arXiv:astro-ph/0112232v1 10 Dec 2001
ASP Conference Series, Vol. **VOLUME**, **PUBLICATION YEAR**
The Wisconsin H-Alpha Mapper Northern Sky Survey
G.J. Madsen, L.M. Haffner, & R.J. Reynolds
Dept. of Astronomy, University of Wisconsin – Madison, 475 N.
Charter Street, Madison, WI, 53706-1582
one-degree resolution, velocity-resolved northern sky survey of Hα emis-
sion from our Galaxy. The unprecedented sensitivity of the instrument
and accurate spectral subtraction of atmospheric features allow us to de-
tect Galactic features as faint as 0.1 Rayleighs (EM ≈ 0.25 cm−6pc).
This survey allows a direct comparison of the ionized and neutral com-
ponents of the ISM on a global scale for the first time. All-sky maps of
Hα emission in select velocity bands highlight the rich kinematic struc-
ture of the Galaxy’s ionized gas. The full set of data from the WHAM
survey is now available at http://www.astro.wisc.edu/wham/.
One surprising result is that the high latitude sky of both the ionized
and neutral components display marked similarity in the location and
radial velocity of emitting regions, especially for many of the previously
identified intermediate velocity clouds. Although there is evidence for
spatial and velocity correlation, in many cases examined so far there is
little evidence for a quantitative correlation between the column density
of H I and the emission measure of H II.
WHAM is also capable of studying the ISM through optical emis-
sion lines other than Hα. Two directions toward the Perseus arm have
been studied in detail through several other optical emission lines. The
multiple velocity component structure toward these directions provides a
selection of ionized environments for study and shows interesting varia-
tions in the ratios of these lines. WHAM also has the ability to study
select regions of the sky at high spatial resolution (3′to 5′) with high
velocity resolution. Such observations will allow an even closer compari-
son between the neutral and ionized components with the advent of high
resolution H I surveys.
The Wisconsin H-Alpha Mapper (WHAM) has completed a
The WHAM instrument is a fully remotely operated facility with a 15 cm, dual-
etalon Fabry-Perot spectrometer at the focal plane of a 0.6 m telescope atop
Kitt Peak in Arizona (Reynolds et al. 1998). The WHAM spectrometer has
a 1◦diameter circular field of view on the sky, and a velocity resolution of 12
kms−1within a 200 kms−1wide spectral window that can be centered on any
wavelength between 4800˚ A and 7300˚ A. WHAM was designed to detect very
weak emission lines from ionized gas.
Madsen, Haffner, & Reynolds
The Wisconsin H-Alpha Mapper Northern Sky Survey (WHAM-NSS) is the
first deep,velocity-resolved survey of interstellar Hα emission over the northern
sky (δ ≥ −30◦). The survey consists of 37,565 individual observations taken over
a span of two years. Each observation recorded the composite spectrum of a one-
degree diameter patch on the sky. The spectral resolution of 12 kms−1made it
possible to separate cleanly the interstellar emission from the terrestrial emission
in every spectrum and to measure the thermal and non-thermal motions of the
interstellar gas. Details about the WHAM-NSS and downloadable versions of
the survey can be found at http://www.astro.wisc.edu/wham/.
These survey maps of interstellar Hα emission provide the first global view
of the distribution and motions of wide spread ionized hydrogen within the
Milky Way (see Figure 1). The small bright knots of Hα are classical emission
nebulae in the vicinities of hot O and B stars located mostly near the Galac-
tic midplane. Between these bright knots and filling most of the sky is fainter
Hα emission from the Warm Ionized Medium (WIM), with a characteristic tem-
perature Te≈ 104K and density ne≈ 0.1 cm−3.
Past studies have shown the WIM to be a significant component of the
interstellar medium, especially in the halos of disk galaxies. In the Milky Way,
the WIM has a mass surface density about one-third that of neutral atomic
hydrogen, a power requirement equal to the kinetic energy injected into the
Galaxy by supernovae, and a characteristic scale height above the midplane
of 1000 pc, approximately five times larger than that of the neutral hydrogen.
This survey of the WIM shows rich structure both on the sky and in velocity,
allowing an exploration of the origin of the ionization and heating of this gas
and its relationship to the other components of the interstellar medium.
2. High Latitude Hα Emitting H I Clouds
The intermediate velocity channel maps (−75 kms−1≤ vLSR≤ −50 kms−1)
from the survey reveal significant amount of high-latitude Hα emission. Much
of this emission is located near HI gas previously identified and classified as In-
termediate Velocity Clouds (IVCs; see the recent compilation by Wakker 2001).
One particularly striking example is the IVC Complex K, reported by Haffner,
Reynolds, & Tufte (2001).
They find that general spatial correlation between NHIand Hα contours,
and an excellent kinematic correlation between Hα and H I spectra toward
Complex K, demonstrating that they probe the same structure. However, the
spatial intensity distribution of the lines does not appear to be related, i.e.
peaks in H I emission do not necessarily correspond to peaks in H II emission.
