arXiv subjects

Thomas G. Bisbas

Publications and source records attributed to Thomas G. Bisbas.

At least 19 recordsLinked to original sources

arXiv2026

Resolving dense photodissociation regions: the structure of photochemical fronts in three-dimensional gas distributions

For decades, the Orion Bar has been the prototypical photodissociation region. Viewed nearly edge-on, it offers a unique window into the stratified chemical structure of the atomic-to-molecular transition of the interstellar medium. Understanding its photochemistry is essential to interpreting key observations originating from dense photo-dissociation regions. ALMA and JWST observations reveal that H2 photodissociation front overlaps with the C+ recombination front and exhibits a complex spatial morphology. Despite considerable theoretical effort, existing modeling approaches based on simplified geometrical assumptions have difficulties reproducing the spatial emission structure. Our aim is to investigate the response of photochemistry in realistic three-dimensional density distributions, using the Orion Bar as a representative application. We present the first fully three-dimensional high-resolution model of an Orion Bar analogue that resolves the relevant photochemical fronts using the upgraded steady-state 3D-PDR photo-dissociation region code, which allows for the treatment of plane-irradiatation and for the solution of the non-LTE H2 rovibrational levels. We find that the H2 dissociation front is characterized by a complex surface that overlaps with the C+ recombination front. Our 3D model can reproduce the complex morphology of H2 emission seen in observations, in particular the arc- and filament-like features, and provides a physical explanation of its chemistry. The overlapping H2 dissociation and C+ recombination fronts and the spatial emission morphology can be explained due to the three-dimensional gas distribution, resulting in shadowing and shielding by dense substructures. Our results mark a turning point for astrochemistry, where three-dimensional steady-state models can deliver fundamentally new insights into the chemistry of the interstellar medium. (Abridged)

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arXiv2026

Anomalously high deuterium fractionation in a galactic translucent cloud: a challenge to chemical models

Deuterated (D-) species have long been proposed to diagnose the physical conditions and chemical evolution of cold dense molecular clouds. While deuterium fractionation has been extensively measured in dense cores, observations in diffuse and translucent clouds remain rare. We report here the detection of DCN and DNC toward a translucent cloud ($A_{\rm V} =1.2\pm0.2$ mag, $n_{\rm H_2}$ = $3.9\pm0.2\times10^2$ cm$^{-3}$) through sensitive absorption observations with the IRAM NOrthern Extended Millimeter Array (NOEMA). This detection reaches the lowest column-density and volume-density regime in which deuteration has been observed so far. Interestingly, the observed DCN/HCN and DNC/HNC abundance ratios ($3.3\pm0.6\times10^{-3}$ and $3.6\pm1.2\times10^{-3}$, respectively), which are more than two orders of magnitude higher than the element abundance [D]/[H] (1.5$\times$10$^{-5}$), suggest an unexpected enhancement of deuterium fractionation in the translucent cloud. These results represent a significant departure from established chemical models considering deuterium fractionation, which predict negligible formation of D-molecules in such environments. Although it remains unclear how D-molecules built up their abundances in translucent gas, a dispersed dense core scenario could potentially explain the observed high deuterium fraction. This interpretation is consistent with the idea proposed by Price et al. (2003) more than two decades ago: a translucent cloud may be a transient, dynamically evolving structure formed through the dissipation of a dense molecular cloud.

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arXiv2026

A HINSA view of cosmic-ray ionization in IC 348 and NGC 1333: evidence for a strong low-energy cosmic-ray disparity

The cosmic-ray ionization rate (CRIR) is one of the fundamental parameters influencing the chemical and dynamical evolution of molecular clouds. Although observations in recent years have revealed high CRIR values in massive star-forming regions and in the vicinity of protostars, the sources and acceleration mechanisms of cosmic rays remain uncertain. In this work, we present our new estimates of CRIR using the HI narrow self-absorption (HINSA) technique towards two nearby low-mass star-forming clouds, IC 348 and NGC 1333. In both clouds, the CRIR decreases with increasing H$_2$ column density, but IC 348 exhibits values that are roughly an order of magnitude higher than those in NGC 1333. To interpret this contrast, we model the low-energy spectrum of CRs in a finite slab attenuation framework, using additional constraints from the high-energy CR spectrum inferred from Fermi $\gamma$-ray observations. The best-fit spectra reproduce the observed CRIR profiles and the contrast between IC 348 and NGC 1333 suggests an order of magnitude difference in low-energy CR populations, likely originating from local acceleration sources beyond protostars (e.g., stellar-wind termination shocks), and partly from the same sources responsible for the GeV $\gamma$-ray excess. Although uncertainties in cloud structure and gas density may affect the absolute CRIR values, they do not erase the pronounced disparity between the two regions.

