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Register a new userThe fraction of ionizing radiation from massive stars that escapes to the intergalactic medium
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The part played by stars in the ionization of the intergalactic medium remains an open question. A key issue is the proportion of the stellar ionizing radiation that escapes the galaxies in which it is produced. Spectroscopy of gamma-ray burst afterglows can be used to determine the neutral hydrogen column-density in their host galaxies and hence the opacity to extreme ultra-violet radiation along the lines-of-sight to the bursts. Thus, making the reasonable assumption that long-duration GRB locations are representative of the sites of massive stars that dominate EUV production, one can calculate an average escape fraction of ionizing radiation in a way that is independent of galaxy size, luminosity or underlying spectrum. Here we present a sample of NH measures for 138 GRBs in the range 1.6<z<6.7 and use it to establish an average escape fraction at the Lyman limit of <fesc>~0.005, with a 98% confidence upper limit of ~0.015. This analysis suggests that stars provide a small contribution to the ionizing radiation budget of the IGM at z<5, where the bulk of the bursts lie. At higher redshifts, z>5, firm conclusions are limited by the small size of the GRB sample, but any decline in average HI column-density seems to be modest. We also find no indication of a significant correlation of NH with galaxy UV luminosity or host stellar mass, for the subset of events for which these are available. We discuss in some detail a number of selection effects and potential biases. Drawing on a range of evidence we argue that such effects, while not negligible, are unlikely to produce systematic errors of more than a factor ~2, and so would not affect the primary conclusions. Given that many GRB hosts are low metallicity, high specific star-formation rate, dwarf galaxies, these results present a particular problem for the hypothesis that such galaxies dominated the reionization of the universe.
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We discuss two important effects for the astrospheres of runaway stars: the propagation of ionizing photons far beyond the astropause, and the rapid evolution of massive stars (and their winds) near the end of their lives. Hot stars emit ionizing photons with associated photoheating that has a significant dynamical effect on their surroundings. 3D simulations show that HII regions around runaway O stars drive expanding conical shells and leave underdense wakes in the medium they pass through. For late O stars this feedback to the interstellar medium is more important than that from stellar winds. Late in life, O stars evolve to cool red supergiants more rapidly than their environment can react, producing transient circumstellar structures such as double bow shocks. This provides an explanation for the bow shock and linear bar-shaped structure observed around Betelgeuse.
We present some results of an on-going project aimed at studying a sample of Galactic HII regions ionized by a single massive star to test the predictions of modern generation stellar atmosphere codes in the H Lyman continuum. The observations collected for this study comprise the optical spectra of the corresponding ionizing stars, along with imaging and long-slit spatially resolved nebular observations. The analysis of the stellar spectra allows to obtain the stellar parameters of the ionizing star, while the nebular observations provide constraints on the nebular abundances and gas distribution. All this information is then used to construct tailored photoionization models of the HII regions. The reliability of the stellar ionizing fluxes is hence tested by comparing the photoionization model results with the observations in terms of the spatial variation across the nebula of an appropriate set of nebular line ratios.
We describe a new method for simulating ionizing radiation and supernova feedback in the analogues of low-redshift galactic disks. In this method, which we call star-forming molecular cloud (SFMC) particles, we use a ray-tracing technique to solve the radiative transfer equation for ultraviolet photons emitted by thousands of distinct particles on the fly. Joined with high numerical resolution of 3.8 pc, the realistic description of stellar feedback helps to self-regulate star formation. This new feedback scheme also enables us to study the escape of ionizing photons from star-forming clumps and from a galaxy, and to examine the evolving environment of star-forming gas clumps. By simulating a galactic disk in a halo of 2.3e11 Msun, we find that the average escape fraction from all radiating sources on the spiral arms (excluding the central 2.5 kpc) fluctuates between 0.08% and 5.9% during a ~20 Myr period with a mean value of 1.1%. The flux of escaped photons from these sources is not strongly beamed, but manifests a large opening angle of more than 60 degree from the galactic pole. Further, we investigate the escape fraction per SFMC particle, f_esc(i), and how it evolves as the particle ages. We discover that the average escape fraction f_esc is dominated by a small number of SFMC particles with high f_esc(i). On average, the escape fraction from a SFMC particle rises from 0.27% at its birth to 2.1% at the end of a particle lifetime, 6 Myrs. This is because SFMC particles drift away from the dense gas clumps in which they were born, and because the gas around the star-forming clumps is dispersed by ionizing radiation and supernova feedback. The framework established in this study brings deeper insight into the physics of photon escape fraction from an individual star-forming clump, and from a galactic disk.
