Galactic Disk Research Articles

Can Supernovae from Runaway Stars Mimic the Signs of Absorbing “Supervirial” Gas?

The recent detection of large column density absorption lines from highly ionized gas in a few directions through the circumgalactic medium (CGM) of the Milky Way (MW) has been puzzling. The inferred temperature from these absorption lines far exceeds the virial temperature of the MW, and the column densities are also too large to be easily explained. In this paper, we propose a novel idea to explain these observations and claim that they may not have originated from the CGM, but from a totally different type of source, namely, stellar ejecta from supernovae (SNe) above the Galactic disk that happen to lie in the line of sight to the background quasars. About ∼20% of massive OB stars (progenitors of core-collapse supernovae) are known to be runaway stars that have high ejection velocities near the Galactic plane and can end up exploding as SNe above the Galactic disk. We show that the associated reverse shock in the supernova remnant in the early nonradiative phase can heat the ejecta to temperatures of ≳107 K and can naturally explain the observed high column density of ions in the observed “supervirial” phase along with α-enriched supersolar abundance that is typical of core-collapse supernovae. However, SNe from runaway stars has a covering fraction of ≲0.7% and thus can only explain the observations along limited sightlines.

The Distance to Dobashi 6193—A Dark Cloud Shadowing the eROSITA Bubbles

We focus on a prominent gas cloud, Dobashi 6193, toward the western base of the northern eROSITA bubble (eRObub). It can potentially put a tight distance constraint on the eRObubs and clarify the emissions of the Loop I superbubble from the eRObubs. We found its distance to be 700 pc based on Gaia DR3 stellar extinction and parallax measurements, making it one of the most distant clouds shadowing the eRObubs well above the Galactic disk.

A Potential Dynamical Origin of the Galactic Disk Warp: The Gaia–Sausage–Enceladus Major Merger

Previous studies have revealed that the Galactic warp is a long-lived, nonsteady, and asymmetric structure. There is a need for a model that accounts for the warp’s long-term evolution. Given that this structure has persisted for over 5 Gyr, its timeline may coincide with the completion of the Gaia–Sausage–Enceladus (GSE) merger. Recent studies indicate that the GSE, the significant merger of our Galaxy, was likely a gas-rich merger and the large amount of gas introduced could have created a profound impact on the Galactic morphology. This study utilizes GIZMO simulation code to construct a gas-rich GSE merger. By reconstructing the observed characteristics of the GSE, we successfully reproduce the disk warp and capture nearly all of its documented features, which align closely with observational data from both stellar and gas disks. This simulation demonstrates the possibility that a single major merger could generate the Galactic warp amplitude and precession. Furthermore, the analysis of the warp’s long-term evolution may offer more clues into the formation history of the Milky Way.

Assessing the robustness of the Galactic rotation curve inferred from the Jeans equations using Gaia DR3 and cosmological simulations

Several authors have recently applied Jeans modelling to -based datasets to infer the circular velocity curve for the Milky Way. These works have consistently found evidence for a continuous decline in the rotation curve beyond $ kpc $, which may indicate the existence of a light dark matter (DM) halo. Using a large sample of Gaia DR3 data, we aim to derive the rotation curve of the Milky Way using the Jeans equations, and to quantify the role of systematic effects, both in the data and those inherent to the Jeans methodology under the assumptions of axisymmetry and time independence. We used data from the Gaia DR3 radial velocity spectrometer sample, supplemented with distances inferred through Bayesian frameworks, to determine the radial variation of the second moments of the velocity distribution for stars close to the Galactic plane. We used these profiles to determine the rotation curve using the Jeans equations under the assumption of axisymmetry and explored how they vary with azimuth and position above and below the plane of the Galactic disc. We applied the same methodology to an N-body simulation of a Milky Way-like galaxy impacted by a satellite akin the Sagittarius dwarf, and to the Auriga suite of cosmological simulations. The circular velocity curve we infer for the Milky Way is consistent with previous findings out to $ 15$ kpc, where our statistics are robust. Due to the larger number of stars in our sample, we are able to reveal evidence of disequilibrium and deviations from axisymmetry closer in. For example, we find that the second moment of $V_R$ flattens out at $R kpc, and that the second moment of $V_ is different above and below the plane for $R kpc. Our exploration of the simulations indicates that these features are typical of galaxies that have been perturbed by external satellites. From the simulations, we also estimate that the difference between the true circular velocity curve and that inferred from Jeans equations can be as high as $15<!PCT!>$, but that it is likely of the order of $10<!PCT!>$ for the Milky Way. This is higher than the systematic uncertainties associated with the observations or those linked to most modelling assumptions when using the Jeans equations. However, if the density of the tracer population were truncated at large radii instead of being exponential as often assumed, this could lead to the erroneous conclusion of a steeply declining rotation curve. We find that steady-state axisymmetric Jeans modelling becomes less robust at large radii, indicating that particular caution must be exercised when interpreting the rotation curve inferred in those regions. A more careful and sophisticated approach may be necessary for precision measurements of the DM content of our Galaxy.

