1. Christopher M. Witt1
, B. Wakker2
, T. D. Engel1
, M. C. Gostisha1
, E. Thomson1
, L. Stratman1
, R. A. Benjamin1
1
University of Wisconsin-Whitewater, 2
University of Wisconsin-Madison.
Abstract: We present progress towards the creation of a new all-sky catalog of intermediate velocity clouds using the Leiden/Argentina/Bonn (LAB) Galactic HI survey. We have developed a Gaussian fitting program to fit
individual spectra. Each spectra is initially fit automatically with a set of Gaussians, and then reviewed and adjusted, if necessary, by hand by our undergraduate team. When a satisfactory fit is found, it is submitted for
review and adjustment by the senior team member. Intermediate clouds and complexes are formed by grouping Gaussian components by velocity and section of the sky. When complete, this will be the first all-sky catalog
of intermediate velocity clouds, which can be compared to dynamical models of the Galactic fountain flows. We present preliminary results for the catalog in the sky with Galactic latitude greater than 45 degrees. This
research was supported by NASA ATP grant NNX10AI70G to the University of Wisconsin-Whitewater.
ACKNOWLEDGEMENTS
We would like to acknowledge the Astronomy REU program at the University of Wisconsin
Madison for its support during the summer of 2009, to National Science Foundation and the
NASA ATP grant NNX10AI70G given to the University of Wisconsin-Whitewater.
The data set used for this research is the Leiden/Argentine/Bonn Galactic H I Survey (LAB). LAB
is the union of two previous surveys: the Instituto Argentino de Radioastronomía Survey (IAR:
Arnal et al. 2000 and Bajaja et al. 2005) which covered the sky south of Galactic Latitude = -25°
and the Leiden/Dwingeloo Survey (LDS: Hartmann & Burton 1997) which covered the sky north
of Galactic Latitude = -30°. The data was processed to correct for instrumental artifacts, like
"stray radiation", in Bonn by Kalberla et al 2005.
This is the most sensitive all-sky Milky Way H I survey to date. It maps emission from the 21 cm
line of H I. It has an angular resolution HPBW ~ 0.6°, LSR velocity span of -400 km/s to +400
km/s, and resolution of 1.3 km/s. This data will allow us to create a higher resolution IVC and
HVC map than previous surveys (Wakker 1999). We expect that new clouds and cloud
structure will be discovered because of LABs greater angular resolution.
Data Source
What Intermediate Velocity Clouds Are
Intermediate Velocity Clouds (IVCs) are large Hydrogen clouds above and below the plane of the Milky Way Galaxy
which are moving at velocities that cannot be explained by galactic rotation. The velocity range they span is +/- 30 to
90 [km/s] from the local standard rest (LSR), the average velocity of nearby stars. IVCs are thought to have originated
from in a “Galactic Fountain”. In this model, gas within the disk of the Milky Way Galaxy is heated by supernovae
forming superbubbles. The resulting bubble then ejects hot gas out of the galactic disk. Some of this gas can fragment
to form IVCs. Over the past few decades these clouds have grown something of a curiosity into an area of active
research. It is in the spirit of continuing this research that we set out to create a full sky map of IVCs.
Figure 3. A spectra
containing two components
of Intermediate Velocity
Gas.Intermediate
Velocity
Cloud’s
LSR velocity [km/s]
Antenna
Temperature
Figure 1. The
initial automated
fit. Data is
shown in black,
the individual
gaussians in blue,
and the
combined
spectrum in red.
Figure 2. The
same spectra as
figure 1 but
corrected.
Note the faint
component
centered at
v=-102 km/s
which was
missed by the
automated fit.
Computer Fit
Peaks and Gaussians
Using a program developed by Dr. Bart Wakker we are fitting Gaussian curves to
the LAB survey in order to break each spectrum up into individual velocity
delimited clumps of gas. In Figure 3 is an example of a LAB spectrum fitted by
Gaussian curves. Along the x-axis is the LSR velocity [km/s] of the gas and along
the y-axis is the antenna temperature which represents energy flux. Each peak
corresponds to a clump of gas moving at a given velocity. In Figure 3 two such
bumps have been marked. They are Intermediate Velocity Clouds because they
fall within the range vLSR= -30 to -90 km/s.
Human Involvement
Each spectrum is fit individually, by hand. One may ask why we have not
automated the entire process. The difficulty lies in the computer estimates. No
matter how intelligent we make our Gaussian fitting program, it inevitably
encounters spectra it is unable to fit. Some areas of the sky have distinct peaks
which the program easily picks up. Other areas have broad peaks that fit poorly,
blended peaks, or small peaks at large velocities which the program misses
completely. Often the program will even create fits that perfectly follow the
spectrum but don't make physical sense. It is due to these difficulties that we
look at each spectrum individually.
Procedure
Future Work
Once the entire sky has been mapped and categorized, we will compare what we have found to
older catalogs and create a new IVC catalogue which covers the entire sky. New clouds will be
given new designations and the names of previously discovered clouds may change since the
angular resolution of the data is making more subtle structure visible.
Preliminary Results
As of Jan 2011 we have fit ~ 45 000 out of 203 000 total positions. The maps shown
in the figures below. Some interesting results so far include the following:
1. Evidence for “Compact IVCs”. In general, intermediate velocity gas tends to
be found in large associations, like the two features discussed below. However,
we have found a few clear examples of small, isolated clouds at intermediate
velocities, two of which are shown in Figure 4. These objects may be analogous
to “Compact HVCs” but with projected space velocities not large enough to be
identified in previous analyses, or they may be small scale local features.
2. Correlation of velocity extrema with integrated column density in the
large scale Pegasus-Pisces Arch (Figure 5) and the Intermediate Velocity Arch
(Figure 6). We find a correlation between the HI column density and location of
the most extreme velocity peaks. In certain locations, structure found in column
density is adjacent to structure found in velocity peaks. This will constrain
models for the origin of these two features. Detection of intermediate velocity
interstellar absorption towards the central concentration (L=85, b=-37) in the
Pegasus-Pisces arch indicates this gas is closer than 1.1 kpc. Absorption line
studies along the IV arch indicate a distance range of 0.4-2.4 kpc (Wakker 2001).
Figure 6 Integrated column densities of HI at V=-80 to -30 km/s
[upper], and map of the velocity of peak emission [lower] for the
Intermediate Velocity Arch (Kuntz & Danly 1996).
The structure arcing from (l,b)=(100,40) to (160,70) to (220,40) is
the Intermediate Velocity Arch. Note the general correlation
between the column density field and the velocity field. However
the section with the sharpest divergence in the velocity field
(narrow ellipse) is offset to the edge of the column density
maximum (wider ellipse).
Figure 5 Integrated column densities of HI at V=-80 to -30 km/s
[upper], and map of the velocity of peak emission [lower] for the
Pegasus-Pisces Intermediate Velocity Arch (Wakker 2001).
The structure indicated is part of the Pegasus-Pisces intermediate
velocity cloud, showing a general correspondence, but slight offset
between the column density (larger ellipse) and velocity extremes.
Figure 4 Map of the velocity of peak emission in the high-latitude fourth
quadrant. Although much of the intermediate velocity gas is in complexes,
we have found several small isolated intermediate velocity clouds, two of
which are noted in the figure to the right.