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Lecture Series sponsored by the National Science Foundation through
CAREER Grant AST-9984073 awarded to Prof. Kim Venn.
Webpage developed by Chrissy Blank (Macalester Physics/Astronomy '03)
Presented in collaboration with Prof. Evan Skillman and the
Department of Astronomy, University of Minnesota.
| Visiting Lecturers: | ||
| Professor Sidney van den Bergh | January 31, 2002 | Dark Matter in the Local Group |
| Professor Lisa Young | February 7, 2002 | Gas in Dwarf Spheroidal (dSph) Galaxies |
| Dr. Chris Stubbs | February 28, 2002 | MACHO (MAssive Compact Halo Objects) Surveys |
| Dr. Abi Saha | March 7, 2002 | Hubble's Constant From Type Ia Supernovae |
| Dr. Stephen Smartt | March 14, 2002 | Supernovae Progenitors & Massive Stars |
| Dr. Eline Tolstoy | April 4, 2002 | Global Star Formation History |
| Professor Liliya Williams | April 9, 2002 | Hubble's Constant from Gravitational Lensing |
| Dr. Matthew Shetrone | April 18, 2002 | Chemical Evolution of dSph Galaxies |
| Dr. Taft Armandroff | April 25, 2002 | Survey for Dwarf Galaxies |
| Professor Mario Mateo | May 2, 2002 | The Past, Present & Future of the Local Group |
Dark Matter in the Local Group
Sidney van den Bergh
(at left)
Dominion Astrophysical Observatory (DAO), NRC of Canada, Victoria, British Columbia
*Professor van den Bergh wrote the text used in this course, Galaxies of the Local Group (1999, Cambridge University Press), so it was appropriate that he was the first speaker to visit the class.
Balance Sheet for Galactic Matter:
1900: 100% stars
2000: 1% stars and cold gas, 3% hot gas, about 30% cold dark matter and 66% dark energy (we lost 97% of what we thought we knew in 100 years!)
Who "saw" it first?
Zwicky, 1933 (at right; in his most famous pose): first paper mentioning dark matter:
Looked at 8 Coma galaxies. Assuming visual equilbrium, calculated mass-to-light ratio and determined that about 90% of the mass necessary to account for observed ratio was missing and therefore invisible, or "dark".
"The discrepancy appears to be real and important." --Edwin Hubble, 1936
The dissenters:
deVaucouleurs, 1960 (at left; second from left): Clusters are expanding like stellar associations, thus the visual theorem is not justified in clusters of galaxies. The picture, taken in 1963, shows the first astronomers to work at the University of Texas. (Incidentally, Professor Venn received her PhD at Texas!)
Yet 75% of all ellptical and 50% of all spiral & irregular galaxies are members of clusters--therefore clusters are not all that unstable after all!!
More Evidence:
Oort, 1940: Mass-to-light ratio also shows 90% of local group mass is "missing." This discovery was independent of the discovery of missing mass in clusters; Oort didn't cite Zwicky's original 1933 paper.
Rubin & Ford, 1970; Roberts & White..., 1975: radial velocity curve plots radius vs. velocity and flattens out rather than trailing down. This implies that mass continues to increase with radius. At right, see a plot of the radial velocity versus distance for the Milky Way Galaxy. The Keplerian Motion line shows the predicted drop-off for the curve; while the red line is the plot of actual data. Plots of many other galaxies have shown the same trend to a flat, rather than Keplerian curve.
"Cold Dark Matter"
In closing, cold dark matter is not a trivial hypothesis. Cold dark matter could account for up to 30% of the closure density of the universe and is therefore pertinent to study of the past, present and future of the universe.
Gas in Dwarf Spheroidal (dSph) Galaxies
Lisa Young
New Mexico Tech, Socorro, New Mexico
Is there hydrogen gas in dwarf spheroidal galaxies?
Typically dwarf spheroidals are thought to have formed all of their stars in old and intermediate-aged bursts, and to be devoid of gas today. This is unlike transition galaxies and the gas rich dwarf irregular galaxies.
Some 21 cm detections towards dwarf spheroidals with the correct radial velocities, e.g., Carignan et al. (1998) who find H I associated with the Sculptor dwarf spheroidal galaxy. Young noted that a wider field shows similar H I, suggestive of a high velocity cloud complex. Thus, perhaps Sculptor does not have H I.
The image at right, of Sculptor Dwarf Spheroidal Galaxy (the cluster in the center), shows that extensive high velocity clouds are all around the relgion and perhaps not associated directly with Sculptor.
For more information:
MACHO Surveys
Chris Stubbs
University of Washington, Seattle, Washington
See Stubbs, at right, looking through a MACHO telescope.