Apostle–auriga: effects of stellar feedback subgrid models on the evolution of angular momentum in disc galaxies

ABSTRACT Utilizing the apostle–auriga simulations, which start from the same zoom-in initial conditions of Local Group-like systems, but run with different galaxy formation subgrid models and hydrodynamic solvers, we study the impact of stellar feedback models on the evolution of angular momentum in disc galaxies. At $z = 0$, auriga disc galaxies tend to exhibit higher specific angular momenta compared to their cross-matched apostle counterparts. By tracing the evolution history of the Lagrangian mass tracers of the in-situ star particles in the $z = 0$ galaxies, we find that the specific angular momentum distributions of the gas tracers from the two simulations at the halo accretion time are relatively similar. The present-day angular momentum difference is mainly driven by the physical processes occurring inside dark matter haloes, especially galactic fountains. Due to the different subgrid implementations of stellar feedback processes, auriga galaxies contain a high fraction of gas that has gone through recycled fountain (${\sim } 65$ per cent) which could acquire angular momentum through mixing with the high angular momentum circumgalactic medium (CGM). In apostle, however, the fraction of gas that has undergone the recycled fountain process is significantly lower (down to ${\sim } 20$ per cent for Milky Way-sized galaxies) and the angular momentum acquisition from the CGM is marginal. As a result, the present-day auriga galaxies overall have higher specific angular momenta.

CHANG-ES. XXXIII. A 20 kpc radio bubble in the halo of the star-forming galaxy NGC 4217

Cosmic rays may be dynamically very important in driving large-scale galactic winds. Edge-on galaxies give us an outsider's view of radio haloes, and of their extra-planar cosmic-ray electrons and magnetic fields. We present a new radio continuum imaging study of the nearby edge-on galaxy NGC\,4217. We examine the distribution of extra-planar cosmic rays and magnetic fields. We observed it with both the Jansky Very Large Array (JVLA) in the $S$ band (2--4\,GHz) and the LOw Frequency ARray (LOFAR) at 144\,MHz. We measured vertical intensity profiles and exponential scale heights. We re-imaged both the JVLA and LOFAR data at matched angular resolution in order to measure radio spectral indices between 144\,MHz and 3\,GHz. Confusing point-like sources were subtracted prior to imaging. We then fitted intensity profiles with cosmic-ray electron advection models, using an isothermal wind model that is driven by a combination of pressure from the hot gas and cosmic rays. We discover a large-scale radio halo on the north-western side of the galactic disc. The morphology is reminiscent of a bubble extending up to 20\,kpc from the disc. We find spectral ageing in the bubble, which allowed us to measure the advection speeds of the cosmic-ray electrons, which accelerate from 300 to $600\ $. Assuming energy equipartition between the cosmic rays and the magnetic field, we estimate the bubble may have been inflated by a modest 10\,<!PCT!> of the kinetic energy injected by supernovae over its dynamical timescale of 35\,Myr. While no active galactic nucleus (AGN) has been detected, such activity in the recent past cannot be ruled out. Non-thermal bubbles with sizes of tens of kiloparsecs may be a ubiquitous feature of star-forming galaxies, and if so this would demonstrate the influence of feedback. Determining possible contributions by AGN feedback will require deeper observations.

