Alex S. Hill

Magnetized Gas in the Smith High Velocity Cloud

We report the first detection of magnetic fields associated with the Smith High Velocity Cloud. We use a catalog of Faraday rotation measures toward extragalactic radio sources behind the Smith Cloud, new H I observations from the Robert C. Byrd Green Bank Telescope, and a spectroscopic map of Hα from the Wisconsin H-Alpha Mapper Northern Sky Survey. There are enhancements in rotation measure (RM) of 100 rad m–2 which are generally well correlated with decelerated Hα emission. We estimate a lower limit on the line-of-sight component of the field of 8 μG along a decelerated filament; this is a lower limit due to our assumptions about the geometry. No RM excess is evident in sightlines dominated by H I or Hα at the velocity of the Smith Cloud. The smooth Hα morphology of the emission at the Smith Cloud velocity suggests photoionization by the Galactic ionizing radiation field as the dominant ionization mechanism, while the filamentary morphology and high (1 Rayleigh) Hα intensity of the lower-velocity magnetized ionized gas suggests an ionization process associated with shocks due to interaction with the Galactic interstellar medium. The presence of the magnetic field may contribute to the survival of high velocity clouds like the Smith Cloud as they move from the Galactic halo to the disk. We expect these data to provide a test for magnetohydrodynamic simulations of infalling gas.

Steady-state hadronic gamma-ray emission from 100-MYR-old Fermi Bubbles

Fermi Bubbles are enigmatic γ-ray features of the Galactic bulge. Both putative activity (within few × Myr) connected to the Galactic center super-massive black hole and, alternatively, nuclear star formation have been claimed as the energizing source of the Bubbles. Likewise, both inverse-Compton emission by non-thermal electrons (leptonic models) and collisions between non-thermal protons and gas (hadronic models) have been advanced as the process supplying the Bubbles γ-ray emission. An issue for any steady state hadronic model is that the very low density of the Bubbles plasma seems to require that they accumulate protons over a multi-gigayear timescale, much longer than other natural timescales occurring in the problem. Here we present a mechanism wherein the timescale for generating the Bubbles γ-ray emission via hadronic processes is ∼few × 10{sup 8} yr. Our model invokes the collapse of the Bubbles thermally unstable plasma, leading to an accumulation of cosmic rays and magnetic field into localized, warm (∼10{sup 4} K), and likely filamentary condensations of higher-density gas. Under the condition that these filaments are supported by non-thermal pressure, the hadronic emission from the Bubbles is L {sub γ} ≅ 2 × 10{sup 37} erg s{sup –1} M-dot {sub in}/(0.1 M{sub ⊙} yr{sup –1} ) T{sub FB}{sup 2}/(3.5×10{sup 7} K){sup 2}morexa0» M {sub fil}/M {sub pls}, equal to their observed luminosity (normalizing to the star-formation-driven mass flux into the Bubbles and their measured plasma temperature and adopting the further result that the mass in the filaments, M {sub fil} is approximately equal to the that of the Bubbles plasma, M {sub pls})«xa0less

Photoionization and heating of a supernova-driven turbulent interstellar medium

The diffuse ionized gas (DIG) in galaxies traces photoionization feedback from massive stars. Through three-dimensional photoionization simulations, we study the propagation of ionizing photons,photoionizationheatingandtheresultingdistributionofionizedandneutralgaswithin snapshots of magnetohydrodynamic simulations of a supernova-driven turbulent interstellar medium. We also investigate the impact of non-photoionization heating on observed optical emission line ratios. Inclusion of a heating term which scales less steeply with electron density than photoionization is required to produce diagnostic emission line ratios similar to those observed with the Wisconsin Hα Mapper. Once such heating terms have been included, we are also able to produce temperatures similar to those inferred from observations of the DIG, with temperatures increasing to above 15000 K at heights |z| 1 kpc. We find that ionizing photons travel through low-density regions close to the mid-plane of the simulations, while travelling through diffuse low-density regions at large heights. The majority of photons travel small distances (100 pc); however some travel kiloparsecs and ionize the DIG.

GSH 006−15+7: a local Galactic supershell featuring transition from H i emission to absorption

We report on the discovery of a new Galactic supershell, GSH 006−15+7, from the Galactic All-Sky Survey data. Observed and derived properties are presented, and we find that GSH 006−15+7 is one of the nearest physically large supershells known, with dimensions of ∼780 × 520xa0pc at a distance of ∼ 1.5xa0kpc. The shell wall appears in Hi emission at b≲−65 and in Hi self-absorption (HiSA) at b≳−65. We use this feature along with HiSA diagnostics to estimate an optical depth of τ∼ 3, a spin temperature of ∼40xa0K and a swept-up mass of M∼ 3 × 106xa0M⊙. We also investigate the origin of GSH 006−15+7, assessing the energy contribution of candidate powering sources and finding evidence in favour of a formation energy of ∼1052 erg. We find that this structure provides evidence for the transfer of mass and energy from the Galactic disc into the halo.

Hα AND [SII] EMISSION FROM WARM IONIZED GAS IN THE SCUTUM-CENTAURUS ARM

We present Wisconsin H-Alpha Mapper [SII] λ6716 and Hα spectroscopic maps of the warm ionized medium (WIM) in the Scutum-Centaurus Arm at Galactic longitudes 310° < l < 345°. Using extinction-corrected Hα intensities (I{sub Hα}{sup c}), we measure an exponential scale height of electron density squared in the arm of H{sub n{sub e{sup 2}}}=0.30 kpc (assuming a distance of 3.5 kpc), intermediate between that observed in the inner Galaxy and in the Perseus Arm. The [S II]/Hα line ratio is enhanced at large |z| and in sightlines with faint I{sub Hα}{sup c}. We find that the [S II]/Hα line ratio has a power-law relationship with I{sub Hα}{sup c} from a value of ≈1.0 at I{sub Hα}{sup c}<0.2 R (Rayleighs) to a value of ≈0.08 at I{sub Hα}{sup c}≳100 R. The line ratio is better correlated with Hα intensity than with height above the plane, indicating that the physical conditions within the WIM vary systematically with electron density. We argue that the variation of the line ratio with height is a consequence of the decrease of electron density with height. Our results reinforce the well-established picture in which the diffuse Hα emission is due primarily to emission from in situ photoionizedmorexa0» gas, with scattered light only a minor contributor.«xa0less