arXiv subjects

Christoph Federrath

Publications and source records attributed to Christoph Federrath.

At least 109 records · Page 6Linked to original sources

arXiv2021

Velocity structure functions in multiphase turbulence: interpreting kinematics of H$α$ filaments in cool core clusters

The central regions of cool-core galaxy clusters harbour multiphase gas, with gas temperatures ranging from $10$ $\mathrm{K}$--$10^7$$\mathrm{K}$. Feedback from active galactic nuclei (AGNs) jets prevents the gas from undergoing a catastrophic cooling flow. However, the exact mechanism of this feedback energy input is unknown, mainly due to the lack of velocity measurements of the hot phase gas. However, recent observations have measured the velocity structure functions ($\mathrm{VSF}$s) of the cooler molecular ($\sim10$$\mathrm{K}$) and H$α$ filaments ($\sim10^4$$\mathrm{K}$) and used them to indirectly estimate the motions of the hot phase. In the first part of this study, we conduct high-resolution ($384^3$--$1536^3$ resolution elements) simulations of homogeneous isotropic subsonic turbulence, without radiative cooling. We analyse the second-order velocity structure functions ($\mathrm{VSF}_2$) in these simulations and study the effects of varying spatial resolution, the introduction of magnetic fields, and the effect of projection along the line of sight (LOS) on it. In the second part of the study, we analyse high-resolution ($768^3$ resolution elements) idealised simulations of multiphase turbulence in the intracluster medium (ICM) from Mohapatra et al 2021. We compare the $\mathrm{VSF}_2$ for both the hot ($T\sim10^7$$\mathrm{K}$) and cold ($T\sim10^4$$\mathrm{K}$) phases and find that their amplitude depends on the density contrast between the phases. They have similar scaling with separation, but introducing magnetic fields steepens the $\mathrm{VSF}_2$ of only the cold phase. We also find that projection along the LOS steepens the $\mathrm{VSF}_2$ for the hot phase and mostly flattens it for the cold phase.

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arXiv2021

A new method for measuring the 3D turbulent velocity dispersion of molecular clouds

The structure and star formation activity of a molecular cloud are fundamentally linked to its internal turbulence. However, accurately measuring the turbulent velocity dispersion is challenging due to projection effects and observational limitations, such as telescope resolution, particularly for clouds that include non-turbulent motions, such as large-scale rotation. Here we develop a new method to recover the three-dimensional (3D) turbulent velocity dispersion ($σ_{v,3D}$) from position-position-velocity (PPV) data. We simulate a rotating, turbulent, collapsing molecular cloud and compare its intrinsic $σ_{v,3D}$ with three different measures of the velocity dispersion accessible in PPV space: 1) the spatial mean of the 2nd-moment map, $σ_i$, 2) the standard deviation of the gradient/rotation-corrected 1st-moment map, $σ_{(c-grad)}$, and 3) a combination of 1) and 2), called the 'gradient-corrected parent velocity dispersion', $σ_{(p-grad)}=(σ^2_i+σ^2_{(c-grad)})^{1/2}$. We show that the gradient correction is crucial in order to recover purely turbulent motions of the cloud, independent of the orientation of the cloud with respect to the line of sight (LOS). We find that with a suitable correction factor and appropriate filters applied to the moment maps, all three statistics can be used to recover $σ_{v,3D}$, with method 3 being the most robust and reliable. We determine the correction factor as a function of the telescope beam size for different levels of cloud rotation, and find that for a beam FWHM $f$ and cloud radius $R$, the 3D turbulent velocity dispersion can best be recovered from the gradient-corrected parent velocity dispersion via $σ_{v,3D}=[(-0.29\pm0.26)f/R+1.93\pm0.15]σ_{(p-grad)}$ for $f/R<1$, independent of the level of cloud rotation or LOS orientation.

