arXiv subjects

Andreas Burkert

Publications and source records attributed to Andreas Burkert.

At least 91 records · Page 5Linked to original sources

arXiv2019

The Outer Stellar Halos of Galaxies: how Radial Merger Mass Deposition, Shells and Streams depend on Infall-Orbit Configurations

Galaxy mergers are a fundamental part of galaxy evolution. To study the resulting mass distributions of different kinds of galaxy mergers, we present a simulation suite of 36 high-resolution isolated merger simulations, exploring a wide range of parameter space in terms of mass ratios (mu = 1:5, 1:10, 1:50, 1:100) and orbital parameters. We find that mini mergers deposit a higher fraction of their mass in the outer halo compared to minor mergers, while their contribution to the central mass distribution is highly dependent on the orbital impact parameter: for larger pericentric distances we find that the centre of the host galaxy is almost not contaminated by merger particles. We also find that the median of the resulting radial mass distribution for mini mergers differs significantly from the predictions of simple theoretical tidal-force models. Furthermore, we find that mini mergers can increase the size of the host disc significantly without changing the global shape of the galaxy, if the impact occurs in the disc plane, thus providing a possible explanation for extended low-surface brightness disks reported in observations. Finally, we find clear evidence that streams are a strong indication of nearly circular infall of a satellite (with large angular momentum), whereas the appearance of shells clearly points to (nearly) radial satellite infall.

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arXiv2019

The Evolution and Origin of Ionized Gas Velocity Dispersion from $z\sim2.6$ to $z\sim0.6$ with KMOS$^{\rm 3D}$

We present the $0.6<z<2.6$ evolution of the ionized gas velocity dispersion in 175 star-forming disk galaxies based on data from the full KMOS$^{\rm 3D}$ integral field spectroscopic survey. In a forward-modelling Bayesian framework including instrumental effects and beam-smearing, we fit simultaneously the observed galaxy velocity and velocity dispersion along the kinematic major axis to derive the intrinsic velocity dispersion $σ_0$. We find a reduction of the average intrinsic velocity dispersion of disk galaxies as a function of cosmic time, from $σ_0\sim45$ km s$^{-1}$ at $z\sim2.3$ to $σ_0\sim30$ km s$^{-1}$ at $z\sim0.9$. There is substantial intrinsic scatter ($σ_{σ_0, {\rm int}}\approx10$ km s$^{-1}$) around the best-fit $σ_0-z$-relation beyond what can be accounted for from the typical measurement uncertainties ($δσ_0\approx12$ km s$^{-1}$), independent of other identifiable galaxy parameters. This potentially suggests a dynamic mechanism such as minor mergers or variation in accretion being responsible for the scatter. Putting our data into the broader literature context, we find that ionized and atomic+molecular velocity dispersions evolve similarly with redshift, with the ionized gas dispersion being $\sim10-15$ km s$^{-1}$ higher on average. We investigate the physical driver of the on average elevated velocity dispersions at higher redshift, and find that our galaxies are at most marginally Toomre-stable, suggesting that their turbulent velocities are powered by gravitational instabilities, while stellar feedback as a driver alone is insufficient. This picture is supported through comparison with a state-of-the-art analytical model of galaxy evolution.

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arXiv2019

Droplets I: Pressure-Dominated Sub-0.1 pc Coherent Structures in L1688 and B18

We present the observation and analysis of newly discovered coherent structures in the L1688 region of Ophiuchus and the B18 region of Taurus. Using data from the Green Bank Ammonia Survey (GAS), we identify regions of high density and near-constant, almost-thermal, velocity dispersion. Eighteen coherent structures are revealed, twelve in L1688 and six in B18, each of which shows a sharp "transition to coherence" in velocity dispersion around its periphery. The identification of these structures provides a chance to study the coherent structures in molecular clouds statistically. The identified coherent structures have a typical radius of 0.04 pc and a typical mass of 0.4 Msun, generally smaller than previously known coherent cores identified by Goodman et al. (1998), Caselli et al. (2002), and Pineda et al. (2010). We call these structures "droplets." We find that unlike previously known coherent cores, these structures are not virially bound by self-gravity and are instead predominantly confined by ambient pressure. The droplets have density profiles shallower than a critical Bonnor-Ebert sphere, and they have a velocity (VLSR) distribution consistent with the dense gas motions traced by NH3 emission. These results point to a potential formation mechanism through pressure compression and turbulent processes in the dense gas. We present a comparison with a magnetohydrodynamic simulation of a star-forming region, and we speculate on the relationship of droplets with larger, gravitationally bound coherent cores, as well as on the role that droplets and other coherent structures play in the star formation process.

