S. L. Tufte

Evidence for an Additional Heat Source in the Warm Ionized Medium of Galaxies

Spatial variations of the [S ii]/Halpha and [N ii]/Halpha line intensity ratios observed in the gaseous halo of the Milky Way and other galaxies are inconsistent with pure photoionization models. They appear to require a supplemental heating mechanism that increases the electron temperature at low densities, ne. This would imply that in addition to photoionization, which has a heating rate per unit volume proportional to n2e, there is another source of heat with a rate per unit volume proportional to a lower power of ne. One possible mechanism is the dissipation of interstellar plasma turbulence, which, according to Minter & Spangler, heats the ionized interstellar medium in the Milky Way at a rate of approximately 1x10-25ne ergs cm-3 s-1. If such a source were present, it would dominate over photoionization heating in regions where ne less, similar0.1 cm-3, producing the observed increases in the [S ii]/Halpha and [N ii]/Halpha intensity ratios at large distances from the galactic midplane as well as accounting for the constancy of [S ii]/[N ii], which is not explained by pure photoionization. Other supplemental heating sources, such as magnetic reconnection, cosmic rays, or photoelectric emission from small grains, could also account for these observations, provided they supply approximately 10-5 ergs s-1 per square centimeter of the Galactic disk to the warm ionized medium.

Faint, Large-Scale Hα Filaments in the Milky Way

During the initial data reduction of the Wisconsin H-Alpha Mapper (WHAM) H-Alpha Sky Survey, we have discovered several very long (~30--80 deg) filaments superimposed on the diffuse H-Alpha background. These features have no clear correspondence to the other phases of the interstellar medium revealed by 21 cm, X-ray, IR, or radio continuum surveys, and they have no readily identifiable origin or source of ionization. In this letter, the data for two of these faint (I_{H-Alpha} = 0.5--1.5 R) structures are presented. The first is an 80 deg-long, 2 deg-wide arch that extends nearly perpendicular to the Galactic plane at l = 225 deg and attains a maximum latitude of +51 deg near l = 240 deg. Where this feature appears to meet the Galactic plane near l = 225 deg, it is directly above the H II region surrounding CMa R1/OB1. A second filament consists of a ~25--30 deg-long arc spanning l = 210--240 deg at b = +30 deg to +40 deg. Both features have measurable velocity trends with position. However, they have rather constant intensities along their entire lengths, ranging from 0.5--1.5 R (EM = 1--3 cm^{-6} pc) with no obvious trends.

Geocoronal Hα intensity measurements using the Wisconsin Hα Mapper Fabry-Perot facility

The Wisconsin Hα Mapper (WHAM), a remotely operable, semi-automated Fabry-Perot located at Kitt Peak Observatory, has been making an all-sky survey of interstellar hydrogen Balmer α(Hα) emissions since 1997. Using the annular summing spectroscopy technique, WHAM has acquired ∼37,000 spectra to date, spanning almost 100 nights of observations. Since all of the galactic emission spectral data contain the terrestrial Hα (6562.7 A) emission line, these measurements constitute a rich source of geocoronal data for investigating natural variability in the upper atmosphere. The WHAM observations also serve as a benchmark for comparison with future data. Analysis of the first year of WHAM data shows only small day-to-day variations after shadow altitude variations are taken into account. For example, at shadow altitudes of 2000 and 3000 km, the RMS scatter is within approximately +/− 20%; this variability is expected to be reduced with accurate accounting of the smaller-scale effects of observational slant path, zenith angle, and azimuth on the Hα intensity. This result is consistent with past midlatitude Wisconsin data sets but different from observations made by other observers and instruments at the low-latitude Arecibo site. The multiple viewing geometries of the observations provide stringent modeling constraints, useful in testing current modeling capabilities. Modeling of the WHAM data with a global nonisothermal resonance radiation transport code (lyao_rt) indicates that the signal-to-noise of the data is sufficient to determine relative variations in upper atmospheric atomic hydrogen column densities to better than 5%. This paper describes the WHAM aeronomy program and its observational scheme, analysis procedures, and results from data taken in 1997. Case study comparisons are made with past data sets and with predictions from the lyao_rt resonant radiation transport modeling code of Bishop [1999].