What is out there?
| Ordinary Stuff | Exotic Stuff |
| Neutrinos? very little rest mass and moving very quickly | WIMPs-Weakly Interacting Massive Particles |
| Matter? stars-we'd see it as gas or plasma | Critical density universe? |
| Planets? don't emit or absorb much light | Axions? As-yet undiscovered, hypothetical particle |
| MACHOs? Jupiter-size planets, tiny black holes, and brown dwarves | |
| Massive neutrinos? |
How do we look for these objects? Microlensing!!
The diagram at left shows how an object, such as a brown dwarf, when between our eye and a star, will bend the light from said star and bring a distorted image to the eye. This image illustrates what it might look like to an observer to ''see'' a brown dwarf pass through their field of vision. The brown dwarf itself is invisible, but it creates a visible disturbance in the light paths it passes through. For the source of this diagram as well as a number of real images showing gravitational lensing, click here.
Sadly, the objects the MACHO surveys are looking for will not be large enough to resolve into multiple images. There is, however, a noticable increase in brightness, just as would be observed with a classical lens and as can be seen in the central simulation of the image at left.
Results?
The number of observed microlensing events leads to conclusions about the density of dark matter in the Galaxy.
For more information:
Hubble's Constant From Type Ia Supernovae
Abi Saha
National Optical and Astronomical Observatories (NOAO), Tucson, Arizona
Saha stands in the snow, at left, to show his true Minnesota experience!
How reliable are SNeIa as standard candles?
Establish ABSOLUTE scale by looking at galaxies that we know have SNeIa and use the cepheids in those galaxies to determine the distance and thus calculate absolute magnitudes of supernovae.
Problems involve extinction and the need to use HST to see cepheids due to distance and uncommonness.
Additionally, errors in measurement are magnified and influenced by systematic color errors that change perceived reddening.
Finally, plot calibrated cepheids and supernovae with estimated value for H0. The value that came out for this experiment using SNeIa as standard candles was H0~60 km/s/Mpc, and definitely less than 65.
The Cepheid Key Project, however, suggests values up to H0~72 km/s/Mpc.
The chart below shows the absolute magnitude (MB) of SNeIa (open circles) and of cepheid calibrated SNeIa (filled circles) vs. velocity distance using different values for Hubble's constant. Saha argues that H0 must be less than 60 because if H0 is greater than or equal to 70, distant SNeIa must be, on average, less luminous than nearby calibrators. This is highly unlikely!
From Saha, et al. The Astrophysical Journal, 1997 486:1-20.
For more information:
Supernovae Progenitors & Massive Stars
Stephen Smartt
University of Cambridge, Cambridge, United Kingdom
At right, see Smartt, Kim Venn, and Clare Venn Skillman with an Irish Elvis on Saint Patrick's Day.
Close up of supernova 1987A, at left. Courtesy of Hubble Space Telescope.
How do massive stars evolve? Which ones will go supernovae?
Supernovae drive the chemical evolution of gasses because they are the main sites for nucleosynthisis and are responsible for blowing matter out into the universe which can then recombine into new stars. Supernovae are thus associated with gamma-ray bursts as well.
Only 1-2 supernovae occur every 100 years! Therefore it can be rather difficult to find them. Many have actually been seen by accident on slides from other, unrelated observations that just happened to catch the supernova in the corner.
Sher25 can be seen as the bright star with the faint ring around it in the upper left of the image below.
Classifying Supernovae:
Supernovae are classified based on the presence of hydrogen gas in the parent galaxy as well as the presence or absence of other elements in the particular progenitor star. Following the table below, the first question is whether there was hydrogen or not. Hydrogen gas implies Type II, and no hydrogen implies Type I. Then within Type I the three types a, b, and c are determined by presence of either silicon, helium, or neither. Type II are classified by other properties determined by photometry.
| Hydrogen? | No H2 present: | H2 present: | ||||
| General group? | Type I supernovae | Type II supernovae | ||||
| Other properties? | Si | He | neither | Photometry/spectral properties | ||
| Specific type? | Ia | Ib | Ic | IIp | IIn | II... |
Conclusions:
Global Star Formation History
Eline Tolstoy
Clare doing Tolstoy's hair, at right.
How far back in history can we see?
The horizon of optical observations is at a redshift of about z=6.
What are the different indicators of star formation rates?
The question of global stare formation rates is complicated because we don't know if historical objects look like objects we can recognize from the present or near past.
Star Formation Rate Density vs. Redshift: Looking back in time
A.M. Hopkins did us the service of compiling the star formation densities measured at different redshifts in 13 different papers. As shown in the chart below, the gray area covers to the extremes of uncertainty. Click on the chart to go to the full refereed journal article from The Astrophysical Journal.
