Molecular Interstellar Medium Research Articles

GOALS-JWST: Mid-infrared Molecular Gas Excitation Probes the Local Conditions of Nuclear Star Clusters and the Active Galactic Nucleus in the LIRG VV 114

The enormous increase in mid-IR sensitivity and spatial and spectral resolution provided by the JWST spectrographs enables, for the first time, detailed extragalactic studies of molecular vibrational bands. This opens an entirely new window for the study of the molecular interstellar medium in luminous infrared galaxies (LIRGs). We present a detailed analysis of rovibrational bands of gas-phase CO, H2O, C2H2, and HCN toward the heavily obscured eastern nucleus of the LIRG VV 114, as observed by NIRSpec and the medium resolution spectrograph on the Mid-InfraRed Instrument (MIRI MRS). Spectra extracted from apertures of 130 pc in radius show a clear dichotomy between the obscured active galactic nucleus (AGN) and two intense starburst regions. We detect the 2.3 μm CO bandheads, characteristic of cool stellar atmospheres, in the star-forming regions, but not toward the AGN. Surprisingly, at 4.7 μm, we find highly excited CO (T ex ≈ 700–800 K out to at least rotational level J = 27) toward the star-forming regions, but only cooler gas (T ex ≈ 200 K) toward the AGN. We conclude that only mid-infrared pumping through the rovibrational lines can account for the equilibrium conditions found for CO and H2O in the deeply embedded starbursts. Here, the CO bands probe regions with an intense local radiation field inside dusty young massive star clusters or near the most massive young stars. The lack of high-excitation molecular gas toward the AGN is attributed to geometric dilution of the intense radiation from the bright point source. An overview of the relevant excitation and radiative transfer physics is provided in an appendix.

Two shell- and wing-shaped supernova remnants

Supernova remnants (SNRs) are profoundly affected by their ambient medium. In particular, SNRs with a mixed morphology (characterised by a shell-like radio morphology and centrally filled X-ray emission) are thought to be the result of the interaction of a supernova explosion with a dense environment. In this work, we present carbon monoxide (CO) observations around two mixed morphology SNRs, VRO 42.05.01 and G 350.0-2.0, that look remarkably similar in continuum radio emission, showing what we refer to as a shell and wing shape. It has been proposed that the shell and wing shape is the result of environmental effects, in the form of a sharp density gradient or discontinuity. Therefore, our motivation for studying these two sources jointly is that if the dense molecular environment causes the development of these sources’ shell and wing shape, then these two sources’ environments must be similar. This is contrary to what we observe. In the case of VRO 42.05.01, we have found direct evidence of an interaction with its molecular environment, in the form of broadened 12CO line profiles, high 12CO (J = 2−1) to 12CO (J = 1−0) line ratios, and arc features in position-velocity space. We interpret some of these features to be associated with the SNR shock, and some of them to be due to the presence of a pre-supernova stellar wind. We have found no such features in the abundant molecular gas surrounding G 350.02.0. In addition to the spectral line analysis, we have used radio continuum data to make a spectral index map of G 350.0-2.0, and we see that the radio spectrum of G 350.0-2.0 steepens significantly at frequencies <200 MHz, much like that of VRO 42.05.01. In spite of their spectral and morphological similarities, these two sources look substantially different in their observed optical and infrared emission. The lack of large-scale correspondence between the radio continuum and the molecular material, in either case, as well as the differences in the excitation and morphological properties of the molecular gas surrounding both sources, lead us to conclude that the shell and wing morphology of these two sources is not due to interactions with a similar ambient molecular interstellar medium.

