arXiv subjects

Volker Bromm

Publications and source records attributed to Volker Bromm.

At least 127 records · Page 7Linked to original sources

arXiv2018

The Far Infrared Emission of the First Massive Galaxies

Massive Population II galaxies undergoing the first phase of vigorous star formation after the initial Population III stage should have high energy densities and silicate-rich interstellar dust. We have modeled the resulting far-infrared spectral energy distributions (SEDs), demonstrating that they are shifted substantially to bluer (`warmer') wavelengths relative to the best fitting ones at z ~ 3, and with strong outputs in the 10 - 40 micron range. When combined with a low level of emission by carbon dust, their SEDs match that of Haro 11, a local moderately-low-metallicity galaxy undergoing a very young and vigorous starburst that is likely to approximate the relevant conditions in young Population II galaxies. We expect to see similar SEDs at high redshifts (z >= 5) given the youth of galaxies at this epoch. In fact, we find a progression with redshift in observed galaxy SEDs, from those resembling local ones at 2 < z < 4 to a closer resemblance with Haro 11 at 5 < z < 7. In addition to the insight on conditions in high redshift galaxies, this result implies that estimates of the total infrared luminosities at z ~ 6 based on measurements near lambda ~ 1 mm can vary by factors of 2 - 4, depending on the SED template used. Currently popular modified blackbodies or local templates can result in significant underestimates compared with the preferred template based on the SED of Haro 11.

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arXiv2018

The physics of Lyman-alpha escape from high-redshift galaxies

Lyman-alpha (Lyα) photons from ionizing sources and cooling radiation undergo a complex resonant scattering process that generates unique spectral signatures in high-redshift galaxies. We present a detailed Lyα radiative transfer study of a cosmological zoom-in simulation from the Feedback In Realistic Environments (FIRE) project. We focus on the time, spatial, and angular properties of the Lyα emission over a redshift range of z = 5-7, after escaping the galaxy and being transmitted through the intergalactic medium (IGM). Over this epoch, our target galaxy has an average stellar mass of $M_{\rm star} \approx 5 \times 10^8 {\rm M}_\odot$. We find that many of the interesting features of the Lyα line can be understood in terms of the galaxy's star formation history. The time variability, spatial morphology, and anisotropy of Lyα properties are consistent with current observations. For example, the rest frame equivalent width has a ${\rm EW}_{{\rm Ly}α,0} > 20 {\rm Å}$ duty cycle of 62% with a non-negligible number of sightlines with $> 100 {\rm Å}$, associated with outflowing regions of a starburst with greater coincident UV continuum absorption, as these conditions generate redder, narrower (or single peaked) line profiles. The lowest equivalent widths correspond to cosmological filaments, which have little impact on UV continuum photons but efficiently trap Lyα and produce bluer, broader lines with less transmission through the IGM. We also show that in dense self-shielding, low-metallicity filaments and satellites Lyα radiation pressure can be dynamically important. Finally, despite a significant reduction in surface brightness with increasing redshift, Lyα detections and spectroscopy of high-$z$ galaxies with the upcoming James Webb Space Telescope is feasible.

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arXiv2018

Assembly of supermassive black hole seeds

We present a suite of six fully cosmological, three-dimensional simulations of the collapse of an atomic cooling halo in the early Universe. We use the moving-mesh code arepo with an improved primordial chemistry network to evolve the hydrodynamical and chemical equations. The addition of a strong Lyman-Werner background suppresses molecular hydrogen cooling and permits the gas to evolve nearly isothermally at a temperature of about 8000 K. Strong gravitational torques effectively remove angular momentum and lead to the central collapse of gas, forming a supermassive protostar at the center of the halo. We model the protostar using two methods: sink particles that grow through mergers with other sink particles, and a stiff equation of state that leads to the formation of an adiabatic core. We impose threshold densities of $10^8$, $10^{10}$, and $10^{12}\,\text{cm}^{-3}$ for the sink particle formation and the onset of the stiff equation of state to study the late, intermediate, and early stages in the evolution of the protostar, respectively. We follow its growth from masses $\simeq 10\,\text{M}_\odot$ to $\simeq 10^5\,\text{M}_\odot$, with an average accretion rate of $\langle\dot{M}_\star\rangle \simeq 2\,\text{M}_\odot\,\text{yr}^{-1}$ for sink particles, and $\simeq 0.8 - 1.4\,\text{M}_\odot\,\text{yr}^{-1}$ for the adiabatic cores. At the end of the simulations, the HII region generated by radiation from the central object has long detached from the protostellar photosphere, but the ionizing radiation remains trapped in the inner host halo, and has thus not yet escaped into the intergalactic medium. Fully coupled, radiation-hydrodynamics simulations hold the key for further progress.

