Browsing by Subject "hydrogen"

Now showing 1 - 4 of 4
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    Open Access
    A high-dispersion molecular gas component in nearby galaxies
    (2013) Caldú-Primo, Anahi; Schruba, Andreas; Walter, Fabian; Leroy, Adam; Sandstrom, Karin; de Blok, W J G; Ianjamasimanana, R; Mogotsi, K M
    We present a comprehensive study of the velocity dispersion of the atomic (H I) and molecular (H2) gas components in the disks (R R 25) of a sample of 12 nearby spiral galaxies with moderate inclinations. Our analysis is based on sensitive high-resolution data from the THINGS (atomic gas) and HERACLES (molecular gas) surveys. To obtain reliable measurements of the velocity dispersion, we stack regions several kiloparsecs in size, after accounting for intrinsic velocity shifts due to galactic rotation and large-scale motions. We stack using various parameters: the galactocentric distance, star formation rate surface density, H I surface density, H2 surface density, and total gas surface density. We fit single Gaussian components to the stacked spectra and measure median velocity dispersions for H I of 11.9 ± 3.1 km s–1 and for CO of 12.0 ± 3.9 km s–1. The CO velocity dispersions are thus, surprisingly, very similar to the corresponding ones of H I, with an average ratio of σH I /σCO= 1.0 ± 0.2 irrespective of the stacking parameter. The measured CO velocity dispersions are significantly higher (factor of ~2) than the traditional picture of a cold molecular gas disk associated with star formation. The high dispersion implies an additional thick molecular gas disk (possibly as thick as the H I disk). Our finding is in agreement with recent sensitive measurements in individual edge-on and face-on galaxies and points toward the general existence of a thick disk of molecular gas, in addition to the well-known thin disk in nearby spiral galaxies.
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    Open Access
    Evidence for a clumpy, rotating gas disk in a submillimeter galaxy at z = 4
    (2012) Hodge, J A; Carilli, C L; Walter, F; de Blok, W J G; Riechers, D; Daddi, E; Lentati, L
    We present Karl G. Jansky Very Large Array observations of the CO(2-1) emission in the z = 4.05 submillimeter galaxy (SMG) GN20. These high-resolution data allow us to image the molecular gas at 1.3 kpc resolution just 1.6 Gyr after the big bang. The data reveal a clumpy, extended gas reservoir, 14 {+-} 4 kpc in diameter, in unprecedented detail. A dynamical analysis shows that the data are consistent with a rotating disk of total dynamical mass 5.4 {+-} 2.4 Multiplication-Sign 10{sup 11} M {sub Sun }. We use this dynamical mass estimate to constrain the CO-to-H{sub 2} mass conversion factor ({alpha}{sub CO}), finding {alpha}{sub CO} = 1.1 {+-} 0.6 M {sub Sun }(K km s{sup -1} pc{sup 2}){sup -1}. We identify five distinct molecular gas clumps in the disk of GN20 with masses a few percent of the total gas mass, brightness temperatures of 16-31K, and surface densities of >3200-4500 Multiplication-Sign ({alpha}{sub CO}/0.8) M {sub Sun} pc{sup -2}. Virial mass estimates indicate they could be self-gravitating, and we constrain their CO-to-H{sub 2} mass conversion factor to be <0.2-0.7 M {sub Sun }(K km s{sup -1} pc{sup 2}){sup -1}. A multiwavelength comparison demonstrates that the molecular gas is concentrated in a region of the galaxy that is heavily obscured in the rest-frame UV/optical. We investigate the spatially resolved gas excitation and find that the CO(6-5)/CO(2-1) ratio is constant with radius, consistent with star formation occurring over a large portion of the disk. We discuss the implications of our results in the context of different fueling scenarios for SMGs.
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    Open Access
    Oxygen-mediated mitigation of Ru/Al2O3 deactivation during propane steam reforming
    (2023) Cotterrell, Stephen; Fletcher, Jack
    PEMFCs promise efficient and low emission energy generation, compared to internal combustion engines (ICEs). If operated using pure hydrogen, the only by-products of a PEMFC system are heat and water. Indeed, even when the hydrogen is generated from hydrocarbon fuels, emissions are limited to CO2 (and small amount of hydrocarbons during start up), in contrast to ICEs which typically may produce significant environmental pollutants, such as NOx. A pivotal issue in the deployment of PEMFC systems is the availability of hydrogen feedstock. While much research has been done into hydrogen storage technologies, there are significant shortcomings, primarily low volumetric and gravimetric storage density. An alternative is the small-scale on-site production of hydrogen by catalytic processing of a readily available hydrocarbon feed. Reforming is an important step in this process and industrial applications typically employ nickel- based catalysts which suffer from complicated pre-operational reduction procedures and may be pyrophoric on exposure to air. In contrast, the requirements for small-scale distributed hydrogen production are significantly different from those in industrial settings. This study focuses on the performance of an air-stable ruthenium catalyst, Ru/Al2O3, which avoids the needs to extended pre-operational activation/reduction procedure, specifically catalyst stability and the introduction of a limited oxygen (air) in the reformer feed for purposes of mitigating deactivation. The study included the construction of an experimental apparatus to perform catalytic performance evaluation and an investigation of the effect of changing certain key variables (air co-feed and steam- to-carbon ratio) on catalyst performance and stability. Further work was performed to characterise the catalyst before and after use to determine the cause of deactivation. Catalytic performance testing results indicated that the addition of an air co-feed at 1-3% of the total dry feed composition had a very limited effect on the deactivation of the catalyst, but appeared to slightly shift the composition of the reaction products favourably, with a greater effect incurred by a larger co-feed. The favourable shift in reaction products was reversible on removal of the air co-feed. Characterisation of the spent catalyst from these catalytic performance tests indicated a significant extent of coking, supporting the hypothesis that coking is the primary driver of deactivation.
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    Open Access
    SPIRITS: Uncovering Unusual Infrared Transients with Spitzer
    (2017) Kasliwal, Mansi M; Bally, John; Masci, Frank; Cody, Ann Marie; Bond, Howard E; Jencson, Jacob E; Cao, Yi; Boyer, Martha; Cantiello, Matteo; Cook, David; Hsiao, Eric; Khan, Rubab M; Milne, Peter; Morrell, Nidia; Ofek, Eran O; Perley, Daniel A; Phillips, Mark; Prince, Thomas A; Shenoy, Dinesh; Surace, Jason; Dyk, Schuyler D Van; Whitelock, Patricia A; Williams, Robert
    We present an ongoing, five-year systematic search for extragalactic infrared transients, dubbed SPIRITS—SPitzer InfraRed Intensive Transients Survey. In the first year, using Spitzer /IRAC, we searched 190 nearby galaxies with cadence baselines of one month and six months. We discovered over 1958 variables and 43 transients. Here, we describe the survey design and highlight 14 unusual infrared transients with no optical counterparts to deep limits, which we refer to as SPRITEs (eSPecially Red Intermediate-luminosity Transient Events). SPRITEs are in the infrared luminosity gap between novae and supernovae, with [4.5] absolute magnitudes between −11 and −14 (Vega-mag) and [3.6]–[4.5] colors between 0.3 mag and 1.6 mag. The photometric evolution of SPRITEs is diverse, ranging from <0.1 mag yr{sup −1} to >7 mag yr{sup −1}. SPRITEs occur in star-forming galaxies. We present an in-depth study of one of them, SPIRITS 14ajc in Messier 83, which shows shock-excited molecular hydrogen emission. This shock may have been triggered by the dynamic decay of a non-hierarchical system of massive stars that led to either the formation of a binary or a protostellar merger.