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arXiv2026

Star Formation Drives Production of Low Energy Cosmic Rays

For over a century, the origin of low-energy cosmic rays (LECRs), the dominant heaters and ionizers of dense interstellar gas, remains elusive owing to solar modulation and uncertain transport processes. In this study, we introduce a new astrophysical approach based on HI Narrow Self-Absorption (HINSA) to obtain spatially resolved measurements of LECR ionization rates using high-fidelity HI observations toward the Orion region from the FAST telescope. The LECR ionization rate is found to scale with local star formation rate (SFR) as $log_{10}\zeta = (1.4\pm 0.70)log_{10}\mathrm{SFR} + (-10.5\pm 2.9)$. Moreover, it increases with visual extinction, and is found to exceed, toward active star-forming regions, the value predicted for diffuse regions based on \textit{Voyager} measurements and an external propagation model. These findings demonstrate that LECRs are generated in situ by star-forming activities rather than penetrating from the broader Galactic cosmic-ray population. This is further supported by \textit{Fermi}-LAT gamma-ray observations toward the Orion region. Together, these results resolve a key uncertainty in cosmic-ray origin and establish a new avenue for quantifying the energetic feedback that regulates the interstellar medium.

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arXiv2026

RAYTHEIA: A high-performance ray-tracing algorithm for three-dimensional direction-dependent equations in astronomical simulations

We present RAYTHEIA, a high-performance reverse ray-tracing algorithm designed to efficiently solve three-dimensional direction-dependent equations in astronomical simulations. The algorithm uses a dual-grid framework in which the native simulation mesh -- serving as the source grid for ray emission -- and an adaptive mesh refinement (AMR) Cartesian contribution grid are constructed for efficient ray-walking and contribution accumulation. The core of the algorithm integrates a leaf-only linear-octree data structure to reduce memory overhead, the digital differential analyzer (DDA) traversal method to efficiently determine the ray-walking path, Morton Code indexing to fast leaf cell lookup during traversal, and the slab method to analytically compute the path length. Furthermore, RAYTHEIA employs a hybrid (MPI/OpenMP) distributed parallel framework with a chunk-to-chunk communication strategy, achieving exceptional, near-ideal linear speed-up ratio and delivering high-end performance. We integrate RAYTHEIA with the 3D-PDR code to solve the complex chemistry and radiation transfer in photodissociation regions (PDRs). This allowed the modelling of three-dimensional PDR chemistry in a turbulent, star-forming cloud at an unprecedented resolution of $512^3$ grid cells. The algorithm demonstrates accuracy and convergence even at low angular resolutions. We further showcase the capabilities of RAYTHEIA by producing high-resolution synthetic emission maps of key diagnostic lines of a star-forming region capturing physical effects such as [O I] $63μ$m self-absorption, measuring the [C I]-bright but CO-dark molecular gas, and deriving a CO-to-H$_2$ conversion factor in agreement with observations.

astro-ph.IM↗

arXiv2026

The SOMA Atomic Outflow Survey. I. An Atomic OI and Highly Ionized OIII Outflow from Massive Protostar G11.94-00.62

