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\markboth{Claus Leitherer}{Massive Stellar Populations in the IR}
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\begin{document}
\vspace*{1cm}
\title{Massive Stellar Populations in the Infrared}
 \author{Claus Leitherer}
\affil{STScI, 3700 San Martin Drive, Baltimore,
    MD 21218~$^1$}
\altaffiltext{1}{Operated by AURA, Inc., for NASA under contract NAS5-26555}

%\author{Co-Author}
%\affil{The Name of My Institution, The Full Address of My Institution}

\begin{abstract}
An overview of some key results related to hot, massive
stars and starbursts in the infrared is given. The wavelength range covers the 
ground- and space-based near-infrared, the mid- and far-IR as
probed by several space missions, and the sub-millimeter regime.
I discuss how these observations permit an unbiased view of
dust-obscured stars and how they are related to data at optical and ultraviolet
wavelengths. 
\end{abstract}

\section{Introduction}
Observational studies of massive stars have traditionally been biased
toward the optical/ultraviolet (UV), where the most significant spectral lines are
 and where the stellar energy distribution peaks. 
This is also the domain where
hot stars dominate over their cooler descendants in a stellar population.
Quite ironically, the optical/UV  can be the
least favorable wavelength region for observations of hot, massive stars
in starburst galaxies. Stars, gas, and dust are co-spatial, and the most
intense star-formation sites are often deeply obscured by dust. Only 
observations at sufficiently long wavelengths can unveil the underlying
stars, either directly, or via the radiation from surrounding gas and dust. 
I will address a handful
of topics, arranged by wavelength domain from the near-infrared
(IR) to sub-millimeters, 
highlighting the impact of IR data on our understanding of massive
stellar populations. The subjects cover the determination of starburst masses,
the IMF of massive stars, the heating of the interstellar medium (ISM),
the connection between starbursts and AGN, and the global energy budget
of starbursts.

 
\section{Red-supergiant features and starburst masses}

Determining the stellar mass in a starburst and/or the low-mass end of the 
initial mass function (IMF) is notoriously challenging. Massive stars are
overwhelmingly luminous for a typical population mix, precluding any
direct detection of the light from low-mass stars. Yet, it is the low-mass
population where most of the stellar mass is concentrated and whose
mass fraction determines the star-formation efficiency. 

The most direct technique to obtain the starburst mass relies on gravity,
not on radiation. Velocity dispersion measurements of the gas in the center
of M82 provide strong support for a stellar population which is overluminous for
its mass, and therefore for a {\em top-heavy} IMF (Rieke et al. 1993). 
{\em Stellar} velocity dispersions of starburst regions have remained elusive
until very recently. In a pioneering study, Ho \& Filippenko (1996a,b) detected
and resolved faint absorption features from red supergiants (RSG) in optical spectra
of the super star clusters in NGC~1569 and NGC~1705. This technique has
been applied to clusters in other starburst galaxies, such as M82 (Smith \&
Gallagher (2001). RSGs are the only species in a young stellar population
whose spectral lines are detectable and sufficiently narrow to provide the
velocity dispersion, whose value is of order 10~\kms\ for a $10^6$~\Ms\
cluster.

Observations of RSG features in the near-IR have two distinct advantages over 
the optical: (i) an approximately tenfold decrease of dust attenuation
and (ii) an almost complete absence of dilution from stars not in the RSG
phase. The most prominent features in
the H and K bands are
the first and second overtones of CO at 2.29~$\mu$m (2--0 band head) and 
1.62~$\mu$m (6--3 band head) and the Si~I absorption at 1.59~$\mu$m (Oliva
et al. 1995).

B\"oker et al. (1999) observed the  
nuclear stellar cluster in the nearby spiral galaxy IC~342. Their 
high-resolution ($R= 20000$) spectra of the CO band head at 2.29~$\mu$m 
allowed them to derive a line-of-sight stellar velocity 
dispersion of 33~\kms.  Under the assumption of an isotropic 
velocity distribution,  a mass-to-light ratio in the K band 
of $ M/L_{\rm K} = 0.05$ and a cluster mass near $10^7$~\Ms\
are found. Comparison of the derived 
mass-to-light ratio with stellar population synthesis models 
suggests an age around $10^7$~yr. 


