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.