Artist's concept of the Milky Way galaxy, with the "galactic bar" visible in the center. (Image by R. Hurt)

The Exoplanet Exploration Program has a broad scope of science that includes galactic and extragalactic astrophysics in addition to exoplanet science. The ability to carry out general astrophysics observations follows naturally from the design requirements of the Exoplanet Exploration missions, which provide unprecedented high-angular resolution and high dynamic-range sensitivity.

The following are among the key questions that challenge and motivate future missions:

• What is dark matter?
• What is dark energy?
• How did the universe begin?
• How did the first galaxies form?
• How has the intergalactic medium evolved?
• How do the feedback cycles between galaxies and the intergalactic medium (IGM) work?
• When did the first black holes form, and how are black holes and galaxies related?
• How do stars form and what governs their mass distribution?

Such issues have been identified by the astronomical community in the National Academies studies -- e.g., the McKee-Taylor decadal survey report Astronomy and Astrophysics in the New Millennium (2001), the Turner report of the committee on the physics of the universe Connecting Quarks with the Cosmos (2003), and in the NASA strategic roadmaps.

General astrophysics objectives

The following is a summary of some of the key astrophysics objectives currently conceived for SIM PlanetQuest, TPF-C, and TPF-I:

1. Stellar Evolution. SIM distance measurements to nearby stars and globular clusters will resolve one of the major impediments to accurate determination of the ages of stars. Measurements of stellar oscillations by Kepler and COROT will also test stellar-evolution models. If these observations and the subsequent refinement of models yield an absolute accuracy of better than 5 percent in the ages of the oldest stars, then stellar chronology will emerge once again as an important cosmological tool -- testing dark energy models and/or models for early galaxy formation and re-ionization.
2. Cosmology. SIM will remove the major uncertainties in the lowest rungs of the cosmic distance ladder by determining precise distances to galactic Cepheids and RR-Lyrae stars and local-group galaxies. Cepheid observations of nearby SN Ia host galaxies with TPF-C could reduce the uncertainty in the Hubble constant H0 to ~2 percent, providing a critical test of dark-energy models. Both TPF-C and TPF-I will test galaxy-formation models via the highest-resolution measurements of the structure of high-redshift galaxies.
3. Dark matter. SIM will measure proper motions and rotational parallaxes of nearby galaxies, allowing a full reconstruction of galaxy orbits in the Local Group, thereby vastly improving our understanding of the distribution of dark matter within and around galaxies. TPF-C will be able to measure proper motions in more distant galaxies and will also probe dark matter through high-resolution images of gravitational-lens systems.
4. Exotica. SIM and TPF-C will measure the astrometric shifts induced by microlensing events that, combined with observations from the distant Earth, will allow the detection of isolated black holes toward and massive stellar remnants toward the center of our Galaxy. TPF-I will provide definitive measurements of the structure of the dusty tori that surround black holes in active-galactic nuclei.