arXiv subjects

Natalia Lahén

Publications and source records attributed to Natalia Lahén.

At least 19 recordsLinked to original sources

arXiv2026

FROST-CLUSTERS -- III. Metallicity-dependent intermediate mass black hole formation by runaway collisions in dense star clusters

We explore the formation of intermediate mass black holes (IMBHs), potential seeds for supermassive black holes (SMBHs), via runaway stellar collisions for a wide range of star cluster (surface) densities ($4\times10^3 M_\odot$ pc$^{-2} \lesssim Σ_\mathrm{h} \lesssim 4\times10^6 M_\odot$ pc$^{-2}$) and metallicities $(0.01 Z_\odot \lesssim Z \lesssim 1.0 Z_\odot)$. Our sample of isolated (>1400) and hierarchical (30) simulations of young, massive star clusters with up to $N=1.8\times10^6$ stars includes collisional stellar dynamics, stellar evolution, and post-Newtonian equations of motion for black holes using the BIFROST code. High stellar wind rates suppress IMBH formation at high metallicities ($Z\gtrsim0.2 Z_\odot$) and low collision rates prevent their formation at low densities ($Σ_\mathrm{h}\lesssim 3\times10^4 M_\odot$ pc$^{-2}$). The assumptions about stellar wind loss rates strongly affect the maximum final IMBH masses ($M_\bullet\sim 6000 M_\odot$ vs. $25000 M_\odot$). The total stellar mass loss from collisions and collisionally boosted winds before $t=3$ Myr can together reach up to $5$--$10\%$ of the final cluster mass. We present fitting formulae for IMBH masses as a function of host star cluster $Σ_\mathrm{h}$ and $Z$ which can be used to seed SMBHs in high resolution cosmological hydrodynamical simulations and in semi-analytic models for galaxy formation. Our results favour IMBH formation in dense low metallicity environments similar to $z\sim10$ James Webb Space Telescope (\textit{JWST}) proto globular clusters. IMBH formation is suppressed in the high metallicity and low density conditions of the local Universe.

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arXiv2026

Origins of Extreme Emission-Line Ratios in z > 3 Galaxies: Insights from the Lumen Model

Optical emission-line ratios in star-forming galaxies at $z \sim 3$-8, such as [OIII]/H$β$ and [OIII]/[OII], are strongly offset from those at $z \sim 0$-2, pointing to more extreme ionization and ISM conditions in the early Universe. To constrain the physical origin of these offsets, we developed Lumen, a framework for modelling nebular emission from spatially distributed HII regions in cosmological simulations. We apply Lumen to IllustrisTNG50, validate its predictions at low redshift, and test a suite of proposed mechanisms for producing extreme line ratios at $z = 3$-8. We focus on the [NII]/H$α$ versus [OIII]/H$β$ (N2-BPT) diagram, the [SII]/H$α$ versus [OIII]/H$β$ (S2-VO87) diagram, and the [OIII]/[OII] versus ([OII]+[OIII])/H$β$ (O32-R23) diagram. We find that $α$-enhancement alone cannot explain the bulk of observations. Moderate offsets emerge from the combined effects of $α$-enhancement, a higher IMF upper-mass cutoff, and AGN contributions. The most extreme [OIII]/H$β$ and [OIII]/[OII] values require high ionization parameters powered by massive star clusters of $\gtrsim 10^5$-$10^6\,\mathrm{M}_\odot$, consistent with recent JWST observations. Reproducing the highest [NII]/H$α$ ratios additionally requires enhanced nitrogen abundances. Although gas densities of $n \sim 10^4\,\mathrm{cm}^{-3}$ can boost several diagnostic ratios, they suppress [SII]/H$α$ and are therefore in tension with current observations. Overall, models combining harder ionizing spectra, elevated ionization parameters from massive star clusters, and enhanced nitrogen abundances reproduce the observed high-$z$ galaxy population across the N2-BPT, S2-VO87, and O32-R23 diagrams. This successful model also motivates new demarcation lines for star-forming galaxies in the N2-BPT and S2-VO87 diagrams.

