Lyman-alpha Forest Absorption Research Articles

THE PHOTON UNDERPRODUCTION CRISIS

We examine the statistics of the low-redshift Lyman-alpha forest from smoothed particle hydrodynamic simulations in light of recent improvements in the estimated evolution of the cosmic ultraviolet background (UVB) and recent observations from the Cosmic Origins Spectrograph (COS). We find that the value of the metagalactic photoionization rate required by our simulations to match the observed properties of the low-redshift Lyman-alpha forest is a factor of 5 larger than the value predicted by state-of-the art models for the evolution of this quantity. This mismatch results in the mean flux decrement of the Lyman-alpha forest being underpredicted by at least a factor of 2 (a 10-sigma discrepancy with observations) and a column density distribution of Lyman-alpha forest absorbers systematically and significantly elevated compared to observations over nearly two decades in column density. We examine potential resolutions to this mismatch and find that either conventional sources of ionizing photons (galaxies and quasars) must be significantly elevated relative to current observational estimates or our theoretical understanding of the low-redshift universe is in need of substantial revision.

ON MODELING AND MEASURING THE TEMPERATURE OF THEz∼ 5 INTERGALACTIC MEDIUM

The temperature of the low-density intergalactic medium (IGM) at high redshift is sensitive to the timing and nature of hydrogen and HeII reionization, and can be measured from Lyman-alpha forest absorption spectra. Since the memory of intergalactic gas to heating during reionization gradually fades, measurements as close as possible to reionization are desirable. In addition, measuring the IGM temperature at sufficiently high redshifts should help to isolate the effects of hydrogen reionization since HeII reionization starts later, at lower redshift. Motivated by this, we model the IGM temperature at z>5 using semi-numeric models of patchy reionization. We construct mock Lyman-alpha forest spectra from these models and consider their observable implications. We find that the small-scale structure in the Lyman-alpha forest is sensitive to the temperature of the IGM even at redshifts where the average absorption in the forest is as high as 90%. We forecast the accuracy at which the z~5 IGM temperature can be measured using existing samples of high resolution quasar spectra, and find that interesting constraints are possible. For example, an early reionization model in which reionization ends at z~10 should be distinguishable -- at high statistical significance -- from a lower redshift model where reionization completes at z~6. We discuss improvements to our modeling that may be required to robustly interpret future measurements.

Absorption signatures of warm-hot gas at low redshift: broad H i Lyα absorbers

We investigate the physical state of HI absorbing gas at low redshift (z = 0.25) using a subset of cosmological, hydrodynamic simulations from the OWLS project, focusing in particular on broad (b_HI > 40 km/s) HI Lyman-Alpha absorbers (BLAs), which are believed to originate in shock-heated gas in the warm-hot intergalactic medium (WHIM). Our fiducial model, which includes radiative cooling by heavy elements and feedback by supernovae and active galactic nuclei, predicts that by z = 0.25 nearly 60 per cent of the gas mass ends up at densities and temperatures characteristic of the WHIM and we find that half of this fraction is due to outflows. The standard HI observables (distribution of HI column densities N_HI, distribution of Doppler parameters b_HI, b_HI - N_HI correlation) and the BLA line number density predicted by our simulations are in remarkably good agreement with observations. BLAs arise in gas that is hotter, more highly ionised and more enriched than the gas giving rise to typical Lyman-Alpha forest absorbers. The majority of the BLAs arise in warm-hot (log (T/K) ~ 5) gas at low (log Delta < 1.5) overdensities. On average, thermal broadening accounts for at least 60 per cent of the BLA line width, which in turn can be used as a rough indicator of the thermal state of the gas. Detectable BLAs account for only a small fraction of the true baryon content of the WHIM at low redshift. In order to detect the bulk of the mass in this gas phase, a sensitivity at least one order of magnitude better than achieved by current ultraviolet spectrographs is required. We argue that BLAs mostly trace gas that has been shock-heated and enriched by outflows and that they therefore provide an important window on a poorly understood feedback process.

