Lyman-alpha Flux Research Articles

Probing the first galaxies with the Square Kilometer Array

Observations of anisotropies in the brightness temperature of the 21 cm line of neutral hydrogen from the period before reionization would shed light on the dawn of the first stars and galaxies. In this paper, we use large-scale semi-numerical simulations to analyse the imprint on the 21 cm signal of spatial fluctuations in the Lyman-alpha flux arising from the clustering of the first galaxies. We show that an experiment such as the Square Kilometer Array (SKA) can probe this signal at the onset of reionization, giving us important information about the UV emission spectra of the first stars and characterizing their host galaxies. SKA-pathfinders with ~ 10% of the full collecting area should be capable of making a statistical detection of the 21 cm power spectrum at redshifts z < 20 (corresponding to frequencies $\nu$ > 67 MHz). We then show that the SKA should be able to measure the three dimensional power spectrum as a function of the angle with the line of sight and discuss the use of the redshift space distortions as a way to separate out the different components of the 21 cm power spectrum. We demonstrate that, at least on large scales where the Lyman-alpha fluctuations are linear, they can be used as a model independent way to extract the power spectra due to these Lyman-alpha fluctuations.

A CORRELATION BETWEEN STELLAR ACTIVITY AND HOT JUPITER EMISSION SPECTRA

We present evidence for a correlation between the observed properties of hot Jupiter emission spectra and the activity levels of the host stars measured using Ca II H & K emission lines. We find that planets with dayside emission spectra that are well-described by standard 1D atmosphere models with water in absorption (HD 189733, TrES-1, TrES-3, WASP-4) orbit chromospherically active stars, while planets with emission spectra that are consistent with the presence of a strong high-altitude temperature inversion and water in emission orbit quieter stars. We estimate that active G and K stars have Lyman alpha fluxes that are typically a factor of 4-7 times higher than quiet stars with analogous spectral types, and propose that the increased UV flux received by planets orbiting active stars destroys the compounds responsible for the formation of the observed temperature inversions. In this paper we also derive a model-independent method for differentiating between these two atmosphere types using the secondary eclipse depths measured in the 3.6 and 4.5 micron bands on the Spitzer Space Telescope, and argue that the observed correlation is independent of the inverted/non-inverted paradigm for classifying hot Jupiter atmospheres.

O2 density and temperature profiles retrieving from direct solar Lyman-alpha radiation measurements

The resonance transition 2P-2S of the atomic hydrogen (Lyman-alpha emission) is the strongest and most conspicuous feature in the solar EUV spectrum. The Lyman-alpha radiation transfer depends on the resonance scattering from the hydrogen atoms in the atmosphere and on the O2 absorption. Since the Lyman-alpha extinction in the atmosphere is a measure for the column density of the oxygen molecules, the atmospheric O2 density and temperature profiles can be calculated thereof. A detector of solar Lyman-alpha radiation was manufactured in the Stara Zagora Department of the Solar-Terrestrial Influences Laboratory (STIL). Its basic part is an ionization camera, filled in with NO. A 60 V power supply is applied to the chamber. The produced photoelectric current from the sensor is fed to a two-channel amplifier, providing analog signal. The characteristics of the Lyman-alpha detector were studied. It passed successfully all tests and the results showed that the so-designed instrument could be used in rocket experiments to measure the Lymanalpha flux. From the measurements of the detector, the Lyman-alpha vertical profile can be obtained. Programs are created to compute the O2 density, atmospheric power and temperature profiles based on Lymanalpha data. The detector design appertained to ASLAF project (Attenuation of the Solar Lyman-Alpha Flux), a scientific cooperation between STIL—Bul.Acad.Sci., Stara Zagora Department and the Atmospheric Physics Group at the Department of Meteorology (MISU), Stockholm University, Sweden. The joint project was part of the rocket experiment HotPay I, in the ALOMAR eARI Project, EU’s 6th Framework Programme, Andøya Rocket Range, Andenes, Norway. The project is partly financed by the Bulgarian Ministry of Science and Education.

