arXiv subjects

A. T. Barnes

Publications and source records attributed to A. T. Barnes.

At least 19 recordsLinked to original sources

arXiv2026

Mapping CO Ice in a Star-Forming Filament in the 3 kpc Arm with JWST

CO gas emission is a fundamental tool for measuring column density, but in cold, dark clouds, much of the CO is locked away in ice. We present JWST results from observations of a star forming filament (G0.342+0.024) that that appears to be associated with the 3 kpc arm. This filament is backlit by the Galactic Center, which has allowed us to construct a high-resolution extinction map (mean separation between stars of ~1" outside the filament, ~2" in the filament). ALMA Band 3 data reveals embedded star formation within the cloud. Using the CO ice feature covered by the F466N band, we map the CO ice column density of the filament. By combining the extinction map, CO ice column density map, and archival CO observations, we examine the efficacy of standard CO X-factor measurements of mass in star forming gas. We find that 50-88% of the CO is locked away in ice at large column densities ($N_{\rm \rm H_2} \gtrsim 10^{22} \rm ~cm^{-2}, 200 \rm ~M_{\odot} \rm ~pc^{-2}$) in the filament. The primary sources of uncertainty in this estimate are due to uncertainty in the ice composition and lab measurements of ice opacities. This shows that systematic corrections are needed for mass measurements in the Milky Way and nearby galaxies at high column densities.

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arXiv2026

The Colors of Ices: Measuring ice column density through photometry

Ices imprint strong absorption features in the near- and mid-infrared, but until recently they have been studied almost exclusively with spectroscopy toward small samples of bright sources. We show that JWST photometry alone can reveal and quantify interstellar ices, and we present a new open-source modeling tool, icemodels, to produce synthetic photometry of ices based on laboratory measurements. We provide reference tables indicating which filters are likely to be observably affected by ice absorption. Applying these models to NIRCam data of background stars behind several Galactic Center (GC) clouds (dust ridge clouds A [the Brick], C, and D), and validating against NIRSpec spectra of Galactic disk sources, we find clear signatures of CO, H$_2$O, and CO$_2$ ices and evidence for excess absorption in the F356W filter likely caused by CH-bearing species such as methanol. The ice ratios differ between the Galactic disk and Center, with GC clouds showing a higher H$_2$O fraction. A large ice abundance is observed in CO, H2O, and possibly complex molecules, which implies that there is substantial freezeout and therefore potential for ice-phase chemistry in non-star-forming gas. Accounting for all likely ices, we infer that $>25%$ of the total carbon is frozen into CO ice in the GC, which exceeds the entire solar-neighborhood carbon budget. By assuming the freezeout fraction is the same in GC and disk clouds, we obtain a metallicity measurement indicating that $Z_{GC}\gtrsim2.5Z_\odot$. These results demonstrate that photometric ice measurements are feasible with JWST and capable of probing the metallicity structure of the cold interstellar medium.

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arXiv2026

Uncovering the multi-scale structure of dust distribution in nearby galaxies

High-resolution JWST-MIRI images now allow us to resolve in great detail the multi-scale nature of the emission in nearby star-forming galaxies, from compact star-forming regions to large-scale diffuse emission, giving new insights into dust emission, its composition, and the surrounding interstellar medium (ISM). We aim to understand at which scale the different processes driving dust emission in mid-infrared (7.7-21 um) wavelengths take place and if we can disentangle dense regions' emission from emission linked to a more diffuse component. We use and enhance the constrained diffusion decomposition (CDD) algorithm, an alternative to the wavelet transform decomposition, to disentangle the emission coming from compact regions from the emission originating from diffuse sources. This allows us to cleanly quantify the mid-IR spectral properties of the ISM at intervals within a continuum of physical scales. We find a transition scale of PAH emission around 300 pc, with weaker PAH fraction at smaller scales, highlighting the destruction of PAHs in HII regions. We also show variations in the PAH fraction in different morphological environments, with a smaller fraction in bright and star-forming environments. Studying and comparing the probability distribution functions (PDFs) of HII regions and diffuse ISM with the PDFs at different scales, we find a similar separation scale around 200 pc at which we observe a transition from a power-law PDF for dense structures to a log-normal one for the diffuse ISM.