Star-forming galaxies at z≈ 8-9 from Hubble Space Telescope/WFC3: implications for reionization

We present a search for galaxies at 7.6<z<9.8 using the latest HST WFC3 near-infrared data, based on the Lyman-break technique. We search for galaxies which have large (Y-J) colours (the "Y-drops") on account of the Lyman-alpha forest absorption, and with (J-H) colours inconsistent with being low-redshift contaminants. We identify 24 candidates at redshift z~8-9 (15 are robust and a further 9 more marginal but consistent with being high redshift) over an area of ~50 square arcminutes. Previous searches for Y-drops with WFC3 have focussed only on the Hubble Ultra Deep Field (HUDF), and our larger survey (involving two other nearby deep fields and a wider area survey) has trebelled the number of robust Y-drop candidates. For the first time, we have sufficient z~8-9 galaxies to fit both phi^* and M^* of the UV Schechter luminosity function. There is evidence for evolution in this luminosity function from z=6-7 to z=8-9, in the sense that there are fewer UV-bright galaxies at z~8-9, consistent with an evolution mainly in M^*. The candidate z~8-9 galaxies we detect have insufficient ionizing flux to reionize the Universe, and it is probable that galaxies below our detection limit provide a significant UV contribution. The faint-end slope, alpha, is not well constrained. However, adopting a similiar faint-end slope to that determined at z=3-6 (alpha=-1.7) and a Salpeter initial mass function, then the ionizing photon budget still falls short if f_escape<0.5, even integrating down to M(UV)=-8. A steeper faint end slope or a low-metallicity population (or a top-heavy IMF) might still provide sufficient photons for star-forming galaxies to reionize the Universe, but confirmation of this might have to await the James Webb Space Telescope.

A SEARCH FOR OXYGEN IN THE LOW-DENSITY Lyα FOREST USING THE SLOAN DIGITAL SKY SURVEY

We use 2167 Sloan Digital Sky Survey (SDSS) quasar spectra to search for low-density oxygen in the Intergalactic Medium (IGM). Oxygen absorption is detected on a pixel-by-pixel basis by its correlation with Lyman-alpha forest absorption. We have developed a novel Locally Calibrated Pixel (LCP) search method that uses adjacent regions of the spectrum to calibrate interlopers and spectral artifacts, which would otherwise limit the measurement of OVI absorption. Despite the challenges presented by searching for weak OVI within the Lyman-alpha forest in spectra of moderate resolution and signal-to-noise, we find a highly significant detection of absorption by oxygen at 2.7 < z < 3.2 (the null hypothesis has a chi^2=80 for 9 data points). We interpret our results using synthetic spectra generated from a lognormal density field assuming a mixed quasar-galaxy photoionizing background (Haardt & Madau 2001) and that it dominates the ionization fraction of detected OVI. The LCP search data can be fit by a constant metallicity model with [O/H] = -2.15_(-0.09)^(+0.07), but also by models in which low-density regions are unenriched and higher density regions have a higher metallicity. The density-dependent enrichment model by Aguirre et al. (2008) is also an acceptable fit. All our successful models have similar mass-weighted oxygen abundance, corresponding to [<O/H>_MW] = -2.45+-0.06. This result can be used to find the cosmic oxygen density in the Lyman-alpha forest, Omega_(Oxy, IGM) = 1.4(+-0.2)x10^(-6) = 3x10^(-4) Omega_b. This is the tightest constraint on the mass-weighted mean oxygen abundance and the cosmic oxygen density in the Lyman-alpha forest to date and indicates that it contains approximately 16% of metals produced by star formation (Bouch\'e et al. 2008) up to z = 3.

