1. July 25th, 2000WFC3 Critical Science Review1 WFC3 Science Themes The WFC3 White Paper highlights a few science projects where one may expects the contribution of WFC3 to be ground-breaking. We have considered programs in each of the following themes: High-z Universe (Sect. 4.2 and 5.1) Nearby Galaxies (Sect. 6.1) Resolved Stellar Populations (Sect. 4.1) Stars and Interstellar Medium (Sect. 6.2) Solar System (Sect. 5.2 and 6.3) In the following we will review each of these themes individually and identify the major science requirements which can be derived from them.

2. July 25th, 2000WFC3 Critical Science Review2 WFC3 Unique Science For each theme we will also indicate the relevant DRM programs highlighting those which are enabled by WFC3 and would otherwise be impossible from the ground or with other existing space observatories. This uniqueness can be due to either one or a combination of the following factors: wavelength region inaccessible from the ground filters unavailable in other space instruments high angular resolution combined to a large FOV high sensitivity

3. July 25th, 2000WFC3 Critical Science Review3 High-z Universe WFC3 can address a number of questions within the NASA Origins of Galaxies theme:  How do galaxies assemble?  When and how is gas converted into stars?  When is the Hubble sequence established?  What are the first luminous objects in the Universe? It is essential to efficiently identify high redshift galaxies in a range of redshift as wide as possible. The panchromatic capabilities of WFC3 allow the exploration from redshift z=1 to 12 with the Lyman-break technique or photometric redshifts.

4. July 25th, 2000WFC3 Critical Science Review4 High-z Universe – cont’d  The most effective method to identify high-z galaxies is the so-called Lyman break technique which relies on three filters to identify objects with:  a relatively flat continuum longward of the Lyman  line and  a strong reduction in flux shortwards of the Lyman break.  The strong reduction in flux shortwards of the Lyman break is caused by absorption due to intervening neutral hydrogen. The Lyman break technique has been applied extensively in the last five years and it has proven both efficient and reliable for finding high redshift galaxies regardless of whether Lyman  is present in emission or in absorption.

5. July 25th, 2000WFC3 Critical Science Review5 High-z Universe – cont’d Both the low and high z ends are unique to WFC3. The redshift interval z = 1-3 is unexplored because a systematic study requires high sensitivity below 3000 A over a large FOV. The figure illustrates a color selection criterion to isolate low redshift objects through the Lyman break technique. Plotted are about 4,000 models including a range of metallicity, dust content, age, star formation history as well as intergalactic opacity due to neutral hydrogen clouds along the line of sight. Galaxy models identified by red points have redshift within the specified range. Blue points represent models with a different redshift. Galaxies found within the red box would have a high probability of being between redshift z=1.3 and z=2.4

6. July 25th, 2000WFC3 Critical Science Review6 High-z Universe – cont’d Redshifts above z=5 are unexplored because they require high sensitivity in the near-IR over a large FOV. The figure illustrates a color selection criterion to isolate high redshift objects through the Lyman break technique. Plotted are about 4,000 models including a range of metallicity, dust content, age, star formation history as well as intergalactic opacity due to neutral hydrogen clouds along the line of sight. Different colors correspond to different redshifts. The dark green box identifies objects with redshift z>7.4.

7. July 25th, 2000WFC3 Critical Science Review7 High-z Universe – cont’d The sensitivity of the Lyman-break technique to high redshift galaxies is limited by the depth of the shortest wavelength image, i.e. shortwards of the Lyman break. The figure below illustrates the limiting magnitudes required to detect the UV continuum of galaxies with various star formation rates. Star formation rates and rest frame fluxes for starburst galaxies at different redshift. The calculation requires Lyman break galaxy candidates to show at least a 1.6 magnitudes jump at the Lyman break. The blue line and points represent the expected performance of WFC3.