Observational Evidence of Supershell Blowout in GS 018–04+44: The Scutum Supershell

Emission in the H I 21 cm line has been mapped for a region of the Galaxy that includes two known supershells, GS 018(04)44 and GS 034(06)65. We focus on the GS 018(04)44, hereafter referred to as the Scutum Supershell, which is an elongated shell about 5i in diameter extending to (7i below the Galactic plane. The Scutum shell lies at a kinematic distance of D3300 pc, implying a shell diameter of D290 pc with a vertical extension of D400 pc away from the Galactic plane. The Scutum shell con- tains 6.2 ) 105 swept into the walls. We observe that the top of the shell is missing, and a substan- M _ tial column of H I rises from the shell walls to b \( 11i, culminating in a large cloud of neutral hydrogen, 3.74 ) 104 located D630 pc from the plane. ROSAT data show X-ray emission that M _ , closely anticorrelates with the 21 cm emission. This emission probably originates from hot gas within the Scutum Supershell. After approximately correcting for the foreground absorption, we —nd that the 1.5 keV X-rays peak at the base of the shell, the 0.75 keV emission peaks in the interior and at the top of the shell, and the 0.25 keV emission extends to high latitudes above the shell. The X-ray luminosity is roughly D5 ) 1036 ergs s~1. The Wisconsin H-Alpha Mapper (WHAM) survey shows the presence of Ha emission that exhibits a morphology similar to that of the H I. Spectra indicate the presence of ionized hydrogen at velocities similar to the H I, placing ionized material at the same kinematic distance as the neutral material. IRAS images in the 60 and 100 km wavebands reveal the presence of dust corre- lated with the neutral hydrogen. Infrared surface brightness indicates an excess in the 100 km emission, which could indicate a molecular hydrogen component with a column density of 2.4 ) 1021 cm~2 in the densest regions of the high-latitude cloud of neutral hydrogen. IUE ultraviolet high dispersion spectra of HD 177989 and HD 175754 reveal the presence of very (l \ 17i.89, b \( 11i.88) (l \ 16i.40, b \( 9i.92) strong absorption by highly ionized gas at a velocity that associates the absorption with the ejecta of the Scutum Supershell. In the case of HD 177989, the high ion column density ratios suggest an origin in a turbulent mixing layer where hot and cool gases mix in the presence of shear —ows. The Ha and X-ray emission suggest that a multitude of energetic phenomena exist in this region, providing the necessary ionizing radiation. Indeed, there are multiple supernova remnants, H II regions, and hot stars, which could all contribute sizeable amounts of energy and ionizing radiation. The combination of these data sets indicates observational evidence of a ii blowout ˇˇ phenomena whereby hot material produced within the Scutum Supershell has blown through the top of the shell and been pushed to high latitude. Subject headings: Galaxy: structureISM: bubblesISM: individual (Scutum Supershell) ¨ ISM: structureradio lines: ISMsupernova remnants

Detection of [O [CSC]i[/CSC]] λ6300 Emission from the Diffuse Interstellar Medium

The Wisconsin H? Mapper facility was used to obtain spectra of [O I] ?6300 in three directions that sample the warm ionized component of the interstellar medium at the Galactic midplane and at z-300 pc. Weak interstellar [O I] emission was clearly detected toward all three directions, with [O I] ?6300/H? intensity ratios for individual radial velocity components that range from less than 0.01 to approximately 0.04. According to photoionization models of the warm ionized medium, these [O I]/H? ratios suggest that most of the H? originates from density-bounded, nearly fully ionized regions along the lines of sight rather than from partially ionized H I clouds or layers of H II on the surfaces of H I clouds.

Detection of [O I] ?6300 Emission from the Diffuse Interstellar Medium

The Wisconsin H? Mapper facility was used to obtain spectra of [O I] ?6300 in three directions that sample the warm ionized component of the interstellar medium at the Galactic midplane and at z-300 pc. Weak interstellar [O I] emission was clearly detected toward all three directions, with [O I] ?6300/H? intensity ratios for individual radial velocity components that range from less than 0.01 to approximately 0.04. According to photoionization models of the warm ionized medium, these [O I]/H? ratios suggest that most of the H? originates from density-bounded, nearly fully ionized regions along the lines of sight rather than from partially ionized H I clouds or layers of H II on the surfaces of H I clouds.

Detection of [O I] λ6300 Emission from the Diffuse Interstellar Medium

The Wisconsin H? Mapper facility was used to obtain spectra of [O I] ?6300 in three directions that sample the warm ionized component of the interstellar medium at the Galactic midplane and at z-300 pc. Weak interstellar [O I] emission was clearly detected toward all three directions, with [O I] ?6300/H? intensity ratios for individual radial velocity components that range from less than 0.01 to approximately 0.04. According to photoionization models of the warm ionized medium, these [O I]/H? ratios suggest that most of the H? originates from density-bounded, nearly fully ionized regions along the lines of sight rather than from partially ionized H I clouds or layers of H II on the surfaces of H I clouds.