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10 Cards in this Set
- Front
- Back
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Top-down |
Large structures (galaxies) form first. Involves hot dark matter (neutrinos). |
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Bottom-up |
Galaxies form first, then clusters of galaxies. Involves cold dark matter (WIMPs). |
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Most updated version |
After the time of recombination baryonic matter falls into the potential wells composed of dark matter. Simulations show that dwarf galaxies form first, and support bottom-up with cold dark matter and dark energy. |
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Why do galaxies look different? |
Rapidly rotating giant molecular cloud -> collapse to form a disk and therefore a spiral galaxy. Cloud with little internal rotation -> elliptical galaxy. |
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Elliptical galaxies |
Formed stars early on during a couple of burst of particularly intense star formation. Narrow metallicity range (red old). Exhausted of gas and dust. Irregular star motion. Found inside clusters of galaxies. E0 is almost round, E7 is very elliptical. |
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Spiral galaxies |
Have spiral arms, disk and bulge Still form stars in the disk, though at a low rate. Broad metallicity range. Much gas and dust. Regular rotation (same direction). Small letter: compactness of arms. a: most compact. The bulge size increase with compactness. |
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Barred spiral galaxies |
Many more SB than S. The formation of bars is strongly correlated with the mass of the dark matter halo. ---> Very massive halo -> no bar formation. Small letter: compactness of arms.a: most compact. The bulge size increase with compactness. The spial arms in SB usually start at the end of the bar instead of from the bulge. |
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Lenticular galaxies (S0 and SB0) |
Like spiral galaxies, but without spiral arms. Have a disk and bulge. |
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Irregular galaxies |
Galaxies that do not fit into Hubble's tuning fork. Neither spiral or elliptical, can have any shape. Frequently a product of two galaxies colliding with each other, or at least affecting each other through gravity. |
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Hubble's tuning fork diagram - extras |
The classification is based on the galaxy's appearance viewed from Earth. Since their orientation in space is random, a flat E7 galaxy can be mistaken for a S0 galaxy from a different view. It is not an evolutionary sequence. Spiral galaxies rotate quickly, while elliptical galaxies do not. There is no way a elliptical galaxy could spontaneously begin to rotate, so there is no way an elliptical galaxy could turn into a spiral galaxy. Milky Way is an SB. |