H band echelle observations of the super star clusters A and B in
NGC~1569, together with appropriate RSG template spectra were obtained
by Gilbert \& Graham (2002). The data have a resolution and
S/N comparable to those of the optical spectra analyzed by Ho
\& Filippenko (1996a; see above), but are superior in two ways:
the spatial information allows the separation of the two cluster
components in NGC~1569-A (Maoz et al. 2001), and the longer
wavelength minimizes the contamination from non-RSG stars.
Cluster~B has a velocity dispersion of
$7.5 \pm 0.2$~\kms, which implies a dynamic
mass of $2.3 \times 10^5$~\Ms\ from the virial relation. The
cross-correlation function of cluster~A is asymmetric and 
can be fit by two Gaussian components separated by 21.4~\kms,
each with a velocity dispersion of $10.4 \pm 0.4$~\kms.
The implied dynamic masses for the two components are
$3.9 \times 10^5$ and $4.4 \times 10^5$~\Ms. 
This may be compared with a total mass of cluster~A
of about $10^6$ derived by Ho \& Filippenko under the
assumption of a single component. The studied clusters have
a normal IMF, with no evidence for a low-mass truncation from
the dynamic mass determination.

Near-IR velocity dispersions, both from absorption and emission lines,
are still around the instrumentation limits. Rapid
technological progress will make this technique feasible for
even less massive and more distant starburst clusters.   



\section{Hot, massive stars in dusty starbursts}

In contrast to RSGs, hot stars have no sufficiently strong features 
that would be detectable in a population longward of $\sim$1~$\mu$m.
However, the mid-IR includes numerous nebular fine-structure lines with
ionization energies of tens of eV. These lines are suitable tracers for the
shape of the stellar ionizing spectrum, and therefore for the 
properties of the upper end of the IMF. 
 
Thornley et al. (2000) carried out an ISO spectroscopic
survey of 27 starburst galaxies 
with a range of luminosities from $10^8$ to $10^{12}$~\Ls. 
The [Ne~III] 15.6~$\mu$m and [Ne~II] 12.8~$\mu$m lines are
particularly useful. The ionization
 potentials of neutral and ionized Ne are 22 and 41~eV, respectively,
the two lines are very close in wavelength (12.8 and 15.6~$\mu$m), 
and they have similar critical densities. This makes the line ratio a sensitive
probe for various star formation parameters, in particular the upper mass 
cutoff of the IMF (\mup) and the age and duration of the starburst. The
result of the ISO survey is in Fig.~1 (left).
The neon ratios of the sample objects are generally found to be 
between 0.05 and 1.0; however, the metal-poor starburst galaxies NGC~5253 
and II~Zw~40 have neon ratios of 3.5 and 12, respectively. It is instructive
to compare these neon ratios to observations of nearby high-mass star-formation
sites, such as W51, 30~Doradus, and the central parsec of the Galaxy. The 
neon ratios for W51 and 30~Doradus are higher than for the starburst 
galaxy sample, while that for the Galactic center is lower.  The ISO-SWS data 
thus generally confirm that powerful dusty starbursts exhibit low nebular 
excitation (e.g., Doherty et al. 1994). 
The average [Ne~III]/[Ne~II] ratio in starburst galaxies 
is a factor of 2 to 3 lower than in individual Galactic compact HII regions. 
 
\begin{figure}
\plottwo{IAU212_Ne.eps}{IAU212_IMF.eps}
\caption{Left: Ratio of [Ne~III] 15.6~$\mu$m over [Ne~II] 12.8~$\mu$m
vs. $L_{\rm{IR}}$ for a sample of starburst galaxies and nearby
star formation regions measured with ISO. Arrows in the 
upper right indicate the effects of dust attenuation. Right:
[Ne~III]/[Ne~II] ratios (upper)  and $L_{\rm{IR}}/L_{\rm{Lyc}}$
line ratios (lower) for starburst models with \mup\ 
between  25 and 100~\Ms. For each \mup, three burst models are shown 
 for time scales of 1, 5, and 20 Myr.  The vertical bar on the right 
side indicates the range of measured ratios. From Thornley et al. (2000).}
\end{figure}

Thornley et al. (2000) modeled the H~II regions as ionization-bounded 
gas clouds photoionized by central evolving star clusters. 
 The computed Ne ratios (for a Salpeter IMF 
with different values for \mup) as a function of burst age and duration
 are shown in Fig.~1 (right). The important result is that on average stars more 
massive than about 35 to 40~\Ms\ are deficient in the observed sample. 
Either they never formed because of a peculiar IMF, or they have already 
disappeared as a result of aging effects. 




