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Dwarf Spheroidal Galaxies Orbiting the Milky Way

Dwarf Spheroidal Galaxies Orbiting the Milky Way. Edward W Olszewski, Steward Obs. The Milky Way is a fairly typical large spiral galaxy. Most of the young stars and gas are in the spiral disk. Here are some schematics. Sun 8000 pc (25000 light y) from center.

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Dwarf Spheroidal Galaxies Orbiting the Milky Way

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  1. Dwarf Spheroidal Galaxies Orbiting the Milky Way Edward W Olszewski, Steward Obs

  2. The Milky Way is a fairly typical large spiral galaxy. Most of the young stars and gas are in the spiral disk. Here are some schematics. Sun 8000 pc (25000 light y) from center. Total size ROUGHLY 30000 pc (probably more precisely, half distance to M31).

  3. Milky Way is one of two large members of Local Group. Milky Way and M31 (Andromeda Galaxy) are roughly 800,000 pc apart (2.5 million light y). There are intermediate-mass and dwarfs in Local Group, too.

  4. Are the big galaxies important because they’re big, or are the little ones important because There are so many of them? The answer is both… As you’ll see from the movie of the evolution of structure (Matthias Steinmetz), big galaxies swallow up many smaller ones over time, and are still “forming” today…

  5. There are medium-sized galaxies like M33 and LMC and SMC, and smaller gas-rich galaxies like NGC 6822, and many gas-poor, low-luminosity dwarfs. The first dwarf spheroidal (dSph) galaxies were discovered by Shapley in the 1930s, and Their nature was deduced by Baade and Hubble in 1939. These are Fornax and Sculptor.

  6. They were deduced to be sortof like globular clusters, only much larger in radius and much lower in stellar density. Next slide will show an H-R diagram (or color-magnitude diagram) of a globular and of a dwarf spheroidal. These are modern data, not the discovery data.

  7. Scl and For were discovered from Harvard Patrol Plates from South Africa. In 1950s, Leo I, Leo II, Umi, and Dra were discovered from Palomar Sky Survey. In 1970s, Carina was discovered from ESO/SRC Southern Sky survey. In 1995, Sgr was discovered by Ibata.

  8. Sgr was different, it’s very elongated and is clearly being tidally destroyed by Milky Way. We’ll show two figures from several years after discovery, both by the group that includes Majewski and Johnston.

  9. Given the evolution of structure movie, and the Sgr results, this brings up the question of “how much of Milky Way came from outer space,” and “how much remains to be swallowed?”

  10. Now to go back to 1982 or so. Marc Aaronson of Steward Obs showed that the motions of stars (the radial velocities, more precisely) in the Draco and Umi dwarfs Were too large to come from “only the gravity of stars.” So something else is adding mass. It’s apparently “dark matter.” Aaronson died tragically in 1987, but he left behind a postdoc, me, and lots of collaborators and competitors

  11. We have confirmed “extra gravity” in all the dwarf spheroidals. Dark Matter distinguishes them from globular clusters, and makes them galaxies in their own right. And remember, there are lots of them, even though each one doesn’t weigh a lot.

  12. The next few slides will show modern data, how it’s collected, how we deduce the mass… Typically, in 2009 we collect 100-200 velocities of individual stars in 3 hours.

  13. Observations & Data: Magellan + MMFS • 256 fibers over 30 arcmin field • Magnesium Triplet 5140-5180 Angstroms at resolution 20000-25000 • +/- 2 km/s velocities for V~20.5 star in 2.5 hours • [Fe/H] to +/- 0.2 dex • Up to 600 spectra per night

  14. There are lots of dwarf spheroidals, but there aren’t enough! Those models of evolution of structure predict hundreds in the Milky Way halo. Where are the missing ones? Are they faint, completely dark, nonexistant?

  15. People have been data mining the Sloan Digital Sky Survey (SDSS). What they do is to convolve the CMD of an old population with the survey. If there is such a population it’ll be a mathematical peak. One can then use big telescopes and big spectrographs to see if these objects are real.

  16. These new galaxies can’t be “seen” easily, but can be discovered from densities of old stars, velocities of stars, and from chemical composition of the stars compared to those in the general halo “field.” Ursa Minor is 1/millionth the luminosity of the Milky Way. Some of these new galaxies are 1/hundredth Umi’s luminosity.

  17. They are the luminosity of one red giant! They are probably tidal fragments, but they seem to contain dark matter. We announced Leo V a year ago, and will announce Aries next month. We have observing time in March to study 17 more candidate dwarfs.

  18. Finally, these galaxies are being used today To deduce the nature of the dark matter. If they are a particle, you have to be able to stuff enough of them in the galaxy (Pauli exclusion) and they can’t be moving too fast. Modern bets are that such particles will self annihilate, so the nearby dwarfs will glow In gamma rays or some such.

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