Cold Disk Research Articles

A Milky Way-like barred spiral galaxy at a redshift of 3

The majority of massive disk galaxies in the local Universe show a stellar barred structure in their central regions, including our Milky Way1,2. Bars are supposed to develop in dynamically cold stellar disks at low redshift, as the strong gas turbulence typical of disk galaxies at high redshift suppresses or delays bar formation3,4. Moreover, simulations predict bars to be almost absent beyond z = 1.5 in the progenitors of Milky Way-like galaxies5,6. Here we report observations of ceers-2112, a barred spiral galaxy at redshift zphot ≈ 3, which was already mature when the Universe was only 2 Gyr old. The stellar mass (M★ = 3.9 × 109 M⊙) and barred morphology mean that ceers-2112 can be considered a progenitor of the Milky Way7–9, in terms of both structure and mass-assembly history in the first 2 Gyr of the Universe, and was the closest in mass in the first 4 Gyr. We infer that baryons in galaxies could have already dominated over dark matter at z ≈ 3, that high-redshift bars could form in approximately 400 Myr and that dynamically cold stellar disks could have been in place by redshift z = 4–5 (more than 12 Gyrs ago)10,11.

Local models of two-temperature accretion disc coronae – I. Structure, outflows, and energetics

ABSTRACT We use local stratified shearing-box simulations to elucidate the impact of two-temperature thermodynamics on the thermal structure of coronae in radiatively efficient accretion flows. Rather than treating the coronal plasma as an isothermal fluid, we use a simple, parametrized cooling function that models the collisional transfer of energy from the ions to the rapidly cooling leptons. Two-temperature models naturally form temperature inversions, with a hot, magnetically dominated corona surrounding a cold disc. Simulations with net vertical flux (NF) magnetic fields launch powerful magnetocentrifugal winds that would enhance accretion in a global system. The outflow rates are much better converged with increasing box height than analogous isothermal simulations, suggesting that the winds into two-temperature coronae may be sufficiently strong to evaporate a thin disc and form a radiatively inefficient accretion flow under some conditions. We find evidence for multiphase structure in the corona, with broad density and temperature distributions, and we propose criteria for the formation of a multiphase corona. The fraction of cooling in the surface layers of the disc is substantially larger for NF fields compared to zero net-flux configurations, with moderate NF simulations radiating ≳30 per cent of the flow’s total luminosity above two mid-plane scale heights. Our work shows that NF fields may efficiently power the coronae of luminous Seyfert galaxies and quasars, providing compelling motivation for future studies of the heating mechanisms available to NF fields and the interplay of radiation with two-temperature thermodynamics.

Accretion flows in the hard state of black hole X-ray binaries: the effect of hot gas condensation

ABSTRACT It is commonly believed that accretion discs are truncated and their inner regions are described by advection dominated accretion flows (ADAFs) in the hard spectral state of black hole X-ray binaries. However, the increasing occurrence of a relativistically blurred Fe K α line together with a hard continuum points to the existence of a thin disc located near the innermost stable circular orbit (ISCO). Assuming the accretion in the hard state is via an ADAF extending to near 100 Schwarzschild radii, which is supplied by either a stellar wind from a companion star or resulting from an evaporated disc, we study the possible condensation of the hot gas during its accretion towards the black hole. It is found that a small fraction of the ADAF condenses into a cold disc as a consequence of efficient radiative cooling at small distances, forming a disc-corona configuration near the ISCO. This takes place for low accretion rates corresponding to luminosities ranging from ∼10−3 to a few per cent of the Eddington luminosity. The coexistence of the weak inner disc and the dominant hot accretion flow provides a natural explanation of the broad K α line in the hard state. Detailed computations demonstrate that such accretion flows produce a hard X-ray spectrum accompanied by a weak disc component with a negative correlation between the 2 and 10 keV photon index and the Eddington ratio. The predicted spectrum of Cygnus X-1 and the correlation between the photon index and the Eddington ratio are in good agreement with observations.