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arXiv2026

The emerging timescale of young star clusters regulated by cluster stellar mass

Quantifying the timescales of star cluster emergence from their natal clouds remains one of the main challenges in understanding the star formation process. These timescales are fundamental measurements of the star formation cycle within galaxies, yet are difficult to constrain due to the complex interplay between stellar feedback and star formation across multiple physical scales. Here we present Hubble Space Telescope and James Webb Space Telescope observations of thousands of young star clusters in four nearby galaxies (M51, M83, NGC 628 and NGC 4449). A substantial fraction of these clusters are still embedded within their natal gas and remain invisible at optical wavelengths. We constrain their emergence process by measuring the timescales required to disperse the surrounding material. We find a strong correlation between dispersal timescale and cluster stellar mass, with massive clusters emerging faster than their lower-mass counterparts. This is a critical constraint on star formation and stellar feedback simulations, which struggle to fully reproduce star clusters formation and emergence. Our results emphasize the central role of massive clusters in driving the escape of ionizing radiation into the galactic medium. Finally, they impose time limitations for planet formation in massive cluster environments where disks get exposed to ultraviolet irradiation and further gas infall is halted.

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arXiv2025

Where do stars explode in the ISM? -- The distribution of dense gas around evolved massive stars in M33

The effect of supernovae (SNe) on star-formation in the interstellar medium (ISM) depends sensitively on where SNe explode with respect to ISM clouds. Observationally, SN ISM environments characterized by spatially-resolved gas maps can empirically guide the placement of SNe in subgrid models, but unfortunately such measurements remain scarce, as SNe are rare and often distant. Here we demonstrate a new approach -- mapping the ISM around evolved massive stars that are soon to explode. These provide a substantially larger sample of `explosion sites' (than just historical SNe) in nearby galaxies that have high-resolution atomic and molecular ISM maps from Jansky VLA and ALMA. We demonstrate this technique in the well-resolved Local Group spiral M33 by analyzing the 50 pc-scale projected ISM densities around red supergiants (RSGs, 8-30 M$_{\odot}$ stars) Wolf-Rayet stars (WRs, $>$30 M$_{\odot}$ stars), and supernova remnants (SNRs). We find a \emph{mass-dependent} correlation between stars and gas clouds, with atleast 45\% of WRs and upto 77\% of RSGs having no detectable H$_2$ at their pixel locations. In the sample with H$_2$ detections, we find that more massive younger progenitors are coincident with denser gas. We show that the density distributions for stars $>$15 M$_{\odot}$ are statistically distinct from random alignment of stars and gas in M33. Our work provides the first observationally-derived estimate of the fraction of the SN-producing stellar population correlated with ISM density peaks. We demonstrate how this can be compared with galaxy simulations, and advocate similar comparisons to the community for constraining sub-grid models.

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arXiv2025

Mergers all the way down: stellar collisions and kinematics of a dense hierarchically forming massive star cluster in a dwarf starburst

Recent observations indicate that the progenitors of globular clusters (GCs) at high redshifts had high average stellar surface densities above $10^5\, \mathrm{M}_\odot\, \mathrm{pc}^{-2}$. The internal structure and kinematics of the clusters, however, remain out of reach. Numerical simulations are necessary to decipher the origin of spatio-kinematic features in present-day GCs. Here we study star cluster formation in a star-by-star hydrodynamical simulation of a low-metallicity starburst in a merger of two gas-rich dwarf galaxies. The simulation accounts for the multiphase interstellar medium, stellar radiation, winds and supernovae, and the accurate small-scale gravitational dynamics near massive stars. We also include prescriptions for stellar collisions and tidal disruption events by black holes. Gravitationally bound star clusters up to $\sim2\times10^5\, \mathrm{M}_\odot$ form dense with initial half-mass radii of $\sim0.1\unicode{x2013}1\, \mathrm{pc}$. The most massive cluster approaches the observed high-redshift surface densities throughout its hierarchical and dissipative assembly. The cluster also hosts a collisionally growing very massive star of $\sim1000\, \mathrm{M}_\odot$ that will eventually collapse, forming an intermediate mass black hole. The assembly leaves an imprint in the spatio-kinematic structure of the cluster. The youngest stars are more centrally concentrated, they show significant bulk rotation and have radially biased velocity components at outer radii. The older population is more round in shape, rotates slowly, its velocity distribution is isotropic and exhibits higher dispersion. If chemically enriched star formation proceeds mainly in the later stages of cluster assembly, these results provide a possible explanation for some of the multiple population features observed in dynamically young GCs.