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arXiv2018

Minimum star-forming halo mass in axion cosmology

Elucidating the particle physics nature of dark matter (DM) is one of the great challenges in modern science. The current lack of any direct DM detections in the laboratory heightens the need for astrophysical constraints, extending the search to DM models beyond the popular weakly interacting massive particle (WIMP) scenario. We here apply the classical Rees-Ostriker-Silk cooling criterion for galaxy formation to models with ultralight axion DM, also known as fuzzy dark matter (FDM). The resulting constraints provide a heuristic framework for upcoming observations, and our approximate analysis motivates the need for future, self-consistent simulations of FDM structure formation. We use observational constraints for the DM hosts of ultra faint dwarf (UFD) galaxies in the Local Group, together with the redshift constraints for the onset of primordial star formation from the recent EDGES 21-cm cosmology measurement, to illustrate this approach. We find that the existing constraints are straightforward to reconcile with standard $Λ$CDM, but disfavour FDM axion masses below $\sim 10^{-21}\,{\rm eV}/c^2$. The future potential for harnessing astrophysical probes of DM particle physics is compelling.

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arXiv2018

Baryon-dark matter scattering and first star formation

The recent detection of the sky-averaged 21-cm cosmological signal indicates a stronger absorption than the maximum allowed value based on the standard model. One explanation for the required colder primordial gas is the energy transfer between the baryon and dark matter fluids due to non-gravitational scattering. Here, we explore the thermal evolution of primordial gas, collapsing to form Population III (Pop III) stars, when this energy transfer is included. Performing a series of one-zone calculations, we find that the evolution results in stars more massive than in the standard model, provided that the dark matter is described by the best-fit parameters inferred from the 21-cm observation. On the other hand, a significant part of the dark matter parameter space can be excluded by the requirement to form massive Pop III stars sufficiently early in cosmic history. Otherwise, the radiation background needed to bring about the strong Wouthuysen-Field coupling at z >~ 17, inferred to explain the 21-cm absorption feature, could not be builtup. Intriguingly, the independent constraint from the physics of first star formation at high densities points to a similarly narrow range in dark matter properties. This exploratory study has to be followed-up with self-consistent three-dimensional simulations for a more rigorous derivation.

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arXiv2018

Discrete diffusion Lyman-alpha radiative transfer

Due to its accuracy and generality, Monte Carlo radiative transfer (MCRT) has emerged as the prevalent method for Ly$α$ radiative transfer in arbitrary geometries. The standard MCRT encounters a significant efficiency barrier in the high optical depth, diffusion regime. Multiple acceleration schemes have been developed to improve the efficiency of MCRT but the noise from photon packet discretization remains a challenge. The discrete diffusion Monte Carlo (DDMC) scheme has been successfully applied in state-of-the-art radiation hydrodynamics (RHD) simulations. Still, the established framework is not optimal for resonant line transfer. Inspired by the DDMC paradigm, we present a novel extension to resonant DDMC (rDDMC) in which diffusion in space and frequency are treated on equal footing. We explore the robustness of our new method and demonstrate a level of performance that justifies incorporating the method into existing Ly$α$ codes. We present computational speedups of $\sim 10^2$-$10^6$ relative to contemporary MCRT implementations with schemes that skip scattering in the core of the line profile. This is because the rDDMC runtime scales with the spatial and frequency resolution rather than the number of scatterings - the latter is typically $\propto τ_0$ for static media, or $\propto (a τ_0)^{2/3}$ with core-skipping. We anticipate new frontiers in which on-the-fly Ly$α$ radiative transfer calculations are feasible in 3D RHD. More generally, rDDMC is transferable to any computationally demanding problem amenable to a Fokker-Planck approximation of frequency redistribution.

