arXiv subjects

Jarrett L. Johnson

Publications and source records attributed to Jarrett L. Johnson.

At least 55 records · Page 3Linked to original sources

arXiv2010

Observational Signatures of the First Galaxies

Detection of the radiation emitted from some of the earliest galaxies will be made possible in the next decade, with the launch of the James Webb Space Telescope (JWST). A significant fraction of these galaxies may host Population (Pop) III star clusters. The detection of the recombination radiation emitted by such clusters would provide an important new constraint on the initial mass function (IMF) of primordial stars. Here I review the expected recombination line signature of Pop III stars, and present the results of cosmological radiation hydrodynamics simulations of the initial stages of Pop III starbursts in a first galaxy at z ~ 12, from which the time-dependent luminosities and equivalent widths of IMF-sensitive recombination lines are calculated. While it may be unfeasible to detect the emission from Pop III star clusters in the first galaxies at z > 10, even with next generation telescopes, Pop III star clusters which form at lower redshifts (i.e. at z < 6) may be detectable in deep surveys by the JWST.

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arXiv2010

Supernovae-induced accretion and star formation in the inner kiloparsec of a gaseous disk

We consider the effects of supernovae (SNe) on accretion and star formation in a massive gaseous disk in a large primeval galaxy. The gaseous disk we envisage, roughly 1 kiloparsec (kpc) in size with >~ 10^8 M_Sun of gas, could have formed as a result of galaxy mergers where tidal interactions removed angular momentum from gas at larger radius and thereby concentrated it within the central ~ 1 kpc region. We find that SNe lead to accretion in the disk at a rate of roughly 0.1 - 1 M_Sun per year and induce star formation at a rate of ~ 10 - 100 M_Sun per year which contributes to the formation of a bulge; a part of the stellar velocity dispersion is due to the speed of SNa shells from which stars are formed and a part due to the repeated action of the stochastic gravitational field of the network of SNa remnants on stars. The rate of SNe in the inner kpc is shown to be self- regulating, and it cycles through phases of low and high activity. The supernova-assisted accretion transports gas from about one kpc to within a few pc of the center. If this accretion were to continue down to the central black hole (BH) then the resulting ratio of BH mass to the stellar mass in the bulge would be of order ~ 10^-2 - 10^-3, in line with the observed Magorrian relation.

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arXiv2010

Population III Star Clusters in the Reionized Universe

In reionized regions of the Universe, gas can only collapse to form stars in dark matter (DM) haloes which grow to be sufficiently massive. If star formation is prevented in the minihalo progenitors of such DM haloes at redshifts z >~ 20, then these haloes will not be self-enriched with metals and so may host Population (Pop) III star formation. We estimate an upper limit for the abundance of Pop III star clusters which thus form in the reionized Universe, as a function of redshift. Depending on the minimum DM halo mass for star formation, between of the order of one and of the order of a thousand Pop III star clusters per square degree may be observable at 2 <~ z <~ 7. Thus, there may be a sufficient number density of Pop III star clusters for detection in surveys such as the Deep-Wide Survey (DWS) to be conducted by the James Webb Space Telescope (JWST). We predict that Pop III clusters formed after reionization are most likely to be found at z >~ 3 and within ~ 40 arcsec (~ 1 Mpc comoving) of DM haloes with masses of ~ 10^11 M_Sun, the descendants of the haloes at z ~ 20 which host the first galaxies that begin reionization. If the stellar initial mass function (IMF) is top-heavy the clusters may have sufficiently high luminosities in both Ly_alpha and He II lambda1640 to be detected and for constraints to be placed on the Pop III IMF. While a small fraction of DM haloes with masses as high as ~10^9 M_Sun at redshifts z <~ 4 are not enriched due to star formation in their progenitors, external metal enrichment due to galactic winds is likely to preclude Pop III star formation in a large fraction of otherwise unenriched haloes, perhaps even preventing star formation in pristine haloes altogether after reionization is complete.

