Cold Dense Gas Research Articles

On the Origin of Quenched but Gas-rich Regions at Kiloparsec Scales in Nearby Galaxies

We use resolved spectroscopy from MaNGA to investigate the significance of both local and global properties of galaxies to the cessation of star formation at kpc scales. Quenched regions are identified from a sample of isolated disk galaxies by a single-parameter criterion D n (4000) - log EW(Hα) >1.6−log2=1.3 , and are divided into gas-rich quenched regions (GRQRs) and gas-poor quenched regions according to the surface density of cold gas (Σgas). Both types of quenched regions tend to be hosted by non-AGN galaxies with relatively high mass (M * ≳ 1010 M ⊙) and red colors (NUV − r ≳ 3), as well as low star formation rate and high central density at fixed mass. They span wide ranges in other properties including structural parameters that are similar to the parent sample, indicating that the conditions responsible for quenching in gas-rich regions are largely independent of the global properties of galaxies. We train random forest classifiers and regressors for predicting quenching in our sample with 15 local/global properties. Σ* is the most important property for quenching, especially for GRQRs. These results strongly indicate the important roles of low-mass hot evolved stars, which are numerous and long-lived in quenched regions and can provide substantial radiation pressure to support the surrounding gas against gravitational collapse. The different feature importance for quenching, as found previously by A. F. L. Bluck et al., is partly due to the different definitions of quenched regions, particularly the different requirements on EW(Hα).

On the Origin of Star Formation Quenching of Galaxies in Group Environments Using the NewHorizon Simulation

We study star formation (SF) quenching of satellite galaxies with M * > 107 M ⊙ within two low-mass groups (M vir = 1012.9 and 1012.7 M ⊙) using the NewHorizon simulation. We confirm that satellite galaxies (M * ≲ 1010 M ⊙) are more prone to quenching than their field counterparts. This quenched fraction decreases with increasing stellar mass, consistent with recent studies. Similar to the findings in cluster environments, we note a correlation between the orbital motions of galaxies within these groups and the phenomenon of SF quenching. Specifically, SF is suppressed at the group center, and for galaxies with M * > 109.1 M ⊙, there is often a notable rejuvenation phase following a temporary quenching period. The SF quenching at the group center is primarily driven by changes in SF efficiency and the amount of gas available, both of which are influenced by hydrodynamic interactions between the interstellar medium and surrounding hot gas within the group. Conversely, satellite galaxies with M * < 108.2 M ⊙ experience significant gas removal within the group, leading to SF quenching. Our analysis highlights the complexity of SF quenching in satellite galaxies in group environments, which involves an intricate competition between the efficiency of SF (which depends on the dynamical state of the gas) on the one hand, and the availability of cold dense gas on the other hand. This challenges the typical understanding of environmental effects based on gas stripping through ram pressure, suggesting a need for a new description of galaxy evolution under mild environmental effects.

Simulations of early structure formation: Properties of halos that host primordial star formation

Context. Population III (pop III) stars were born in halos characterised by a pristine gas composition. In such a halo, once the gas density reaches nH ∼ 1 cm−3, molecular cooling leads to the collapse of the gas and the birth of pop III stars. Halo properties, such as the chemical abundances, mass, and angular momentum can affect the collapse of the gas, thereby leading to the pop III initial mass function (IMF) of star formation. Aims. We want to study the properties of primordial halos and how halos that host early star formation differ from other types of halos. The aim of this study is to obtain a representative population of halos at a given redshift hosting a cold and massive gas cloud that enables the birth of the first stars. Methods. We investigated the growth of primordial halos in a ΛCDM Universe in a large cosmological simulation. We used the hydrodynamic code RAMSES and the chemical solver KROME to study halo formation with non-equilibrium thermochemistry. We then identified structures in the dark and baryonic matter fields, thereby linking the presence or absence of dense gas clouds to the mass and the physical properties of the hosting halos. Results. In our simulations, the mass threshold for a halo for hosting a cold dense gas cloud is ≃7 × 105 M⊙ and the threshold in the H2 mass fraction is found to be ≃2 × 10−4. This is in agreement with previous works. We find that the halo history and accretion rate play a minor role. Here, we present halos with higher HD abundances, which are shown to be colder, as the temperature in the range between 102 − 104 cm−3 depends on the HD abundance to a large extent. The higher fraction of HD is linked to the higher spin parameter that is seen for the dense gas.