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arXiv2025

Cosmic Threads: Interlinking the Stellar Initial Mass Function from Star-Birth to Galaxies

The stellar initial mass function (sIMF) describes the distribution of stellar masses formed in a single star formation event in a molecular cloud clump. It is fundamental to astrophysics and cosmology, shaping our understanding of unresolved stellar populations, galactic chemical enrichment and habitable zones, and black hole growth. This White Paper reviews studies on the core mass function, stellar multiplicity, and dynamical processes affecting sIMF determinations, as well as the link between star-forming clumps and the galaxy-wide IMF (gIMF). The evidence gleaned from observed systems for the dependency of the sIMF on the metallicity and density of the clump is portrayed. We examine evidence from gravitational lensing, stellar and gas kinematics, and spectral diagnostics to assess environmental dependencies of the gIMF. Theoretical perspectives provide further insights into the sIMF's variability. Beyond summarizing current knowledge, this work aims to establish a shared framework and define strategies for studying a variable IMF in the era of near-infrared integral-field spectroscopy, 30m-class telescopes and major space-based observatories.

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arXiv2025

FROST-CLUSTERS -- II. Massive stars, binaries and triples boost supermassive black hole seed formation in assembling star clusters

Observations and high-resolution hydrodynamical simulations indicate that massive star clusters form through a complex hierarchical assembly. We use simulations including post-Newtonian dynamics (the BIFROST code) and stellar evolution (the SEVN module) to investigate this collisional assembly. With a full initial stellar mass function, we study the effect of initial binary, triple and massive single stars (450 $M_\odot$) on the assembly, structure, and kinematics of massive ($M_\mathrm{cl}\sim10^6 M_\odot$, $N=1.8 \times 10^6$) star clusters. Simultaneously, intermediate mass black holes (IMBHs), potential seeds for supermassive black holes, can form and grow in our models by stellar collisions, tidal disruption events (TDEs) and black hole (BH) mergers. At a fixed cluster mass, stellar multiplicity or a high mass limit increase the numbers (up to $\sim$ 10) and masses (up to $10^4 M_\odot$) of the formed IMBHs within the first 10 Myr of cluster evolution. The TDE rates peak at $Γ_\mathrm{tde}\sim 5 \times 10^{-5}$ yr$^{-1}$ after IMBH formation at $\sim 2$ Myr. In all simulations, we find gravitational wave driven mergers involving stellar BHs and IMBHs. Initial multiplicity or a high mass limit also result in IMBH-IMBH mergers. The IMBH masses correlate with the initial cluster masses, surface densities and velocity dispersions approximately as $M_\bullet \propto M_\mathrm{cl}$, $M_\bullet\proptoΣ_\mathrm{h}^\mathrm{3/2}$ and $M_\bullet\proptoσ^\mathrm{3}$. Our results suggest the dense $z\sim10$ star clusters recently observed by the James Webb Space Telescope host IMBHs with masses above $M_\bullet \gtrsim 10^4 M_\odot$.

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arXiv2025

Massive Interacting Binaries Enhance Feedback in Star-Forming Regions

We present a new framework to incorporate feedback from massive interacting binaries in simulations of star cluster formation. Our new feedback model adds binary stellar evolution to the cluster formation code Torch, and couples it in AMUSE to the pre-existing modules for collisional stellar dynamics, magnetohydrodynamics, and mechanical and radiative feedback. Our model accounts for the effects of mass transfer on the stars' mass loss rates, their radiation spectra, and the timing of core-collapse supernovae. It also injects mass lost through non-conservative mass transfer and common envelope ejection into the interstellar medium. We demonstrate the use of our feedback model through simulations of isolated binaries in a gaseous medium, and of embedded clusters of massive binaries. Feedback from interacting binaries efficiently couples with the surrounding interstellar medium. It increases the size of HII regions, increases the kinetic and thermal energy of the gas, and increases the pressure within HII regions compared to models that use single star stellar evolution. Those differences arise from the ionizing radiation, which increases by three orders of magnitude, resulting in HII regions that expand due to thermal pressure rather than radiation pressure. The effects of stellar dynamics and the gravitational potential of the background gas cause the evolution of individual binaries to deviate from the predictions made by secular evolution, impacting the subsequent feedback from the binary. We conclude that massive interacting binaries are an important source of feedback in cluster-forming regions, and must be considered when studying the emerging timescales of young star clusters.

