arXiv subjects

Volker Bromm

Publications and source records attributed to Volker Bromm.

At least 145 records · Page 8Linked to original sources

arXiv2017

Magnetar-Powered Supernovae in Two Dimensions. II. Broad-Line Supernovae Ic

Nascent neutron stars with millisecond periods and magnetic fields in excess of $10^{16}$ Gauss can drive highly energetic and asymmetric explosions known as magnetar-powered supernovae. These exotic explosions are one theoretical interpretation for supernovae Ic-BL which are sometimes associated with long gamma-ray bursts. Twisted magnetic field lines extract the rotational energy of the neutron star and release it as a disk wind or a jet with energies greater than 10$^{52}$ erg over $\sim 20$ sec. What fractions of the energy of the central engine go into the wind and the jet remain unclear. We have performed two-dimensional hydrodynamical simulations of magnetar-powered supernovae (SNe) driven by disk winds and jets with the CASTRO code to investigate the effect of the central engine on nucleosynthetic yields, mixing, and light curves. We find that these explosions synthesize less than 0.05 Msun of Ni and that this mass is not very sensitive to central engine type. The morphology of the explosion can provide a powerful diagnostic of the properties of the central engine. In the absence of a circumstellar medium these events are not very luminous, with peak bolometric magnitudes $M_b \sim -16.5 $ due to low Ni production.

astro-ph.HE↗

arXiv2017

Formation and survival of Population III stellar systems

The initial mass function of the first, Population III (Pop III), stars plays a vital role in shaping galaxy formation and evolution in the early Universe. One key remaining issue is the final fate of secondary protostars formed in the accretion disc, specifically whether they merge or survive. We perform a suite of hydrodynamic simulations of the complex interplay between fragmentation, protostellar accretion, and merging inside dark matter minihaloes. Instead of the traditional sink particle method, we employ a stiff equation of state approach, so that we can more robustly ascertain the viscous transport inside the disc. The simulations show inside-out fragmentation because the gas collapses faster in the central region. Fragments migrate on the viscous timescale, over which angular momentum is lost, enabling them to move towards the disc centre, where merging with the primary protostar can occur. This process depends on the fragmentation scale, such that there is a maximum scale of $(1 - 5) \times 10^4$ au, inside which fragments can migrate to the primary protostar. Viscous transport is active until radiative feedback from the primary protostar destroys the accretion disc. The final mass spectrum and multiplicity thus crucially depends on the effect of viscosity in the disc. The entire disc is subjected to efficient viscous transport in the primordial case with viscous parameter $α\le 1$. An important aspect of this question is the survival probability of Pop III binary systems, possible gravitational wave sources to be probed with the Advanced LIGO detectors.

astro-ph.GA↗

arXiv2017

Reionization and Galaxy Formation in Warm Dark Matter Cosmologies

We compare model results from a semi-analytic (merger-tree based) framework for high-redshift (z ~ 5-20) galaxy formation against reionization indicators, including the Planck electron scattering optical depth and the ionizing photon emissivity, to shed light on the reionization history and sources in Cold (CDM) and Warm Dark Matter (WDM; particle masses of $m_x = 1.5,3$ and 5 keV) cosmologies. This model includes all the key processes of star formation, supernova feedback, the merger/accretion/ejection driven evolution of gas and stellar mass and the effect of the ultra-violet background (UVB) in photo-evaporating the gas content of low-mass galaxies. We find that the delay in the start of reionization in light (1.5 keV) WDM models can be compensated by a steeper redshift evolution of the ionizing photon escape fraction and a faster mass assembly, resulting in reionization ending at comparable redshifts (z~5.5) in all the DM models considered. We find the bulk of the reionization photons come from galaxies with a halo mass $M_h < 10^9 M_\odot$ and a UV magnitude $ -15 < M_{UV} < -10$ in CDM. The progressive suppression of low-mass halos with decreasing $m_x$ leads to a shift in the reionization population to larger halo masses of $M_h > 10^9 M_\odot$ and $ -17 < M_{UV} < -13$ for 1.5 keV WDM. We find that current observations of the electron scattering optical depth and the Ultra-violet luminosity function are equally compatible with all the (cold and warm) DM models considered in this work. We propose that global indicators including the redshift evolution of the stellar mass density and the stellar mass-halo mass relation, observable with the James Webb Space Telescope, can be used to distinguish between CDM and WDM (1.5 keV) cosmologies.

