The students attending this lecture course are living at a special time! During your lifetime you can reasonably confidently expect to witness the location of the sources responsible for cosmic reionization, to determine the redshift range over which they were active and perhaps even witness directly the `cosmic dawn' when the first stellar systems shone and terminated the `Dark Ages'!
We have made such remarkable progress in the past decade, reviewed here, that such a bold prediction seems reasonable, even for a cautious individual like myself! We have extended our fundamental knowledge of how various populations of galaxies from 0 < z < 3 combine to give us a broad picture of galaxy evolution, while extending the frontiers to z 7 and possibly beyond. Certainly many issues remain, including the apparent early assembly of certain classes of quiescent galaxy, the abundance patterns in the intergalactic medium and the apparent `downsizing' signatures seen over a variety of redshifts. However, the progress has been rapid and driven by observations. Accompanying this is a much greater synergy between theoretical predictions and observations than ever before.
We have seen that the question of `First Light' - the subject of this course - is the remaining frontier for observations of galaxy formation. The physical processes involved are poorly understood and thus observations will continue to be key to making progress. In this final lecture, I take out my crystal ball and consider the likely progress we can expect at optical and near-infrared wavelengths with current and near-term facilities ahead of those possible with the more ambitious new observatories such as the James Webb Space Telescope (JWST) and a new generation of extremely large ground-based telescopes.
9.2. The Next Five Years
Panoramic imaging with large optical and near-infrared cameras on telescopes such as Subaru, UKIRT and VISTA will enable continued exploration of the abundance of luminous drop-outs and Ly emitters over 5 < z < 7, reducing the currently troublesome issues of cosmic variance. Deeper data will continue to be provided from further fields taken with ACS and NICMOS. With further spectroscopic surveys on large telescopes, we can expect improved stellar mass density estimates at z 5-6 (Eyles et al 2006). However, strong gravitational lensing may still be the only route to probing intrinsically fainter sources, particularly beyond z 7.
More detailed characterization of the properties of the most massive galaxies at z 5-6 will also be worthwhile, in addition to continuing to deduce statistical properties such as abundances and luminosity functions.. Laser-guide star adaptive optics is now in widespread use on our 8-10 meter class telescopes and, with integral field spectrographs, can be used to probe the resolved dynamics of distant galaxies (Genzel et al 2006). Application of these techniques to the most intense star-forming lensed systems at z > 4 (Swinbank et al 2006, Figure 61) is already providing unique insight into the physical state of the earliest galaxies. Extending the same techniques with adaptive optics will advance progress and address process on remarkably small scales ( 200 pc).
Figure 61. Detailed studies of a lensed z 5 galaxy (Swinbank et al 2006). (Top) Reconstructed color HST VI image of the z = 4.88 arc in the cluster RCS0224-002. The inset shows the effect of 0.8 arcsec seeing on the reconstruction, thereby demonstrating the advantage of lensing. In the source plane, the galaxy is 2.0 × 0.8 kpc. (Bottom) [O II] velocity field obtained during a 12 hr VLT SINFONI exposure without AO. Spatial comparison with the Ly field gives clear evidence of significant bi-polar outflows.
A number of special purpose ground-based instruments are also being developed, both with and without adaptive optics, to probe for z > 7 dropouts and Lyman emitters. These include:
The exciting redshift range 7 < z < 12 will also be the province of improved drop-out searches using the instrument WFC3 slated to be installed on Hubble Space Telescope in 2008-9 11. The infrared channel of this instrument spans 850 - 1170 nm with a field of 2 arcmin at an angular resolution of 0.13 arcsec pixel-1. This resolution is coarser than that of adaptive optics-assisted instruments such as F2T2. The principal gain over ground-based instruments will be in deep broad-band imaging free from airglow. The survey efficiency is about an order of magnitude better than that of NICMOS. WFC3 also has two infrared grisms which will be helpful in source discrimination.
