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. Total size ROUGHLY 30000 pc (probably more precisely, half distance to M31).

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.

Are the big galaxies important because they’rebig, or are the little ones important becauseThere are so many of them?

The answer is both…

As you’ll see from the movie of the evolution ofstructure (Matthias Steinmetz), big galaxiesswallow up many smaller ones over time, andare still “forming” today…

There are medium-sized galaxies like M33and LMC and SMC, and smaller gas-richgalaxies like NGC 6822, and many gas-poor,low-luminosity dwarfs.

The first dwarf spheroidal (dSph) galaxieswere discovered by Shapley in the 1930s, andTheir nature was deduced by Baade and Hubblein 1939. These are Fornax and Sculptor.

They were deduced to be sortof like globularclusters, only much larger in radius andmuch lower in stellar density.

Next slide will show an H-R diagram (or color-magnitude diagram) of a globularand of a dwarf spheroidal. These aremodern data, not the discovery data.

Scl and For were discovered from HarvardPatrol Plates from South Africa.

In 1950s, Leo I, Leo II, Umi, and Dra werediscovered from Palomar Sky Survey.

In 1970s, Carina was discovered from ESO/SRC Southern Sky survey.

In 1995, Sgr was discovered by Ibata.

Sgr was different, it’s very elongated andis clearly being tidally destroyed by MilkyWay.

We’ll show two figures from several yearsafter discovery, both by the group thatincludes Majewski and Johnston.

Given the evolution of structure movie,and the Sgr results, this brings up thequestion of “how much of Milky Way camefrom outer space,” and “how much remainsto be swallowed?”

Now to go back to 1982 or so.Marc Aaronson of Steward Obs showed thatthe motions of stars (the radial velocities, moreprecisely) in the Draco and Umi dwarfsWere too large to come from “only the gravityof stars.”

So something else is adding mass.It’s apparently “dark matter.”

Aaronson died tragically in 1987, but he left behinda postdoc, me, and lots of collaborators and

competitors

We have confirmed “extra gravity” in allthe dwarf spheroidals. Dark Matter distinguishesthem from globular clusters, and makes themgalaxies in their own right.

And remember, there are lots of them, eventhough each one doesn’t weigh a lot.

The next few slides will show modern data,how it’s collected, how we deduce the mass…

Typically, in 2009 we collect 100-200 velocitiesof individual stars in 3 hours.

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

There are lots of dwarf spheroidals,but there aren’t enough!

Those models of evolution of structurepredict hundreds in the Milky Way halo.

Where are the missing ones? Are theyfaint, completely dark, nonexistant?

People have been data mining the SloanDigital Sky Survey (SDSS).

What they do is to convolve the CMD ofan old population with the survey. Ifthere is such a population it’ll be a mathematicalpeak. One can then use big telescopesand big spectrographs to see if these objectsare real.

These new galaxies can’t be “seen” easily,but can be discovered from densities of old stars, velocities of stars, and from chemicalcomposition of the stars compared to thosein the general halo “field.”

Ursa Minor is 1/millionth the luminosityof the Milky Way. Some of these new galaxiesare 1/hundredth Umi’s luminosity.

They are the luminosity of one red giant!They are probably tidal fragments, butthey seem to contain dark matter.

We announced Leo V a year ago, and willannounce Aries next month.

We have observing time in March to study17 more candidate dwarfs.

Finally, these galaxies are being used todayTo deduce the nature of the dark matter.

If they are a particle, you have to be able to stuffenough of them in the galaxy (Pauli exclusion)and they can’t be moving too fast.

Modern bets are that such particles will selfannihilate, so the nearby dwarfs will glowIn gamma rays or some such.