REU program-Summer 2004
Advisor: Dr. Bart Wakker
My Final Presentation (.ppt)
Research projects of other
2004 REU students
Introduction | Detection | Origin | Distance
My Project | Understanding Spectra | Measurements
Conclusions | References | Useful Links
Metallicities and Abundances of
High Velocity Clouds
Most of the gas and stars in the Milky Way rotate about the center of the galaxy at a velocity of 220km/s. However, clouds of mostly hydrogen have been detected in the halo of the Milky Way that travel at velocities greater than can be explained by the rotation of the galaxy. The range of velocities was broken into two groups to create what are now called intermediate-velocity clouds (IVCs) and high-velocity clouds (HVCs). Using the sun as the frame of reference, which is referred to as the local standard of rest or LSR, IVCs appear to be moving at 30 < |v| < 90km/s and HVCs move at |v| > 90km/s.
In the diagram on the left, we can see that the sun is located at the edge of one of the Milky Way's spiral arms. The edge-on view shows the halo, a large but thin sphere of very hot, highly ionized gas, where many HVCs are thought to be located.
Finding HVCs starts with being able to locate clouds of neutral hydrogen, or HI. This is usually done by detecting 21cm emission, which is given off by the cloud when electrons in the hydrogen atoms switch their direction of spin. Once clouds of HI have been found, they are categorized according to their velocities. Further analysis of HVCs and IVCs is done by looking at spectral absorption lines. (See Figure 1 just below.)
When doing spectral analysis, a good background source of light must first be found. Quasars (QSO), active galactic nuclei (AGN) and starburst galaxies are often used because they are so bright in a wide range of wavelengths. When a cloud is in front of a background source, the elements in the cloud absorb some of the light passing through it. The electrons in each element only absorb certain wavelengths of light, due to specific electron transitions, so when looking at a spectrum, a dip will occur where there has been an absorption. The spectra toward different background sources, called sight lines, are recorded by satellites, because ground based detectors require the light to travel through our atmosphere.
Fig. 1 The figure above is an example of an absorption line from an HVC. The units on the bottom are in Angstroms, which are interchangeable with velocity, and the unit on the side are of flux, Rayleighs. The title across the top of the figure gives the background source, the element, and the wavelength. In this case, neutral oxygen at a wavelength of 1039.2303 Angstroms was detected toward PG1011-040, a Seyfert galaxy.
The origin of these clouds is still unknown, but there are several leading theories, each of which may be valid. First is the Galactic Fountain, where gas from the Milky Way was blown out of the disk and into the galactic halo by a superbubble, caused by supernovae. This gas then cooled and condensed, creating HVCs. As these clouds fall back toward the disk of the Milky Way, friction from the Galaxy's rotation slowed them to velocities we consider IVCs. In this case, the abundance of elements heavier than hydrogen and helium, called a cloud's metallicity, should be about the same as the Galaxy's, or solar metallicity. This theory is supported by the detection of low latitude IVCs with abundances similar to that of the gas in the disk.
Another theory is that when the Milky Way was forming about 10 billion years ago, it left small clumps of gas behind. These clumps are now falling back toward the galaxy and will eventually mix with the gas in the disk, infusing it with fresh hydrogen. Since heavy elements are created by nuclear fusion in stars, this infalling gas should have very low abundances of heavy elements. Complex C, a large HVC, supports this theory because of its low metallicity and the fact that it is falling into the disk of the galaxy.
|In this image you can see both the Large and Small Magellanic Clouds with the smooth trail of the Magellanic Stream behind them. The leading arm, however, does not have a nice, stream-like look to it due to gravity from the LMC.||
The Large and Small Magellanic Clouds (LMC and SMC, respectively) are two small galaxies that orbit the Milky Way every 2 billion years. They have an elliptical orbit, and it is thought that the last time they passed close to the Milky Way, tidal forces from both the LMC and Milky Way pulled gas from the SMC. Much of this gas was pulled into the LMC, but the rest was either accelerated, to form the irregular leading arm of the Magellanic Stream, or decelerated to form the trailing arm. The metallicity of the Magellanic Clouds is slightly lower than that of the Milky Way, so the slightly less than solar metallicity of the Magellanic Stream is a strong indicator that this theory is correct.
|http://spaceflightnow.com/news/n0305/06clouds/||This is an actual radio image of the Magellanic Clouds. Notice the area between the two clouds where it looks like matter is being ripped off the smaller cloud and pulled up into the larger cloud. You can also see the Magellanic Stream being left behind.|
The final theory is much more uncertain than the other three listed
above, but could still be a possible explanation of the origin of HVCs. It
proposes that HVCs are farther away than originally thought and are not even
within the Milky Way's halo. Instead, they are intergalactic clouds, but this
increased distance means they must also be much larger than originally
thought. The problem with this lies in the fact that there is not enough
matter in an extremely large cloud of gas to hold itself together
gravitationally. Therefore, there must be dark matter mixed in with the neutral
hydrogen to keep the cloud from dispersing. An obvious limitation of this
theory is that we still don't know what dark matter is. Nonetheless, it is
a probable explanation for certain HVCs.