Radiation hydrodynamical self-similar accretion disc winds

ABSTRACT Self-similar supersonic winds from an accretion disc around a central object are examined with/without radiation fields under the fully self-similar treatment of two-dimensional outflows. The thermal pressure and viscosity are ignored for simplicity. In the case without radiation fields, many solutions are not outflowing winds, but bound flows, except for limited cases which satisfy the escape condition with sufficiently large initial launch speed and rotational one. In the case with radiation fields, the radiation force acts as a driving force, and therefore, the winds blow easier. However, several solutions are unphysical, since the radiative energy density becomes negative. As a result, depending on the rotational motion and boundary conditions, the radiatively driven self-similar cold disc winds are divided into three types; bound solutions lacking launch speed or radiative acceleration, unphysical ones with a negative energy density, and escaping winds confined in the conical region or extending in the whole space as a special type.

Barless flocculent galaxies: a dynamic puzzle

ABSTRACT We draw attention to the bright galaxies that do not show a bar in their structure but have a flocculent spiral structure. Using the THINGS and HERACLES kinematic data for four barless galaxies (NGC 2841, NGC 3512, NGC 5055, NGC 7331), we built their mass models including dark haloes. We concluded that the fraction of the dark matter does not exceed 50 per cent within the optical radii of the galaxies. This is too little to explain the lack of a bar in these galaxies. In an attempt to understand the featureless structure of these galaxies, we constructed several N-body models with an initially reduced content of dark matter. We concluded that, in addition to the low mass of the dark halo, the decisive factor that leads to a barless disc is the start from an initially unstable state. An isolated dynamically cold disc (with the Toomre parameter Q < 0.5) settled into rotational equilibrium passes through the short stage of violent instability with fragmentation and formation of stellar clumps. After that, it evolves passively and ends up with a featureless structure. We assume that the barless flocculent galaxies studied in this work may be descendants of galaxies at high redshifts with rotation curves that are consistent with the high mass fraction of baryons relative to the total dark matter halo.

Mass Accretion, Spectral, and Photometric Properties of T Tauri Stars in Taurus Based on TESS and LAMOST

We present the analysis of 16 classical T Tauri stars (CTTSs) using LAMOST and TESS data, investigating spectral properties, photometric variations, and mass accretion rates. All 16 stars exhibit emissions in Hα lines, from which the average mass accretion rate of 1.76 × 10−9 M ⊙ yr−1 is derived. Two of the stars, DL Tau and Haro 6-13, show mass accretion bursts simultaneously in TESS, ASAS-SN, and/or the ZTF survey. Based on these observations, we find that the mass accretion rates of DL Tau and Haro 6-13 reach their maxima of 2.5 × 10−8 M ⊙ yr−1 and 2 × 10−10 M ⊙ yr−1, respectively, during the TESS observation. We detect 13 flares among these stars. The flare frequency distribution shows that the CTTSs’ flare activity is not only dominated by strong flares with high energy but also much more active than those of solar-type and young low-mass stars. By comparing the variability classes reported in the literature, we find that the transition timescale between different classes of variability in CTTSs, such as from stochastic (S) to bursting (B) or from quasi-periodic symmetric to quasi-periodic dipping, may range from 1.6 to 4 yr. We observe no significant correlation between inclination and mass accretion rates derived from the emission indicators. This suggests that inner disk properties may be more important than those of outer disks. Finally, we find a relatively significant positive correlation between the asymmetric metric M and the cold disk inclination compared to the literature. A weak negative correlation between the periodicity metric Q value and inclination has also been found.

Dwarfs in Void Environments (DIVE): The Stellar Kinematics of Void Dwarf Galaxies Using the Keck Cosmic Web Imager