Lenses in the Forest: Cross Correlation of the Lyman-αFlux with Cosmic Microwave Background Lensing

We present a theoretical estimate for a new observable: the cross correlation between the Lyman-alpha flux fluctuations in quasar spectra and the convergence of the cosmic microwave background as measured along the same line of sight. As a first step toward the assessment of its detectability, we estimate the signal-to-noise ratio using linear theory. Although the signal-to-noise is small for a single line of sight and peaks at somewhat smaller redshifts than those probed by the Lyman-alpha forest, we estimate a total signal-to-noise of 9 for cross correlating quasar spectra of SDSS-III with Planck and 20 for cross correlating with a future polarization based cosmic microwave background experiment. The detection of this effect would be a direct measure of the neutral hydrogen-matter cross correlation and could provide important information on the growth of structures at large scales in a redshift range which is still poorly probed.

The simulated 21 cm signal during the epoch of reionization: full modeling of the Ly-αpumping

The 21 cm emission of neutral hydrogen is the most promising probe of the epoch of reionization (EoR). In the next few years, the SKA pathfinders will provide statistical measurements of this signal, such as its power spectrum. Within one decade, SKA should produce a full tomography of the signal. Numerical simulations predicting these observations are necessary to optimize the design of the instruments and help in the interpretation of the data. Simulations are reaching a reasonable level in terms of scale range, but still often rely on simplifications to compute the strength of the signal. The main difficulty is the computation of the spin temperature of neutral hydrogen which depends on the gas kinetic temperature and on the level of the local Lyman-alpha flux (The Wouthuysen-Field effect). A T-S >> T-CMB assumption is usual. However, this assumption does not apply early in the reionization history, or even later in the history as long as the sources of X-rays are too weak to heat the intergalactic medium significantly. This work presents the first EoR numerical simulations including, beside dynamics and ionizing continuum radiative transfer, a self-consistent treatment of the Ly-alpha radiative transfer. This allows us to compute the spin temperature more accurately. We use two different box sizes, 20 h(-1) Mpc and 100 h(-1) Mpc, and a star source model. Using the redshift dependence of average quantities, maps, and power spectra, we quantify the effect of using different assumptions to compute the spin temperature and the influence of the box size. The first effect comes from allowing for a signal in absorption (i.e. not making the T-S >> T-CMB approximation). The magnitude of this effect depends on the amount of heating by hydrodynamic shocks and X-rays in the intergalactic medium (IGM). With our source model we have little heating so regions seen in absorption survive almost until the end of reionization. The second effects comes from using the real, local, Lyman-alpha flux. This effect is important for an average ionization fraction of less than similar to 10%: it changes the overall amplitude of the 21 cm signal, and adds its own fluctuations to the power spectrum.

Primordial non-Gaussianities in the intergalactic medium

We present results from the first high-resolution hydrodynamical simulations of non-Gaussian cosmological models. We focus on the statistical properties of the transmitted Lyman-alpha flux in the high redshift intergalactic medium. Imprints of non-Gaussianity are present and are larger at high redshifts. Differences larger than 20 % at z>3 in the flux probability distribution function for high transmissivity regions (voids) are expected for values of the non linearity parameter f_NL=\pm 100 when compared to a standard LCDM cosmology with f_NL=0. We investigate also the one-dimensional flux bispectrum: at the largest scales (corresponding to tens of Mpc) we expect deviations in the flux bispectrum up to 20% at z~4 (for f_NL=\pm 100), significantly larger than deviations of ~ 3% in the flux power spectrum. We briefly discuss possible systematic errors that can contaminate the signal. Although challenging, a detection of non-Gaussianities in the interesting regime of scales and redshifts probed by the Lyman-alpha forest, could be possible with future data sets.

The Flux Auto‐ and Cross‐Correlation of the Lyα Forest. II. Modeling Anisotropies with Cosmological Hydrodynamic Simulations