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arXiv2019

From kpcs to the central parsec of NGC 1097: feeding star formation and a black hole at the same time

A panchromatic view of the star forming ring and feeding process in the central kpc of the galaxy NGC 1097 is presented. The assembled IR to UV images at $\sim$10 pc resolution allow us to characterise the population of circa 250 clusters in the ring and disentangle the network of filaments of dust and gas that enshroud and feed them. The ring is a place of intermittent star bursts over the last 100 Myr. Four major episodes covering a proto-cluster phase of eleven mid-IR sources at the molecular clouds core, and two (three) previous bursts with a time separation of 20 - 30 Myr are identified. The extinction map of the inner few kpc resolves NGC1097's two major dust lanes in bundles of narrow, $<$25 pc width, filaments running along the galaxy's bar. As they approach the ring, some circularise along it, others curve to the centre to produce a nuclear spiral. We believe these are kpc-scale dust-gas streamers feeding the ring and the black-hole. The total mass in clusters formed in the ring in the last 100 Myr is $< 10^7\, \rm{M_\odot}$, i.e. $< 1\% $ of the $10^{9} M_\odot$ of molecular gas in the ring; yet, at its current star formation rate, $\sim1.8\, \rm{M_\odot \, yr^{-1}}$, an order of magnitude more in stellar mass should have been produced over that period. This means that the availability of gas in the ring is not the sole star formation driver, perhaps the rate at which dense gas accumulates in the ring is key.

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arXiv2019

Kiloparsec Scale Properties of Star-Formation Driven Outflows at z~2.3 in the SINS/zC-SINF AO Survey

We investigate the relationship between star formation activity and outflow properties on kiloparsec scales in a sample of 28 star forming galaxies at $z\sim$ 2-2.6, using adaptive optics assisted integral field observations from SINFONI on the VLT. The narrow and broad components of the H$α$ emission are used to simultaneously determine the local star formation rate surface density ($Σ_{\rm SFR}$), and the outflow velocity $v_{\rm out}$ and mass outflow rate $\dot{M}_{\rm out}$, respectively. We find clear evidence for faster outflows with larger mass loading factors at higher $Σ_{\rm SFR}$. The outflow velocities scale as $v_{\rm out}$ $\propto$ $Σ_{\rm SFR}^{0.34 \pm 0.10}$, which suggests that the outflows may be driven by a combination of mechanical energy released by supernova explosions and stellar winds, as well as radiation pressure acting on dust grains. The majority of the outflowing material does not have sufficient velocity to escape from the galaxy halos, but will likely be re-accreted and contribute to the chemical enrichment of the galaxies. In the highest $Σ_{\rm SFR}$ regions the outflow component contains an average of $\sim$45% of the H$α$ flux, while in the lower $Σ_{\rm SFR}$ regions only $\sim$10% of the H$α$ flux is associated with outflows. The mass loading factor, $η$ = $\dot{M}_{\rm out}$/SFR, is positively correlated with $Σ_{\rm SFR}$ but is relatively low even at the highest $Σ_{\rm SFR}$: $η\lesssim$ 0.5 $\times$ (380 cm$^{-3}$/n$_e$). This may be in tension with the $η$ $\gtrsim$ 1 required by cosmological simulations, unless a significant fraction of the outflowing mass is in other gas phases and has sufficient velocity to escape the galaxy halos.

