Jarrett L. Johnson

Probing the formation of the first low-mass stars with stellar archaeology

ABSTRACT We investigate the conditions under which the first low-mass stars formed in theUniverse by confronting theoretical predictions governing the transition from massivePopulationIII to low-mass PopulationII stars with recent observational C and/or Oabundance data of metal-poor Galactic stars. We introduce a new “observer-friendly”function, the transition discriminant D trans , which provides empirical constraints aswell as a powerful comparison between the currently available data of metal-poor halostars and theoretical predictions of the formation of the first low-mass stars (. 1M ⊙ ).Specifically, we comparethe empirical stellarresults with the theorythat fine-structurelines of C and O dominate the transition from PopIII to PopII in the early Universe.We find the currently-available data for halo stars as well as for dSph galaxies andglobularclusters to be consistent with this theory. An explanationfor the observedlackof metal-poor stars in dSph galaxies and globular clusters is also suggested. Finally, wepredict that any star to be found with [Fe/H] . −4 should have enhanced C and/orO abundances. The high C and O abundances of the two most iron-poor stars are inline with our prediction.Key words: cosmology: early Universe — stars: abundances — stars: Population II— Galaxy: halo — Galaxy: stellar content — techniques: spectroscopic

Ubiquitous seeding of supermassive black holes by direct collapse

We study for the first time the environment of massive black hole (BH) seeds (∼10 4−5 M� ) formed via the direct collapse of pristine gas clouds in massive haloes (≥10 7 M� )a tz > 6. Our model is based on the evolution of dark matter haloes within a cosmological N-body simulation, combined with prescriptions for the formation of BH along with both Population III (Pop III) and Population II (Pop II) stars. We calculate the spatially varying intensity of Lyman–Werner (LW) radiation from stars and identify the massive pristine haloes in which it is high enough to shut down molecular hydrogen cooling. In contrast to previous BH seeding models with a spatially constant LW background, we find that the intensity of LW radiation due to local sources, Jlocal, can be up to ∼10 6 times the spatially averaged background in the simulated volume and exceeds the critical value, Jcrit, for the complete suppression of molecular cooling, in some cases by four orders of magnitude. Even after accounting for possible metal pollution in a halo from previous episodes of star formation, we find a steady rise in the formation rate of direct collapse BHs (DCBHs) with decreasing redshift from 10 −3 Mpc −3 z −1 at z = 12 to 10 −2 Mpc −3 z −1 at z = 6. The onset of Pop II star formation at z ≈ 16 simultaneously marks the onset of the epoch of DCBH formation, as the increased level of LW radiation from Pop II stars is able to elevate the local levels of the LW intensity to Jlocal > Jcrit, while Pop III stars fail to do so at any time. The number density of DCBHs is sensitive to the number of LW photons and can vary by over an order of magnitude at z = 7 after accounting for reionization feedback. Haloes hosting DCBHs are more clustered than similar massive counterparts that do not host DCBHs, especially at redshifts z 10. Also, the DCBHs that form at z > 10 are found to reside in highly clustered regions, whereas the DCBHs formed around z ∼ 6a re more common. We also show that planned surveys with James Webb Space Telescope should be able to detect the supermassive stellar precursors of DCBHs.

Mass Fall-back and Accretion in the Central Engine of Gamma-Ray Bursts

We calculate the rate of in-fall of stellar matter on an accretion disc during the collapse of a rapidly rotating massive star and estimate the luminosity of the relativistic jet that results from accretion on to the central black hole. We find that the jet luminosity remains high for about 102 s, at a level comparable to the typical luminosity observed in gamma-ray bursts (GRBs). The luminosity then decreases rapidly with time for about ∼103 s, roughly as ∼t−3; the duration depends on the size and rotation speed of the stellar core. The rapid decrease of the jet power explains the steeply declining X-ray flux observed at the end of most long-duration GRBs. Observations with the Swift satellite show that, following the steep decline, many GRBs exhibit a plateau in the X-ray light curve (XLC) that lasts for about 104 s. We suggest that this puzzling feature is due to continued accretion in the central engine. A plateau in the jet luminosity can arise when the viscosity parameter α is small, ∼10−2 or less. A plateau is also produced by continued fall-back of matter – either from an extended stellar envelope or from material that failed to escape with the supernova ejecta. In a few GRBs, the XLC is observed to drop suddenly at the end of the plateau phase, while in others the XLC declines more slowly as ∼t−1−t−2. These features arise naturally in the accretion model depending on the radius and mean specific angular momentum of the stellar envelope. The total energy in the disc-wind accompanying accretion is found to be about 1052 erg. This is comparable to the energy observed in supernovae associated with GRBs, suggesting that the wind might be the primary agent responsible for the explosion. The accretion model thus provides a coherent explanation for the diverse and puzzling features observed in the early XLC of GRBs. It might be possible to use this model to invert gamma-ray and X-ray observations of GRBs and thereby infer basic properties of the core and envelope of the GRB progenitor star.