SUNRISE: The rich molecular inventory of high-redshift dusty galaxies revealed by broadband spectral line surveys

Understanding the nature of high-redshift dusty galaxies requires a comprehensive view of their interstellar medium (ISM) and molecular complexity. However, the molecular ISM at high redshifts is commonly studied using only a few species beyond 12C16O, limiting our understanding. In this paper, we present the results of deep 3 mm spectral line surveys using the NOrthern Extended Millimeter Array (NOEMA) targeting two strongly lensed dusty galaxies observed when the Universe was less than 1.8 Gyr old: APM 08279+5255, a quasar at redshift z = 3.911, and NCv1.143 (H-ATLAS J125632.7+233625), a z = 3.565 starburst galaxy. The spectral line surveys cover rest-frame frequencies from about 330 to 550 GHz for both galaxies. We report the detection of 38 and 25 emission lines in APM 08279+5255 and NCv1.143, respectively. These lines originate from 17 species, namely CO, 13CO, C18O, CN, CCH, HCN, HCO+, HNC, CS, C34S, H2O, H3O+, NO, N2H+, CH, c-C3H2, and the vibrationally excited HCN and neutral carbon. The spectra reveal the chemical richness and the complexity of the physical properties of the ISM. By comparing the spectra of the two sources and combining the analysis of the molecular gas excitation, we find that the physical properties and the chemical imprints of the ISM are different: the molecular gas is more excited in APM 08279+5255, which exhibits higher molecular gas temperatures and densities compared to NCv1.143; the molecular abundances in APM 08279+5255 are akin to the values of local active galactic nuclei (AGN), showing boosted relative abundances of the dense gas tracers that might be related to high-temperature chemistry and/or the X-ray-dominated regions, while NCv1.143 more closely resembles local starburst galaxies. The most significant differences between the two sources are found in H2O: the 448 GHz ortho-H2O(423 − 330) line is significantly brighter in APM 08279+5255, which is likely linked to the intense far-infrared radiation from the dust powered by AGN. Our astrochemical model suggests that, at such high column densities, far-ultraviolet radiation is less important in regulating the ISM, while cosmic rays (and/or X-rays and shocks) are the key players in shaping the molecular abundances and the initial conditions of star formation. Both our observed CO isotopologs line ratios and the derived extreme ISM conditions (high gas temperatures, densities, and cosmic-ray ionization rates) suggest the presence of a top-heavy stellar initial mass function. From the ∼330–550 GHz continuum, we also find evidence of nonthermal millimeter flux excess in APM 08279+5255 that might be related to the central supermassive black hole. Such deep spectral line surveys open a new window into the physics and chemistry of the ISM and the radiation field of galaxies in the early Universe.

The Quasar Feedback Survey: characterizing CO excitation in quasar host galaxies

ABSTRACT We present a comprehensive study of the molecular gas properties of 17 Type 2 quasars at z < 0.2 from the Quasar Feedback Survey (L$_{\rm [O~{\small III}]}$ > 1042.1 $\rm ergs^{-1}$), selected by their high [O iii] luminosities and displaying a large diversity of radio jet properties, but dominated by LIRG-like galaxies. With these data, we are able to investigate the impact of AGN and AGN feedback mechanisms on the global molecular interstellar medium. Using Atacama Pathfinder EXperiment and ALMA ACA observations, we measure the total molecular gas content using the CO(1-0) emission and homogeneously sample the carbon monoxide (CO) spectral line energy distributions, observing CO transitions (Jup = 1, 2, 3, 6, 7). We observe high r21 ratios (r21 = L’CO(2-1)/L’CO(1-0)) with a median r21 = 1.06, similar to local (U)LIRGs (with r21 ∼ 1) and higher than normal star-forming galaxies (with r21 ∼ 0.65). Despite the high r21 values, for the seven targets with the required data, we find low excitation in CO(6-5) & CO(7-6) (r61 and r62 < 0.6 in all but one target), unlike high-redshift quasars in the literature, which are far more luminous and show higher line ratios. The ionized gas traced by [O iii] exhibits systematically higher velocities than the molecular gas traced by CO. We conclude that any effects of quasar feedback (e.g. via outflows and radio jets) do not have a significant instantaneous impact on the global molecular gas content and excitation and we suggest that it only occurs on more localized scales.