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arXiv2021

Impact of relativistic jets on the star formation rate: a turbulence-regulated framework

We apply a turbulence-regulated model of star formation to calculate the star formation rate (SFR) of dense star-forming clouds in simulations of jet-ISM interactions. The method isolates individual clumps and accounts for the impact of virial parameter and Mach number of the clumps on the star formation activity. This improves upon other estimates of the SFR in simulations of jet--ISM interactions, which are often solely based on local gas density, neglecting the impact of turbulence. We apply this framework to the results of a suite of jet-ISM interaction simulations to study how the jet regulates the SFR both globally and on the scale of individual star-forming clouds. We find that the jet strongly affects the multi-phase ISM in the galaxy, inducing turbulence and increasing the velocity dispersion within the clouds. This causes a global reduction in the SFR compared to a simulation without a jet. The shocks driven into clouds by the jet also compress the gas to higher densities, resulting in local enhancements of the SFR. However, the velocity dispersion in such clouds is also comparably high, which results in a lower SFR than would be observed in galaxies with similar gas mass surface densities and without powerful radio jets. We thus show that both local negative and positive jet feedback can occur in a single system during a single jet event, and that the star-formation rate in the ISM varies in a complicated manner that depends on the strength of the jet-ISM coupling and the jet break-out time-scale.

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arXiv2021

First extragalactic measurement of the turbulence driving parameter: ALMA observations of the star-forming region N159E in the Large Magellanic Cloud

Studying the driving modes of turbulence is important for characterizing the impact of turbulence in various astrophysical environments. The driving mode of turbulence is parameterized by $b$, which relates the width of the gas density PDF to the turbulent Mach number; $b\approx 1/3$, $1$, and $0.4$ correspond to driving that is solenoidal, compressive, and a natural mixture of the two, respectively. In this work, we use high-resolution (sub-pc) ALMA $^{12}$CO ($J$ = $2-1$), $^{13}$CO ($J$ = $2-1$), and C$^{18}$O ($J$ = $2-1$) observations of filamentary molecular clouds in the star-forming region N159E (the Papillon Nebula) in the Large Magellanic Cloud (LMC) to provide the first measurement of turbulence driving parameter in an extragalactic region. We use a non-local thermodynamic equilibrium (NLTE) analysis of the CO isotopologues to construct a gas density PDF, which we find to be largely log-normal in shape with some intermittent features indicating deviations from lognormality. We find that the width of the log-normal part of the density PDF is comparable to the supersonic turbulent Mach number, resulting in $b \approx 0.9$. This implies that the driving mode of turbulence in N159E is primarily compressive. We speculate that the compressive turbulence could have been powered by gravo-turbulent fragmentation of the molecular gas, or due to compression powered by H I flows that led to the development of the molecular filaments observed by ALMA in the region. Our analysis can be easily applied to study the nature of turbulence driving in resolved star-forming regions in the local as well as the high-redshift Universe.

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arXiv2021

Quantifying stochasticity-driven uncertainties in H II region metallicities

With the advent of Integral Field Units (IFUs), surveys can now measure metallicities across the discs of nearby galaxies at scales $\lesssim 100~$ pc. At such small scales, many of these regions contain too few stars to fully sample all possible stellar masses and evolutionary states, leading to stochastic fluctuations in the ionising continuum. The impact of these fluctuations on the line diagnostics used to infer galaxy metallicities is poorly understood. In this paper, we quantify this impact for six most commonly-used diagnostics. We generate stochastic stellar populations for galaxy patches with star formation rates varying over a factor of $1000$, compute the nebular emission that results when these stars ionise gas at a wide range of densities, metallicities, and determine how much inferred metallicities vary with fluctuations in the driving stellar spectrum. We find that metallicities derived from diagnostics that measure multiple ionisation states of their target elements (e.g.~electron temperature methods) are weakly affected (variation $< 0.1$ dex), but that larger fluctuations ($\sim 0.4$ dex) occur for diagnostics that depend on a single ionisation state. Scatter in the inferred metallicity is generally largest at low star formation rate and metallicity, and is larger for more sensitive observations than for shallower ones. The main cause of the fluctuations is stochastic variation in the ionisation state in the nebula in response to the absence of Wolf-Rayet stars, which dominate the production of $\gtrsim 2-3$ Ryd photons. Our results quantify the trade-off between line brightness and diagnostic accuracy, and can be used to optimise observing strategies for future IFU campaigns.