Dwarf galaxies located in extremely underdense cosmic voids are excellent test beds for disentangling the effects of large-scale environments on galaxy formation and evolution. We present the first results of the Dwarfs in Void Environments Survey, which has obtained integral field spectroscopy for low-mass galaxies (M ⋆ = 107–109 M ⊙) located inside (N = 21) and outside (N = 9) cosmic voids using the Keck Cosmic Web Imager. Using measurements of stellar line-of-sight rotational velocity v rot and velocity dispersion σ ⋆, we test the tidal stirring hypothesis, which posits that dwarf spheroidal galaxies are formed through tidal interactions with more massive host galaxies. We measure low values of v rot/σ ⋆ ≲ 2 for our sample of isolated dwarf galaxies, and we find no trend between v rot/σ ⋆ and the distance from a massive galaxy out to Mpc. These suggest that dwarf galaxies can become dispersion-supported, “puffy” systems even in the absence of environmental effects like tidal interactions. We also find indications of an upward trend between v rot/σ ⋆ and galaxy stellar mass, perhaps implying that stellar disk formation depends on mass rather than environment. Although some of our conclusions may be slightly modified by systematic effects, our main result still holds: that isolated low-mass galaxies may form and remain as puffy systems rather than the dynamically cold disks predicted by classical galaxy formation theory.

Study of recently discovered Be/X-ray pulsar MAXI J0655-013 using NuSTAR

Abstract We study the recently discovered Be/X-ray pulsar MAXI J0655–013 using the 2022 NuSTAR observations. The paper is the first detailed study of the timing and spectral properties of the source. The pulse profiles of the pulsar vary with energy. The pulsed fraction is found to increase monotonically with energy. In between the two NuSTAR observations, a large spin-up rate of ∼ -1.23 s d−1 is observed, which can be due to large spin-up torque acting on the pulsar during an outburst. Such a large spin-up rate is observed for the first time in an X-ray pulsar during an outburst. The variation of the spin period with time can be employed to obtain the orbital parameters of the binary system, and we found the orbital period to be ∼ 27.9 d. The second NuSTAR observation is done in a low luminosity state (LX ∼ 3.9 × 1034 erg s−1 ). We have detected the pulsation of the pulsar in such a low luminosity state. In such a low luminosity state, the pulsar MAXI J0655–013 might be accreting from the cold disk.

Atomic Hydrogen in the Milky Way: A Stepping Stone in the Evolution of Galaxies

Atomic hydrogen (Hi) is a critical stepping stone in the gas evolution cycle of the interstellar medium (ISM) of the Milky Way. Hi traces both the cold, premolecular state before star formation and the warm, diffuse ISM before and after star formation. This review describes new, sensitive Hi absorption and emission surveys, which, together with high angular and spectral resolution Hi emission data, have revealed the physical properties of Hi, its structure, and its association with magnetic fields. We give an overview of the Hi phases and discuss how Hi properties depend on the environment and what its structure can tell us about feedback in the ISM. Key findings include the following: ▪The mass fraction of the cold neutral medium is ≲40% on average, increasing with A V due to the increase of mean gas density.▪The cold disk extends to at least R ∼ 25 kpc.▪Approximately 40% of the Hi is warm, with structural characteristics that derive from feedback events.▪Cold Hi is highly filamentary, whereas warm Hi is more smoothly distributed. We summarize future observational and simulation opportunities that can be used to unravel the 3D structure of the atomic ISM and the effects of heating and cooling on Hi properties.

The impact of early massive mergers on the chemical evolution of Milky Way-like galaxies: insights from NIHAO-UHD simulations

ABSTRACT Recent observations of the Milky Way (MW) found an unexpected steepening of the star-forming gas metallicity gradient around the time of the Gaia–Sausage–Enceladus (GSE) merger event. Here, we investigate the influence of early (tmerger ≲ 5 Gyr) massive ($M_{\rm {gas}}^{\rm {merger}}/M_{\rm {gas}}^{\rm {main}}(t_{\rm {merger}})\gtrsim 10~{{\ \rm per\ cent}}$) merger events such as the Gaia–Sausage–Enceladus merger in the MW on the evolution of the cold gas metallicity gradient. We use the NIHAO-UHD suite of cosmological hydrodynamical simulations of MW-mass galaxies to study the frequency of massive early mergers and their detailed impact on the morphology and chemistry of the gaseous discs. We find a strong steepening of the metallicity gradient at early times for all four galaxies in our sample which is caused by a sudden increase in the cold gas disc size (up to a factor of 2) in combination with the supply of unenriched gas (∼0.75 dex lower compared to the main galaxy) by the merging dwarf galaxies. The mergers mostly affect the galaxy outskirts and lead to an increase in cold gas surface density of up to 200 per cent outside of ∼8 kpc. The addition of unenriched gas breaks the self-similar enrichment of the inter-stellar-medium and causes a dilution of the cold gas in the outskirts of the galaxies. The accreted stars and the ones formed later out of the accreted gas inhabit distinct tracks offset to lower [α/Fe] and [Fe/H] values compared to the main galaxy’s stars. We find that such mergers can contribute significantly to the formation of a second, low-α sequence as is observed in the MW.