For more information:
Hubble's Constant from Gravitational Lensing
Liliya Williams
University of Minnesota, Minneapolis, Minnesota
After the first few talks, particularly those of Stubbs and Saha, it became clear that we needed more background on gravitational lensing and Hubble's constant. Thus we turned to our neighbor, Liliya Williams, at the University of Minnesota, to help us understand Hubble's constant and its relationship to gravitational lensing.
Gravitational Lensing:
Gravitational lensing (see Chris Stubbs' talk on MACHO surveys, above, for other information on gravitational lensing) creates images in only odd numbers. In the image at right, you can see how the light from a single object can be bent around a galaxy and create multiple images. Turns out that what is happening is that the light is following the distortion, or the curve, of spacetime due to the dense region of matter and its associated gravitational force that is a galaxy or a star or any other massive object.
How does H0 ''Pop Out,'' Exactly?!
The project uses curved (as opposed to Euclidian) geometry to map the path between the object and the observer. A saddle point is defined in the length of time it takes for the light to reach the observer. Different lengths of time pop out due to the curvature of spacetime that results from the uneven distribution of matter density in space. The time delay surface is the difference between the times with and without the occurence of lensing. This surface defines the cosmology of the region of space and, using a complicated formula, ejects a value for H0.
Chemical Evolution of Dwarf Spheroidal (dSph) Galaxies
Constraints on Galaxy Formation from Abundance Ratios in Nearby Galaxies
Matt Shetrone
Hobby-Eberly Telescope, University of Texas at Austin, Texas
Shetrone and Clare walk Brown Bear around, left.
Questions about galaxy formation:
| What clues do we have to answer these questions? | How do they pan out? |
| Halo metallicity: | Doesn't tell the whole story. |
| High resolution, high signal-to-noise (S/N) spectra: | Only way to reach weak line features--Cu, Eu, Ba; [Ba/Eu] then plot gives turn-on point for AGB stars. Also, assumptions about Ca lines tracing Fe lines are removed |
| Type II supernovae produce more magnesium than iron: | Type Ia supernovae ''turn on'' at a certain point and drive ratio down |
| Determine ratios of s- to r-process elements (heavy elements) from meteorites: | More reliable than supernova model; if [Ba/Eu] < 0, r-processes dominate, not AGB star. Thus restricts the timescale for star bursts and formation. |
| Mg to Fe ratio: | Changes the number of free electrons in the atmosphere; thus changes both opacity and temperature |
| Alpha to Fe ratio: | Alpha elements come from Type II supernovae and Fe from type Ia, therefore their ratio gives the ratio of the history of II:Ia supernovae in galaxies. |
Results?
The chart above demonstrates the previous conclusions by showing the abundances of alpha elements in dwarf spheroidal galaxies versus solar metallicity. The symbols are as follows:
For more information:
Survey for Dwarf Galaxies
Searches for Low Surface Brightness dSph Galaxies in the Local Group
Taft Armandroff
National Optical and Astronomical Observatories (NOAO) Tucson, Arizona
At left, Armandroff and Clare read together.
Motivation:
How to find dwarfs:
The best way is with deep CCD imaging of faint, clumped stars. Take the radial velocity of every star and look for clumping in radial velocities.
At left, the image shows a map of the local group sky with the Roman numerals pointing out the locations of Armandroff's new Andromeda galaxy findings.
Impact of the search:
For more information:
The Present, Past and Future of the Local Group
Mario Mateo
University of Michigan, Ann Arbor, Michigan
Clare and Mateo at the baseball game, at left.
The class watched anxiously as Mario Mateo, Kim Venn, and Barron Koralesky tried, in vain, to project Mateo's talk from his laptop onto the screen. Alas, it was not to be and the students sat back to see what Mateo could manage off the top of his head while his talk remained trapped in technology. Most expected a loose, general, non-specific talk. Yet all were pleasantly surprised as Mateo went through sheet after sheet of overhead transparency plastic. He gave us charts and graphs, statistics, and a fully cohesive and highly impressive talk right from his head. Not an event to have missed!
Paleontology vs. Astronomy:
Both scientists are looking back in time, but astronomers really can ''see'' back in time, while paleontologists can actually touch their objects of study.
Present:
Primary nearby sample:
The Hubble Sequence of galaxies?
The Hubble sequence is not apparent through the Hubble deep field surveys; but it is possible that we are in the middle of it and the rest of the sequence will become clear in the future.
The figure at right is a 3-D map of the current Local Group of Galaxies.
Past:
Density fluctuations in early universe evolved to present-day local group.
Cold Dark Matter Models:
Future:
Main Conclusions:
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