Infrared Astronomical Satellite (IRAS) observations show that there is a maximum in the continuum energy distribution of the broad-line radio galaxy 3C 390.3 near 25µm and that this active galaxy emits most of its energy in the infrared. If the 25µm component is thermal, its temperature is approximately 180 K, and its size must exceed tens of parsecs

Clegg, P. E.

Harris, S.

Miley, G.

Neugebauer, G.

Rowan-Robinson, M.

Soifer, B. T.

Young, E.

Miley, G.K.

Soifer, B.T.

Clegg, P.E.

Leiden University Scholary Publications

A 25 micron component in 3C 390.3Miley, G.K.; Neugebauer, G.; Soifer, B.T.; Clegg, P.E.; Harris, S.; Rowan-Robinson, M.; Young,E.CitationMiley, G. K., Neugebauer, G., Soifer, B. T., Clegg, P. E., Harris, S., Rowan-Robinson, M., &Young, E. (1984). A 25 micron component in 3C 390.3. Astrophysical Journal, 278, L79-L81.Retrieved from https://hdl.handle.net/1887/6463 Version: Not Applicable (or Unknown)License:Downloaded from: https://hdl.handle.net/1887/6463 Note: To cite this publication please use the final published version (if applicable).1984ApJ...278L..79M1984ApJ...278L..79M1984ApJ...278L..79M1984ApJ...278L..79M