Clusters, clouds, and correlations: relating young clusters to giant molecular clouds in M33 and M31

ABSTRACT We use young clusters and giant molecular clouds (GMCs) in the galaxies M33 and M31 to constrain temporal and spatial scales in the star formation process. In M33, we compare the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER) catalogue of 1214 clusters with ages measured via colour–magnitude diagram (CMD) fitting to 444 GMCs identified from a new 35 pc resolution Atacama Large Millimeter/submillimeter Array (ALMA) 12CO(2–1) survey. In M31, we compare the Panchromatic Hubble Andromeda Treasury (PHAT) catalogue of 1249 clusters to 251 GMCs measured from a Combined Array for Research in Millimeter-wave Astronomy (CARMA) 12CO(1–0) survey with 20 pc resolution. Through two-point correlation analysis, we find that young clusters have a high probability of being near other young clusters, but correlation between GMCs is suppressed by the cloud identification algorithm. By comparing the positions, we find that younger clusters are closer to GMCs than older clusters. Through cross-correlation analysis of the M33 cluster data, we find that clusters are statistically associated when they are ≤10 Myr old. Utilizing the high precision ages of the clusters, we find that clusters older than ≈18 Myr are uncorrelated with the molecular interstellar medium (ISM). Using the spatial coincidence of the youngest clusters and GMCs in M33, we estimate that clusters spend ≈4–6 Myr inside their parent GMC. Through similar analysis, we find that the GMCs in M33 have a total lifetime of ≈11–15 Myr. We also develop a drift model and show that the above correlations can be explained if the clusters in M33 have a 5–10 km s−1 velocity dispersion relative to the molecular ISM.

Multiple gas phases in supernova remnant IC 443: mapping shocked H2 with VLT/KMOS

ABSTRACT Supernovae and their remnants provide energetic feedback to the ambient interstellar medium (ISM), which is often distributed in multiple gas phases. Among them, warm molecular hydrogen (H2) often dominates the cooling of the shocked molecular ISM, which has been observed with the H2 emission lines at near-infrared wavelengths. Such studies, however, were either limited in narrow filter imaging or sparsely sampled mid-infrared spectroscopic observations with relatively poor angular resolutions. Here we present near-infrared (H and K bands) spectroscopic mosaic observations towards the A, B, C, and G regions of the supernova remnant (SNR) IC 443, with the K-band Multi-Object Spectrograph (KMOS) onboard the Very Large Telescope (VLT). We detected 20 rotational–vibrational transitions of H2, one H line (Brγ), and two [Fe ii] lines, which dominate broad-band images at both H and K bands. The spatial distribution of H2 lines at all regions is clumpy on scales from ∼0.1 down to ∼0.008 pc. The fitted excitation temperature of H2 is between 1500 and 2500 K, indicating warm shocked gas in these regions. The multigas phase comparison shows stratified shock structures in all regions, which explains the coexistence of multiple types of shocks in the same regions. Lastly, we verify the candidates of young stellar objects previously identified in these regions with our spectroscopic data, and find none of them are associated with young stars. This sets challenges to the previously proposed scenario of triggered star formation by SNR shocks in IC 443.

Importance of radiative pumping for the excitation of the H2O submillimeter lines in galaxies

H2O submillimeter emission is a powerful diagnostic of the molecular interstellar medium in a variety of sources, including low- and high-mass star-forming regions of the Milky Way, and from local to high-redshift galaxies. However, the excitation mechanism of these lines in galaxies has been debated, preventing a basic consensus on the physical information that H2O provides. Radiative pumping due to H2O absorption of far-infrared photons emitted by dust and collisional excitation in dense shocked gas have both been proposed to explain the H2O emission. Here we propose two basic diagnostics to distinguish between the two mechanisms: First, the ortho-H2O 321 − 212 75 μm and the para-H2O 220 − 111 101 μm rotational lines in shock-excited regions are expected to be in emission, while when radiative pumping dominates, the two far-infrared lines are expected to be in absorption. Second, the radiative pumping scenario predicts, based on the statistical equilibrium of H2O level populations, that the apparent isotropic net rate of far-infrared absorption in the 321 ← 212 (75 μm) and 220 ← 111 (101 μm) lines should be higher than or equal to the apparent isotropic net rate of submillimeter emission in the 321 → 312 (1163 GHz) and 220 → 211 (1229 GHz) lines, respectively. Applying both criteria to all 16 galaxies and several Galactic high-mass star-forming regions in which the H2O 75 μm and submillimeter lines have been observed with Herschel/PACS and SPIRE, we show that in most (extra)galactic sources, the H2O submillimeter line excitation is dominated by far-infrared pumping, combined in some cases with collisional excitation of the lowest energy levels. Based on this finding, we revisit the interpretation of the correlation between the luminosity of the H2O 988 GHz line and the source luminosity in the combined Galactic and extragalactic sample.