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arXiv2025

The formation, evolution and disruption of star clusters with improved gravitational dynamics in simulated dwarf galaxies

So far, even the highest resolution galaxy formation simulations with gravitational softening have failed to reproduce realistic life cycles of star clusters. We present the first star-by-star galaxy models of star cluster formation to account for hydrodynamics, star formation, stellar evolution and collisional gravitational interactions between stars and compact remnants using the updated SPHGAL+KETJU code, part of the GRIFFIN-project. Gravitational dynamics in the vicinity of $>3$ M$_\odot$ stars and their remnants are solved with a regularised integrator (KETJU) without gravitational softening. Comparisons of idealised star cluster evolution with SPHGAL+KETJU and direct N-body show broad agreement and the failure of simulations that use gravitational softening. In the hydrodynamical simulations of idealised dwarf galaxies run with SPHGAL+KETJU, clusters up to $\sim900$ M$_\odot$ form compact (effective radii 0.1$\unicode{x2013}$1 pc) and their sizes increase by up to a factor of ten in agreement with previous N-body simulations and the observed sizes of exposed star clusters. The sizes increase rapidly once the clusters become exposed due to photoionising radiation. On average 63% of the gravitationally bound clusters disrupt during the first 100 Myr of evolution in the galactic tidal field. The addition of collisional dynamics reduces the fraction of supernovae in bound clusters by a factor of $\sim1.7$, however the global star formation and outflow histories change by less than 30%. We demonstrate that the accurate treatment of gravitational encounters with massive stars enables more realistic star cluster life cycles from the earliest stages of cluster formation until disruption in simulated low-mass galaxies.

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arXiv2024

The importance of nuclear star clusters for massive black hole growth and nuclear star formation in simulated low-mass galaxies

Observed low-mass galaxies with nuclear star clusters (NSCs) can host accreting massive black holes (MBH). We present simulations of dwarf galaxies ($M_{\mathrm{baryon}} \sim 0.6 - 2.4 \times 10^8 \rm \, M_\odot$) at solar mass resolution ($0.5\rm \, M_\odot < m_{\mathrm{gas}} < 4 \rm \, M_\odot$) with a multi-phase interstellar medium (ISM) and investigate the impact of NSCs on MBH growth and nuclear star formation (SF). The Griffin simulation model includes non-equilibrium low temperature cooling, chemistry and the effect of HII regions and supernovae (SN) from massive stars. Individual stars are sampled down to 0.08 $\rm M_\odot$ and their non-softened gravitational interactions with MBHs are computed with the regularised Ketju integrator. MBHs with masses in the range of $10^2 - 10^5 \, \rm M_\odot$ are represented by accreting sink particles without feedback. We find that the presence of NSCs boost nuclear SF (i.e. NSC growth) and MBH accretion by funneling gas to the central few parsecs. Low-mass MBHs grow more rapidly on $\sim 600$ Myr timescales, exceeding their Eddington rates at peak accretion. MBH accretion and nuclear SF is episodic (i.e. leads to multiple stellar generations), coeval and regulated by SN explosions. On 40 - 60 Myr timescales the first SN of each episode terminates MBH accretion and nuclear SF. Without NSCs, low-mass MBHs do not grow and MBH accretion and reduced nuclear SF become irregular and uncorrelated. This study gives the first insights into the possible co-evolution of MBHs and NSCs in low-mass galaxies and highlights the importance of considering dense NSCs in galactic studies of MBH growth.