Characterizing the line emission from molecular clouds

Aims. We aim to characterize and compare the molecular-line emission of three clouds whose star-formation rates span one order of magnitude: California, Perseus, and Orion A. Methods. We used stratified random sampling to select positions representing the different column density regimes of each cloud and observed them with the IRAM 30 m telescope. We covered the 3 mm wavelength band and focused our analysis on CO, HCN, CS, HCO+, HNC, and N2H+. Results. We find that the line intensities depend most strongly on the H2 column density, with which they are tightly correlated. A secondary effect, especially visible in Orion A, is a dependence of the line intensities on the gas temperature. We explored a method that corrects for temperature variations and show that, when it is applied, the emission from the three clouds behaves very similarly. CO intensities vary weakly with column density, while the intensity of traditional dense-gas tracers such as HCN, CS, and HCO+ varies almost linearly with column density. N2H+ differs from all other species in that it traces only cold dense gas. The intensity of the rare HCN and CS isotopologs reveals additional temperature-dependent abundance variations. Overall, the clouds have similar chemical compositions that, as the depth increases, are sequentially dominated by photodissociation, gas-phase reactions, molecular freeze-out, and stellar feedback in the densest parts of Orion A. Our observations also allowed us to calculate line luminosities for each cloud, and a comparison with literature values shows good agreement. We used our HCN(1–0) data to explore the behavior of the HCN conversion factor, finding that it is dominated by the emission from the outermost cloud layers. It also depends strongly on the gas kinetic temperature. Finally, we show that the HCN/CO ratio provides a gas volume density estimate, and that its correlation with the column density resembles that found in extragalactic observations.

The Kinematic Bimodality: Efficient Feedback and Cold Gas Deficiency in Slow-rotating Galaxies

The bimodality in the stellar spin of low-redshift (massive) galaxies, ubiquitously existing at all star formation levels and in diverse environments, suggests that galaxies grow and quench through two diverged evolutionary pathways. For spheroid-dominated galaxies of slow stellar rotation, the age composition and metallicity of their stellar populations are evidence of a fast quenching history with significant gas outflows. In this work, we measure the spin parameter , i.e., the normalized specific angular momentum of stars, out of the MaNGA integral field spectroscopy for about 10,000 galaxies. Among the two-thirds with H i follow-up observations (z ≲ 0.05), we find that, compared to fast-rotating galaxies of the same stellar mass and star formation, the galaxy populations with slower rotation are generally more H i gas-poor, robust against further environmental restriction and with nondetections properly taken into account using the stacking technique. This cold gas deficit of slow-rotating galaxies is most apparent at high mass below the star formation main sequence, supporting the pivotal role of gas outflows in their quenching history. With hints from H i velocity distributions, we suspect that massive gas outflows among the slow-rotating population are facilitated by high ejective feedback efficiency, which is a result of extensive coupling between disturbed volume-filling cold gas and (commonly) biconical feedback from central black holes. By contrast, in fast-rotating disk galaxies the feedback energy mostly goes to the hot circumgalactic medium rather than directly impacting the dense and planar cold gas, thus making the feedback mainly preventive against further gas inflow.