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arXiv2026

Projection effects in star-forming regions: I. Nearest-neighbour statistics and observational biases

Stars form as molecular clouds fragment into networks of dense cores, filaments, and subclusters. The characteristic spacing of these cores is a key observable imprint of fragmentation physics and is commonly measured using nearest-neighbour (NN) statistics. However, NN separations are derived from projected two-dimensional (2D) positions, while fragmentation occurs in three dimensions (3D). Using spherical and fractal toy models, we show that the standard geometric deprojection factor of $4/π\simeq1.27$ is inadequate because projection not only foreshortens separations but also rewires the NN network, while finite angular resolution merges close neighbours and inflates apparent spacings. We quantify these competing biases with Monte Carlo experiments spanning a wide range of morphologies, sample sizes, and effective resolutions. From these we derive an empirical correction factor that depends on both sample size and resolution: for small ($N\lesssim10$) or poorly resolved samples ($\lesssim$10 resolution elements across the field), intrinsic NN spacings exceed projected values by only 20 to 40%, whereas for well-sampled ($N\gtrsim100$), well-resolved data ($\gtrsim$30-50 resolution elements), true 3D separations are typically larger by a factor of $\sim$2. This calibration enables observers to convert measured 2D NN spacings into corresponding 3D estimates, with typical morphology-driven uncertainties of order 30 to 40%, and we demonstrate how it alters inferred fragmentation scales in observed and simulated core populations. [abridged]

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arXiv2026

PHANGS-JWST: the largest extragalactic molecular cloud catalog traced by polycyclic aromatic hydrocarbon emission

High-resolution JWST images of nearby spiral galaxies reveal polycyclic aromatic hydrocarbon (PAH) emission structures that trace molecular gas, including CO-dark regions. We identify ISM cloud structures in PHANGS-JWST 7.7 $μ$m PAH maps for 66 galaxies, smoothed to 30 pc and at native resolution, extracting 108,466 and 146,040 clouds, respectively. Molecular properties were inferred using a linear conversion from PAH to CO. Given the tendency for clouds in galaxy centers to overlap in velocity space, we opted to flag these and omit them from the analysis in this work. The remaining clouds correspond to giant molecular clouds, such as those detected in CO(2-1) emission by ALMA, or lower surface density clouds that either fall below the ALMA detection limits of existing maps or genuinely have no molecular counterpart. Cross-matching with ALMA CO maps at 90 pc in 27 galaxies shows that 41 % of PAH clouds have CO associations. The converted molecular properties vary little across environments, but the most massive clouds are preferentially found in spiral arms. Fitting lognormal mass distributions down to $2\times10^{3} M_{\odot}$ shows that spiral arms host the highest-mass clouds, consistent with enhanced formation in arm gravitational potentials. Cloud molecular surface densities decline by a factor of $\sim 1.5-2$ toward $2 - 3 R_{e}$. However, the trend largely varies in individual galaxies, with flat, decreasing, and even no trend as a function of galactocentric radius. Factors like large-scale processes and morphologies might influence the observed trends. We publish two catalogs online, one at the common resolution of 30 pc and another at the native resolution. We expect them to have broad utility for future PAH clouds, molecular clouds, and star formation studies.

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arXiv2025

Chemical complexity in star formation induced by stellar feedback: cores shock-formed by the supernova remnant W44