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arXiv2018

Detection strategies for the first supernovae with JWST

Pair-instability supernovae (PISNe) are very luminous explosions of massive, low metallicity stars. They can potentially be observed out to high redshifts due to their high explosion energies, thus providing a probe of the Universe prior to reionization. The near-infrared camera, NIRCam, on board the James Webb Space Telescope is ideally suited for detecting their redshifted ultraviolet emission. We calculate the photometric signature of high-redshift PISNe and derive the optimal detection strategy for identifying their prompt emission and possible afterglow. We differentiate between PISNe and other sources that could have a similar photometric signature, such as active galactic nuclei or high-redshift galaxies. We demonstrate that the optimal strategy, which maximizes the visibility time of the PISN lightcurve per invested exposure time, consists of the two wide-band filters F200W and F356W with an exposure time of 600s. For such exposures, we expect one PISN at $z \lesssim 7.5$ per at least 50,000 different field of view, which can be accomplished with parallel observations and an extensive archival search. The PISN afterglow, caused by nebular emission and reverberation, is very faint and requires unfeasibly long exposure times to be uniquely identified. However, this afterglow would be visible for several hundred years, about two orders of magnitude longer than the prompt emission, rendering PISNe promising targets for future, even more powerful telescopes.

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arXiv2018

Opacity limit for supermassive protostars

We present a model for the evolution of supermassive protostars from their formation at $M_\star \simeq 0.1\,\text{M}_\odot$ until their growth to $M_\star \simeq 10^5\,\text{M}_\odot$. To calculate the initial properties of the object in the optically thick regime we follow two approaches: based on idealized thermodynamic considerations, and on a more detailed one-zone model. Both methods derive a similar value of $n_{\rm F} \simeq 2 \times 10^{17} \,\text{cm}^{-3}$ for the density of the object when opacity becomes important, i.e. the opacity limit. The subsequent evolution of the growing protostar is determined by the accretion of gas onto the object and can be described by a mass-radius relation of the form $R_\star \propto M_\star^{1/3}$ during the early stages, and of the form $R_\star \propto M_\star^{1/2}$ when internal luminosity becomes important. For the case of a supermassive protostar, this implies that the radius of the star grows from $R_\star \simeq 0.65 \,{\rm AU}$ to $R_\star \simeq 250 \,{\rm AU}$ during its evolution. Finally, we use this model to construct a sub-grid recipe for accreting sink particles in numerical simulations. A prime ingredient thereof is a physically motivated prescription for the accretion radius and the effective temperature of the growing protostar embedded inside it. From the latter, we can conclude that photo-ionization feedback can be neglected until very late in the assembly process of the supermassive object.

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arXiv2018

Angular momentum transfer in primordial discs and the rotation of the first stars

We investigate the rotation velocity of the first stars by modelling the angular momentum transfer in the primordial accretion disc.Assessing the impact of magnetic braking, we consider the transition in angular momentum transport mode at the Alfv$\acute{\rm e}$n radius, from the dynamically dominated free-fall accretion to the magnetically dominated solid-body one.The accreting protostar at the centre of the primordial star-forming cloud rotates with close to breakup speed in the case without magnetic fields.Considering a physically-motivated model for small-scale turbulent dynamo amplification, we find that stellar rotation speed quickly declines if a large fraction of the initial turbulent energy is converted to magnetic energy ($\gtrsim 0.14$). Alternatively, if the dynamo process were inefficient, for amplification due to flux-freezing, stars would become slow rotators if the pre-galactic magnetic field strength is above a critical value, $\simeq 10^{-8.2}$G, evaluated at a scale of $n_{\rm H} = 1 {\rm cm}^{-3}$, which is significantly higher than plausible cosmological seed values ($\sim 10^{-15}$G). Because of the rapid decline of the stellar rotational speed over a narrow range in model parameters, the first stars encounter a bimodal fate: rapid rotation at almost the breakup level, or the near absence of any rotation.