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arXiv2021

The Effects of Magnetic Fields and Outflow Feedback on the Shape and Evolution of the Density PDF in Turbulent Star-Forming Clouds

Using a suite of 3D hydrodynamical simulations of star-forming molecular clouds, we investigate how the density probability distribution function (PDF) changes when including gravity, turbulence, magnetic fields, and protostellar outflows and heating. We find that the density PDF is not lognormal when outflows and self-gravity are considered. Self-gravity produces a power-law tail at high densities and the inclusion of stellar feedback from protostellar outflows and heating produces significant time-varying deviations from a lognormal distribution at the low densities. The simulation with outflows has an excess of diffuse gas compared to the simulations without outflows, exhibits increased average sonic Mach number, and maintains a slower star formation rate over the entire duration of the run. We study the mass transfer between the diffuse gas in the lognormal peak of the PDF, the collapsing gas in the power-law tail, and the stars. We find that the mass fraction in the power-law tail is constant, such that the stars form out of the power-law gas at the same rate at which the gas from the lognormal part replenishes the power-law. We find that turbulence does not provide significant support in the dense gas associated with the power-law tail. When including outflows and magnetic fields in addition to driven turbulence, the rate of mass transfer from the lognormal to the power-law, and then to the stars, becomes significantly slower, resulting in slower star formation rates and longer depletion times.

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arXiv2021

Saturation mechanism of the fluctuation dynamo in supersonic turbulent plasmas

Magnetic fields in several astrophysical objects are amplified and maintained by a dynamo mechanism, which is the conversion of the turbulent kinetic energy to magnetic energy. A dynamo that amplifies magnetic fields at scales $<$ the driving scale of turbulence is known as the fluctuation dynamo. We study the properties of the fluctuation dynamo in supersonic turbulent plasmas, which is of relevance to the ISM, structure formation, and lab experiments of laser-plasma turbulence. Using simulations, we explore the properties of the exponentially growing and saturated state of the fluctuation dynamo for subsonic and supersonic turbulence. We confirm that the fluctuation dynamo efficiency decreases with compressibility. We show that the fluctuation dynamo generated magnetic fields are spatially intermittent and the level of intermittency decreases as the field saturates. We find a stronger back reaction of the magnetic field on the velocity for the subsonic case as compared to the supersonic case. Locally, we find that the level of alignment between vorticity and velocity, velocity and magnetic field, and current density and magnetic field in the saturated stage is enhanced in comparison to the exponentially growing phase for the subsonic case, but only the current density and magnetic field alignment is enhanced for the supersonic case. We show that both the magnetic field amplification (due to weaker stretching of field lines) and diffusion decreases when the field saturates, but the diffusion is enhanced relative to amplification. This occurs throughout the volume in the subsonic turbulence, but primarily in the strong-field regions for the supersonic case. This leads to the saturation of the fluctuation dynamo. Overall, both the amplification and diffusion of magnetic fields are affected and thus a drastic change in either of them is not required for the saturation. [Abstract abridged]

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arXiv2021

On the origin of the mass-metallicity gradient relation in the local Universe

In addition to the well-known gas phase mass-metallicity relation (MZR), recent spatially-resolved observations have shown that local galaxies also obey a mass-metallicity gradient relation (MZGR) whereby metallicity gradients can vary systematically with galaxy mass. In this work, we use our recently-developed analytic model for metallicity distributions in galactic discs, which includes a wide range of physical processes -- radial advection, metal diffusion, cosmological accretion, and metal-enriched outflows -- to simultaneously analyse the MZR and MZGR. We show that the same physical principles govern the shape of both: centrally-peaked metal production favours steeper gradients, and this steepening is diluted by the addition of metal-poor gas, which is supplied by inward advection for low-mass galaxies and by cosmological accretion for massive galaxies. The MZR and the MZGR both bend at galaxy stellar mass $\sim 10^{10} - 10^{10.5}\,\rm{M_{\odot}}$, and we show that this feature corresponds to the transition of galaxies from the advection-dominated to the accretion-dominated regime. We also find that both the MZR and MZGR strongly suggest that low-mass galaxies preferentially lose metals entrained in their galactic winds. While this metal-enrichment of the galactic outflows is crucial for reproducing both the MZR and the MZGR at the low-mass end, we show that the flattening of gradients in massive galaxies is expected regardless of the nature of their winds.

