139
THE INFRARED SPECTRA OF QUASARS AS SEEN BY ISO: PROSPECTS FOR FIRST
Martin Haas1, Ulrich Klaas1, Klaus Meisenheimer1, Dietrich Lemke1, Sven Müller2, and Rolf Chini2
1
Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany, E-mail: [e-mail]
2
Astronomisches Institut, Ruhr-Universität Bochum, 44780 Bochum, Germany
Abstract The SEDs of the PG-quasars are shown in Fig. 2. The
new observations increase the fraction of optically bright
ISOPHOT observations supplemented by mm photom-
quasars detected in the mid- and far-IR from about 30% to
etry with IRAM gained new insights on dust and syn-
70% (12 out of 17), providing evidence that most  if not
chrotron emission in quasars and radio galaxies. They con-
all  quasars are surrounded by considerable amounts of
firmed former hypotheses, but also raised new suggestions
dust similar to normal spiral galaxies. For non-PG-quasars
as well: While evidence is growing for the dustiness of
the dust content may be even higher, since the Palomar-
quasars, the AGN-starburst connection and the unifica-
Green sample is biased against bright host galaxies be-
ton between radio galaxies and steep stectrum radio-loud
cause of the restriction to stellar appearance on images.
quasars, new trends like the redshift dependence of the
Nevertheless, if the evolutionary chain merger of two dust
mid-to-far-IR (AGN-to-starburst) luminosity contribution
rich galaxies  quasar   naked elliptical galaxy is correct,
and the IR-to-radio power ratio are puzzling. Larger data
then the dust has to disappear at some stage between the
bases from the ISO archive and from the upcoming IR
quasar and the elliptical galaxy period. Here FIRST could
satellites SIRTF and ASCA during the next years will
reveal the  dust-disappearing-stage .
support the findings from the first samples with ISO. How-
Also, of cosmological interest is the possible trend that
ever, for most of the new suggestions  in particular those
the coldest dust component appears warmer in quasars at
related to cosmic evolution  a clarification will have to
high redshift. This issue is related to the SED maximum
wait until the next generation of larger IR telescopes like
in the FIR and the shape of the Rayleigh Jeans tail in
FIRST.
the sub-mm range, therefore it can ideally be investigated
with FIRST.
Key words: Galaxies: photometry  Quasars: general 
From the shape of the SEDs the hypothesis can be
Infrared: galaxies
confirmed, that the AGN powers mainly the MIR emission
and that bursts of starformation are required in order to
explain the FIR and sub-mm output.
For the study of the AGN-starburst connection both
the MIR and the FIR ranges are necessary. Therefore
1. Introduction
FIRST should extend its wavelength coverage as far as
ISOPHOT (Lemke et al. 1996) the photometer on board
possible into the MIR range, otherwise a direct study of
ISO provided mid- and far-IR observations of unprece-
the AGN-starburst connection via the SEDs would be re-
dented sensitivity. These data were supplemented by mm
stricted to high redshifts.
photometry with the IRAM 30-m-telescope. Within the
As a new alternative, at low redshifts the steepness of
ISO European Central Quasar Programme various samples
the Rayleigh Jeans tail (measured by the dust emissivity
of quasars and related objects were observed. The main
exponent ²) could provide clues to discriminate between
emphasis was on clarifying the nature of the IR emission
mergers in an early status and more evolved AGN: As a
 dust versus synchrotron  and the unified schemes (see
trend from ISOPHOT observations, ULIRGs seem to have
Fig. 1). More details can be found in Haas et al. (1998),
a lower ² than quasars (Klaas et al. 2001). Furthermore,
Chini et al. (1999), Haas et al. (2000) and Meisenheimer et
the PAH 7.7µm strength seems to be better correlated
al. (2001). Here we summarize the results for two samples
with the sub-mm than with the FIR emission (Haas et al.
with regards to the prospects for FIRST.
2001). Thus, the shape of the Rayleigh Jeans tail could
reveal essential AGN-starburst informations  a challenge
for FIRST.