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arXiv2009

Towards the First Galaxies

The formation of the first galaxies at redshifts z~10-15 signaled the transition from the simple initial state of the universe to one of ever increasing complexity. We here review recent progress in understanding their assembly process with numerical simulations, starting with cosmological initial conditions and modelling the detailed physics of star formation. In particular, we study the role of HD cooling in ionized primordial gas, the impact of UV radiation produced by the first stars, and the propagation of the supernova blast waves triggered at the end of their brief lives. We conclude by discussing promising observational diagnostics that will allow us to probe the properties of the first galaxies, such as their contribution to reionization and the chemical abundance pattern observed in extremely low-metallicity stars.

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arXiv2009

The formation of the first galaxies and the transition to low-mass star formation

The formation of the first galaxies at redshifts z ~ 10-15 signaled the transition from the simple initial state of the universe to one of ever increasing complexity. We here review recent progress in understanding their assembly process with numerical simulations, starting with cosmological initial conditions and modelling the detailed physics of star formation. In this context we emphasize the importance and influence of selecting appropriate initial conditions for the star formation process. We revisit the notion of a critical metallicity resulting in the transition from primordial to present-day initial mass functions and highlight its dependence on additional cooling mechanisms and the exact initial conditions. We also review recent work on the ability of dust cooling to provide the transition to present-day low-mass star formation. In particular, we highlight the extreme conditions under which this transition mechanism occurs, with violent fragmentation in dense gas resulting in tightly packed clusters.

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arXiv2009

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_sun 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 HII region, and in this process sweeps up a total gas mass of 2.5*10^5 M_sun. 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 M_vir > 10^8 M_sun must be assembled to recollect all components of the swept-up gas.

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arXiv2009

The First Galaxies: Assembly, Cooling and the Onset of Turbulence

We investigate the properties of the first galaxies at z > 10 with highly resolved numerical simulations, starting from cosmological initial conditions and taking into account all relevant primordial chemistry and cooling. A first galaxy is characterized by the onset of atomic hydrogen cooling, once the virial temperature exceeds 10^4 K, and its ability to retain photoheated gas. We follow the complex accretion and star formation history of a 5*10^7 M_sun system by means of a detailed merger tree and derive an upper limit on the number of Population III (Pop III) stars formed prior to its assembly. We investigate the thermal and chemical evolution of infalling gas and find that partial ionization at temperatures > 10^4 K catalyses the formation of H2 and hydrogen deuteride, allowing the gas to cool to the temperature of the cosmic microwave background. Depending on the strength of radiative and chemical feedback, primordial star formation might be dominated by intermediate-mass Pop III stars formed during the assembly of the first galaxies. Accretion on to the nascent galaxy begins with hot accretion, where gas is accreted directly from the intergalactic medium and shock-heated to the virial temperature, but is quickly accompanied by a phase of cold accretion, where the gas cools in filaments before flowing into the parent halo with high velocities. The latter drives supersonic turbulence at the centre of the galaxy and could lead to very efficient chemical mixing. The onset of turbulence in the first galaxies thus likely marks the transition to Pop II star formation.

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arXiv2009

The observational signature of the first H II regions

We use three-dimensional smoothed particle hydrodynamics simulations together with a dynamical ray-tracing scheme to investigate the build-up of the first H ii regions around massive Population III stars in minihaloes. We trace the highly anisotropic breakout of the ionising radiation into the intergalactic medium, allowing us to predict the resulting recombination radiation with greatly increased realism. Our simulations, together with Press-Schechter type arguments, allow us to predict the Population III contribution to the radio background at 100 MHz via bremsstrahlung and 21 cm emission. We find a global bremsstrahlung signal of around 1 mK, and a combined 21 cm signature which is an order of magnitude larger. Both might be within reach of the planned Square Kilometer Array experiment, although detection of the free-free emission is only marginal. The imprint of the first stars on the cosmic radio background might provide us with one of the few diagnostics to test the otherwise elusive minihalo star formation site.