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arXiv2018

Effect of lithium hydride on the cooling of primordial gas

We complete the formulation of the standard model of first star formation by exploring the possible impact of $\mathrm{LiH}$ cooling, which has been neglected in previous simulations of non-linear collapse. Specifically, we find that at redshift $z\gtrsim 5$, the cooling by $\mathrm{LiH}$ has no effect on the thermal evolution of shocked primordial gas, and of collapsing primordial gas into minihaloes or relic HII regions, even if the primordial lithium abundance were enhanced by one order of magnitude. Adding the most important lithium species to a minimum network of primordial chemistry, we demonstrate that insufficient $\mathrm{LiH}$ is produced in all cases considered, about $[\mathrm{LiH/Li}]\sim 10^{-9}$ for $T\lesssim 100$ K. Indeed, $\mathrm{LiH}$ cooling would only be marginally significant in shocked primordial gas for the highly unlikely case that the $\mathrm{LiH}$ abundance were increased by nine orders of magnitude, implying that $all$ lithium would have to be converted into $\mathrm{LiH}$. In this study, photo-destruction processes are not considered, and the collisional disassociation rate of $\mathrm{LiH}$ is possibly underestimated, rendering our results an extreme upper limit. Therefore, the cooling by $\mathrm{LiH}$ can safely be neglected for the thermal evolution of Population~III star-forming gas.

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arXiv2018

Dust in the first galaxies

Using cosmological volume simulations and a custom built sub-grid model for Pop~III star formation, we examine the baseline dust extinction in the first galaxies due to Pop~III metal enrichment in the first billion years of cosmic history. We find that while the most enriched, high-density lines of sight in primordial galaxies can experience a measurable amount of extinction from Pop~III dust ($E(B-V)_{\rm max}=0.07,\ A_{\rm V,max}\approx0.28$), the average extinction is very low with $\left< E(B-V) \right> \lesssim 10^{-3}$. We derive a power-law relationship between dark matter halo mass and extinction of $E(B-V)\propto M_{\rm halo}^{0.80}$. Performing a Monte Carlo parameter study, we establish the baseline reddening of the UV spectra of dwarf galaxies at high redshift due to Pop~III enrichment only. With this method, we find $\left<β_{\rm UV}\right>-2.51\pm0.07$, which is both nearly halo mass and redshift independent.

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arXiv2018

Baseline Metal Enrichment from Population III Star Formation in Cosmological Volume Simulations

We utilize the hydrodynamic and N-body code {\small GIZMO} coupled with our newly developed sub-grid Population~III (Pop~III) Legacy model, designed specifically for cosmological volume simulations, to study the baseline metal enrichment from Pop~III star formation at $z>7$. In this idealized numerical experiment, we only consider Pop~III star formation. We find that our model Pop~III star formation rate density (SFRD), which peaks at $\sim 10^{-3}\ {\rm M_\odot yr^{-1} Mpc^{-1}}$ near $z\sim10$, agrees well with previous numerical studies and is consistent with the observed estimates for Pop~II SFRDs. The mean Pop~III metallicity rises smoothly from $z=25-7$, but does not reach the critical metallicity value, $Z_{\rm crit}=10^{-4}\ Z_\odot$, required for the Pop~III to Pop~II transition in star formation mode until $z\simeq7$. This suggests that, while individual halos can suppress in-situ Pop~III star formation, the external enrichment is insufficient to globally terminate Pop~III star formation. The maximum enrichment from Pop~III star formation in star forming dark matter halos is $Z\sim10^{-2}\ Z_\odot$, whereas the minimum found in externally enriched haloes is $Z\gtrsim10^{-7}\ Z_\odot$. Finally, mock observations of our simulated IGM enriched with Pop~III metals produce equivalent widths similar to observations of an extremely metal poor damped Lyman alpha (DLA) system at $z=7.04$, which is thought to be enriched by Pop~III star formation only.

