Max Planck Institute for Astronomy - Annual Report 2005

42 II. Highlights
II.7 Observations of Distant Galaxies with Spitzer
Recent observations have convincingly established that
over the last eight billion years, the mean star formation
rate in the universe has declined by almost one order of
magnitude. Both the cause of this phenomenon and which
galaxy types are responsible for it are largely unclear. An
international team of astronomers under the leadership
of the MPIA investigated these questions by identifying
almost 8000 galaxies on an infrared image taken with
the Spitzer Space Telescope and combining their infrared
fluxes with data from the CoMbo-17 and other surveys.
Studies of local galaxies had already shown the infrared
luminosities of galaxies to be a very good measure of the
star formation rate. Using a special analysis technique,
the scientists at the MPIA have now measured the mean
infrared luminosities of galaxies (even of faint dwarf
galaxies that would normally be undetectable) in great
detail. This enabled them to determine the most accurate
estimate to date of the evolution of the global star formation
rate over the last eight billion years. These new
results show that this rate has decreased by a factor of
nine. Furthermore, the new Spitzer observations showed
that this is mainly caused by the decline of luminous and
ultra luminous infrared galaxies.
The massive stars in a young stellar population dominate
the UV luminosity of a galaxy and often even its
overall energy output. Large quantities of dust present
in star-forming regions, however, absorb most of the
UV emission. As a result, the dust heated by young stars
radiates in the thermal infrared range. All observational
estimates of the star formation rate therefore are based
Fig. II.7.1: The mean thermal–IR emission for classes of galaxies
in four different redshift bins, 0.1 � z � 0.8. As hardly any of
these galaxies are detected individually, the MipS images for all
–17 � M B � –16
0.1 � z � 0.2
f stack = 6.07 �Jy
� bg = 0.87 �Jy
–18.7 � M B � –17.7
0.2 � z � 0.3
f stack = 8.64 �Jy
� bg = 1.1 �Jy
either on the measurement of the UV or the infrared luminosity.
A complete census of the light from young stars
in distant galaxies actually should include both UV and
infrared fluxes.
Until recently, IR telescopes lacked the sensitivity and
resolution needed for detailed observations of distant
galaxies, particularly in the low-mass range. Now the
Spitzer Space Telescope allows such observations in the
wavelength range between 3.6 and 160 µm.
An international team under the leadership of the
MPIA used the image of a field of 90� � 30� taken with
the »Multiband Imaging Photometer on Spitzer« (MipS) at
a wavelength of 24 µm. It is considered a mosaic image,
since the field of view of MipS is only 5�.4 � 5�.4. The MipS
field covers the area of the Chandra Deep Field South Xray
image and also coincides with the CoMbo-17 survey
(Classifying Objects by Medium-Band Observations with
17 Filters) of MPIA and the geMS image from the hubble
Space Telescope.
Within CoMbo-17, a large area of the sky was imaged
through 17 filters and the magnitudes of galaxies were
measured in the corresponding color bands. Up to a
redshift of z�1 (about half the age of the Universe), this
allows us to classify galaxies and determine their redshifts
with an accuracy of a few percent. The decisive prerequisite
for the project was the large field of view of the Wide
Field Imager camera (WFI) developed under the leadership
of the MPIA and built in collaboration (ed) with eSo.
It is operated at the 2.2-m MPG/eSo telescope on la Silla,
Chile and has a field of view of 32� � 33�, corresponding
approximately to the area of the full moon.
galaxies of a given B-band in a redshift bin were »stacked«,
providing a clear statistical detection of the dust heated by
young stars.
–20 � M B � –19
0.6 � z � 0.7
f stack = 6.48 �Jy
� bg = 0.44 �Jy
–20 � M B � –19
0.7 � z � 0.8
f stack = 6.68 �Jy
� bg = 0.56 �Jy