The Survey of Lines in M31 (SLIM): The Drivers of the [CII]/TIR Variation

The ratio of the [CII] 158$\,\mu$m emission line over the total infrared emission (TIR) is often used as a proxy for the photoelectric (PE) heating efficiency ($\epsilon_{\rm PE}$) of the far-ultraviolet (FUV) photons absorbed by dust in the interstellar medium. In the nearby galaxy M31, we measure a strong radial variation of [CII]/TIR that we rule out as being due to an intrinsic variation in $\epsilon_{\rm PE}$. [CII]/TIR fails as a proxy for $\epsilon_{\rm PE}$, because the TIR measures all dust heating, not just the contribution from FUV photons capable of ejecting electrons from dust grains. Using extensive multiwavelength coverage from the FUV to far-infrared (FIR), we infer the attenuated FUV emission ($\rm UV_{att}$), and the total attenuated flux ($\rm TOT_{att}$). We find [CII]/TIR to be strongly correlated with $\rm UV_{att}$/$\rm TOT_{att}$, indicating that, in M31 at least, one of the dominant drivers for [CII]/TIR variation is the relative hardness of the absorbed stellar radiation field. We define $\rm{ \epsilon_{PE}^{UV}}$, [CII]/$\rm{ UV_{att}}$ which should be more closely related to the actual PE efficiency, which we find to be essentially constant ($1.85 \pm 0.8 \%$) in all explored fields in M31. This suggests that part of the observed variation of [CII]/TIR in other galaxies is likely due to a change in the relative hardness of the absorbed stellar radiation field, caused by a combination of variations in the stellar population, dust opacity and galaxy metallicity, although PE efficiency may also vary across a wider range of environments.Comment: 19 pages, 16 figures, accepted for publication in Ap

PHANGS CO kinematics: disk orientations and rotation curves at 150 pc resolution

We present kinematic orientations and high resolution (150 pc) rotation curves for 67 main sequence star-forming galaxies surveyed in CO (2-1) emission by PHANGS-ALMA. Our measurements are based on the application of a new fitting method tailored to CO velocity fields. Our approach identifies an optimal global orientation as a way to reduce the impact of non-axisymmetric (bar and spiral) features and the uneven spatial sampling characteristic of CO emission in the inner regions of nearby galaxies. The method performs especially well when applied to the large number of independent lines-of-sight contained in the PHANGS CO velocity fields mapped at 1'' resolution. The high resolution rotation curves fitted to these data are sensitive probes of mass distribution in the inner regions of these galaxies. We use the inner slope as well as the amplitude of our fitted rotation curves to demonstrate that CO is a reliable global dynamical mass tracer. From the consistency between photometric orientations from the literature and kinematic orientations determined with our method, we infer that the shapes of stellar disks in the mass range of log($\rm M_{\star}(M_{\odot})$)=9.0-10.9 probed by our sample are very close to circular and have uniform thickness.Comment: 19 figures, 36 pages, accepted for publication in ApJ. Table of PHANGS rotation curves available from http://phangs.org/dat

PHANGS-MUSE: Detection and Bayesian classification of ~40000 ionised nebulae in nearby spiral galaxies

In this work, we present a new catalogue of >40000 ionised nebulae distributed across the 19 galaxies observed by the PHANGS-MUSE survey. The nebulae have been classified using a new model-comparison-based algorithm that exploits the odds ratio principle to assign a probabilistic classification to each nebula in the sample. The resulting catalogue is the largest catalogue containing complete spectral and spatial information for a variety of ionised nebulae available so far in the literature. We developed this new algorithm to address some of the limitations of the traditional classification criteria, such as their binarity, the sharpness of the involved limits, and the limited amount of data they rely on for the classification. The analysis of the catalogue shows that the algorithm performs well when selecting H II regions. We can recover their luminosity function, and its properties are in line with what is available in the literature. We also identify a rather significant population of shock-ionised regions (mostly composed of supernova remnants), an order of magnitude larger than any other homogeneous catalogue of supernova remnants currently available in the literature. The number of supernova remnants we identify per galaxy is in line with results in our Galaxy and other very nearby sources. However, limitations in the source detection algorithm result in an incomplete sample of planetary nebulae, even though their classification seems robust. Finally, we demonstrate how applying a correction for the contribution of the diffuse ionised gas to the nebulae's spectra is essential to obtain a robust classification of the objects and how a correct measurement of the extinction using DIG-corrected line fluxes prompts the use of a higher theoretical Ha/Hb ratio (3.03) than what is commonly used when recovering the E(B-V) via the Balmer decrement technique in massive star-forming galaxies.Comment: 58 pages, 46 figures. Paper accepted for pubblications in A&A. The catalogue will be available via the CDS or at the following link: http://dx.doi.org/10.11570/23.000