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arXiv2017

Connecting the first galaxies with ultra faint dwarfs in the Local Group: chemical signatures of Population~III stars

We investigate the star formation history and chemical evolution of isolated analogues of Local Group (LG) ultra faint dwarf galaxies (UFDs; stellar mass range of 10^2 solar mass < M_star <10^5 solar mass) and gas rich, low mass dwarfs (Leo P analogs; stellar mass range of 10^5 solar mass < M_star <10^6 solar mass). We perform a suite of cosmological hydrodynamic zoom-in simulations to follow their evolution from the era of the first generation of stars down to z=0. We confirm that reionization, combined with supernova (SN) feedback, is primarily responsible for the truncated star formation in UFDs. Specifically, haloes with a virial mass of M_vir < 2 x 10^9 solar mass form> 90\% of stars prior to reionization. Our work further demonstrates the importance of Population~III (Pop~III) stars, with their intrinsically high $\rm [C/Fe]$ yields, and the associated external metal-enrichment, in producing low-metallicity stars ($\rm [Fe/H]\lesssim-4$) and carbon-enhanced metal-poor (CEMP) stars. We find that UFDs are composite systems, assembled from multiple progenitor haloes, some of which hosted only Population~II (Pop~II) stars formed in environments externally enriched by SNe in neighboring haloes, naturally producing, extremely low-metallicity Pop~II stars. We illustrate how the simulated chemical enrichment may be used to constrain the star formation histories (SFHs) of true observed UFDs. We find that Leo P analogs can form in haloes with M_vir ~ 4 x 10^9 solar mass (z=0). Such systems are less affected by reionization and continue to form stars until z=0, causing higher metallicity tails. Finally, we predict the existence of extremely low-metallicity stars in LG UFD galaxies that preserve the pure chemical signatures of Pop~III nucleosynthesis.

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arXiv2017

First star formation in ultra-light particle dark matter cosmology

The formation of the first stars in the high-redshift Universe is a sensitive probe of the small-scale, particle physics nature of dark matter (DM). We carry out cosmological simulations of primordial star formation in ultra-light, axion-like particle DM cosmology, with masses of $10^{-22}$ and $10^{-21}\,{\rm eV}$, with de Broglie wavelengths approaching galactic scales ($\sim$kpc). The onset of star formation is delayed, and shifted to more massive host structures. For the lightest DM particle mass explored here, first stars form at $z \sim 7$ in structures with $\sim 10^9\,{\rm M}_\odot$, compared to the standard minihalo environment within the $Λ$ cold dark matter ($Λ$CDM) cosmology, where $z \sim 20 - 30$ and $\sim 10^5 - 10^6\,{\rm M}_\odot$. Despite this greatly altered DM host environment, the thermodynamic behaviour of the metal-free gas as it collapses into the DM potential well asymptotically approaches a very similar evolutionary track. Thus, the fragmentation properties are predicted to remain the same as in $Λ$CDM cosmology, implying a similar mass scale for the first stars. These results predict intense starbursts in the axion cosmologies, which may be amenable to observations with the {\it James Webb Space Telescope}.

