Condition for low-mass star formation in shock-compressed metal-poor clouds

Shocks may have been prevalent in the early Universe, associated with virialization and supernova explosions, etc. Here, we study thermal evolution and fragmentation of shock-compressed clouds, by using a one-zone model with detailed thermal and chemical processes. We explore a large range of initial density (1-1e5 /cm^3), metallicity (0-1e-2 Z_sun), UV strength (0-500 times Galactic value), and cosmic microwave background temperature (10 and 30 K). Shock-compressed clouds contract isobarically via atomic and molecular line cooling, until self-gravitating clumps are formed by fragmentation. If the metals are only in the gas-phase, the clump mass is higher than ~ 3 M_sun in any conditions we studied. Although in some cases with a metallicity higher than ~ 1e-3 Z_sun, re-fragmentation of a clump is caused by metal-line cooling, this fragment mass is higher than ~ 30 M_sun. On the other hand, if about half the mass of metals is condensed in dust grains, as in the Galactic interstellar medium, dust cooling triggers re-fragmentation of a clump into sub-solar mass pieces, for metallicities higher than ~ 1e-5 Z_sun. Therefore, the presence of dust is essential in low-mass (< M_sun) star formation from a shock-compressed cloud.Comment: 15 pages, 8 figures, accepted for publication in MNRA

Super-Eddington growth of the first black holes

The assembly of the first super massive black holes (SMBHs) at z ≳ 6 is still a subject of intense debate. If black holes (BHs) grow at their Eddington rate, they must start from ≳104 M⊙ seeds formed by the direct collapse of gas. Here, we explore the alternative scenario where ˜100 M⊙ BH remnants of the first stars grow at super-Eddington rate via radiatively inefficient slim accretion discs. We use an improved version of the cosmological, data-constrained semi-analytic model GAMETE/QSODUST, where we follow the evolution of nuclear BHs and gas cooling, disc and bulge formation of their host galaxies. Adopting SDSS J1148+5251 (J1148) at z = 6.4 as a prototype of luminous z ≳ 6 quasars, we find that ˜80 per cent of its SMBH mass is grown by super-Eddington accretion, which can be sustained down to z ˜ 10 in dense, gas-rich environments. The average BH mass at z ˜ 20 is MBH ≳ 104 M⊙, comparable to that of direct collapse BHs. At z = 6.4 the AGN-driven mass outflow rate is consistent with the observations and the BH-to-bulge mass ratio is compatible with the local scaling relation. However, the stellar mass in the central 2.5 kpc is closer to the value inferred from CO observations. Finally, ˜20 per cent of J1148 progenitors at z = 7.1 have BH luminosities and masses comparable to ULAS J1120+0641, suggesting that this quasar may be one of the progenitors of J1148

Cosmic stellar relics in the Galactic halo

We study the stellar population history and chemical evolution of the Milky Way (MW) in a hierarchical LCDM model for structure formation. Using a Monte Carlo method based on the semi-analytical EPS formalism, we reconstruct the merger tree of our Galaxy and follow the evolution of gas and stars along the hierarchy. Our approach allows us to compare the observational properties of the MW with model results, exploring different properties of primordial stars, such as their IMF and the critical metallicity for low-mass star formation, Zcr. By matching our predictions to the Metallicity Distribution Function (MDF) of metal-poor stars in the Galactic halo we find that: (i) supernova feedback is required to reproduce the observed properties of the MW; (ii) stars with [Fe/H]<-2.5 form in halos accreting Galactic Medium (GM) enriched by earlier supernova explosions; (iii) the fiducial model (Zcr=10^-4Zsun,m_PopIII=200Msun) provides an overall good fit to the MDF but cannot account for the two stars with [Fe/H]<-5; the latter can be accommodated if Zcr<10^-6Zsun but such model overpopulates the range -5.3<[Fe/H]<-4 in which no stars have been detected; (iv) the current non-detection of metal-free stars robustly constrains either Zcr>0 or the masses of the first stars m_PopIII>0.9Msun; (v) the statistical impact of second generation stars, i.e stars forming out of gas polluted only by metal-free stars, is negligible in current samples; (vi) independently of Zcr, 60% of metals in the GM are ejected through winds by halos with masses M<6x10^9 Msun, showing that low-mass halos are the dominant population contributing to cosmic metal enrichment.Comment: 18 pages, 12 figures, submitted to MNRA

