Research Highlights

March 2019: The influence of streaming velocities on the formation of the first stars

Schauer, Anna T. P., Glover, Simon C. O., Klessen, Ralf S., Ceverino, Daniel: MNRAS, 484, 3510-3521 (2019)  [ADS link]

Baryon fraction in haloes with masses larger than 105 M as a function of redshift. In the case of zero streaming velocity (light green), the value approaches the cosmic mean (dashed line). For all simulations with non-zero streaming velocities (blue, dark green, grey), it is well below that value. The shaded regions show the respective standard deviations.

How, when, and where the first stars formed are fundamental questions regarding the epoch of cosmic dawn. A second-order effect in the fluid equations was recently found to make a significant contribution: an offset velocity between gas and dark matter, the so-called streaming velocity. Previous simulations of a limited number of low-mass dark matter haloes suggest that this streaming velocity can delay the formation of the first stars and decrease halo gas fractions and the halo mass function in the low-mass regime. However, a systematic exploration of its effects in a large sample of haloes has been lacking until now. In this paper, we present results from a set of cosmological simulations of regions of the Universe with different streaming velocities performed with the moving mesh code AREPO. Our simulations have very high mass resolution, enabling us to accurately resolve minihaloes as small as 105 M. We show that in the absence of streaming, the least massive halo that contains cold gas has a mass Mhalo, min = 5 × 105 M, but that cooling only becomes efficient in a majority of haloes for halo masses greater than Mhalo, 50% = 1.6 × 106 M. In regions with non-zero streaming velocities, Mhalo, min and Mhalo, 50% both increase significantly, by around a factor of a few for each one sigma increase in the value of the local streaming velocity. As a result, in regions with streaming velocities v_stream ≥ 3 σrms, cooling of gas in minihaloes is completely suppressed, implying that the first stars in these regions form within atomic cooling haloes.

FEBRUARY 2019: FirstLight III: rest-frame UV-optical spectral energy distributions of simulated galaxies at cosmic dawn

Ceverino, Daniel, Klessen, Ralf S., Glover, Simon C. O.: MNRAS, 484, 1366-1377 (2019)  [ADS link]

Stellar mass versus M1500, coloured by sSFR at z=6 (from Ceverino et al. 2019).

Using the FirstLight data base of 300 zoom-in cosmological simulations we provide rest-frame UV-optical spectral energy distributions of galaxies with complex star-formation histories that are coupled to the non-uniform gas accretion history of galactic haloes during cosmic dawn. The population at any redshift is very diverse ranging from starbursts to quiescent galaxies even at a fixed stellar mass. This drives a redshift-dependent relation between UV luminosity and stellar mass with a large scatter, driven by the specific star formation rate. The UV slope and the production efficiency of Lyman continuum photons have high values, consistent with dust-corrected observations. This indicates young stellar populations with low metallicities. The FirstLight simulations make predictions on the rest-frame UV-optical absolute magnitudes, colours, and optical emission lines of galaxies at z = 6-12 that will be observed for the first time with James Webb Space Telescope and the next generation of telescopes in the coming decade.

JANUARY 2019: WARPFIELD 2.0 — feedback-regulated minimum star formation efficiencies of giant molecular clouds

Rahner, Daniel, Pellegrini, Eric W., Glover, Simon C. O., Klessen, Ralf S.: MNRAS, 483, 2547-2560 (2019)   [ADS link]

Schematic view of the cloud / cluster model used in WARPFIELD (Figure 1 of Rahner et al. 2019)