Pressure support versus thermal broadening in the Lyman α forest - II. Effects of the equation of state on transverse structure

We examine the impact of gas pressure on the transverse coherence of high-redshift (2 <= z <= 4) Lyman-alpha forest absorption along neighboring lines of sight that probe the gas Jeans scale (projected separation Delta r <= 500 kpc/h comoving; angular separation Delta theta <= 30"). We compare predictions from two smoothed particle hydrodynamics (SPH) simulations that have different photoionization heating rates and thus different temperature-density relations in the intergalactic medium (IGM). We also compare spectra computed from the gas distributions to those computed from the pressureless dark matter. The coherence along neighboring sightlines is markedly higher for the hotter, higher pressure simulation, and lower for the dark matter spectra. We quantify this coherence using the flux cross-correlation function and the conditional distribution of flux decrements as a function of transverse and line-of-sight (velocity) separation. Sightlines separated by Delta theta <= 15" are ideal for probing this transverse coherence. Higher pressure decreases the redshift-space anisotropy of the flux correlation function, while higher thermal broadening increases the anisotropy. In contrast to the longitudinal (line-of-sight) structure of the Lya forest, the transverse structure on these scales is dominated by pressure effects rather than thermal broadening. With the rapid recent growth in the number of known close quasar pairs, paired line-of-sight observations offer a promising new route to probe the IGM temperature-density relation and test the unexpectedly high temperatures that have been inferred from single sightline analyses.

A star-forming galaxy atz= 5.78 in theChandraDeep Field South

We report the discovery of a luminous z=5.78 star-forming galaxy in the Chandra Deep Field South. This galaxy was selected as an `i-drop' from the GOODS public survey imaging with HST/ACS (object 3 in Stanway, Bunker & McMahon 2003, astro-ph/0302212). The large colour of (i'-z')_AB=1.6 indicated a spectral break consistent with the Lyman-alpha forest absorption short-ward of Lyman-alpha at z~6. The galaxy is very compact (marginally resolved with ACS with a half-light radius of 0.08arcsec, so r_hl<0.5kpc/h_70). We have obtained a deep (5.5-hour) spectrum of this z'(AB)=24.7 galaxy with the DEIMOS optical spectrograph on Keck, and here we report the discovery of a single emission line centred on 8245Ang detected at 20sigma with a flux of f~2E-17 ergs/s/cm^2. The line is clearly resolved with detectable structure at our resolution of better than 55km/s, and the only plausible interpretation consistent with the ACS photometry is that we are seeing Lyman-alpha emission from a z=5.78 galaxy. This is the highest redshift galaxy to be discovered and studied using HST data. The velocity width Delta(v)_FWHM=260km/s and rest-frame equivalent width (W=20Ang) indicate that this line is most probably powered by star formation, as an AGN would typically have larger values. The starburst interpretation is supported by our non-detection of the high-ionization NV1240Ang emission line, and the absence of this source from the deep Chandra X-ray images. The star formation rate inferred from the rest-frame UV continuum is 34M_sun/yr/h^2_70 (Omega_M=0.3, Omega_Lambda=0.7). This is the most luminous starburst known at z>5. Our spectroscopic redshift for this object confirms the validity of the i'-drop technique of Stanway, Bunker & McMahon (2003) to select star-forming galaxies at z~6.

