1. Astrophysics 2020: STScI November 13, 2007 Page 1 Mapping the Structure of All Matter in the Universe Steven M. Kahn Kavli Institute for Particle Astrophsics and Cosmology Stanford University

2. Astrophysics 2020: STScI November 13, 2007 Page 2 Challenges for the Future in Space Astrophysics *When asked to think about large space astrophysics missions for the next decade and beyond, I find it useful to first think about “where we have been”, and then move to “where we are going”. *I entered the field in the mid-1970s. At that time, space astrophysics mostly involved exploratory investigations in new wavelength bands - not visible from below the atmosphere (X-rays,  -rays, ultraviolet, infrared). *The main goal of those missions was to see what is out there. A range of interesting phenomena were found.

3. Astrophysics 2020: STScI November 13, 2007 Page 3 Challenges for the Future of Space Astrophysics *In the 80’s and 90’s, arguments for new missions were tied to “discovery-space”: –The development of new technical capability was directed at “opening windows” - e.g. high resolution, imaging, spectroscopy, polarization, new wavelength bands, etc. –While a set of planned measurements was used to guide the design, the real motivation was the enabling of unanticipated discoveries, which were expected to provide the most excitement. –Our understanding of the basic structure of the Universe was still fairly crude - observations in new wavelength bands led to the discovery of new source populations, and radically changed our concepts of how galaxies were born and evolved.

4. Astrophysics 2020: STScI November 13, 2007 Page 4 The Great Observatories Hubble Space Telescope - April, 1990 Chandra X-ray Observatory - July, 1999 Compton Gamma-Ray Observatory - April, 1991 Spitzer Space Telescope - August 2003

5. Astrophysics 2020: STScI November 13, 2007 Page 5 Challenges for the Future of Space Astrophysics *As things evolved into the mid- to late 90’s, the emphasis changed from further expanding discovery space, to addressing grand themes and questions. *Infrared, optical, and ultraviolet missions became captured under the umbrella theme of “Origins”, while the high energy astrophysics and gravitational missions were collected into the “Beyond Einstein” program. *Whether or not this approach was successful is still unclear. Only a small fraction of the Origins missions have moved into development, and none of the Beyond Einstein missions has yet been really started.

6. Astrophysics 2020: STScI November 13, 2007 Page 6 Challenges for the Future of Space Astrophysics *In my view, the transition away from discovery-space and toward well-focused missions had two major causes: –First, all government agencies were subjected to an increased emphasis on “metrics” around that time. Major science programs were not immune - they were asked to define metrics in advance of deployment that could be used to evaluate the latter success of their experiments and missions. It is hard to define a useful metric for “unanticipated discovery”. Answering well-posed questions was an easier concept to get across. –Second, the phenomenal success of COBE and later WMAP proved that space astrophysics missions could be used to test and verify detailed theoretical predictions, with profound implications. This success was seductive. It became fashionable to try to formulate all astrophysical issues in terms of specific quantitative questions that could be used to drive requirements for future missions.

7. Astrophysics 2020: STScI November 13, 2007 Page 7 Challenges for the Future of Space Astrophysics *But too strong an emphasis on answering specific questions has its drawbacks. –Often the questions get “old”. It takes 15+ years to develop a major space mission from concept to launch, even when the funding is there. 15 years is a long time - fashions in science change on that timescale. A good example is the search for the Higgs boson, which was used to motivate the Tevatron and the SSC. This went on so long, it fell out of fashion. Particle physicists now don’t emphasize it as much in PR materials for the Large Hadron Collider or the International Linear Collider. –Quantitative questions impose quantitative requirements. Graceful descopes become more difficult to accommodate. As costs start to grow, and mission reach starts to decline, we run the risk of losing the mission entirely, if the quantitative requirement can no longer be met.