Low-velocity shocks from Supernova Remnants (SNRs) may set the physical and chemical conditions of star formation in molecular clouds. Recent evidence suggests that the Sun might have formed through this process. However, the chemical conditions of shock-induced star forming region remain poorly constrained. We study the chemical complexity of a shock-impacted clump, with potential to yield star formation, named the Clump, and located at the interface between the SNR W44 and the infrared dark cloud G034.77-00.55. We test whether the Clump has chemical properties consistent with those observed in star forming regions unaffected by SNRs. We use high-sensitivity, broad spectral surveys at 3 and 7 mm obtained with the 30m antenna at IIRAM and the 40 m YEBES antenna, to identify D-bearing species and complex organic molecules (COMs) toward the Clump. For all species, we estimate molecular abundances and compare them with those observed across star forming regions at different evolutionary stages and masses, as well as comets. We detect multiple deuterated molecules (DCO+, DNC, DCN, CH2DOH) and COMs (CH3OH, CH3CHO, CH3CCH, CH3CN, CH3SH) with excitation temperatures of 5-13 K. To the best of our knowledge, this is the first detection of COMs toward a site of SNR-cloud interaction. The derived D/H ratios (0.01-0.04) and COM abundances are consistent with those reported toward typical low-mass starless cores and comparable to cometary values. The overall level of chemical complexity is relatively low, in line with an early evolutionary stage. We suggest that the Clump is a early stage shock-induced low-mass star forming region, not yet protostellar. We speculate that SNR shocks may set the physical and chemical conditions to form stars. The resulting chemical budget may be preserved along the formation process of a planetary system, being finally incorporated into planetesimals and cometesimals.

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arXiv2025

The PHANGS-MUSE/HST-Halpha Nebulae Catalogue

We present the PHANGS-MUSE/HST-Halpha nebulae catalogue, comprising 5177 spatially resolved nebulae across 19 nearby star-forming galaxies (< 20 Mpc), based on high-resolution Halpha imaging from HST, homogenised to a fixed 10 pc resolution and sensitivity. Combined with MUSE spectroscopy, this enables robust classification of 4882 H II regions and separation of planetary nebulae and supernova remnants. Electron densities for 2544 H II regions are derived using [S II] diagnostics, and nebular sizes measured via circularised radii and second moments yield a median of 20 pc, extending to sub-parsec scales. A structural complexity score traces substructure, showing that about a third of regions are H II complexes, with a higher fraction in galaxy centres. A luminosity-size relation calibrated from the HST sample is applied to 30,790 MUSE nebulae, recovering sizes down to 1 pc. Observed sizes exceed classical Stromgren radii, implying typical volume filling factors of 0.22. We associate 3349 H II regions with stellar populations from PHANGS-HST, finding median ages of 3 Myr and masses of 4-5 log(Msun). The dataset provides a detailed, spatially resolved link between nebular structure and ionising sources, serving as a benchmark for future studies of feedback, diffuse ionised gas, and star formation regulation in the interstellar medium. The full catalogue is made publicly available in machine-readable format.

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arXiv2025

CO Depletion in Infrared Dark Clouds

Infrared Dark Clouds (IRDCs) are cold, dense structures representative of the initial conditions of star formation. Many studies of IRDCs employ CO to investigate cloud dynamics. However, CO can be highly depleted from the gas phase in IRDCs, impacting its fidelity as tracer. CO depletion is also of great interest in astrochemistry, since CO ice in dust grain mantles provides the raw material for forming complex organic molecules. We study CO depletion toward four IRDCs to investigate how it correlates with volume density and dust temperature, calculated from Herschel images. We use 13CO(1-0) and (2-1) maps to measure CO depletion factor, $f_D$, across IRDCs G23.46-00.53, G24.49-00.70, G24.94-00.15, and G25.16-00.28. We also consider a normalized CO depletion factor, f_D', which takes a value of unity, i.e., no depletion, in the outer, lower density, warmer regions. We then investigate the dependence of f_D and f_D' on gas density, $n_H$ and dust temperature, $T_{dust}$. We find CO depletion rises as density increases, reaching maximum values of f_D'$\sim$10 in regions with $n_H>3\times10^5\:{cm}^{-3}$, although with significant scatter at a given density. We find a tighter, less scattered relation of f_D' with temperature, rising rapidly for temperatures <18 K. We propose a functional form $f_D^\prime = \:{exp}(T_0/[T_{dust}-T_1])$ with $T_0\simeq4\:$K and $T_1\simeq12\:$K to reproduce this behaviour. We conclude that CO is heavily depleted from the gas phase in cold, dense regions of IRDCs. Thus CO depletion can lead to underestimation of total cloud masses based on CO line fluxes by factors up to 5. These results indicate a dominant role for thermal desorption in setting near equilibrium abundances of gas phase CO in IRDCs, providing important constraints for both astrochemical models and the chemodynamical history of gas during the early stages of star formation.

