Here I summarize the status of a recent debate within the field over the last four or five years: which population of galaxies contributes the most to the total star formation rate density at high redshift? The two top contenders are the optically selected LBG population and the "SCUBA" sub-mm and mm galaxies. Table 2 summarizes some properties of these contenders and their estimated contribution to the sub-mm background S850µm = 44 Jy deg-2 (Fixsen et al. 1998).
Property | LBGs | SCUBA galaxies |
Detectability | "easy" & numerous | difficult & rare |
~ 6000 deg2 to R ~ 25.5 | ~ 300 deg2 to ~ 6.5 mJy | |
confusion limited | ||
(Steidel et al. 2003) | (e.g. Hughes et al. 1998) | |
Luminosity | LUV ~ 1010-11
L![]() |
LFIR ~ 1012-13.5
L![]() |
(without dust correction) | ||
LFIR / LUV | ![]() |
30 - 3000 a |
local analog | UV-bright starbursts | ULIRGs (Goldader |
(MHC99) | et al. 2002; AS00) | |
Contribution to S850 | 93% | 100% (by definition...) |
after corrections | ~ 1.3 completeness | ~ 5 completeness |
~ 6 dust absortion | ||
(AS00) | Chapman et al. (2000) | |
a One deviant sub-mm galaxy is SMMJ16358+4057 with LFIR / LUV ~ 10 (Smail et al. 2003). |
As noted above, LBGs strongly resemble UV bright starbursts. One of the
strongest correlations seen in local UV bright starbursts is the
so-called IRX-
relationship, shown in Fig. 1
(Meurer et al. 1995;
MHC99).
This relationship shows that the ratio of dust emission in the
FIR to (residual) UV emission (infra-red excess or IRX) correlates with
the UV spectral slope
(defined by
the spectrum
f
). Since IRX
is basically a measure of dust
extinction this means that starburst redden as they become more
extincted. The simplest explanation for this correlation is that the
dust has a strong diffuse foreground contribution to its distribution,
i.e. it behaves like a foreground screen (e.g.
Witt & Gordon 2000;
MHC99;
Calzetti et al. 1994;
Witt et al. 1992).
Indeed, correcting
survey data for dust absorption using simple reddening models greatly
improves the consistency of multi-wavelength SFRD(z) plots
(Calzetti 1999).
![]() |
Figure 1. The
IRX- |
It should be noted that not all local star forming galaxies obey this
relationship. While it works well for the UV-bright calibrating sample
whose members have Lbol
1011.5
it does not work well for ULIRGs which typically have
log(FFIR / F1600) > 2 and
fall well above the
IRX-
relationship
(Goldader et al. 2002).
A significant fraction (~ 30%) of normal (i.e. non starburst)
galaxies fall below the
IRX-
relationship, presumably due to
strong contamination from intermediate age populations in the near UV
(Seibert, 2003).
Be that as it may, the
IRX-
correlation seems to work well for
strongly star forming galaxies with modest amounts of dust absorption.
While there is some distaste for the foreground screen geometry in the
literature (e.g.
Charlot & Fall 2000),
we need not fixate on the
interpretation to use the correlation. We have seen the myriad ways
that LBGs resemble local starbursts, so it seems reasonable to suppose
that they obey the same
IRX-
correlation. If so, then we can
estimate the total extinction correction UV flux (and integrated cosmic
UV flux density), and hence star formation rate of LBGs from just their
rest-frame UV-flux and colors. This was done by
MHC99. An even better
job, using similar and other methods, was done by
AS00. Both papers
find that only 12% - 20% of the UV flux emitted at rest
1600Å reaches
the earth (cf.
Vijh et al. 2003,
and references therein).
It is hard to test whether the
IRX- relation
holds for LBGs,
since the predicted sub-mm fluxes are below the SCUBA confusion limit
(MHC99).
AS00
show that there are a few individual LBGs that have been
directly detected with SCUBA and these largely obey the
IRX-
relation. One lensed LBG, MS1512+36-cB58, falls somewhat below the
IRX-
relationship, albeit with large error bars
(Baker et al. 2001).
If a large fraction of LBGs have similar properties, then the
IRX-
relationship would over predict their contribution to the
total SFRD at z ~ 3.
Fortunately, while LBGs in general are not individually visible at other dust-insensitive wavelengths, stacked measurements of fluxes in the radio and X-ray can be used to test whether LBGs have similar dust extinction properties as nearby UV-bright starbursts.
The radio result is given as a note in proof to
MHC99 and repeated in
Table 3. Radio fluxes of the the ten
U-dropout galaxies
in the Hubble Deep Field North (HDF-N) with the highest predicted
850 µm SCUBA fluxes were computed by assuming that the
galaxies obey
the local FIR to radio correlation, have a radio spectral slope
= - 0.7
(f
) and that the FIR flux
is the reprocessed UV flux. The stacked radio fluxes, kindly provided by
E. Richards, agree remarkably well with the predictions for these ten
sources.