Acetaldehyde binding energies: a coupled experimental and theoretical study

ABSTRACT Acetaldehyde is one of the most common and abundant gaseous interstellar complex organic molecules found in cold and hot regions of the molecular interstellar medium. Its presence in the gas-phase depends on the chemical formation and destruction routes, and its binding energy (BE) governs whether acetaldehyde remains frozen on to the interstellar dust grains or not. In this work, we report a combined study of the acetaldehyde BE obtained via laboratory temperature programmed desorption (TPD) experiments and theoretical quantum chemical computations. BEs have been measured and computed as a pure acetaldehyde ice and mixed with both polycrystalline and amorphous water ice. Both calculations and experiments found a BE distribution on amorphous solid water that covers the 4000–6000 K range when a pre-exponential factor of $1.1\times 10^{18}\, \mathrm{s}^{-1}$ is used for the interpretation of the experiments. We discuss in detail the importance of using a consistent couple of BE and pre-exponential factor values when comparing experiments and computations, as well as when introducing them in astrochemical models. Based on the comparison of the acetaldehyde BEs measured and computed in the present work with those of other species, we predict that acetaldehyde is less volatile than formaldehyde, but much more than water, methanol, ethanol, and formamide. We discuss the astrochemical implications of our findings and how recent astronomical high spatial resolution observations show a chemical differentiation involving acetaldehyde, which can easily explained due to the different BEs of the observed molecules.

FAUST

Aims. Methanol is a ubiquitous species commonly found in the molecular interstellar medium. It is also a crucial seed species for the build-up of chemical complexity in star forming regions. Thus, understanding how its abundance evolves during the star formation process and whether it enriches the emerging planetary system is of paramount importance. Methods. We used new data from the ALMA Large Program FAUST (Fifty AU STudy of the chemistry in the disc/envelope system of solar protostars) to study the methanol line emission towards the [BHB2007] 11 protobinary system (sources A and B), where a complex structure of filaments connecting the two sources with a larger circumbinary disc has previously been detected. Results. Twelve methanol lines have been detected with upper energies in the [45–537] K range along with one 13CH3OH transition and one methyl formate (CH3OCHO) line blended with one of the methanol transitions. The methanol emission is compact (FWHM ~ 0.5″) and encompasses both protostars, which are separated by only 0.2″ (28 au). In addition, the overall methanol line emission presents three velocity components, which are not spatially resolved by our observations. Nonetheless, a detailed analysis of the spatial origin of these three components suggests that they are associated with three different spatial regions, with two of them close to 11B and the third one associated with 11A. A radiative transfer analysis of the methanol lines gives a kinetic temperature of [100–140] K, an H2 volume density of 106–107 cm−3 and column density of a few 1018 cm−2 in all three components with a source size of ~0.15″. Thus, this hot and dense gas is highly enriched in methanol with an abundance as high as 10−5. Using previous continuum data, we show that dust opacity can potentially completely absorb the methanol line emission from the two binary objects. Conclusions. Although we cannot firmly exclude other possibilities, we suggest that the detected hot methanol is resulting from the shocked gas from the incoming filaments streaming towards [BHB2007] 11A and B, respectively. Higher spatial resolution observations are necessary to confirm this hypothesis.