NGC 1436: the making of a lenticular galaxy in the Fornax Cluster

ABSTRACT We study the evolutionary path of the Fornax Cluster galaxy NGC 1436, which is known to be currently transitioning from a spiral into a lenticular morphology. This galaxy hosts an inner star-forming disc and an outer quiescent disc, and we analyse data from the MeerKAT Fornax Survey, ALMA, and the Fornax 3D survey to study the interstellar medium and the stellar populations of both disc components. Thanks to the combination of high resolution and sensitivity of the MeerKAT data, we find that the H i is entirely confined within the inner star-forming disc, and that its kinematics is coincident with that of the CO. The cold gas disc is now well settled, which suggests that the galaxy has not been affected by any environmental interactions in the last ∼1 Gyr. The star-formation history derived from the Fornax 3D data shows that both the inner and outer discs experienced a burst of star formation ∼5 Gyr ago, followed by rapid quenching in the outer disc and by slow quenching in the inner disc, which continues forming stars to this day. We claim that NGC 1436 has begun to effectively interact with the cluster environment 5 Gyr ago, when a combination of gravitational and hydrodynamical interactions caused the temporary enhancement of the star-formation rate. Furthermore, due to the weaker gravitational binding H i was stripped from the outer disc, causing its rapid quenching. At the same time, accretion of gas onto the inner disc stopped, causing slow quenching in this region.

The Fornax3D project: Environmental effects on the assembly of dynamically cold disks in Fornax cluster galaxies

We apply a population-orbit superposition metho1d to 16 galaxies in the Fornax cluster observed with MUSE/VLT in the context of the Fornax3D project. By fitting the luminosity distribution, stellar kinematics, and age and metallicity maps simultaneously, we obtained the internal stellar orbit distribution, as well as the age and metallicity distribution of stars on different orbits for each galaxy. Based on the model, we decompose each galaxy into a dynamically cold disk (orbital circularity λz ≥ 0.8) and a dynamically hot non-disk component (orbital circularity λz < 0.8), and obtain the surface-brightness, age, and metallicity radial profiles of each component. The galaxy infall time into the cluster is strongly correlated with galaxy cold-disk age with older cold disks in ancient infallers. We quantify the infall time tinfall of each galaxy with its cold-disk age using a correlation calibrated with TNG50 cosmological simulations. For galaxies in the Fornax cluster, we found that the luminosity fraction of cold disk in galaxies with tinfall > 8 Gyr are a factor of ∼4 lower than in more recent infallers while controlling for total stellar mass. Nine of the 16 galaxies have spatially extended cold disks, and most of them show positive or zero age gradients; stars in the inner disk are ∼2 − 5 Gyr younger than that in the outer disk, in contrast to the expectation of inside-out growth. Our results indicate that the assembly of cold disks in galaxies is strongly affected by their infall into clusters, by either removal of gas in outer regions or even tidally stripping or heating part of the pre-existing disks. Star formation in outer disks can stop quickly after the galaxy falls into the cluster, while star formation in the inner disks can last for a few Gyrs more, building the positive age gradient measured in cold disks.