astro-ph.CO↗

arXiv2016

Contribution of the first galaxies to the cosmic far-infrared/sub-millimeter background - I. Mean background level

We study the contribution of the first galaxies to the far-infrared/sub-millimeter (FIR/sub-mm) extragalactic background light (EBL) by implementing an analytical model for dust emission. We explore different dust models, assuming different grain size distributions and chemical compositions. According to our findings, observed re-radiated emission from dust in dwarf-size galaxies at $z \sim 10$ would peak at a wavelength of $\sim 500 μ{\rm m}$ with observed fluxes of $\sim 10^{-3} - 10^{-2}$ nJy, which is below the capabilities of current observatories. In order to be detectable, model sources at these high redshifts should exhibit luminosities of $\gtrsim 10^{12} L_{\odot}$, comparable to that of local ultra-luminous systems. The FIR/sub-mm EBL generated by primeval galaxies peaks at $\sim 500 μ{\rm m}$, with an intensity ranging from $\sim 10^{-4}$ to $10^{-3} {\rm nW \ m^{-2} \ sr^{-1}}$, depending on dust properties. These values are $\sim 3 - 4$ orders of magnitude below the absolute measured cosmic background level, suggesting that the first galaxies would not contribute significantly to the observed FIR/sub-mm EBL. Our model EBL exhibits a strong correlation with the dust-to-metal ratio, where we assume a fiducial value of $D = 0.005$, increasing almost proportionally to it. Thus, measurements of the FIR/sub-mm EBL could provide constraints on the amount of dust in the early Universe. Even if the absolute signal from primeval dust emission may be undetectable, it might still be possible to obtain information about it by exploring angular fluctuations at $\sim 500 μ{\rm m}$, close to the peak of dust emission from the first galaxies.

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arXiv2016

Lyman-alpha radiation hydrodynamics of galactic winds before cosmic reionization

The dynamical impact of Lyman-alpha (Lyα) radiation pressure on galaxy formation depends on the rate and duration of momentum transfer between Lyα photons and neutral hydrogen gas. Although photon trapping has the potential to multiply the effective force, ionizing radiation from stellar sources may relieve the Lyα pressure before appreciably affecting the kinematics of the host galaxy or efficiently coupling Lyα photons to the outflow. We present self-consistent Lyα radiation-hydrodynamics simulations of high-$z$ galaxy environments by coupling the Cosmic Lyα Transfer code (COLT) with spherically symmetric Lagrangian frame hydrodynamics. The accurate but computationally expensive Monte-Carlo radiative transfer calculations are feasible under the one-dimensional approximation. The initial starburst drives an expanding shell of gas from the centre and in certain cases Lyα feedback significantly enhances the shell velocity. Radiative feedback alone is capable of ejecting baryons into the intergalactic medium (IGM) for protogalaxies with a virial mass of $M_{\rm vir} \lesssim 10^8~{\rm M}_\odot$. We compare the Lyα signatures of Population III stars with $10^5$ K blackbody emission to that of direct collapse black holes with a nonthermal Compton-thick spectrum and find substantial differences if the Lyα spectra are shaped by gas pushed by Lyα radiation-driven winds. For both sources, the flux emerging from the galaxy is reprocessed by the IGM such that the observed Lyα luminosity is reduced significantly and the time-averaged velocity offset of the Lyα peak is shifted redward.

