Draft (Jan. 18, 2010) Executive Summary

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4.7Early light houses and cosmic reionization

Haojing Yan (Ohio State), Liang Gao (NAOC), Xiaohui Fan (Arizona)
Key questions:

  1. What are the origins of first stars, first galaxies, and first supermassive blackholes?

  2. When and how did the first light sources cause the cosmic reionization, and what is the impact of the reionization to the cosmic galaxy formation processes afterwards?

  3. How were the early galaxies assembled?

  4. What is the chemical enrichment history of the early Universe?

While we have not yet found the "first light sources", we are approaching this goal. Currently, quasars and star-forming galaxies have been unambiguously identified out to z~6.4-7 ([1][2]), an early epoch that is only ~ 700-800 million years after the Big Bang. Studies of galaxies at only slightly later epochs (z~5.5-6.5) suggest that the Universe must have started actively forming galaxies well before z~7 ([3][4]), and indeed we have found candidate galaxies out to z~10 ([5-13]). Notably, there is also strong evidence that gamma-ray bursts (GRBs), which are believed to be intimately connected to very massive stars, have been identified out to z~8.2 ([14,15]), an epoch that extends well into the cosmic reionization. It is expected that more and better candidate objects will be discovered to z~10 within the next decade; however, any detailed studies of such objects, or even just the unambiguous confirmation of their high-redshift nature will have to wait for the next generation of large facilities such as the TMT. The unprecedented light gathering power of TMT will enable us to push to the very moment when the first stars in the Universe began to form, and to study in detail the history of galaxy and supermassive black hole formation. With adaptive optics (AO), TMT will have higher spatial resolution than ALMA and JWST, and will also be able to carry out very high resolution spectroscopic studies which JWST will not be capable of. All this will place TMT to a unique position in the synergy of future facilities in the high-redshift frontier.
Main science areas:
1. Direct detection of Population III stars

One of the most exciting subjects in the study of the early Universe is the detection of the very first generation of luminous objects formed out of the primordial gas, or the metal-free ``Population III" (Pop-III) stars. In the popular Cold Dark Matter (CDM) scenario, Pop-III stars could begin to form at z>50 within dark matter halos that are as massive as 100,000 times of the Sun, and could have formed in large number by z~10-15 (e.g., [16]) such that the characteristic He II 1640A line ([17]) could be strong enough for detection. IRMS, which is one of the first-light instruments at TMT, will be sufficiently sensitive for such experiments.

Direct observations of Pop-III stars are not only of fundamental importance in understanding early star/galaxy formation but could also be critical in constraining the nature of dark matter. For instance, an attractive alternative to the CDM model is the so-called ``Warm Dark Matter" (WDM) model in which dark matter has a larger intrinsic velocity than that in the CDM. In this model, the very first stars would form in a huge burst along thin and straight filaments that could stretch over some 24 million light years in length (about half the size of the Milky Way galaxy) ([18]). In contrast, the CDM model predicts that first stars are distributed in quasi-spherical clumps of individual dark matter halos. The high luminosity generated by such a huge star burst in the WDM model would significantly boost the probability of the direct detection of the earliest firework of star formation, and the distinct filamentary geometry, which will be discernable by the TMT with AO, could be used to distinguish the two cosmological models.
2. Detailed spectroscopic studies of galaxies and quasars at z  6

By design, TMT will be uniquely suited for detailed spectroscopic studies of early galaxies and quasars. The most obvious science to pursue at TMT is to do spectroscopic confirmation of well-defined galaxy candidate samples at z  6. Spectroscopic identification is the only unambiguous way to confirm the high-z nature of such candidates and to obtain their precise redshifts. Such samples at z ~ 6 are already in place, and similar samples at z ~ 7--10 are being gathered by the on-going near-IR surveys from the ground (e.g., UKIDSS, VISTA) and from the space (HST/WFC3 IR surveys). After JWST is launched, more and better samples will soon be available. The mere identification of very high-z objects will already have great impact to a number of important questions, e.g., whether low-luminosity star-forming galaxies could be the major sources of reionization ([19]), when the precise epoch of reionization is, etc.