Massive stars regulate galaxy evolution and star formation through their physical and chemical feedback, but their formation remains poorly understood. Accretion-powered outflows provide important diagnostics of massive star formation. We present first results from the SOMA Atomic Outflow Survey, a far-infrared massive star formation survey using the FIFI-LS instrument on SOFIA. We report detection of \OIII\ $^3P_2\rightarrow^3P_1$ emission at 52 \micron\ from the massive protostar G11.94-0.62, tracing highly ionized gas. We also detect \OI\ $^3P_2\rightarrow^3P_1$ and $^3P_1\rightarrow^3P_0$ at 63 and 145 \micron\ tracing atomic gas, as well as CO $J=14\rightarrow13$ at 186 \micron\ from highly excited molecular gas. The \OIII\ and \OI\ lines exhibit large line widths ($\sim200$ and $\sim40-80$ \kms, respectively) and their morphologies are consistent with a wide-angle bipolar outflow. The properties of molecular tracers ($^{12}$CO, $^{13}$CO, C$^{18}$O, H$_2$CO, and CH$_3$OH) observed with ALMA support this interpretation. Ionized nebula and PDR modeling imply an ionized outflow mass flux of $\sim8\times10^{-5}\:M_\odot$ yr$^{-1}$ and an atomic outflow mass flux of $\sim5\times10^{-6}\:M_\odot$ yr$^{-1}$, while the molecular outflow traced by CO has an implied mass flux of $\sim3\times10^{-4}\:M_\odot$ yr$^{-1}$. The mass and momentum flux in the ionized outflow are consistent with the primary disk wind, while the molecular component is mainly swept-up, secondary outflow gas. We also observe G11.94-0.62 with the LBT in the near-infrared, potentially tracing the base of wide-angle outflow cavities. SED modeling implies a protostellar mass $m_* = 22.4^{+21}_{-11}\:M_\odot$, while the \OIII\ emission implies $m_*\gtrsim30\:M_\odot$ and that the protostar is in the final stages of its accretion.

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arXiv2026

Large scale mapping of [CI] and the [CI]-to-CO transition in $ρ$ Ophiuchus molecular cloud

Atomic carbon ([CI]) is a key species in the carbon chemistry of the interstellar medium (ISM). Using the Submillimeter Wave Astronomy Satellite (SWAS), we conducted a [CI]($^3$P$_1$--$^3$P$_0$) 492 GHz survey covering approximately 4 deg$^2$ of the L1688 and L1689 regions in the $ρ$ Oph molecular cloud, achieving a spatial resolution of 4.25$\hbox{$^{\prime}$}$. The derived [CI] column densities, N([CI]), range from 4.85 $\times$ 10$^{14}$ cm$^{-2}$ to 6.29 $\times$ 10$^{17}$ cm$^{-2}$, corresponding to an abundance ratio N([CI])/N($H_2$) of 2.24$\times$ 10$^{-7}$ to 2.39$\times$ 10$^{-4}$, with a median value of 1.8$\times$ 10$^{-5}$. Combining observations with photodissociation region (PDR) modeling, we find that [CI] abundance varies less than CO in regions with UV intensity G$_0$ $> 16$ and N(H$_2$) $<$ 4.6 $\times$ 10$^{21}$ cm$^{-2}$, suggesting [CI] is a more reliable tracer of molecular hydrogen in low-density, high-radiation environments where the [CI]-to-CO transition occurs. Utilizing [CI] as direct H$_2$ tracer, the CO-dark gas fraction is estimated to be 0.43 , meaning that 43% of the total cloud mass will be missed by conventional calculation based on CO observations but can be calibrated by [CI] emission. The [CI] line widths are systematically broader than those of $^{13}$CO, possibly due to contributions from atomic carbon. These findings provide key insights into Galactic [CI] emission and the carbon cycle evolution in the interstellar medium. Future high-sensitivity [CI] ($^3$P$_1$--$^3$P$_0$) surveys with the Chinese Survey Space Telescope (CSST) will significantly advance our understanding of the carbon cycle evolution.

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arXiv2025

JWST observations of cosmic-ray-excited H$_2$ in Barnard 68: spatial variations and constraints on cosmic-ray attenuation

We present James Webb Space Telescope (JWST) NIRSpec observations of the starless dark cloud Barnard 68 that reveal the spatially-resolved signature of cosmic-ray excited molecular hydrogen (CRXH$_2$) emissions for the first time. Following up on our initial detection of CRXH$_2$ emissions from B68 (Bialy et al. 2025), we now exploit JWST's sensitivity and spatial multiplexing to map CRXH$_2$ rovibrational lines across 16 sight lines through the cloud. By disentangling the CRXH$_2$ and UV-pumped H$_2$ components, we isolate the para-H$_2$-dominated spectrum attributable to cosmic-ray excitation. We find that there are significant spatial variations in the ratio of the CRXH$_2$ line intensity to the line-of-sight H$_2$ column density; these cannot be accounted for by dust extinction alone and demonstrate a clear attenuation of the cosmic-ray flux with increasing shielding column. Modeling B68 as a Bonnor-Ebert sphere, we constrain both the unshielded cosmic-ray ionization rate, $ζ_{\rm H_2}$, and how it decreases with shielding column. At a reference depth of $N({\rm H}_2) = 3 \times 10^{21}$ cm$^{-2}$, we infer $ζ_{\rm H_2} \approx 1.4 \times 10^{-16}$ s$^{-1}$, a factor of $\approx 3$ higher than the average value derived from H$_3^+$ absorption studies. These results provide the most direct probe to date of cosmic-ray penetration into cold, dense gas, offering new constraints on both the microphysics of CR-H$_2$ interactions and the attenuation of low-energy cosmic rays in molecular clouds. Our findings establish CRXH$_2$ emission as a powerful new diagnostic of the cosmic-ray environment in interstellar space.