Extinction towards the cluster R136 in the Large Magellanic Cloud

Context. The cluster R136 in the giant star-forming region 30 Doradus in the Large Magellanic Cloud (LMC) offers a unique opportunity to resolve a stellar population in a starburst-like environment. Knowledge of the extinction towards this region is key for the accurate determination of stellar masses, and for the correct interpretation of observations of distant, unresolved starburst galaxies. Aims. Our aims are to construct an extinction law towards R136, and to measure the extinction towards individual sources inside the cluster. This will allow us to map the spatial distribution of the dust, to learn about dust properties, and to improve mass measurements of the very massive WNh stars inside the cluster. Methods. We obtain the near-infrared to ultraviolet extinction towards 50 stars in the core of R136, employing the ‘extinction without standards’ method. To assure good fits over the full wavelength range, we combine and modify existing extinction laws. Results. We detect a strong spatial gradient in the extinction properties across the core of R136, coinciding with a gradient in density of cold gas that is part of an extension of the Stapler Nebula, a molecular cloud lying northeast of the cluster. In line with previous measurements of R136 and the 30 Doradus region, we obtain a high total-to-relative extinction (RV = 4.38 ± 0.87). However, the high values of RV are accompanied by relatively strong extinction in the ultraviolet, contrary to what is observed for Galactic sightlines. Conclusions. The relatively strong ultraviolet extinction towards R136 suggests that the properties of the dust towards R136 differ from those in the Milky Way. For RV ~ 4.4, about three times fewer ultraviolet photons can escape from the ambient dust environment relative to the canonical Galactic extinction at the same RV and AV. Therefore, if dust in the R136 star-bursting environment is characteristic for cosmologically distant star-bursting regions, the escape fraction of ultraviolet photons from such regions is overestimated by a factor of three relative to the standard Milky Way assumption for the total-to-selective extinction. Furthermore, a comparison with average curves tailored to other regions of the LMC shows that large differences in ultraviolet extinction exist within this galaxy. Further investigation is required in order to decipher whether or not there is a relation between RV and ultraviolet extinction in the LMC.

Accelerated Growth of Seed Black Holes by Dust in the Early Universe

We explore the effect of dust on the growth of seed black holes (BHs) in the early universe. Previous 1D radiation-hydrodynamic (RHD) simulations show that increased radiation pressure on dust further suppresses the accretion rate than the case for the chemically pristine gas. Using the Enzo+Moray code, we perform a suite of 3D RHD simulations of accreting BHs in a dusty interstellar medium (ISM). We use the modified Grackle cooling library to consider dust physics in its nonequilibrium chemistry. The BH goes through an early evolutionary phase, where ionizing BH radiation creates an oscillating H ii region as it cycles between accretion and feedback. As the simulations proceed, dense cold gas accumulates outside the ionized region where inflow from the neutral medium meets the outflow driven by radiation pressure. In the late phase, high-density gas streams develop and break the quasi-spherical symmetry of the ionized region, rapidly boosting the accretion rate. The late phase is characterized by the coexistence of strong ionized outflows and fueling high-density gas inflows. The mean accretion rate increases with metallicity reaching a peak at Z ∼ 0.01–0.1 Z ☉, one order of magnitude higher than the one for pristine gas. However, as the metallicity approaches the solar abundance, the mean accretion rate drops as the radiation pressure becomes strong enough to drive out the high-density gas. Our results indicate that a dusty metal-poor ISM can accelerate the growth rate of BHs in the early universe, but can also stun its growth as the ISM is further enriched toward the solar abundance.

Spatially resolved gas-phase metallicity in FIRE-2 dwarfs: late-time evolution of metallicity relations in simulations with feedback and mergers

ABSTRACT We present an analysis of spatially resolved gas-phase metallicity relations in five dwarf galaxies ($\rm \mathit{M}_{halo} \approx 10^{11}\, {\rm M}_\odot$, $\rm \mathit{M}_\star \approx 10^{8.8}{-}10^{9.6}\, {\rm M}_\odot$) from the FIRE-2 (Feedback in Realistic Environments) cosmological zoom-in simulation suite, which include an explicit model for sub-grid turbulent mixing of metals in gas, near z ≈ 0, over a period of 1.4 Gyr, and compare our findings with observations. While these dwarf galaxies represent a diverse sample, we find that all simulated galaxies match the observed mass–metallicity (MZR) and mass–metallicity gradient (MZGR) relations. We note that in all five galaxies, the metallicities are effectively identical between phases of the interstellar medium (ISM), with 95 ${{\ \rm per\ cent}}$ of the gas being within ±0.1 dex between the cold and dense gas (T &amp;lt; 500 K and nH &amp;gt; 1 cm−3), ionized gas (near the H αT ≈ 104 K ridge-line), and nebular regions (ionized gas where the 10 Myr-averaged star formation rate is non-zero). We find that most of the scatter in relative metallicity between cold dense gas and ionized gas/nebular regions can be attributed to either local starburst events or metal-poor inflows. We also note the presence of a major merger in one of our galaxies, m11e, with a substantial impact on the metallicity distribution in the spatially resolved map, showing two strong metallicity peaks and triggering a starburst in the main galaxy.