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arXiv2025

Low-velocity large-scale shocks in the infrared dark cloud G035.39-00.33: bubble-driven cloud-cloud collisions

Low-velocity large-scale shocks impacting on the ISM may efficiently shape molecular clouds and trigger star formation within them. These shocks, both driven by galactic bubbles and/or cloud-cloud collisions, leave specific signatures in the gas morphology and kinematics. Observational studies of such signatures are crucial to investigate if and how shocks affect the clouds formation process and trigger their future star formation. We have analysed the shocked and dense gas tracers SiO(2-1) and H13CO+(1-0) emission toward the IRDC G035.39-00.33, using new, larger-scale maps obtained with the 30m telescope at the Instituto de Radioastronomìa Millimétrica. We find that the dense gas is organised into a northern and a southern filament having different velocities and tilted orientation with respect to each other. The two filaments are spatially separated yet connected by a faint bridge feature also seen in a position-velocity diagram extracted across the cloud. This bridge-feature, typical of cloud-cloud collisions, also coincides with a very spectrally narrow SiO-traced emission. The northern filament is suggested to be interacting with the nearby supernova remnant G035.6-0.4. Toward the southern filament, we also report the presence of a parsec-scale, spectrally narrow SiO emission likely driven by the interaction between this filament and a nearby expanding shell. The shell is visible in the 1.3 GHz and 610 MHz continuum images and our preliminary analysis suggests it may be the relic of a supernova remnant. We conclude that the two filaments represent the densest part of two colliding clouds, pushed toward each other by nearby Supernova Remnants. We speculate that this cloud-cloud collision driven by stellar feedback may have assembled the infrared dark cloud. We also evaluate the possibility that star formation may have been triggered within G035.39-00.33 by the collision.

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arXiv2025

The Galactic-Centre Arms inferred from ACES (ALMA CMZ Exploration Survey)

Analyzing longitude-velocity diagrams (LVDs) in the CS(J=2-1) and H13CN(J=1-0) molecular lines from the internal release data of the ALMA Central-Molecular-Zone Exploration Survey (ACES) and in the 13CO (J=1-0) line from the Nobeyama Galactic-Centre (GC) survey, we identify six GC Arms as prominent straight LV ridges. In addition to the currently known Arms I to IV, we identify a new inner arm, Arm V, and further highlight the circum-nuclear disc (CND) as Arm VI. Integrated intensity maps of the Arms on the sky suggest that most of the Arms compose ring-like structures inclined from the Galactic plane. We determine the radii (curvatures) of the Arms using the velocity-gradient ($dv/dl$) method, assuming that the arms are rotating on circular orbits at a constant velocity of $\sim 150$ km/s. We show that Arms I and II compose the main ring structure of the CMZ with radii $\sim 100$--120 pc; Arm III is a dense arm 42 pc from the GC; Arm IV is a clear and narrow arm 20 pc from the GC; and Arm V is a faint, long arm of 8.2 pc radius. We show that the circum-nuclear disc (CND) composes the sixth arm, Arm VI, of radius $\sim 2.3$ pc associated with bifurcated spiral fins. We also discuss the association of the 20- and 50-km/s clouds with these Arms. The radii of the arms fall on an empirical relation $R\sim 630 (2/5)^N$ for $N=1$ (Arm I) to 6 (VI), suggesting either discrete rings or a logarithmic spiral with pitch angle $\sim 22^\circ$. The vertical full extent of the arm increases with radius and is represented by $z\sim 0.7 (R/1 {\rm pc})^{0.7}$ pc. The tilt angle of the arms from the Galactic plane, or the warping, increases rapidly toward the GC.

