Massive clouds of gas are orbiting around and, occasionally, plummeting into our Galaxy at speeds in excess of 200,000 miles per hour. If your eyes could see radio-waves or ultraviolet light, you would see that these clouds cover 2/3 of the sky. You would also see a variety of shapes, ranging from tiny dots to a 100,000 lightyear long streamer of clouds that weighs 10 million times the mass of the Sun. One of my research goals is to understand these clouds, called High Velocity Clouds (HVCs) and how they affect our Galaxy, the Milky Way. My research group uses computer clusters in the Georgia Advanced Computing Resource Center to simulate the hydrodynamic interaction between cloud gas and the gas in our own Galaxy. You can see one of our simulations here. Through these simulations, we've determined how mixing between cloud gas and hot gas in our Galaxy's outskirts heats up the cloud gas, fragments it, and, ultimately, causes it to merge with our Galaxy. Our simulational results compare well with ultraviolet observations of cloud gas and provide a new explanation for semi-hot gas in our Galaxy's outskirts (the halo).
The gas that is already in our Galaxy has been studied for several decades. From such work, we know that Galactic gas is the material from which new stars form and the material into which dying stars eject their ashes. We also know that the gas is most concentrated in a pancake-shaped disk that forms the spine of the Milky Way and least concentrated above and below the disk, where our Galaxy's gas blends with sparce intergalactic gas. My group studies the hottest gas in our galaxy, material that is about 1 to 2 million degrees Fahrenheit (roughly 1/2 to 1 million degrees Celsius). At these temperatures, the atoms in the gas glow brightly in ultraviolet and X-ray light. My group has a history of observing and analyzing the ultraviolet and X-ray photons and of using computers to simulate the interstellar gas. A small sample of our recent results follows. We have determined the quantity and temperature of hot gas in the halo of our galaxy by analyzing data taken by the X-ray Multi-Mirror observatory. We then compared these numbers with simulations made by our collaborators, finding that the hot halo gas was probably heated by the energy ejected into the halo from stars, most of which were in the disk of our galaxy. Using data from another X-ray telescope, the Suzaku telescope, we may have found the highest supernova remnant (an enormous bubble left behind after a star explodes) ever observed. Also, by comparing observations made of the same directions on the sky, but made at different points in time, we have shown that the solar heliosphere erratically produces X-rays. A selection of our X-ray observations can be seen here.
Gritton, J. A., Shelton, R. L., and Kwak, K., Mixing between High Velocity Clouds and the Galactic Halo, 2014, the Astrophysical Journal, 795, article id 99 (9 pages)
Henley, D. B., Shelton, R. L.,and Kwak, K.,The Origin of the X-ray Emission from the High-velocity Cloud MS30.7-81.4-118, 2014, the Astrophysical Journal, 791, article id 41 (16 pages)
Cumbee, R. S., Henley, D. B., Stancil, P. C., Shelton, R. L., Nolte, J. L., Wu, Y., and Schultz, D. R., Can Charge Exchange Explain Anomalous Soft X-ray Emission in the Cygnus Loop, 2014, the Astrophysical Journal Letters, 787L, article id 31 (5 pages)
Henley, D. B., and Shelton, R. L., Is the Milky Way's Hot Halo Convectively Unstable?, 2014, the Astrophysical Journal, 784, article id 54 (8 pages)
Henley, D. B., and Shelton, R. L., An XMM-Newton Survey of the Soft X-Ray Background. III. The Galactic Halo X-Ray Emission, 2013, The Astrophysical Journal, 773, 92 (21 pages)
Henley, D. B., and Shelton, R. L.,An XMM-Newton Survey of the Soft X-rayBackground. II. An All-Sky Catalog of Diffuse O VII and O VIIIEmission Intensities, 2012, The Astrophysical Journal Supplement, 202, article id 14 (27 pages)
Henley, D. B., Kwak, K., and Shelton, R. L., Simulations of High Velocity-Clouds. II. Ablation from High-VelocityClouds as a Source of Low-Velocity High-Ions, 2012, The Astrophysical Journal, 753, article id 58 (16 pages)
Shelton, R. L., Kwak, K.,and Henley, D. B., 2011, Modeling the X-rays Resulting From High Velocity Clouds,2012, The Astrophysical Journal, 751, article id 120 (14 pages)
Kwak, K., Henley, D. B., and Shelton, R. L., Simulations of High-Velocity Clouds. I. Hydrodynamics and High-VelocityHigh Ions, 2011, The Astrophysical Journal, 739, article id 30 (17 pages)