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arXiv2009

The First Galaxies: Signatures of the Initial Starburst

Detection of the radiation emitted from the first galaxies at z > 10 will be made possible in the next decade, with the launch of the James Webb Space Telescope (JWST). We carry out cosmological radiation hydrodynamics simulations of Population III (Pop III) starbursts in a 10^8 M_Sun dwarf galaxy at z = 12.5. For different star formation efficiencies and stellar initial mass functions (IMFs), we calculate the luminosities and equivalent widths (EWs) of the recombination lines H_alpha, Ly_alpha, and He II 1640, under the simple assumption that the stellar population does not evolve over the first ~3 Myr of the starburst. Although only < 40 percent of the gas in the central 100 pc of the galaxy is photoionized, we find that photoheating by massive stars causes a strong dynamical response, which results in a weak correlation between luminosity emitted in hydrogen recombination lines and the total mass in stars. However, owing to the low escape fraction of He II-ionizing photons, the luminosity emitted in He II 1640 is much more strongly correlated with the total stellar mass. The ratio of the luminosity in He II 1640 to that in Ly_alpha or H_alpha is found to be a good indicator of the IMF in many cases. The ratio of observable fluxes is F_1640/F_Halpha ~ 1 for clusters of 100 M_Sun Pop III stars and F_1640/F_Halpha ~ 0.1 for clusters of 25 M_Sun Pop III stars. The EW of the He II 1640 emission line is the most reliable IMF indicator, its value varying between ~ 20 and ~ 200 angstrom for a massive and very massive Pop III IMF, respectively. Even the bright, initial stages of Pop III starbursts in the first dwarf galaxies will likely not be directly detectable by the JWST. Instead, the JWST may discover only more massive, and hence more chemically evolved, galaxies which host normal, Pop I/II, star formation.

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arXiv2008

The minimum stellar metallicity observable in the Galaxy

The first stars fundamentally transformed the early Universe through their production of energetic radiation and the first heavy chemical elements. The impact on cosmic evolution sensitively depends on their initial mass function (IMF), which can be empirically constrained through detailed studies of ancient, metal-poor halo stars in our Galaxy. We compare the lowest magnesium and iron abundances measured in Galactic halo stars with theoretical predictions for the minimum stellar enrichment provided by Population III stars under the assumption of a top-heavy IMF. To demonstrate that abundances measured in metal-poor stars reflect the chemical conditions at their formation, and that they can thus be used to derive constraints on the primordial IMF, we carry out a detailed kinematic analysis of a large sample of metal-poor stars drawn from the SDSS survey. We assess whether interstellar accretion has altered their surface abundances. We find that accretion is generally negligible, even at the extremely low levels where the primordial IMF can be tested. We conclude that the majority of the first stars were very massive, but had likely masses below ~140 M.

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arXiv2008

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 disk 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 100 seconds, at a level comparable to the typical luminosity observed in gamma-ray bursts (GRBs). The luminosity then decreases rapidly with time for about 10^3 seconds, 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. A X-ray 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 X-ray light curve is observed to drop suddenly at the end of the plateau phase, while in others the decline is ~ 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 accretion model thus provides a coherent explanation for the diverse and puzzling features observed in the early X-ray light curves of GRBs. (Abridged)

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arXiv2008

Properties of Gamma-Ray Burst Progenitor Stars

We determine some basic properties of stars that produce spectacular gamma-ray bursts at the end of their life. We assume that accretion of the outer portion of the stellar core by a central black hole fuels the prompt emission, and that fall-back and accretion of the stellar envelope later produces the plateau in the X-ray light curve seen in some bursts. Using X-ray data for three bursts we estimate the radius of the stellar core to be ~ 1-3 x 10^10 cm, and that of the stellar envelope to be ~ 1-2 x 10^11 cm. The density profile in the envelope is fairly shallow, with ρ~ r^-2. The rotation speeds of the core and envelope are ~ 0.05 and ~ 0.2 of the local Keplerian speed, respectively.

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arXiv2008

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 Lyman-Werner (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 T > 10^4 K and total mass > 10^8 M_Sun at z > 10, and find that complete suppression of previous Pop III star formation is unlikely, with stars of > 100 M_Sun (Pop III.1) and > 10 M_Sun (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.

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arXiv2008

The First Stars

The formation of the first generations of stars at redshifts z > 15-20 signaled the transition from the simple initial state of the universe to one of increasing complexity. We here review recent progress in understanding the assembly process of the first galaxies, starting with cosmological initial conditions and modelling the detailed physics of star formation. In particular, we study the role of HD cooling in ionized primordial gas, the impact of UV radiation produced by the first stars, and the propagation of the supernova blast waves triggered at the end of their brief lives. We conclude by discussing how the chemical abundance patterns observed in extremely low-metallicity stars allow us to probe the properties of the first stars.

