Annual Report 2011 Max Planck Institute for Astronomy

34 II. Highlights
II.4 Starbursting Dwarf Galaxies at High Redshift
Using the Hubble Space Telescope/Wide Field Camera
3 a team of astronomers led by MPIA scientists uncovered
a previously unknown population of dwarf galaxies
at z ~ 2 displaying extraordinary star formation activity.
They are doubling their stellar masses in 10 million years,
a pace that exceeds that of the Milky Way by three
orders of magnitude.
Until now, the formation history of dwarf galaxies
could only be studied through meticulously constructing
color-magnitude diagrams of individual stars in
nearby objects. Such case studies have been carried out
for the past two decades and it is by now clear that the
majority of stars living in present-day dwarf galaxies
are old. In essence, the star formation history of dwarf
galaxies does not appear to be markedly different from
that of the universe as a whole: all galaxies, including
dwarf galaxies, were forming stars at higher rates in the
more distant past.
Observing high-redshift galaxies is the commonly
adopted strategy to examine the galaxy formation process
in more detail and also in a more direct manner.
This has been done successfully for quite some time
now, but results have been limited almost exclusively
to massive, luminous galaxies. Dwarf galaxies are,
usually, too faint to detect at any interestingly high
redshift
CandelS: an eye opener
This was suddenly changed by the unexpected discovery
of an unusual yet apparently quite common type
of object in new deep observations with the HuBBle
Space Telescope (HST). These observations were part
of the largest HST survey ever undertaken: CaNdelS
(Fig.II.4.1). This international project aims at obtaining
optical and near- infrared images over ‘large’ parts
of the sky (about 700 square arcminutes) at exquisite
depth (27.5 AB mag). The combination of depth, area,
spatial resolution and wavelength coverage make
this a gold mine for examining the origin of galaxy
structure and morphology (the HuBBle sequence), the
defining properties of the first galaxies that formed during
(and presumably caused) reionization, and putting
stronger constraints on the dynamical state of the universe
through the discovery of Type 1a supernova at
even larger cosmological distances than before.
This dramatic expansion of discovery space has immediately
resulted in a number of interesting results, the
most unexpected revelation being the topic of this article.
Among the tens of thousands of high-redshift gal-
axies in the currently available images – the survey is
about 50 % complete at the time of writing, and will be
finished in 2013 – we noticed a number of H 25 AB
mag objects with very strange colors (Fig. II.4.1 and 2).
Whereas they are red in I – J ( 0.5 in AB mag, which
is typical for a distant galaxy) they are extremely blue
in J – H ( 0.5).
Extreme Emission Lines
Such extreme variations in the spectral energy distribution
cannot be explained by any known continuum radiation
processes, and we arrived at the conclusion that
enormously strong emission lines must contribute considerably
to the integrated light in the J band. The implication
is that the equivalent width of this line (or lines
combined) exceeds 1000 Å, which is unusual indeed.
Spectroscopic confirmation is, of course, essential to
justify such an extraordinary claim. Usually, 25 th magnitude
objects are far too faint to obtain spectra for,
but in the purported presence of bright emission lines
(flux 10 –16 erg s -1 cm -2 ) this should, in fact, be quite
straightforward. By chance, HST had already taken
spectra for four of the 69 ‘extreme emission line galaxy’
candidates, and all four candidates display bright
emission lines at 1.3 – 1.4 μm (Fig. II.4.3). Moreover,
the line fluxes are consistent with the excess light observed
in the J band: emission lines explain the odd
broad-band colors.
Interestingly, in all four cases the bright emission
lines could readily be identified as the [OIII] doublet
at restframe wavelengths λ 4969 Å and λ 5007 Å,
flanked by Hb at 4861 Å. This puts these objects in the
redshift range z 1.6 – 1.8, and from now on the working
hypothesis will be that all 69 candidates we identified
are in fact [OIII] emitters at z 1.7. However, we
should keep in mind that some of the candidates may be
Hα emitters at z 1, whereas other emission lines can
be ruled out based on additional broad-band photometry
at shorter wavelengths: none of the objects are so-called
U or B-band dropouts, meaning that the Lyman break
lies blueward of the U and B bands, limiting the redshift
to z 2. This rules out the other potentially very bright
Ly α and [OII] emission lines.
In short, we have made a very strong case that we
have identified a previously unknown population of
z 1 – 2 objects with extremely bright [OIII] (or Hα)
emission lines. The question naturally arises what could
cause emission lines with equivalent widths in excess of
500 Å in the rest frame. The two options are starbursts
and active nuclei, that is, accreting black holes.