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arXiv2024

The masses, structure and lifetimes of cold clouds in a high-resolution simulation of a low metallicity starburst

We present an analysis of the cold gas phase in a low metallicity starburst generated in a high-resolution hydrodynamical simulation of a gas-rich dwarf galaxy merger as part of the GRIFFIN project. The simulations resolve (4 M$_\odot$ gas phase mass resolution, $\sim$ 0.1 pc spatial resolution) the multi-phase interstellar medium with a non-equilibrium chemical heating/cooling network at temperatures below $10^4$ K. Massive stars are sampled individually and interact with the ISM through the formation of HII regions and supernova explosions. In the extended starburst phase, the ISM is dominated by cold ($T_\mathrm{gas} < 300$ K) filamentary clouds with self-similar internal structures. The clouds have masses of $10^{2.6}$ - $10^{5.6}$ M$_\odot$ with a power law mass function, $dN/dM \propto M^α$ with $α= -1.78 (\pm 0.08)$. They also follow the Larson relations, in good agreement with observations. We trace the lifecycle of the cold clouds and find that they follow an exponential lifetime distribution and an e-folding time of $\sim$ 3.5 Myr. Clouds with peak masses below $10^4$ M$_\odot$ follow a power law relation with their average lifetime $τ_\mathrm{life} \propto M^{0.3}_\mathrm{max}$ which flattens out for higher cloud masses at $ < 10$ Myr. A similar relation exists between cloud size at peak mass and lifetime. This simulation of the evolution of a realistic galactic cold cloud population supports the rapid formation and disruption of star-forming clouds by stellar radiation and supernovae on a timescale less than 10 Myr.

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arXiv2024

FROST-CLUSTERS -- I. Hierarchical star cluster assembly boosts intermediate-mass black hole formation

Observations and high-resolution hydrodynamical simulations indicate that massive star clusters assemble hierarchically from sub-clusters with a universal power-law cluster mass function. We study the consequences of such assembly for the formation of intermediate-mass black holes (IMBHs) at low metallicities ($Z=0.01\;Z_\mathrm{\odot}$) with our updated N-body code BIFROST based on the hierarchical fourth-order forward integrator. BIFROST integrates few-body systems using secular and regularized techniques including post-Newtonian equations of motion up to order PN3.5 and gravitational-wave recoil kicks for BHs. Single stellar evolution is treated using the fast population synthesis code SEVN. We evolve three cluster assembly regions with $N_\mathrm{tot} = 1.70$--$2.35 \times 10^6$ stars following a realistic IMF in $\sim$1000 sub-clusters for $t=50$ Myr. IMBHs with masses up to $m_\bullet \sim 2200\:M_\mathrm{\odot}$ form rapidly mainly via the collapse of very massive stars (VMSs) assembled through repeated collisions of massive stars followed by growth through tidal disruption events and BH mergers. No IMBHs originate from the stars in the initially most massive clusters. We explain this by suppression of hard massive star binary formation at high velocity dispersions and the competition between core collapse and massive star life-times. Later the IMBHs form subsystems resulting in gravitational-wave BH-BH, IMBH-BH and IMBH-IMBH mergers with a $m_\bullet\sim1000\:M_\mathrm{\odot}$ gravitational-wave detection being the observable prediction. Our simulations indicate that the hierarchical formation of massive star clusters in metal poor environments naturally results in formation of potential seeds for supermassive black holes.

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arXiv2024

Star clusters forming in a low metallicity starburst -- rapid self-enrichment by (very) massive stars

Stellar winds of massive ($\gtrsim9\,\mathrm{M_\odot}$) and very massive ($\gtrsim100\,\mathrm{M_\odot}$) stars may play an important role in the metal-enrichment during the formation of star clusters. With novel high-resolution hydrodynamical \textsc{griffin}-project simulations, we investigate the rapid recycling of stellar wind-material during the formation of massive star clusters up to $M_\mathrm{cluster}\sim2\times10^5\,\mathrm{M_\odot}$ in a low-metallicity dwarf galaxy starburst. The simulation realises new stars from a stellar initial mass function (IMF) between $0.08\,\mathrm{M_\odot}$ and $\sim400\,\mathrm{M_\odot}$ and follows stellar winds, radiation and supernova-feedback of single massive stars with evolution tracks. Star clusters form on timescales less than $\sim5$ Myr, and their supernova-material is very inefficiently recycled. Stellar wind-material, however, is trapped in massive clusters resulting in the formation of stars self-enriched in Na, Al, and N within only a few Myr. Wind-enriched (second population) stars can be centrally concentrated in the most massive clusters ($\gtrsim10^4\,\mathrm{M_\odot}$) and the locked wind-material increases approximately as $M_\mathrm{cluster}^{2}$. These trends resemble the characteristics of observed second population stars in globular clusters. We fit scaling relations to the log-normal distributed wind-mass fractions and extrapolate to possible globular cluster progenitors of $M_\mathrm{cluster}=10^7\,\mathrm{M_\odot}$ to investigate whether a dominant second population could form. This can only happen if the IMF is well sampled, single massive stars produce at least a factor of a few more enriched winds e.g. through a top-heavy IMF, and a significant fraction of the first population (unenriched) stars is lost during cluster evolution.