In this image, created by Dr. Bart Wakker, the large form of Complex C is circled with Complex A, another large HVC, off to the left. Below the Milky Way, the Magellanic Clouds can be seen as two small white dots surrounded by blue, with the trailing edge of the Magellanic Stream curving behind, or to the left of them.
Determining the distance to each HVC is fairly difficult. There are many ways it can be done indirectly, but a variety of assumptions must be made. On the other hand, there is a direct way to find a range for the possible distance to a cloud called the absorption-line method. This method uses the idea that if a cloud is in front of a background source, there will be an absorption, but if the cloud is behind the source, no absorption will be detected. (See image to right.) When the distance to the background sources is known, a limit on the distance to the HVC is possible. This process is very limited by the number of possible background sources and the certainty of non-detections.
|  Image Credit: Ingrid Kallick|
I spent most off the summer analyzing data from several different sight
lines. The majority of the data came from the Far Ultraviolet Spectroscopic
Explorer (FUSE), but a few extra absorption lines were outside of the
wavelength range of FUSE, so data from the high resolution spectrograph (HRS)
and the space telescope imaging spectrocgraph (STIS) on Hubble were also
needed. Using a program that my advisor wrote, we went through and measured all
the absorptions in each of the following sight lines: ESO265-G23 (l=285.91,
b=16.59), PG1011-040 (l=246.50, b=40.75), HE1143-1810 (l=281.85, b=47.71),
MRK509 (l=35.97, b=-29.86), NGC5253 (l=314.86, b=30.10), NGC7714 (l=88.22,
b=-55.56) and MRK205 (l=125.45, b=41.67). The majority of these were Seyfert I
galaxies and were toward positive velocity HVCs and IVCs. NGC7714, on the other
hand, was a startburst galaxies and was toward a negative velocity component
of the Magellanic Stream. MRK205 was also toward negative velocity clouds,
including a CHVC, Complex C-south and part of the lower latitude intermediate
velocity arch (LLIV).
In Figure 2, below, notice the large dip in the spectrum right where the
velocity is zero. This is local oxygen absorption. To the right of this line
there is another fairly large dip, which was shaded in Figure 1. This is the
redshifted HVC absorption. Based on the plot, its velocity is a little over
+100 kilometers per second, and we know from the Leiden Dwingeloo HI Survey
that its velocity is 128km/s.
The dotted line across the top of Figure 3 is called the continuum and is
an estimate of the original amount of light being emitted by the background
source. When we know the original light level is relatively constant, the
continuum is usually pretty simple. In most cases, we looked at a relatively
wide velocity range and selected the parts of the spectrum that were flat. The
program then calculated the best-fit line and printed it out as a red dotted
line. When there are a lot of absorptions and blending between the lines, the
continuum may not be as easy to see. Looking at a wider velocity range
sometimes helps, but other times, you just can't figure out where the
continuum should be.
Fig. 3 This is a simple example of fitting a continuum. The black lines above the spectrum show the sections of the plot that the program used to create the continuum. Notice the Nitrogen triplet in the middle of the plot with HVC absorptions to the right of the second and third lines.
The next step is manually measuring the equivalent width of each absorption in a sight line. All of these should be in a similar velocity range, but after all of the initial measurements have been made, a fixed velocity range is selected. This takes some of the subjectivity out this process and gives the final equivalent width for each absorption line. These values are then used to create a Curve of Growth.