SIRIUS Project – IV. The formation history of the Orion Nebula Cluster driven by clump mergers

ABSTRACT The Orion Nebula Cluster (ONC) is an excellent example for understanding the formation of star clusters. Recent studies have shown that ONC has three distinct age populations and anisotropy in velocity dispersions, which are key characteristics for understanding the formation history of the ONC. In this study, we perform a smoothed-particle hydrodynamics/N-body simulation of star cluster formation from a turbulent molecular cloud. In this simulation, stellar orbits are integrated using a high-order integrator without gravitational softening; therefore, we can follow the collisional evolution of star clusters. We find that hierarchical formation causes episodic star formation that is observed in the ONC. In our simulation, star clusters evolve due to mergers of subclumps. The mergers bring cold gas with the clumps into the forming cluster. This enhances the star formation in the cluster centre. The dense cold gas in the cluster centre continues to form stars until the latest time. This explains the compact distribution of the youngest stars observed in the ONC. Subclump mergers also contribute to the anisotropy in the velocity dispersions and the formation of runaway stars. However, the anisotropy disappears within 0.5 Myr. The virial ratio of the cluster also increases after a merger due to the runaways. These results suggest that the ONC recently experienced a clump merger. We predict that most runaways originated from the ONC have already been found, but walkaways have not.

Cosmic rays in molecular clouds probed by H2 rovibrational lines

Context. Low-energy cosmic rays (&lt;1 TeV) play a fundamental role in the chemical and dynamical evolution of molecular clouds, as they control the ionisation, dissociation, and excitation of H2. Their characterisation is therefore important both for the interpretation of observations and for the development of theoretical models. However, the methods used so far for estimating the cosmic-ray ionisation rate in molecular clouds have several limitations due to uncertainties in the adopted chemical networks. Aims. We refine and extend a previously proposed method to estimate the cosmic-ray ionisation rate in molecular clouds by observing rovibrational transitions of H2 at near-infrared wavelengths, which are mainly excited by secondary cosmic-ray electrons. Methods. Combining models of interstellar cosmic-ray propagation and attenuation in molecular clouds with the rigorous calculation of the expected secondary electron spectrum and updated electron-H2 excitation cross sections, we derive the intensity of the four H2 rovibrational transitions observable in cold dense gas: (1−0)O(2), (1−0)Q(2), (1−0)S(0), and (1−0)O(4). Results. The proposed method allows the estimation of the cosmic-ray ionisation rate for a given observed line intensity and H2 column density. We are also able to deduce the shape of the low-energy cosmic-ray proton spectrum impinging upon the molecular cloud. In addition, we present a look-up plot and a web-based application that can be used to constrain the low-energy spectral slope of the interstellar cosmic-ray proton spectrum. We finally comment on the capability of the James Webb Space Telescope to detect these near-infrared H2 lines, which will make it possible to derive, for the first time, spatial variation in the cosmic-ray ionisation rate in dense gas. Besides the implications for the interpretation of the chemical-dynamic evolution of a molecular cloud, it will finally be possible to test competing models of cosmic-ray propagation and attenuation in the interstellar medium, as well as compare cosmic-ray spectra in different Galactic regions.

SOFIA/GREAT observations of OD and OH rotational lines towards high-mass star forming regions