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arXiv2023

Formation of star clusters and enrichment by massive stars in simulations of low-metallicity galaxies with a fully sampled initial stellar mass function

We present new GRIFFIN project hydrodynamical simulations that model the formation of galactic star cluster populations in low-metallicity ($Z=0.00021$) dwarf galaxies, including radiation, supernova and stellar wind feedback of individual massive stars. In the simulations, stars are sampled from the stellar initial mass function (IMF) down to the hydrogen burning limit of $0.08$ M$_\odot$. Mass conservation is enforced within a radius of $1$ pc for the formation of massive stars. We find that massive stars are preferentially found in star clusters and follow a correlation set at birth between the highest initial stellar mass and the star cluster mass that differs from pure stochastic IMF sampling. With a fully sampled IMF, star clusters lose mass in the galactic tidal field according to mass-loss rates observed in nearby galaxies. Of the released stellar feedback, $60\%$ of the supernova material and up to $35\%$ of the wind material reside either in the hot interstellar medium (ISM) or in gaseous, metal enriched outflows. While stellar winds (instantaneously) and supernovae (delayed) start enriching the ISM right after the first massive stars form, the formation of supernova-enriched stars and star clusters is significantly delayed (by $>50$ Myr) compared to the formation of stars and star clusters enriched by stellar winds. Overall, supernova ejecta dominate the enrichment by mass, while the number of enriched stars is determined by continuous stellar winds. These results present a concept for the formation of chemically distinct populations of stars in bound star clusters, reminiscent of multiple populations in globular clusters.

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arXiv2022

A panchromatic view of star cluster formation in a simulated dwarf galaxy starburst

We present a photometric analysis of star and star cluster (SC) formation in a high-resolution simulation of a dwarf galaxy starburst that allows the formation of individual stars to be followed. Previous work demonstrated that the properties of the SCs formed in the simulation are in good agreement with observations. In this paper, we create mock spectral energy distributions and broad-band photometric images using the radiative transfer code SKIRT 9. We test several observational star formation rate (SFR) tracers and find that $24$ $μ$m, total infrared and H$α$ trace the underlying SFR during the (post)starburst phase, while UV tracers yield a more accurate picture of star formation during quiescent phases prior to and after the merger. We then place the simulated galaxy at distances of $10$ and $50$ Mpc and use aperture photometry at Hubble Space Telescope resolution to analyse the simulated SC population. During the starburst phase, a hierarchically forming set of SCs leads inaccurate source separation because of crowding. This results in estimated SC mass function slopes that are up to $\sim0.3$ shallower than the true slope of $\sim-1.9$ to $-2$ found for the bound clusters identified from the particle data in the simulation. The masses of the largest clusters are overestimated by a factor of up to $2.9$ due to unresolved clusters within the apertures. The aperture-based analysis also produces a relation between cluster formation efficiency and SFR surface density that is slightly flatter than that recovered from bound clusters. The differences are strongest in quiescent SF environments.