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arXiv2019

DASH: a library of dynamical subhalo evolution

The abundance and demographics of dark matter substructure is important for many areas in astrophysics and cosmological $N$-body simulations have been the primary tool used to investigate them. However, it has recently become clear that the simulations are subject to numerical artefacts, which hampers a proper treatment of the tidal evolution of subhaloes. Unfortunately, no analytical models that accurately describe subhalo evolution exist either. We therefore present a library of idealized, high resolution $N$-body simulations of the tidal evolution of individual subhaloes that can be used to calibrate semi-analytical models and to complement cosmological simulations. The simulations focus on minor mergers, i.e., the mass of the subhalo is much smaller than that of the host halo, such that the impact of dynamical friction is negligible. This setup allows the adoption of a fixed analytical potential for modelling the host halo. The dynamical evolution of subhaloes is followed with $N$-body computations. In the library, four parameters, two of which characterize the subhalo orbit with respect to the host halo, and the two concentrations of the host- and subhalo, are varied over the ranges encountered in cosmological simulations. We show several representative examples from the library that illustrate the evolution of the subhalo mass and velocity dispersion profiles. Additionally, we make publicly available a pre-trained non-parametric model of the subhalo mass evolution based on random forest regression. This model is able to interpolate the simulation data at the 0.1\,dex level and provides efficient access to the data for further use in modelling.

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arXiv2018

Kinematics of Simulated Galaxies I: Connecting Dynamical and Morphological Properties of Early-Type Galaxies at Different Redshifts

State-of-the-art integral field surveys like $\mathrm{ATLAS^{3D}}$, SLUGGS, CALIFA, SAMI, and MaNGA provide large data sets of kinematical observations of early-type galaxies (ETGs), yielding constraints on the formation of ETGs. Using the cosmological hydrodynamical \textit{Magneticum Pathfinder} simulations, we investigate the paradigm of fast and slow rotating ETGs in a fully cosmological context. We show that the ETGs within the \textit{Magneticum} simulation are in remarkable agreement with the observations, revealing fast and slow rotators quantified by the angular momentum proxy $λ_{\mathrm{R}}$ and the flattening $ε$ with the observed prevalence. Taking full advantage of the three-dimensional data, we demonstrate that the dichotomy between fast and slow rotating galaxies gets enhanced, showing an upper and lower population separated by an underpopulated region in the edge-on $λ_{\mathrm{R}}$-$ε$ plane. Following the evolution of the $λ_{\mathrm{R}}$-$ε$ plane through cosmic time, we find that, while the upper population is already in place at $z=2$, the lower population gets statistically significant below $z=1$ with a gradual increase. At least $50\%$ of the galaxies transition from fast to slow rotators on a short timescale, in most cases associated to a significant merger event. Furthermore, we connect the $M_{*}$-$j_{*}$ plane, quantified by the $b$-value, with the $λ_{\mathrm{R}}$-$ε$ plane, revealing a strong correlation between the position of a galaxy in the $λ_{\mathrm{R}}$-$ε$ plane and the $b$-value. Going one step further, we classify our sample based on features in their velocity map, finding all five common kinematic groups, also including the recently observed group of prolate rotators, populating distinct regions in the $λ_{\mathrm{R}}$-$b$ plane.

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arXiv2018

3D shape of Orion A from Gaia DR2

We use the $\mathit{Gaia}$ DR2 distances of about 700 mid-infrared selected young stellar objects in the benchmark giant molecular cloud Orion A to infer its 3D shape and orientation. We find that Orion A is not the fairly straight filamentary cloud that we see in (2D) projection, but instead a cometary-like cloud oriented toward the Galactic plane, with two distinct components: a denser and enhanced star-forming (bent) Head, and a lower density and star-formation quieter $\sim$75 pc long Tail. The true extent of Orion A is not the projected $\sim$40 pc but $\sim$90 pc, making it by far the largest molecular cloud in the local neighborhood. Its aspect ratio ($\sim$30:1) and high column-density fraction ($\sim45\%$) make it similar to large-scale Milky Way filaments ("bones"), despite its distance to the galactic mid-plane being an order of magnitude larger than typically found for these structures.