The SEDIGISM survey: Molecular cloud morphology

The Structure, Excitation, and Dynamics of the Inner Galactic InterStellar Medium (SEDIGISM) survey has produced high (spatial and spectral) resolution 13CO (2−1) maps of the Milky Way. It has allowed us to investigate the molecular interstellar medium in the inner Galaxy at an unprecedented level of detail and characterise it into molecular clouds (MCs). In a previous paper, we classified the SEDIGISM clouds into four morphologies. However, how the properties of the clouds vary for these four morphologies is not well understood. Here, we use the morphological classification of SEDIGISM clouds to find connections between the cloud morphologies, their integrated properties, and their location on scaling relation diagrams. We observe that ring-like clouds show the most peculiar properties, having, on average, higher masses, sizes, aspect ratios, and velocity dispersions, compared to other morphologies. We speculate that this is related to the physical mechanisms that regulate their formation and evolution; for example, turbulence from stellar feedback can often result in the creation of bubble-like structures. We also see a trend of morphology with the virial parameter, whereby ring-like, elongated, clumpy, and concentrated clouds have virial parameters in decreasing order. Our findings provide a foundation for a better understanding of MC behaviour, based on their measurable properties.

Impact of H2-driven star formation and stellar feedback from low-enrichment environments on the formation of spiral galaxies

ABSTRACT The reservoir of molecular gas (H2) represents the fuel for the star formation (SF) of a galaxy. Connecting the star formation rate (SFR) to the available H2 is key to accurately model SF in cosmological simulations of galaxy formation. We investigate how modifying the underlying modelling of H2 and the description of stellar feedback in low-metallicity environments (LMF, i.e. low-metallicity stellar feedback) in cosmological zoomed-in simulations of a Milky Way-size halo influences the formation history of the forming, spiral galaxy, and its final properties. We exploit two different models to compute the molecular fraction of cold gas ($f_{\rm H_{2}}$): (i) the theoretical model by Krumholz et al. (2009b) and (ii) the phenomenological prescription by Blitz and Rosolowsky (2006). We find that the model adopted to estimate $f_{\rm H_{2}}$ plays a key role in determining final properties and in shaping the morphology of the galaxy. The clumpier interstellar medium (ISM) and the more complex H2 distribution that the Krumholz et al. model predicts result in better agreement with observations of nearby disc galaxies. This shows how crucial it is to link the SFR to the physical properties of the star-forming, molecular ISM. The additional source of energy that LMF supplies in a metal-poor ISM is key in controlling SF at high redshift and in regulating the reservoir of SF across cosmic time. Not only is LMF able to regulate cooling properties of the ISM, but it also reduces the stellar mass of the galaxy bulge. These findings can foster the improvement of the numerical modelling of SF in cosmological simulations.

Weather forecast of the Milky Way: shear and stellar feedback determine the lives of Galactic-scale filaments

ABSTRACT The interstellar medium (ISM) is an inseparable part of the Milky Way ecosystem whose evolutionary history remains a challenging question. We trace the evolution of the molecular ISM using a sample of young stellar objects (YSO) association–molecular cloud complex (YSO–MC complex). We derive their three-dimensional (3D) velocities by combining the Gaia astrometric measurements of the YSO associations and the CO observations of the associated molecular clouds. Based on the 3D velocities, we simulate the motions of the YSO–MC complexes in the Galactic potential and forecast the ISM evolution by tracing the motions of the individual complexes, and reveal the roles of shear and stellar feedback in determining ISM evolution: Galactic shear stretches Galactic-scale molecular cloud complexes, such as the G120 Complex, into Galactic-scale filaments, and it also contributes to the destruction of the filaments; while stellar feedback creates interconnected superbubbles whose expansion injects peculiar velocities into the ISM. The Galactic-scale molecular gas clumps are often precursors of the filaments and the Galactic-scale filaments are transient structures under a constant stretch by shear. This evolutionary sequence sets a foundation to interpret other gas structures.