Compact ellipticals in the Antlia cluster of galaxies

Compact ellipticals (cEs) are rare objects in the local Universe, but relevant for the evolution of galaxies. Their origin is not completely understood, with evidence equally suggesting that they are relics of early epochs, and remnants of tidal disruptions. This article aims to characterise two cEs, members of the nearby Antlia cluster, providing insights into the formation channels of cEs residing in high-density environments. This dataset consists of archival ACS/HST images and Gemini/GMOS slit spectra. The isophotal analysis of the cEs was compared with their kinematics and stellar population synthesis. The results were also compared with a sample of cEs and `relic' galaxies from the literature, to put them into context. Their characterisations highlight clear differences between the two cEs. Although the stellar populations for both cEs are mainly old, with mass-weighted ages above 10\,Gyr, FS90\,110, the most massive one, has a minor contribution from an intermediate-age population. Besides, it presents rotational evidence, a remarkably bright nucleus for its luminosity, and a clear dichotomy between the inner and outer components. It is probably the remnant of a low-mass disc galaxy that has been tidally disrupted by the close massive elliptical NGC\,3258. On the contrary, FS90\,192 does not present evidence for either rotation or discy isophotes, and its population synthesis leads to a stellar population with ages above 12\,Gyr. It is probably a low-mass analogue to `relic' galaxies.

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

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

The masses of open star clusters and their tidal tails and the stellar initial mass function

Unresolved binaries have a strong influence on the observed parameters of SC We quantify this influence and compute the resulting mass underestimates and MF N-body simulations of realistic SC were used to investigate the evolution of the binary population in a SC and its tidal tails. Together with an empirically gauged stellar mass-luminosity relation, the results were then used to determine how the presence of binaries changes the photometric mass and MF of the SC and its tails as deduced from observations. T1 which is the tidal tail caused by gas expulsion, contains a larger fraction of binaries than both the SC and T2 which forms after gas expulsion. Additionally T1 has a larger velocity dispersion. Using the luminosity of an unresolved binary, an observer would underestimate its mass. This bias sensitively depends on the companion masses due to the structure of the stellar mass-luminosity relation. Combining the effect of all binaries in the simulation, the total photometric mass of the SC is underestimated by 15<!PCT!>. Dark objects (black holes and neutron stars) increase the difference between the real and observed mass of the SC further. For both the SC and the tails, the observed power-law index of the MF between a stellar mass of 0.3 and 0.7 $M_ is smaller by up to 0.2 than the real one, the real IMF being steeper by this amount. This difference is larger for stars with a larger velocity dispersion or binary fraction. Since the stars formed in SC are the progenitors of the Galactic field stars, this work suggests that the binary fractions of different populations of stars in the Galactic disc will differ as a function of the velocity dispersion. However, the direction of this correlation is currently unclear, and a complete population synthesis will be needed to investigate this effect. Variations in the binary fractions of different clusters can lead to perceived variations of the deduced stellar MF

Comparison of a new type of Dark Matter with the Milky Way and M31 grand rotation curves

In the electron Born self-energy (eBse) model, free electrons are of finite-size and possess both a rest mass, me, as well as, a Born mass, meB = 74,000 me. The Born mass, which originates from the energy contained within the electric field that surrounds a finite-sized electron, serves as a Dark Matter (DM) particle in this theory (designated eBDM, electron Born Dark Matter). The equation of state for meB is w = -1, which implies that two Born masses experience a repulsive gravitational interaction. This repulsive gravitational interaction stabilizes the formation of a DM halo of meB particles, of typical halo size ~ 100 kpc, around a central mass M (e.g. a galaxy), where this gravitational stability arises from the competing attractive M - meB and repulsive meB - meB interactions. A solution of the linearized Poisson-Boltzmann equation, for this system, allows one to derive an expression for the rotational velocity VeBDM(R), as a function of radius R from the galactic center. A composite model composed of rotational velocity contributions from the galactic bulge, galactic disk, as well as, VeBDM(R) is found to provide a good description of the Grand Rotation Curves for the Milky Way and M31 galaxies.