Star formation is an inefficient process and in general only a small fraction of the gas in a giant molecular cloud (GMC) is turned into stars. This is partly due to the negative effect of stellar feedback from young massive star clusters. Recently, we introduced a novel 1D numerical treatment of the effects of stellar feedback from young massive clusters on their natal clouds, which we named WARPFIELD. Here, we present version 2 of the WARPFIELD code, containing improved treatments of the thermal evolution of the gas and the fragmentation of the feedback-driven shell. As part of this update, we have produced new cooling and heating tables that account for the combined effects of photoionization and collisional ionization on the cooling rate, which we now make publicly available. We employ our updated version of WARPFIELD to investigate the impact of stellar feedback on GMCs with a broad range of masses and surface densities and a variety of density profiles. We show that the minimum star formation efficiency (SFE) ɛmin, i.e. the SFE above which the cloud is destroyed by feedback, is mainly set by the average cloud surface density. An SFE of 1-6 per cent is generally sufficient to destroy a GMC. We also find an SFE per free-fall time ɛff ˜ 0.3 per cent, in good agreement with recent observations. Our results imply that feedback alone is sufficient to explain the low observed SFE of GMCs. Finally, we show that very massive clouds with steep density profiles – possible proxies of the giant clumps observed in galaxies at z ≈ 2 – are more resilient to feedback than typical GMCs, with ɛmin between 1 and 12 per cent. 

December 2018: Fingerprint of the first stars: multi-enriched extremely metal-poor stars in the TOPOS

Hartwig, Tilman, Ishigaki, Miho, Klessen, Ralf S., Yoshida, Naoki, MNRAS, 482, 1204–1210 (2019)  [ADS LINK]

Divergence of the chemical displacement (DCD) as a function of the abundances of calcium and magnesium relative (Figure 1 of Hartwig et al. 2019)

Extremely metal-poor (EMP) stars in the Milky Way inherited the chemical composition of the gas out of which they formed. They therefore carry the chemical fingerprint of the first stars in their spectral lines. It is commonly assumed that EMP stars form from gas that was enriched by only one progenitor supernova (‘mono-enriched’). However, recent numerical simulations show that the first stars form in small clusters. Consequently, we expect several supernovae to contribute to the abundances of an EMP star (‘multi-enriched’). We analyse seven recently observed EMP stars from the TOPoS survey by applying the divergence of the chemical displacement and find that J1035+0641 is mono-enriched (⁠pmono=53 per cent⁠) and J1507+0051 is multi-enriched (⁠pmono=4 per cent⁠). For the remaining five stars, we cannot make a distinct prediction (⁠pmono≲50 per cent⁠) due to theoretical and observational uncertainties. Further observations in the near-UV will help to improve our diagnostic and therefore contribute to constrain the nature of the first stars.

NoVember 2018: A dynamical mechanism for the origin of nuclear rings

Sormani, Mattia C., Sobacchi, Emanuele, Fragkoudi, Francesca, Ridley, Matthew, Treß, Robin G., Glover, Simon C. O., Klessen, Ralf S., MNRAS, 481, 2 -19 (2018)  [ADS LINK]

Velocity field in the center of the model galaxy (Figure 10 from Sormani et al. 2018)

We develop a dynamical theory for the origin of nuclear rings in barred galaxies. In analogy with the standard theory of accretion discs, our theory is based on shear viscous forces among nested annuli of gas. However, the fact that gas follows non-circular orbits in an external barred potential has profound consequences: it creates a region of reverse shear in which it is energetically favourable to form a stable ring that does not spread despite dissipation. Our theory allows us to approximately predict the size of the ring given the underlying gravitational potential. The size of the ring is loosely related to the location of the Inner Lindblad Resonance in the epicyclic approximation, but the predicted location is more accurate and is also valid for strongly barred potentials. By comparing analytical predictions with the results of hydrodynamical simulations, we find that our theory provides a viable mechanism for ring formation if the effective sound speed of the gas is low (⁠cs≲1kms−1⁠), but that nuclear spirals/shocks created by pressure destroy the ring when the sound speed is high (⁠cs≃10kms−1⁠). We conclude that whether this mechanism for ring formation is relevant for real galaxies ultimately depends on the effective equation of state of the interstellar medium (ISM). Promising confirmation comes from simulations in which the ISM is modelled using state-of-the-art cooling functions coupled to live chemical networks, but more tests are needed regarding the role of turbulence driven by stellar feedback. If the mechanism is relevant in real galaxies, it could provide a powerful tool to constrain the gravitational potential, in particular the bar pattern speed.

