t Caralyn Flack's REU homepage, Summer 2004

Caralyn Flack

REU program-Summer 2004
Univ. of Wisconsin - Madison
Madison, WI 53706

flack@astro.wisc.edu

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

Introduction

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.

http://www.timstouse.com/milkyway.htm

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.

Detection

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.

Origin

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.

MS and Milky
Way http://www.atnf.csiro.au/news/press/images/magellanic_pics/

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.

Distance

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

My Project

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).

Understanding Spectra

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.

Fig. 2 This is the same plot as Figure 1, with a few simple changes. Most notably, the wavelength scale across the bottom has been converted to a velocity scale. You can now see the local OI absorption at 0km/s and the HVC absorption at 128km/s. There are also molecular hydrogen absorptions in this particular spectrum that are identified by H2 labels at the bottom of the plot.

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.

Measurements

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)

Fig. 4 This is an example of a Curve of Growth. The bottom is the actual curve and the top is a contour plot that the column density is derived from. This particular example is of SiII from the Mrk509 sight line.

Conclusions

WD (ESO265-G23)

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 measurements meaningless.

WD (NGC5253)

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.

Image Credit: Dr. Bart Wakker

In this map of HVCs, the sight lines I analyzed have been overlaid. The colors correspond to each clouds' velocity, with the scale wedge at the bottom. ESO265-G23 is in the middle of a group of small clouds, WD, and MRK205 is at the edge of Complex C. The line of sight toward PG1011-040 goes through another complex of small clouds called WB, and in the lower right, NGC7714 is at the edge of the Magellanic Stream.

WB (PG1011-040)

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.

Magellanic Stream (NGC7714)

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 (Mrk205)

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 from?

Image Credit: Dr. Bart Wakker

This is a map of IVCs, once again, with the sight lines I reduced overlaid. HE1143-1810 is in the same area of the sky as PG1011-040. There is a possibility of a connection between the clouds in these two sight lines. Also, MRK509 is close to the Magellanic Stream, but it's metallicity might be a bit too high for it to be part of the Stream.

IV-WA (HE1143-1810)

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.

gp (Mrk509)

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.

Final Comments

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.

References

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