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arXiv2016

Toward $ab\,initio$ extremely metal poor stars

Extremely metal poor stars have been the focus of much recent attention owing to the expectation that their chemical abundances can shed light on the metal and dust yields of the earliest supernovae. We present our most realistic simulation to date of the astrophysical pathway to the first metal enriched stars. We simulate the radiative and supernova hydrodynamic feedback of a $60\,M_\odot$ Population III star starting from cosmological initial conditions realizing Gaussian density fluctuations. We follow the gravitational hydrodynamics of the supernova remnant at high spatial resolution through its freely-expanding, adiabatic, and radiative phases, until gas, now metal-enriched, has resumed runaway gravitational collapse. Our findings are surprising: while the Population III progenitor exploded with a low energy of $10^{51}\,\text{erg}$ and injected an ample metal mass of $6\,M_\odot$, the first cloud to collapse after the supernova explosion is a dense surviving primordial cloud on which the supernova blastwave deposited metals only superficially, in a thin, unresolved layer. The first metal-enriched stars can form at a very low metallicity, of only $2-5\times10^{-4}\,Z_\odot$, and can inherit the parent cloud's highly elliptical, radially extended orbit in the dark matter gravitational potential.

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arXiv2016

Exploring the nature of the Lyman-$α$ emitter CR7

CR7 is the brightest Lyman-$α$ emitter observed at $z>6$, which shows very strong Lyman-$α$ and HeII 1640Å line luminosities, but no metal line emission. Previous studies suggest that CR7 hosts either young primordial stars with a total stellar mass of $\sim 10^7\,\mathrm{M}_\odot$ or a black hole of $\gtrsim 10^6\,\mathrm{M}_\odot$. Here, we explore different formation scenarios for CR7 with a semianalytical model, based on the random sampling of dark matter merger trees. We are unable to reproduce the observational constraints with a primordial stellar source, given our model assumptions, due to the short stellar lifetimes and the early metal enrichment. Black holes that are the remnants of the first stars are either not massive enough, or reside in metal-polluted haloes, ruling out this possible explanation of CR7. Our models instead suggest that direct collapse black holes, which form in metal-free haloes exposed to large Lyman-Werner fluxes, are more likely the origin of CR7. However, this result is derived under optimistic assumptions and future observations are necessary to further constrain the nature of CR7.

astro-ph.GA↗

arXiv2016

Building up the Population III initial mass function from cosmological initial conditions

We simulate the growth of a Population III stellar system, starting from cosmological initial conditions at z=100. We follow the formation of a minihalo and the subsequent collapse of its central gas to high densities, resolving scales as small as ~ 1 AU. Using sink particles to represent the growing protostars, we model the growth of the photodissociating and ionizing region around the first sink, continuing the simulation for ~ 5000 yr after initial protostar formation. Along with the first-forming sink, several tens of secondary sinks form before an ionization front develops around the most massive star. The resulting cluster has high rates of sink formation, ejections from the stellar disc, and sink mergers during the first ~ 2000 yr, before the onset of radiative feedback. By this time a warm ~ 5000 K phase of neutral gas has expanded to roughly the disc radius of 2000 AU, slowing mass flow onto the disc and sinks. By 5000 yr the most massive star grows to 20 M_sol, while the total stellar mass approaches 75 M_sol. Out of the ~ 40 sinks, approximately 30 are low-mass (M_* < 1 M_sol), and if the simulation had resolved smaller scales an even greater number of sinks might have formed. Thus, protostellar radiative feedback is insufficient to prevent rapid disc fragmentation and the formation of a high-member Pop III cluster before an ionization front emerges. Throughout the simulation, the majority of stellar mass is contained within the most massive stars, further implying that the Pop III initial mass function is top-heavy.

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arXiv2016

Gravitational Waves from the Remnants of the First Stars

Gravitational waves (GWs) provide a revolutionary tool to investigate yet unobserved astrophysical objects. Especially the first stars, which are believed to be more massive than present-day stars, might be indirectly observable via the merger of their compact remnants. We develop a self-consistent, cosmologically representative, semi-analytical model to simulate the formation of the first stars. By extrapolating binary stellar-evolution models at 10% solar metallicity to metal-free stars, we track the individual systems until the coalescence of the compact remnants. We estimate the contribution of primordial stars to the merger rate density and to the detection rate of the Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO). Owing to their higher masses, the remnants of primordial stars produce strong GW signals, even if their contribution in number is relatively small. We find a probability of $\gtrsim1\%$ that the current detection GW150914 is of primordial origin. We estimate that aLIGO will detect roughly 1 primordial BH-BH merger per year for the final design sensitivity, although this rate depends sensitively on the primordial initial mass function (IMF). Turning this around, the detection of black hole mergers with a total binary mass of $\sim 300\,\mathrm{M}_\odot$ would enable us to constrain the primordial IMF.