Early-type galaxy speciation: elliptical (E) and ellicular (ES) galaxies in the Mbh–M*,sph diagram, and a merger-driven explanation for the origin of ES galaxies, antitruncated stellar discs in lenticular (S0) galaxies, and the Sérsicification of E galaxy light profiles

ABSTRACT In a recent series of papers, supermassive black holes were used to discern pathways in galaxy evolution. By considering the black holes’ coupling with their host galaxy’s bulge/spheroid, the progression of mass within each component has shed light on the chronological sequence of galaxy speciation. Offsets between the galaxy-morphology-dependent $M_{\rm bh}$–$M_{\rm \star ,sph}$ scaling relations trace a pattern of ‘punctuated equilibrium’ arising from merger-driven transitions between galaxy types, such as from spirals to dust-rich lenticulars and further to ‘ellicular’ and elliptical galaxies. This study delves deeper into the distinction between the ellicular galaxies – characterised by their intermediate-scale discs – and elliptical galaxies. Along the way, it is shown how some antitruncated large-scale discs in lenticular galaxies can arise from the coexistence of a steep intermediate-scale disc and a relatively shallow large-scale disc. This observation undermines application of the popular exponential-disc plus Sérsic-bulge model for lenticular galaxies and suggests some past bulge mass measurements have been overestimated. Furthermore, it is discussed how merger-driven disc-heating and blending likely leads to the spheroidalisation of discs and the conglomeration of multiple discs leads to the (high-n) Sérsicification of light profiles. The ellicular and elliptical galaxy distribution in the $M_{\rm bh}$–$M_{\rm \star ,sph}$ diagram is explored relative to major-merger-built lenticular galaxies and brightest cluster galaxies. The super-quadratic $M_{\rm bh}$–$M_{\rm \star }$ relations, presented herein, for merger-built systems should aid studies of massive black hole collisions and the gravitational wave background. Finally, connections to dwarf compact elliptical and ultracompact dwarf galaxies, with their 100–1000 times higher $M_{\rm bh}/M_{\rm \star ,sph}$ ratios, are presented.

The formation and survival of the Milky Way’s oldest stellar disk

The formation and survival of the Milky Way’s oldest stellar disk

SDSS. IV. MaNGA: The Impact of the Acquisition of Gas with Opposite Angular Momentum on the Evolution of Galaxies

A gaseous counterrotating galaxy is a galaxy containing a gas component with opposite angular momentum to the main stellar disk. The counterrotating gas provides direct evidence for the accretion of external material, a key aspect in hierarchical galaxy evolution. We identified 303 gaseous counterrotators out of 9992 galaxies in MaNGA. The majority of the counterrotators are early types. This implies their formation is highly correlated with early-type galaxies, although it is still difficult to know if one leads to the other. To disentangle which of the galaxy characteristics within a morphological class were changed by the accretion of counterrotating gas, we carefully selected a comparison sample with similar fundamental galactic properties but corotation in gas. This comparison shows that gaseous counterrotation correlates with weak rotation in the stellar component, the high central concentration of star-forming regions, if present, and a higher fraction of central low ionization emission regions (cLIER) galaxies. The light distributions of the stellar components, dust and H i content (both low), and overall suppressed star formation rates are similar for both samples and seem typical for the morphological class. We claim that elliptical and about half of the lenticular counterrotators, those with weak rotation in the stellar component in the outskirts and central regions, likely have a major merger origin for the gas acquisition, and the other half of lenticulars, with stronger stellar rotation, may have a minor merger or pure gas accretion origin.