Modified models of radiation pressure instability applied to 10, 105, and 107 M⊙ accreting black holes

Context. Some accreting black holes exhibit much stronger variability patterns than the usual stochastic variations. Radiation pressure instability is one of the proposed mechanisms that might account for this effect. Aims. We model luminosity changes for objects with a black hole mass of 10, 105, and 107 solar masses, using the time-dependent evolution of an accretion disk that is unstable as a result of the dominant radiation pressure. We concentrate on the outburst timescales. We explore the influence of the hot coronal flow above the cold disk, the inner purely hot flow, and the effect of the magnetic field on the time evolution of the disk-corona system. For intermediate-mass black holes and active galactic nuclei, we also explore the role of the disk outer radius because a disk that is fed by tidal disruption events (TDE) can be quite small. Methods. We used a 1D vertically integrated time-dependent numerical scheme that models the simultaneous evolution of the disk and corona, which is coupled by the vertical mass exchange. We parameterized the strength of the large-scale toroidal magnetic fields according to a local accretion rate. We also discuss a possible inner optically thin flow, the advection-dominated accretion flow (ADAF). This flow would require modification of the inner boundary condition of the cold disk flow. For the set of the global parameters, we calculated the variability timescales and outburst amplitudes of the disk and the corona. Results. We found that the role of the inner ADAF and the accreting corona are relatively unimportant, but the outburst character strongly depends on the magnetic field and on the outer radius of the disk if this radius is smaller (due to the TDE phenomenon) than the size of the instability zone in a stationary disk with infinite radius. For microquasars, the dependence on the magnetic field is monotonic, and the period decreases with the field strength. For higher black hole masses, the dependence is nonmonotonic, and an initial rise of the period is later replaced with a relatively rapid decrease as the magnetic field continues to rise. A still stronger magnetic field stabilizes the disk. When we assumed a smaller disk outer radiusfor 105 and 107 M⊙, the outbursts were shorter and led to complex multiscale outbursts for some parameters, thus approaching the behavior of deterministic chaos. Conclusions. Our computations confirm that the radiation pressure instability model can account for heartbeat states in microquasars. The rapid variability detected in intermediate-mass black holes in the form of quasi-periodic eruptions can be consistent with the model, but only when it is combined with the TDE phenomenon. The yearly repeating variability in changing-look active galactic nuclei in our model also requires a small outer radius either due to the recent TDE or due to the gap in the disk that is related to a secondary black hole.

The Apparent Absence of Forward Scattering in the HD 53143 Debris Disk

HD 53143 is a mature Sun-like star and host to a broad disk of dusty debris, including a cold outer ring of planetesimals near 90 au. Unlike most other inclined debris disks imaged at visible wavelengths, the cold disk around HD 53143 appears as disconnected “arcs” of material, with no forward-scattering side detected to date. We present new, deeper Hubble Space Telescope Imaging Spectrograph coronagraphic observations of the HD 53143 debris disk and show that the forward-scattering side of the disk remains undetected. By fitting our KLIP-reduced observations via forward modeling with an optically thin disk model, we show that fitting the visible wavelength images with an azimuthally symmetric disk with unconstrained orientation results in an unphysical edge-on orientation that is at odds with recent ALMA observations, while constraining the orientation to that observed by ALMA results in nearly isotropically scattering dust. We show that the HD 53143 host star exhibits significant stellar variations due to spot rotation and revisit age estimates for this system.

Toward Horizon-scale Accretion onto Supermassive Black Holes in Elliptical Galaxies

We present high-resolution, three-dimensional hydrodynamic simulations of the fueling of supermassive black holes in elliptical galaxies from a turbulent medium on galactic scales, taking M87* as a typical case. The simulations use a new GPU-accelerated version of the Athena++ AMR code, and they span more than six orders of magnitude in radius, reaching scales similar to that of the black hole horizon. The key physical ingredients are radiative cooling and a phenomenological heating model. We find that the accretion flow takes the form of multiphase gas at radii less than about a kpc. The cold gas accretion includes two dynamically distinct stages: the typical disk stage in which the cold gas resides in a rotationally supported disk, and relatively rare chaotic stages (≲10% of the time) in which the cold gas inflows via chaotic streams. Though cold gas accretion dominates the time-averaged accretion rate at intermediate radii, accretion at the smallest radii is dominated by hot virialized gas at most times. The accretion rate scales with radius as when hot gas dominates, and we obtain near the event horizon, similar to what is inferred from EHT observations. The orientation of the cold gas disk can differ significantly on different spatial scales. We propose a subgrid model for accretion in lower-resolution simulations in which the hot gas accretion rate is suppressed relative to the Bondi rate by . Our results can also provide more realistic initial conditions for simulations of black hole accretion at the event horizon scale.