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arXiv2017

Warm dark matter constraints from high-$z$ Direct Collapse Black Holes using the JWST

We use a semi-analytic model, ${\it Delphi}$, that jointly tracks the dark matter and baryonic assembly of high-redshift ($z \simeq 4-20$) galaxies to gain insight on the number density of Direct Collapse Black Hole (DCBH) hosts in three different cosmologies: the standard Cold Dark Matter (CDM) model and two Warm Dark Matter (WDM) models with particle masses of 3.5 and 1.5 keV. Obtaining the Lyman-Werner (LW) luminosity of each galaxy from ${\it Delphi}$, we use a clustering bias analysis to identify all, pristine halos with a virial temperature $T_{vir}>=10^4$ K that are irradiated by a LW background above a critical value as, DCBH hosts. In good agreement with previous studies, we find the DCBH number density rises from $\sim10^{-6.1}$ to $\sim 10^{-3.5}\, \mathrm{cMpc^{-3}}$ from $z\simeq 17.5$ to $8$ in the CDM model using a critical LW background value of $30 J_{21}$ (where $J_{21}= 10^{-21} \, {\rm erg\, s^{-1}\, Hz^{-1} \, cm^{-2} \, sr^{-1}}$). We find that a combination of delayed structure formation and an accelerated assembly of galaxies results in a later metal-enrichment and an accelerated build-up of the LW background in the 1.5 keV WDM model, resulting in DCBH hosts persisting down to much lower redshifts ($z \simeq 5$) as compared to CDM where DCBH hosts only exist down to $z \simeq 8$. We end by showing how the expected colours in three different bands of the Near Infrared Camera (NIRCam) onboard the forthcoming James Webb Space Telescope (${\it JWST}$) can be used to hunt for potential $z \simeq 5-9$ DCBHs, allowing hints on the WDM particle mass.

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arXiv2017

Radiative effects during the assembly of direct collapse black holes

We perform a post-processing radiative feedback analysis on a 3D ab initio cosmological simulation of an atomic cooling halo under the direct collapse black hole (DCBH) scenario. We maintain the spatial resolution of the simulation by incorporating native ray-tracing on unstructured mesh data, including Monte Carlo Lyman-alpha (Lyα) radiative transfer. DCBHs are born in gas-rich, metal-poor environments with the possibility of Compton-thick conditions, $N_H \gtrsim 10^{24} {\rm cm}^{-2}$. Therefore, the surrounding gas is capable of experiencing the full impact of the bottled-up radiation pressure. In particular, we find that multiple scattering of Lyα photons provides an important source of mechanical feedback after the gas in the sub-parsec region becomes partially ionized, avoiding the bottleneck of destruction via the two-photon emission mechanism. We provide detailed discussion of the simulation environment, expansion of the ionization front, emission and escape of Lyα radiation, and Compton scattering. A sink particle prescription allows us to extract approximate limits on the post-formation evolution of the radiative feedback. Fully coupled Lyα radiation hydrodynamics will be crucial to consider in future DCBH simulations.

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arXiv2017

The first supermassive black holes

We briefly review the historical development of the ideas regarding the first supermassive black hole seeds, the physics of their formation and radiative feedback, recent theoretical and observational progress, and our outlook for the future.

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arXiv2017

Low-energy Population III supernovae and the origin of extremely metal-poor stars

Some ancient, dim, metal-poor stars may have formed in the ashes of the first supernovae (SNe). If their chemical abundances can be reconciled with the elemental yields of specific Population III (Pop III) explosions, they could reveal the properties of primordial stars. But multidimensional simulations of such explosions are required to predict their yields because dynamical instabilities can dredge material up from deep in the ejecta that would otherwise be predicted to fall back on to the central remnant and be lost in one-dimensional (1D) models. We have performed two-dimensional (2D) numerical simulations of two low-energy Pop III SNe, a 12.4 Msun explosion and a 60 Msun explosion, and find that they produce elemental yields that are a good fit to those measured in the most iron-poor star discovered to date, SMSS J031300.36-670839.3 (J031300). Fallback on to the compact remnant in these weak explosions accounts for the lack of measurable iron in J031300 and its low iron-group abundances in general. Our 2D explosions produce higher abundances of heavy elements (atomic number Z > 20) than their 1D counterparts due to dredge-up by fluid instabilities. Since almost no Ni is ejected by these weak SNe, their low luminosities will prevent their detection in the near-infrared with the James Webb Space Telescope and future 30-m telescopes on the ground. The only evidence that they ever occurred will be in the fossil abundance record.

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