The cooling of shock-compressed primordial gas

We find that at redshifts z? 10, HD line cooling allows strongly shocked primordial gas to cool to the temperature of the cosmic microwave background (CMB). This temperature is the minimum value attainable via radiative cooling. Provided that the abundance of HD, normalized to the total number density, exceeds a critical level of ∼10 -8 , the CMB temperature floor is reached in a time which is short in comparison to the Hubble time. We estimate the characteristic masses of stars formed out of shocked primordial gas in the wake of the first supernovae, and resulting from the virialization of dark matter haloes during hierarchical structure formation to be ∼ 10 M ⊙ . In addition, we show that cooling by HD enables the primordial gas in relic H II regions to cool to temperatures considerably lower than those reached via H 2 cooling alone. We confirm that HD cooling is unimportant in cases where the primordial gas does not go through an ionized phase, as in the formation process of the very first stars in z? 20 minihaloes of mass ∼10 6 M ⊙ .

THE FIRST SUPERNOVA EXPLOSIONS : ENERGETICS, FEEDBACK, AND CHEMICAL ENRICHMENT

We perform three-dimensional smoothed particle hydrodynamics simulations in a realistic cosmological setting to investigate the expansion, feedback, and chemical enrichment properties of a 200 M⊙ pair-instability supernova in the high-redshift universe. We find that the SN remnant propagates for a Hubble time at z ≃ 20 to a final mass-weighted mean shock radius of 2.5 kpc (proper), roughly half the size of the H ii region, and in this process sweeps up a total gas mass of 2.5 × 10 5 M⊙. The morphology of the shock becomes highly anisotropic once it leaves the host halo and encounters filaments and neighboring minihalos, while the bulk of the shock propagates into the voids of the intergalactic medium. The SN entirely disrupts the host halo and terminates further star formation for at least 200 Myr, while in our specific case it exerts positive mechanical feedback on neighboring minihalos by shock-compressing their cores. In contrast, we do not observe secondary star formation in the dense shell via gravitational fragmentation, due to the previous photoheating by the progenitor star. We find that cooling by metal lines is unimportant for the entire evolution of the SN remnant, while the metal-enriched, interior bubble expands adiabatically into the cavities created by the shock, and ultimately into the voids with a maximum extent similar to the final mass-weighted mean shock radius. Finally, we conclude that dark matter halos of at least Mvir & 10 8 M⊙ must be assembled to recollect all components of the swept-up gas. Subject headings: cosmology: theory — galaxies: formation — galaxies: high-redshift — H ii regions — hydrodynamics — intergalactic medium — supernovae: general

The aftermath of the first stars : massive black holes

We investigate the evolution of the primordial gas surrounding the first massive black holes formed by the collapse of Population III stars at redshifts z≥ 20. Carrying out three-dimensional hydrodynamical simulations using GADGET, we study the dynamical, thermal and chemical evolution of the first relic H II regions. We also carry out simulations of the mergers of relic H II regions with neighbouring neutral minihaloes, which contain high-density primordial gas that could accrete on to a Population III remnant black hole. We find that there may have been a significant time delay, of the order of ∼10 8 yr, between black hole formation and the onset of efficient accretion. The build-up of supermassive black holes, believed to power the z ≥ 6 quasars observed in the Sloan Digital Sky Survey, therefore faces a crucial early bottleneck. More massive seed black holes may thus be required, such as those formed by the direct collapse of a primordial gas cloud facilitated by atomic line cooling. The high optical depth to Lyman-Werner (LW) photons that results from the high fraction of H 2 molecules that form in relic H II regions, combined with the continued formation of H 2 inside the dynamically expanding relic H II region, leads to shielding of the molecules inside these regions at least until a critical background LW flux of ∼10 -24 erg s -1 cm -2 Hz -1 sr -1 is established. Furthermore, we find that a high fraction of deuterium hydride (HD) molecules, X HD ≥ 10 -7 , is formed, potentially enabling the formation of Population 11.5 stars, with masses of the order of ∼ 10 M ⊙ , during later stages of structure formation when the relic H region gas is assembled into a sufficiently deep potential well to gravitationally confine the gas again.

Local Radiative Feedback in the Formation of the First Protogalaxies

The first galaxies form under the influence of radiative feedback from the first generations of stars. This feedback acts to heat and ionize the gas within the H II regions surrounding the first stars, as well as to photodissociate hydrogen molecules within the larger Lyman-Werner (LW) bubbles that surround these sources. Using a ray-tracing method in three-dimensional cosmological simulations, we self-consistently track the formation of, and radiative feedback from, individual stars in the formation of a protogalaxy. We compute in detail the H II regions of each of these sources, as well as the regions affected by their molecule-dissociating radiation. We follow the thermal, chemical, and dynamical evolution of the primordial gas as it becomes incorporated into the protogalaxy. While the IGM is, in general, optically thick to LW photons only over physical distances of 30 kpc at redshifts z 20, the high molecule fraction that is built up in relic H II regions and their increasing volume-filling factor renders even the local IGM optically thick to LW photons over physical distances of a few kiloparsecs. We find that Population III relic black holes may begin accreting efficiently after ~60 Myr from the time of their formation, when the photo-heated relic H II region gas can cool and recollapse into the 106 M☉ minihalo which hosts the black hole. Population II.5 stars, postulated to have masses of the order of 10 M☉, can also likely form from this recollapsing relic H II region gas. Overall, we find that the local radiative feedback from Population III stars suppresses the star formation rate only slightly.