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arXiv2025

Full disc [CII] mapping of nearby star-forming galaxies: SOFIA FIFI/LS observations of NGC 3627, NGC 4321, and NGC 6946

As a major cooling line of interstellar gas, the far-infrared 158 μm line from singly ionised carbon [CII] is an important tracer of various components of the interstellar medium in galaxies across all spatial and morphological scales. Yet, there is still not a strong constraint on the origins of [CII] emission. In this work, we derive the resolved [CII] star formation rate relation and aim to unravel the complexity of the origin of [CII]. We used the Field-Imaging Far-Infrared Line Spectrometer on board the Stratospheric Observatory for Infrared Astronomy to map [CII] in three nearby star-forming galaxies at sub-kiloparsec scales, namely, NGC 3627, NGC 4321, and NGC 6946, and we compared these [CII] observations to the galactic properties derived from complementary data from the literature. We find that the relationship between the [CII] fine structure line and star formation rate shows variations between the galaxies as well as between different environments within each galaxy. Our results show that the use of [CII] as a tracer for star formation is much more tangled than has previously been suggested within the extragalactic literature, which typically focuses on small regions of galaxies and/or uses large-aperture sampling of many different physical environments. As found within resolved observations of the Milky Way, the picture obtained from [CII] observations is complicated by its local interstellar medium conditions. Future studies will require a larger sample and additional observational tracers, obtained on spatial scales within galaxies, in order to accurately disentangle the origin of [CII] and calibrate its use as a star formation tracer.

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arXiv2024

Tracing the earliest stages of star and cluster formation in 19 nearby galaxies with PHANGS-JWST and HST: compact 3.3 $μ$m PAH emitters and their relation to the optical census of star clusters

The earliest stages of star and cluster formation are hidden within dense cocoons of gas and dust, limiting their detection at optical wavelengths. With the unprecedented infrared capabilities of JWST, we can now observe dust-enshrouded star formation with $\sim$10 pc resolution out to $\sim$20 Mpc. Early findings from PHANGS-JWST suggest that 3.3 $μ$m polycyclic aromatic hydrocarbon (PAH) emission can identify star clusters in their dust-embedded phases. Here, we extend this analysis to 19 galaxies from the PHANGS-JWST Cycle 1 Treasury Survey, providing the first characterization of compact sources exhibiting 3.3$μ$m PAH emission across a diverse sample of nearby star-forming galaxies. We establish selection criteria, a median color threshold of F300M-F335M=0.67 at F335M=20, and identify of 1816 sources. These sources are predominantly located in dust lanes, spiral arms, rings, and galaxy centers, with $\sim$87% showing concentration indices similar to optically detected star clusters. Comparison with the PHANGS-HST catalogs suggests that PAH emission fades within $\sim$3 Myr. The H$α$ equivalent width of PAH emitters is 1-2.8 times higher than that of young PHANGS-HST clusters, providing evidence that PAH emitters are on average younger. Analysis of the bright portions of luminosity functions (which should not suffer from incompleteness) shows that young dusty clusters may increase the number of optically visible $\leq$ 3 Myr-old clusters in PHANGS-HST by a factor between $\sim$1.8x-8.5x.

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arXiv2024

Interaction between the Supernova Remnant W44 and the Infrared Dark Cloud G034.77-00.55: shock induced star formation?

How Supernova Remnant (SNR) shocks impact nearby molecular clouds is still poorly observationally constrained. It is unclear if SNRs can positively or negatively affect clouds star formation potential. We have studied the dense gas morphology and kinematics toward the Infrared Dark Cloud (IRDC) G034.77-00.55, shock-interacting with the SNR W44, to identify evidence of early stage star formation induced by the shock. We have used high-angular resolution N2H+(1-0) images across G034.77-00.55, obtained with ALMA. N2H+ is a well known tracer of dense and cold material, optimal to identify gas with the highest potential to harbour star formation. The N2H+ emission is distributed into two elongated structures, one toward the dense ridge at the edge of the source and one toward the inner cloud. Both elongations are spatially associated with well-defined mass-surface density features. The velocities of the gas in the two structures i.e., 38-41 km s-1 and 41-43 km s-1 are consistent with the lowest velocities of the J- and C-type parts of the SNR-driven shock, respectively. A third velocity component is present at 43-45.5 km s-1. The dense gas shows a fragmented morphology with core-like fragments of scales consistent with the Jeans lengths, masses $\sim$1-20 M$_{\odot}$, densities (n(H$_2$)$\geq$10$^5$ cm$^{-3}$) sufficient to host star formation in free-fall time scales (few 10$^4$ yr) and with virial parameters that hint toward possible collapse. The W44 driven shock may have swept up the encountered material which is now seen as a dense ridge, almost detached from the main cloud, and an elongation within the inner cloud, well constrained in both N2H+ emission and mass surface density. This shock compressed material may have then fragmented into cores that are either in a starless or pre-stellar stage. Additional observations are needed to confirm this scenario and the nature of the cores.

