Max Planck Institute for Astronomy - Annual Report 2005

The CoMbo-17 colors are used to identify stars, quasars,
and different types of galaxies such as elliptical,
spiral and starburst galaxieswith specially developed
software. A total of roughly 25000 galaxies are recorded,
making CoMbo-17 one of the most extensive and deepest
surveys in the world.
Even with the improved sensitivity of Spitzer (IR
flux larger than 83 µJy at 24 µm), only galaxies with
unusually high star formation rates can be measured
as individual objects in the thermal infrared range. At
a redshift of z � 0.7, the lower limit is about six solar
masses per year – a few times larger than the SFR in the
Milky Way. However, the combination of the MipS data
with the redshifts and positions from CoMbo-17 was used
to significantly improve the detection threshold with the
help of »stacking«. This method works according to the
following principle: based on the CoMbo-17 data, a set
of one hundred galaxies is defined (for example blue
galaxies in the redshift range of z � 0.7 that are as bright
as the Milky Way system). Then the MipS frames are
centered on the coordinates of these galaxies and added
up. Most of the time, even when none of the objects are
visible individually, the stacking yields a significant measurement
of the mean infrared flux. This technique was
previously used for X-ray and submillimeter images, but
it is novel for infrared images.
In this way, it was possible to reduce the lower limit
for the measured infrared fluxes by a factor of five (for
0.7�z�1.0) and ten (z �0.7), respectively, below the
previous limit for the MipS images and to determine star
formation rates less than one solar mass per year. Finally,
the mean redshifts and mean magnitudes at 24 µm and
2800 Angström as well as in the Johnson-U-, V-, and Bbands
were available for various sub-classes for a total of
almost 8000 galaxies.
This sample was divided into nine groups with
redshifts between z � 0.1 and z � 1, corresponding to a
maximum look-back time of almost eight billion years.
Fig. II.7.1 shows examples of four sub-classes of galaxies
with their infrared fluxes in four redshift ranges.
Notice the low infrared fluxes lying far below the previous
threshold of 83 µJy.
The Relationship Between IR Luminosity and Star
Formation Rate
In nearby galaxies that can be studied in detail, a relationship
was found between the ratio of the IR and UV
luminosities (L IR /L UV ) and the absolute blue-band magnitude
(B). Galaxies with a low B magnitude also show a
low L IR /L UV ratio: the higher the star formation rate, the
higher the dust extinction, and the higher the fraction of
UV radiation converted into IR radiation.
With the data set available, this empirical relationship
can now be tested for more distant galaxies. In order to
do this, however, the overall infrared luminosity from 8
log L IR / L UV
1
0.5
0
–0.5
–1
1
0.5
0
–0.5
–1
1
0.5
0
–0.5
–1
II.7 Observations of Distant Galaxies with Spitzer 43
0.9 � z � 1.0
0.8 � z � 0.9
0.7 � z � 0.8
0.6 � z � 0.7
0.5 � z � 0.6
0.4 � z � 0.5
0.3 � z � 0.4
0.2 � z � 0.3
0.1 � z � 0.2
4 3 2 1 0 –1
MB [M*+] [mag]
Fig. II.7.2: Correlation between the L IR /L UV ratio (i.e. the absorbed
vs. escaping radiation from young stars) and the B magnitude
for various redshift ranges: the more luminous a galaxy is,
the higher the fraction of IR radiation. This holds at all redhifts
(bottom to top panel).
to 1000 µm had to be calculated from the 24-µm fluxes.
This was achieved by fitting typical spectral distributions
for various galaxy types to the measured spectral data.
This method yields rather imprecise infrared fluxes, but
within the uncertainties, the ratio of L IR /L UV to B seems