A first direct measurement of the intergalactic medium temperature around a quasar at z=6

The thermal state of the intergalactic medium (IGM) provides an indirect probe of both the HI and HeII reionisation epochs. Current constraints on the IGM temperature from the Lya forest are restricted to the redshift range 2<z<4.5, limiting the ability to probe the thermal memory of HI reionisation toward higher redshift. In this work, we present the first direct measurement of the IGM temperature around a z=6 quasar by analysing the Doppler widths of Lya absorption lines in the proximity zone of SDSS J0818+1722. We use a high resolution (R= 40000) Keck/HIRES spectrum in combination with detailed numerical modelling to obtain the temperature at mean density, T_0=23600\pm^5000_6900K (\pm^9200_9300K) at 68 (95) per cent confidence assuming a prior probability 13500K<T_0<38500 K following HI and HeII reionisation. This enables us to place an upper limit on the redshift of HI reionisation, z_H, within 33 comoving Mpc of SDSS J0818+1722. If the quasar reionises the HeII in its vicinity, then in the limit of instantaneous reionisation we infer z_H<9.0 (11.0) at 68 (95) per cent confidence assuming photoheating is the dominant heat source and that HI reionisation is driven by ionising sources with soft spectra, typical of population II stars. If the HI and HeII in the IGM around SDSS J0818+1722 are instead reionised simultaneously by a population of massive metal-free stars, characterised by very hard ionising spectra, we obtain a tighter upper limit of z_H<8.4 (9.4). Initiating reionisation at higher redshifts produces temperatures which are too low with respect to our constraint unless the HI ionising sources or the quasar itself have spectra significantly harder than typically assumed.Comment: 15 pages, 9 figures, accepted to MNRA

Observations of Chemically Enriched QSO Absorbers near z ~ 2.3 Galaxies: Galaxy-Formation Feedback Signatures in the IGM

We present a study of galaxies and intergalactic gas toward the z=2.73 quasar HS1700+6416, to explore the effects of galaxy formation feedback on the IGM. Our observations and ionization simulations indicate that the volume within 100-200 h_71^{-1} physical kpc of high-redshift galaxies contains very small, dense, and metal-rich absorption-line regions. These systems often contain shock-heated gas seen in OVI, and may exhibit [Si/C] abundance enhancements suggestive of Type II supernova enrichment. We argue that the absorbers resemble thin sheets or bubbles, whose physical properties can be explained with a simple model of radiatively efficient shocks propegating through the IGM. Their high metallicities suggest that these shocks are being expelled from--rather than falling into--star forming galaxies. There is a dropoff in the IGM gas density at galaxy impact parameters beyond ~300 physical kpc that may trace boundaries of gas structures where the galaxies reside. The local heavy-element enhancement covers 100-200 kpc; beyond this the observed abundances blend into the general IGM. Supernova-driven winds or dynamical stripping of interstellar gas appears to affect the IGM near massive galaxies, even at R>~100 kpc. However, these feedback systems represent only a few percent of the Lya forest mass at z~2.5. Their mass could be larger if the more numerous metal-poor CIV systems at >~200 kpc are tepid remnants of very powerful winds. Based on present observations it is not clear that this scenario is to be favored over one involving pre-enrichment by smaller galaxies at z>~6.Comment: Accepted for publication in the Astrophysical Journal. 26 pages emulateapj, incl. 5 pages tables, 15 figure

Spectral mapping of brain functional connectivity from diffusion imaging.

Understanding the relationship between the dynamics of neural processes and the anatomical substrate of the brain is a central question in neuroscience. On the one hand, modern neuroimaging technologies, such as diffusion tensor imaging, can be used to construct structural graphs representing the architecture of white matter streamlines linking cortical and subcortical structures. On the other hand, temporal patterns of neural activity can be used to construct functional graphs representing temporal correlations between brain regions. Although some studies provide evidence that whole-brain functional connectivity is shaped by the underlying anatomy, the observed relationship between function and structure is weak, and the rules by which anatomy constrains brain dynamics remain elusive. In this article, we introduce a methodology to map the functional connectivity of a subject at rest from his or her structural graph. Using our methodology, we are able to systematically account for the role of structural walks in the formation of functional correlations. Furthermore, in our empirical evaluations, we observe that the eigenmodes of the mapped functional connectivity are associated with activity patterns associated with different cognitive systems

Thermal X-rays from Millisecond Pulsars: Constraining the Fundamental Properties of Neutron Stars

