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  • 1
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    The structure function of Galactic H I opacity fluctuations on au scales based on MERLIN, VLA and VLBA data (2014)
    Oxford University Press
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    Publication Date: 2014-06-10
    Description: We use MERLIN, VLA and VLBA observations of Galactic H  i absorption towards 3C 138 to estimate the structure function of the H  i opacity fluctuations at au scales. Using Monte Carlo simulations, we show that there is likely to be a significant bias in the estimated structure function at signal-to-noise ratios characteristic of our observations, if the structure function is constructed in the manner most commonly used in the literature. We develop a new estimator that is free from this bias and use it to estimate the true underlying structure function slope on length-scales ranging from 5 to 40 au. From a power-law fit to the structure function, we derive a slope of $0.81^{+0.14}_{-0.13}$ , i.e. similar to the value observed at parsec scales. The estimated upper limit for the amplitude of the structure function is also consistent with the measurements carried out at parsec scales. Our measurements are hence consistent with the H  i opacity fluctuation in the Galaxy being characterized by a power-law structure function over length-scales that span six orders of magnitude. This result implies that the dissipation scale has to be smaller than a few au if the fluctuations are produced by turbulence. This inferred smaller dissipation scale implies that the dissipation occurs either in (i) regions with densities 10 3 cm – 3 (i.e. similar to that inferred for ‘tiny scale’ atomic clouds or (ii) regions with a mix of ionized and atomic gas (i.e. the observed structure in the atomic gas has a magnetohydrodynamic origin).
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 2
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    Accurate measurement of the H I column density from H I 21 cm absorption-emission spectroscopy (2013)
    Oxford University Press
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    Publication Date: 2013-06-18
    Description: We present a detailed study of an estimator of the H i column density, based on a combination of H i 21 cm absorption and H i 21cm emission spectroscopy. This ‘isothermal’ estimate is given by N H I , ISO = 1.823 x 10 18 [ tot x T B / [ 1 – e – tot ]dV, where tot is the total H i 21cm optical depth along the sightline and T B is the measured brightness temperature. We have used a Monte Carlo simulation to quantify the accuracy of the isothermal estimate by comparing the derived N H I , ISO with the true H i column density N H I . The simulation was carried out for a wide range of sightlines, including gas in different temperature phases and random locations along the path. We find that the results are statistically insensitive to the assumed gas temperature distribution and the positions of different phases along the line of sight. The median value of the ratio of the true H i column density to the isothermal estimate, N H I / N H I , ISO , is within a factor of 2 of unity while the 68.2 per cent confidence intervals are within a factor of 3 of unity, out to high H i column densities, ≤5  x 10 23 cm –2 per 1 km s –1 channel, and high total optical depths, ≤1000. The isothermal estimator thus provides a significantly better measure of the H i column density than other methods, within a factor of a few of the true value even at the highest columns, and should allow us to directly probe the existence of high H i column density gas in the Milky Way.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 3
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    Bioenergetic flux, mitochondrial mass and mitochondrial morphology dynamics in AD and MCI cybrid cell lines (2013)
    Oxford University Press
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    Publication Date: 2013-09-08
    Description: Bioenergetic dysfunction occurs in Alzheimer's disease (AD) and mild cognitive impairment (MCI), a clinical syndrome that frequently precedes symptomatic AD. In this study, we modeled AD and MCI bioenergetic dysfunction by transferring mitochondria from MCI, AD and control subject platelets to mtDNA-depleted SH-SY5Y cells. Bioenergetic fluxes and bioenergetics-related infrastructures were characterized in the resulting cytoplasmic hybrid (cybrid) cell lines. Relative to control cybrids, AD and MCI cybrids showed changes in oxygen consumption, respiratory coupling and glucose utilization. AD and MCI cybrids had higher ADP/ATP and lower NAD + /NADH ratios. AD and MCI cybrids exhibited differences in proteins that monitor, respond to or regulate cell bioenergetic fluxes including HIF1α, PGC1α, SIRT1, AMPK, p38 MAPK and mTOR. Several endpoints suggested mitochondrial mass increased in the AD cybrid group and probably to a lesser extent in the MCI cybrid group, and that the mitochondrial fission–fusion balance shifted towards increased fission in the AD and MCI cybrids. As many of the changes we observed in AD and MCI cybrid models are also seen in AD subject brains, we conclude reduced bioenergetic function is present during very early AD, is not brain-limited and induces protean retrograde responses that likely have both adaptive and mal-adaptive consequences.
