Chemical constraints on the contribution of Population III stars to cosmic reionization

Recent studies have highlighted that galaxies at z = 6-8 fall short of producing enough ionizing photons to reionize the IGM, and suggest that Population III stars could resolve this tension, because their harder spectra can produce ~10x more ionizing photons than Population II. But this argument depends critically on the duration of the Population III era, and because Population III stars form from pristine gas, in turn depends on the rate of galactic enrichment. We use a semi-analytic model of galaxy formation which tracks galactic chemical evolution, to gauge the impact of Population III stars on reionization. Population III SNe produce distinct metal abundances, and we argue that the duration of the Population III era can be constrained by precise relative abundance measurements in high-z damped Ly{\alpha} absorbers (DLAs), which provide a chemical record of past star-formation. We find that a single generation of Population III stars can self-enrich galaxies above the critical metallicity Zcrit=10^-4 Zsun for the Population III-to-II transition, on a very short timescale of ~10^6 yr, owing to the large metal yields and short lifetimes of Population III stars. This subsequently terminates the Population III era, hence they contribute >~ 50% of the ionizing photons only for z >~ 30, and at z=10 contribute <1%. The Population III contribution can be increased by delaying metal mixing into the ISM. However comparing the resulting metal abundance pattern to existing measurements in z <~ 6 DLAs, we show that the fractional contribution of high-mass Population III stars to the ionization rate must be <~ 10% at z = 10. Future abundance measurements of z~7-8 QSOs and GRBs should probe the era when the chemical vestiges of Population III star formation become detectable.Comment: 12 pages, 5 figures; Submitted to ApJ; Comments welcom

Chemical enrichment of Damped Lyman Alpha systems as a direct constraint on Population III star formation

Damped Ly-alpha absorbers (DLAs) can be used to measure gas-phase metallicities at large cosmological lookback times with high precision. Relative abundances can still be measured accurately deep into the reionization epoch (z > 6) using transitions redward of Ly-alpha. Here we study chemical evolution of DLAs using a constrained model for evolution of galaxies and IGM to determine the degree to which DLA abundance measurements can probe Population III enrichment. We find that if the critical metallicity of Population III to II transition is < 10^-4 Zsun, the cosmic Population III SFR is zero for z<8. Nevertheless, at high redshift (z ~ 6) Population III chemical signatures are retained in low-mass galaxies (halo mass < 10^9 Msun). This is because photoionization feedback suppresses star formation in these galaxies until relatively low redshift (z ~ 10), and the chemical record of early Population III star formation is retained. We model DLAs as these galaxies by assigning to them a mass-dependent H I absorption cross section and predict distribution of DLA abundance ratios. We find that these distributions are anchored towards abundance ratios set by Population II yields, but exhibit a tail that depends on the Population III IMF for z > 5. Thus, a sample of DLA abundance measurements at high redshift holds the promise to constrain Population III IMF. A sample of just 10 DLAs with relative abundances measured to an accuracy of 0.1 dex is sufficient to constrain the Population III IMF at 4-sigma. These constraints may prove stronger than other probes such as metal-poor stars and individual DLAs. Our results provide a global picture of the cosmic thermal, ionization, and chemical evolution, and can rule out certain Population III scenarios.Comment: 21 pages, 12 figures; this version accepted in Ap

Incarner la notion de vitesse en classe de 6e

International audienceCet article présente la mise en place d'un apprentissage original de la notion de vitesse en 6e, à travers l'utilisation d'un « planétaire humain », ressource pédagogique qui permet d'incarner les mouvements des planètes du Système Solaire. En insistant sur le lien entre la cognition et l'interaction corps-monde extérieur, nous proposons un autre regard sur les difficultés liées à l'apprentissage de la vitesse. Une activité spécifique a été mise en place avec deux classes d'un collège parisien. L'engagement des élèves dans cette activité, étudié à travers des interviews est clairement favorisé par leur immersion dans le mouvement étudié. La description détaillée de la séance menée et un dictionnaire associant mouvements sur le planétaire et vocabulaire de la cinématique sont donnés dans l'objectif de favoriser la mise en place, l'évaluation et l'amélioration de cette approche originale dans l'avenir

