arXiv subjects

Volker Bromm

Publications and source records attributed to Volker Bromm.

At least 217 records · Page 12Linked to original sources

arXiv2007

GRB Cosmology

Current observations are about to open up a direct window into the final frontier of cosmology: the first billion years in cosmic history when the first stars and galaxies formed. Even before the launch of the James Webb Space Telescope, it might be possible to utilize Gamma-ray Bursts (GRBs) as unique probes of cosmic star formation and the state of the intergalactic medium (IGM) up to redshifts of several tens, when the first (Population III) stars had formed. The Swift mission, or future satellites such as EXIST, might be the first observatories to detect individual Population III stars, provided that massive metal-free stars were able to trigger GRBs. Spectroscopic follow-up observations of the GRB afterglow emission would allow to probe the ionization state and metal enrichment of the IGM as a function of redshift.

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arXiv2007

Impact of Cosmic Rays on Population III Star Formation

We explore the implications of a possible cosmic ray (CR) background generated during the first supernova explosions that end the brief lives of massive Population III stars. We show that such a CR background could have significantly influenced the cooling and collapse of primordial gas clouds in minihaloes around redshifts of z ~ 15 - 20, provided the CR flux was sufficient to yield an ionization rate greater than about 10^-19 s^-1 near the center of the minihalo. The presence of CRs with energies less than approximately 10^7 eV would indirectly enhance the molecular cooling in these regions, and we estimate that the resulting lower temperatures in these minihaloes would yield a characteristic stellar mass as low as ~ 10 M_sun. CRs have a less pronounced effect on the cooling and collapse of primordial gas clouds inside more massive dark matter haloes with virial masses greater than approximately 10^8 M_sun at the later stages of cosmological structure formation around z ~ 10 - 15. In these clouds, even without CR flux the molecular abundance is already sufficient to allow cooling to the floor set by the temperature of the cosmic microwave background.

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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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arXiv2006

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 neighboring neutral minihaloes, which contain high density primordial gas that can accrete onto a Pop III remnant black hole. We find that there may have been a significant time delay, of order ~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} ergs s^{-1} cm^{-2} Hz^{-1} sr^{-1}, is established. Furthermore, we find that a high fraction of HD molecules, X_{HD} > 10^{-7}, is formed, potentially enabling the formation of Pop II.5 stars during later stages of structure formation when the relic H II region gas is assembled into a sufficiently deep potential well to gravitationally confine the gas again.

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arXiv2006

GRB Cosmology and the First Stars

Gamma-Ray Bursts (GRBs) are unique probes of the cosmic star formation history and the state of the intergalactic medium up to the redshifts of the first stars. In particular, the ongoing {\it Swift} mission might be the first observatory to detect individual Population~III stars, provided that the massive, metal-free stars were able to trigger GRBs. {\it Swift} will empirically constrain the redshift at which Population~III star formation was terminated, thus providing crucial input to models of cosmic reionization and metal enrichment.

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arXiv2006

High-Redshift Gamma-Ray Bursts from Population III Progenitors

Detection of gamma-ray bursts (GRBs) from redshifts z > 7 would open a new window into the earliest epoch of cosmic star formation. We construct separate star formation histories at high redshifts for normal (Pop I and II) stars, and for predominantly massive (Pop III) stars. Based on these separate histories, we predict the GRB redshift distribution to be observed by the {\it Swift} mission. Regardless of whether Pop III progenitors are able to trigger GRBs, we find that a fraction 10% of all bursts detected by {\it Swift} will originate at z > 5. This baseline contribution is due to Pop I/II star formation which must have extended out to high redshifts in rare massive galaxies that were enriched by heavy elements earlier than the typical galaxies. In addition, we consider the possible contribution of Pop III progenitors to the observable GRB rate. Pop III stars are viable progenitors for long-duration GRBs which are triggered by the collapsar mechanism, as long as they can lose their outer envelope through mass transfer to a companion star in a close binary. We find that the likelihood of Pop III binaries to satisfy the conditions required by the collapsar mechanism could be enhanced significantly relative to Pop I/II binaries. If Pop III binaries are common, {\it Swift} will be first observatory to probe Pop III star formation at redshifts z > 7.