PHANGS CO Kinematics: Disk Orientations and Rotation Curves at 150 pc Resolution

We present kinematic orientations and high-resolution (150 pc) rotation curves for 67 main-sequence star-forming galaxies surveyed in CO (2-1) emission by PHANGS-ALMA. Our measurements are based on the application of a new fitting method tailored to CO velocity fields. Our approach identifies an optimal global orientation as a way to reduce the impact of nonaxisymmetric (bar and spiral) features and the uneven spatial sampling characteristic of CO emission in the inner regions of nearby galaxies. The method performs especially well when applied to the large number of independent lines of sight contained in the PHANGS CO velocity fields mapped at 1'' resolution. The high-resolution rotation curves fitted to these data are sensitive probes of mass distribution in the inner regions of these galaxies. We use the inner slope as well as the amplitude of our fitted rotation curves to demonstrate that CO is a reliable global dynamical mass tracer. From the consistency between photometric orientations from the literature and kinematic orientations determined with our method, we infer that the shapes of stellar disks in the mass range of log(${M}_{\star }({M}_{\odot })$) = 9.0-10.9 probed by our sample are very close to circular and have uniform thickness.P.L., E.S., C.F., and D.L. acknowledge support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement No. 694343). E.R. acknowledges the support of the Natural Sciences and Engineering Research Council of Canada (NSERC), funding reference number RGPIN-2017- 03987. J.M.D.K. and M.C. gratefully acknowledge funding from the Deutsche Forschungsgemeinschaft (DFG) through an Emmy Noether Research Group (grant No. KR4801/1-1) and the DFG Sachbeihilfe (grant No. KR4801/2-1). J.M.D.K. gratefully acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program via the ERC Starting Grant MUSTANG (grant agreement No. 714907). S.C.O.G. acknowledges support from the Deutsche Forschungsgemeinschaft via SFB 881 “The Milky Way System” (Project-ID 138713538; subprojects B1, B2, and B8) and via Germany’s Excellence Strategy EXC 2181/1-390900948 (the Heidelberg STRUCTURES Excellence Cluster). C.H., A.H., and J.P. acknowledge support from the Programme National “Physique et Chimie du Milieu Interstellaire” (PCMI) of CNRS/INSU with INC/INP co-funded by CEA and CNES, and from the Programme National Cosmology and Galaxies (PNCG) of CNRS/INSU with INP and IN2P3, co-funded by CEA and CNES. J.P. and F.B. acknowledge funding from the European Union’s Horizon 2020 research and innovation program (grant agreement No. 726384