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arXiv2024

Disruption of a massive molecular cloud by a supernova in the Galactic Centre: Initial results from the ACES project

The Milky Way's Central Molecular Zone (CMZ) differs dramatically from our local solar neighbourhood, both in the extreme interstellar medium conditions it exhibits (e.g. high gas, stellar, and feedback density) and in the strong dynamics at play (e.g. due to shear and gas influx along the bar). Consequently, it is likely that there are large-scale physical structures within the CMZ that cannot form elsewhere in the Milky Way. In this paper, we present new results from the Atacama Large Millimeter/submillimeter Array (ALMA) large programme ACES (ALMA CMZ Exploration Survey) and conduct a multi-wavelength and kinematic analysis to determine the origin of the M0.8$-$0.2 ring, a molecular cloud with a distinct ring-like morphology. We estimate the projected inner and outer radii of the M0.8$-$0.2 ring to be 79" and 154", respectively (3.1 pc and 6.1 pc at an assumed Galactic Centre distance of 8.2 kpc) and calculate a mean gas density $> 10^{4}$ cm$^{-3}$, a mass of $\sim$ $10^6$ M$_\odot$, and an expansion speed of $\sim$ 20 km s$^{-1}$, resulting in a high estimated kinetic energy ($> 10^{51}$ erg) and momentum ($> 10^7$ M$_\odot$ km s$^{-1}$). We discuss several possible causes for the existence and expansion of the structure, including stellar feedback and large-scale dynamics. We propose that the most likely cause of the M0.8$-$0.2 ring is a single high-energy hypernova explosion. To viably explain the observed morphology and kinematics, such an explosion would need to have taken place inside a dense, very massive molecular cloud, the remnants of which we now see as the M0.8$-$0.2 ring. In this case, the structure provides an extreme example of how supernovae can affect molecular clouds.

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arXiv2024

Magnetic field morphology and evolution in the Central Molecular Zone and its effect on gas dynamics

The interstellar medium in the Milky Way's Central Molecular Zone (CMZ) is known to be strongly magnetised, but its large-scale morphology and impact on the gas dynamics are not well understood. We explore the impact and properties of magnetic fields in the CMZ using three-dimensional non-self gravitating magnetohydrodynamical simulations of gas flow in an external Milky Way barred potential. We find that: (1) The magnetic field is conveniently decomposed into a regular time-averaged component and an irregular turbulent component. The regular component aligns well with the velocity vectors of the gas everywhere, including within the bar lanes. (2) The field geometry transitions from parallel to the Galactic plane near $z=0$ to poloidal away from the plane. (3) The magneto-rotational instability (MRI) causes an in-plane inflow of matter from the CMZ gas ring towards the central few parsecs of $0.01-0.1$ M$_\odot$ yr$^{-1}$ that is absent in the unmagnetised simulations. However, the magnetic fields have no significant effect on the larger-scale bar-driven inflow that brings the gas from the Galactic disc into the CMZ. (4) A combination of bar inflow and MRI-driven turbulence can sustain a turbulent vertical velocity dispersion of $σ_z \simeq 5$ km s$^{-1}$ on scales of $20$ pc in the CMZ ring. The MRI alone sustains a velocity dispersion of $σ_z \simeq 3$ km s$^{-1}$. Both these numbers are lower than the observed velocity dispersion of gas in the CMZ, suggesting that other processes such as stellar feedback are necessary to explain the observations. (5) Dynamo action driven by differential rotation and the MRI amplifies the magnetic fields in the CMZ ring until they saturate at a value that scales with the average local density as $B \simeq 102 (n/10^3 cm^{-3})^{0.33}$ $μ$G. Finally, we discuss the implications of our results within the observational context in the CMZ.