Context. Only recently, OD, the deuterated isotopolog of hydroxyl, OH, has become accessible in the interstellar medium; spectral lines from both species have been observed in the supra-Terahertz and far infrared regime. Studying variations of the OD/OH abundance amongst different types of sources can deliver key information on the formation of water, H2O. Aims. With observations of rotational lines of OD and OH towards 13 Galactic high-mass star forming regions, we aim to constrain the OD abundance and infer the deuterium fractionation of OH in their molecular envelopes. For the best studied source in our sample, G34.26+0.15, we were able to perform detailed radiative transfer modelling to investigate the OD abundance profile in its inner envelope. Methods. We used the Stratospheric Observatory for Infrared Astronomy (SOFIA) to observe the 2Π3∕2 J = 5∕2−3∕2 ground-state transition of OD at 1.3 THz (215 μm) and the rotationally excited OH line at 1.84 THz (163 μm). We also used published high-spectral-resolution SOFIA data of the OH ground-state transition at 2.51 THz (119.3 μm). Results. Absorption from the 2Π3∕2 OD J = 5∕2−3∕2 ground-state transition is prevalent in the dense clumps surrounding active sites of high-mass star formation. Our modelling suggests that part of the absorption arises from the denser inner parts, while the bulk of it as seen with SOFIA originates in the outer, cold layers of the envelope for which our constraints on the molecular abundance suggest a strong enhancement in deuterium fractionation. We find a weak negative correlation between the OD abundance and the bolometric luminosity to mass ratio, an evolutionary indicator, suggesting a slow decrease of OD abundance with time. A comparison with HDO shows a similarly high deuterium fractionation for the two species in the cold envelopes, which is of the order of 0.48% for the best studied source, G34.26+0.15. Conclusions. Our results are consistent with chemical models that favour rapid exchange reactions to form OD in the dense cold gas. Constraints on the OD/OH ratio in the inner envelope could further elucidate the water and oxygen chemistry near young high-mass stars.

The AGN fuelling/feedback cycle in nearby radio galaxies – IV. Molecular gas conditions and jet–ISM interaction in NGC 3100

ABSTRACT This is the fourth paper of a series investigating the AGN fuelling/feedback processes in a sample of 11 nearby low-excitation radio galaxies (LERGs). In this paper, we present follow-up Atacama Large Millimeter/submillimeter Array (ALMA) observations of one source, NGC 3100, targeting the 12CO(1-0), 12CO(3-2), HCO+(4-3), SiO(3-2), and HNCO(6-5) molecular transitions. 12CO(1-0) and 12CO(3-2) lines are nicely detected and complement our previous 12CO(2-1) data. By comparing the relative strength of these three CO transitions, we find extreme gas excitation conditions (i.e. Tex ≳ 50 K) in regions that are spatially correlated with the radio lobes, supporting the case for a jet–ISM interaction. An accurate study of the CO kinematics demonstrates that although the bulk of the gas is regularly rotating, two distinct non-rotational kinematic components can be identified in the inner gas regions: one can be associated to inflow/outflow streaming motions induced by a two-armed spiral perturbation; the second one is consistent with a jet-induced outflow with vmax ≈ 200 km s−1 and $\dot{M}\lesssim 0.12$ M⊙ yr−1. These values indicate that the jet-CO coupling ongoing in NGC 3100 is only mildly affecting the gas kinematics, as opposed to what expected from existing simulations and other observational studies of (sub-)kpc scale jet–cold gas interactions. HCO+(4-3) emission is tentatively detected in a small area adjacent to the base of the northern radio lobe, possibly tracing a region of jet-induced gas compression. The SiO(3-2) and HNCO(6-5) shock tracers are undetected: this – along with the tentative HCO+(4-3) detection – may be consistent with a deficiency of very dense (i.e. ncrit &amp;gt; 106 cm−3) cold gas in the central regions of NGC 3100.

Resolving the Formation of Cold H i Filaments in the High-velocity Cloud Complex C

Abstract The physical properties of galactic halo gas have a profound impact on the life cycle of galaxies. As gas travels through a galactic halo, it undergoes dynamical interactions, influencing its impact on star formation and the chemical evolution of the galactic disk. In the Milky Way halo, considerable effort has been made to understand the spatial distribution of neutral gas, which is mostly in the form of large complexes. However, the internal variations of their physical properties remain unclear. In this study, we investigate the thermal and dynamical state of the neutral gas in high-velocity clouds. High-resolution observations (1.′1) of the 21 cm line emission in the EN field of the DHIGLS H i survey are used to analyze the physical properties of the bright concentration CIB located at an edge of a large HVC complex, complex C. We use the Gaussian decomposition code ROHSA to model the multiphase content of CIB and perform a power spectrum analysis to analyze its multiscale structure. The physical properties of some 200 structures extracted using dendrograms are examined. Each phase exhibits different thermal and turbulent properties. We identify two distinct regions, one of which has a prominent protrusion extending from the edge of complex C that exhibits an ongoing phase transition from warm diffuse gas to cold dense gas and filaments. The scale at which the warm gas becomes unstable and undergoes thermal condensation is about 15 pc, corresponding to a cooling time of about 1.5 Myr. Our study characterizes the statistical properties of turbulence in the fluid of an HVC for the first time. We find that a transition from subsonic to transonic turbulence is associated with the thermal condensation, going from large to small scales. A large-scale perspective of complex C suggests that hydrodynamic instabilities are involved in creating the structured concentration CIB and the phase transition therein. However, the details of the dynamical and thermal processes remain unclear and will require further investigation through both observations and numerical simulations.