How do the successive buckling events affect a galaxy bar and stellar disc? Potential observable signatures for spotting the buckling action – I

ABSTRACT Until now, observations have caught up only a handful of galaxies in ongoing buckling action. Interestingly, N-body simulations over the past several decades show that almost every bar buckles or vertically thickens as soon as it reaches its peak strength during its evolution and leads to box/peanut/x (BPX) shapes. In order to understand the effect of multiple buckling events on the observable properties of galactic bar and disc, we perform an N-body simulation of a Milky Way-type disc. The axisymmetric galaxy disc forms a bar within a Gyr of its evolution and the bar undergoes two successive buckling events. We report that the time-spans of these two buckling events are 220 Myr and 1 Gyr, which have almost similar strengths of the bending modes. As a result of these two buckling events, the full lengths of BPX shapes are around 5.8 and 8.6 kpc, which are around two-thirds of the full bar length at the end of each buckling event. We find that the first buckling occurs at a smaller scale (radius $\lt $3 kpc) with a shorter time-span affecting the larger length-scales of the disc, which is quantified in terms of changes in $m=$2 and $m=$ 4 Fourier modes. While the second buckling occurs at larger scales (radius $\approx$6 kpc) affecting the inner disc the most. Finally, we provide observable kinematic signatures (i.e. quadrupolar patterns of the line-of-sight velocities), which can potentially differentiate the successive buckling events.

Compact and high excitation molecular clumps in the extended ultraviolet disk of M83

The extended ultraviolet (XUV) disks of nearby galaxies show ongoing massive-star formation, but their parental molecular clouds remain mostly undetected despite searches in CO(1-0) and CO(2-1). The recent detection of 23 clouds in the higher excitation transition CO(3-2) within the XUV disk of M83 thus requires an explanation. We test the hypothesis introduced to explain the non-detections and recent detection simultaneously: The clouds in XUV disks have a clump-envelope structure similar to those in Galactic star-forming clouds, having dense star-forming clumps (or concentrations of multiple clumps) at their centers, which predominantly contribute to the CO(3-2) emission and are surrounded by less dense envelopes, where CO molecules are photo-dissociated due to the low-metallicity environment there. We utilized new high-resolution ALMA CO(3-2) observations of a subset (11) of the 23 clouds in the XUV disk of M83. We confirm the compactness of the CO(3-2)-emitting dense clumps (or their concentrations), finding clump diameters below the spatial resolution of 6-9 pc. This is similar to the size of the dense gas region in the Orion A molecular cloud, a local star-forming cloud with massive-star formation. The dense star-forming clumps are common between normal and XUV disks. This may also indicate that once the cloud structure is set, the process of star formation is governed by the cloud's internal physics rather than by external triggers. This simple model explains the current observations of clouds with ongoing massive-star formation, although it may require some adjustment, for example including the effect of cloud evolution, to describe star formation in molecular clouds more generally.

The structure of massive star-forming galaxies from JWST and ALMA: Dusty, high-redshift disc galaxies