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arXiv2005

The H II Region of the First Star

Simulations predict that the first stars in an LCDM universe formed at redshifts z>20 in minihalos with masses of about 10^6 M_sun. We have studied their radiative feedback by simulating the propagation of ionization fronts (I-fronts) created by these first Population III stars (M_* = 15-500 M_sun) at z=20, within the density field of a cosmological simulation of primordial star formation, outward thru the host minihalo and into the surrounding gas. A three-dimensional ray-tracing calculation tracks the I-front once the H II region evolves a "champagne flow'' inside the minihalo, after the early D-type I-front detaches from the shock and runs ahead, becoming R-type. We take account of the hydrodynamical back-reaction by an approximate model of the central wind. We find that the escape fraction of ionizing radiation from the host halo increases with stellar mass, with 0.7 < f_esc <0.9 for 80 < M_*/M_sun < 500. To quantify the ionizing efficiency of these stars as they begin cosmic reionization, we find that, for M_* > 80 M_sun, the ratio of gas mass ionized to stellar mass is ~60,000, roughly half the number of ionizing photons released per stellar baryon. Nearby minihalos are shown to trap the I-front, so their centers remain neutral. This is contrary to the recent suggestion that these stars would trigger formation of a second generation by fully ionizing neighboring minihalos, stimulating H_2 formation in their cores. Finally, we discuss how the evacuation of gas from the host halo reduces the growth and luminosity of "miniquasars'' that may form from black hole remnants of the first stars.

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arXiv2005

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 compared 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 mergers of dark matter haloes during hierarchical structure formation to be ~ 10 M_{solar}. 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_{solar}.

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arXiv2005

Formation of the First Stars

How and when did the first generation of stars form at the end of the cosmic dark ages? Quite generically, within variants of the cold dark matter model of cosmological structure formation, the first sources of light are expected to form in ~ 10^{6} M_sun dark matter potential wells at redshifts z > 20. I discuss the physical processes that govern the formation of the first stars. These so-called Population~III stars are predicted to be predominantly very massive, and to have contributed significantly to the early reionization of the intergalactic medium. Such an early reionization epoch is inferred from the recent measurement of the Thomson optical depth by the {\it WMAP} satellite. I address the importance of heavy elements in bringing about the transition from an early star formation mode dominated by massive stars, to the familiar mode dominated by low mass stars, at later times, and present possible observational probes. This transition could have been gradual, giving rise to an intermediate-mass population of still virtually metal-free stars (``Population II.5''). These stars could have given rise to the peculiar class of black-hole forming supernovae inferred from the abundance pattern of extremely iron-poor stars.

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arXiv2004

Accretion onto a Primordial Protostar

We present a three-dimensional numerical simulation that resolves the formation process of a Population III star down to a scale of ~100 AU. The simulation is initialized on the scale of a dark matter halo of mass ~10^6 M_sun that virializes at z~20. It then follows the formation of a fully-molecular central core, and traces the accretion from the diffuse dust-free cloud onto the protostellar core for as long as ~10^4 yr, at which time the core has grown to ~50 M_sun. We find that the accretion rate starts very high, ~0.1 M_sun yr^-1, and declines rapidly thereafter approaching a power-law temporal scaling. Asymptotically, at times t > 10^3 yr after core formation, the stellar mass grows approximately as M_star=20 M_sun (t/10^3 yr)^0.4. Earlier on, accretion is faster with M_star \propto t^0.75. By extrapolating this growth over the full lifetime of very massive stars, t~3x10^6 yr, we obtain the conservative upper limit M_star < 500 M_sun. The actual stellar mass is, however, likely to be significantly smaller than this mass limit due to radiative and mechanical feedback from the protostar.

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arXiv2004

The Era of Massive Population III Stars: Cosmological Implications and Self-Termination

The birth and death of the first generation of stars have important implications for the thermal state and chemical properties of the intergalactic medium (IGM) in the early universe. Sometime after recombination, the neutral, chemically pristine gas was reionized by ultraviolet photons emitted from the first stars, but also enriched with heavy elements when these stars ended their lives as energetic supernovae. Using the results from previous high-resolution cosmological simulations of early structure formation that include radiative transfer, we show that a significant volume fraction of the IGM can be metal-polluted, as well as ionized, by massive Population III stars formed in small-mass (10^6-10^7 Msun) halos early on. If most of the early generation stars die as pair-instability supernovae with energies up to 10^{53} ergs, the volume-averaged mean metallicity will quickly reach Z ~ 10^{-4}Zsun by a redshift of 15-20, possibly causing a prompt transition to the formation of a stellar population that is dominated by low-mass stars. In this scenario, the early chemical enrichment history should closely trace the reionization history of the IGM, and the end of the Population III era is marked by the completion of reionization and pre-enrichment by z=15. We conclude that, while the pre-enrichment may partially account for the ``metallicity-floor'' in high-redshift Lyman-alpha clouds, it does not significantly affect the elemental abundance in the intracluster medium.