    Print ISSN: 0964-6906
    Electronic ISSN: 1460-2083
    Topics: Biology , Medicine
    Published by Oxford University Press
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  • 4
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    Towards a census of supercompact massive galaxies in the Kilo Degree Survey (2016)
    Oxford University Press
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    Publication Date: 2016-02-20
    Description: The abundance of compact, massive, early-type galaxies (ETGs) provides important constraints to galaxy formation scenarios. Thanks to the area covered, depth, excellent spatial resolution and seeing, the ESO Public optical Kilo Degree Survey (KiDS), carried out with the VLT Survey Telescope, offers a unique opportunity to conduct a complete census of the most compact galaxies in the Universe. This paper presents a first census of such systems from the first 156 deg 2 of KiDS. Our analysis relies on g -, r - and i -band effective radii ( R e ), derived by fitting galaxy images with point spread function (PSF)-convolved Sérsic models, high-quality photometric redshifts, z phot , estimated from machine learning techniques, and stellar masses, M * , calculated from KiDS aperture photometry. After massiveness ( ${M_{\star }}\gtrsim 8 \times 10^{10}\, \rm {\text{M}_{\odot }}$ ) and compactness ( ${R_{\rm e}}\lesssim 1.5 \, \rm kpc$ in g , r and i bands) criteria are applied, a visual inspection of the candidates plus near-infrared photometry from VIKING-DR1 are used to refine our sample. The final catalogue, to be spectroscopically confirmed, consists of 92 systems in the redshift range z ~ 0.2–0.7. This sample, which we expect to increase by a factor of 10 over the total survey area, represents the first attempt to select massive supercompact ETGs ( MSCGs ) in KiDS. We investigate the impact of redshift systematics in the selection, finding that this seems to be a major source of contamination in our sample. A preliminary analysis shows that MSCGs exhibit negative internal colour gradients, consistent with a passive evolution of these systems. We find that the number density of MSCGs is only mildly consistent with predictions from simulations at z  〉 0.2, while no such system is found at z  〈 0.2.
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    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    Insights into the evolution of symbiotic recurrent novae from radio synchrotron emission: V745 Scorpii and RS Ophiuchi (2015)
    Oxford University Press
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    Publication Date: 2015-11-28
    Description: We present observations at 610 and 235 MHz using the Giant Metrewave Radio Telescope (GMRT) of the recurrent nova V745 Scorpii which recorded its last outburst on 2014 February 6. This is the second symbiotic recurrent nova whose light curve at low frequencies has been followed in detail, the first being RS Ophiuchi in 2006. We fitted the 610 MHz light curve by a model of synchrotron emission from an expanding shell being modified by radiative transfer effects due to local absorbing gas consisting of a uniformly distributed and a clumpy component. Using our model parameters, we find that the emission at 235 MHz peaked around day 35 which is consistent with our GMRT observations. The two main results of our study are (1) The radio emission at a given frequency is visible sooner after the outburst in successive outbursts of both V745 Scorpii and RS Ophiuchi. The earlier detection of radio emission is interpreted to be caused by decreasing foreground densities. (2) The clumpy material, if exists, is close to the white dwarf and can be interpreted as being due to the material from the hot accretion disc. The uniform density gas is widespread and attributed to the winds blown by the white dwarf. We present implications of these results on the evolution of both novae. Such studies along with theoretical understanding have the potential of resolving several outstanding issues such as why all recurrent novae are not detectable in synchrotron radio and whether recurrent novae are progenitor systems of Type 1a supernova.
    Print ISSN: 1745-3925
    Electronic ISSN: 1745-3933
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    Oxaloacetate activates brain mitochondrial biogenesis, enhances the insulin pathway, reduces inflammation and stimulates neurogenesis (2014)
    Oxford University Press
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    Publication Date: 2014-11-21
    Description: Brain bioenergetic function declines in some neurodegenerative diseases, this may influence other pathologies and administering bioenergetic intermediates could have therapeutic value. To test how one intermediate, oxaloacetate (OAA) affects brain bioenergetics, insulin signaling, inflammation and neurogenesis, we administered intraperitoneal OAA, 1–2 g/kg once per day for 1–2 weeks, to C57Bl/6 mice. OAA altered levels, distributions or post-translational modifications of mRNA and proteins (proliferator-activated receptor-gamma coactivator 1α, PGC1 related co-activator, nuclear respiratory factor 1, transcription factor A of the mitochondria, cytochrome oxidase subunit 4 isoform 1, cAMP-response element binding, p38 MAPK and adenosine monophosphate-activated protein kinase) in ways that should promote mitochondrial biogenesis. OAA increased Akt, mammalian target of rapamycin and P70S6K phosphorylation. OAA lowered nuclear factor B nucleus-to-cytoplasm ratios and CCL11 mRNA. Hippocampal vascular endothelial growth factor mRNA, doublecortin mRNA, doublecortin protein, doublecortin-positive neuron counts and neurite length increased in OAA-treated mice. 1 H-MRS showed OAA increased brain lactate, GABA and glutathione thereby demonstrating metabolic changes are detectable in vivo . In mice, OAA promotes brain mitochondrial biogenesis, activates the insulin signaling pathway, reduces neuroinflammation and activates hippocampal neurogenesis.