astro-ph.GA↗

arXiv2016

Evidence for a direct collapse black hole in the Lyman-alpha source CR7

Throughout the epoch of reionization the most luminous Lyα emitters are capable of ionizing their own local bubbles. The CR7 galaxy at $z \approx 6.6$ stands out for its combination of exceptionally bright Lyα and HeII 1640 Angstrom line emission but absence of metal lines. As a result CR7 may be the first viable candidate host of a young primordial starburst or direct collapse black hole. High-resolution spectroscopy reveals a +160 km s$^{-1}$ velocity offset between the Lyα and HeII line peaks while the spatial extent of the Lyα emitting region is $\sim 16$ kpc. The observables are indicative of an outflow signature produced by a strong central source. We present one-dimensional radiation-hydrodynamics simulations incorporating accurate Lyα feedback and ionizing radiation to investigate the nature of the CR7 source. We find that a Population III star cluster with $10^5$ K blackbody emission ionizes its environment too efficiently to generate a significant velocity offset. However, a massive black hole with a nonthermal Compton-thick spectrum is able to reproduce the Lyα signatures as a result of higher photon trapping and longer potential lifetime. For both sources, Lyα radiation pressure turns out to be dynamically important.

astro-ph.GA↗

arXiv2016

Gamma-ray bursts and Population III stars

Gamma-ray bursts (GRBs) are ideal probes of the epoch of the first stars and galaxies. We review the recent theoretical understanding of the formation and evolution of the first (so-called Population III) stars, in light of their viability of providing GRB progenitors. We proceed to discuss possible unique observational signatures of such bursts, based on the current formation scenario of long GRBs. These include signatures related to the prompt emission mechanism, as well as to the afterglow radiation, where the surrounding intergalactic medium might imprint a telltale absorption spectrum. We emphasize important remaining uncertainties in our emerging theoretical framework.

astro-ph.HE↗

arXiv2016

The First Stars: formation under cosmic ray feedback

We explore the impact of a cosmic ray (CR) background generated by supernova explosions from the first stars on star-forming metal-free gas in a minihalo at $z\sim25$. Starting from cosmological initial conditions, we use the smoothed particle hydrodynamics code GADGET-2 to follow gas collapsing under the influence of a CR background up to densities of $n=10^{12}\,{\rm cm}^{-3}$, at which point we form sink particles. Using a suite of simulations with two sets of initial conditions and employing a range of CR background models, we follow each simulation for $5000\,$yr after the first sink forms. CRs both heat and ionise the gas, boosting ${\rm H}_2$ formation. Additional ${\rm H}_2$ enhances the cooling efficiency of the gas, allowing it to fulfil the Rees-Ostriker criterion sooner and expediting the collapse, such that each simulation reaches high densities at a different epoch. As it exits the loitering phase, the thermodynamic path of the collapsing gas converges to that seen in the absence of any CR background. By the time the gas approaches sink formation densities, the thermodynamic state of the gas is thus remarkably similar across all simulations. This leads to a robust characteristic mass that is largely independent of the CR background, of order $\sim$ a few $\times10\,{\rm M}_{\odot}$ even as the CR background strength varies by five orders of magnitude.

astro-ph.GA↗

arXiv2015

Preserving chemical signatures of primordial star formation in the first low-mass stars