COALAS

We report a detailed CO(1−0) survey of a galaxy protocluster field at z = 2.16, based on 475 h of observations with the Australia Telescope Compact Array. We constructed a large mosaic of 13 individual pointings, covering an area of 21 arcmin2 and ±6500 km s−1 range in velocity. We obtained a robust sample of 46 CO(1−0) detections spanning z = 2.09 − 2.22, constituting the largest sample of molecular gas measurements in protoclusters to date. The CO emitters show an overdensity at z = 2.12 − 2.21, suggesting a galaxy super-protocluster or a protocluster connected to large-scale filaments of ∼120 cMpc in size. We find that 90% of CO emitters have distances &gt;0.′5−4′ to the center galaxy, indicating that small area surveys would miss the majority of gas reservoirs in similar structures. Half of the CO emitters have velocities larger than escape velocities, which appears gravitationally unbound to the cluster core. These unbound sources are barely found within the R200 radius around the center, which is consistent with a picture in which the cluster core is collapsed while outer regions are still in formation. Compared to other protoclusters, this structure contains a relatively higher number of CO emitters with relatively narrow line widths and high luminosities, indicating galaxy mergers. We used these CO emitters to place the first constraint on the CO luminosity function and molecular gas density in an overdense environment. The amplitude of the CO luminosity function is 1.6 ± 0.5 orders of magnitude higher than that observed for field galaxy samples at z ∼ 2, and one order of magnitude higher than predictions for galaxy protoclusters from semi-analytical SHARK models. We derive a high molecular gas density of 0.6 − 1.3 × 109 M⊙ cMpc−3 for this structure, which is consistent with predictions for cold gas density of massive structures from hydro-dynamical DIANOGA simulations.

The Critical Dark Matter Halo Mass for Population III Star Formation: Dependence on Lyman–Werner Radiation, Baryon-dark Matter Streaming Velocity, and Redshift

Abstract A critical dark matter halo mass (M crit) for Population III stars can be defined as the typical minimum halo mass that hosts sufficient cold-dense gas required for the formation of the first stars. The presence of Lyman–Werner (UV) radiation, which can dissociate molecular hydrogen, and the baryon-dark matter streaming velocity both delay the formation of Population III stars by increasing M crit. In this work, we constrain M crit as a function of Lyman–Werner flux (including self-shielding), baryon-dark matter streaming, and redshift using cosmological simulations with a large sample of halos utilizing the adaptive mesh refinement code enzo. We provide a fit for M crit as a function of these quantities, which we expect to be particularly useful for semi-analytical models of early galaxy formation. In addition, we find (i) the measured redshift dependence of M crit in the absence of radiation or streaming is (1 + z)−1.58, consistent with a constant virial temperature; (ii) increasing the UV background increases M crit while steepening the redshift dependence, up to (1 + z)−5.7; (iii) baryon-dark matter streaming boosts M crit but flattens the dependence on redshift; (iv) the combination of the two effects is not simply multiplicative.