8. July 25th, 2000WFC3 Critical Science Review8 High-z Universe – cont’d Summary of requirements (DRM225):  assembly of galaxies at redshift 1-3  large field of view  requires sensitivity at 2000-4000  point source limiting magnitude U AB = 27.5  search for very high-redshift galaxies  large field of view  requires sensitivity in the near-IR point source limiting magnitude J AB = 28.5

9. July 25th, 2000WFC3 Critical Science Review9 High-z Universe – cont’d  Identifying the most distant galaxies DRM218 - Grism survey for faint Compact Em. Line Objects DRM225 - A WFC3 Study of Faint Galaxies at z=1-12. DRM233 - Search for High-z Em. Line Objects with the IR grism  Galaxy Evolution from high-z to the present DRM102 - Ultra-deep Imaging DRM208 - Evolution of Distant Cluster Galaxies in restframe UV DRM210 - Tracing the evolution of Ellipticals: the UV-upturn DRM222 - An Optical-Near IR Medium deep survey DRM240 - UV Parallel Survey Items in purple-red are unique science areas for WFC3

10. July 25th, 2000WFC3 Critical Science Review10 Nearby Galaxies Understanding the Origin of Galaxies requires also understanding phenomena and processes in the local Universe. Namely:  How universal are the processes of star formation in galaxies?  What are the dust and gas properties of star forming galaxies in the local Universe?  What are the implications for observations of high-z galaxies? Gas, dust, and stars of different ages display features at a variety of ultraviolet, visible and near infrared wavelengths. A complete studies of the complex environments in galaxies requires a panchromatic approach.

11. July 25th, 2000WFC3 Critical Science Review11 Nearby Galaxies – cont’d An example of unique WFC3 science is the study of the 2175 A bump of the extinction curve with narrow band imaging of distant galaxies through the disks of nearby galaxies (DRM 219). The shaded area represent the change in the observed spectrum due to the 2175 A bump

12. July 25th, 2000WFC3 Critical Science Review12 Nearby Galaxies – cont’d Summary of requirements (DRM219 and DRM 232):  panchromatic dissection of nearby-galaxies  sensitivity in the UV and in the near-IR UV point source limiting magnitude U=27 flux in IR iron lines = 1.5 x 10 -16 erg s -1 cm -2 arcsec -2 ([FeII]1.26  m and [FeII]1.64  m)  high angular resolution  large field of view  narrow band filters

13. July 25th, 2000WFC3 Critical Science Review13 Nearby Galaxies – cont’d  Morphology and Evolution of Galaxies DRM206 - Wide Field UV Imaging of nearby galaxies DRM211 - UV color gradients in Elliptical Galaxies DRM223 - The search for low surface brightness objects  Starbursts and mergers DRM103 - Multi-wavelength Observations of Merging Galaxies DRM229 – Nearby starbursts and their connection with high-z gal. DRM239 – Structure of Starburst Galaxies  The interstellar medium in other galaxies DRM219 – The universality of the 2175 A bump DRM232 – Nature of the diffuse-ionized medium in local SF gal.

14. July 25th, 2000WFC3 Critical Science Review14 Nearby Galaxies – cont’d  Distance scales DRM101 - Cepheid Observations in Virgo cluster DRM104 - Distance estimates from SB Fluctuations DRM224 - Post-AGB stars in the Virgo Cluster  Microlensing DRM204 – Detection of Earth-like planets through microlensing DRM231 - Pixel microlensing in M87

15. July 25th, 2000WFC3 Critical Science Review15 Resolved Stellar Populations The star formation history of a galaxy is locked into its stellar population and can be revealed through stellar archeology:  What is the baryonic content of galaxies?  How much mass is locked into low mass stars?  Is the Initial Mass Function universal?  How old are globular clusters?  What are the effects of age, abundances, and rotation on stellar evolution? One of the techniques to address these issues relies on studying color- magnitude diagrams using UV and near-IR bands.

16. July 25th, 2000WFC3 Critical Science Review16 Resolved Stellar Populations – cont’d The fraction of mass locked into low mass stars and the IMF universality can be studied by deriving the lower end of the mass function in the galactic bulge (DRM 227). The large field of view of WFC3 allows one to reduce the effect of crowding by observing areas with lower densities of stars while still preserving sufficient statistics. NICMOS/CAM2 Color-magnitude diagram of low mass stars in the galactic bulge. The NICMOS data reach stars of mass twice the hydrogen-burning limit. 26 WFC3 will go 2 magnitudes deeper thus pushing toward the hydrogen-burning limit.