We model early star forming regions and their chemical enrichment by Population III (Pop III) supernovae with nucleosynthetic yields featuring high [C/Fe] ratios and pair-instability supernova (PISN) signatures. We aim to test how well these chemical abundance signatures are preserved in the gas prior to forming the first long-lived low-mass stars (or second-generation stars). Our results show that second-generation stars can retain the nucleosynthetic signature of their Pop III progenitors, even in the presence of nucleosynthetically normal Pop III core-collapse supernovae. We find that carbon-enhanced metal-poor stars are likely second-generation stars that form in minihaloes. Furthermore, it is likely that the majority of Pop III supernovae produce high [C/Fe] yields. In contrast, metals ejected by a PISN are not concentrated in the first star forming haloes, which may explain the absence of observed PISN signatures in metal-poor stars. We also find that unique Pop III abundance signatures in the gas are quickly wiped out by the emergence of Pop II supernovae. We caution that the observed fractions of stars with Pop III signatures cannot be directly interpreted as the fraction of Pop III stars producing that signature. Such interpretations require modelling the metal enrichment process prior to the second-generation stars' formation, including results from simulations of metal mixing. The full potential of stellar archaeology can likely be reached in ultra-faint dwarf galaxies, where the simple formation history may allow for straightforward identification of second-generation stars.

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arXiv2015

On the Physical Requirements for a Pre-Reionization Origin of the Unresolved Near-Infrared Background

The study of the Cosmic Near-Infrared Background (CIB) light after subtraction of resolved sources can push the limits of current observations and infer the level of galaxy and black hole activity in the early universe. However, disentangling the relative contribution from low- and high-redshift sources is not trivial. Spatial fluctuations of the CIB exhibit a clustering excess at angular scales $\sim 1^\circ$ whose origin has not been conclusively identified. We explore the likelihood that this signal is dominated by emission from galaxies and accreting black holes in the early Universe. We find that, if the first small mass galaxies have a normal IMF, the light of their ageing stars (fossils) integrated over cosmic time contributes a comparable amount to the CIB as their pre-reionization progenitors. However, the measured fluctuation signal is too large to be produced by galaxies at redshifts $z>8$ unless their star formation efficiencies are much larger than those inferred from the observed Lyman-dropout population. In order to produce the observed level of CIB fluctuation without violating constraints from galaxy counts and the electron optical depth of the IGM, minihalos at $z>12$ must form stars with efficiency $f_\star \gtrsim 0.1$ and, although a top-heavy IMF is preferred, have a very low escape fraction of ionizing radiation, $f_{\rm esc}<0.01$. If instead the CIB fluctuations are produced by high-$z$ black holes, one requires vigorous accretion in the early universe reaching $ρ_{\rm acc} \gtrsim 10^5M_\odot{\rm Mpc^{-3}}$ by $z\simeq 10$. This growth must stop by $z \sim 6$ and be significantly obscured not to overproduce the soft cosmic X-ray background (CXB) and its observed coherence with the CIB. We therefore find the range of suitable possibilities at high-$z$ to be narrow, but could possibly be widened by including additional physics and evolution at those epochs.

astro-ph.CO↗

arXiv2015

The First Stars: formation under X-ray feedback

We investigate the impact of a cosmic X-ray background (CXB) on Population III stars forming in a minihalo at $z\simeq25$. Using the smoothed particle hydrodynamics code GADGET-2, we attain sufficient numerical resolution to follow gas collapsing into the centre of the minihalo from cosmological initial conditions up to densities of $10^{12}\,{\rm cm}^{-3}$, at which point we form sink particles. This allows us to study how the presence of a CXB affects the formation of H$_2$ and HD in the gas prior to becoming fully molecular. Using a suite of simulations for a range of possible CXB models, we follow each simulation for 5000\yr after the first sink particle forms. The CXB provides two competing effects, with X-rays both heating the gas and increasing the free electron fraction, allowing more H$_2$ to form. X-ray heating dominates below $n\sim1\,{\rm cm}^{-3}$, while the additional H$_2$ cooling becomes more important above $n\sim10^2\,{\rm cm}^{-3}$. The gas becomes optically thick to X-rays as it exits the quasi-hydrostatic `loitering phase,' such that the primary impact of the CXB is to cool the gas at intermediate densities, resulting in an earlier onset of baryonic collapse into the dark matter halo. At the highest densities, self-shielding results in similar thermodynamic behaviour across a wide range of CXB strengths. Consequently, we find that star formation is relatively insensitive to the presence of a CXB; both the number and the characteristic mass of the stars formed remains quite similar even as the strength of the CXB varies by several orders of magnitude.