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arXiv2021

The IMF and multiplicity of stars from gravity, turbulence, magnetic fields, radiation and outflow feedback

We perform a series of three-dimensional, magnetohydrodynamical (MHD) simulations of star cluster formation including gravity, turbulence, magnetic fields, stellar radiative heating and outflow feedback. We observe that the inclusion of protostellar outflows (1) reduces the star formation rate by a factor of $\sim2$, (2) increases fragmentation, and (3) shifts the initial mass function (IMF) to lower masses by a factor of $2.0\pm0.2$, without significantly affecting the overall shape of the IMF. The form of the sink particle (protostellar objects) mass distribution obtained from our simulations matches the observational IMFs reasonably well. We also show that turbulence-based theoretical models of the IMF agree well with our simulation IMF in the high-mass and low-mass regime, but do not predict any brown dwarfs, whereas our simulations produce a considerable number of sub-stellar objects, which are produced by dynamical interactions (ejections). We find that these dynamical interactions also play a key role for the binary separation distribution and stellar kinematics in general. Our numerical model of star cluster formation also reproduces the observed mass dependence of multiplicity. Our multiplicity fraction estimates generally concur with the observational estimates for different spectral types. We further calculate the specific angular momentum of all the sink particles and find that the average value of $1.5 \times 10^{19}\, \mathrm{cm^2\, s^{-1}}$ is consistent with observational data. The specific angular momentum of our sink particles lies in the range typical of protostellar envelopes and binaries. We conclude that the IMF is controlled by a combination of gravity, turbulence, magnetic fields, radiation and outflow feedback.

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arXiv2021

The Dependence of the Hierarchical Distribution of Star Clusters on Galactic Environment

We use the angular Two Point Correlation Function (TPCF) to investigate the hierarchical distribution of young star clusters in 12 local (3--18 Mpc) star-forming galaxies using star cluster catalogues obtained with the \textit{Hubble Space Telescope} (\textit{HST}) as part of the Treasury Program LEGUS (Legacy ExtraGalactic UV Survey). The sample spans a range of different morphological types, allowing us to infer how the physical properties of the galaxy affect the spatial distribution of the clusters. We also prepare a range of physically motivated toy models to compare with and interpret the observed features in the TPCFs. We find that, conforming to earlier studies, young clusters ($T \la 10\, \mathrm{Myr}$) have power-law TPCFs that are characteristic of fractal distributions with a fractal dimension $D_2$, and this scale-free nature extends out to a maximum scale $l_{\mathrm{corr}}$ beyond which the distribution becomes Poissonian. However, $l_{\mathrm{corr}}$, and $D_2$ vary significantly across the sample, and are correlated with a number of host galaxy physical properties, suggesting that there are physical differences in the underlying star cluster distributions. We also find that hierarchical structuring weakens with age, evidenced by flatter TPCFs for older clusters ($T \ga 10\, \mathrm{Myr}$), that eventually converges to the residual correlation expected from a completely random large-scale radial distribution of clusters in the galaxy in $\sim 100 \, \mathrm{Myr}$. Our study demonstrates that the hierarchical distribution of star clusters evolves with age, and is strongly dependent on the properties of the host galaxy environment.