8. Astrophysics 2020: STScI November 13, 2007 Page 8 Challenges for the the Future of Space Astrophysics *The biggest challenge we face as we look to the next decade and beyond is undoubtedly associated with the tremendous growth in mission cost. *“Opening up discovery space” is too ill-defined to justify billions of dollars in the current climate. Answering specific questions might work if the questions are both sufficiently profound and sufficiently accessible to the public. As particle physics has entered the tens of $B regime, however, both of those approaches are failing. It is unclear whether space astrophysics will do any better. *Instead, my belief is that a new transition will be required - a transition to “Beyond Science”. We must couple our field to goals that the public can viscerally attach themselves to, whether or not they understand the scientific measurements and analyses that will be performed with the data.

9. Astrophysics 2020: STScI November 13, 2007 Page 9 Some of these “Beyond Science” Themes are not new… *Discovering life on another planet. –Would have deep philosophical implications affecting our view of our place in the Universe *Imaging of the event horizon of a black hole. –Would yield direct “contact” with the extremes of space- time, providing a gut feeling for relativistic effects, far outside of normal human experience.

10. Astrophysics 2020: STScI November 13, 2007 Page 10 Beyond Science *In this category, I would add a new challenge: “Mapping the structure of all matter in the Universe”. *The drive to map and catalog has obvious scientific utility. But I would argue that this is also a larger human endeavor. “We should map it because it is out there, and it is the only universe we have.” The database will be there for generations to come, and would probably ultimately be used in ways we can cannot imagine today. *Mapping missions form an intermediate case between discovery- space driven missions and question-driven missions. The scientific requirements can be well-formulated in quantitative fashion. The goal is to assemble the archive. However, the science is not predetermined. The questions that the archive will answer can be formulated later.

11. Astrophysics 2020: STScI November 13, 2007 Page 11 Analogy to the U.S. Census *The U.S. Census Bureau conducts a complete census of all U.S. households every ten years. *They have specific analyses in mind, but the primary goal is to assemble the data into an archive that can be queried in many different ways. *The figure at the right is an example of the simplest form of output, showing population density as a function of location in Arizona. But, once familiar with the tools, you can use the database to plot all sorts of interesting things, e.g. fractional excess of women over men in rich households as a function of location. *The discoveries come from the formulation of the questions, not just from the original measurements.

12. Astrophysics 2020: STScI November 13, 2007 Page 12 Contents of the Universe Dark Energy Dark Matter Diffuse Baryons Baryons in Galaxies

13. Astrophysics 2020: STScI November 13, 2007 Page 13 What Does it Mean to Map? *Mapping will mean charting the distribution of each component over the whole sky in 3-dimensional space with sufficient resolution to discern the “important structure” relevant to that scale. *For a galaxy, we want to know where the gas is, where the stars are, etc., but it is perhaps less important to discern the internal structure of an individual molecular cloud. *On the scale of the whole Universe, we would certainly want to at least be able to chart the explicit distribution of dark matter, diffuse gas, and galaxies. At this stage, we don’t really know what it means to map “dark energy”, but we should at least try to ascertain whether there are any small variations in the universal expansion across the sky.

14. Astrophysics 2020: STScI November 13, 2007 Page 14 An Example of What We Would Measure

15. Astrophysics 2020: STScI November 13, 2007 Page 15 The “Saul Steinberg Problem” *The illustration at the right is a map of the world, as constructed by Saul Steinberg from his apartment window on 9th Avenue in Manhattan. (It was used as a cover by the New Yorker in March, 1976.) *The map is quite detailed close in, but the resolution (and accuracy) degrades rapidly with distance. *Nevertheless, it sill maintains the essential features, e.g. there are mountains just west of Nebraska, the Pacific Ocean separates California from Japan. *Since we will always make astronomical maps from a given vantage point in space, we will always suffer this problem. The trick is in deciding how much to invest to improve the more distant renderings. (If Saul had a telescope in his window, he would have done a somewhat better job.)

16. Astrophysics 2020: STScI November 13, 2007 Page 16 Fortunately, Structure Evolves Rapidly With Time *As we look further out, we look further back in time. *But the structure in the Universe grew rapidly with cosmic time. *Depending on the question, high resolution over the whole sky may be less crucial at high redshifts. This is fortunate, because extending detailed maps out the most distant redshifts is the most demanding technically.

