L. M. Haffner

The Wisconsin Hα Mapper Northern Sky Survey

The Wisconsin H-Alpha Mapper (WHAM) has completed a one-degree resolution, velocity-resolved northern sky survey of H-alpha emission from our Galaxy. The unprecedented sensitivity of the instrument and accurate spectral subtraction of atmospheric features allow us to detect Galactic features as faint as 0.1 Rayleighs (EM ~ 0.25 cm^{-6} pc). This survey allows a direct comparison of the ionized and neutral components of the ISM on a global scale for the first time. All-sky maps of H-alpha emission in select velocity bands highlight the rich kinematic structure of the Galaxys ionized gas. The full set of data from the WHAM survey is now available at this http URL (abridged)The Wisconsin Hα Mapper (WHAM) has surveyed the distribution and kinematics of ionized gas in the Galaxy above declination -30°. The WHAM Northern Sky Survey (WHAM-NSS) has an angular resolution of 1° and provides the first absolutely calibrated, kinematically resolved map of the Hα emission from the warm ionized medium (WIM) within ~±100 km s-1 of the local standard of rest. Leveraging WHAMs 12 km s-1 spectral resolution, we have modeled and removed atmospheric emission and zodiacal absorption features from each of the 37,565 spectra. The resulting Hα profiles reveal ionized gas detected in nearly every direction on the sky with a sensitivity of 0.15 R (3 σ). Complex distributions of ionized gas are revealed in the nearby spiral arms up to 1-2 kpc away from the Galactic plane. Toward the inner Galaxy, the WHAM-NSS provides information about the WIM out to the tangent point down to a few degrees from the plane. Ionized gas is also detected toward many intermediate velocity clouds at high latitudes. Several new H II regions are revealed around early B stars and evolved stellar cores (sdB/O). This work presents the details of the instrument, the survey, and the data reduction techniques. The WHAM-NSS is also presented and analyzed for its gross properties. Finally, some general conclusions are presented about the nature of the WIM as revealed by the WHAM-NSS.

The warm ionized medium in spiral galaxies

This article reviews observations and models of the diffuse ionized gas that permeates the disk and halo of our Galaxy and others. It was inspired by a series of invited talks presented during an afternoon scientific session of the 65th birthday celebration for Professor Carl Heiles held at Arecibo Observatory in August 2004. This review is in recognition of Carls long-standing interest in and advocacy for studies of the ionized as well as the neutral components of the interstellar medium.

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.

A Multiwavelength Optical Emission Line Survey of Warm Ionized Gas in the Galaxy

We report on observations of several optical emission lines toward a variety of newly revealed faint, large-scale H?-emitting regions in the Galaxy. The lines include [N II] ?6583, [N II] ?5755, [S II] ?6716, [O III] ?5007, and He I ?5876 obtained with the Wisconsin H? Mapper (WHAM) toward sight lines that probe superbubbles, high-latitude filamentary features, and the more diffuse warm ionized medium (WIM). Our observations include maps covering thousands of square degrees toward the well-known Orion-Eridanus bubble, a recently discovered 60? ? 20? bipolar superbubble centered in Perseus, plus several classical H II regions surrounding OB stars and hot evolved stellar cores. We use the emission-line data to explore the temperature and ionization conditions within the emitting gas and their variations between the different emission regions. We find that in the diffuse WIM and in the faint high-latitude filamentary structures the line ratios of [N II]/H? and [S II]/H? are generally high, while [O III]/H? and He I/H? are generally low compared to the bright classical H II regions. This suggests that the gas producing this faint widespread emission is warmer, in a lower ionization state, and ionized by a softer spectrum than gas in classical H II regions surrounding O stars, the presumed ionization source for the WIM. In addition, we find differences in physical conditions between the large bubble structures and the more diffuse WIM, suggesting that the ionization of superbubble walls by radiation from interior O associations does not account entirely for the range of conditions found within the WIM, particularly the highest values of [N II]/H? and [S II]/H?.

A NEW SPIN ON GALACTIC DUST

We present a new puzzle involving Galactic microwave emission and attempt to resolve it. On one hand, a cross-correlation analysis of the Wisconsin Hα Mapper map with the Tenerife 10 and 15 GHz maps shows that the well-known DIRBE correlated microwave emission cannot be dominated by free-free emission. On the other hand, recent high-resolution observations in the 8-10 GHz range with the Green Bank 140 foot telescope by Finkbeiner et al. failed to find the corresponding 8 σ signal that would be expected in the simplest spinning-dust models. So what physical mechanism is causing this ubiquitous dust-correlated emission? We argue for a model predicting that spinning dust is the culprit after all, but that the corresponding small grains are well correlated with the larger grains seen at 100 μm only on large angular scales. In support of this grain-segregation model, we find that the best spinning-dust template involves higher frequency maps in the range 12-60 μm, in which emission from transiently heated small grains is important. Upcoming cosmic microwave background experiments such as ground-based interferometers, the Microwave Anisotropy Probe, and the Planck low-frequency interferometer with high resolution at low frequencies should allow a definitive test of this model.