17. July 25th, 2000WFC3 Critical Science Review17 Resolved Stellar Populations – cont’d Summary of requirements (DRM 220 and 227):  high precision photometry in a crowded field  sensitivity in the UV and in the near-IR UV point source limiting magnitude U = 25 with S/N=20 IR point source limiting magnitude J = 27  high angular resolution at both UV and IR wavelengths  large field of view  stable PSF

18. July 25th, 2000WFC3 Critical Science Review18 Resolved Stellar Populations – cont’d  Ages and chemical evolution of stars DRM202 - The Age of the LMC bar DRM203 - Ages and abundances of M31 open clusters DRM207 - Direct determination of metallicities in nearby galaxies DRM228 - Near-IR Imaging of star forming regions in the LMC  Evolution of globular clusters DRM105 - Identifying globular cluster stars with proper motions DRM214 - Ages of globular clusters from WD cooling sequence DRM220 - Hot stars in globular clusters DRM226 - Near-IR Lum. Function of G.C. main sequences  Star formation in the Galactic bulge DRM227 - The lower main sequence in the Galactic bulge DRM235 - Star Formation History of the Galactic Center

19. July 25th, 2000WFC3 Critical Science Review19 Stars and ISM The origin of stars and planetary systems is another theme in the NASA Origins program that can be addressed by WFC3. Namely:  How does the star formation rate depend on the environmental conditions?  Do OB stars trigger or abort the star formation process?  What is the frequency of brown dwarfs and super-jovian companions?  How frequent are the very low-mass stars?  How do disks evolve and when are they dissipated?  What is the interplay between accretion, outflows and final properties (mass, rotation, multiplicity) of a stellar system

20. July 25th, 2000WFC3 Critical Science Review20 Stars and ISM – cont’d Herbig-Haro objects are collimated mass outflows (jets) removing material and angular momentum from the accreting protostars and energizing the interstellar medium. The UVIS imaging capabilities of WFC3 allows to study with unparalled accuracy the physics of the shock processes occurring when they interact with the interstellar medium (Herbig-Haro objects, bow shocks). The IR capabilities of WFC3 will also allow to study ([FeII], Pa  lines) the outflows hidden within the dense molecular clouds, powered by protostellar objects in an earlier evolutionary phase. WFC3 extends the time baseline up to the NGST era. HST image quality is constant with time. Coupled with spectroscopy (radial velocity), proper motion measurements allow to reconstruct the real 3-d kinematics of the outflows.

21. July 25th, 2000WFC3 Critical Science Review21 Stars and ISM – cont’d Summary of requirements (DRM201):  large filter set to study emission lines and high sensitivity  requires strategically chosen filters  sensitivity in the UV, visible, and near-IR UV extended source flux = 10 -16 erg s -1 cm -2 arcsec -2 visible extended source flux = 10 -16 erg s -1 cm -2 arcsec -2 IR extended source flux = 10 -16 erg s -1 cm -2 arcsec -2

22. July 25th, 2000WFC3 Critical Science Review22 Stars and ISM – cont’d  Formation DRM236 – Searches for Brown Dwarfs in SF regions DRM237 – Very low mass luminosity function in young clusters  The disk-planet connection DRM106 – Search for disks around stars DRM201 – WFC3 observations of Herbig-Haro objects  Death and transfiguration DRM205 – Origin of Elements: Ejecta-dominated SN remnants DRM212 – Survey of the Crab nebula DRM216, 217 – Ejecta from Old Stellar Objects

23. July 25th, 2000WFC3 Critical Science Review23 Solar System Studying our own solar system and its evolution is a required step in addressing the origin of planetary systems and their evolution as well as understanding whether and how the habilitability of a planet evolves.  what is the water cycle on Mars?  what are the atmospheric cycles of the giant planets?  what are the properties of the relic remnants of the early solar system?

24. July 25th, 2000WFC3 Critical Science Review24 Solar System – cont’d Summary of requirements:  ices of outer planetary satellites  near-IR sensitivity and suitable filters  meteorology of the outer planets  requires UV, visible and near-IR sensitivity  requires suitable filters never flown on HST

25. July 25th, 2000WFC3 Critical Science Review25 Solar System – cont’d  Planetary atmospheres DRM215 – Atmospheric structure of the Outer Planets DRM238 – Mars Surface and Atmospheric studies using WFC3  Minor planets and comets DRM230 – Determination of cometary and KBO sizes using WFC3 DRM234 – Determination of Asteroid Size and Surface Composition