AAS Acknowledges Achievements in Astronomy

The American Astronomical Society recently announced the winners of several of its awards for 2001. These awards recognize contributions to astronomy and astrophysics. The Henry Norris Russell Lectureship, the society’s highest honor, will be presented to Wallace L. W. Sargent, Ira S. Bowen Professor of Astronomy at Caltech. AAS praises Sargent for being an “intellectual leader and educator” throughout his career and recognizes his contributions to astronomical spectroscopy, including his work in stellar spectroscopy of A-type stars that “led to the discovery of the helium-3 isotope in star 3 Centauri A.” He involved many of his students in his work in extragalactic spectroscopy, “which produced the first evidence for a black hole in galaxy M87 and culminated with his studies of quasar absorption lines.” His demonstration that the Lymanalpha forest absorption arises from intergalactic primordial gas clouds “provided a fundamental new probe of primeval gas in the early universe.”The society is presenting a new award this year. The inaugural AAS Education Prize, given in recognition of outstanding contributions to the education of the public, of students, and the next generation of professional astronomers, will go to Frank D. Drake for his “inspiration and leadership in many areas of education and public outreach in astronomy.” Drake’s “wide-ranging popularizations, his tireless help for journalists, and his championship of education and public information through the SETI Institute have helped scientists, educators, and the world at large to think rationally about life in the cosmos.” The citation also noted that his course work for and mentorship of students, and his development of the science of SETI (the search for extraterrestrial intelligence) through Project Ozma, the Drake Equation, and Project Phoenix, have “brought the excitement of the cosmic quest to several generations.” Drake is a professor emeritus of astronomy and astrophysics at the University of California, Santa Cruz, and distinguished scientist and chairman of the board of trustees of the SETI Institute. Bruce Elmgreen, a research staff member at IBM’s T. J. Watson Research Center in Yorktown Heights, New York, will receive the Dannie Heineman Prize for Astrophysics, given jointly by AAS and the American Institute of Physics. Elmgreen is being recognized for contributions that “span a remarkable range from theoretical studies of key processes in the interstellar medium to the physics of galaxy-wide starbursts, to investigations of dynamical features, including spiral arms and bars in galaxies.” The Newton Lacy Pierce Prize will be given this year to Kenneth Sembach, a research scientist at Johns Hopkins University. Sembach is being acknowledged for work “that has been important in increasing our understanding of the structure and elemental abundances of the gaseous component of the Galaxy, especially of the galactic halo, as well as in discovering new facets of the high-velocity cloud phenomenon in the galactic periphery.” Uros Seljak will receive the Helen B. Warner Prize for Astronomy for his “contributions to the theoretical understanding of the cosmic microwave background anisotropies and to the development of numerical and analytical tools that have been widely adopted for the comparison of observational data and cosmological models in that area.” Seljak is an assistant professor of physics at Princeton University. Michael J. Kurtz, an astronomer and computer scientist at the Harvard–Smithsonian Center for Astrophysics in Cambridge, Massachusetts, will be awarded the George Van Biesbroeck Prize. AAS acknowledges Kurtz’s contributions as “the visionary designer of the Astrophysics Data System, which clearly has revolutionized for over a decade the speed and thoroughness in which astronomers now can search and access the vast and still growing technical literature.”The Bruno Rossi Prize, given by AAS’s high-energy astrophysics division, will be shared by Andrew Fabian and Yasuo Tanaka for “their discovery, with the ASCA satellite [Advanced Satellite for Cosmology and Astrophysics], of broad iron K lines in active galactic nuclei, which demonstrate the effects of the strong gravitational field characteristic of black holes.” Fabian is a Royal Society Research Professor with the Institute of Astronomy at the University of Cambridge, UK. Tanaka is a professor emeritus of astronomy and astrophysics at the Institute of Space and Astronautical Science in Sagamihara, Japan. Sargent PPT|High resolution© 2001 American Institute of Physics.

Lyα absorption in the nearby Universe: the sightline to Q1821+643

We present the first results of a survey designed to understand the origin of the Lyman alpha-forest absorption systems at low redshift. Using the WYFFOS and HYDRA multi-fibre spectrographs on the WHT and WIYN telescopes, we have identified 51 galaxies brighter than b_j=18.5 within 30 arcmins of the sightline of the QSO 1821+643. We find three galaxies within 500 h-1 kpc of the QSO sightline; the nearest galaxy is 104 h-1 kpc away from the line of sight, and is at the same redshift as a strong Lyman alpha absorption line. The remaining two galaxies have no corresponding absorption to extremely low equivalent width limits (< 0.05 \AA). Beyond 500 h-1 kpc Lyman-alpha absorption lines are found at similar redshifts as several galaxies, but we show that these coincidences are likely to be accidental. Half of the Lyman alpha systems for which we could have found at least an L^* galaxy have no galaxies at the absorbers' redshifts. For the majority of the remainder, we show that any apparent association with galaxies is probably coincidental. These \lya systems are characterised by their weak equivalent widths (W_r < 0.2 \AA), and we conclude that this population of absorbers is uncorrelated, or at best, weakly correlated, with galaxies.

Spectroscopy of the double quasars Q1343+266A, B: A new determination of the size of Lyman-alpha forest absorbers

Spectroscopy of the double quasars Q1343+266A, B: A new determination of the size of Lyman-alpha forest absorbers