We present an analysis of the NIRCam and MIRI morphological and structural properties of 80 massive ($ (M_ M_ odot )$\,=\,11.2\,pm \,0.1) dusty star-forming galaxies at $, identified as sub-millimetre galaxies (SMGs) by ALMA, which have been observed as part of the PRIMER project. To compare the structure of these massive, active galaxies to more typical, less actively star-forming galaxies, we defined two comparison samples. The first of 850 field galaxies matched in specific star formation rate and redshift and the second of 80 field galaxies matched in stellar mass . From the visual classification of the SMGs, we have identified 20\,pm \,5<!PCT!> as candidate late-stage major mergers, a further 40\,pm \,10<!PCT!> as potential minor mergers, and 40\,pm \,10<!PCT!> that have comparatively undisturbed disc-like morphologies, with no obvious massive neighbours on lesssim \,20\,--\,30\,kpc (projected) scales. These rates are comparable to those for the field samples and indicate that the majority of the sub-millimetre-detected galaxies are not late-stage major mergers, but have interaction rates similar to the general field population at $z$\,sim \,2--3. Through a multi-wavelength morphological analysis, using parametric and non-parametric techniques, we establish that SMGs have comparable near-infrared, mass-normalised sizes to the less active population, $ F444W $\,=\,2.7\,pm \,0.2\,kpc versus $ F444W $\,=\,3.1\,pm \,0.1\,kpc, but exhibit lower S\'ersic indices, consistent with bulge-less discs: F444W $\,=\,1.1\,pm \,0.1, compared to F444W $\,=\,1.9\,pm \,0.1 for the less active field galaxies and $n_ F444W $\,=\,2.8\,pm \,0.2 for the most massive field galaxies . The SMGs exhibit greater single-S\'ersic fit residuals and their morphologies are more structured at 2 relative to 4 when compared to the field galaxies. This appears to be caused by significant structured dust content in the SMGs and we find evidence for dust reddening as the origin of the morphological differences by identifying a strong correlation between the F200W$-$F444W pixel colour and the 870 surface brightness using high-resolution ALMA observations. We conclude that SMGs and both massive and less massive star-forming galaxies at the same epochs share a common disc-like structure, but the weaker bulge components (and potentially lower black hole masses) of the SMGs result in their gas discs being less stable. Consequently, the combination of high gas masses and instabilities triggered either secularly or by minor external perturbations results in higher levels of activity (and dust content) in SMGs compared to typical star-forming galaxies.

z ∼ 2 dual AGN host galaxies are disky: stellar kinematics in the ASTRID Simulation

We study dual AGN host galaxy morphologies at z=2 using the ASTRID simulation, selecting black hole (BH) pairs with small separation (), high mass ( MBH,12>107M⊙), and luminosity (). We kinematically decompose (using MORDOR) ∼1000 dual AGN hosts into standard components - a ‘disk’ (thin and thick disk, pseudo-bulge) and ‘bulge’ (bulge and halo) and define disk-dominated galaxies by the disk-to-total D/T≥0.5. In ASTRID, 60.9±2.1% of dual AGN hosts (independent of separation) are disk-dominated, with the D/T distribution peaking at ∼0.7. In dual-AGN hosts, the D/T increases from $17% $ at to $ 64% $ for , and the pseudo-bulge is the dominant component of the disk fraction at the high mass end. Moreover, dual AGN hosts exhibit a higher fraction of disk/large pseudo-bulge than single-AGN hosts. The Disk-to-Total ratio is approximately constant with BH mass or AGN luminosity. We also create mock images of dual AGN host galaxies, employing morphological fitting software Statmorph to calculate morphological parameters and compare them with our kinematic decomposition results. Around 83.3±2.4% of galaxies display disk-like profiles, of which ∼60.7±2.2% are kinematically confirmed as disks. Se'rsic indices and half-mass radii of dual AGN host galaxies align with observational measurements from HST at z∼2. Around 34% are identified as mergers from the Gini−M20 relation. We find two dual AGN hosted by galaxies that exhibit disk-like se'rsic index n12<1 and (D/T)12>0.5, which are in remarkable agreement with properties of recently discovered dual quasars in disk galaxies at z∼2.