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arXiv2025

The first detection of cosmic-ray excited H$_2$ in interstellar space

Stars and planets form within cold, dark molecular clouds. In these dense regions, where starlight cannot penetrate, cosmic rays (CRs) are the dominant source of ionization -- driving interstellar chemistry(Dalgarno (2006, PNAS, 103, 12269)), setting the gas temperature(Goldsmith et al. (1969, ApJ, 158, 173)), and enabling coupling to magnetic fields(McKee & Ostriker (2007, ARA&A, 45, 565; arXiv:0707.3514)). Together, these effects regulate the collapse of clouds and the onset of star formation. Despite this importance, the cosmic-ray ionization rate, $ζ$, has never been measured directly. Instead, this fundamental parameter has been loosely inferred from indirect chemical tracers and uncertain assumptions, leading to published values that span nearly two orders of magnitude and limiting our understanding of star formation physics. Here, we report the first direct detection of CR-excited vibrational H$_2$ emission, using \textit{James Webb Space Telescope} (JWST) observations of the starless core Barnard 68 (B68). The observed emission pattern matches theoretical predictions for CR excitation precisely, confirming a decades-old theoretical proposal long considered observationally inaccessible. This result enables direct measurement of $ζ$, effectively turning molecular clouds into natural, light-year-sized, cosmic-ray detectors. It opens a transformative observational window into the origin, propagation, and role of cosmic rays in star formation and galaxy evolution.

astro-ph.GA↗

arXiv2025

Tight correlation of star formation with [Ci] and CO lines across cosmic time

Context. Cold molecular gas tracers, such as CI and CO lines, have been widely used to infer specific characteristics of the ISM and to derive star-formation relations among galaxies. Aims. However, there is still a lack of systematic studies of the star-formation scaling relation of CO and [CI] lines across cosmic time on multiple physical scales. Methods. We used observations of the ground state transitions of [CI], CO, and [CII], for 885 sources collected from the literature, to infer possible correlations between line luminosities of $\rm L^{'}_{[CI](1-0)}, \rm L^{'}_{CO(1-0)}$, and $\rm L^{'}_{[CII]}$ with star formation rates (SFR). With linear regression, we fit the relations between SFR and molecular mass derived from CO, CI, and CII lines. Results. The relation between [CI] and CO-based total molecular masses is weakly superlinear. Nevertheless, they can be calibrated against each other. For $\rm α_{CO} = 0.8$ and $4.0\ \rm {M}_{\odot}\,({K}\,{km}\,{s}^{-1}\,{pc}^2)^{-1}$ we derive $α_{\rm [CI]} = 3.9$ and $\sim$$17\ \rm {M}_{\odot}\,({K}\,{km}\,{s}^{-1}\,{pc}^2)^{-1}$ , respectively. Using the \emph{lmfit} package, we derived relation slopes of SFR--$\rm L^{'}_{[CI](1-0)}$, SFR--$\rm L^{'}_{CO(1-0)}$, and SFR--$\rm L^{'}_{[CII](1-0)}$ to be $\rm β$ = 1.06 $\pm$ 0.02, 1.24 $\pm$ 0.02, and 0.74 $\pm$ 0.02, respectively. With a Bayesian-inference \emph{linmix} method, we find consistent results. Conclusions. Our relations for [CI](1-0) and CO(1-0) indicate that they trace similar molecular gas contents, across different redshifts and different types of galaxies. This suggests that these correlations do not have strong evolution with cosmic time.