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.

Estimating the Porosity of the Interstellar Medium from Three-dimensional Photoionization Modeling of H II Regions

We apply our three-dimensional photoionization code to model Wisconsin HMapper observations of the H ii regionsurroundingtheO9.5VstarOph.Our modelsinvestigate theporosityoftheinterstellarmediumaroundOph andtheeffectsofthree-dimensionaldensitiesontheHsurfacebrightnessandvariationinthe(Nii)k6583/Hline ratio. TheOph H ii region has a well-characterized ionizing source, so it is an excellent starting point for three- dimensional models of diffuse ionized gas. We investigate various hierarchically clumped density structures, varying the overall smoothness within the clumping algorithm. Bysimulating the observations, we estimate the porosity of the medium in the vicinity ofOph and find that within the context of our hierarchically clumped models, around 50%-80% of the volume is occupied by clumps surrounded by a low-density smooth medium. We also conclude that in order for O stars to ionize the diffuse warm ionized medium, the O star environment must be more porous than that surroundingOph, with clumps occupying less than one-half of the interstellar volume. Our clumpy models have irregular boundaries, similar to observed H ii regions. However, in observed H ii regions, it is difficult to identify the precise location of the boundary because of the foreground and/or background emission from the widespreadwarmionizedmedium.Thiscomplicatestheinterpretationofthepredictedrapidriseofsomeemission- line ratios near the edge of uniformdensity Hiiregions and, combined with the three-dimensional clumpy nature of the interstellar medium, may explain the apparent lack of distinctive emission-line ratios near H i-Hii interfaces. Subject headingg H ii regions — ISM: structure — radiative transfer — stars: individual (� Ophiuchi)

Interstellar Hα Line Profiles toward HD 93521 and the Lockman Window

We have used the Wisconsin Hα Mapper (WHAM) facility to measure the interstellar Hα emission toward the high Galactic latitude O star HD 93521 (l = 1831, b = +622). Three emission components were detected along this line of sight. These components have radial velocities of -10, -51, and -90 km s-1 with respect to the local standard of rest (LSR) and have Hα intensities of 0.20, 0.15, and 0.023 R, respectively, corresponding to emission measures of 0.55, 0.42, and 0.06 cm-6 pc. We have also measured the Hα emission toward the direction l = 1485, b = +530, which lies in the region of exceptionally low H I column density known as the Lockman Window. An emission component is detected in this direction at -1 km s-1 (LSR), with an intensity of 0.20 R (0.55 cm-6 pc). In addition, we studied the direction l = 1635, b = +535. No Galactic emission was detected along this line of sight, and upper limits on the possible intensity of Galactic emission toward this direction are 0.11 R at the LSR and 0.06 R at -50 km s-1. As part of the process of separating the interstellar from the terrestrial emission, we also detected and characterized 12 faint (~0.03-0.15 R), unidentified atmospheric lines present in WHAM Hα spectra. Finally, we have used WHAM to obtain [O I] λ6300 spectra along the line of sight toward HD 93521. We do not conclusively detect interstellar [O I] emission toward the star, but place an upper limit of 0.060 R on the [O I] intensity of the intermediate-velocity (-51 km s-1) component. If the temperature of the gas is 10,000 K, this limit implies that, within the Hα-emitting region, the hydrogen ionization fraction n(H+)/n(H total) > 0.6.

WHAM Observations of Hα from High-Velocity Clouds: Are They Galactic or Extragalactic?

It has been suggested that high-velocity clouds may be distributed throughout the Local Group and are therefore not in general associated with the Milky Way. With the aim of testing this hypothesis, we have made observations in the Hα line of high-velocity clouds selected as the most likely candidates for being at larger than average distances. We have found Hα emission from four out of five of the observed clouds, suggesting that the clouds under study are being illuminated by a Lyman continuum flux greater than that of the metagalactic ionizing radiation. Therefore, it appears likely that these clouds are in the Galactic halo and not distributed throughout the Local Group.

Detection of a Large Arc of Ionized Hydrogen Far above the Cassiopeia OB6 Association: A Superbubble Blowout into the Galactic Halo?

The Wisconsin Ha Mapper Northern Sky Survey has revealed a loop of H ii reaching 1300 pc from the Galactic midplane above the Cassiopeia OB6 association in the Perseus spiral arm. This enormous feature surrounds and extends far above the “W4 chimney” identified by Normandeau, Taylor, and Dewdney and appears to be associated with the star formation activity near the W3/W4/W5 H ii region complex. The existence of this ionized structure suggests that past episodes of massive star formation have cleared the H i from an enormous volume above the Perseus arm, allowing Lyman continuum photons from O stars near the Galactic midplane to reach into the halo. Subject headings: galaxies: halos — galaxies: ISM — H ii regions — ISM: bubbles — ISM: general — ISM: structure