AS00
also do an analysis of the stacked LBG fluxes at 20cm
using the Richards et al. radio map. Their total observed and predicted
fluxes are 105 ± 81 µJy and
114 ± 36 µJy respectively,
similar to what is reported in
MHC99
but with larger errors. The difference is in part due to
AS00 stacking all
their 46 HDF LBGs for the analysis. This includes many with
very little predicted flux which contribute the majority of the error
budget. In addition,
AS00
derive a higher error per
beam by integrating over the beam, which overestimates the error per
source. Hence the stacked radio flux estimate is probably more
significant than implied by
AS00.
Wavelength | Flux density (µ-Jy) | |
(cm) | predicted | observed |
3.5 | 28 | 27 ± 5 |
20 | 100 | 105 ± 24 |
X-rays can also be used to probe high-z star formation. The Chandra
soft X-ray band corresponds to 2-8 Kev at z = 3, "hard" enough to
pass through the ISM of galaxies virtually unattenuated.
Seibert, Heckman &
Meurer (2002)
compared the stacked soft X-ray flux of HDF-N AGN free
U-dropouts given by
Brandt et al. (2001)
with predictions from dust
reddening models. The results are consistent with a variety of
plausible dust reddening laws including the
IRX-
correlation from
MHC99, the
Calzetti et al. (2000)
starburst "obscuration curve", and
the homogeneous and clumpy foreground dust screen models from
Witt & Gordon (2000).
The results are not consistent with the rest-frame UV
flux from LBGs being like that of ULIRGs which have IRX values on the
order of 102 to 103.5
(Goldader et al. 2002).
In fact, if LBGs
had similar values they would be easily observable individually in
X-rays, the sub-mm, and radio (and they are not). Similarly the
scenario that LBGs are not extincted at all under-predicts the stacked
X-ray flux by a factor of six.
Nandra et al. (2002)
do an independent analysis of stacked X-ray results in the HDF-N from
Chandra and reach similar conclusions.
The stacked radio and X-ray analyses are both consistent with the local
starburst reddening relation applying to high-z LBGs. If so, then
they would dominate total SFR density at
z 3
(AS00). However,
there may be a fly in the ointment. As earlier mentioned, in the
MHC99
and AS00
picture we would expect the LBGs to have 850 µm
fluxes up to ~ 1 mJy. While such fluxes are at or below the confusion limit
of blank field SCUBA observations it may be possible to detect such
sources through gravitational lensing.
Smail et al. (2002b)
constrain
the faint counts at 850 µm using SCUBA observations towards
lensing clusters. A little over half of their sample are undetected in
their HST I images, with the non-detections preferentially being at
the (sub-mm) faint end. These faint end sources have magnification
corrected 850 µm micron fluxes like the predictions for
LBGs. The implication is that the LBGs are not showing up in SCUBA
observations presumably because the local
IRX-
relationship overpredicts
their 850 µm flux as it does for MS1512+36-cB58. Instead the
850 µm background is actually dominated by star forming galaxies
like local ULIRGs
(Goldader et al. 2002)
- totally hidden by dust, as is also the case with the brightest SCUBA
sources.
While this scenario may be correct, a careful examination of Smail et al. (2002b) suggests that it is built on a shaky foundation. Their claim to having resolved the 850 µm background rests on four faint (S/N = 3 to 5) 850 µm detections. These have only have lower limits to their magnifications (presumably because their positions are uncertain, since they have no optical, NIR, or radio counterparts). Their Monte-Carlo simulations indicate that at least one of these sources should have a source plane 850 µm flux in the 0.5 to 1 Jy range, the only sources in their survey that could be that faint (or fainter). However, using the lower limit magnifications shows that none of the source plane fluxes need actually be fainter than 1.6 Jy, while the brightest predicted 850 µm flux for the LBGs in the HDF-N is 1.8 mJy (MHC99). Smail et al. are not convincingly dipping deep into the expected realm of LBGs with their SCUBA observations. The lack of optical counterparts at the faint end also does not thoroughly rule out the presence of LBGs. They state a detection limit of I ~ 26 in their HST images which corresponds to a typical brightness seen in LBGs. However, this is the detection limit at S/N = 2 for a point source. To do this well the detection limit should be stated at a higher S/N (at least 3) and be calculated for typical LBG sizes (corrected for lensing in their case). I expect this would lower the limiting mag to I ~ 25 or brighter. Figure 15 of AS00 implies that the optical counterparts of the ~ 1 - 2 Jy sources have magnitudes over a wide range I ~ 24 to 27 ABmag in the source plane. Smail et al. have not convincingly ruled this out yet.
I conclude that the nature of the sources dominating the sub-mm
background is not yet well determined. It is clear that the leading
contenders are galaxies at
z 2 whose
bolometric output is
dominated by dust, and that these galaxies dominate the star formation
rate density at these redshifts. It is not yet settled whether these
galaxies are detectable in the rest-frame UV. Additional observations
of lensing clusters in the mm and sub-mm would improve the statistics on
the 850 µm counts at and below 2 mJy, while deeper optical
observations of the faintest SCUBA sources (e.g. with ACS on HST) are
needed to make a fair and convincing test of the LBG scenario.