october 2018: FirstLight – II. Star formation rates of primeval galaxies from z=5-15

Ceverino, Daniel, Klessen, Ralf S., Glover, Simon C. O., MNRAS, 480, 4842-4850 (2018)  [ADS LINK]

Image of FirstLight galaxy (from D. Ceverino)

In the FirstLight project, we have used ˜300 cosmological, zoom-in simulations to determine the star formation histories of distinct first galaxies with stellar masses between M* = 106 and 3 × 10^9 M_{⊙} during cosmic dawn (z = 5-15). The evolution of the star formation rate (SFR) in each galaxy is complex and diverse, characterized by bursts of star formation. Overall, first galaxies spend 70 per cent of their time in star formation bursts. A sample of 1000 of these bursts indicates that the typical burst at z ≃ 6 has a specific SFR (sSFR) maximum of 5-}15 Gyr^{-1} with an effective width of ˜100 Myr, one-tenth of the age of the Universe at that redshift. A quarter of the bursts populate a tail with very high sSFR maxima of 20-}30 Gyr^{-1} and significantly shorter time-scales of ˜40-80 Myr. This diversity of bursts sets the mean and the mass-dependent scatter of the star-forming main sequence. This scatter is driven by a population of low-mass, M_*≤10^8 M_{⊙}, quiescent galaxies. The mean sSFR and the burst maximum at fixed mass increase with redshift, with the later always being a factor of ˜2 higher than the former. This implies sSFR maxima of {˜ } 20-60 Gyr^{-1} at z = 9-10. The SFR histories are publicly available at the FirstLight website.

September 2018: Hubble Tarantula Treasury Project – VI. Identification of pre-main-sequence stars using machine-learning techniques

Ksoll, Victor F., Gouliermis, Dimitrios A., Klessen, Ralf S., Grebel, Eva K., Sabbi, Elena, Anderson, Jay, Lennon, Daniel J., Cignoni, Michele, de Marchi, Guido, Smith, Linda J., Tosi, Monica, van der Marel, Roeland P., MNRAS, 479, 2389-2414 (2018)  [ADS LINK]

Optical CMD of the HTTP data, where each star is colored according to the mean predicted PMS probability (Figure 20 of Ksoll et al. 2018)

The Hubble Tarantula Treasury Project (HTTP) has provided an unprecedented photometric coverage of the entire starburst region of 30 Doradus down to the half Solar mass limit. We use the deep stellar catalogue of HTTP to identify all the pre-main-sequence (PMS) stars of the region, i.e. stars that have not started their lives on the main-sequence yet. The photometric distinction of these stars from the more evolved populations is not a trivial task due to several factors that alter their colour-magnitude diagram positions. The identification of PMS stars requires, thus, sophisticated statistical methods. We employ machine-learning classification techniques on the HTTP survey of more than 800 000 sources to identify the PMS stellar content of the observed field. Our methodology consists of (1) carefully selecting the most probable low-mass PMS stellar population of the star-forming cluster NGC 2070, (2) using this sample to train classification algorithms to build a predictive model for PMS stars, and (3) applying this model in order to identify the most probable PMS content across the entire Tarantula Nebula. We employ decision tree, random forest (RF), and support vector machine (SVM) classifiers to categorize the stars as PMS and non-PMS. The RF and SVM provided the most accurate models, predicting about 20 000 sources with a candidateship probability higher than 50 per cent, and almost 10 000 PMS candidates with a probability higher than 95 per cent. This is the richest and most accurate photometric catalogue of extragalactic PMS candidates across the extent of a whole star-forming complex.