The First Billion Years project: the impact of stellar radiation on the co-evolution of Populations II and III

With the first metal enrichment by Population (Pop) III supernovae (SNe), the formation of the first metal-enriched, Pop II stars becomes possible. In turn, Pop III star formation and early metal enrichment are slowed by the high energy radiation emitted by Pop II stars. Thus, through the SNe and radiation they produce, Populations II and III coevolve in the early Universe, one regulated by the other. We present large (4 Mpc) 3 , high resolution cosmological simulations in which we self-consistently model early metal enrichment and the stellar radiation responsible for the destruction of the coolants (H2 and HD) required for Pop III star formation. We find that the moleculedissociating stellar radiation produced both locally and over cosmological distances reduces the Pop III star formation rate (SFR) at z > 10 by up to an order of magnitude, to a rate per comoving volume of < 10 −4 M⊙ yr −1 Mpc −3 , compared to the case in which this radiation is not included. However, we find that the effect of LW feedback is to enhance the amount of Pop II star formation. We attribute this to the reduced rate at which gas is blown out of dark matter haloes by SNe in the simulation with LW feedback, which results in larger reservoirs for metal-enriched star formation. Even accounting for metal enrichment, molecule-dissociating radiation and the strong suppression of low-mass galaxy formation due to reionization at z < 10, we find that Pop III stars are still formed at a rate of » 10 −5 M⊙ yr −1 Mpc −3 down to z » 6. This suggests that the majority of primordial pair-instability SNe that may be uncovered in future surveys will be found at z < 10. We also find that the molecule-dissociating radiation emitted from Pop II stars may destroy H2 molecules at a high enough rate to suppress gas cooling and allow for the formation of supermassive primordial stars which collapse to form » 10 5 M⊙ black holes.

Occurrence of metal-free galaxies in the early Universe

The character of the first galaxies at redshifts z > 10 strongly depends on their level of pre-enrichment, which is in turn determined by the rate of primordial star formation prior to their assembly. In order for the first galaxies to remain metal-free, star formation in minihaloes must be highly suppressed, most likely by H2-dissociating LymanWerner (LW) radiation. We show that the build-up of such a strong LW background is hindered by two effects. Firstly, the level of the LW background is self-regulated, being produced by the Population III (Pop III) star formation which it, in turn, suppresses. Secondly, the high opacity to LW photons which is built up in the relic H II regions left by the first stars acts to diminish the global LW background. Accounting for a self-regulated LW background, we estimate a lower limit for the rate of Pop III star formation in minihaloes at z > 15. Further, we simulate the formation of a ’first galaxy’ with virial temperature Tvir > 10 4 K and total mass > 10 8 M⊙ at z > 10, and find that complete suppression of previous Pop III star formation is unlikely, with stars of > 100 M⊙ (Pop III.1) and > 10 M⊙ (Pop III.2) likely forming. Finally, we discuss the implications of these results for the nature of the first galaxies, which may be observed by future missions such as the James Webb Space Telescope.

The interplay between chemical and mechanical feedback from the first generation of stars

We study cosmological simulations of early structure formation, including non-equilibrium molecular chemistry, metal pollution from stellar evoluti on, transition from population III (popIII) to population II (popII) star formation, regulate d by a given critical metallicity, and feedback effects. We perform analyses of the properties of t he gas, and use the popIII and popII populations as tracers of the metallicity. This allow s us to investigate the properties of early metal spreading from the different stellar populatio ns and its interplay with pristine molecular gas, in terms of the initial mass function and crit ical metallicity. We find that, independently of the details about popIII modeling, after t he onset of star formation, regions enriched below the critical level are mostly found in isolat ed environments, while popII star formation regions are much more clumped. Typical star forming haloes, at z � 15 10, with masses between � 10 7 10 8 M⊙, show average SN driven outflow rates of up to � 10 −4 M⊙/yr in enriched gas, initially leaving the original star format ion regions almost devoid of metals. The polluted material, which is gravitati onally incorporated in over-dense environments on timescales of � 10 7 yr, is mostly coming from external, nearby star forming sites (“gravitational enrichment”). In parallel, the pris tine-gas inflow rates are some orders of magnitudes larger, between � 10 −3 10 −1 M⊙/yr. However, thermal feedback from SN destroys molecules within the pristine gas hindering its ab ility to cool and to condense into high-density star forming regions. Only the polluted mater ial incorporated via gravitational enrichment can continue to cool by atomic fine-structure tra nsitions on short time scales, short enough to end the initial popIII regime within less tha n 10 8 yr. Moreover, the interplay between the pristine, cold, infalling gas and the ejected, h ot, metal-rich gas leads to turbulent Reynolds numbers of the order of � 10 8 10 10 , and contributes to the suppression of pristine inflow rates into the densest, star forming regions.