astro-ph.GA↗

arXiv2015

The first galaxies: simulating their feedback-regulated assembly

We investigate the formation of a galaxy reaching a virial mass of $~ 10^8$ solar mass at $z=10$ by carrying out a zoomed radiation-hydrodynamical cosmological simulation. This simulation traces Population~III (Pop~III) star formation, characterized by a modestly top-heavy initial mass function (IMF), and considers stellar feedback such as photoionization heating from Pop III and Population~II (Pop~II) stars, mechanical and chemical feedback from supernovae (SNe), and X-ray feedback from accreting black holes (BHs) and high-mass X-ray binaries (HMXBs). We self-consistently impose a transition in star formation mode from top-heavy Pop III to low-mass Pop~II, and find that the star formation rate in the computational box is dominated by Pop~III until $z=13$, and by Pop~II thereafter. The simulated galaxy experiences bursty star formation, with a substantially reduced gas content due to photoionization heating from Pop~III and Pop~II stars, together with SN feedback. All the gas within the simulated galaxy is metal-enriched above $10^{-5}$ solar, such that there are no remaining pockets of primordial gas. The simulated galaxy has an estimated observed flux of $~10^{-3} nJy$, which is too low to be detected by the James Webb Space Telescope (JWST) without strong lensing amplification. We also show that our simulated galaxy is similar in terms of stellar mass to Segue 2, the least luminous dwarf known in the Local Group.

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arXiv2015

Abundance anomalies in metal-poor stars from Population III supernova ejecta hydrodynamics

We present a simulation of the long-term evolution of a Population III supernova remnant in a cosmological minihalo. Employing passive Lagrangian tracer particles, we investigate how chemical stratification and anisotropy in the explosion can affect the abundances of the first low-mass, metal-enriched stars. We find that reverse shock heating can leave the inner mass shells at entropies too high to cool, leading to carbon-enhancement in the re-collapsing gas. This hydrodynamic selection effect could explain the observed incidence of carbon-enhanced metal-poor (CEMP) stars at low metallicity. We further explore how anisotropic ejecta distributions, recently seen in direct numerical simulations of core-collapse explosions, may translate to abundances in metal-poor stars. We find that some of the observed scatter in the Population II abundance ratios can be explained by an incomplete mixing of supernova ejecta, even in the case of only one contributing enrichment event. We demonstrate that the customary hypothesis of fully-mixed ejecta clearly fails if post-explosion hydrodynamics prefers the recycling of some nucleosynthetic products over others. Furthermore, to fully exploit the stellar-archaeological program of constraining the Pop III initial mass function from the observed Pop II abundances, considering these hydrodynamical transport effects is crucial. We discuss applications to the rich chemical structure of ultra-faint dwarf satellite galaxies, to be probed in unprecedented detail with upcoming spectroscopic surveys.

astro-ph.GA↗

arXiv2015

Constraining the primordial initial mass function with stellar archaeology

We present a new near-field cosmological probe of the initial mass function (IMF) of the first stars. Specifically, we constrain the lower-mass limit of the Population III (Pop III) IMF with the total number of stars in large, unbiased surveys of the Milky Way. We model the early star formation history in a Milky Way-like halo with a semi-analytic approach, based on Monte-Carlo sampling of dark matter merger trees, combined with a treatment of the most important feedback mechanisms. Assuming a logarithmically flat Pop III IMF and varying its low mass limit, we derive the number of expected survivors of these first stars, using them to estimate the probability to detect any such Pop III fossil in stellar archaeological surveys. Following our analysis, the most promising region to find possible Pop III survivors is the stellar halo of the Milky Way, which is the best target for future surveys. We find that if no genuine Pop III survivor is detected in a sample size of $4 \times 10^6$ ($2 \times 10^7$) halo stars with well-controlled selection effects, then we can exclude the hypothesis that the primordial IMF extended down below $0.8 M_\odot$ at a confidence level of 68% (99%). With the sample size of the Hamburg/ESO survey, we can tentatively exclude Pop III stars with masses below $0.65 M_\odot$ with a confidence level of 95%, although this is subject to significant uncertainties. To fully harness the potential of our approach, future large surveys are needed that employ uniform, unbiased selection strategies for high-resolution spectroscopic follow-up.

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