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arXiv2018

Surround & Squash: The Interstellar Medium around Scorpius Centaurus OB2

We exploited observational constraints on stars, gas and nucleosynthesis ashes for the closest region of recent massive-star formation, Scorpius-Centaurus OB2, and combined them with 3D hydrodynamical simulations, in order to address physics and history for the case of the Scorpius-Centaurus superbubble. We used published cold gas observations through PLANCK survey data processing, HERSCHEL and APEX, continuum and molecular line observations. We analysed the Galactic All Sky Survey (GASS) to investigate shell structures in atomic hydrogen, and used HIPPARCOS and Gaia data in combination with interstellar absorption against stars to obtain new constraints for the distance to the Hi features. Hot gas is traced in soft X-rays via the ROSAT all sky survey. Nucleosynthesis ejecta from massive stars were traced with new INTEGRAL spectrometer observations via 26Al radioactivity. We also performed 3D hydrodynamical simulations for the Sco-Cen superbubble. To investigate the impact of massive star feedback on extended clouds, we simulate the interaction of a turbulent cloud with the hot, pressurised gas in a superbubble. The hot gas fills the tenuous regions of the cloud and compresses the denser parts. Stars formed in these dense clumps would have distinct spatial and kinematic distributions. The combined results from observations and simulations are consistent with a scenario where dense gas was initially distributed in a band elongated in the direction now occupied by the OB association. Superbubbles powered by massive stars would then repeatedly break out of the elongated parent cloud, surround and squash the denser parts of the gas sheet and thus induce more star formation. The expected spatial and kinematic distribution of stars is consistent with observations of Sco-Cen. The scenario might apply to many similar regions in the Galaxy and also to AGN-related superbubbles.

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arXiv2018

Accretion driven turbulence in filaments I: Non-gravitational accretion

We study accretion driven turbulence for different inflow velocities in star forming filaments using the code ramses. Filaments are rarely isolated objects and their gravitational potential will lead to radially dominated accretion. In the non-gravitational case, accretion by itself can already provoke non-isotropic, radially dominated turbulent motions responsible for the complex structure and non-thermal line widths observed in filaments. We find that there is a direct linear relation between the absolute value of the total density weighted velocity dispersion and the infall velocity. The turbulent velocity dispersion in the filaments is independent of sound speed or any net flow along the filament. We show that the density weighted velocity dispersion acts as an additional pressure term supporting the filament in hydrostatic equilibrium. Comparing to observations, we find that the projected non-thermal line width variation is generally subsonic independent of inflow velocity.

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arXiv2018

Morphology of prestellar cores in pressure confined filaments

Observations of prestellar cores in star-forming filaments show two distinct morphologies. While molecular line measurements often show broad cores, submillimeter continuum observations predominantly display pinched cores compared to the bulk of the filament gas. In order to explain how different morphologies arise, we use the gravitational instability model where prestellar cores form by growing density perturbations. The radial extent at each position is set by the local line-mass. We show that the ratio of core radius to filament radius is determined by the initial line-mass of the filament. Additionally, the core morphology is independent of perturbation length scale and inclination, which makes it an ideal diagnostic for observations. Filaments with a line-mass of less than half its critical value should form broad cores, whereas filaments with more than half its critical line-mass value should form pinched cores. For filaments embedded in a constant background pressure, the dominant perturbation growth times significantly differ for low and high line-mass filaments. Therefore, we predict that only one population of cores is present if all filaments within a region begin with similar initial perturbations.