2. PG quasars
17 optically bright quasars randomly selected from the
3. 3CR radio galaxies and quasars
Palomar-Green (PG) catalog were observed in order to
probe the existence and properties of dust in these sources 20 radio galaxies and radio-loud quasars from the 3CR
and the link to ultraluminous IR galaxies (ULIRGs): catalog with powerful FR II lobes were observed in order
Proc. Symposium  The Promise of the Herschel Space Observatory 12 15 December 2000, Toledo, Spain
ESA SP-460, July 2001, eds. G.L. Pilbratt, J. Cernicharo, A.M. Heras, T. Prusti, & R. Harris
140
Figure 1. Illustration of the pure aspect-angle unified schemes between ULIRGs, radio-quiet (PG) quasars, flat and steep spectrum
radio-loud quasars and radio galaxies. The dashed arrows indicate the directions (pole-on, intermediate and edge-on aspect-angles)
from which the observer looks at the system, resulting in SEDs with different relative dust and synchrotron contributions.
to discriminate between dust emission and synchrotron (Meisenheimer et al. 2001). FIRST s longwavelengths cov-
radiation, and to check the unification of radio galaxies erage and sensitivity  also for the faint high redshift
and quasars. sources  could confirm these trends of cosmic evolution,
and find clues to its reason.
The SEDs of the 3CR radio galaxies and quasars are
shown in Fig. 3. For the first time a sufficient number of ra-
Acknowledgements
dio galaxies (6 out of 10) and radio-loud quasars (7 out of
The development and operation of ISOPHOT were supported
10) could be detected individually in the mid- and/or far-
by MPIA and funds from Deutsches Zentrum für Luft- und
IR to allow for a statistically meaningful trend. 12 sources
Raumfahrt e.V. (DLR, formerly DARA). The ISOPHOT Data
exhibit a clear thermal bump rising above the smooth Centre at MPIA is supported by DLR with funds of Bun-
synchrotron spectrum, and in the remaining sources the desministerium für Bildung und Forschung, grant 50 QI 9801 3.
The authors are responsible for the contents of this publication.
upper limits are still compatible with the presence of lu-
minous dust emission. Considering pairs of quasars and
galaxies which match in redshift and isotropic 178 MHz
References
radio power of the FR II lobes, both samples show the
Haas M., Chini R., Meisenheimer K., et al. 1998, ApJ Letters
same mid- and far-IR detection statistics  in agreement
503, L109
with the predictions of the unified scheme.
Haas M., Müller S.A.H., Chini R., et al. 2000, A&A 354, 453
In addition to the purely geometric unification, also a
Haas M., Klaas U., Müller S.A.H., et al. 2001, A&A in press
trend with cosmic evolution can be seen: While at high
Chini R., Haas M., Klaas U., et al. 1999, in ISO Surveys of a
redshift (z > 0.8) both the radio galaxies and the quasars
Dusty Universe eds. Lemke D., Stickel M., Wilke K.,
show a similar IR-to-radio 178 MHz luminosity ratio in
Lecture Notes in Physics 548, Springer-Verlag p. 169
accordance with pure unification, at low redshift (z<0.8)
Klaas U., Haas M., Müller S.A.H., et al. 2001, A&A submitted
some of the radio galaxies exhibit relatively fainter IR
Lemke D., Klaas U., Abolins J., et al. 1996, A&A 315, L64
fluxes than the quasars. This difference seems to be mainly