Signatures of large-scale cold fronts in the optically-selected merging cluster HSC J085024+001536

Abstract We represent a joint X-ray, weak-lensing, and optical analysis of the optically-selected merging cluster, HSC J085024+001536, from the Subaru HSC-SSP survey. Both the member galaxy density and the weak-lensing mass map show that the cluster is composed of south-east and north-west components. The two-dimensional weak-lensing analysis shows that the south-east component is the main cluster, and the sub-cluster and main cluster mass ratio is $0.32^{+0.75}_{-0.23}$. The north-west sub-cluster is offset by ∼700 kpc from the main cluster center, and their relative line-of-sight velocity is ∼1300 km s−1 from spectroscopic redshifts of member galaxies. The X-ray emission is concentrated around the main cluster, while the gas mass fraction within a sphere of 1′ radius of the sub-cluster is only $f_{\mathrm{gas}}=4.0^{+2.3}_{-3.3}\%$, indicating that the sub-cluster gas was stripped by ram pressure. An X-ray residual image shows three arc-like excess patterns, of which two are symmetrically located at ∼550 kpc from the X-ray morphological center, and the other is close to the X-ray core. The excess close to the sub-cluster has a cold-front feature where dense-cold gas and thin-hot gas contact. The two outer excesses are tangentially elongated about ∼450–650 kpc, suggesting that the cluster is merged with a non-zero impact parameter. Overall features revealed by the multi-wavelength datasets indicate that the cluster is at the second impact or later. Since the optically-defined merger catalog is unbiased for merger boost of the intracluster medium, X-ray follow-up observations will pave the way to understand merger physics at various phases.

Weighing the Galactic disk in sub-regions of the solar neighbourhood using Gaia DR2

Aims. We infer the gravitational potential of the Galactic disk by analysing the phase-space densities of 120 stellar samples in 40 spatially separate sub-regions of the solar neighbourhood, using Gaia’s second data release (DR2), in order to quantify spatially dependent systematic effects that bias this type of measurement. Methods. The gravitational potential was inferred under the assumption of a steady state in the framework of a Bayesian hierarchical model. We performed a joint fit of our stellar tracers’ three-dimensional velocity distribution, while fully accounting for the astrometric uncertainties of all stars as well as dust extinction, and we also masked angular areas of known open clusters. The inferred gravitational potential is compared, post-inference, to a model for the baryonic matter and halo dark matter components. Results. We see an unexpected but clear trend for all 40 spatially separate sub-regions: Compared to the potential derived from the baryonic model, the inferred gravitational potential is significantly steeper close to the Galactic mid-plane (≲60 pc), but flattens such that the two agree well at greater distances (∼400 pc). The inferred potential implies a total matter density distribution that is highly concentrated to the Galactic mid-plane and decays quickly with height. We see a dependence on the Galactic radius that is consistent with a disk scale length of a few kiloparsecs. Apart from this, there are discrepancies between stellar samples, implying spatially dependent systematic effects which are, at least in part, explained by substructures in the phase-space distributions. Conclusions. In terms of the inferred matter density distribution, the very low matter density that is inferred at greater heights (≳300 pc) is inconsistent with the observed scale height and matter distribution of the stellar disk, which cannot be explained by a misunderstood density of cold gas or other hidden mass. Our interpretation is that these results must be biased by a time-varying phase-space structure, possibly a breathing mode, that is large enough to affect all stellar samples in the same manner.

Early Science from POSSUM: Shocks, turbulence, and a massive new reservoir of ionised gas in the Fornax cluster