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arXiv2021

The density structure of supersonic self-gravitating turbulence

We conduct numerical experiments to determine the density probability distribution function (PDF) produced in supersonic, isothermal, self-gravitating turbulence of the sort that is ubiquitous in star-forming molecular clouds. Our experiments cover a wide range of turbulent Mach number and virial parameter, allowing us for the first time to determine how the PDF responds as these parameters vary, and we introduce a new diagnostic, the dimensionless star formation efficiency versus density ($ε_{\rm ff}(s)$) curve, which provides a sensitive diagnostic of the PDF shape and dynamics. We show that the PDF follows a universal functional form consisting of a log-normal at low density with two distinct power law tails at higher density; the first of these represents the onset of self-gravitation, and the second reflects the onset of rotational support. Once the star formation efficiency reaches a few percent, the PDF becomes statistically steady, with no evidence for secular time-evolution at star formation efficiencies from about five to 20 percent. We show that both the Mach number and the virial parameter influence the characteristic densities at which the log-normal gives way to the first power-law, and the first to the second, and we extend (for the former) and develop (for the latter) simple theoretical models for the relationship between these density thresholds and the global properties of the turbulent medium.

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arXiv2021

Shock-multicloud interactions in galactic outflows -- II. Radiative fractal clouds and cold gas thermodynamics

Galactic winds are crucial to the cosmic cycle of matter, transporting material out of the dense regions of galaxies. Observations show the coexistence of different temperature phases in such winds, which is not easy to explain. We present a set of 3D shock-multicloud simulations that account for radiative heating and cooling at temperatures between $10^2\,\rm K$ and $10^7\,\rm K$. The interplay between shock heating, dynamical instabilities, turbulence, and radiative heating and cooling creates a complex multi-phase flow with a rain-like morphology. Cloud gas fragments and is continuously eroded, becoming efficiently mixed and mass loaded. The resulting warm mixed gas then cools down and precipitates into new dense cloudlets, which repeat the process. Thus, radiative cooling is able to sustain fast-moving dense gas by aiding condensation of gas from warm clouds and the hot wind. In the ensuing outflow, hot gas with temperatures $\gtrsim 10^6\,\rm K$ outruns the warm and cold phases, which reach thermal equilibrium near $\approx 10^4\,\rm K$ and $\approx 10^2\,\rm K$, respectively. Although the volume filling factor of hot gas is higher in the outflow, most of the mass is concentrated in dense gas cloudlets and filaments with these temperatures. More porous multicloud layers result in more vertically extended outflows, and dense gas is more efficiently produced in more compact layers. The cold phase is not accelerated by ram-pressure, but, instead, precipitates from warm and mixed gas out of thermal equilibrium. This cycle can explain the presence of high-velocity H\,{\sc i} gas with $N_{\rm H\,{\scriptstyle I}}=10^{19-21}\,\rm cm^{-2}$ and $Δv_{\rm FWHM}\lesssim37\,\rm km\,s^{-1}$ in the Galactic centre outflow.

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arXiv2021

The role of gas kinematics in setting metallicity gradients at high redshift

In this work, we explore the diversity of ionised gas kinematics (rotational velocity $v_ϕ$ and velocity dispersion $σ_{\mathrm{g}}$) and gas-phase metallicity gradients at $0.1 \leq z \leq 2.5$ using a compiled data set of 74 galaxies resolved with ground-based integral field spectroscopy. We find that galaxies with the highest and the lowest $σ_{\mathrm{g}}$ have preferentially flat metallicity gradients, whereas those with intermediate values of $σ_{\mathrm{g}}$ show a large scatter in the metallicity gradients. Additionally, steep negative gradients appear almost only in rotation-dominated galaxies ($v_ϕ/σ_{\mathrm{g}} > 1$), whereas most dispersion-dominated galaxies show flat gradients. We use our recently developed analytic model of metallicity gradients to provide a physical explanation for the shape and scatter of these observed trends. In the case of high $σ_{\mathrm{g}}$, the inward radial advection of gas dominates over metal production and causes efficient metal mixing, thus giving rise to flat gradients. For low $σ_{\mathrm{g}}$, it is the cosmic accretion of metal-poor gas diluting the metallicity that gives rise to flat gradients. Finally, the reason for intermediate $σ_{\mathrm{g}}$ showing the steepest negative gradients is that both inward radial advection and cosmic accretion are weak as compared to metal production, which leads to the creation of steeper gradients. The larger scatter at intermediate $σ_{\mathrm{g}}$ may be due in part to preferential ejection of metals in galactic winds, which can decrease the strength of the production term. Our analysis shows how gas kinematics play a critical role in setting metallicity gradients in high-redshift galaxies.