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arXiv2008

Uncovering the Chemical Signature of the First Stars in the Universe

The chemical abundance patterns observed in metal-poor Galactic halo stars contain the signature of the first supernovae, and thus allows us to probe the first stars that formed in the universe. We construct a theoretical model for the early chemical enrichment history of the Milky Way, aiming in particular at the contribution from pair-instability supernovae (PISNe). These are a natural consequence of current theoretical models for primordial star formation at the highest masses. However, no metal-poor star displaying the distinct PISN signature has yet been observed. We here argue that this apparent absence of any PISN signature is due to an observational selection effect. Whereas most surveys traditionally focus on the most metal-poor stars, we predict that early PISN enrichment tends to `overshoot', reaching enrichment levels of [Ca/H] ~= -2.5 that would be missed by current searches. We utilize existing observational data to place constraints on the primordial initial mass function (IMF). The number fraction of PISNe in the primordial stellar population is estimated to be <0.07, or ~< 40% by mass, assuming that metal-free stars have masses in excess of 10 M_sun. We further predict, based on theoretical estimates for the relative number of PISNe, that the expected fraction of second generation stars below [Ca/H]=-2 with a dominant (i.e., >90%) contribution from PISNe is merely ~1e-4 - 5e-4. The corresponding fraction of stars formed from gas exclusively enriched by PISNe is a factor of ~4 smaller. With the advent of next generation telescopes and new, deeper surveys, we should be able to test these predictions.

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arXiv2007

Visualization of Cosmological Particle-Based Datasets

We describe our visualization process for a particle-based simulation of the formation of the first stars and their impact on cosmic history. The dataset consists of several hundred time-steps of point simulation data, with each time-step containing approximately two million point particles. For each time-step, we interpolate the point data onto a regular grid using a method taken from the radiance estimate of photon mapping. We import the resulting regular grid representation into ParaView, with which we extract isosurfaces across multiple variables. Our images provide insights into the evolution of the early universe, tracing the cosmic transition from an initially homogeneous state to one of increasing complexity. Specifically, our visualizations capture the build-up of regions of ionized gas around the first stars, their evolution, and their complex interactions with the surrounding matter. These observations will guide the upcoming James Webb Space Telescope, the key astronomy mission of the next decade.

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arXiv2007

Probing the Formation of the First Low-Mass Stars with Stellar Archaeology

We investigate the conditions under which the first low-mass stars formed in the universe by confronting theoretical predictions governing the transition from massive Population III to low-mass Population II stars with recent observational C and/or O abundance data of metal-poor Galactic stars. Based on an existing theory, we introduce a new "observer-friendly" function, the transition discriminant D_trans, which provides empirical constraints as well as a powerful comparison between the currently available data of metal-poor halo stars and theoretical predictions of the formation of the first low-mass stars (<1 M_sun). Specifically, we compare the empirical stellar results with the theory that fine-structure lines of C and O dominate the transition from Pop III to Pop II in the early universe. We find the observational data for halo objects as well as for dSph galaxies and globular clusters to be consistent with this theory. An explanation for the observed lack of metal-poor stars in dSph galaxies and globular clusters is also suggested. Finally, we predict that any star to be found with [Fe/H]<-4 should have enhanced C and/or O abundances. The high C and O abundances of the two most iron-poor stars are in line with our prediction.

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arXiv2007

Local radiative feedback in the formation of the first protogalaxies

The formation of the first galaxies is influenced by the radiative feedback from the first generations of stars. This feedback is manisfested by the heating and ionization of the gas which lies within the H II regions surrounding the first stars, as well as by the photodissociation of 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 course of 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 fraction renders even the local IGM optically thick to LW photons over physical distances of the order of a few kiloparsecs. We find that efficient accretion onto Population III relic black holes may occur after ~ 60 Myr from the time of their formation, by which time the photo-heated relic H II region gas can cool and re-collapse into the 10^6 M_solar minihalo which hosts the black hole. Also, Pop II.5 stars, postulated to have masses of the order of 10 M_solar, can likely form from this re-collapsing relic H II region gas. Overall, we find that the local radiative feedback from the first generations of stars suppresses the star formation rate by only a factor of, at most, a few.

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