August 2018: On the indeterministic nature of star formation on the cloud scale

Geen, Sam, Watson, Stuart K., Rosdahl, Joakim, Bieri, Rebekka, Klessen, Ralf S., Hennebelle, Patrick, MNRAS, 481, 2548-2569 (2018)  [ADS LINK]

Radius of an H II region(s) in one of the Yule simulations (Figure 5 of Geen et al. 2018)

Molecular clouds are turbulent structures whose star formation efficiency (SFE) is strongly affected by internal stellar feedback processes. In this paper, we determine how sensitive the SFE of molecular clouds is to randomized inputs in the star formation feedback loop, and to what extent relationships between emergent cloud properties and the SFE can be recovered. We introduce the YULE suite of 26 radiative magnetohydrodynamic simulations of a 10 000 solar mass cloud similar to those in the solar neighbourhood. We use the same initial global properties in every simulation but vary the initial mass function sampling and initial cloud velocity structure. The final SFE lies between 6 and 23 per cent when either of these parameters are changed. We use Bayesian mixed-effects models to uncover trends in the SFE. The number of photons emitted early in the cluster’s life and the length of the cloud provide the strongest predictors of the SFE. The H II regions evolve following an analytic model of expansion into a roughly isothermal density field. The more efficient feedback is at evaporating the cloud, the less the star cluster is dispersed. We argue that this is because if the gas is evaporated slowly, the stars are dragged outwards towards surviving gas clumps due to the gravitational attraction between the stars and gas. While star formation and feedback efficiencies are dependent on non-linear processes, statistical models describing cloud-scale processes can be constructed.

July 2018: SPRAI: coupling of radiative feedback and primordial chemistry in moving mesh hydrodynamics

Jaura, O., Glover, S. C. O., Klessen, R. S., Paardekooper, J.-P., MNRAS, 475, 2822-2834 (2018)  [ADS LINK]

Schematic of photon transport in SPRAI (Figure 2 in Jaura et al. 2018)

In this paper, we introduce a new radiative transfer code SPRAI (Simplex Photon Radiation in the Arepo Implementation) based on the SIMPLEX radiation transfer method. This method, originally used only for post-processing, is now directly integrated into the AREPO code and takes advantage of its adaptive unstructured mesh. Radiated photons are transferred from the sources through the series of Voronoi gas cells within a specific solid angle. From the photon attenuation, we derive corresponding photon fluxes and ionization rates and feed them to a primordial chemistry module. This gives us a self-consistent method for studying dynamical and chemical processes caused by ionizing sources in primordial gas. Since the computational cost of the SIMPLEX method does not scale directly with the number of sources, it is convenient for studying systems such as primordial star-forming haloes that may form multiple ionizing sources.

June 2018: A theoretical explanation for the Central Molecular Zone asymmetry

Sormani, Mattia C., Treß, Robin G., Ridley, Matthew, Glover, Simon C. O., Klessen, Ralf S., Binney, James, Magorrian, John, Smith, Rowan, MNRAS, 475, 2383-2402 (2018)   [ADS LINK]

Numerical model of the gas in the inner parts of the Milky Way (Figure 10 of Sormani et al. 2018)
Numerical model of the gas in the inner parts of the Milky Way (Figure 10 of Sormani et al. 2018)

It has been known for more than 30 yr that the distribution of molecular gas in the innermost 300 parsecs of the Milky Way, the Central Molecular Zone, is strongly asymmetric. Indeed, approximately three quarters of molecular emission come from positive longitudes, and only one quarter from negative longitudes. However, despite much theoretical effort, the origin of this asymmetry has remained a mystery. Here, we show that the asymmetry can be neatly explained by unsteady flow of gas in a barred potential. We use high-resolution 3D hydrodynamical simulations coupled to a state-of-the-art chemical network. Despite the initial conditions and the bar potential being point symmetric with respect to the Galactic Centre, asymmetries develop spontaneously due to the combination of a hydrodynamical instability known as the `wiggle instability‘ and the thermal instability. The observed asymmetry must be transient: observations made tens of megayears in the past or in the future would often show an asymmetry in the opposite sense. Fluctuations of amplitude comparable to the observed asymmetry occur for a large fraction of time in our simulations, and suggest that the present is not an exceptional moment in the life of our Galaxy.