Small scale structure in diffuse molecular gas from repeated FUSE and visible spectra of HD 34078

We present preliminary results from an ongoing program devoted to a study of small scale structure in the spatial distribution of molecular gas. Our work is based on multi-epoch FUSE and visible observations of HD34078. A detailed comparison of H2, CH and CH+ absorption lines is performed. No short term variations are seen (except for highly excited H2) but long-term changes in N(CH) are clearly detected when comparing our data to spectra taken about 10 years ago.Comment: 4 pages, 2 figures, To appear in the Proceedings of the XVII IAP Colloquium "Gaseous Matter in Galaxies and Intergalactic Space

Cosmological Cosmic Rays and the observed Li6 plateau in metal poor halo stars

Very recent observations of the Li6 isotope in halo stars reveal a Li6 plateau about 1000 times above the predicted BBN abundance. We calculate the evolution of Li6 versus redshift generated from an initial burst of cosmological cosmic rays (CCRs) up to the formation of the Galaxy. We show that the pregalactic production of the Li6 isotope can account for the Li6 plateau observed in metal poor halo stars without additional over-production of Li7. The derived relation between the amplitude of the CCR energy spectra and the redshift of the initial CCR production puts constraints on the physics and history of the objects, such as pop III stars, responsible for these early cosmic rays. Consequently, we consider the evolution of Li6 in the Galaxy. Since Li6 is also produced in Galactic cosmic ray nucleosynthesis, we argue that halo stars with metallicities between [Fe/H] = -2 and -1, must be somewhat depleted in Li6.Comment: 8 pages, 6 figures, version accepted for publication in Ap

Evidence for overdensity around z_em > 4 quasars from the proximity effect.

MNRAS (accepted) 11 pages, 12 figuresInternational audienceWe study the density field around z_em > 4 quasars using high quality medium spectral resolution ESI-Keck spectra (R~4300, SNR > 25) of 45 high-redshift quasars selected from a total of 95 spectra. This large sample considerably increases the statistics compared to previous studies. The redshift evolution of the mean photo-ionization rate and the median optical depth of the intergalactic medium (IGM) are derived statistically from the observed transmitted flux and the pixel optical depth probability distribution function respectively. This is used to study the so-called proximity effect, that is, the observed decrease of the median optical depth of the IGM in the vicinity of the quasar caused by enhanced photo-ionization rate due to photons emitted by the quasar. We show that the proximity effect is correlated with the luminosity of the quasars, as expected. By comparing the observed decrease of the median optical depth with the theoretical expectation we find that the optical depth does not decrease as rapidly as expected when approaching the quasar if the gas in its vicinity is part of the standard IGM. We interpret this effect as revealing gaseous overdensities on scales as large as ~15 Mpc/h. The mean overdensity is of the order of two and five within, respectively, 10 and 3 Mpc/h. If true, this would indicate that high redshift quasars are located in the center of overdense regions that could evolve with time into massive clusters of galaxies. The overdensity is correlated with luminosity: brighter quasars show higher overdensities

Cosmic Ray production of Beryllium and Boron at high redshift

Recently, new observations of Li6 in Pop II stars of the galactic halo have shown a surprisingly high abundance of this isotope, about a thousand times higher than its predicted primordial value. In previous papers, a cosmological model for the cosmic ray-induced production of this isotope in the IGM has been developed to explain the observed abundance at low metallicity. In this paper, given this constraint on the Li6, we calculate the non-thermal evolution with redshift of D, Be, and B in the IGM. In addition to cosmological cosmic ray interactions in the IGM, we include additional processes driven by SN explosions: neutrino spallation and a low energy component in the structures ejected by outflows to the IGM. We take into account CNO CRs impinging on the intergalactic gas. Although subdominant in the galactic disk, this process is shown to produce the bulk of Be and B in the IGM, due to the differential metal enrichment between structures (where CRs originate) and the IGM. We also consider the resulting extragalactic gamma-ray background which we find to be well below existing data. The computation is performed in the framework of hierarchical structure formation considering several star formation histories including Pop III stars. We find that D production is negligible and that a potentially detectable Be and B plateau is produced by these processes at the time of the formation of the Galaxy (z ~ 3).Comment: 9 pages, 7 figure

