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.Comment: Published in ApJ Letter

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.Comment: Version accepted by ApJ. Minor revisions and a few citations adde

Dark Matter Halo Environment for Primordial Star Formation

We study the statistical properties (such as shape and spin) of high-z halos likely hosting the first (PopIII) stars with cosmological simulations including detailed gas physics. In the redshift range considered ($11 < z < 16$) the average sphericity is $= 0.3 \pm 0.1$, and for more than 90% of halos the triaxiality parameter is $T \lesssim 0.4$, showing a clear preference for oblateness over prolateness. Larger halos in the simulation tend to be both more spherical and prolate: we find $s \propto M_h^{\alpha_s}$ and $T \propto M_h^{\alpha_T}$, with $\alpha_s \approx 0.128$ and $\alpha_T= 0.276$ at z = 11. The spin distributions of dark matter and gas are considerably different at $z=16$, with the baryons rotating slower than the dark matter. At lower redshift, instead, the spin distributions of dark matter and gas track each other almost perfectly, as a consequence of a longer time interval available for momentum redistribution between the two components. The spin of both the gas and dark matter follows a lognormal distribution, with a mean value at z=16 of $=0.0184$, virtually independent of halo mass. This is in good agreement with previous studies. Using the results of two feedback models (MT1 and MT2) by McKee & Tan (2008) and mapping our halo spin distribution into a PopIII IMF, we find that at high-$z$ the IMF closely tracks the spin lognormal distribution. Depending on the feedback model, though, the distribution can be centered at $\approx 65 M_\odot$ (MT1) or $\approx 140 M_\odot$ (MT2). At later times, model MT1 evolves into a bimodal distribution with a second prominent peak located at $35-40 M_\odot$ as a result of the non-linear relation between rotation and halo mass. We conclude that the dark matter halo properties might be a key factor shaping the IMF of the first stars.Comment: 10 pages, 6 figures, accepted for publication in MNRA

Population III stars and the Long Gamma Ray Burst rate

Because massive, low-metallicity population III (PopIII) stars may produce very powerful long gamma-ray bursts (LGRBs), high-redshift GRB observations could probe the properties of the first stars. We analyze the correlation between early PopIII stars and LGRBs by using cosmological N-body/hydrodynamical simulations, which include detailed chemical evolution, cooling, star formation, feedback effects and the transition between PopIII and more standard population I/II (PopII/I) stars. From the Swift observed rate of LGRBs, we estimate the fraction of black holes that will produce a GRB from PopII/I stars to be in the range 0.028<f_{GRB}<0.140, depending on the assumed upper metallicity of the progenitor. Assuming that as of today no GRB event has been associated to a PopIII star, we estimate the upper limit for the fraction of LGRBs produced by PopIII stars to be in the range 0.006<f_{GRB}<0.022. When we apply a detection threshold compatible with the BAT instrument, we find that the expected fraction of PopIII GRBs (GRB3) is ~10% of the full LGRB population at z>6, becoming as high has 40% at z>10. Finally, we study the properties of the galaxies hosting our sample of GRB3. We find that the average metallicity of the galaxies hosting a GRB3 is typically higher than the critical metallicity used to select the PopIII stars, due to the efficiency in polluting the gas above such low values. We also find that the highest probability of finding a GRB3 is within galaxies with a stellar mass <10^7 Msun, independently from the redshift.Comment: 8 pages,3 figures. Submitted to MNRAS, revised version after referee's comment

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.Comment: 11 pages, 9 figures, revised version, ApJ in press (October 10, 2003

Astrophysics: Most distant cosmic blast seen

The most distant -ray burst yet sighted is the earliest astronomical object ever observed in cosmic history. This ancient beacon offers a glimpse of the little-known cosmic dark ages.Comment: Published in Nature News & View

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.Comment: 16 pages, 14 figures, published in Ap

Forming the First Stars in the Universe: The Fragmentation of Primordial Gas

In order to constrain the initial mass function (IMF) of the first generation of stars (Population III), we investigate the fragmentation properties of metal-free gas in the context of a hierarchical model of structure formation. We investigate the evolution of an isolated 3-sigma peak of mass 2x10^6 M_solar which collapses at z_coll=30 using Smoothed Particle Hydrodynamics. We find that the gas dissipatively settles into a rotationally supported disk which has a very filamentary morphology. The gas in these filaments is Jeans unstable with M_J~10^3 M_solar. Fragmentation leads to the formation of high density (n>10^8 cm^-3) clumps which subsequently grow in mass by accreting surrounding gas and by merging with other clumps up to masses of ~10^4 M_solar. This suggests that the very first stars were rather massive. We explore the complex dynamics of the merging and tidal disruption of these clumps by following their evolution over a few dynamical times.Comment: 7 pages, 3 figures, uses emulateapj.sty. Accepted for publication in the Astrophysical Journal Letter