May 2018: Spectral shifting strongly constrains molecular cloud disruption by radiation pressure on dust

Reißl, Stefan, Klessen, Ralf S., Mac Low, Mordecai-Mark, Pellegrini, Eric W., A&A, 611, A70, 1-19 (2018)   [ADS LINK]

Gravitational (red) and radiative (blue) forces, and the ratio between the two (purple). Radiation pressure never dominates cloud evolution in Milky Way clouds (Reißl et al. 2018)

Aim. We aim to test the hypothesis that radiation pressure from young star clusters acting on dust is the dominant feedback agent disrupting the largest star-forming molecular clouds and thus regulating the star-formation process.
Methods: We performed multi-frequency, 3D, radiative transfer calculations including both scattering and absorption and re-emission to longer wavelengths for model clouds with masses of 104-107 M, containing embedded clusters with star formation efficiencies of 0.009-91%, and varying maximum grain sizes up to 200 μm. We calculated the ratio between radiative and gravitational forces to determine whether radiation pressure can disrupt clouds.
Results: We find that radiation pressure acting on dust almost never disrupts star-forming clouds. Ultraviolet and optical photons from young stars to which the cloud is optically thick do not scatter much. Instead, they quickly get absorbed and re-emitted by the dust at thermal wavelengths. As the cloud is typically optically thin to far-infrared radiation, it promptly escapes, depositing little momentum in the cloud. The resulting spectrum is more narrowly peaked than the corresponding Planck function, and exhibits an extended tail at longer wavelengths. As the opacity drops significantly across the sub-mm and mm wavelength regime, the resulting radiative force is even smaller than for the corresponding single-temperature blackbody. We find that the force from radiation pressure falls below the strength of gravitational attraction by an order of magnitude or more for either Milky Way or moderate starbust conditions. Only for unrealistically large maximum grain sizes, and star formation efficiencies far exceeding 50% do we find that the strength of radiation pressure can exceed gravity.
Conclusions: We conclude that radiation pressure acting on dust does not disrupt star-forming molecular clouds in any Local Group galaxies. Radiation pressure thus appears unlikely to regulate the star-formation process on either local or global scales.

April 2018: Predicting the locations of possible long-lived low-mass first stars: importance of satellite dwarf galaxies

Magg, Mattis, Hartwig, Tilman, Agarwal, Bhaskar, Frebel, Anna, Glover, Simon C. O., Griffen, Brendan F., Klessen, Ralf S., MNRAS, 473, 5308-5323 (2018)   [ADS LINK]

Simulated Milky Way satellites with the color indicating the expected fraction of Pop III survivors (Figure 7 of Magg et al. 2018)

The search for metal-free stars has so far been unsuccessful, proving that if there are surviving stars from the first generation, they are rare, they have been polluted or we have been looking in the wrong place. To predict the likely location of Population III (Pop III) survivors, we semi-analytically model early star formation in progenitors of Milky Way-like galaxies and their environments. We base our model on merger trees from the high-resolution dark matter only simulation suite Caterpillar. Radiative and chemical feedback are taken into account self-consistently, based on the spatial distribution of the haloes. Our results are consistent with the non-detection of Pop III survivors in the Milky Way today. We find that possible surviving Pop III stars are more common in Milky Way satellites than in the main Galaxy. In particular, low-mass Milky Way satellites contain a much larger fraction of Pop III stars than the Milky Way. Such nearby, low-mass Milky Way satellites are promising targets for future attempts to find Pop III survivors, especially for high-resolution, high signal-to-noise spectroscopic observations. We provide the probabilities of finding a Pop III survivor in the red giant branch phase for all known Milky Way satellites to guide future observations.