The isotropy of the Lyman-alpha forest in real-space uniquely provides a measurement of cosmic geometry at z > 2. The angular diameter distance for which the correlation function along the line of sight and in the transverse direction agree corresponds to the correct cosmological model. However, the Lyman-alpha forest is observed in redshift-space where distortions due to Hubble expansion, bulk flows, and thermal broadening introduce anisotropy. Similarly, a spectrograph's line spread function affects the autocorrelation and cross-correlation differently. In this the second paper of a series on using the Lyman-alpha forest observed in pairs of QSOs for a new application of the Alcock-Paczynski (AP) test, these anisotropies and related sources of potential systematic error are investigated with cosmological hydrodynamic simulations. Three prescriptions for galactic outflow were compared and found to have only a marginal effect on the Lyman-alpha flux correlation (which changed by at most 7% with use of the currently favored variable-momentum wind model vs. no winds at all). An approximate solution for obtaining the zero-lag cross-correlation corresponding to arbitrary spectral resolution directly from the zero-lag cross-correlation computed at full-resolution (good to within 2% at the scales of interest) is presented. Uncertainty in the observationally determined mean flux decrement of the Lyman-alpha forest was found to be the dominant source of systematic error; however, this is reduced significantly when considering correlation ratios. We describe a simple scheme for implementing our results, while mitigating systematic errors, in the context of a future application of the AP test.

How Cold Is Cold Dark Matter? Small-Scales Constraints from the Flux Power Spectrum of the High-Redshift Lyman-αForest

We present constraints on the mass of warm dark matter (WDM) particles derived from the Lyman-alpha flux power spectrum of 55 high-resolution HIRES spectra at 2.0<z<6.4. From the HIRES spectra, we obtain a lower limit of m(WDM) > or approximately 1.2 keV (2sigma) if the WDM consists of early decoupled thermal relics and m(WDM) > or approximately 5.6 keV (2sigma) for sterile neutrinos. Adding the Sloan Digital Sky Survey Lyman-alpha flux power spectrum, we get m(WDM) > or approximately 4 keV and m(WDM) > or approximately 28 keV (2sigma) for thermal relics and sterile neutrinos. These results improve previous constraints by a factor of 2.

Can Sterile Neutrinos Be Ruled Out as Warm Dark Matter Candidates?

We present constraints on the mass of warm dark matter (WDM) particles from a combined analysis of the matter power spectrum inferred from the Sloan Digital Sky Survey Lyman-alpha flux power spectrum at 2.2<z<4.2, cosmic microwave background data, and the galaxy power spectrum. We obtain a lower limit of mWDM greater than or similar to 10 keV (2sigma) if the WDM consists of sterile neutrinos and mWDM greater than or similar to 2 keV (2sigma) for early decoupled thermal relics. If we combine this bound with the constraint derived from x-ray flux observations of the Coma cluster, we find that the allowed sterile neutrino mass is approximately 10 keV (in the standard production scenario). Adding constraints based on x-ray fluxes from the Andromeda galaxy, we find that dark matter particles cannot be sterile neutrinos, unless they are produced by a nonstandard mechanism (resonant oscillations, coupling with the inflation) or get diluted by a large entropy release.

SOLARNET &amp; LAIME: Imaging &amp; Spectroscopy in the Far Ultraviolet

SOLARNET is a medium size high resolution solar physics mission proposed to CNES and ESA for a new start in 2007 and a possible launch in 2012 (CNES) or later (ESA Cosmic Vision framework: 2015–2016). Partnerships with India and China are under discussion, and several European contributions are considered. At the center of the SOLARNET mission is a 3-telescope interferometer of 1 meter baseline capable to provide 40 times the best ever spatial resolution achieved in Space with previous, current or even planned solar missions: 20 mas - 20 km on the Sun in the FUV. The interferometer is associated to an on-axis Subtractive Double Monochromator coupled to an Imaging Fourier Transform Spectrometer capable of high spectral (0.01 nm) and high temporal resolutions (50 ms) on a field of view of 40 arcsec and covering the FUV and UV spectral domains (from 117.5 to 400 nm). This will allow to access process scales of magnetic reconnection, dissipation, emerging flux and much more, from the chromosphere to the low corona with emphasis on the transition zone where the magnetic confinement is expected to be maximum. A whole new chapter of the physics of solar magnetic field structuring, evolution and mapping from the photosphere to the high atmosphere will be opened. The interferometer is completed by instruments providing larger field of view and higher temperature (EUV-XUV coronal imaging &amp; spectroscopy) to define the context and extension of the solar phenomena. The 3-telescope interferometer design results of an extensive laboratory demonstration program of interferometric imaging of extended objects. We will review the scientific program of SOLARNET, describe the interferometer concept and design, present the results of the breadboard and give a short overview of the mission aspects. In a different category, LAIME, the Lyman Alpha Imaging-Monitor Experiment, is a remarkably simple (no mechanisms) and compact full Sun imager to be flown with TESIS on the CORONAS-PHOTON mission in 2008. It could be the only chromospheric imager to be flown in the next years, supporting Solar-B, STEREO, SDO and the Belgian LYRA Lyman Alpha flux monitor. We will give a short description of this unique 60 mm aperture imaging telescope, dedicated to the investigation of the UV sources of solar variability and of the chromospheric and coronal disruptive events (Moreton waves, prominences, CMEs, etc.).