REBELS-25: discovery of a dynamically cold disc galaxy at z = 7.31

ABSTRACT We present high-resolution ($\sim 0.14$ arcsec = 710 pc) Atacama Large Millimetre/submillimetre Array [C ii] 158 $\mu$m and dust continuum follow-up observations of REBELS-25, a [C ii]-luminous ($L_{\mathrm{[CII]}}=(1.7\pm 0.2)\times 10^9\, \mathrm{L_{\odot }}$) galaxy at redshift $z=7.3065\pm 0.0001$. These high-resolution, high signal-to-noise observations allow us to study the sub-kpc morphology and kinematics of this massive ($M_* = 8^{+4}_{-2} \times 10^9 \mathrm{{\rm M}_{\odot }}$) star-forming (SFR$_{\mathrm{UV+IR}} = 199^{+101}_{-63} \mathrm{{\rm M}_{\odot }} \mathrm{yr}^{-1}$) galaxy in the Epoch of Reionization. By modelling the kinematics with $^{\mathrm{3D}}$BAROLO, we find it has a low-velocity dispersion ($\bar{\sigma } = 33^{+9}_{-7}$ km s$^{-1}$) and a high ratio of ordered-to-random motion ($V_{\mathrm{rot, ~max}}/\bar{\sigma } = 11 ^{+6}_{-5}$), indicating that REBELS-25 is a dynamically cold disc. Additionally, we find that the [C ii] distribution is well fit by a near-exponential disc model, with a Sérsic index, n, of $1.3 \pm 0.2$, and we see tentative evidence of more complex non-axisymmetric structures suggestive of a bar in the [C ii] and dust continuum emission. By comparing to other high spatial resolution cold gas kinematic studies, we find that dynamically cold discs seem to be more common in the high-redshift Universe than expected based on prevailing galaxy formation theories, which typically predict more turbulent and dispersion-dominated galaxies in the early Universe as an outcome of merger activity, gas accretion, and more intense feedback. This higher degree of rotational support seems instead to be consistent with recent cosmological simulations that have highlighted the contrast between cold and warm ionized gas tracers, particularly for massive galaxies. We therefore show that dynamically settled disc galaxies can form as early as 700 Myr after the big bang

General relativistic self-gravitating equilibrium discs around rotating neutron stars

ABSTRACT In modelling a relativistic disc around a compact object, the self-gravity of the disc is often neglected while it needs to be incorporated for more accurate descriptions in several circumstances. Extending the Komatsu–Eriguchi–Hachisu self-consistent field method, we present numerical models of a rapidly rotating neutron star with a self-gravitating disc in stationary equilibrium. In particular, our approach allows us to obtain numerical solutions involving a massive disc with the rest mass $\mathcal {O}(10^{-1})-\mathcal {O}(10^0)\, \mathrm{ M}_\odot$ closely attached to a rotating neutron star, given that the disc is mainly supported by the relativistic electron degeneracy pressure. We also assess the impact of self-gravity on the internal structure of the disc and the neutron star. These axisymmetric, stationary solutions can be employed for simulations involving the neutron star–disc system in the context of high-energy transients and gravitational-wave emissions.

Magnetic field morphology and evolution in the Central Molecular Zone and its effect on gas dynamics

The interstellar medium in the Milky Way's Central Molecular Zone (CMZ) is known to be strongly magnetised, but its large-scale morphology and impact on the gas dynamics are not well understood. We explore the impact and properties of magnetic fields in the CMZ using three-dimensional non-self gravitating magnetohydrodynamical simulations of gas flow in an external Milky Way barred potential. We find that: (1) The magnetic field is conveniently decomposed into a regular time-averaged component and an irregular turbulent component. The regular component aligns well with the velocity vectors of the gas everywhere, including within the bar lanes. (2) The field geometry transitions from parallel to the Galactic plane near $z=0$ to poloidal away from the plane. (3) The magneto-rotational instability (MRI) causes an in-plane inflow of matter from the CMZ gas ring towards the central few parsecs of $0.01 that is absent in the unmagnetised simulations. However, the magnetic fields have no significant effect on the larger-scale bar-driven inflow that brings the gas from the Galactic disc into the CMZ. (4) A combination of bar inflow and MRI-driven turbulence can sustain a turbulent vertical velocity dispersion of $ on scales of $20 in the CMZ ring. The MRI alone sustains a velocity dispersion of $ Both these numbers are lower than the observed velocity dispersion of gas in the CMZ, suggesting that other processes such as stellar feedback are necessary to explain the observations. (5) Dynamo action driven by differential rotation and the MRI amplifies the magnetic fields in the CMZ ring until they saturate at a value that scales with the average local density as $B 102 \, (n/10^3 \ G $. Finally, we discuss the implications of our results within the observational context in the CMZ.