March 2018: Shape and spin of minihaloes: from large scales to the centeRs

Druschke, Maik, Schauer, Anna T. P., Glover, Simon C. O., Klessen, Ralf S., MNRAS, 481, 3266-3277 (2018)   [ADS LINK]

Examples of minihalos at redshift 14 (Figure 4 from Druschke et al. 2019)

The spin and shape of galaxies at the present day have been well studied both observationally and theoretically. At high redshifts, however, we have to rely on numerical simulations. In this study, we investigate the shape and spin of minihaloes with masses of M ˜ 105-10^7 M_{⊙} which are of particular interest as they are the sites where the first stars in the Universe form. We analyse a large sample of these minihaloes, selected from a high-resolution cosmological simulation. The first minihaloes form at z ≃ 24 and by the end of the simulation at z ≃ 14 our sample includes ˜9000 minihaloes. We find that the spin parameter of the minihaloes follows a log-normal distribution with minimal dependence on redshift. Most minihaloes are prolate, but those formed at the highest redshifts are more prolate than those formed at lower redshifts. On the scale of the virial radius, there is a good correlation between the shape and spin of the gas and that of the dark matter. However, this correlation breaks down in gas which is cooling and undergoing gravitational collapse. We show, contrary to previous assumptions, that although the direction of the spin of the central dense gas correlates well with that of the halo, the magnitude of the spin of the dense gas is uncorrelated with that of the halo. Therefore, measurements of the spin of minihaloes on large scales tell us little about the angular momentum of the gas responsible for forming the first stars.

February 2018: Forming clusters within clusters: how 30 Doradus recollapsed and gave birth again

Rahner, Daniel, Pellegrini, Eric W., Glover, Simon C. O., Klessen, Ralf S., MNRAS, 473, L11-L15 (2018)   [ADS LINK]

Model of the recollapse of 30 Doradus (Figure 1 of Rahner et al. 2018)

The 30 Doradus nebula in the Large Magellanic Cloud (LMC) contains the massive starburst cluster NGC 2070 with a massive and probably younger stellar sub clump at its centre: R136. It is not clear how such a massive inner cluster could form several million years after the older stars in NGC 2070, given that stellar feedback is usually thought to expel gas and inhibit further star formation. Using the recently developed 1D feedback scheme WARPFIELD to scan a large range of cloud and cluster properties, we show that an age offset of several million years between the stellar populations is in fact to be expected given the interplay between feedback and gravity in a giant molecular cloud with a density ≳500 cm-3 due to re-accretion of gas on to the older stellar population. Neither capture of field stars nor gas retention inside the cluster have to be invoked in order to explain the observed age offset in NGC 2070 as well as the structure of the interstellar medium around it.

January 2018: On the Rotation of Supermassive Stars

Haemmerlé, Lionel, Woods, Tyrone E., Klessen, Ralf S., Heger, Alexander, Whalen, Daniel J., ApJ, 853, L3, 1-5 (2018)   [ADS LINK]

Internal structure of differentially rotating primordial star with accretion rate of 1 Msun/year (from Haemmerlé et al. 2018)

Supermassive stars (SMSs) born from pristine gas in atomically cooled halos are thought to be the progenitors of supermassive black holes at high redshifts. However, the way they accrete their mass is still an unsolved problem. In particular, for accretion to proceed, a large amount of angular momentum has to be extracted from the collapsing gas. Here, we investigate the constraints stellar evolution imposes on this angular momentum problem. We present an evolution model of a supermassive Population III star simultaneously including accretion and rotation. We find that, for SMSs to form by accretion, the accreted angular momentum has to be about 1% of the Keplerian angular momentum. This tight constraint comes from the ΩΓ-limit, at which the combination of radiation pressure and centrifugal force cancels gravity. It implies that SMSs are slow rotators, with a surface velocity less than 10%-20% of their first critical velocity, at which the centrifugal force alone cancels gravity. At such low velocities, the deformation of the star due to rotation is negligible.