Dynamics of Molecular Gas in the Central Region of the Quasar I Zwicky 1

We present a study of the molecular gas distribution and kinematics in the cicumnuclear region (radii ≲2 kpc) of the z ≈ 0.061 quasar I Zwicky 1 using a collection of available Atacama Large Millimeter/submillimeter Array observations of the carbon monoxide (CO) emission. With an angular resolution of ∼0.″36 (corresponding to ∼400 pc), the host-galaxy substructures including the nuclear molecular gas disk, spiral arms, and a compact bar-like component are resolved. We analyzed the gas kinematics based on the CO image cube and obtained the rotation curve and radial distribution of velocity dispersion. The velocity dispersion is about 30 km s−1 in the outer CO disk region and rises up to ≳100 km s−1 at radius ≲1 kpc, suggesting that the central region of the disk is dynamically hot. We constrain the CO-to-H2 conversion factor, α CO, by modeling the cold gas disk dynamics. We find that, with prior knowledge about the stellar and dark matter components, the α CO value in the circumnuclear region of this quasar host galaxy is , which is between the value reported in ultraluminous infrared galaxies and in the Milky Way. The central 1 kpc region of this quasar host galaxy has significant star formation activity, which can be identified as a nuclear starburst. We further investigate the high-velocity dispersion in the central region. We find that the interstellar medium (ISM) turbulent pressure derived from the gas velocity dispersion is in equilibrium with the weight of the ISM. This argues against extra power from active galactic nuclei feedback that significantly affects the kinematics of the cold molecular gas.

First Look at z > 1 Bars in the Rest-frame Near-infrared with JWST Early CEERS Imaging

Stellar bars are key drivers of secular evolution in galaxies and can be effectively studied using rest-frame near-infrared (NIR) images, which trace the underlying stellar mass and are less impacted by dust and star formation than rest-frame UV or optical images. We leverage the power of JWST CEERS NIRCam images to present the first quantitative identification and characterization of stellar bars at z > 1 based on rest-frame NIR F444W images of high resolution (∼1.3 kpc at z ∼ 1–3). We identify stellar bars in these images using quantitative criteria based on ellipse fits. For this pilot study, we present six examples of robustly identified bars at z > 1 with spectroscopic redshifts, including the two highest-redshift bars at z ∼ 2.136 and 2.312 quantitatively identified and characterized to date. The stellar bars at z ∼ 1.1–2.3 presented in our study have projected semimajor axes of ∼2.9–4.3 kpc and projected ellipticities of ∼0.41–0.53 in the rest-frame NIR. The barred host galaxies have stellar masses ∼1 × 1010 to 2 × 1011 M ⊙ and star formation rates of ∼21–295 M ⊙ yr−1, and several have potential nearby companions. Our finding of bars at z ∼ 1.1–2.3 demonstrates the early onset of such instabilities and supports simulations where bars form early in massive dynamically cold disks. It also suggests that if these bars at lookback times of 8–11 Gyr survive out to present epochs, bar-driven secular processes may operate over a long time and have a significant impact on some galaxies by z ∼ 0.

The jet emitting disk‐standard accretion disk model applied to the active galactic nuclei ultra violet–X‐ray correlation

AbstractThe non‐linear correlation between the UV and X‐ray emission observed in active galactic nuclei remains a puzzling question that challenges accretion models. While the UV emission originates from the cold disk, the X‐ray emission is emitted by a hot corona whose physical characteristics and geometry are still highly debated. The jet emitting disk‐standard accretion disk (JED‐SAD) is a spectral model stemming from self‐similar accretion‐ejection solutions. It is composed of an inner highly magnetized and hot accretion flow launching jets, the JED, and an outer SAD. The model has been successfully applied to X‐ray binaries outbursts. The AGN UV–X‐ray correlation represents another essential test for the JED‐SAD model. We use multiple AGN samples to explore the parameter space and identify the regions able to reproduce the observations. In this first paper, we show that the JED‐SAD model is able to reproduce the UV–X‐ray correlation.