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arXiv2021

The Global Magneto-Ionic Medium Survey (GMIMS): The brightest polarized region in the Southern sky at 75cm and its implications for Radio Loop II

Using the Global Magneto-Ionic Medium Survey (GMIMS) Low-Band South (LBS) southern sky polarization survey, covering 300 to 480 MHz at 81 arcmin resolution, we reveal the brightest region in the Southern polarized sky at these frequencies. The region, G150-50, covers nearly 20deg$^2$, near (l,b)~(150 deg,-50 deg). Using GMIMS-LBS and complementary data at higher frequencies (~0.6--30 GHz), we apply Faraday tomography and Stokes QU-fitting techniques. We find that the magnetic field associated with G150-50 is both coherent and primarily in the plane of the sky, and indications that the region is associated with Radio Loop II. The Faraday depth spectra across G150-50 are broad and contain a large-scale spatial gradient. We model the magnetic field in the region as an expanding shell, and we can reproduce both the observed Faraday rotation and the synchrotron emission in the GMIMS-LBS band. Using QU-fitting, we find that the Faraday spectra are produced by several Faraday dispersive sources along the line-of-sight. Alternatively, polarization horizon effects that we cannot model are adding complexity to the high-frequency polarized spectra. The magnetic field structure of Loop II dominates a large fraction of the sky, and studies of the large-scale polarized sky will need to account for this object. Studies of G150-50 with high angular resolution could mitigate polarization horizon effects, and clarify the nature of G150-50.

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arXiv2021

A multi-shock model for the density variance of anisotropic, highly-magnetised, supersonic turbulence

Shocks form the basis of our understanding for the density and velocity statistics of supersonic turbulent flows, such as those found in the cool interstellar medium (ISM). The variance of the density field, $σ^2_{ρ/ρ_0}$, is of particular interest for molecular clouds (MCs), the birthplaces of stars in the Universe. The density variance may be used to infer underlying physical processes in an MC, and parameterises the star formation (SF) rate of a cloud. However, models for $σ^2_{ρ/ρ_0}$ all share a common feature -- the variance is assumed to be isotropic. This assumption does not hold when a trans/sub-Alfvénic mean magnetic field, $\vec{B}_0$, is present in the cloud, which observations suggest is relevant for some MCs. We develop an anisotropic model for $σ_{ρ/ρ_0}^2$, using contributions from hydrodynamical and fast magnetosonic shocks that propagate orthogonal to each other. Our model predicts an upper bound for $σ_{ρ/ρ_0}^2$ in the high Mach number $(\mathcal{M})$ limit as small-scale density fluctuations become suppressed by the strong $\vec{B}_0$. The model reduces to the isotropic $σ_{ρ/ρ_0}^2-\mathcal{M}$ relation in the hydrodynamical limit. To validate our model, we calculate $σ_{ρ/ρ_0}^2$ from 12~high-resolution, three-dimensional, supersonic, sub-Alfvénic magnetohydrodynamical (MHD) turbulence simulations and find good agreement with our theory. We discuss how the two MHD shocks may be the bimodally oriented over-densities observed in some MCs and the implications for SF theory in the presence of a sub-Alfvénic $\vec{B}_0$. By creating an anisotropic, supersonic density fluctuation model, this study paves the way for SF theory in the highly anisotropic regime of interstellar turbulence.