The Curve of Growth (See Figure 4) is a curve that relates all the
equivalent widths for each ion and gives the expected column density for a
particular dispersion velocity, or b-value. The log of the HI column density
is then subtracted from the log of the column density from the curve of
growth, giving the abundance of that ion in the cloud. Next, the abundances
are compared to those of our Galaxy. If they are similar, the cloud is
probably gas from the Milky Way. If the abundances are much lower, a couple of
other theories may apply. (See section on Origins)
The ESO265-G23 sight line went through a section of complex WD with an LSR
velocity of 117km/s. WD is a group of small, positive velocity clouds with
similar positions and HI characteristics. The data had a low signal to noise
ratio (S/N), which made actual absorptions difficult to distinguish from
noise. OI-1039, FeII-1144 and FeII-1096 were the only three sigma detections,
but upper limits were possible for CIII, PII and ArI. Neutral oxygen and FeII
abundances were only 0.1 solar, which implies very little dust. Both the
abundances and lack of dust are similar to Complex C. However, Complex C is in
a different part of the sky and is a negative velocity HVC. Also, a previously
undetected IVC was found at 40km/s. Unfortunately, the low S/N made any
NGC5253 was another sight line through Complex WD, but the results are very
different. The cloud's velocity is 102km/s and its abundances are 0.72
solar. This is much higher than the abundances calculated toward ESO265-G23.
Therefore, the differences in the clouds in this area of the sky cannot be
distinguished by their HI characteristics.
PG1011-040 goes through complex WB, another group of small clouds just
above WD and has a LSR velocity of 128km/s. This sight line started out
looking like it would give some very good results because of its high S/N and
many absorptions. Unfortunately, the equivalent width measurements fell on the
critical part of the curve of growth, so the errors were very large. Regardless
of the errors, OI, SiII and FeII produced three very consistent curves of
growth and we could get a measurement on the column density. The abundances
turned out to be 0.38 solar, which is higher than the Magellanic Cloud but
lower than the surrounding clouds.
This sight line looked through the edge of the Magellanic Stream,
v=-311km/s, and toward NGC7714, as starburst galaxy, which means the level of
light coming from the galaxy is not constant. The continuum should end up
looking like a wavy line, so it can be very difficult to figure out. There was
also a lot of blending of the lines and low S/N so only two absorptions were
detected, FeII-1144, 1063. Upper limits were measured for PII, ArI and a
few other FeII lines. The curve of growth was not restricted enough to give a
good column density, so only a lower limit was possible. Consequently, no
useful abundances were calculated from this sight line.
WW84 is a compact high velocity cloud (CHVC) traveling at -206km/s. CHVCs
are a subset of HVCs that cover <2 degrees of the sky and have a compact
core. They may be the remnants of the formation of the Local Group, which
would put them in intergalactic space. A limit on the distance of WW84 was
given by Braun & Burton (2001) and Bruens, et al. (2001) to be 150-900kpcs. At
this distance, the cloud cannot be gravitationally stable, which means cold
dark matter (CDM) must be stabilizing it. The abundances we measured came out
to be ~0.1 solar, similar to that of Complex C. The abundances may be low, but
if this cloud was never part of the local galaxies, where did the metals come
When I started analyzing the cloud in the line of sight of HE1143-1810, its
name had not been specified in any liturature. Because it is in the same
area of the sky as WB and WD and is an IVC (v=56km/s), Dr. Wakker proposed
naming it IV-WA. This keeps with the same naming scheme in that part of the
sky and identifies it as an IVC. The abundances of IV-WA turned out to be
about solar, which is very strong evidence that the source of this cloud is
the Galactic Fountain. Since it is so close to Complex WB, there is a
possibility that the two are part of one large complex. The PG1011-040 sight
line had similar abundances, within the errors, and if a velocity gradient is
introduced, it is not a stretch linking the two clouds.
This cloud, in complex gp, has a velocity of 60km/s and had many apborption
lines. OI, SiII and FeII produced relatively good curves of growth which lead
to abundance measurements around 0.3 solar. Although this seems to be
consistant with abundances calculated for WB and a sightline to HD215733 that
Dr. Wakker is working on, it does not match any theory. The Magellanic Stream
has abundances that are close, but gp is not at a position or velocity
predicted by models of the stream.
Abundances can be difficult to determine. Accurate measurements depend on the signal to noise ratio, blending of lines and where the equivalent widths fall on the curve of growth. Nonetheless, abundances are a valuable tool in determining the origins of HVCs and IVCs. For Complex WD, they showed that similar HI properties do not necessarily mean a connection between clouds. For IV-WA and WB, however, abundances can imply a possible relationship, even if a velocity gradient in the HI is necessary.
Tripp, T.M., et al. 2003, AJ, 125, 3122
Wakker, B.P. 2001, ApJS, 136, 463
Wakker, B.P. & Richter, R. 2004, Sci. Amer., 290, 28
Wakker, B.P. & van Woerden H. 1997, ARA&A, 35, 217
Wakker, B.P., et al. 2003, ApJS, 146, 1
Introduction | Detection | Origin | Distance
My Project | Understanding Spectra | Measurements
Conclusions | References | Useful Links
Last edited: August 6, 2004