    Print ISSN: 0964-6906
    Electronic ISSN: 1460-2083
    Topics: Biology , Medicine
    Published by Oxford University Press
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  • 7
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    Tapering the sky response for angular power spectrum estimation from low-frequency radio-interferometric data (2016)
    Oxford University Press
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    Publication Date: 2016-04-14
    Description: It is important to correctly subtract point sources from radio-interferometric data in order to measure the power spectrum of diffuse radiation like the Galactic synchrotron or the Epoch of Reionization 21-cm signal. It is computationally very expensive and challenging to image a very large area and accurately subtract all the point sources from the image. The problem is particularly severe at the sidelobes and the outer parts of the main lobe where the antenna response is highly frequency dependent and the calibration also differs from that of the phase centre. Here, we show that it is possible to overcome this problem by tapering the sky response. Using simulated 150 MHz observations, we demonstrate that it is possible to suppress the contribution due to point sources from the outer parts by using the Tapered Gridded Estimator to measure the angular power spectrum C of the sky signal. We also show from the simulation that this method can self-consistently compute the noise bias and accurately subtract it to provide an unbiased estimation of C .
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 8
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    The temperature of the diffuse H I in the Milky Way - I. High resolution H I-21 cm absorption studies (2013)
    Oxford University Press
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    Publication Date: 2013-11-17
    Description: We have carried out deep, high velocity resolution, interferometric Galactic H i -21 cm absorption spectroscopy towards 32 compact extragalactic radio sources with the Giant Metrewave Radio Telescope (GMRT) and the Westerbork Synthesis Radio Telescope (WSRT). The optical depth spectra for most sources have root-mean-square noise values 10 –3 per 1 km s –1 velocity channel and are thus sufficiently sensitive to detect absorption by warm neutral hydrogen with H i column densities N H i 10 20 cm –2 , spin temperatures T s  ≤ 5000 K and line widths equal to the thermal width (20 km s –1 ). H i -21 cm absorption was detected against all background sources but one, B0438–436. The spectra of sources observed separately with GMRT and WSRT show excellent agreement, indicating that spectral baseline problems and contamination from H i -21 cm emission are negligible. This paper presents the absorption spectra, the emission spectra along neighbouring sightlines from the Leiden–Argentine–Bonn survey and the derived spin temperature spectra. On every sightline, the maximum spin temperature detected (at ≥3 significance) even at a velocity resolution of 1 km s –1 is 1000 K, indicating that we are detecting the warm neutral medium along most sightlines. This is by far the largest sample of Galactic H i -21 cm absorption spectra of this quality, providing a sensitive probe of physical conditions in the neutral atomic interstellar medium.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 9
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    The temperature of the diffuse H I in the Milky Way - II. Gaussian decomposition of the H I-21 cm absorption spectra (2013)
    Oxford University Press
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    Publication Date: 2013-11-17
    Description: We discuss physical conditions in Galactic neutral hydrogen based on deep, high-velocity resolution interferometric H i -21 cm absorption spectroscopy towards 33 compact extragalactic radio sources. The H i -21 cm optical depth spectra have root-mean-square noise values 10 –3 per 1 km s –1 velocity channel, i.e. sufficiently sensitive to detect H i -21 cm absorption by the warm neutral medium (WNM). Comparing these spectra with H i -21 cm emission spectra from the Leiden–Argentine–Bonn (LAB) survey, we show that some of the absorption detected on most sightlines must arise in gas with temperatures higher than that in the stable cold neutral medium (CNM). A multi-Gaussian decomposition of 30 of the H i -21 cm absorption spectra yielded very few components with linewidths in the temperature range of stable WNM, with no such WNM components detected for 16 of the 30 sightlines. We find that some of the detected H i -21 cm absorption along 13 of these sightlines must arise in gas with spin temperatures larger than the CNM range. For these sightlines, we use very conservative estimates of the CNM spin temperature and the non-thermal broadening to derive strict upper limits to the gas column densities in the CNM and WNM phases. Comparing these upper limits to the total H i column density, we find that typically at least 28 per cent of the gas must have temperatures in the thermally unstable range (200–5000 K). Our observations hence robustly indicate that a significant fraction of the gas in the Galactic interstellar medium has temperatures outside the ranges expected for thermally stable gas in two-phase models.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 10
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    Stringent constraints on the H I spin temperature in two z 〉 3 damped Lyman {alpha} systems from redshifted 21 cm absorption studies (2013)
    Oxford University Press
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    Publication Date: 2013-11-23
    Description: Physical properties of damped Lyman α (DLA) absorbers and their evolution are closely related to galaxy formation and evolution theories and have important cosmological implications. H i 21 cm absorption study is one useful way of measuring the temperature of these systems. In this work, very strong constraints on the temperature of two DLA absorbers at z  〉 3 are derived from low radio frequency observations. The H i spin temperature is found to be greater than 2000 K for both the absorbers. The high spin temperature of these high-redshift systems is in agreement with the trend found in a compilation of temperatures for other DLA absorbers. We also argue that the temperature–metallicity relation, reported earlier in the literature, is unlikely to be a spurious line-of-sight effect, and that the redshift evolution of the spin temperature does not arise due to a selection effect. All of these are consistent with a redshift evolution of the warm gas fraction in DLA systems.
    Print ISSN: 1745-3925
    Electronic ISSN: 1745-3933
    Topics: Physics
    Published by Oxford University Press
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