An upper-mass cutoff as low as 40~\Ms\ is difficult to reconcile with 
the ubiquitous evidence of very massive stars with
masses of up to 100~\Ms\ in many starburst regions (e.g., 
Leitherer et al. 1996; Massey \&
Hunter 1998; Gonz\'alez Delgado et al. 2002). 
Therefore the alternative explanation,
an aged starburst seems more plausible. Under this assumption, stars of 
masses 50~--~100~\Ms\ are initially formed in most galaxies, but the
starbursts are observed at an epoch when these stars are no longer present.
This implies that the inferred burst durations must be less than a few Myr 
(Fig.~1). Such short burst time scales are surprising,
in particular for luminous, starburst galaxies whose the dynamical time scales 
can exceed tens of Myr. However, supernovae and stellar winds can
 induce large negative 
feedback. Starburst models (Leitherer et al. 1999; Thornley et al. 2000) 
as well as observations of galactic superwinds (Heckman et al. 1990) 
indicate that 1\% to 2\% of the bolometric luminosity of starbursts 
is converted into mechanical energy in superwinds. This mechanical 
energy input could remove interstellar gas from 
the star-formation site on a time scale much shorter than the gas 
consumption time scale, which can be hundreds of Myr.

Both the peculiar IMF or the short starburst time scales in dusty,
IR-bright starbursts are quite unexpected and pose a challenge to 
conventional models in which the starburst is fed by gas inflow to
the nucleus over tens of Myr as a result of angular momentum loss. 
Clearly, additional
observational verification is desired since this interpretation rests on 
the assumption that the soft radiation field results from a 
deficit of massive stars. A case in point is the UV+optical
study of NGC~3049 by Gonz\'alez Delgado et al. (2002). This metal-rich
starburst galaxy exhibits an emission-line spectrum of relatively
low excitation, whereas the UV stellar-wind lines clearly prove the
presence of massive, ionizing stars following a normal Salpeter-type
IMF. Indirect star-formation tracers, like nebular lines, still
require careful calibration, in particular when applied to dusty,
metal-rich starbursts.




\section{The heating mechanism of the interstellar medium}

The stellar ionizing thermal luminosity
decreases with time since the overall evolutionary trend is
towards lower stellar temperature. Therefore the relative
proportion of the non-thermal (i.e., energy input from
winds and supernovae) and thermal luminosities 
increases with time, with the two quantities being comparable
in an instantaneous starburst of age 10~Myr. After that 
epoch, the powering of the
interstellar environment becomes almost totally determined 
by the non-thermal energy input.  
Thus,  the emission-line characteristics of the 
ionized gas change from a  photoionized 
 to a shock-ionized spectrum.

The high-ionization species of [S~IV] 10.5$\mu$m and [O~IV] 25.9$\mu$m
are of special interest for constraining the presence of very hot
stars (\Teff~$\approx 50000$~K) and shock-ionized gas in the star forming
regions.  The [O~IV] line is a powerful diagnostic because
it is collisionally de-excited in stellar winds and hence serves as
a purely nebular hardness indicator for photons with energies above 55~eV. 

In the sample of [O~IV] emitting starbursts of Lutz et al. (1998), II~Zw~40 
and NGC~5253 show the strongest excitation (measured by [Ne~III]/[Ne~II]) and
the largest [O~IV] strength,
 and stand out in their nebular He~II 4686 emission, indicative of high excitation. 
A significant number of Wolf-Rayet (WR) stars has been detected in these galaxies 
(Vacca \& Conti 1992; Schaerer et al. 1999). The optical
detection of photons above 54~eV and their spatial association with the mid-IR
[O~IV] line suggests a direct link between the nebular He~II and [O~IV] emission.
The optical He~II 4686 has been used before to model the WR content in
starburst galaxies  (e.g., Schaerer \& Vacca 1998). Its usefulness is compromised
by dust obscuration effects and by its dual formation mechanism: as a resonance
line, it can form in the ISM
 and in the winds of the ionizing stars
themselves. Although the latter contribution has a much broader profile, typical
spectra often have insufficient quality to separate the two components. The
mid-IR [O~IV] line traces the same energy regime as the He~II line ($>54$~eV)
but does not form in winds and is free of dust obscuration effects.