AbstractWe present the first Faraday rotation measure (RM) grid study of an individual low-mass cluster—the Fornax cluster—which is presently undergoing a series of mergers. Exploiting commissioning data for the POlarisation Sky Survey of the Universe’s Magnetism (POSSUM) covering a${\sim}34$square degree sky area using the Australian Square Kilometre Array Pathfinder (ASKAP), we achieve an RM grid density of${\sim}25$RMs per square degree from a 280-MHz band centred at 887 MHz, which is similar to expectations for forthcoming GHz-frequency${\sim}3\pi$-steradian sky surveys. These data allow us to probe the extended magnetoionic structure of the cluster and its surroundings in unprecedented detail. We find that the scatter in the Faraday RM of confirmed background sources is increased by$16.8\pm2.4$rad m−2within 1$^\circ$(360 kpc) projected distance to the cluster centre, which is 2–4 times larger than the spatial extent of the presently detectable X-ray-emitting intracluster medium (ICM). The mass of the Faraday-active plasma is larger than that of the X-ray-emitting ICM and exists in a density regime that broadly matches expectations for moderately dense components of the Warm-Hot Intergalactic Medium. We argue that forthcoming RM grids from both targeted and survey observations may be a singular probe of cosmic plasma in this regime. The morphology of the global Faraday depth enhancement is not uniform and isotropic but rather exhibits the classic morphology of an astrophysical bow shock on the southwest side of the main Fornax cluster, and an extended, swept-back wake on the northeastern side. Our favoured explanation for these phenomena is an ongoing merger between the main cluster and a subcluster to the southwest. The shock’s Mach angle and stand-off distance lead to a self-consistent transonic merger speed with Mach 1.06. The region hosting the Faraday depth enhancement also appears to show a decrement in both total and polarised radio emission compared to the broader field. We evaluate cosmic variance and free-free absorption by a pervasive cold dense gas surrounding NGC 1399 as possible causes but find both explanations unsatisfactory, warranting further observations. Generally, our study illustrates the scientific returns that can be expected from all-sky grids of discrete sources generated by forthcoming all-sky radio surveys.

Spatially resolved star formation and fuelling in galaxy interactions

ABSTRACT We investigate the spatial structure and evolution of star formation and the interstellar medium (ISM) in interacting galaxies. We use an extensive suite of parsec-scale galaxy-merger simulations (stellar mass ratio = 2.5:1), which employs the ‘Feedback In Realistic Environments-2’ model (fire-2). This framework resolves star formation, feedback processes, and the multiphase structure of the ISM. We focus on the galaxy-pair stages of interaction. We find that close encounters substantially augment cool (H i) and cold-dense (H2) gas budgets, elevating the formation of new stars as a result. This enhancement is centrally concentrated for the secondary galaxy, and more radially extended for the primary. This behaviour is weakly dependent on orbital geometry. We also find that galaxies with elevated global star formation rate (SFR) experience intense nuclear SFR enhancement, driven by high levels of either star formation efficiency (SFE) or available cold-dense gas fuel. Galaxies with suppressed global SFR also contain a nuclear cold-dense gas reservoir, but low SFE levels diminish SFR in the central region. Concretely, in the majority of cases, SFR enhancement in the central kiloparsec is fuel-driven (55 per cent for the secondary, 71 per cent for the primary) – while central SFR suppression is efficiency-driven (91 per cent for the secondary, 97 per cent for the primary). Our numerical predictions underscore the need of substantially larger, and/or merger-dedicated, spatially resolved galaxy surveys – capable of examining vast and diverse samples of interacting systems – coupled with multiwavelength campaigns aimed to capture their internal ISM structure.

VLA–ALMA Spectroscopic Survey in the Hubble Ultra Deep Field (VLASPECS): Total Cold Gas Masses and CO Line Ratios for z = 2–3 Main-sequence Galaxies

Abstract Using the NSF’s Karl G. Jansky Very Large Array (VLA), we report six detections of CO(J = 1 → 0) emission and one upper limit in z = 2–3 galaxies originally detected in higher-J CO emission in the Atacama Large Millimeter/submillimeter Array Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS). From the CO(J = 1 → 0) line strengths, we measure total cold molecular gas masses of M gas = (2.4–11.6) × 1010 (α CO/3.6)M ⊙. We also measure a median CO(J = 3 → 2) to CO(J = 1 → 0) line brightness temperature ratio of r 31 = 0.84 ± 0.26, and a CO(J = 7 → 6) to CO(J = 1 → 0) ratio range of r 71 &lt; 0.05 to r 71 = 0.17. These results suggest that CO(J = 3 → 2) selected galaxies may have a higher CO line excitation on average than CO(J = 1 → 0) selected galaxies, based on the limited, currently available samples from the ASPECS and VLA CO Luminosity Density at High Redshift (COLDz) surveys. This implies that previous estimates of the cosmic density of cold gas in galaxies based on CO(J = 3 → 2) measurements should be revised down by a factor of ≃2 on average based on assumptions regarding CO excitation alone. This correction further improves the agreement between the best currently existing constraints on the cold gas density evolution across cosmic history from line scan surveys, and the implied characteristic gas depletion times.