A new relation between the central spectral solar H I Lyman α irradiance and the line irradiance measured by SUMER/SOHO during the cycle 23

The spectral irradiance at the center of the solar H I Lyman α ( λ 0 = 121.5664 nm , referred to as Ly α in this paper) line profile is the main excitation source responsible for the atomic hydrogen resonant scattering of cool material in our Solar System. It has therefore to be known with the best possible accuracy in order to model the various Ly α emissions taking place in planetary, cometary, and interplanetary environments. Since the only permanently monitored solar irradiance is the total one (i.e. integrated over the whole Ly α line profile), Vidal-Madjar [1975. Evolution of the solar Lyman alpha flux during four consecutive years. Solar Phys. 40, 69–86] using Orbiting Solar Observatory 5 (OSO-5) satellite Ly α data, established a semi-empirical formula allowing him to deduce the central spectral Ly α irradiance from the total one. This relation has been extensively used for three decades. But, at the low altitude of the OSO-5 orbit, the central part of the solar line profile was deeply absorbed by a large column of exospheric atomic hydrogen. Consequently, the spectral irradiance at the center of the line was obtained by a complex procedure confronting the observations with simulations of both the geocoronal absorption and the self-reversed shape of the solar Ly α profile. The SUMER spectrometer onboard SOHO positioned well outside the hydrogen geocorona, provided full-Sun Ly α profiles, not affected by such an absorption [Lemaire et al., 1998. Solar H I Lyman α full disk profile obtained with the SUMER/SOHO spectrometer. Astron. Astrophys. 334, 1095–1098; 2002. Variation of the full Sun Hydrogen Lyman α and β profiles with the activity cycle. Proc. SOHO 11 Symposium, ESA SP-508, 219–222; 2004. Variation of the full Sun Hydrogen Lyman profiles through solar cycle 23. COSPAR 2004 Meeting], making it—for the first time—possible to measure the spectral and total Ly α solar irradiances directly and simultaneously. A new relation between these two quantities is derived in an expression that is formally similar to the previous one, but with significantly different parameters. After having discussed the potential causes for such differences, it is suggested that the new relation should replace the old one for any future modeling of the numerous Ly α absorptions and emissions observed in the Solar System.

Noctilucent clouds as possible indicators of global change in the mesosphere

Noctilucent clouds (NLC) are ice clouds near the high-latitude mesopause region, occurring in summer. These clouds have been implicated as possible harbingers of permanent changes in the upper atmosphere. Their year-to-year variations have been characterized by a semi-quantitative index, the number of nights per season in which they were reported. In this paper we compare early studies of NLC year-to-year variability with data from the modern era, including observations from Europe, the USSR and North America. This review focuses on the possible influence on NLC of water vapor variability. We compare the noctilucent cloud time series, which indicate a strong upward trend in the 1964–1986 period, with changes expected of atmospheric water vapor at the mean height of the clouds (83 km). At this height, both methane-induced changes and 11-year solar-UV induced changes are expected to be the main forcings. Using the data available for surface methane and for solar Lyman-alpha fluxes, we estimate the water vapor changes due to methane oxidation, and to Lyman-α induced photodissociation of water vapor. For the periodic (10-year) component, the NLC time series was found to significantly correlate with Lyman-α flux data, for nearly all available multi-decadal NLC data sets. As first shown by Gadsden for the European data, the correlations are highest when the time lag of NLC following solar cycle minimum is two to three years. This result places into considerable doubt the hypothesis of direct solar Lyman-α control of NLC. Volcanism appears to have had a negligible influence, with the possible exception of the 1883 Krakatoa eruption.