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arXiv2025

Extended multi-phase gas reservoirs in the z=4.3 protocluster SPT2349-56: non-stellar ionisation sources?

We aim to characterize the multi-phase gas in the SPT2349-56 protocluster at z=4.3, known to host one of the most starbursting and AGN-rich high redshift environments.For this purpose we conducted APEX single dish observations of the [CII]158 micron (hereafter [CII]) line towards the Core and North components, previously imaged with the ALMA 12-m array. We also present the first [OIII]88 micron (hereafter [OIII]) line observations in such high redshift protocluster system. We obtain a [CII] line luminosity $\sim$1.7$\times$ more than the one recovered by ALMA towards the Core, while remarkably we recover 4$\times$ more [CII] line emission than the one found in deep ALMA images towards the North component, suggesting that the most massive gas reservoirs lie in the less extreme regions of this protocluster system. A minimum ionised gas mass of $\mathrm M_{\rm min}(H^+)$$ \sim$$3.7\times 10^{10}$\,\Msun\, is deduced from the [OIII] line, amounting to 30\% of the molecular gas mass in the same area. Finally we obtain star formation rate (SFR) estimates using the [OIII] line luminosity, and the corresponding ionised gas mass. These yield values that can surpass the far-IR continuum-derived SFR (under the assumption of a standard stellar IMF), which can be reconciled only if non-stellar ionising sources contribute to the [OIII] line luminosity, or a top-heavy stellar IMF produces a larger fraction of O stars per total stellar mass, a distinct possibility in High-Energy-Particle (HEP) rather than (UV-photon)-dominated environments in clusters. Future work using far-IR fine-structure and molecular/neutral-atomic lines is necessary for determining the thermal/ionisation states of the multi-phase medium and these line ratios must be measured over a wide range of spatial scales, which ultimately requires combining wide-field single-dish and high resolution interferometric observations.

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arXiv2025

NeuralPDR: Neural Differential Equations as surrogate models for Photodissociation Regions

Computational astrochemical models are essential for helping us interpret and understand the observations of different astrophysical environments. In the age of high-resolution telescopes such as JWST and ALMA, the substructure of many objects can be resolved, raising the need for astrochemical modeling at these smaller scales, meaning that the simulations of these objects need to include both the physics and chemistry to accurately model the observations. The computational cost of the simulations coupling both the three-dimensional hydrodynamics and chemistry is enormous, creating an opportunity for surrogate models that can effectively substitute the chemical solver. In this work we present surrogate models that can replace the original chemical code, namely Latent Augmented Neural Ordinary Differential Equations. We train these surrogate architectures on three datasets of increasing physical complexity, with the last dataset derived directly from a three-dimensional simulation of a molecular cloud using a Photodissociation Region (PDR) code, 3D-PDR. We show that these surrogate models can provide speedup and reproduce the original observable column density maps of the dataset. This enables the rapid inference of the chemistry (on the GPU), allowing for the faster statistical inference of observations or increasing the resolution in hydrodynamical simulations of astrophysical environments.

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arXiv2025

Metallicity dependence of the CO-to-H$_2$ and the [CI]-to-H$_2$ conversion factors in galaxies

Understanding the molecular gas content in the interstellar medium (ISM) is crucial for studying star formation and galaxy evolution. The CO-to-H$_2$ ($X_{\rm CO}$) and the [CI]-to-H$_2$ ($X_{\rm CI}$) conversion factors are widely used to estimate the molecular mass content in galaxies. However, these factors depend on many ISM environmental parameters. This work investigates the dependence of $X_{\rm CO}$ and $X_{\rm CI}$ on these parameters, with a focus on the low-metallicity $α$-enhanced ISM ($\rm [C/O]<0$), to provide improved tracers of molecular gas in diverse conditions. We used the PDFchem algorithm, coupled with a database of 3D-PDR models. These account for a wide range of metallicities, dust-to-gas mass ratios, FUV intensities, and cosmic-ray ionization rates. The conversion factors were computed by integrating the PDR properties over log-normal column density distributions ($A_{\rm V}$-PDFs) representing various cloud types. The $X_{\rm CO}$ factor increases significantly with decreasing metallicity, exceeding $\sim\!\!1000$ times the Galactic value at ${\rm [O/H] = -1.0}$ under $α$-enhanced conditions, as opposed to $\sim\!\!300$ times under non-$α$-enhanced conditions (${\rm [C/O]=0}$). In contrast, $X_{\rm CI}$ shows a more gradual variation with metallicity, making it a more reliable tracer of molecular gas in metal-poor environments under most conditions. The fraction of `CO-dark' molecular gas increases dramatically in low-metallicity regions, exceeding 90\% at ${\rm [O/H] = -1.0}$, particularly in diffuse clouds and environments with strong FUV radiation fields. We recommend the use of the $\log_{10}X_{\rm CO}\simeq-2.41Z+41.3$ relation for the CO-to-H$_2$ conversion factor, and the $\log_{10}X_{\rm CI}\simeq-0.99Z+29.7$ relation for the [CI]-to-H$_2$, where $Z=12+\log_{10}({\rm O/H})$.