Previous photoionization models indicate
a short phase in the evolution of a stellar population with sufficient
photons above 54.4~eV to naturally produce the He~II, [O~IV], or [S~IV] lines 
at levels comparable to the observations. The emission is due to the presence of 
hot WR stars at ages  3~--~4~Myr (Schaerer \& Stasi\'nska 1999). However, these
older models rely on the first generation of expanding WR atmospheres calculated
by Schmutz et al. (1992), which are suspected to produce overly hard 
radiation fields (Bresolin \& Kennicutt 2002). Models with the new generation
of blanketed atmospheres discussed by Smith et al. (2002) may lead to
some revision of these results.
Nevertheless, it is commonly accepted that hot stars in unevolved
starbursts are at least partially responsible for the [O~IV] and related lines.


Alternatively, the observed relation between [O~IV] and 
[Ne~III]/[Ne~II] is well reproduced by models accounting consistently for 
both shock and photoionization effects occurring in starburst regions. The
shock component becomes increasingly more important in older starbursts, such
as M82 (Viegas et al. 1999), as expected from the higher non-thermal energy
input from supernovae. The critical parameters which reproduce the observed 
trend of the mid-IR line ratios are the pre-shock density and the shock velocity. 
Most of the shocks are produced in low-density and low-velocity clouds,
which represent the bulk of the ionized gas in 
starburst galaxies.  
The difference between young (H~II galaxies) and older starbursts (M82-like)
can be interpreted in terms of an evolutionary sequence. In the former, 
the starburst is too young to release large amounts of mechanical energy 
through supernova explosions whereas in the latter the kinetic energy competes
with the ionizing radiation in the heating of the ISM.


\section{The starburst-AGN connection}

 The central accretion rate in the 
prototypical Seyfert2 galaxy NGC~1068 is about 0.1~\Myr\ (Veilleux 2001),
close to the star-formation rate in M82. Therefore it is
natural to suggest a  common feeding mechanism for both
a starburst and an AGN, i.e., infall of gas to the galaxy center.
If so, starbursts and AGN should coexist and exhibit a close
connection.

Hot stars are a significant component of
the UV light in many bright Type~2 AGN.
The stars in these AGN are
typically obscured and reddened by dust, with attenuation factors of
5~--~10 at 1500~\AA. The implied UV luminosities of the starbursts
range from $10^8 - 10^9$~\Ls\ in low-luminosity AGN
 and $10^{10} - 10^{11}$~\Ls\
in Seyfert galaxies. Massive stars are clearly energetically significant in
 AGN. However, stars have been {\em directly} detected in relatively few 
cases, and these are biased in favor of galaxies with high
UV surface brightness. Cid Fernandes et al. (2001), applying
detailed semi-empirical population synthesis models, find about
40\% of Seyfert2 galaxies to have composite starburst+AGN spectra. 

Analysis of stellar features is preferable over nebular diagnostics, which
can often be degenerate. Nevertheless, since the nebular emission lines
are by far the strongest features in an AGN spectrum, numerous efforts
have been made to utilize them as a tracer of the starburst population.
The fundamental challenge is to disentangle between extreme-UV radiation
field of stars vs. the power-law spectrum emitted by the gas accreting
onto a black hole. Often, both types of ionizing spectra can produce very
similar optical and IR emission lines,
and a carefully fine-tuned starburst spectrum can often mimic the
same nebular emission-line spectrum as does a power law spectrum from an
AGN. 

An important result of the ISO mission was
a set of mid-IR diagnostics which can break the degeneracy. 
The differences between
the spectra of starbursts and AGN in the mid-IR are striking. 
Starbursts have strong
low-excitation fine-structure lines, few and/or weak high-excitation
lines, little dust emission below 10~$\mu$m, and very strong 
polycyclic aromatic hydrocarbon (PAH) bands. In contrast, Seyfert2
galaxies have fainter emission lines, but with much higher excitation
(e.g., the much stronger [O~IV] 25.9~$\mu$m line). The dust continuum is
much stronger in the AGN-type spectra as well. These trends
hold for even more powerful AGN up to the highest observed 
luminosities (Genzel \& Cesarsky 2000).