The large-scale Quasar-Lyman \alpha\ Forest Cross-Correlation from BOSS

We measure the large-scale cross-correlation of quasars with the Lyman \alpha\ forest absorption in redshift space, using ~ 60000 quasar spectra from Data Release 9 (DR9) of the Baryon Oscillation Spectroscopic Survey (BOSS). The cross-correlation is detected over a wide range of scales, up to comoving separations r of 80 Mpc/h. For r > 15 Mpc/h, we show that the cross-correlation is well fitted by the linear theory prediction for the mean overdensity around a quasar host halo in the standard \Lambda CDM model, with the redshift distortions indicative of gravitational evolution detected at high confidence. Using previous determinations of the Lyman \alpha\ forest bias factor obtained from the Lyman \alpha\ autocorrelation, we infer the quasar bias factor to be b_q = 3.64^+0.13_-0.15 at a mean redshift z=2.38, in agreement with previous measurements from the quasar auto-correlation. We also obtain a new estimate of the Lyman \alpha\ forest redshift distortion factor, \beta_F = 1.1 +/- 0.15, slightly larger than but consistent with the previous measurement from the Lyman \alpha\ forest autocorrelation. The simple linear model we use fails at separations r < 15 Mpc/h, and we show that this may reasonably be due to the enhanced ionization due to radiation from the quasars. We also provide the expected correction that the mass overdensity around the quasar implies for measurements of the ionizing radiation background from the line-of-sight proximity effect.Comment: 24 pages, 6 figures, published in JCA

Population III Generated Cosmic Rays and the Production of Li6

We calculate the evolution of Li6 generated from cosmic rays produced by an early population of massive stars. The computation is performed in the framework of hierarchical structure formation and is based on cosmic star formation histories constrained to reproduce the observed star formation rate at redshift z \la 6, the observed chemical abundances in damped Lyman alpha absorbers and in the intergalactic medium, and to allow for an early reionization of the Universe at z\sim 11 by Pop III stars as indicated by the third year results released by WMAP. We show that the pregalactic production of the Li6 isotope in the IGM via these Pop III stars can account for the Li6 plateau observed in metal poor halo stars without additional over-production of Li7. Our results depend on the efficiency of cosmic rays to propagate out of minihalos and the fraction of supernovae energy deposited in cosmic rays. We also compute the cosmic ray heating of the IGM gas. In general, we find somewhat high temperatures (of order 10^5 K) implying that the cosmic rays production of Li6 may be required to be confined to the so-called warm-hot IGM.Comment: 9 pages 8 figure

The Eighth Data Release of the Sloan Digital Sky Survey: First Data from SDSS-III

The Sloan Digital Sky Survey (SDSS) started a new phase in August 2008, with new instrumentation and new surveys focused on Galactic structure and chemical evolution, measurements of the baryon oscillation feature in the clustering of galaxies and the quasar Ly alpha forest, and a radial velocity search for planets around ~8000 stars. This paper describes the first data release of SDSS-III (and the eighth counting from the beginning of the SDSS). The release includes five-band imaging of roughly 5200 deg^2 in the Southern Galactic Cap, bringing the total footprint of the SDSS imaging to 14,555 deg^2, or over a third of the Celestial Sphere. All the imaging data have been reprocessed with an improved sky-subtraction algorithm and a final, self-consistent photometric recalibration and flat-field determination. This release also includes all data from the second phase of the Sloan Extension for Galactic Understanding and Evolution (SEGUE-2), consisting of spectroscopy of approximately 118,000 stars at both high and low Galactic latitudes. All the more than half a million stellar spectra obtained with the SDSS spectrograph have been reprocessed through an improved stellar parameters pipeline, which has better determination of metallicity for high metallicity stars.Comment: Astrophysical Journal Supplements, in press (minor updates from submitted version