Galileo Ultraviolet Spectrometer Experiment: Initial Venus and Interplanetary Cruise Results

The Galileo Extreme Ultraviolet Spectrometer obtained a spectrum of Venus atmospheric emissions in the 55.0- to 125.0-nanometer (nm) wavelength region. Emissions of helium (58.4 nm), ionized atomic oxygen (83.4 nm), and atomic hydrogen (121.6 nm), as well as a blended spectral feature of atomic hydrogen (Lyman-beta) and atomic oxygen (102.5 nm), were observed at 3.5-nm resolution. During the Galileo spacecraft cruise from Venus to Earth, Lyman-alpha emission from solar system atomic hydrogen (121.6 nm) was measured. The dominant source of the Lyman-alpha emission is atomic hydrogen from the interstellar medium. A model of Galileo observations at solar maximum indicates a decrease in the solar Lyman-alpha flux near the solar poles. A strong day-to-day variation also occurs with the 27-day periodicity of the rotation of the sun.

Stellar Lyman-alpha emission and the local interstellar medium

Under the auspices of this ADP program, a systematic study was made of IUE archival images in order to extract spectra of the Lyman alpha region and to measure the stellar Lyman alpha flux for as many late-type stars as possible. The Lyman alpha resonance line is a powerful cooling channel for the hot chromospheres of solar-type stars, but has not been studied before in any systematic fashion across the H-R diagram. A major deterrent which has limited the use of Lyman alpha in the study of stellar chromospheres is the contamination of this spectral feature caused by the scattering of solar Lyman alpha photons in the Earth's exosphere. This scattered light is monochromatically imaged through the entrance slot of the IUE telescope and is superposed onto the stellar spectrum. In all but the shortest exposures with IUE, this 'geocoronal emission' overwhelms the stellar flux and makes it impossible to directly measure the strength of the stellar chromospheric feature. The IUESIPS processing contains no provision for correcting standard G.O. output products for this contamination. The first task was to develop a scheme for removing the geocoronal flux, specifically from low-dispersion spectra taken with the Short-Wavelength Camera of IUE. The strategy adopted was to fit a 'sky model' to the spatially-resolved geocoronal emission observed through the large science aperture of the telescope, using the spectral orders on either side of the central ones where the stellar emission is concentrated. The model emission was then subtracted from the observed image, leaving behind the corrected stellar Lyman alpha emission. The details of this fitting procedure are described. Having devised a successful method for removing the unwanted geocoronal emission, the correction procedure was applied to 366 archival images which, from inspection of the photowrites in the IUE browse file, seemed especially promising. In this survey, Lyman alpha emission were eventually detected in the spectra of 227 stars representing a wide range in age, temperature, and luminosity throughout the cool half of the H-R diagram. Previously fewer than 30 such stars had been measured, and an order of magnitude increase in the numbers of stars having Lyman alpha flux measurements is provided. Multiple measurements were made for 52 stars and upper limits on chromospheric flux were derived for another 48 stars.

Comparison of 10.7 cm radio flux with SME solar Lyman alpha flux

Measurements of the solar Lyman alpha flux that were made over a seven‐and‐one‐half‐year period between October 11, 1981 and April 13, 1989 have been compared with ground‐based measurements of the solar 10.7 cm radio flux made over the same time period. There is a long‐term correlation between these two measures of solar flux during the declining part of the solar cycle. During the solar minimum period, there is only a poor correlation between the two solar fluxes because the 10.7 cm radio flux reaches a minimum of 65 × 10−22 W m−2 Hz−1 and does not vary below this value while the Lyman alpha flux continues to decline and show long‐term and short‐term variations. During the early ascending phase of the new solar cycle, there is again a correlation between the two fluxes, although the constant of proportionality between the two is different from the constant during the declining phase of the previous solar cycle. Somewhat later, during the period November 25, 1988 — April 13, 1989 (last period when observations of Lyman alpha were made) a medium‐term correlation exists and the proportionality of the two indices is once again similar to what it was during the declining phase of the previous solar cycle. A study of the correlation of the 10.7 cm flux with the Lyman alpha for a 999‐day period during the declining phase showed that for the short‐term (27‐day) variation there is a correlation between the two fluxes but the proportionality between them varies from one solar rotation to the next. The conclusion is that the solar 10.7 cm radio flux is not a useful index for the prediction of solar Lyman alpha flux for the short‐term, 27‐day variations.