Diagnostic diagrams are powerful tools for an empirical characterization
of the excitation state of a source. Genzel et al. (1998) introduced the
[O~IV] 25.9~$\mu$m to [Ne~II] 12.8~$\mu$m line flux ratio versus the
strength of the PAH bands to discriminate between starbursts and AGN.
This ISO-based diagnostic diagram
clearly separates known star forming galaxies from AGN. The diagram is
insensitive to dust extinction if the emission lines and the bands are
affected by a similar amount of extinction. [O~IV], although weak, is
often detected in starburst galaxies. As discussed before,  
its origin is due to a combination of fast, ionizing shocks powered 
by supernovae and stellar winds and of the hard UV radiation from
hot Wolf-Rayet
stars.

The  mid-IR diagnostic diagram  is most powerful when
applied to dusty, IR-luminous galaxies, where the nature of the underlying
source is not known a priori. Ultraluminous IR galaxies (ULIRGs)
are of particular
interest, as the starburst versus AGN debate is still ongoing in these
objects. When studied with this method, starbursts dominate up to
luminosities of about $2 \times 10^{12}$~\Ls, above which AGN become
energetically more and more important. The most luminous starbursts are
detected in galaxies with luminosities close to $\sim$10$^{13}$~\Ls. This
limit corresponds to the maximum star formation rate of $\sim$10$^3$~\Myr\
of a gas-rich spheroid undergoing a monolithic collapse on a dynamical
time scale.

There is now ubiquitous evidence for starbursts in IR-luminous
AGN over several
decades in luminosity. While the precise evolutionary scheme is still
unknown, it seems likely that the gas supply feeding the central AGN
is capable of sustaining the circumnuclear starburst as well.





\section{The energy budget of dusty starbursts}

Energy conservation dictates that
the attenuated light in the UV must be re-emitted in a different waveband. 
The dust
grains scattering and absorbing the stellar light have typical sizes of
about 0.1~$\mu$m and equilibrium temperatures of tens of K.
The corresponding emission peaks in the far-IR around 60~$\mu$m.
Therefore, the net
effect of dust obscuration is a redistribution of the
stellar UV/optical flux to the far-IR. In the local universe,
about 80\% of the stellar UV photons
are hidden from direct view and can only be recovered as reprocessed light
in the far-IR (Calzetti 2001).
The starburst energy distribution always peaks around 60~$\mu$m, irrespective 
of the amount of reddening.  This property has important
consequences for the prospects of detecting starburst galaxies at high
redshift(see below).

\begin{figure}
\centerline{\psfig{figure=IAU212_UVIR.eps,height=2.5in,angle=90}}%{IAU212_UVIR.eps}{2.5in}{90}{42}{42}{150}{-30}
\caption{ Comparison of  
$L_{\rm{abs}}$, the absorbed UV luminosity between 912 and 3650~\AA, and the IRAS far-IR luminosity
for a sample of star-forming galaxies.
Galaxy identifiers: 
1 -- NGC~1097,
2 -- NGC~1365,
3 -- NGC~2403,
4 -- IRAS~08339+6517,
5 -- NGC~2903,
6 -- NGC~3310,
7 -- NGC~3351,
8 -- NGC~4214,
9 -- NGC~4449,
10 -- NGC~5055,
11 -- Mrk~66,
12 -- NGC~5194,
13 -- NGC~5236,
14 -- NGC~5253,
15 -- NGC~6090. From Leitherer et al. (2002).}
\end{figure}



If dust acts like a foreground screen, we expect a simple scaling between
the far-IR and UV luminosities. First,
 a well-defined relation exists between the 
monochromatic luminosity at 1500~\AA\ and the total UV luminosity
of a young starburst:
\begin{equation}
\log L_{\rm{UV}} = \log L({\rm 1500}) + 3.2 \pm 0.05,
\end{equation}
where $L_{\rm{UV}}$ is in erg~s$^{-1}$ and $L({\rm 1500})$ in 
erg~s$^{-1}$~\AA$^{-1}$. Eq.~(1) simply tells us that the total
UV luminosity in a starburst can be obtained from $L({\rm 1500})$
after multiplication by 1600 (Leitherer et al. 2002).
This observational relation
agrees with synthetic models and has its roots in the age independence of
the spectrum. $L_{\rm{UV}}$ in turn
is tightly related to the total bolometric luminosity. Starburst99 models predict
\begin{equation}
\label{bolUV}
\log L_{\rm{Bol}} = \log L_{\rm{UV}} + 0.1,
\end{equation}
where both quantities are  in    erg~s$^{-1}$.
The UV luminosity accounts for about 75\% of the total luminosity, with a negligible 
dispersion. The remaining 25\% are mostly in the Lyman continuum, with a small 
contribution from the Paschen continuum. These relations and their small dispersions 
are the physical basis for the usefulness of the 1500~\AA\ luminosity to derive 
star-formation rates from UV continuum fluxes. 