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arXiv2018

The Galactic Centre source G2 was unlikely born in any of the known massive binaries

The source G2 has already completed its pericentre passage around Sgr A*, the super-massive black hole in the centre of our Galaxy. Although it has been monitored for 15 years, its astrophysical nature and origin still remain unknown. In this work, we aim to test the hypothesis of G2 being the result of a stellar wind collision. To do so, we study the motion and final fate of gas clumps formed as a result of collisions of stellar winds in massive binaries. Our approach is based on a test-particle model in order to describe the trajectories of such clumps. The model takes into account the gravitational field of Sgr A*, the interaction of the clumps with the interstellar medium as well as their finite lifetimes. Our analysis allows us to reject the hypothesis based on four arguments: i) if G2 has followed a purely Keplerian orbit since its formation, it cannot have been produced in any of the known massive binaries since their motions are not consistent; ii) in general, gas clumps are evaporated through thermal conduction on very short timescale (< 100yr) before getting close enough to Sgr A*; iii) IRS 16SW, the best candidate for the origin of G2, cannot generate clumps as massive as G2; and iv) clumps ejected from IRS 16SW describe trajectories significantly different to the observed motion of G2.

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arXiv2018

Declining rotation curves at $z=2$ in $Λ$CDM galaxy formation simulations

Selecting disk galaxies from the cosmological, hydrodynamical simulation Magneticum Pathfinder we show that almost half of our poster child disk galaxies at $z=2$ show significantly declining rotation curves and low dark matter fractions, very similar to recently reported observations. These galaxies do not show any anomalous behavior, reside in standard dark matter halos and typically grow significantly in mass until $z = 0$, where they span all morphological classes, including disk galaxies matching present day rotation curves and observed dark matter fractions. Our findings demonstrate that declining rotation curves and low dark matter fractions in rotation dominated galaxies at $z=2$ appear naturally within the $Λ$CDM paradigm and reflect the complex baryonic physics, which plays a role at the peak epoch of star-formation. In addition, we find some dispersion dominated galaxies at $z=2$ which host a significant gas disk and exhibit similar shaped rotation curves as the disk galaxy population, rendering it difficult to differentiate between these two populations with currently available observation techniques.

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arXiv2017

Quantifying the interplay between gravity and magnetic field in molecular clouds - a possible multi-scale energy equipartition in NGC6334

The interplay between gravity, turbulence and the magnetic field determines the evolution of the molecular ISM and the formation of the stars. In spite of growing interests, there remains a lack of understanding of the importance of magnetic field over multiple scales. We derive the magnetic energy spectrum -- a measure that constraints the multi-scale distribution of the magnetic energy, and compare it with the gravitational energy spectrum derived in Li \& Burkert (2016). In our formalism, the gravitational energy spectrum is purely determined by the surface density PDF, and the magnetic energy spectrum is determined by both the surface density PDF and the magnetic-field-density relation. If regions have density PDFs close to $P(Σ)\sim Σ^{-2}$ and a universal magnetic field-density relation $B\sim ρ^{1/2}$, we expect a multi-scale near equipartition between gravity and the magnetic fields. This equipartition is found to be true in NGC6334 where estimates of magnetic fields over multiple scales (from 0.1 pc to a few parsec) are available. However, the current observations are still limited in sample size. In the future, it is necessary to obtain multi-scale measurements of magnetic fields from different clouds with different surface density PDFs and apply our formalism to further study the gravity-magnetic field interplay.

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arXiv2017

Disruption of Dark Matter Substructure: Fact or Fiction?

Accurately predicting the demographics of dark matter (DM) substructure is of paramount importance for many fields of astrophysics, including gravitational lensing, galaxy evolution, halo occupation modeling, and constraining the nature of dark matter. Because of its strongly non-linear nature, DM substructure is typically modeled using N-body simulations, which reveal that large fractions of DM subhaloes undergo complete disruption. In this paper we use both analytical estimates and idealized numerical simulations to investigate whether this disruption is mainly physical, due to tidal heating and stripping, or numerical (i.e., artificial). We show that, contrary to naive expectation, subhaloes that experience a tidal shock $ΔE$ that exceeds the subhalo's binding energy, $|E_{\rm b}|$, do not undergo disruption, even when $ΔE/|E_{\rm b}|$ is as large as 100. Along the same line, and contrary to existing claims in the literature, instantaneously stripping matter from the outskirts of a DM subhalo also does not result in its complete disruption, even when the instantaneous remnant has positive binding energy. In addition, we show that tidal heating due to high-speed (impulsive) encounters with other subhaloes (`harassment'), is negligible compared to the tidal effects due to the host halo. Hence, we conclude that, in the absence of baryonic processes, the complete, physical disruption of CDM substructure is extremely rare, and that most disruption in numerical simulations therefore must be artificial. We discuss various processes that have been associated with numerical overmerging, and conclude that inadequate force-softening is the most likely culprit.