On the scale height of the molecular gas disc in Milky Way-like galaxies

ABSTRACT We study the relationship between the scale height of the molecular gas disc and the turbulent velocity dispersion of the molecular interstellar medium within a simulation of a Milky Way-like galaxy in the moving-mesh code arepo. We find that the vertical distribution of molecular gas can be described by a Gaussian function with a uniform scale height of ∼50 pc. We investigate whether this scale height is consistent with a state of hydrostatic balance between gravity and turbulent pressure. We find that the hydrostatic prediction using the total turbulent velocity dispersion (as one would measure from kiloparsec-scale observations) gives an overestimate of the true molecular disc scale height. The hydrostatic prediction using the velocity dispersion between the centroids of discrete giant molecular clouds (cloud–cloud velocity dispersion) leads to more accurate estimates. The velocity dispersion internal to molecular clouds is elevated by the locally enhanced gravitational field. Our results suggest that observations of molecular gas need to reach the scale of individual molecular clouds in order to accurately determine the molecular disc scale height.

Kinematics of the molecular interstellar medium probed by Gaia: steep velocity dispersion–size relation, isotropic turbulence, and location-dependent energy dissipation

ABSTRACT The evolution of the molecular interstellar medium is controlled by processes such as turbulence, gravity, stellar feedback, and Galactic shear. AL a part of the ISM-6D project, using Gaia astrometric measurements towards a sample of young stellar objects (YSOs), we study the morphology and kinematic structure of the associated molecular gas. We identify 150 YSO associations with distance $d \lesssim 3 \,\,\rm kpc$. The YSO associations are elongated, with a median aspect ratio of 1.97, and are oriented parallel to the disc mid-plane, with a median angle of 30°. The turbulence in the molecular clouds as probed by the YSOs is isotropic, and the velocity dispersions are related to the sizes by σv, 2D = 0.74 (r/pc)0.67 (km s−1) . The slope is on the steeper side, yet consistent with previous measurements. The energy dissipation rate of turbulence $\dot{\epsilon } = \sigma _{v,{\rm 3D}}^3 /L$ decreases with the Galactocentric distance, with a gradient of 0.2 $\rm dex \,\, kpc^{-1}$, which can be explained if turbulence is driven by cloud collisions. In this scenario, the clouds located in the inner Galaxy have higher chances to accrete smaller clouds and are more turbulent. Although the density structures of the complexes are anisotropic, the turbulence is consistent with being isotropic. If the alignment between density structures and the Galactic-disc mid-plane is due to shear, we expect $t_{\rm cloud} \gtrsim t_{\rm shear}\approx 30\,\, \rm Myr$. This cloud lifetime is longer than the turbulence crossing time, and a continuous energy injection is required to maintain the turbulence.

APEX and NOEMA observations of H2S in nearby luminous galaxies and the ULIRG Mrk 231

Context. In order to understand the evolution and feedback of active galactic nuclei (AGN) and star formation, it is important to use molecular lines as probes of physical conditions and chemistry. Aims. We use H2S to investigate the impact of starburst and AGN activity on the chemistry of the molecular interstellar medium in luminous infrared galaxies. Specifically, our aim is to search for evidence of shock enhancement of H2S related to galactic-scale mechanical feedback processes such as outflows. Methods. Using the APEX single-dish telescope, we have observed the 110–101 transition of ortho-H2S at 168 GHz towards the centres of 12 nearby luminous infrared galaxies. We have also observed the same line towards the ultra-luminous infrared galaxy Mrk 231 with the NOEMA interferometer. Results. We detected H2S towards NGC 253, NGC 1068, NGC 3256, NGC 4418, NGC 4826, NGC 4945, Circinus, M 83, and Mrk 231. Upper limits were obtained for NGC 1097, NGC 1377, and IC 860. We also detected line emission from HCN 2–1 in all galaxies in the APEX survey as well as HCO+, HNC, CH3CN, CH3OH, H2CS, HOC+, and SO in several of the sample galaxies. Mrk 231 has a rich 2 mm molecular spectrum and, in addition to H2S, we detect emission from HC3N, CH3OH, HC18O+, C2S, and CH3CCH. Four galaxies show elevated H2S emission relative to HCN: Circinus, NGC 3256, NGC 4826, and NGC 4418. We suggest that the high line ratios are caused by elevated H2S abundances in the dense gas. However, we do not find any clear connection between the H2S/HCN line intensity ratio and the presence (or speed) of molecular outflows in the sample galaxies. Therefore, H2S abundances do not seem to be globally affected by the large-scale outflows. In addition, the H2S/HCN line ratio is not enhanced in the line wings compared to the line core in Mrk 231. This suggests that H2S abundances do not increase in the dense gas in the outflow. However, we do find that the H2S and HCN luminosities (LH2S and LHCN) correlate well with the total molecular gas mass in the outflow, Moutflow(H2), in contrast to LCO and LHCO+. We also find that the line luminosity of H2S correlates with the total infrared luminosity in a similar way as that of H2O. Conclusions. We do not find any evidence of H2S abundance enhancements in the dense gas due to galactic-scale outflows in our sample galaxies, nor in the high-resolution study of Mrk 231. We discuss possible mechanisms behind the suggested H2S abundance enhancements in NGC 4418, Circinus, NGC 3256, and NGC 4826. These include radiative processes (for example X-rays or cosmic rays) or smaller-scale shocks. Further high-resolution and multi-transition studies are required to determine the cause behind the elevated H2S emission in these galaxies. We suggest that LH2S serves as a tracer of the dense gas content, similar to LHCN, and that the correlation between LH2S and Moutflow(H2) implies a relation between the dense gas reservoir and the properties and evolution of the molecular feedback. This potential link requires further study since it holds important keys to our understanding of how the properties of molecular outflows relate to those of their host galaxies. Finally, the similar infrared-correlation coefficients between H2S and H2O may indicate that they originate in the same regions in the galaxy: warm gas in shocks or irradiated by star formation or an AGN.