Klimek, M. D., Points, S. D., Smith, R. C., Shelton, R. L.,Williams, R.,An X-ray Investigation of Three Supernova Remnants in the Large Magellanic Cloud,b2010, The Astrophysical Journal, 725, 2281-2289
Henley, D. B., Shelton, R. L., Kwak, K., Joung, M. R., and Mac Low, M.-M.,The Origin of the Hot Gas in the Galactic Halo: Confronting Models with XMM-Newton Observations, 2010, The Astrophysical Journal, 723, 935 - 953
Shelton, R. L., Henley, D. B., and Dixon, W. V.,Hot Gas in the Galactic Thick Disk and Halo Near the Draco Cloud, 2010, The Astrophysical Journal, 722, 302 - 310
Kwak, K., and Shelton, R. L.,Numerical Study of Turbulent Mixing Layers with Non-equilibrium Ionization Calculations, 2010, The Astrophysical Journal, 719, 523 - 539
Henley, D. B., and Shelton, R. L.,An XMM-Newton Survey of the Soft X-Ray Background. I. The O VII and O VIII Lines Between l = 120 degrees and l = 240 degrees, 2010, The Astrophysical Journal Supplement, 187, 388 - 408
Frisch, P. C., Bzowski, M., Grun, E., Izmodenov, V., Kruger, H., Linsky, J., McComas, D., Mobius, E., Redfield, S., Schwadron, N., Shelton, R., Slavin, J., and Wood, B.,The Galactic Environment of the Sun: Interstellar Matter Inside and Outside of the Heliosphere, This article appeared in both journal and book chapter form. 2009, Space Sciences Reviews, 146, 235 - 273
Henley, D. B. and Shelton, R. L., A Possible SupernovaRemnant High Above the Galactic Disk, 2009, The Astrophysical Journal, 701, 1880 - 1894
Lei, S.-J., Shelton, R. L., and Henley, D. B.,Determining the Galactic Halo's Emission Measure from UV and X-rayObservations, 2009, The Astrophysical Journal, 699, 1891 - 1905
Kwak, K. J., Shelton, R. L., and Raley, E. R.,The Evolution of Gas Clouds Falling in the Magnetized Galactic Halo: High Velocity Clouds (HVCs) Originated in the Galactic Fountain, 2009, The Astrophysical Journal, 699, 1775 - 1788
Shelton, R. L., "The Local Bubble Debate", Space Sciences Reviews, DOI: 10.1007/s11214-008-9359-7 (2008)
Shelton, R. L., "Revising the Local Bubble Model due to Solar Wind Charge Exchange X-ray Emission", Space Sciences Reviews, DOI: 10.1007/s11214-008-9358-8 (2008)
Henley, D. B., Shelton, R. L., "Comparing Suzaku and XMM-Newton Observations of the Soft X-Ray Background: Evidence for Solar Wind Charge Exchange Emission", Astrophysical Journal, 676, 335 (2008)
Henley, D. B., Shelton, R. L., and Kuntz, K. D., "An XMM-Newton Observation of the Local Bubble Using a Shadowing Filament in the Southern Galactic Hemisphere", Astrophysical Journal, 661, 304 (2007)
Raley, E. A., Shelton, R. L., and Plewa, T., "A Study of the Vertical Motion of Supernova Remnant Bubbles in the Interstellar Medium Drawn from the Results of Three-Dimensional MHD Simulations", Astrophysical Journal, 661, 222 (2007)
Shelton, R. L., Sallmen, S. M., and Jenkins, E. B., "The Galactic Halo's O VI Resonance Line Intensity" Astrophysical Journal, 659, 365 (2007)
R. J. Chastain, Shelton, R. L., Raley, E. A., Magnani, L., "High-Latitude Molecular Structure in Pegasus-Pisces", Astronomical Journal, 132, 1964 (2006)
R. Shelton, "Simulations of Supernova Remnants in Diffuse Media. III. The Population of Buoyant Remnants above the Milky Way's Disk", Astrophysical Journal, 638, 206 (2006)
W. R. Oegerle, Jenkins, E. B., Shelton, R.L., Bowen, D. V., Chayer, P., "A Survey of O VI Absorption in the Local Interstellar Medium", Astrophysical Journal, 622, 377 (2005)
R. M. Williams, R. M., Chu, Y.-H., Dickel, J. R., Gruendl, R. A., Shelton, R., Points, S. D., Smith, R. C., "Remnants in the Magellanic Clouds. IV. X-Ray Emission from the Largest Supernova Remnant in the Large Magellanic Cloud", Astrophysical Journal, 613, 948 (2004)
R.L. Shelton, K. D. Kuntz, & R. Petre, "A Thermal Composite Supernova Remnant with a Cool Shell", Astrophys. J. 615 (2004).
R.L. Shelton, K. D. Kuntz, & R. Petre,"'Chandra' Observations and Models of the Mixed-Morphology Supernova Remnant W44: Global Trends", Astrophys. J. 611, 906 (2004).
B.-G. Andersson, D. C. Knauth, S. L. Snowden, R. L. Shelton, & P. G. Wannier, "A Hot Envelope Around the Southern Coalsack: X-ray and Far-Ultraviolet Observations", Astrophys. J. 606, 341 (2004).
The IX Brazilian Meeting on Simulational Physics included participants from Argentina, France, Germany, Italy, Japan, and the U.S.A, in addition to multiple Brazilian institutions.
Center Director David P. Landau has been awarded a "Doctor Honoris Causa" by the Universidade Federal de Minas Gerais in Belo Horizonte, Brazil.
The American Astronomical Society prize was given to Kyle Walker for his thesis "Molecular Collisional Excitation in Astrophysical Environments and Modeling the Early Universe."