Emission Line Maps of the Milky Way

On August 8, 1996, WHAM obtained its first test map from the Galaxy. Shortly after, it was moved down to Kitt Peak and began it's main task: mapping the entire sky in Hα. Most of this data was obtained during 1997 with more complete southern coverage and re-observations in 1998. After Hα was well in hand, we began mapping select portions of the sky in other emission lines, including [S II] 6717Å, [N II] 6583Å, and Hβ (see additional notes below). Some initial maps of interesting sections of the sky have already trickled out into publications. See Papers for our current selections. The total intensity map is now available from the Survey section of our site.

WHAM's velocity-resolved maps nicely complement the narrow-band filter imaging projects such as the Virginia Tech Spectral Line Sky Survey and a southern Hα Sky Survey spearheaded by John Gaustad of Swarthmore.

High-Velocity Clouds

Unfortunately, the WHAM survey only maps emission at ±100 km/s from the Local Standard of Rest (LSR). However, we have made many additional observations toward known neutral and highly-ionized HVCs. WHAM has detected Hα and [S II] 6716Å, from several of these regions (see Papers for details). Until recently, these mysterious structures were studied almost exclusively through maps of the 21 cm line of neutral hydrogen and a handful of absorption line studies toward more distant objects. Emission line studies of HVCs promises to provide many new clues about the nature of these elusive objects. Hα emission from intermediate-velocity gas is also being investigated.

[S II] 6717Å
[N II] 6583Å

These lines are equal or elevated in WIM gas and are good tracers to discriminate between WIM gas and more traditional H II regions—although a smooth transition is most often observed. Combined with the Hα observations, they allow us to start studing the physics of this phase in addition to its distribution. One of the striking things that we and other researchers have found is that these lines tend to increase in intensity relative to Hα as the Hα emission decreases. In two recent papers, we propose that these rises are due to increasing temperatures. Near 10,000 K, the emissivity of these collisionally excited lines decreases much more slowly with temperature than that of Hα, which arises from recombination. Thus, the smooth increase of these ratios with decreasing Hα intensity seems to indicate a gradual rise in the gas temperature. This argument can be taken one step further since, for many cases, the decrease in Hα intensity is due to a decrease in electron density. For example, as we look toward regions above the Galactic plane, the Hα intensity is decreases smoothly with distance from the plane due to the limited scale height of the ionized layer. In this particular case, we can then infer that the temperature of the WIM rises into the halo of our Galaxy.

[O I] 6300Å

Due to similar first ionization potentials, the fraction of neutral and singly-ionized oxygen and hydrogen are locked together from charge-exchange reactions in many astrophysical plasmas. WHAM has detected this line from the WIM for the first time in the Galactic plane. Measurements of this line relative to Hα give the fraction of neutral oxygen and thus, the fraction of neutral hydrogen along the line of sight. See the paper for details.

[N II] 5755Å

A tentative first detection of this weak line, when combined with the measurement of the strong 6584Å line provides the first direct measurement of the temperature in the WIM. Some comments on our initial results appear in Ron Reynold's paper from the recent Mexico City Astrophysical Plasmas conference proceedings.

He I 5876Å

Using WHAM, we have detected this line for the first time from the warm ionized medium (WIM). This recombination line probes the degree of helium ionization in the WIM. Comparing the helium ionization fraction to the hydrogen ionization fraction yields valuable information on the spectrum of the WIM's unknown source of ionization. These tantalizing results still require a bit more observing before publishing, although some preliminary results can be found in Steve Tufte's thesis and were presented by him at a AAS meeting.

[O III] 5007Å

Emission from the WIM of this classic H II region line had only been detected in the Galactic plane (b = 0) prior to WHAM. With WHAM, we have found that 5007Å emission extends to much higher Galactic latitudes (|b| ~ 45). Observations of this gas at even higher latitudes provides upper limit measurements of the contribution of 5007Å emission from hot, Galactic coronal gas. Matt Haffner presented preliminary results of [O III] observations at a AAS meeting, but further observations were needed because of serious atmospheric contamination from Madison skies. New observations were taken in 1999 at Kitt Peak but have yet to be fully analyzed.

Hβ

Since most of the Hα emission we detect arises from hydrogen recombination, atomic physics is the only thing that dictates the ratio of Hα to Hβ emission from most ionized intersteller gases. Although it is a slight function of temperature, near 10,000 K, the ratio is about 3:1 in favor of Hα. However, intersteller dust absorbs more blue light than red so that ratios greater than this are typical in observations. Observed ratios of Hα/Hβ should be an intersesting probe of dust in front of and within the ionized gas. Our current plan is to cover at least +/- 30 degrees about the Galactic plane along with several brighter ionized regions, such as the entire Orion-Eridanus complex. The observations began in December, 1999 but may take much of 2000 to complete since we have doubled the Hα exposure time for Hβ observations.

H II Regions

Aside from being interesting studies in their own right, H II regions can be used as probes for the ionizing radition of their parent star(s). Since interstellar hydrogen is particularly efficient at attenuating radiation shortward of 912Å, direct observations of the far-ultraviolet radiation from hot stars is rare. WHAM may be able to fill an interesting niche here by detecting faint H II regions around isolated O and B stars. Since we can also map these regions in other emission lines, these new finds may be good constraints for theorists trying to model the spectrum of hot stars.

Geocoronal Studies

Unfortunately for Galactic observers, the earth provides it's own Hα and Hβ emission line which varies in time and location on the sky. However, this is precisely the interest of a group of Wisconsin Physicists! In collaboration with Susan Nossal, Fred Roesler, Frank Scherb, and Ed Mierkiewicz, nearly every photon collected from WHAM is being used for scientific research. The WHAM observations of the geocoronal line are helping to shape models of the earth's exosphere, the very outer reaches of our atmosphere. A recent publication by Susan Nossal is a good introduction to how WHAM is contributing to this area of physics.

Comet Hyakutake

In collaboration with Frank Scherb and Fred Roesler the WHAM group collected [O I] 6300Å, Hα, Hβ, and NH₂ data in March and April of 1996 during the close passage of the comet. The [O I] data provided a sensitive measurement of the water production rate in the comet. The Hβ data revealed the first detection of this line from a comet and, combined with the Hα data, provide interesting information on the solar Lymanβ emission line and how it affects the comet.

Comet Hale-Bopp

This spectacular comet was also observed by WHAM in February - April of 1997. The [O I] distribution around the comet was mapped out to explore water production rates. The water ion, H₂O⁺, was also observed this time, and provides a sensitive tracer of the comet's ion tail. Using WHAM's extremely narrow-band imaging mode (~12 km/s passband), we obtained a data cube of velocity slices, which should provide detailed information about the motion of ions down the comet's tail.