WISDOM Project – X. The morphology of the molecular ISM in galaxy centres and its dependence on galaxy structure

ABSTRACT We use high-resolution maps of the molecular interstellar medium (ISM) in the centres of 86 nearby galaxies from the millimetre-Wave Interferometric Survey of Dark Object Masses (WISDOM) and Physics at High Angular Resolution in Nearby GalaxieS (PHANGS) surveys to investigate the physical mechanisms setting the morphology of the ISM at molecular cloud scales. We show that early-type galaxies tend to have smooth, regular molecular gas morphologies, while the ISM in spiral galaxy bulges is much more asymmetric and clumpy when observed at the same spatial scales. We quantify these differences using non-parametric morphology measures (Asymmetry, Smoothness, and Gini), and compare these measurements with those extracted from idealized galaxy simulations. We show that the morphology of the molecular ISM changes systematically as a function of various large-scale galaxy parameters, including galaxy morphological type, stellar mass, stellar velocity dispersion, effective stellar mass surface density, molecular gas surface density, star formation efficiency, and the presence of a bar. We perform a statistical analysis to determine which of these correlated parameters best predicts the morphology of the ISM. We find the effective stellar mass surface (or volume) density to be the strongest predictor of the morphology of the molecular gas, while star formation and bars maybe be important secondary drivers. We find that gas self-gravity is not the dominant process shaping the morphology of the molecular gas in galaxy centres. Instead effects caused by the depth of the potential well, such as shear, suppression of stellar spiral density waves, and/or inflow, affect the ability of the gas to fragment.

The Role of Neutral Hydrogen in Setting the Abundances of Molecular Species in the Milky Way’s Diffuse Interstellar Medium. II. Comparison between Observations and Theoretical Models

Abstract We compare observations of H i from the Very Large Array (VLA) and the Arecibo Observatory and observations of HCO+ from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Northern Extended Millimeter Array (NOEMA) in the diffuse (A V ≲ 1) interstellar medium (ISM) to predictions from a photodissociation region (PDR) chemical model and multiphase ISM simulations. Using a coarse grid of PDR models, we estimate the density, FUV radiation field, and cosmic-ray ionization rate (CRIR) for each structure identified in HCO+ and H i absorption. These structures fall into two categories. Structures with T s < 40 K, mostly with N(HCO+) ≲ 1012 cm−2, are consistent with modest density, FUV radiation field, and CRIR models, typical of the diffuse molecular ISM. Structures with spin temperature T s > 40 K, mostly with N(HCO+) ≳ 1012 cm−2, are consistent with high density, FUV radiation field, and CRIR models, characteristic of environments close to massive star formation. The latter are also found in directions with a significant fraction of thermally unstable H i. In at least one case, we rule out the PDR model parameters, suggesting that alternative mechanisms (e.g., nonequilibrium processes like turbulent dissipation and/or shocks) are required to explain the observed HCO+ in this direction. Similarly, while our observations and simulations of the turbulent, multiphase ISM agree that HCO+ formation occurs along sight lines with N(H I) ≳ 1021 cm−2, the simulated data fail to explain HCO+ column densities ≳ few × 1012 cm−2. Because a majority of our sight lines with HCO+ had such high column densities, this likely indicates that nonequilibrium chemistry is important for these lines of sight.