17. Astrophysics 2020: STScI November 13, 2007 Page 17 Mapping Starlight - Wide-field Optical Imaging *The easiest way to find and map the distribution of galaxies, is to observe their starlight. *This is nothing new, imaging surveys of the optical sky date back to the early days of astronomy. *The real challenge is to go faint and wide. To accomplish this in a reasonable period of time requires high étendue.

18. Astrophysics 2020: STScI November 13, 2007 Page 18 Importance of É tendue *The solid angle surveyed per unit time, to some limiting flux, F, at a signal-to-noise ratio, SNR  in exposures of time, t, is given by: Here A  is the étendue of the telescope,  is the efficiency of the system,  sky is the sky intensity,  is the size of the seeing-limited PSF, and  det is proportional to detector noise and trap depth. *Large surveys require high étendue. But with conventional optical designs - high étendue requires large detector area. *The ability to construct very large detector arrays has only recently become available.

19. Astrophysics 2020: STScI November 13, 2007 Page 19 OpticalNear IR QUEST SDSS CFH-L LSST DLS NWDS GOODS BTC CFH-LNWDS Constant A  t UDF HDF VST PS Some Recent Optical and Near IR Surveys

20. Astrophysics 2020: STScI November 13, 2007 Page 20 The Large Synoptic Survey Telescope (LSST) *LSST has the largest étendue for survey science that has ever been proposed - roughly 2 orders of magnitude greater than that of currently operating facilities. *It incorporates an 8.4 m diameter primary mirror, with a 9.6 square degree camera. *LSST will observe 20,000 square degrees of sky down to ~ 27th magnitude, yielding a sample of a few billion galaxies out to z ~ 1 - 1.5.

21. Astrophysics 2020: STScI November 13, 2007 Page 21 LSST Simulation

22. Astrophysics 2020: STScI November 13, 2007 Page 22 Getting the Distances - Photometric Redshifts *Galaxies have distinct spectra, with characteristic features at known rest wavelengths. *Accurate redshifts can be obtained by taking spectra of each galaxy. But this is impractical for the billions of galaxies mapped by LSST. *Instead, we can use the colors of the galaxies obtained from the images themselves. This requires accurate calibration of both the photometry and of the intrinsic galaxy spectra as a function of redshift.

23. Astrophysics 2020: STScI November 13, 2007 Page 23 Would it make sense to extend this to space? *To map the distribution of galaxies at higher redshift will require surveying in the IR, and thus from space. *JWST will get down to m AB ~ 31 in its deep surveys (5 x 10 5 s). At z > 6 or so, there will be ~ a few galaxies per square arcmin in this magnitude range. The NIRcam field of view is 2 x 4.7 square arcmin. *To cover 20,000 square degrees of sky with the current system would take 250,000 years. With an IR camera the size of the visible camera on LSST, this comes down to ~ 30 years. It is a stretch, but not completely infeasible!

24. Astrophysics 2020: STScI November 13, 2007 Page 24 JWST Simulated Deep Image

25. Astrophysics 2020: STScI November 13, 2007 Page 25 Finding galaxies in H I might even be more attractive… *The Square Kilometer Array (SKA) will observe the universe in the 21-cm line of neutral hydrogen. *Predicted sensitivity enables the detection of a z ~ 3 galaxy in a reasonable period of time. *With a 10 square degree field of view, it can survey the whole sky in a year of operation, cataloguing ~ 1 B H I emission galaxies out to z ~ 1.5. *The redshift comes for free, since we are looking at an emission line. *Comparison of SKA and LSST maps will be tremendously useful for tracing the relationship between gas and stars as galaxies evolve.

26. Astrophysics 2020: STScI November 13, 2007 Page 26 Mapping the Dark Matter - Cosmic Shear *The systematic and correlated distortion of the appearance of background galaxies due to weak gravitational lensing allows us to map the clustering of dark matter in the intervening universe. *As light from background galaxies passes through the intergalactic medium, it gets deflected by gravitational potentials associated with intervening structures. A given galaxy image is both displaced and sheared. *The effect is detectable only statistically. The shearing of neighboring galaxies is correlated, because their light follows similar paths on the way to earth.