PHANGS–JWST First Results: Rapid Evolution of Star Formation in the Central Molecular Gas Ring of NGC 1365

Large-scale bars can fuel galaxy centers with molecular gas, often leading to the development of dense ringlike structures where intense star formation occurs, forming a very different environment compared to galactic disks. We pair ∼0.″3 (30 pc) resolution new JWST/MIRI imaging with archival ALMA CO(2–1) mapping of the central ∼5 kpc of the nearby barred spiral galaxy NGC 1365 to investigate the physical mechanisms responsible for this extreme star formation. The molecular gas morphology is resolved into two well-known bright bar lanes that surround a smooth dynamically cold gas disk (R gal ∼ 475 pc) reminiscent of non-star-forming disks in early-type galaxies and likely fed by gas inflow triggered by stellar feedback in the lanes. The lanes host a large number of JWST-identified massive young star clusters. We find some evidence for temporal star formation evolution along the ring. The complex kinematics in the gas lanes reveal strong streaming motions and may be consistent with convergence of gas streamlines expected there. Indeed, the extreme line widths are found to be the result of inter-“cloud” motion between gas peaks; ScousePy decomposition reveals multiple components with line widths of 〈σ CO,scouse〉 ≈ 19 km s−1 and surface densities of , similar to the properties observed throughout the rest of the central molecular gas structure. Tailored hydrodynamical simulations exhibit many of the observed properties and imply that the observed structures are transient and highly time-variable. From our study of NGC 1365, we conclude that it is predominantly the high gas inflow triggered by the bar that is setting the star formation in its CMZ.

Disentangling the protoplanetary disk gas mass and carbon depletion through combined atomic and molecular tracers

Context. The total disk gas mass and elemental C, N, and O composition of protoplanetary disks are crucial ingredients for improving our understanding of planetary formation. Measuring the gas mass is complicated, since H2 cannot be detected in the cold bulk of the disk and the elemental abundances with respect to hydrogen are degenerate with gas mass in all disk models. Aims. We aim to determine the gas mass and elemental abundances ratios C/H and O/H in the transition disk around LkCa 15, one of the few disks for which HD data are available, in combination with as many chemical tracers as possible. Methods. We present new NOrthern Extended Millimeter Array observations of CO, 13CO, C18O, and optically thin C17O J = 2−1 lines, along with high angular-resolution Atacama Large Millimeter Array millimeter continuum and CO data to construct a representative model of LkCa 15. Using a grid of 60 azimuthally symmetric thermo-chemical DALI disk models, we translated the observed fluxes to elemental abundances and constrained the best-fitting parameter space of the disk gas mass. Results. The transitions that constrain the gas mass and carbon abundance the most are C17O J = 2−1, N2H+ J = 3−2 and HD J = 1−0. Using these three molecules, we find that the gas mass in the LkCa 15 disk is Mg = 0.01−0.004+0.01 M⊙, which is a factor of 6 lower than previous estimations. This value is consistent with cosmic ray ionization rates between 10−16−10−18 s−1, where 10−18 s−1 is a lower limit based on the HD upper limit. The carbon abundance is C/H = (3 ± 1.5) × 10−5, implying a moderate depletion of elemental carbon by a factor of 3–9. All other analyzed transitions also agree with these numbers, within a modeling uncertainty of a factor of 2. Using the resolved C2H image we find a C/O ratio of ~1, which is consistent with literature values of H2O depletion in this disk. The absence of severe carbon depletion in the LkCa 15 disk is consistent with the young age of the disk, but stands in contrast to the higher levels of depletion seen in older cold transition disks. Conclusions. Combining optically thin CO isotopologue lines with N2H+ is promising with regard to breaking the degeneracy between gas mass and CO abundance. The moderate level of depletion for this source with a cold, but young disk, suggests that long carbon transformation timescales contribute to the evolutionary trend seen in the level of carbon depletion among disk populations, rather than evolving temperature effects and presence of dust traps alone. HD observations remain important for determining the disk’s gas mass.