Stochastic background of gravitational waves generated by a cosmological population of young, rapidly rotating neutron stars

We estimate the spectral properties of the stochastic background of gravitational radiation emitted by a cosmological population of hot, young, rapidly rotating neutron stars. Their formation rate as a function of redshift is deduced from an observation-based determination of the star formation history in the Universe, and the gravitational energy is assumed to be radiated during the spin-down phase associated to the newly discovered r-mode instability. We calculate the overall signal produced by the ensemble of such neutron stars, assuming various cosmological backgrounds. We find that the spectral strain amplitude has a maximum $\approx (2-4)\times 10^{-26} {Hz}^{-1/2}$, at frequencies $\approx (30-60)$ Hz, while the corresponding closure density, $h^2 \Omega_{GW}$, has a maximum amplitude plateau of $\approx (2.2-3.3) \times 10^{-8}$ in the frequency range $(500-1700)$ Hz. We compare our results with a preliminary analysis done by Owen et al. (1998), and discuss the detectability of this background.Comment: 8 pages, 9 figures, accepted for publication in MNRA

Gravitational Wave Background from a Cosmological Population of Core-Collapse Supernovae

We analyse the stochastic background of gravitational radiation emitted by a cosmological population of core-collapse supernovae. The supernova rate as a function of redshift is deduced from an observation-based determination of the star formation rate density evolution. We then restrict our analysis to the range of progenitor masses leading to black hole collapse. In this case, the main features of the gravitational-wave emission spectra have been shown to be, to some extent, independent of the initial conditions and of the equation of state of the collapsing star, and to depend only on the black hole mass and angular momentum. We calculate the overall signal produced by the ensemble of black-hole collapses throughout the Universe, assuming a flat cosmology with vanishing cosmological constant. Within a wide range of parameter values, we find that the spectral strain amplitude has a maximum at a few hundred Hz with an amplitude between $10^{-28}$ and $10^{-27} Hz^{-1/2}$; the corresponding closure density, $\Omega_{GW}$, has a maximum amplitude ranging between $10^{-11}$ and $10^{-10}$ in the frequency interval $\sim 1.5-2.5$ kHz. Contrary to previous claims, our observation-based determination leads to a duty cycle of order 0.01, making our stochastic backgound a non-continuous one. Although the amplitude of our background is comparable to the sensitivity that can be reached by a pair of advanced LIGO detectors, the characteristic shot-noise structure of the predicted signal might be in principle exploited to design specific detection strategies.Comment: 12 pages, LaTeX (uses mn.sty), 13 figures, 2 tables, accepted for publication in MNRA

Stochastic Backgrounds of Gravitational Waves from Cosmological Populations of Astrophysical Sources

Astrophysical sources of gravitational radiation are likely to have been formed since the beginning of star formation. Realistic source rates of formation throughout the Universe have been estimated from an observation-based determination of the star formation rate density evolution. Both the radiation emitted during the collapse to black holes and the spin-down radiation, induced by the r-mode instability, emitted by hot, young rapidly rotating neutron stars have been considered. We have investigated the overall signal produced by the ensemble of sources exploring the parameter space and discussing its possible detectability.Comment: 9 pages, 7 figures, to appear in the proceedings of the 19th Texas Symposium on Relativistic Astrophysics and Cosmology, Paris, December 14-18, 199

High-redshift quasars host galaxies: is there a stellar mass crisis?