Measuring the mixing scale of the ISM within nearby spiral galaxies

The spatial distribution of metals reflects, and can be used to constrain, the processes of chemical enrichment and mixing. Using PHANGS-MUSE optical integral field spectroscopy, we measure the gas-phase oxygen abundances (metallicities) across 7138 H II regions in a sample of eight nearby disc galaxies. In Paper I, we measure and report linear radial gradients in the metallicities of each galaxy, and qualitatively searched for azimuthal abundance variations. Here, we examine the 2D variation in abundances once the radial gradient is subtracted, (O/H), in order to quantify the homogeneity of the metal distribution and to measure the mixing scale over which H II region metallicities are correlated. We observe low (0.03–0.05 dex) scatter in (O/H) globally in all galaxies, with significantly lower (0.02–0.03 dex) scatter on small (<600 pc) spatial scales. This is consistent with the measurement uncertainties, and implies the 2D metallicity distribution is highly correlated on scales of 600 pc. We compute the two-point correlation function for metals in the disc in order to quantify the scale lengths associated with the observed homogeneity. This mixing scale is observed to correlate better with the local gas velocity dispersion (of both cold and ionized gas) than with the star formation rate. Selecting only H II regions with enhanced abundances relative to a linear radial gradient, we do not observe increased homogeneity on small scales. This suggests that the observed homogeneity is driven by the mixing introducing material from large scales rather than by pollution from recent and on-going star formation.k. KK and FS gratefully acknowledges funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) in the form of an Emmy Noether Research Group (grant number KR4598/2-1, PI: Kreckel). SCOG and RSK acknowledge support from the DFG via SFB 881 ‘The Milky Way System’ (project-ID 138713538; subprojects B1, B2, and B8) and from the Heidelberg cluster of excellence EXC 2181-390900948 ‘STRUCTURES: A unifying approach to emergent phenomena in the physical world, mathematics, and complex data’, funded by the German Excellence Strategy. RSK furthermore thanks for funding from the European Research Council via the ERC Synergy Grant ECOGAL (grant 855130). ER acknowledges the support of the Natural Sciences and Engineering Research Council of Canada (NSERC), funding reference number RGPIN-2017-03987. FB acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 726384/Empire). JMDK and MC gratefully acknowledge funding from the DFG through an Emmy Noether Research Group (grant number KR4801/1-1). JMDK, MC, and JJK gratefully acknowledge funding from the DFG through the DFG Sachbeihilfe (grant number KR4801/2-1). JMDK gratefully acknowledges funding from the ERC under the European Union’s Horizon 2020 research and innovation programme via the ERC Starting Grant MUSTANG (grant agreement number 714907). EW acknowledges support from the DFG via SFB 881 ‘The Milky Way System’ (project-ID 138713538; subproject P2). TGW acknowledges funding from the ERC under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 694343

Quantifying the energy balance between the turbulent ionised gas and young stars

We investigate the ionised gas morphology, excitation properties, and kinematics in 19 nearby star-forming galaxies from the PHANGS-MUSE survey. We directly compare the kinetic energy of expanding superbubbles and the turbulent motions in the interstellar medium with the mechanical energy deposited by massive stars in the form of winds and supernovae, with the aim to answer whether the stellar feedback is responsible for the observed turbulent motions and to quantify the fraction of mechanical energy retained in the superbubbles. Based on the distribution of the flux and velocity dispersion in the H$\alpha$ line, we select 1484 regions of locally elevated velocity dispersion ($\sigma$(H$\alpha$)>45 km/s), including at least 171 expanding superbubbles. We analyse these regions and relate their properties to those of the young stellar associations and star clusters identified in PHANGS-HST data. We find a good correlation between the kinetic energy of the ionised gas and the total mechanical energy input from supernovae and stellar winds from the stellar associations, with a typical efficiency of 10-20%. The contribution of mechanical energy by the supernovae alone is not sufficient to explain the measured kinetic energy of the ionised gas, which implies that pre-supernova feedback in the form of radiation/thermal pressure and winds is necessary. We find that the gas kinetic energy decreases with metallicity for our sample covering Z=0.5-1.0 Zsun, reflecting the lower impact of stellar feedback. For the sample of superbubbles, we find that about 40% of the young stellar associations are preferentially located in their rims. We also find a slightly higher (by ~15%) fraction of the youngest (1-2.5 Myr) stellar associations in the rims of the superbubbles than in the centres, and the opposite for older associations, which implies possible propagation or triggering of star formation.Comment: 31 pages (including 5 pages in appendix), 19 figures, the abstract is abridged; submitted to A&A (in mid May; awaiting report

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$\rm \mu m$, and the emission in the PAH-tracing bands at 3.3, 7.7, 10, and 11.3$\rm \mu m$ as tracers of obscured star formation. We analyse $\sim$ 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$\alpha$ flux as the sum of the observed H$\alpha$ emission and a term proportional to the IR emission, with $a_{IR}$ as the proportionality coefficient. A constant $a_{IR}$ leads to extinction-corrected H$\alpha$ estimates which agree with those obtained with the Balmer decrement with a scatter of $\sim$ 0.1 dex for all bands considered. Among these bands, 21$\rm \mu m$ 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$\rm \mu m$ 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&