A major stumbling block at the moment, even at z 5-6, is efficient spectroscopic follow-up of dropout candidates. As we discussed in Section 6, photometric redshifts have unfortunately become de rigeur in statistical analyses of luminosity densities and luminosity functions (Bouwens et al 2006), yet their precision remains controversial. For z- and J-band dropous beyond z 7, photometric redshifts will be even less reliable. Spitzer detections will be harder and fewer, and the typical source may have only 2-3 detected bands. Candidates may be found in abundance but how will they be confirmed?
The various 8-10m telescopes are now building a new generation of cryogenic near-infrared multi-slit spectrographs. It can be hoped that long exposures with these new instruments will be sufficient to break this impasse.
9.3. Beyond Five Years
A number of facilities are being planned, motivated by the progress discussed in these lectures. These include:
Figure 62. The 6.5m James Webb Space Telescope: Then (2013 in orbit) and Now (2005, full scale model).
Figure 63. The proposed US-Canadian Thirty Meter Telescope (www.tmt.org) now in the detailed design phase.
These will complement redshifted 21cm line tomography with radio facilities and address the key questions of the escape fraction of photons from star-forming sources and how they create ionized bubbles which merge to cause reionization.
It is quite likely that, by 2013, the redshift range containing the earliest galactic sources, estimated at present to be 10 < z < 20 perhaps, will have been refined sufficiently by special-purpose instruments on our existing 8-10 meter class telescopes. Thus one can surmise that that both JWST and future ELTs will be used for much more challenging work related to the physical process of reionization, as well as the chemical maturity of the most luminous sources found at high redshift.
Figure 64. Flexibility in the deployment strategy for the multiple integral field units (yellow squares) for the proposed TMT infrared multi-object adaptive optics assisted instrument IRMOS. The IFUs can be distributed around the full field of 2 arcmin in classic multi-object mode. Alternatively, the IFUs can be configured together to map a small 6 × 6 arcsec field at high angular resolution in `blind' mode. Such flexibility will be important in mapping Ly emission at high redshift in various situations.
An obvious partnership between JWST and TMT, for example, which would complement the 21cm studies, would be to (i) search for the extent and topology of faint Ly emission in ionized bubbles around JWST-selected luminous star forming galaxies and, (ii) pinpoint early sources for spectroscopic scrutiny so as to identify signatures of Population III stars.
At the present time there are so many imponderables in our knowledge of the earliest sources, that even the design parameters for the ELT instruments is a considerable challenge. How big are the faintest Ly emitters? What are the typical line widths in km sec-1? How big are the ionized bubbles at a given redshift? And, crucially, what is the surface density of various types of star-forming galaxies. Flexibility in the design of survey strategies will be crucial with instruments such as TMT's IRMOS (Fig. 64).
Any information we can glean on the properties of z 10 sources in the next 5 years will be valuable in optimizing how to move forward when these magnificent new generation telescopes are made available to us. Indeed, it is foolhardy to wait! Time and again, we can retrospectively look back at what we thought we would accomplish with our planned facilities and we always find that we achieved more than we expected!
I thank Daniel Schaerer, Denis Puy and Angela Hempel for inviting me to give these lectures in such a magnificent location with an enthusiastic group of students. I also thank my fellow lecturers, Avi Loeb and Andrea Ferrara and all of the foregoing for their patience in waiting for the completion of my written lectures. I thank my close colleagues Kevin Bundy, Sean Moran, Mike Santos and Dan Stark for their help and permission to show results in progress as well as Ivan Baldry, Jarle Brinchmann, Andrew Hopkins, John Huchra and Jean-Paul Kneib for valuable input. Finally, I thank Ray Carlberg and his colleagues for their hospitality of the Astronomy Department at the University of Toronto where the bulk of these lectures notes were completed.
9 http://www.ast.cam.ac.uk/~optics/dazle/ Back.
10 http://odysseus.astro.utoronto.ca/ggs-blog/?page_id=2 Back.
11 http://www.stsci.edu/hst/wfc3 Back.