We investigate the evolutionary properties of a sample of quasars at 5<z<6.4 using the semi-analytical hierarchical model GAMETE/QSOdust. We find that the observed properties of these quasars are well reproduced by a common formation scenario in which stars form according to a standard IMF, via quiescent star formation and efficient merger-driven bursts, while the central BH grows via gas accretion and BH-BH mergers. Eventually, a strong AGN driven wind starts to clear up the ISM of dust and gas, damping the star formation and un-obscuring the line of sight toward the QSO. In this scenario, all the QSOs hosts have final stellar masses in the range $(4-6)\times 10^{11} M_{sun}$, a factor 3-30 larger than the upper limits allowed by the observations. We discuss alternative scenarios to alleviate this apparent tension: the most likely explanation resides in the large uncertainties that still affect dynamical mass measurements in these high-z galaxies. In addition, during the transition between the starburst-dominated and the active QSO phase, we predict that about 40% of the progenitor galaxies can be classified as Sub Millimeter Galaxies, although their number rapidly decreases with redshift.Comment: 14 pages, 9 figures, accepted for publication in MNRA

From the first stars to the first black holes

The growth of the first super massive black holes (SMBHs) at z > 6 is still a major challenge for theoretical models. If it starts from black hole (BH) remnants of Population III stars (light seeds with mass ~ 100 Msun) it requires super-Eddington accretion. An alternative route is to start from heavy seeds formed by the direct collapse of gas onto a ~ 10^5 Msun BH. Here we investigate the relative role of light and heavy seeds as BH progenitors of the first SMBHs. We use the cosmological, data constrained semi-analytic model GAMETE/QSOdust to simulate several independent merger histories of z > 6 quasars. Using physically motivated prescriptions to form light and heavy seeds in the progenitor galaxies, we find that the formation of a few heavy seeds (between 3 and 30 in our reference model) enables the Eddington-limited growth of SMBHs at z > 6. This conclusion depends sensitively on the interplay between chemical, radiative and mechanical feedback effects, which easily erase the conditions that allow the suppression of gas cooling in the low metallicity gas (Z Jcr). We find that heavy seeds can not form if dust cooling triggers gas fragmentation above a critical dust-to-gas mass ratio (D > Dcr). In addition, the relative importance of light and heavy seeds depends on the adopted mass range for light seeds, as this dramatically affects the history of cold gas along the merger tree, by both SN and AGN-driven winds.Comment: 16 pages, 13 figures, accepted for publication in MNRA

Chasing the observational signatures of seed black holes at z > 7: candidate statistics

Supermassive black holes (SMBHs) of 10^9-10^10 Msun were already in place ~13 Gyr ago, at z>6. Super-Eddington growth of low-mass BH seeds (~100 Msun) or less extreme accretion onto ~10^5 Msun seeds have been recently considered as the main viable routes to these SMBHs. Here we study the statistics of these SMBH progenitors at z~6. The growth of low- and high-mass seeds and their host galaxies are consistently followed using the cosmological data constrained model GAMETE/QSOdust, which reproduces the observed properties of high-z quasars, like SDSS J1148+5251. We show that both seed formation channels can be in action over a similar redshift range, 15 < z < 18 and are found in dark matter halos with comparable mass, ~5x10^7 Msun. However, as long as the systems evolve in isolation (i.e. no mergers occur), noticeable differences in their properties emerge: at z>= 10 galaxies hosting high-mass seeds have smaller stellar mass and metallicity, the BHs accrete gas at higher rates and star formation proceeds less efficiently than in low-mass seeds hosts. At z<10 these differences are progressively erased, as the systems experience minor or major mergers and every trace of the BH origin gets lost.Comment: accepted for publication in MNRA

The sustainable growth of the first black holes

Super-Eddington accretion has been suggested as a possible formation pathway of $10^9 \, M_\odot$ supermassive black holes (SMBHs) 800 Myr after the Big Bang. However, stellar feedback from BH seed progenitors and winds from BH accretion disks may decrease BH accretion rates. In this work, we study the impact of these physical processes on the formation of $z \sim 6$ quasar, including new physical prescriptions in the cosmological, data-constrained semi-analytic model GAMETE/QSOdust. We find that the feedback produced by the first stellar progenitors on the surrounding does not play a relevant role in preventing SMBHs formation. In order to grow the $z \gtrsim 6$ SMBHs, the accreted gas must efficiently lose angular momentum. Moreover disk winds, easily originated in super-Eddington accretion regime, can strongly reduce duty cycles. This produces a decrease in the active fraction among the progenitors of $z\sim6$ bright quasars, reducing the probability to observe them