The Wisconsin H Mapper (WHAM): A Brief Review of Performance Characteristics and Early Scientific Results

Early Results

H Sky Survey

Between January 1997 and September 1998, WHAM will carry out one of its primary missions, a northern sky H survey of the warm ionized medium. This survey will consist of approximately 33,000 spectra above declination , sampling the sky on a 0.85 0.98 grid with a 1 beam. Each spectrum has a 30 s integration time and covers a 4.4 Å (200 km s) spectral interval centered near the LSR at a resolution of 0.26 Å (12 km s). All observations are carried out during dark of the moon to avoid contamination by features in the solar spectrum. This survey will provide for the first time a detailed view of the distribution and kinematics of the diffuse ionized hydrogen through the optical H line comparable to the large scale survey maps of the neutral hydrogen obtained through the radio 21 cm line.

  
Figure 2: WHAM H data for . a) the raw CCD camera image (30 s exposure); b) the spectrum produced by annular binning of the CCD image ( denotes the geocoronal line); c) the final, pure interstellar H spectrum after flat fielding, the removal of the geocoronal line, and the subtraction of the sky background continuum. The intensities of the two blended interstellar lines near -15 km s and +15 km s (LSR) are approximately 4 R and 2 R, respectively.

Figure 2 illustrates a sample spectrum from the survey at , , showing both the raw CCD image and the resulting H spectra. The geocoronal line is the thin, bright annulus in the CCD ``ring spectrum'', which appears as a prominent, relatively narrow emission line in the center frame of Figure 2. The interstellar emission is the broader feature inside the geocoronal ring, appearing in this case to consist of two blended velocity components at +30 km s and +60 km s with respect to the geocoronal line. In general, the separation between the interstellar emission and the geocoronal line is due to a combination of the earth's orbital velocity, the sun's peculiar velocity, and intrinsic motions of the interstellar gas, including Galactic differential rotation. The two interstellar components have intensities of about 2 R and 4 R, where a Rayleigh, 1 R = 2.41 erg cm s sr at H and corresponds to an emission measure of 2.3 cm pc for a temperature of 8000 K (Reynolds 1991). The geocoronal line is removed from the data by fitting each spectrum with gaussian components and then subtracting from the spectrum the fitted gaussian associated with the geocorona. The resulting pure interstellar spectrum is shown in the third frame of Figure 2. The absolute intensity calibration is obtained by comparison with standard astronomical sources (e.g., Scherb 1981).

A portion of the survey data is presented in Figure 3 as a gray scale map of the total intensity of the interstellar H emission. This map, synthesized from approximately 7000 H spectra, covers the region of the sky between about 170 to 240 Galactic longitude and 50 Galactic latitude. The gray scaling and stretch have been adjusted to reveal the fainter high latitude emission along with the brighter regions near the plane. Interstellar H emission is detected in every direction, with intensities that range from thousands of Rayleighs near the Orion nebula ( 209,) and 100-200 R in Barnard's loop and the large Ori H II region (195, ) to 0.5 R in some of the fainter high latitude regions (e.g., 220, +45). The map reveals numerous large scale filaments superposed on a fainter H background. A number of ``classical'' H II regions also dot the map near the Galactic equator. Some of the filamentary features are associated with the Orion-Eridanus bubble (Reynolds & Ogden 1979; Sivan 1976), which fills the sky from l = 180 to 210 and to and includes Barnard's Loop near its northern boundary. Many of the Orion-Eridanus filaments appear to be correlated with emission features at 21 cm and x-ray wavelengths (see, for example, Brown, Hartmann, & Burton 1995, Burrows et al 1993, and Reynolds & Ogden 1979). Other filaments on the map have no obvious correspondence to any previously known structures, for example, the 10 R feature that extends north from l = 220 , across the Galactic midplane to 215, +5 or the fainter (1 R) feature rising vertically from 226, +10 to 229, +50. Narrow velocity interval maps have also been constructed from these data, revealing significant kinematic variations among the various emission features (Haffner et al 1997, in preparation).

  
Figure 3: Total intensity map of the diffuse interstellar H background in the Galactic anticenter region (extracted from the WHAM sky survey). The map is in Galactic coordinates, with the Galactic equator running horizontally through the center. The map is bounded on the left by the declination limit of the survey. Barnard's Loop is the bright arc near , and the large Ori H II region is centered near , . The small black dots are pixels with missing data or spectra contaminated by a bright star within the beam.

High Velocity Clouds

Although they have been observed via their 21 cm emission for many years, the origin of the High Velocity Clouds (HVCs) is still not known (Wakker et al 1996). The detection of HVCs in H (Kutyrev & Reynolds 1989; Songaila, Bryant, & Cowie 1989; Tufte et al 1996) and [S II] 6716 (Tufte 1997) has opened a new window through which to explore the nature of these objects. Furthermore, the M I and A clouds have been found to be located far above the Galactic midplane, z = 1.5-4.4 kpc (Danly et al 1993) and 3-7 kpc (Wakker et al 1996; van Woerden et al, private communication), respectively, making HVCs excellent probes of the environment outside the Galactic disk. WHAM has detected H from the M I, M II, A, and C clouds with an intensity 0.1 R (Tufte et al, in preparation). If the H II in these clouds is produced by photoionization, then the required ionizing flux is 1-2 10 photons cm s (Tufte 1997), which is about 5-10% that required to produce the warm ionized component of the interstellar medium in the disk. Observations in many directions toward and near the M I cloud suggest that the H II is confined primarily to the region of 21 cm emission (at least in projection) and does not form an extended halo about the cloud (Tufte et al 1996).

Diagnostic Lines

In addition to H, WHAM is able to observe other emission lines that probe the temperature and ionization conditions within the gas, including [S II] 6716, [N II] 6584, [N II] 5755, He I 5876, [O I] 6300, and [O III] 5007 (Tufte et al 1996; Haffner et al 1996; Reynolds et al 1997). Some of these lines are too faint to have been detected previously. For example, the [O I] 6300 line intensity relative to H is a measure of the amount of neutral hydrogen within the warm ionized regions and provides an important constraint on photoionization models (e.g., Domgörgen & Mathis 1994; Reynolds 1989). Observations with WHAM have provided the first detections of this line from the diffuse interstellar medium, with an intensity that varies from less than 1% to about 4% that of the interstellar H (Reynolds et al 1997). According to the photoionization models (Domgörgen & Mathis 1994), these low [O I]/H intensity ratios suggest that most of the H originates from density bounded, nearly fully ionized regions rather than from partially ionized H I clouds or H II ``skins'' on the surfaces of H I clouds.

© Copyright Astronomical Society of Australia 1997