Abridged) We model the X-ray properties of millisecond pulsars (MSPs) by considering hot spot emission from a weakly magnetized rotating neutron star (NS) covered by an optically-thick hydrogen atmosphere. We investigate the limitations of using the thermal X-ray pulse profiles of MSPs to constrain the mass-to-radius ($M/R$) ratio of the underlying NS. The accuracy is strongly dependent on the viewing angle and magnetic inclination. For certain systems, the accuracy is ultimately limited only by photon statistics implying that future X-ray observatories could, in principle, achieve constraints on $M/R$ and hence the NS equation of state to better than $\sim$5%. We demonstrate that valuable information regarding the basic properties of the NS can be extracted even from X-ray data of fairly limited photon statistics through modeling of archival spectroscopic and timing observations of the nearby isolated PSRs J0030+0451 and J2124--3358. The X-ray emission from these pulsars is consistent with the presence of a hydrogen atmosphere and a dipolar magnetic field configuration, in agreement with previous findings for PSR J0437--4715. For both MSPs, the favorable geometry allows us to place interesting limits on the allowed $M/R$ of NSs. Assuming 1.4 M$_{\odot}$, the stellar radius is constrained to be $R > 9.4$ km and $R > 7.8$ km (68% confidence) for PSRs J0030+0451 and J2124--3358, respectively. We explore the prospects of using future observatories such as \textit{Constellation-X} and \textit{XEUS} to conduct blind X-ray timing searches for MSPs not detectable at radio wavelengths due to unfavorable viewing geometry. Using the observational constraints on the pulsar obliquities we are also able to place strong constraints on the magnetic field evolution model proposed by Ruderman.Comment: 9 pages, 7 figures, published in the Astrophysical Journal (Volume 689, Issue 1, pp. 407-415

A Refined Measurement of the Mean Transmitted Flux in the Ly-alpha Forest over 2 < z < 5 Using Composite Quasar Spectra

We present new measurements of the mean transmitted flux in the Ly-alpha forest over 2 < z < 5 made using 6065 quasar spectra from the Sloan Digital Sky Survey DR7. We exploit the general lack of evolution in the mean quasar continuum to avoid the bias introduced by continuum fitting over the Ly-alpha forest at high redshifts, which has been the primary systematic uncertainty in previous measurements of the mean Ly-alpha transmission. The individual spectra are first combined into twenty-six composites with mean redshifts spanning 2.25 < z_comp < 5.08. The flux ratios of separate composites at the same rest wavelength are then used, without continuum fitting, to infer the mean transmitted flux, F(z), as a fraction of its value at z~2. Absolute values for F(z) are found by scaling our relative values to measurements made from high-resolution data by Faucher-Giguere et al. (2008) at z < 2.5, where continuum uncertainties are minimal. We find that F(z) evolves smoothly with redshift, with no evidence of a previously reported feature at z~3.2. This trend is consistent with a gradual evolution of the ionization and thermal state of the intergalactic medium over 2 < z < 5. Our results generally agree with the most careful measurements to date made from high-resolution data, but offer much greater precision and extend to higher redshifts. This work also improves upon previous efforts using SDSS spectra by significantly reducing the level of systematic error.Comment: 14 pages, 11 figures, MNRAS, in press. Supplementary materials may be downloaded from http://www.ast.cam.ac.uk/~gdb/mean_flu

Testing metallicity indicators at z~1.4 with the gravitationally lensed galaxy CASSOWARY 20

We present X-shooter observations of CASSOWARY 20 (CSWA 20), a star-forming (SFR ~6 Msol/yr) galaxy at z=1.433, magnified by a factor of 11.5 by the gravitational lensing produced by a massive foreground galaxy at z=0.741. We analysed the integrated physical properties of the HII regions of CSWA 20 using temperature- and density-sensitive emission lines. We find the abundance of oxygen to be ~1/7 of solar, while carbon is ~50 times less abundant than in the Sun. The unusually low C/O ratio may be an indication of a particularly rapid timescale of chemical enrichment. The wide wavelength coverage of X-shooter gives us access to five different methods for determining the metallicity of CSWA 20, three based on emission lines from HII regions and two on absorption features formed in the atmospheres of massive stars. All five estimates are in agreement, within the factor of ~2 uncertainty of each method. The interstellar medium of CSWA 20 only partially covers the star-forming region as viewed from our direction; in particular, absorption lines from neutrals and first ions are exceptionally weak. We find evidence for large-scale outflows of the interstellar medium (ISM) with speeds of up 750 km/s, similar to the values measured in other high-z galaxies sustaining much higher rates of star formation.Comment: 18 pages, 11 figures, accepted for publication in MNRA

Handling qualities of a wide-body transport airplane utilizing Pitch Active Control Systems (PACS) for relaxed static stability application

Piloted simulation studies have been conducted to evaluate the effectiveness of two pitch active control systems (PACS) on the flying qualities of a wide-body transport airplane when operating at negative static margins. These two pitch active control systems consisted of a simple 'near-term' PACS and a more complex 'advanced' PACS. Eight different flight conditions, representing the entire flight envelope, were evaluated with emphasis on the cruise flight conditions. These studies were made utilizing the Langley Visual/Motion Simulator (VMS) which has six degrees of freedom. The simulation tests indicated that (1) the flying qualities of the baseline aircraft (PACS off) for the cruise and other high-speed flight conditions were unacceptable at center-of-gravity positions aft of the neutral static stability point; (2) within the linear static stability flight envelope, the near-term PACS provided acceptable flying qualities for static stabilty margins to -3 percent; and (3) with the advanced PACS operative, the flying qualities were demonstrated to be good (satisfactory to very acceptable) for static stabilty margins to -20 percent