Meisenheimer K., Haas M., Müller S.A.H., et al. 2001, A&A in
a function of redshift and not one of lobe radio power press
141
Frequency ½ [Hz] Frequency ½ [Hz] Frequency ½ [Hz]
1014 1013 1012 1011 1010 109 1014 1013 1012 1011 1010 109 1014 1013 1012 1011 1010 109
PG_0003+158 radio loud PG_1206+459 PG_1307+085
101
z=0.450 z=1.158 z=0.155
100
10-1
67K
10-2
23K 46K
10-3
10-4
PG_0026+129 PG_1241+176 radio loud PG_1411+442
101
z=0.142 z=1.273 z=0.090
100
10-1
10-2
107K 36K
10-3
10-4
PG_0043+039 PG_1247+267 PG_1613+658
101
z=0.385 z=2.038 z=0.129
100
10-1
10-2
122K 26K
10-3
10-4
PG_0044+030 radio loud PG_1254+047 PG_1634+706
101
z=0.623 z=1.024 z=1.334
100
10-1
10-2
38K 77K 47K
10-3
10-4
PG_0050+124 PG_1259+593 PG_2112+059
101
z=0.061 z=0.478 z=0.466
100
10-1
10-2
27K 20K
10-3
10-4
100 101 102 103 104 105
PG_0052+251 PG_1302-102 radio loud
101
z=0.155 z=0.278 Wavelength  [µm]
1991 1986
1997
100
10-1
1993
1998
ISO IRAS
10-2
27K 45K IRAM other
10-3
10-4 3Ã upper limit
100 101 102 103 104 105 100 101 102 103 104 105
Wavelength  [µm] Wavelength  [µm]
Figure 2. SEDs of 17 PG quasars from the ISO European Central Quasar Programme. The wavelength and frequency ranges are
as observed and not corrected to the rest frame of the objects. Several modified blackbodies with emissivity proportional to -2 are
eyeball fitted to the data (solid lines) and only the coolest components are plotted individually (dotted lines). The temperatures
listed are corrected for redshift. The long dashed lines indicate extrapolated synchrotron spectra. For the variable radio source
PG 1302 102 different epochs of the data groups are indicated by the year. Reference: Haas et al. (2000).
½
F [Jy]
½
F [Jy]
½
F [Jy]
½
F [Jy]
½
F [Jy]
½
F [Jy]
142
Frequency ½ [Hz] Frequency ½ [Hz] Frequency ½ [Hz] Frequency ½ [Hz]
1014 1013 1012 1011 1010 109 1014 1013 1012 1011 1010 109 1014 1013 1012 1011 1010 109 1014 1013 1012 1011 1010 109
102 102
100 100
10-2 10-2
gal QSO gal QSO
53K 34K 72K
3C405 3C48 3C280 3C286
10-4 10-4
z=0.056 z=0.367 z=0.996 z=0.849
102 102
100 100
10-2 10-2
gal QSO gal QSO
31K 31K 62K 65K
3C20 3C273 3C368 3C309.1
10-4 10-4
z=0.174 z=0.158 z=1.131 z=0.905
102 102
100 100
10-2 10-2
gal QSO gal QSO
54K 32K
3C295 3C47 3C324 3C287
10-4 10-4
z=0.461 z=0.425 z=1.206 z=1.055
102 102
100 100
10-2 10-2
gal QSO gal QSO
81K 44K
3C427.1 3C380 3C65 3C298
10-4 10-4
z=0.572 z=0.692 z=1.176 z=1.436
102 102
100 100
10-2 10-2
gal QSO gal QSO
73K
3C325.0 3C138 3C437 3C9
10-4 10-4
z=0.860 z=0.759 z=1.480 z=2.012
100 101 102 103 104 105 106 100 101 102 103 104 105 106 100 101 102 103 104 105 106 100 101 102 103 104 105 106
Wavelength  [µm] Wavelength  [µm] Wavelength  [µm] Wavelength  [µm]
Figure 3. Observed spectral energy distributions of the pairs of 3C radio galaxies and quasars ordered according to the redshift
of the galaxy. Symbols abd lines as in Fig. 2, the encircled stars represent the total flux at 178 MHz from the 3CR catalogue. In
cases of non detection of one member of a pair, the dashed dotted line illustrates, how the blackbody of the partner would show
up at corresponding redshift and that it would not exceed the upper limits. Reference: Meisenheimer et al. (2001).
½
½
F [Jy]
F [Jy]
½
½
F [Jy]
F [Jy]
½
½
F [Jy]
F [Jy]
½
½
F [Jy]
F [Jy]
½
½
F [Jy]
F [Jy]