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arXiv2025

Inadequate turbulent support in low-metallicity molecular clouds

The dynamic properties of molecular clouds are set by the interplay of their self-gravity, turbulence, external pressure and magnetic fields. Extended surveys of Galactic molecular clouds typically find that their kinetic energy ($E_{\rm k}$) counterbalances their self-gravitational energy ($E_{\rm g}$), setting their virial parameter $α_{\rm vir}=2E_{\rm k}/|E_{\rm g}|\approx1$. However, past studies either have been biased by the use of optically-thick lines or have been limited within the solar neighborhood and the inner Galaxy (Galactocentric radius $R_{\rm gc}<R_{\rm gc,\odot} \approx 8$ kpc). Here we present sensitive mapping observations of optically thin $^{13}$CO lines towards molecular clouds in the low-metallicity Galactic outer disk ($R_{\rm gc}\sim9-24$ kpc). By combining archival data from the inner Galaxy and four nearby metal-poor dwarf galaxies, we reveal a systematic trend of $α_{\rm vir}$, which declines from supervirial dynamic states in metal-rich clouds to extremely subvirial dynamic states in metal-poor clouds. In these metal-poor environments, turbulence alone is insufficient to counterbalance the self-gravity of a cloud. A cloud-volumetric magnetic field may replace turbulence as the dominant cloud-supporting mechanism in low-metallicity conditions, for example, the outermost galactic disks, dwarf galaxies and galaxies in the early Universe, which would then inevitably impact the initial conditions for star formation in such environments.

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arXiv2024

3D-PDR Orion dataset and NeuralPDR: Neural Differential Equations for Photodissociation Regions

We present a novel dataset of simulations of the photodissociation region (PDR) in the Orion Bar and provide benchmarks of emulators for the dataset. Numerical models of PDRs are computationally expensive since the modeling of these changing regions requires resolving the thermal balance and chemical composition along a line-of-sight into an interstellar cloud. This often makes it a bottleneck for 3D simulations of these regions. In this work, we provide a dataset of 8192 models with different initial conditions simulated with 3D-PDR. We then benchmark different architectures, focusing on Augmented Neural Ordinary Differential Equation (ANODE) based models (Code be found at https://github.com/uclchem/neuralpdr). Obtaining fast and robust emulators that can be included as preconditioners of classical codes or full emulators into 3D simulations of PDRs.

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arXiv2024

Gas dynamics in an AGN-host galaxy at $z\simeq2.6$: regular rotation, non-circular motions, and mass models

The gas dynamics of galaxies provide critical insights into the evolution of both baryons and dark matter (DM) across cosmic time. In this context, galaxies at cosmic noon -- the period characterized by the most intense star formation and black hole activities -- are particularly significant. In this work, we present an analysis of the gas dynamics of PKS 0529-549: a galaxy at $z\simeq2.6$, hosting a radio-loud active galactic nucleus (AGN). We use new ALMA observations of the [CI] (2-1) line at a spatial resolution of 0.18$''$ ($\sim$1.5 kpc). We find that (1) the molecular gas forms a rotation-supported disk with $V_{\rm rot}/σ_{\rm v}=6\pm3$ and displays a flat rotation curve out to 3.3 kpc; (2) there are several non-circular components including a kinematically anomalous structure near the galaxy center, a gas tail to the South-West, and possibly a second weaker tail to the East; (3) dynamical estimates of gas and stellar masses from fitting the rotation curve are inconsistent with photometric estimates using standard gas conversion factors and stellar population models, respectively; these discrepancies may be due to systematic uncertainties in the photometric masses, in the dynamical masses, or in the case a more massive radio-loud AGN-host galaxy is hidden behind the gas-rich [CI] emitting starburst galaxy along the line of sight. Our work shows that in-depth investigations of 3D line cubes are crucial for revealing the complexity of gas dynamics in high-$z$ galaxies, in which regular rotation may coexist with non-circular motions and possibly tidal structures.