Estimation of the solar Lyman alpha flux from ground based measurements of the Ca II K line

Measurements of the solar Lyman alpha and CaII K from October 1981 to April 1989 show a strong correlation (r=.95) that allows estimation of the Lyman alpha flux at 1 AU from 1975 to December 1989. Our estimated Lyman alpha strength of 3.9 × 1011±.15 × 1011 photons/s cm2 on 7 December 1989 is at the same maximum levels seen in Cycle 21. Relative to other UV surrogates (sunspot number, 10.7 cm radio flux, and HeI 10830 line strength), Lyman alpha estimates computed from the K line track the SME measurements well from solar maximum, through solar minimum, and into Cycle 22.

Lyman-alpha emission in star-forming galaxies

IUE observations of five blue, low-metallicity, star-forming galaxies sufficiently redshifted to permit detection of Lyman-alpha are reported. The galaxies with metallicities 0.1 time solar or more have weak or absent Lyman-alpha emission. There is evidence for increasing Lyman-alpha emission with decreasing metallicity. The reduction of Lyman-alpha fluxes from recombination values is attributed to absorption of multiply scattered Lyman-alpha by dust.

Temporal variation of the Jovian H I Lyman alpha emission (1979–1986)

Observations monitoring the long‐term spatial and temporal behavior of the Jovian H I Ly α emission have been made with the International Ultraviolet Explorer (IUE) observatory beginning in December 1978, and extending through December 1986. The hydrogen bulge region centered near λIII = 100° has been observed to be a persistent feature of the Jovian upper atmosphere throughout the period of the observations, which includes the maximum of solar cycle 21 and the current minimum in solar activity. New and previously unpublished measurements made since early 1982 show that the average brightness of both the bulge and nonbulge regions has varied with the long‐term solar Ly α output, which has decreased by a factor of ∼ 2 in this time frame. However, the bulge is ∼ 25% brighter on average and exhibits more short‐term variability than the nonbulge region. The factor of ∼ 2 change in Jupiter's Ly α brightness during the descending phase of solar cycle 21 is significantly less than the order of magnitude variability reported for the ascending phase of the same cycle. The IUE results are consistent with pure resonant scattering from an H column of ∼1017 cm−2 and indicate that no observable changes have taken place in the Jovian atomic hydrogen abundance or the average atmospheric conditions during the 8‐year period since December 1978. If the electroglow process recently proposed for the outer planets plays a significant role in the long‐term behavior of the Jovian Ly α emission, the electroglow must also follow the solar cycle at Ly α. Finally, as a result of revisions in both the IUE instrumental sensitivity at Ly α and the dimensions of the spectrograph large aperture, there is no significant difference between Voyager and IUE measurements of the Jovian Ly α brightness made within the same time frame and with approximately the same field of view.

Mesospheric ozone changes associated with 27 day solar ultraviolet flux variations

Solar ultraviolet flux changes associated with the 27‐day solar rotational period cause corresponding variations in mesospheric ozone near the maximum of the 11‐year sunspot cycle. This statement is based on a correlation and spectral analysis of ozone mixing ratios, deduced from Solar Mesospheric Explorer satellite‐based measurements of 1.27‐µm O2 airglow emission and solar flux observations made from the same spacecraft in 1982. With the Lyman α flux taken as an indicator of solar ultraviolet variability, spectral analysis shows a primary period of 27.1 days with a secondary period of 13.5 days. The 27.1‐ day period is observed in the ozone mixing ratio data together with other periods, including 13.5 days. Both a classical statistical analysis and a time series treatment show that for 244 days there is a correlation between ozone and solar flux near 50 km and between 65 and 70 km. Calculations predict a positive correlation over the entire mesosphere if there is no change in temperature accompanying the solar flux. Lack of correlation is temperature induced.

Lyman-alpha emission from low dispersion IUE spectra

In spite of its importance as cooling mechanism of the chromosphere of late type stars, the hydrogen Lyman-alpha line has been neglected in most IUE programmes The high geocoronal background and the saturation often seen in the stellar emission peak are the major problems that prevent accurate determination of the Lyman alpha flux. We have developed a technique that allows us to overcome both of these problems by fitting an ellipsoidal surface to the geocoronal background and gaussian profiles to the stellar signal. Results on a limited sample of stars are presented.