Up: Infrared to millimetre photometry galaxies:
Infrared to millimetre spectral energy distributions (SEDs) have been obtained for 41 bright ultra-luminous
infrared galaxies (ULIRGs). These are the most up-to-date and detailed photometric templates of the nearby
ULIRGs, which are the fundamental ingredient for cosmological studies. Arp 220, which has been considered so
far as a the archetypal ULIRG, is one of the most peculiar objects in our sample.
The SED maxima lie between 60 and 100
m, and for those 22 sources with submm detections or upper
limits the slopes of the Rayleigh-Jeans tails can be well constrained. The FIR and submm parts of the SEDs
between 60 and 1300
m can be fitted in two ways:
Firstly, with a single modified blackbody yielding large ranges for both the opacity (
)
and the emissivity (
). The resulting temperatures
range from 50 to 70 K. However, there are several reasons against the physical relevance of one single
dust component:
- 1)
- The optical-NIR morphology is generally disturbed, suggesting the presence of
diffuse outer dust layers in addition to a dense central dust body.
- 2)
- It is difficult to understand why
is significantly <2 in some ULIRGs,
but
for spiral and Markarian galaxies.
- 3)
- The normalised strength of the 7.7
m PAH features shows no correlation
with the extinction derived formally from
.
- 4)
- The derived dust masses lead to unusually high gas-to-dust mass ratios from 500 to 2300.
Secondly, the FIR-submm SEDs can be decomposed into two or more
modified blackbody
spectra representing cool (30-50 K) and cold (10-30 K) dust components. While the cool component
should be moderately opaque (
)
to match size
constraints inferred from interferometric CO observations, the cold component should be transparent
in order to conform to the observed PAH/100
m and PAH/850
m ratios. Remarkably, this suggests
that the bulk FIR dust component cannot contain the majority of the PAH carriers, rather they are more
closely coupled with the submm emitting dust.
This second way to fit the SEDs provides evidence favouring two dust stages:
- 1)
- the cold dust in more quiet regions, which is mainly responsible for the submm emission, and
- 2)
- the cool dust powered by starbursts (SB) which dominates the FIR and may be partly optically
thick even in the FIR.
The dust masses derived from such a multiple component model yield gas-to-dust mass ratios in the
range between 30 and 300, hence close to the galactic value of 150. ULIRGs classified as either AGNs
or SBs appear indistinguishable at FIR and submm wavelengths. We suppose that the cool FIR emitting dust
is not related to the AGN, in particular since SB-type ULIRGs seem to be more than twice as frequent
in our sample as AGN-type ULIRGs.
In the NIR-MIR the SEDs reveal two basic shapes:
- 1)
- A flat NIR-MIR flux plateau followed by a steep rise at about 10
m.
All objects optically classified as LINERs and HII/SBs exhibit this SED shape,
but so do some of the Seyferts.
- 2)
- A power-law like flux increase starting in the NIR and peaking in the FIR or even MIR.
This SED shape is only seen in Seyferts.
The NIR-MIR power-law-like emission is due to warm and hot dust powered by the AGN, and this is a
third dust stage restricted to AGN-type ULIRGs. From this finding, a NIR photometry diagnostic tool is
established to reveal the presence of an AGN on the basis of the J-K colours (which has red colours,
while SBs have blue colours). This tool is successful in the sense that if an AGN is seen by other
diagnostics (optical or MIR spectroscopy), in most cases (6 out of 8) it is also seen via the NIR colours.
The detection or non-detection of cold dust could reflect different evolutionary states of the ULIRGs
associated with the merger state. However, the current means to determine the merger state, like the
projected distance of the nuclei, appears to be insufficient to verify evolutionary trends.
Acknowledgements
The development and operation of ISOPHOT were supported by MPIA and funds from Deutsches Zentrum
für Luft- und Raumfahrt (DLR, formerly DARA). The ISOPHOT Data Centre at MPIA is supported by
Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR) with funds of Bundesministerium für Bildung
und Forschung, grant No. 50QI98013. The authors are responsible for the contents of this
publication. JCMT is operated by the Joint Astronomy Centre on behalf of the Particle Physics and
Astronomy Research Council of the UK, The Netherlands Organisation for Scientific Research,
and the National Research Council of Canada.
It is a pleasure for us to thank Dr. Uwe Graser for kindly obtaining the optical spectra during
operational tests, and Drs. Robert D. Joseph and José-Míguel Rodríguez-Espinoza for
stimulating discussions. We thank the referee, Dr. Suzanne Madden, for a comprehensive set of
valuable comments.
For literature search and photometry we used the NASA/IPAC Extragalactic Data Base (NED) and the
NASA Astrophysics Data System (ADS).
Up: Infrared to millimetre photometry galaxies:
Copyright ESO 2001