An observational verification of this reasoning is in Fig.~2,
which compares $L_{\rm{abs}}$ and $L_{\rm{IR}}$ for sample of galaxies
observed with HUT and IRAS. $L_{\rm{abs}}$ is absorbed UV luminosity,
derived observationally after applying the attenuation correction.
If eqs.~(1) and (2) hold,  $L_{\rm{abs}}$ should equal $L_{\rm{IR}}$
since $L_{\rm{abs}} \approx L_{\rm{Bol}}$. 
The figure suggests that overall this is the case.  The 
agreement holds over almost three orders of magnitude from 
low-luminosity galaxies like NGC~4214 to the luminous IR galaxy 
NGC~6090. The relation between the absorbed 
and re-emitted flux in the UV and IR, respectively,
is generally well defined in UV-selected galaxies (Meurer et al. 1999). 
On the other hand, it is known to break down
in ULIRGs (Goldader et al. 2002). Using the 
absorbed UV flux would underpredict
the far-IR emission by factors of 10~--~100. Apparently, a simple 
clumpy foreground screen model for the
dust is inappropriate for starburst galaxies with bolometric 
luminosities in excess of
$10^{12}$~\Ls. 

 
\begin{figure}[t]
\centerline{\psfig{figure=IAU212_VV114.eps,height=4.05in}}
%\plotfiddle{IAU212_VV114.eps}{4.05in}{0}{22}{22}{-135}{-0}
\caption{Inner 3~kpc of VV~114~W imaged with HST's STIS (upper)
and NICMOS (lower). The maximum brightness in the IR is marked with
a cross. From Goldader et al. (2002).}
\end{figure}


One possible interpretation is that 
the star-forming regions of ULIRGs are so obscured that 
we detect no UV light from them at all. In this case, 
the UV light we see would come from relatively lightly reddened 
star-forming regions and stars outside the most obscured 
regions, giving blue colors. High-resolution UV and IR
imaging with HST of a sample luminous and ultraluminous IR
galaxies by Goldader et al. (2002) supports this suggestion.
One of the objects, VV~114 ($D=90$~Mpc) was discovered 
as  a luminous far-IR source in the IRAS Bright Galaxy Survey. 
VV~114 is pair of galaxies with a remarkable contrast from the 
UV to the near-IR.  The western galaxy VV~114~W has a large 
number of luminous star-forming regions visible in Fig.~3.
The brightest UV sources are clearly displaced from the 
near-IR peaks by at least hundreds of parsecs. Furthermore,
only a few percent of the total near-UV light is projected within the 
inner 500~pc radius, even though this is the region where most 
of the bolometric energy is generated.


VV~114 highlights the challenge of measuring the rate of star formation
at high redshift where morphological information on galaxies is
limited and where the stellar content of the most luminous galaxies
is often inaccessible to spectroscopic observations. The best studied group
of galaxies at cosmological distance is defined by
the absence of emission in the U band
as a result of the Lyman break at 912~\AA\ (Steidel et al. 1998). They are
usually referred to as Lyman break galaxies. Their 
colors are 
observed to be redder than expected for dust-free star-forming objects. If
due to dust obscuration, the previous line of arguments can be used to
predict their far-IR emission, which is redshifted into the sub-mm and
becomes observable from the ground. A population of high-redshift 
galaxies has indeed been detected at sub-millimeters 
(e.g., Chapman et al. 2001) but
it is presently unknown whether these 
galaxies are drawn from a population similar to the Lyman break 
galaxies, or whether 
the two methods select entirely different types of object at similar redshifts.
Correlating the results from surveys at such widely
spaced wavelengths is frustratingly uncertain. If cases like VV~114 are
{\em not} representative for those galaxies dominating the star formation in the
universe, the 
majority of high-redshift star formation occurs in objects which can at 
least be detected in existing UV-selected surveys, such as Lyman break
galaxies (Adelberger \& Steidel 2000). 
Sub-mm galaxies would then be the dust-rich tail of the star-forming
galaxy population.


\acknowledgments
Scott Chapman provided access to a Keck spectrum of a SCUBA source prior
to publication. Torsten B\"oker's comments helped improve an earlier version
of the manuscript.

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