This is consistent with photoionization by an external flux of radiation. In this
case, the Hα intensity is determined by the incident flux and is unrelated to the
H I column density of the cloud whose outer surface is being ionized.
WHAM has also recently observed very faint Hα emission associated with
the northern tip of the Magellanic Stream. In all five lines of sight observed, we
find Hα emission at the same velocity as the H I gas with intensities ranging
between 0.05 - 0.1 R (EM ≈ 0.1 −0.2 cm−6pc @ 104K). Such observations are
critical for our understanding of high and intermediate velocity clouds, including
WHAM Northern Sky Survey
(right) from WHAM. The spectra toward the two directions labeled
by their titles are marked by an ’X’ in the images on the right. Note
the variations in the line ratios among the velocity components outlined
by the dashed vertical lines in the spectra. The images on the right are
two different velocity channel maps of the same region of the Galaxy,
sampling local gas (top) and more distant Perseus arm gas (bottom).
Multiwavelength spectra (left) and Hα survey images
the ability to constrain models of the escape of Lyman continuum photons from
the Galactic disk (Bland-Hawthorn & Maloney 1999).
3.Probing the Heterogeneous Nature of the WIM
The WHAM Northern Sky Survey has revealed the presence of remarkable large-
scale, Hα-emitting structures, including loops, filaments, bubbles, and bright
“point sources” throughout the WIM superposed on a more diffuse Hα back-
ground (e.g. Figure 1). With the completion of the survey, WHAM can now
be used to study the WIM through observations of several other optical emis-
sion lines, probing the physical properties of the gas, and with higher angular
resolution imaging, exploring its smaller scale structure.
Figure 1 shows the spectra of sight lines towards l = 130◦,b = −7.5◦and
l = 133◦,b = +18◦, sampling filamentary and diffuse gas. The data are given
in units of arbitrary flux versus LSR velocity. Figure 1 also shows two velocity
Madsen, Haffner, & Reynolds
channel WHAM-NSS maps towards these directions, chosen to isolate the local
gas near the LSR and the more distant gas in the Perseus arm near -60 kms−1.
The 0 kms−1frame is dominated by diffuse ionized gas on which is superposed
large, low density O and B star H II regions. The large “bowtie” in the Persues
arm frame is believed to be a superbubble blowout associated with the Cas OB6
association and the H I Normandeau “chimney” (Reynolds, Sterling & Haffner
2001; Normandeau, Taylor, & Dewdney 1996).
The spectra reveal multiple emission line components in each direction,
interpreted as local gas, Perseus spiral arm gas a few hundred parsecs above
the midplane at a distance of 2-3 kpc, and higher velocity gas at even greater
distances. We find significant variations in the ratio of these emission lines
among the different radial velocity components along a single line of sight, as
well as between the two lines of sight. Preliminary analysis reveals interesting
correlations and anti-correlations among the line ratios and raises new questions
about the heterogeneous nature of the WIM. Analysis of the strength of these
emission lines and their ratios reveal important clues about the temperature and
ionization state of the WIM, and indirectly reveal information about the ionizing
spectrum, extinction, density, and heating of the gas (Haffner, Reynolds, & Tufte
1999; Reynolds, Haffner, & Tufte 1999). We also find ionized gas associated
with each of the warm H I components toward (130,-7.5), but not with the cold
H I feature near -30 kms−1. These observations can be used to explore how the
physical conditions in the WIM change with morphology, and how they compare
with classical H II regions.
Another new tool available with WHAM is higher angular resolution imag-
ing. With the insertion of a set of optics, WHAM can obtain very narrow band,
3′resolution images of the sky within its 1◦beam. An adjustable iris mechanism
in the imaging optics can set the width of the narrow spectral window of the
image between 15 kms−1to 200 kms−1. The combination of the multiwave-
length observations of various diagnostic emission lines with the higher angular
resolution imaging will allow us to learn more about the elusive nature of this
important phase of the interstellar medium. The WHAM Survey has already
revealed the rich spatial and kinematic structure of the warm ionized medium.
These multiwavelength and higher spatial resolution observations will hopefully
shed additional light on fundamental questions pertaining to the nature of the
WIM and no doubt will raise even more questions. This work was supported by
National Science Foundation Grant AST96-19424.
Bland-Hawthorn, J., & Maloney, P.R. 1999, ApJ, 510, L33
Haffner, L.M., Reynolds, R.J., & Tufte, S.L. 1999, ApJ, 523, 223
Haffner, L.M., Reynolds, R.J., & Tufte, S.L. 2001, ApJ, 556, L33
Reynolds, R.J., Haffner, L.M, & Tufte, S.L. 1999, ApJ, 525, L21
Reynolds, R.J., Sterling, N.C., & Haffner, L.M. 2001, ApJ, 558, L101
Reynolds, R.J., Tufte, S.L., Haffner, L.M., Jaehnig, K., & Percival, J.W. 1998,
PASA, 15, 14
Normandeau, M., Taylor, A.R., Dewdney, P.E. 1996, Nature, 380, 687
Wakker, B.P. 2001, ApJS, 136, 463