Annual Report 2011 Max Planck Institute for Astronomy

36 II. Highlights
Credit: A. van der Wel
I F814W – J F125W [mag]
1.5
1
0.5
0
–0.5
E(B–V) = 0,3
continuum
emission line
–1.5
Even when adopting the starburst hypothesis, it is
immediately clear that these objects are all but normal.
The only way to get such large line-to-continuum
Flux ratios is to have a stellar population with an age
of about 10 Myr. Usually, galaxy ages are measured in
Gyrs, not Myrs. Given this age constraint, the luminosities
imply stellar masses of around 10 8 M . Although it
is plausible that an older population of stars exists we
can rule out that these starbursts occur in much large,
say, Milky Way type, galaxies. Rather, these must be
truly low-mass galaxies, such that this population constitutes
the first systematic detection of dwarf galaxies
at high redshift.
Cosmological Context
–1 –0.5
JF125W – HF160W [mag]
100 Myr
continuum 1 Myr
Let us now consider the broader implications of the
presence of 69 starbursting dwarf galaxies at z 1.7
in CaNdelS. To be somewhat more precise, assuming
a redshift range of z 1.6 – 1.8, their volume density
is 4 10 –4 Mpc –3 . This makes them about 100 times
rarer than present-day dwarf galaxies and they constitute
only 0.1 % of the 10 Myr young stellar populations
at that redshift if we consider galaxies of all
masses.
While this may sound unimpressive it does not
make these starbursts cosmologically irrelevant once
we realize that they are very short-duration events: for
every objects we see, there must be numerous similarly
massive objects that are not undergoing such an intense
burst of star formation at the time of observation but
have had or will have one at some other time.
Let us formalize this argument and make two reasonable
assumptions. The first assumption is that the
observed bursts do not only occur at at z 1.7 but at
0
Fig. II.4.2: I – J vs. J–H color-color diagram of objects in
CaNdelS, highlighting with large red symbols with error bars
those objects selected as extreme emission line galaxy candidates.
They mainly differ from the main branch of galaxy
colors by their blue J–H colors. The blue line indicates the
colors of the continuum radiation produced by a young population
of stars (the ages are labeled). The red line includes the
effect of a strong emission line on the colors. The selected
objects have colors indicative of strong emission line contributions
in the J filter.
all redshifts z 1 – 4, a period of roughly 4 Gyr. We
know that similarly strong bursts are exceedingly rare
at z 1 but there is no reason to think that they do not
occur over a much broader redshift range. With upcoming
spectroscopic data from HST this assumption can be
tested easily.
The second assumption is that between z 1.7 and
the present day these galaxies will not grow by an order
of magnitude or more. That is, we will rely on the
prediction of ΛCDM-based galaxy formation models
that galaxies typically grow by a factor of several over
that time span. That is, the descendants of the observed
bursting galaxies at z 1.7 are dwarf galaxies with
masses 10 9 M . This assumption is more difficult to test,
and all we can do now is rely on this model prediction.
By integrating the observed burst frequency at
z 1.7 over 4 Gyr (z 1 – 4) we can derive the total
number of bursts per unit cosmic volume as well as the
total number of stars formed in this manner. By comparing
the total number of bursts with the total number of
dwarf galaxies in the present-day universe, it then follows
that each present-day dwarf galaxy must have undergone
two or three of such strong bursts over its life
time, and, moreover, that the majority of the stars in
present-day dwarf galaxies formed in these bursts. In
other words, we propose that the star formation history
of dwarf galaxies has been strongly burst-like: long periods
of relative inactivity are interspersed with a small
number of very strong bursts, mostly occurring at early
times (z 1).
Unsolved Riddles and Future Directions
The observations and our interpretation present a host
of new questions, as well as speculation on the effect of
such strong bursts on the galaxies and the dark matter
halos that host them. First, hydrodynamical simulations
cannot reproduce such strong bursts in such low-mass
galaxies. It appears that something fundamental is lacking
in our description of galaxy formation. Their mere
existence implies the presence of large reservoirs of gas,
and it is entirely unclear how to prevent this gas reservoir
from forming stars before the burst commences, or,
alternatively, how to assemble a large amount of gas on
a very short time scale.