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arXiv2003

Cosmic Renaissance: The First Sources of Light

I review recent progress in understanding the formation of the first stars and quasars. The initial conditions for their emergence are given by the now firmly established model of cosmological structure formation. Numerical simulations of the collapse and fragmentation of primordial gas indicate that the first stars formed at redshifts z ~ 20 - 30, and that they were predominantly very massive, with M_* > 100 M_sun. Important uncertainties, however, remain. Paramount among them is the accretion process, which builds up the final stellar mass by incorporating part of the diffuse, dust-free envelope into the central protostellar core. The first quasars, on the other hand, are predicted to have formed later on, at z ~ 10, in more massive dark matter halos, with total masses, ~ 10^8 M_sun, characteristic of dwarf galaxies.

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arXiv2003

The First Stars

We review recent theoretical results on the formation of the first stars in the universe, and emphasize related open questions. In particular, we discuss the initial conditions for Population III star formation, as given by variants of the cold dark matter cosmology. Numerical simulations have investigated the collapse and the fragmentation of metal-free gas, showing that the first stars were predominantly very massive. The exact determination of the stellar masses, and the precise form of the primordial initial mass function, is still hampered by our limited understanding of the accretion physics and the protostellar feedback effects. We address the importance of heavy elements in bringing about the transition from an early star formation mode dominated by massive stars, to the familiar mode dominated by low mass stars, at later times. We show how complementary observations, both at high redshifts and in our local cosmic neighborhood, can be utilized to probe the first epoch of star formation.

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arXiv2003

The Formation of the First Low-Mass Stars From Gas With Low Carbon and Oxygen Abundances

The first stars in the Universe are predicted to have been much more massive than the Sun. Gravitational condensation accompanied by cooling of the primordial gas due to molecular hydrogen, yields a minimum fragmentation scale of a few hundred solar masses. Numerical simulations indicate that once a gas clump acquires this mass, it undergoes a slow, quasi-hydrostatic contraction without further fragmentation. Here we show that as soon as the primordial gas - left over from the Big Bang - is enriched by supernovae to a carbon or oxygen abundance as small as ~0.01-0.1% of that found in the Sun, cooling by singly-ionized carbon or neutral oxygen can lead to the formation of low-mass stars. This mechanism naturally accommodates the discovery of solar mass stars with unusually low (10^{-5.3} of the solar value) iron abundance but with a high (10^{-1.3} solar) carbon abundance. The minimum stellar mass at early epochs is partially regulated by the temperature of the cosmic microwave background. The derived critical abundances can be used to identify those metal-poor stars in our Milky Way galaxy with elemental patterns imprinted by the first supernovae.

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arXiv2003

The First Supernova Explosions in the Universe

We investigate the supernova explosions that end the lives of massive Population III stars in low-mass minihalos (M~10^6 M_sun) at redshifts z~20. Employing the smoothed particle hydrodynamics method, we carry out numerical simulations in a cosmological set-up of pair-instability supernovae with explosion energies of E_SN=10^51 and 10^53 ergs. We find that the more energetic explosion leads to the complete disruption of the gas in the minihalo, whereas the lower explosion energy leaves much of the halo intact. The higher energy supernova expels > 90% of the stellar metals into a region ~1 kpc across over a timescale of 3-5 Myr. Due to this burst-like initial star formation episode, a large fraction of the universe could have been endowed with a metallicity floor, Z_min>10^-4 Z_sun, already at z>15.

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arXiv2003

Formation of the First Supermassive Black Holes

We consider the physical conditions under which supermassive black holes could have formed inside the first galaxies. Our SPH simulations indicate that metal-free galaxies with a virial temperature ~10^4 K and with suppressed H2 formation (due to an intergalactic UV background) tend to form a binary black hole system which contains a substantial fraction (>10%) of the total baryonic mass of the host galaxy. Fragmentation into stars is suppressed without substantial H2 cooling. Our simulations follow the condensation of ~5x10^6 M_sun around the two centers of the binary down to a scale of < 0.1pc. Low-spin galaxies form a single black hole instead. These early black holes lead to quasar activity before the epoch of reionization. Primordial black hole binaries lead to the emission of gravitational radiation at redshifts z>10 that would be detectable by LISA.

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arXiv2003

The characteristic stellar mass as a function of redshift

We present a model for the star formation process during the initial collapse of dark matter haloes at redshifts z=0-30. We derive a simple expression for the characteristic stellar mass scale during this initial burst of star formation. In our picture, this characteristic scale reflects both the minimum temperature to which the gas can cool (determined by the metallicity and the temperature of the cosmic microwave background) and the pressure of overlying baryons in the collapsing halo. This prescription reproduces both the large mass scales found in simulations of Population III star formation and the near solar values observed for star formation at low redshift.

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