It has been known for decades that the observed number of baryons in the local universe falls about 30-40% short of the total number of baryons predicted by Big-Bang Nucleosynthesis, as inferred from density fluctuations of the Cosmic Microwave Background and seen during the first 2-3 billion years of the universe in the so called Lyman-alpha Forest. A theoretical solution to this paradox locates the missing baryons in the hot and tenuous filamentary gas between galaxies, known as the warm-hot intergalactic medium. However, it is difficult to detect them there because the largest by far constituent of this gas - hydrogen - is mostly ionized and therefore almost invisible in far-ultraviolet spectra with typical signal-to-noise ratios. Indeed, despite the large observational efforts, only a few marginal claims of detection have been made so far. Here we report observations of two absorbers of highly ionized oxygen (OVII) in the high signal-to-noise-ratio X-ray spectrum of a quasar at redshift >0.4. These absorbers show no variability over a 2-year timescale and have no associated cold absorption, making the assumption that they originate from the quasars intrinsic outflow or the host galaxys interstellar medium implausible. The OVII systems lie in regions characterized by large (x4 compared to average) galaxy over-densities and their number (down to the sensitivity threshold of our data), agrees well with numerical simulation predictions for the long-sought warm-hot intergalactic medium (WHIM). We conclude that the missing baryons have been found.
Using the Cosmic Origins Spectrograph aboard the Hubble Space Telescope, we measured the abundances of six ions (C III, C IV, Si III, Si IV, N V, O VI) in the low-redshift (z < 0.4) intergalactic medium and explored C and Si ionization corrections from adjacent ion stages. Both C IV and Si IV have increased in abundance by a factor of ~10 from z = 5.5 to the present. We derive ion mass densities, (rho_ion) = (Omega_ion)(rho_cr) with Omega_ion expressed relative to closure density. Our models of the mass-abundance ratios, (Si III / Si IV) = 0.67(+0.35,-0.19), (C III / C IV) = 0.70(+0.43,-0.20), and (Omega_CIII + Omega_CIV) / (Omega_SiIII + Omega_SiIV) = 4.9(+2.2,-1.1), are consistent with a hydrogen photoionization rate Gamma_H = (8 +/- 2) x 10^{-14} s^{-1} at z < 0.4 and specific intensity I_0 = (3 +/- 1) x 10^{-23} erg/(cm^2 s Hz sr) at the Lyman limit. We find mean photoionization parameter log U = -1.5 +/- 0.4, baryon overdensity Delta_b = 200 +/- 50, and Si/C enhanced to three times its solar ratio (enhancement of alpha-process elements). We compare these metal abundances to the expected IGM enrichment and abundances in higher photoionized states of carbon (C V) and silicon (Si V, Si VI, Si VII). Our ionization modeling infers IGM metal densities of (5.4 +/- 0.5) x 10^5 M_sun / Mpc^3 in the photoionized Lya forest traced by the C and Si ions and (9.1 +/- 0.6) x 10^5 M_sun / Mpc^3 in hotter gas traced by O VI. Combining both phases, the heavy elements in the IGM have mass density rho_Z = (1.5 +/- 0.8) x 10^6 M_sun / Mpc^3 or Omega_Z = 10^{-5}. This represents 10 +/- 5 percent of the metals produced by (6 +/- 2) x 10^8 M_sun / Mpc^3 of integrated star formation with yield y_m = 0.025 +/- 0.010. The missing metals at low redshift may reside within galaxies and in undetected ionized gas in galaxy halos and circumgalactic medium.
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