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arXiv2023

Resolved low-J $^{12}$CO excitation at 190 parsec resolution across NGC 2903 and NGC 3627

The low-$J$ rotational transitions of $^{12}$CO are commonly used to trace the distribution of molecular gas in galaxies. Their ratios are sensitive to excitation and physical conditions in the molecular gas. Spatially resolved studies of CO ratios are still sparse and affected by flux calibration uncertainties, especially since most do not have high angular resolution or do not have short-spacing information and hence miss any diffuse emission. We compare the low-$J$ CO ratios across the disk of two massive, star-forming spiral galaxies NGC2903 and NGC3627 to investigate whether and how local environments drive excitation variations at GMC scales. We use Atacama Large Millimeter Array (ALMA) observations of the three lowest-$J$ CO transitions at a common angular resolution of 4$''$ (190pc). We measure median line ratios of $R_{21}=0.67^{+0.13}_{-0.11}$, $R_{32}=0.33^{+0.09}_{-0.08}$, and $R_{31}=0.24^{+0.10}_{-0.09}$ across the full disk of NGC3627. We see clear CO line ratio variation across the galaxy consistent with changes in temperature and density of the molecular gas. In particular, toward the center, $R_{21}$, $R_{32}$, and $R_{31}$ increase by 35\%, 50\%, and 66\%, respectively compared to their average disk values. The overall line ratio trends suggest that CO(3-2) is more sensitive to changes in the excitation conditions than the two lower-$J$ transitions. Furthermore, we find a similar radial $R_{32}$ trend in NGC2903, albite a larger disk-wide average of $\langle R_{32}\rangle=0.47^{+0.14}_{-0.08}$. We conclude that the CO low-$J$ line ratios vary across environments in such a way that they can trace changes in the molecular gas conditions, with the main driver being changes in temperature.

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arXiv2023

A sensitive, high-resolution, wide-field IRAM NOEMA CO(1-0) survey of the very nearby spiral galaxy IC 342

We present a new wide-field 10.75 x 10.75 arcmin^2 (~11x11 kpc^2), high-resolution (theta = 3.6" ~ 60 pc) NOEMA CO(1-0) survey of the very nearby (d=3.45 Mpc) spiral galaxy IC 342. The survey spans out to about 1.5 effective radii and covers most of the region where molecular gas dominates the cold interstellar medium. We resolved the CO emission into >600 individual giant molecular clouds and associations. We assessed their properties and found that overall the clouds show approximate virial balance, with typical virial parameters of alpha_vir=1-2. The typical surface density and line width of molecular gas increase from the inter-arm region to the arm and bar region, and they reach their highest values in the inner kiloparsec of the galaxy (median Sigma_mol~80, 140, 160, and 1100 M_sun/pc^2, sigma_CO~6.6, 7.6, 9.7, and 18.4 km/s for inter-arm, arm, bar, and center clouds, respectively). Clouds in the central part of the galaxy show an enhanced line width relative to their surface densities and evidence of additional sources of dynamical broadening. All of these results agree well with studies of clouds in more distant galaxies at a similar physical resolution. Leveraging our measurements to estimate the density and gravitational free-fall time at 90 pc resolution, averaged on 1.5 kpc hexagonal apertures, we estimate a typical star formation efficiency per free-fall time of 0.45% with a 16-84% variation of 0.33-0.71% among such 1.5 kpc regions. We speculate that bar-driven gas inflow could explain the large gas concentration in the central kiloparsec and the buildup of the massive nuclear star cluster. This wide-area CO map of the closest face-on massive spiral galaxy demonstrates the current mapping power of NOEMA and has many potential applications. The data and products are publicly available.

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arXiv2023

A constant N$_2$H$^+$(1-0)-to-HCN(1-0) ratio on kiloparsec scales

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, NGC6946. 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 $\sim8\mathrm{kpc}$ 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 ($f_\mathrm{dense}$). We measure an average intensity ratio of N$_2$H$^+$(1-0)/HCN(1-0)$=0.15\pm0.02$ 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 $f_\mathrm{dense}$ and the efficiency of dense gas (SFE$_\mathrm{dense}$) 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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