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arXiv2021

The Single-Cloud Star Formation Relation

One of the most important and well-established empirical results in astronomy is the Kennicutt-Schmidt (KS) relation between the density of interstellar gas and the rate at which that gas forms stars. A tight correlation between these quantities has long been measured at galactic scales. More recently, using surveys of YSOs, a KS relationship has been found within molecular clouds relating the surface density of star formation to the surface density of gas; however, the scaling of these laws varies significantly from cloud to cloud. In this Letter, we use a recently developed, high-accuracy catalog of young stellar objects from $\textit{Spitzer}$ combined with high-dynamic-range gas column density maps of twelve nearby ($<$1.5 kpc) molecular clouds from $\textit{Herschel}$ to re-examine the KS relation within individual molecular clouds. We find a tight, linear correlation between clouds' star formation rate per unit area and their gas surface density normalized by the gas free-fall time. The measured intracloud KS relation, which relates star formation rate to the volume density, extends over more than two orders of magnitude within each cloud and is nearly identical in each of the twelve clouds, implying a constant star formation efficiency per free-fall time $ε_{\rm ff}\approx 0.026$. The finding of a universal correlation within individual molecular clouds, including clouds that contain no massive stars or massive stellar feedback, favors models in which star formation is regulated by local processes such as turbulence or stellar feedback such as protostellar outflows, and disfavors models in which star formation is regulated only by galaxy properties or supernova feedback on galactic scales.

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arXiv2021

The physics of gas phase metallicity gradients in galaxies

We present a new model for the evolution of gas phase metallicity gradients in galaxies from first principles. We show that metallicity gradients depend on four ratios that collectively describe the metal equilibration timescale, production, transport, consumption, and loss. Our model finds that most galaxy metallicity gradients are in equilibrium at all redshifts. When normalized by metal diffusion, metallicity gradients are governed by the competition between radial advection, metal production, and accretion of metal-poor gas from the cosmic web. The model naturally explains the varying gradients measured in local spirals, local dwarfs, and high-redshift star-forming galaxies. We use the model to study the cosmic evolution of gradients across redshift, showing that the gradient in Milky Way-like galaxies has steepened over time, in good agreement with both observations and simulations. We also predict the evolution of metallicity gradients with redshift in galaxy samples constructed using both matched stellar masses and matched abundances. Our model shows that massive galaxies transition from the advection-dominated to the accretion-dominated regime from high to low redshifts, which mirrors the transition from gravity-driven to star formation feedback-driven turbulence. Lastly, we show that gradients in local ultraluminous infrared galaxies (major mergers) and inverted gradients seen both in the local and high-redshift galaxies may not be in equilibrium. In subsequent papers in this series, we show that the model also explains the observed relationship between galaxy mass and metallicity gradients, and between metallicity gradients and galaxy kinematics.

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arXiv2021

Magnetic field amplification in accretion discs around the first stars: implications for the primordial IMF

Magnetic fields play an important role in the dynamics of present-day molecular clouds. Recent work has shown that magnetic fields are equally important for primordial clouds, which form the first stars in the Universe. While the primordial magnetic field strength on cosmic scales is largely unconstrained, theoretical models strongly suggest that a weak seed field existed in the early Universe. We study how the amplification of such a weak field can influence the evolution of accretion discs around first stars, and thus affect the primordial initial mass function (IMF). We perform a suite of 3D ideal magneto-hydrodynamic (MHD) simulations with different initial field strengths and numerical resolutions. We find that, in simulations with sufficient spatial resolution to resolve the Jeans scale during the collapse, even initially weak magnetic fields grow exponentially to become dynamically important due to both the so-called 'small-scale turbulent dynamo' and the 'large-scale mean-field dynamo'. Capturing the small-scale dynamo action depends primarily on how well we resolve the Jeans length, while capturing the large-scale dynamo depends on the Jeans resolution as well as the maximum absolute resolution. Provided enough resolution, we find that fragmentation does not depend strongly on the initial field strength, because even weak fields grow to become strong. However, fragmentation in runs with magnetic fields differs significantly from those without magnetic fields. We conclude that the development of dynamically strong magnetic fields during the formation of the first stars is likely inevitable, and that these fields had a significant impact on the primordial IMF.

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