Mapping electron temperature variations across a spiral arm in ngc 1672

We report one of the first extragalactic observations of electron temperature variations across a spiral arm. Using Multi Unit Spectroscopic Explorer mosaic observations of the nearby galaxy NGC 1672, we measure the [N ii]λ5755 auroral line in a sample of 80 H ii regions in the eastern spiral arm of NGC 1672. We discover systematic temperature variations as a function of distance perpendicular to the spiral arm. The electron temperature is lowest on the spiral arm itself and highest on the downstream side. Photoionization models of different metallicity, pressure, and age of the ionizing source are explored to understand what properties of the interstellar medium drive the observed temperature variations. An azimuthally varying metallicity appears to be the most likely cause of the temperature variations. The electron temperature measurements solidify recent discoveries of azimuthal variations of oxygen abundance based on strong lines, and rule out the possibility that the abundance variations are artifacts of the strong-line calibrations.This work is based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 1100.B-0651. J.M.D.K. gratefully acknowledges funding from the German Research Foundation (DFG) in the form of an Emmy Noether Research Group (grant No. KR4801/1-1) and the DFG Sachbeihilfe (grant No. KR4801/2-1). J.M.D.K. 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 No. 714907). F.B. acknowledges funding from the European Union Horizon 2020 research and innovation programme (grant agreement No. 726384). K.K. gratefully acknowledges funding from the German Research Foundation (DFG) in the form of an Emmy Noether Research Group (grant number KR4598/2-1, PI: Kreckel)

Measuring the mixing scale of the ISM within nearby spiral galaxies

The spatial distribution of metals reflects, and can be used to constrain, the processes of chemical enrichment and mixing. Using PHANGS-MUSE optical integral field spectroscopy, we measure the gas phase oxygen abundances (metallicities) across 7,138 HII regions in a sample of eight nearby disc galaxies. In Paper I (Kreckel et al. 2019) we measure and report linear radial gradients in the metallicities of each galaxy, and qualitatively searched for azimuthal abundance variations. Here, we examine the two-dimensional variation in abundances once the radial gradient is subtracted, Delta(O/H), in order to quantify the homogeneity of the metal distribution and to measure the mixing scale over which HII region metallicities are correlated. We observe low (0.03--0.05 dex) scatter in Delta(O/H) globally in all galaxies, with significantly lower (0.02--0.03 dex) scatter on small (<600 pc) spatial scales. This is consistent with the measurement uncertainties, and implies the two-dimensional metallicity distribution is highly correlated on scales of <600 pc. We compute the two point correlation function for metals in the disc in order to quantify the scale lengths associated with the observed homogeneity. This mixing scale is observed to correlate better with the local gas velocity dispersion (of both cold and ionized gas) than with the star formation rate. Selecting only HII regions with enhanced abundances relative to a linear radial gradient, we do not observe increased homogeneity on small scales. This suggests that the observed homogeneity is driven by the mixing introducing material from large scales rather than by pollution from recent and on-going star formation.Comment: 17 pages, 14 figures. Accepted for publication in MNRA

The lifecycle of molecular clouds in nearby star-forming disc galaxies

It remains a major challenge to derive a theory of cloud-scale (⁠≲100 pc) star formation and feedback, describing how galaxies convert gas into stars as a function of the galactic environment. Progress has been hampered by a lack of robust empirical constraints on the giant molecular cloud (GMC) lifecycle. We address this problem by systematically applying a new statistical method for measuring the evolutionary timeline of the GMC lifecycle, star formation, and feedback to a sample of nine nearby disc galaxies, observed as part of the PHANGS-ALMA survey. We measure the spatially resolved (∼100 pc) CO-to-H α flux ratio and find a universal de-correlation between molecular gas and young stars on GMC scales, allowing us to quantify the underlying evolutionary timeline. GMC lifetimes are short, typically 10−30 Myr⁠, and exhibit environmental variation, between and within galaxies. At kpc-scale molecular gas surface densities Σ_(H₂) ≥ 8 M_⊙ pc⁻²⁠, the GMC lifetime correlates with time-scales for galactic dynamical processes, whereas at Σ_(H₂) ≤ 8 M_⊙ pc⁻² GMCs decouple from galactic dynamics and live for an internal dynamical time-scale. After a long inert phase without massive star formation traced by H α (75–90 per cent of the cloud lifetime), GMCs disperse within just 1−5 Myr once massive stars emerge. The dispersal is most likely due to early stellar feedback, causing GMCs to achieve integrated star formation efficiencies of 4–10 per cent. These results show that galactic star formation is governed by cloud-scale, environmentally dependent, dynamical processes driving rapid evolutionary cycling. GMCs and H II regions are the fundamental units undergoing these lifecycles, with mean separations of 100−300 pc in star-forming discs. Future work should characterize the multiscale physics and mass flows driving these lifecycles