Emission Line Maps of the Milky Way

On August 8, 1996, WHAM obtained its first test map from the Galaxy during final testing at Pine Bluff Observatory in Wisconsin. Shortly after, it was moved to Kitt Peak and began it's primary task: mapping the entire sky in Hα. Most of the data for the northern sky was obtained during 1997 and 1998 while the southern sky was observed in 2009 and 2010. 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 been published (see our publications list for several 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 the Southern Hα Sky Survey Atlas (SHASSA).

Intermediate- and High-velocity Clouds

Emission-line studies of HVCs provide many new clues about the nature of these elusive objects. Until recently, HVCs and IVCs 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. We can tune WHAM's velocity window to observe gas outside the ±100 km/s window about the local standard of rest covered by the sky survey. With this technique, we have made many 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). Hα emission from intermediate-velocity gas appears in the primary survey, which is also highlighted in a few of our publications.

Magellanic System

LMC & SMC: We will be mapping emission in and around these nearby neighbors to see how far their ionized gas extends. The kinematics revealed by these spectral observations may uncover some surprises and should allow us to study connections between the neutral gas in and around the galaxies and any extended ionized structures. WHAM's one-degree beam is too large to provide a great amount of insight on the internal structure of the galaxies themselves.

Bridge: 21 cm observations have shown that a tenuous neutral structure connects the galaxies. Some ionized material and even early stars suggestive of recent star formation has been detected in the portion of the Bridge closest to the SMC. With WHAM, we will be able to detect and map any ionized gas associated with this feature to very faint emission measures.

Stream: An impressive, ~100°-long tail of gas trails the clouds' orbit through the Milky Way's halo. Prior studies have shown there is both ionized gas and Hα emission at certain locations along the Stream, but the full structure has only been surveyed in H I to date. One of our major priorities from Chile is to provide the first survey of the Stream in Hα and to carry out several comprehensive multiline studies along its length. There are many outstanding questions about the origin of the Stream and its evolution, dominant gas phases, ionization structure, and interaction with the halo of the Milky Way.

Leading Arm: Out ahead of the galaxies' orbit, a less organized collection of neutral gas proceed their passage through the halo. No ionized emission has been detected from this structure yet, but WHAM should provide the most sensitive search to date.

[S II] 6717Å
[N II] 6583Å

These lines are nearly as bright as Hα in the WIM and are good tracers to discriminate between the diffuse background emission and H II regions. Combined with Hα, they begin to trace the physics of this ionized phase in addition to its distribution. In both the Milky Way and other spiral galaxies these lines tend to increase in intensity relative to Hα as the Hα emission decreases. In several of our papers, we propose that these rises are due to increasing temperatures. Both the collisional excitation of theses forbidden lines and the recombination that produces Hα are functions of the gas density squared (the emission measure). However near 10,000 K, the emissivity of the forbidden lines increases much more rapidly with temperature than that of Hα decreases. Thus, the smooth increase of [N II]/Hα and [S II]/Hα ratios with decreasing Hα intensity seems to indicate a gradual rise in the gas temperature. This argument can be taken one step further since, in 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 exponential scale height of the ionized layer. In this particular case, we 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 detected this line from the WIM for the first time near the Galactic plane. Measurements of this line relative to Hα provide a good estimate of the average fraction of neutral oxygen (and thus hydrogen) along the line of sight. See Reynolds et al. 1998 and Hausen et al. 2001 in our publications list for details.

[N II] 5755Å

Like the 4363Å "auroral" line of [O III], this upper level transition from the isoelectrically similar [N II] spectrum provides a direct measurement of the temperature of an ionized region. Since [O III] is quite faint in the WIM, the 5755Å line is more likely to be detected from the diffuse background. Although the emission is still very faint, we have several detections that confirm that the WIM has high temperatures for photoionized gas, particularly when compared to diffuse H II regions ionized by single stars (see Reynolds et al. 2001 and Madsen et al. 2006 in our list of papers).

He I 5876Å

Using WHAM, we have detected this line for the first time from the WIM (see Steve Tufte's PhD thesis and Madsen et al. 2006). 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.

[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. Observations of this gas at even higher latitudes provides upper limit measurements of the contribution of 5007Å emission from hot, Galactic coronal gas. Madsen et al. 2006 provides a recent summary of some detections and upper limits of [O III] from the WIM.

Since most of the Hα emission we detect arises from hydrogen recombination, atomic physics dictates a set ratio of Hα to Hβ emission from ionized interstellar gases. Although it is a slight function of temperature, near 10,000 K, the ratio is about 3:1 in favor of Hα. However, interstellar dust absorbs more blue light than red so that ratios greater than this are typical in observations. Observed ratios of Hα/Hβ provide an interesting probe of dust in front of and within the ionized gas. Madsen & Reynolds 2005 present our first application of this technique toward a hole in the local dust toward the northern inner Galaxy, which generally has a substantial amount of obscuration.

H II Regions

Aside from being interesting studies in their own right, H II regions can be used as probes for the ionizing radiation 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 fills an interesting niche here by detecting faint H II regions around isolated main sequence and evolved O and B stars (e.g., Reynolds et al. 2005). 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 with time and location on the sky. However, this is precisely the interest of a group of Wisconsin Physicists. In collaboration with Susan Nossal, Ed Mierkiewicz, and Fred Roesler nearly every photon collected from WHAM is being used for scientific research. WHAM observations of the geocoronal line are helping to shape models of the earth's exosphere, the very outer reaches of our atmosphere.


Hyakutake: In collaboration with Frank Scherb and Fred Roesler the WHAM group collected [O I] 6300Å, Hα, Hβ, and NH 2 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 Lyβ emission line and how it affects the 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 may provide detailed information about the motion of ions down the comet's tail.