) of the PHANGS–ALMA
12
CO (
J
= 2−1) survey, we can now resolve, identify and characterise a statistically representative number of superbubbles using molecular gas in nearby galaxies.
Methods.
We identify superbubbles by requiring spatial correspondence between shells in CO with stellar populations identified in PHANGS–HST. Then, by combining the properties of the stellar populations with the CO, we quantify the energetics of the stars and constrain feedback models. We visually find 325 cavities across 18 PHANGS–ALMA galaxies, 88 of which have clear superbubble signatures (unbroken shells, central clusters, kinematic signatures of expansion). We measure their radii and expansion velocities using CO (2–1) to dynamically derive their ages and the mechanical power driving the bubbles, which we use to compute the expected properties of the parent stellar populations driving the bubbles.
Results.
We find consistency between the predicted and derived stellar ages and masses of the stellar populations if we use a supernova (SN) model that injects energy with a coupling efficiency of ∼10%. Not only does this confirm that molecular gas accurately traces superbubble properties, but it also provides key observational constraints for superbubble models. We also find evidence that the bubbles are sweeping up gas as they expand, and speculate that these sites have the potential to host new generations of stars.
Conclusions.
This work demonstrates that molecular superbubbles provide novel quantitative constraints on SNe feedback efficiencies and gas clearing times, and represent a promising environment to search for the propagation of star formation, all of which are needed to understand what sets the observed star formation rates in galaxies.
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