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arXiv2024

Molecular gas mass measurements of an active, starburst galaxy at $z\approx2.6$ using ALMA observations of the [CI], CO and dust emission

We present new ALMA observations of a starburst galaxy at cosmic noon hosting a radio-loud active galactic nucleus: PKS 0529-549 at $z=2.57$. To investigate the conditions of its cold interstellar medium, we use ALMA observations which spatially resolve the [CI] fine-structure lines, [CI] (2-1) and [CI] (1-0), CO rotational lines, CO (7-6) and CO (4-3), and the rest-frame continuum emission at 461 and 809 GHz. The four emission lines display different morphologies, suggesting spatial variation in the gas excitation conditions. The radio jets have just broken out of the molecular gas but not through the more extended ionized gas halo. The [CI] (2-1) emission is more extended ($\approx8\,{\rm kpc}\times5\,{\rm kpc}$) than detected in previous shallower ALMA observations. The [CI] luminosity ratio implies an excitation temperature of $44\pm16$ K, similar to the dust temperature. Using the [CI] lines, CO (4-3), and 227 GHz dust continuum, we infer the mass of molecular gas $M_{\mathrm{mol}}$ using three independent approaches and typical assumptions in the literature. All approaches point to a massive molecular gas reservoir of about $10^{11}$ $M_{\odot}$, but the exact values differ by up to a factor of 4. Deep observations are critical in correctly characterizing the distribution of cold gas in high-redshift galaxies, and highlight the need to improve systematic uncertainties in inferring accurate molecular gas masses.

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arXiv2024

The origin and evolution of the [CII] deficit in HII regions and star-forming molecular clouds

We analyse synthetic maps of the [CII] 158 $μ$m line and FIR continuum of simulated molecular clouds (MCs) within the SILCC-Zoom project to study the origin of the [CII] deficit, i.e., the drop in the [CII]/FIR intensity ratio. All simulations include stellar radiative feedback and account for further ionisation of C$^+$ into C$^{2+}$ inside HII regions. For individual HII regions, $I_\mathrm{FIR}$ is initially high in the vicinity of young stars, and then moderately decreases as the gas is compressed into shells. In contrast, $I_\mathrm{CII}$ drops strongly over time, to which the second ionisation of C$^+$ into C$^{2+}$ contributes. This leads to a large drop in $I_\mathrm{[CII]}/I_\mathrm{FIR}$ inside HII regions, decreasing from 10$^{-3}$-10$^{-2}$ at scales above 10 pc to 10$^{-6}$-10$^{-4}$ at scales below 2pc. However, projection effects can affect the radial profile of $I_\mathrm{[CII]}$ and $I_\mathrm{FIR}$ and create apparent HII regions without any stars. On MC scales, $L_\mathrm{[CII]}/L_\mathrm{FIR}$ decreases from values $\gtrsim$10$^{-2}$ in MCs without star formation to values around $\sim10^{-3}$ in MCs with star formation. We attribute this and the origin of the [CII] deficit to two main contributors: (i) the saturation of the [CII] line and (ii) the conversion of C$^+$ into C$^{2+}$ by stellar radiation. The drop in $L_\mathrm{[CII]}/L_\mathrm{FIR}$ can be divided into two phases: (i) early on, the saturation of [CII] and the further ionisation of C$^+$ limit the increase in $L_\mathrm{[CII]}$, while $L_\mathrm{FIR}$ increases rapidly, leading to the initial decline of $L_\mathrm{[CII]}/L_\mathrm{FIR}$. (ii) In more evolved HII regions, $L_\mathrm{CII}$ stagnates and even partially drops due to the aforementioned reasons. $L_\mathrm{FIR}$ stagnates as the gas gets pushed into the cooler shells keeping $L_\mathrm{[CII]}/L_\mathrm{FIR}$ at low values of $\sim10^{-3}$.

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