High-resolution spectroscopic observations of galaxies at very high-redshifts, at least for the brightest ones, will be feasible at the TMT with a reasonable amount of time. It will be possible to study their chemical abundances, and to put constraints to the IMF of early galaxies. Furthermore, the IFU capability will enable us to study the kinematic of such galaxies (e.g., through [O II] 3727A emission line), and hence provide invaluable constraints to galaxy formation models.
It is worth keeping in mind that TMT will eventually have PFI, a high-contrast AO imager and spectrograph. While it is designed for extra-solar planet detection, it can also be used to detect quasar host galaxies. The study of quasar hosts at very high-redshifts will reveal the growth of supermassive black holes and its impact to the star-formation processes (a.k.a. "AGN feedback").

3. Spectroscopic studies of IGM through quasars and GRB afterglows

The studies of SDSS quasars at z  6 have revealed the presence of the Gunn-Peterson absorption due to cosmic neutral hydrogen, and have shown that the reionization likely ended by z ~ 6 ([20][1]). With TMT, similar studies can be expanded to many more sightlines and extended to higher redshifts, using fainter and higher-redshift samples that will be available from the wide-field near-IR surveys (e.g., UKIDSS, VISTA). Such studies will enable us to obtain the topology of the IGM, and to probe the inhomogeneity of the reionization. Furthermore, high-dispersion spectroscopy will provide us the chemical abundances of the IGM along the sightline, and hence enable us to study the metal enrichment history of the Universe.

GRB afterglows provide a powerful alternative to quasars in the study of the IGM (e.g., [21]). As long-duration GRBs are believed to be associated with the death of massive stars ([22]), they could exist before the formation epoch of the first quasars and hence could probe earlier epochs than quasars do. The design of TMT is capable of rapid response to Target-of-Opportunity requests, and it is very likely that our understanding of the early IGM in the reionization epoch will come from the observations of a large sample of GRB afterglows.

Possible TMT programs:

  1. IRMS spectroscopic survey of well-defined samples of candidate galaxies and quasars at z~6-15, and establish their relation to the cosmic hydrogen reionization. It is possible to directly detect Pop-III stars during such effort.

  2. Kinematic studies of galaxies at z>5 using IRIS IFU.

  3. High-contrast AO observations of quasar host galaxies at z>7 using PFI to study the interplay of early star-formation process and supermassive black hole growth.

  4. High-resolution spectroscopic observations of quasars and GRB optical afterglows (through target-of-opportunity requests) at z>6 to study the IGM topology and chemical abundance, and the inhomogeneity in reionization.

  5. Imaging and spectroscopy of the host galaxies of GRB at very high redshifts.

China’s strengths and weakness in this area:

Quite a number of groups in China (NAOC, KIAA/PKU, NJU, SHAO, PMO, USTC) already have established record in the studies of the following subjects from theoretical aspect: Population-III stars, reionization, early structure formation, AGN physics, and GRB mechanism. From the observational side, there are a number of on-going or future projects that will complement the TMT science at this forefront and we will seek synergy: the 21CMA experiment will provide valuable experience in detecting the reionization signature, the search for extremely metal-poor stars at the LAMOST will possibly reveal the relics of Pop-III stars, the time-domain study at the South Pole Dome-A will likely result in a large number of GRB afterglows, and the millimeter/sub-millimeter facilities at Dome-A will enable us to study the dust contents of very early galaxies.

Currently, China is still lacking the expertise of observational study of the high-redshift universe. To take full advantage of the opportunity that joining the TMT will bring us in this frontier, we should start fostering such expertise now.


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  12. Yan, H., et al. 2009, arXiv:0910.0077

  13. Capak, P., et al. 2009, arXiv:0910.0444

  14. Salvaterra, R., et al. 2009, arXiv:0906.1578

  15. Tanvir, N. R., et al. 2009, arXiv:0906.1577

  16. Gao, L., et al. 2009, arXiv:0909.1593

  17. Schaerer, D. 2002, A&A, 382, 28

  18. Gao, L & Theuns T., 2007, Sci, 317, 1527

  19. Yan, H. & Windhorst, R. 2004, ApJ, 612, L93

  20. Becker, R., et al. 2001, AJ, 122, 2850

  21. Vreeswijk, P. M, et al. 2004, A&A, 419, 927

  22. Woosley, S. E., & Bloom, J. S. 2006, ARA&A, 44, 507

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