High redshift observational cosmology is one of the most active fields of astronomical research, as can be seen by sifting through the daily offerings on astro-ph. Progress is rapid and happening on many fronts.
The installation of the Advanced Camera for Surveys on HST in 2002 allows routine optical imagining to Hubble Deep Field depths in fields with twice the area and angular resolution of WFPC2 (Ford et al. 2002). The first ACS results from the GOODS project (Giavalisco et al. 2003) are soon to come out in a special ApJ Letters volume. The results include reported color evolution in the LBG population, suggesting less dust is present in the z ~ 4 population compared to the z ~ 3 population (Idzi et al. 2003; Papovich et al. 2003).
The recent successful launches of the GALEX 1 and SIRTF 2 satellites will allow excellent survey capabilities in the vacuum UV (0.13 - 0.3 µm) and infrared (3.5 - 160 µm) respectively. Together they will provide the definitive local calibration of the IRX- relationship. GALEX will extend the redshift range for LBG selection to z < 2. The SIRTF observations of the GOODS project will directly detect normal galaxies and LBGs out to z ~ 5 LBGs at 3.5 - 8 µm (rest frame NIR) which allows probing of their evolved stellar populations. At 24 µm they should also be detect rest frame ~ 7 µm reprocessed PAH emission from galaxies with LFIR 1011 out to z ~ 2.
Recent progress in sub-mm galaxy research includes an improved efficiency in obtaining redshifts. Chapman et al. (2003b) present optical redshifts for 10 sub-mm sources also detected in the radio. Soon that group will publish an expanded sample of about 70 optical spectroscopic redshifts of sources detected at both radio and sub-mm wavelengths. They find that sub-mm galaxies mostly do indeed have the high redshift (z ~ 2.5) that was expected. This work provides the first accurate redshift distribution and space density measurements of the sub-mm population. While more progress could be made in identifying the faintest SCUBA sources (as outlined in Section 3), what is really needed is more and reliable detections of the 1 mJy population, i.e. somewhat below the confusion limit. It seems unlikely that there will be enough clusters not already looked at by the Smail, Blain, Ivison group and other SCUBA researchers to add substantial numbers of faint sources.
Unfortunately, at the longest wavelengths (160 µm), the SIRTF PSF has FWHM = 38" and hence the detections will also be confusion limited at around 7 to 19 mJy (Xu et al., 2001), and thus SIRTF is not likely to settle the debate on the source composition of the sub-mm/FIR background. SOFIA will provide higher resolution imaging (~ 8" at 200 µm) but due to the high sky background will generally be limited to galaxies with z < 1 3. Bolometer arrays larger than SCUBA, such as SCUBA-II 4 also will not address this issue since it is angular resolution, and hence telescope aperture that limits us from resolving the FIR - mm background. Existing or soon to be completed sub-mm to mm arrays such as the SMA 5 will help to pin down the position of the brightest SCUBA sources. However, to truly resolve the sub-mm to mm background we must await significant completion of ALMA - the Atacama Large Millimeter Array 6. When completed in 2012 it will be able to reach flux densities of ~ 0.1 mJy (well into the expected fluxes of LBGs) in about half an hour at 850 µm. With a resolution of 0.1" or better (depending on array configuration) the observations should be well out of the confusion limit. Combining ALMA data with (then ancient archival) data from HST and hopefully new data from JWST (the James Webb Space Telescope) will allow a direct estimate of the bolometric output, and hence star formation rate, of all types of high-z galaxies. This will make the question of whether galaxies obey the IRX- relationship somewhat moot.
1 http://www.galex.caltech.edu/ Back.
2 http://sirtf.caltech.edu/ Back.
3 http://sofia.arc.nasa.gov/Science/science/sci_opport_galaxies.html Back.
4 http://www.jach.hawaii.edu/JACpublic/JCMT/JCMT_developments/SCUBA2/scuba2.html Back.
5 http://sma-www.harvard.edu/ Back.
6 http://www.alma.nrao.edu/ Back.