27. Astrophysics 2020: STScI November 13, 2007 Page 27 COSMOS Dark Matter Maps (Massey et al. 2007) *A beautiful application of weak lensing tomography has now been accomplished by the COSMOS team using ACS data from Hubble. *With multicolor imaging from various bands, they have separated out the lensing contributions from background galaxies at various redshifts, thereby determining the 3D structure of the intervening dark matter.

28. Astrophysics 2020: STScI November 13, 2007 Page 28 Overlays with Baryonic Maps

29. Astrophysics 2020: STScI November 13, 2007 Page 29 This work can only be done from space…

30. Astrophysics 2020: STScI November 13, 2007 Page 30 Requirements to Map the Sky? *COSMOS was a 1.6 square degree survey, using 577 contiguous ACS pointings. It gets down to I F814W < 26.6, yielding ~ 80 resolved galaxies per square arcminute. *The total data set represented 583 orbits, or roughly 1.2 Msec of observing time. Covering 20,000 square degs with this system (assuming the ACS was still available) would take ~ 450 years. *But the ACS field of view is only 11.3 square arcmin. The full survey could be accomplished in ~ few years with a mission with a field of view ~ few tenths of a square degree. This is the characteristic size planned for some of the candidate JDEM missions (e.g. SNAP, DUNE).

31. Astrophysics 2020: STScI November 13, 2007 Page 31 Diffuse Baryons at Low Redshift *Detailed analyses over the past ten years have shown that galaxies can only account for ~ 10% of the baryonic mass at low redshift. This is sometimes called the “missing baryon problem”. *There is no problem at high-z. There, Ly-  clouds plus galaxies account for all of the baryons. *It is now widely believed the missing baryons at low-z must be in the form of a warm hot intergalactic medium, with characteristic temperature in the range 10 5 < T < 10 7 K. *Gas is believed to be shocked up to these temperatures, as it falls into the potential wells formed by the dark matter filaments. From Cen and Ostriker 2006.

32. Astrophysics 2020: STScI November 13, 2007 Page 32 Diffuse Baryons at Low Redshift *This gas is most easily mapped via X- ray emission lines, especially lines of helium-like and hydrogen-like oxygen, which are expected to be the brightest features. *However, the emission is very faint, and is complicated by copious backgrounds and foregrounds associated with distant AGNs, emission from the hot interstellar medium of the galaxy, and particle backgrounds in space. *The optimal detector is an X-ray imaging cryogenic calorimeter, which can provide the requisite energy resolution with high quantum efficiency.

33. Astrophysics 2020: STScI November 13, 2007 Page 33 Requirements for a WHIM Mapping Mission *Again, the system étendue is the key parameter. For a grazing incidence X- ray telescope, the focal ratio is essentially fixed by the graze angle requirements, and the étendue is then independent of focal length - It depends only on the physical size of the detector array. *For typical parameters, one finds: A  ~ 0.012 d 2 cm 2 -sr, where d is the physical size of the detector in cm. *Theoretical predictions of the oxygen line intensity are ~ 0.03 - 0.2 photons cm -2 s -1 sr -1. The backgrounds and foregrounds can be 100 times greater. However, the WHIM should be structured in filaments, following the dark matter. Assuming a typical clumping factor ~ 10, the signal-to-noise ratio in a given energy bin of width  E (in eV) and observation time t (in units of 10 7 s) is given by: SNR ~ 5.6 d t 0.5  E) 0.5. *Clearly, this mission is only feasible, if we can make very large cryogenic detector arrays ~ 100’s of cm in size. To date, the largest ever constructed has been ~ 1 cm in size.

34. Astrophysics 2020: STScI November 13, 2007 Page 34 Summary *Mapping the structure of all the matter in the Universe is a laudable goal for space astrophysics in the future, with significant implications beyond the obvious scientific analyses we can imagine now. *Ground-based experiments will make important contributions to mapping the starlight and cold gas in galaxies. However, space is required for mapping the dark matter through weak lensing, and the diffuse baryons through X-ray emission lines. *In all cases, the primary technical requirement involves the development of very large cameras (visible, IR, X-ray), which directly lead to large system étendue. This should be a priority area for technology investment now, if we hope to pursue these goals in the next decade.