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arXiv2017

A bathtub model for the star-forming interstellar medium

The bathtub model of the star forming interstellar medium is based on the powerful constraint that mass has to be conserved when gas flows through its various thermal and density phases, ending up eventually in a young star or being blown away by stellar feedback. It predicts that the star formation rate of a molecular cloud is not determined by the cloud's mass or its internal collapse timescale, but rather by the accretion rate of new gas. For the most simple case of a constant accretion flow an equilibrium state is reached quickly where the star formation rate equals the accretion rate and where the dense gas mass is constant and independent of time. The mass of the young star cluster, on the other hand, increases linearly with time. The stellar mass fraction therefore represents a sensitive clock to measure the age of the star-forming region. The bathtub model predicts that the efficiency of star formation is small, of order 1%, even in the dense filamentary phases of molecular clouds. It provides a simple explanation for the dense gas fraction of order 10% in molecular clouds and for the large gas depletion timescales of star-forming galaxies of order $5 \times 10^8 - 10^9$ yrs.

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arXiv2017

The Co-Evolution of Total Density Profiles and Central Dark Matter Fractions in Simulated Early-Type Galaxies

We present evidence from cosmological hydrodynamical simulations for a co-evolution of the slope of the total (dark and stellar) mass density profile, gamma_tot, and the dark matter fraction within the half-mass radius, f_DM, in early-type galaxies. The relation can be described as gamma_tot = A f_DM + B for all systems at all redshifts. The trend is set by the decreasing importance of gas dissipation towards lower redshifts and for more massive systems. Early-type galaxies are smaller, more concentrated, have lower f_DM and steeper gamma_tot at high redshifts and at lower masses for a given redshift; f_DM and gamma_tot are good indicators for growth by "dry" merging. The values for A and B change distinctively for different feedback models, and this relation can be used as a test for such models. A similar correlation exists between gamma_tot and the stellar mass surface density Sigma_*. A model with weak stellar feedback and feedback from black holes is in best agreement with observations. All simulations, independent of the assumed feedback model, predict steeper gamma_tot and lower f_DM at higher redshifts. While the latter is in agreement with the observed trends, the former is in conflict with lensing observations, which indicate constant or decreasing gamma_tot. This discrepancy is shown to be artificial: the observed trends can be reproduced from the simulations using observational methodology to calculate the total density slopes.

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arXiv2017

Fast Molecular Cloud Destruction Requires Fast Cloud Formation

A large fraction of the gas in the Galaxy is cold, dense, and molecular. If all this gas collapsed under the influence of gravity and formed stars in a local free-fall time, the star formation rate in the Galaxy would exceed that observed by more than an order of magnitude. Other star-forming galaxies behave similarly. Yet observations and simulations both suggest that the molecular gas is indeed gravitationally collapsing, albeit hierarchically. Prompt stellar feedback offers a potential solution to the low observed star formation rate if it quickly disrupts star-forming clouds during gravitational collapse. However, this requires that molecular clouds must be short-lived objects, raising the question of how so much gas can be observed in the molecular phase. This can occur only if molecular clouds form as quickly as they are destroyed, maintaining a global equilibrium fraction of dense gas. We therefore examine cloud formation timescales. We first demonstrate that supernova and superbubble sweeping cannot produce dense gas at the rate required to match the cloud destruction rate. On the other hand, Toomre gravitational instability can reach the required production rate. We thus argue that, although dense, star-forming gas may last only around a single global free-fall time, the dense gas in star-forming galaxies can globally exist in a state of dynamic equilibrium between formation by gravitational instability, and disruption by stellar feedback. At redshift z >~ 2, the Toomre instability timescale decreases, resulting in a prediction of higher molecular gas fractions at early times, in agreement with observations.

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