Negative and positive feedback from a supernova remnant with SHREC: a detailed study of the shocked gas in IC443

ABSTRACT Supernova remnants (SNRs) contribute to regulate the star formation efficiency and evolution of galaxies. As they expand into the interstellar medium (ISM), they transfer vast amounts of energy and momentum that displace, compress, and heat the surrounding material. Despite the extensive work in galaxy evolution models, it remains to be observationally validated to what extent the molecular ISM is affected by the interaction with SNRs. We use the first results of the ESO–ARO Public Spectroscopic Survey SHREC to investigate the shock interaction between the SNR IC443 and the nearby molecular clump G. We use high-sensitivity SiO(2-1) and H13CO+(1-0) maps obtained by SHREC together with SiO(1-0) observations obtained with the 40-m telescope at the Yebes Observatory. We find that the bulk of the SiO emission is arising from the ongoing shock interaction between IC443 and clump G. The shocked gas shows a well-ordered kinematic structure, with velocities blue-shifted with respect to the central velocity of the SNR, similar to what observed towards other SNR–cloud interaction sites. The shock compression enhances the molecular gas density, n(H2), up to >105 cm−3, a factor of >10 higher than the ambient gas density and similar to values required to ignite star formation. Finally, we estimate that up to 50 per cent of the momentum injected by IC443 is transferred to the interacting molecular material. Therefore, the molecular ISM may represent an important momentum carrier in sites of SNR–cloud interactions.

HCN/HNC ratio: A new chemical thermometer at 3 mm

The gas kinetic temperature (TK) determines the physical and chemical evolution of the interstellar medium (ISM). This work explores the use of HCN/HNC (J=1–0) line ratio as new probe of the gas kinetic temperature in the molecular ISM. Based on a new set of IRAM-30m observations at 30 arcsec resolution towards Orion Nebula Cluster, we find a two-part linear correlation of the observed HCN/HNC line ratio and TK. This empirical calibration allows to obtain direct estimates of the TK values across a wide range of column densities (AV ≳ 5 mag) and up to scales of ~ 10 pc. Comparisons with additional studies highlight the potential use of the HCN/HNC line ratio as novel chemical thermometer at 3mm.

A 30m Large Program: The CO Line Atlas of the Whirlpool Galaxy Survey (CLAWS)

Robust knowledge of the distribution, amount, and physical/chemical state of the cold molecular (H2) gas is key to understanding galaxy evolution. With the help of multi-CO line observations, it is possible to study the molecular gas distribution and disentangle numerous physical and chemical processes that shape and govern the molecular interstellar medium (ISM). For the first time, we obtain full-galaxy mapping data of faint CO isotopologues (13CO, C18O, C17O) at 1mm and 3mm wavelengths across the disk of the nearby spiral galaxy M51. With the help of these CO isotopologues, it is possible to constrain the bulk physical and chemical conditions in the molecular gas. We study potential explanations for why CO isotopologue emission varies. Likely drivers include CO abundance variations due to selective nucleosynthesis and changes in the optical depth. Our analysis concludes that a combination of variation in opacity and relative abundances is the dominant driver for the observed CO isotopologue ratio trends on large (kpc) scales. In contrast, abundance variation due to selective photodissociation and chemical fractionation seem to only play a minor or negligible role on galaxy-wide scales.