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arXiv2022

The origin of the [CII]-deficit in a simulated dwarf galaxies starburst

We present [CII] synthetic observations of smoothed particle hydrodynamics (SPH) simulations of a dwarf galaxy merger. The merging process varies the star-formation rate by more than three orders of magnitude. Several star clusters are formed, the feedback of which disperses and unbinds the dense gas through expanding HII regions and supernova (SN) explosions. For galaxies with properties similar to the modelled ones, we find that the [CII] emission remains optically thin throughout the merging process. We identify the Warm Neutral Medium ($3<\log T_{\rm gas}<4$ with $χ_{\rm HI}>2χ_{\rm H2}$) to be the primary source of [CII] emission ($\sim58\%$ contribution), although at stages when the HII regions are young and dense (during star cluster formation or SNe in the form of ionized bubbles) they can contribute $\gtrsim50\%$ to the total [CII] emission. We find that the [CII]/FIR ratio decreases due to thermal saturation of the [CII] emission caused by strong FUV radiation fields emitted by the massive star clusters, leading to a [CII]-deficit medium. We investigate the [CII]-SFR relation and find an approximately linear correlation which agrees well with observations, particularly those from the Dwarf Galaxy Survey. Our simulation reproduces the observed trends of [CII]/FIR versus $Σ_{\rm SFR}$ and $Σ_{\rm FIR}$, and it agrees well with the Kennicutt relation of SFR-FIR luminosity. We propose that local peaks of [CII] in resolved observations may provide evidence for ongoing massive cluster formation.

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arXiv2022

The Column Densities of Molecular Gas across Cosmic Time: Bridging Observations and Simulations

Observations of the cosmic evolution of different gas phases across time indicate a marked increase in the molecular gas mass density towards $z\sim 2-3$. Such a transformation implies an accompanied change in the global distribution of molecular hydrogen column densities ($N_{\rm{H_2}}$). Using observations by PHANGS-ALMA/SDSS and simulations by GRIFFIN/IllustrisTNG we explore the evolution of this H$_2$ column density distribution function [$f(N_{\rm{H}_2})$]. The H$_2$ (and HI) column density maps for TNG50 and TNG100 are derived in post-processing and are made available through the IllustrisTNG online API. The shape and normalization of $f(N_{\rm{H}_2})$ of individual main-sequence star-forming galaxies are correlated with the star formation rate (SFR), stellar mass (${M_*}$), and H$_2$ mass ($M_{\rm{H}_2}$) in both observations and simulations. TNG100, combined with H$_2$ post-processing models, broadly reproduces observations, albeit with differences in slope and normalization. Also, an analytically modelled $f(N)$, based on exponential gas disks, matches well with the simulations. The GRIFFIN simulation gives first indications that the slope of $f(N_{\rm{H}_2})$ might not majorly differ when including non-equilibrium chemistry in simulations. The $f(N_{\rm{H}_2})$ by TNG100 implies that higher molecular gas column densities are reached at $z=3$ than at $z=0$. Further, denser regions contribute more to the molecular mass density at $z=3$. Finally, H$_2$ starts dominating compared to HI only at column densities above log($N_{\rm{H}_2} / \rm{cm}^{-2}) \sim 21.8-22$ at both redshifts. These results imply that neutral atomic gas is an important contributor to the overall cold gas mass found in the ISM of galaxies including at densities typical for molecular clouds at $z=0$ and $z=3$.

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arXiv2021

The challenge of simulating the star cluster population of dwarf galaxies with resolved interstellar medium

We present results on the star cluster properties from a series of high resolution smoothed particles hydrodynamics (SPH) simulations of isolated dwarf galaxies as part of the GRIFFIN project. The simulations at sub-parsec spatial resolution and a minimum particle mass of 4 $\mathrm{M_\odot}$ incorporate non-equilibrium heating, cooling and chemistry processes, and realise individual massive stars. All the simulations follow feedback channels of massive stars that include the interstellar-radiation field, that is variable in space and time, the radiation input by photo-ionisation and supernova explosions. Varying the star formation efficiency per free-fall time in the range $ε_\mathrm{ff}$ = 0.2 - 50$\%$ neither changes the star formation rates nor the outflow rates. While the environmental densities at star formation change significantly with $ε_\mathrm{ff}$, the ambient densities of supernovae are independent of $ε_\mathrm{ff}$ indicating a decoupling of the two processes. At low $ε_\mathrm{ff}$, more massive, and increasingly more bound star clusters are formed, which are typically not destroyed. With increasing $ε_\mathrm{ff}$ there is a trend for shallower cluster mass functions and the cluster formation efficiency $Γ$ for young bound clusters decreases from $50 \%$ to $\sim 1 \%$ showing evidence for cluster disruption. However, none of our simulations form low mass ($< 10^3$ $\mathrm{M_\odot}$) clusters with structural properties in perfect agreement with observations. Traditional star formation models used in galaxy formation simulations based on local free-fall times might therefore not be able to capture low mass star cluster properties without significant fine-tuning.

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