A 25 micron component in 3C 390.3

Caltech Authors - Main

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THE ASTROPHYSICAL JOURNAL, 278:L79-L81, 1984 March 1 
© 1984. The American Astronomical Society. All rights reserved. Printed in U.S.A. 
A 25 MICRON COMPONENT IN 3C 390.31 
G. MILEY, G. NEUGEBAUER, P. E. CLEGG, S. HARRIS, M. ROWAN-ROBINSON, B. T. SOIFER, AND E. YOUNG 
Received 1983September16; accepted 1983November18 
ABSTRACT 
. In[rar~d Astronomical S~tellite ~IRAS) observations show that there is a maximum in the continuum energy 
distnbution of the broad-line radio galaxy 3C 390.3 near 25 /LID and that this active galaxy emits most of its 
energy in the infrared. If the 25 /LID component is thermal, its temperature is approximately 180 K, and its size 
must exceed tens of parsecs. 
Subject headings: infrared: sources - radio sources: galaxies 
I. INTRODUCTION 
The broad-line radio galaxy 3C 390.3 has been studied 
intensively at radio, optical, and X-ray wavelengths. The radio 
source is identified with a 15th magnitude N type galaxy 
(Longair 1965; Wyndham 1966) at a redshift of 0.0569 (dis-
tance 227 Mpc) 2 (Sandage 1966; Penston and Penston 1973; 
Burbidge and Burbidge 1971; Osterbrock, Koski, and Phillips 
1976; Heckman et al. 1981). It is a strong X-ray source, being 
one of the few active galaxies seen in the original Uhuru 
survey (Giacconi et al. 1972). Until now, there have been no 
published data on 3C 390.3 in the mid-infrared or far-infrared. 
We present the results of IRAS observations which show that 
the continuum energy distribution of 3C 390.3 peaks near 25 
/LID and that 3C 390.3 has its largest energy output in the 
infrared. 
II. OBSERVATIONS AND RESULTS 
The technical details of the IRAS satellite, its various 
observing modes, and the procedures used to reduce the data 
are described by Neugebauer et al. (1984a, hereafter Paper I). 
Pointed observations (see Paper I) of 3C 390.3 in two 
modes were obtained in 1983 March-May. Twenty 1 ?6 x 0?5 
maps of the area surrounding 3C 390.3 were constructed, each 
out of six raster scans 1 ?6 in length and separated by O?l. The 
total observing time for each map was about 6 minutes. The 
various observations were co-added to enhance the signal-to-
noise ratio. In each of the observations, a source was clearly 
detected at 12, 25, 60, and 100 /LID close to the position of 3C 
390.3. The average centroid position obtained from a point-
source extraction algorithm applied to the 12, 25, and 60 /LID 
grids differs less than 10" from the position of the N galaxy 
associated with 3C 390.3 (Hargrave and McEllin 1975). 
The data were calibrated in intensity by observing the 
planetary nebula NGC 6543 at the same relative position and 
in a similar manner (see Paper I). Since each of the IRAS 
1The Infrared Astronomical Satellite (IRAS) used in these observations 
was developed and is operated by the Netherlands Agency for Aerospace 
Programs (NIVR), the US National Aeronautics and Space Administra-
tJ.on (NASA), and the UK Science and Engineering Research Council 
(SERC). 
2 Throughout this Letter, we adopt a Hubble constant of H = 75 
km s- 1 Mpc- 1. 
L79 
bandpasses occupies a finite range of frequency, a small 
correction had to be applied for the difference in infrared 
color between 3C 390.3 and the calibration source (Paper I). 
The resultant flux densities are listed in Table 1. The final 
signal-to-noise ratios exceed 18 in the 12, 25, and 60 /LID 
bands; at 100 /LID, the ratio is approximately 10. The sys-
tematic uncertainties are approximately 15%. The results 
analyzed so far show no evidence for variability at this level. 
The overall spectrum of 3C 390.3 is plotted in Figure 1. 3C 
390.3 shows a clear peak near 25 /LID, different in this respect 
from most of the other 20-30 active galaxies for which IRAS 
measurements have been obtained. These have smooth spectra 
with flux densities at 25 /LID that are smaller than those at 
60 /Lffi. 
·The anomalous nature of the infrared spectrum of 3C 390.3 
is most apparent when the 60 /Lm/25 /LID spectral index ( + 0.40 ± 0.14) is compared with values of the same quantity 
so far measured by IRAS for galaxies ( - 0.8 to - 4.9) (Soifer 
et al. 1984; Young et al. 1984) and quasars (-0.7 to -1.2) 
(Neugebauer et al. 1984b). The presence of an excess 25 /Lm 
flux in 3C 390.3 is indicated by the relative curvature of its 
spectrum. The difference between the 60 /Lm/25 /LID and 25 
/Lm/12 /LID spectral indices can be used as a measure of 
spectral curvature. For 3C 390.3, this difference is + 1.7 ± 0.4, 
which can be compared to -1.3 ± 0.4 and + 0.1 ± 0.2 for the 
mean values of the difference for the six galaxies and five 
quasars measured at all three wavelengths by Soifer et al. 
(1984), Young et al. (1984), and Neugebauer et al. (1984b). 
In showing the radio spectrum in Figure 1, we distinguish 
between the compact nuclear component, whose spectrum is 
flat, and the total radio emission, which is dominated by 
steep-spectrum emission from an extended 220 kpc halo and 
therefore is not directly relevant for the nucleus. 
Typical values for the log10 (flux per logarithmic frequency 
interval [Wm- 2 ]) are -15.65, -13.34, -14.07, and -14.00, 
at 6 cm, 25 /Lm, 0.5 /Lm, and 2-6 keV. It is thus seen that the 
energy output of 3C 390.3 is largest in the infrared. This 
statement is subject to the caveat that the luminosity of 3C 
390.3 has not increased by large amounts at other wavelengths 
since the last reported measurements. Note that 3C 390.3 is 
highly variable in the X-ray and optical region of the spectrum 
(e.g., Barr et al. 1980). 
© American Astronomical Society • Provided by the NASA Astrophysics Data System 
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L80 MILEY ETAL. Vol. 278 
TABLE 1 
MEAN PARAMETERS 
PARAMETER 12 
Flux density (Jy) . . . . . . . . 0.14 ± 0.02 
Log10 luminosity (W) . . . 37.32 
WAVELENGTH (µm) 
25 60 
0.38 ± 0.06 
37.43 
0.25 ± 0.04 
36.86 
100 
0.34 ± 0.06 
36.77 
lm lcm 
WAVELENGTH 
lOOµm lµm 10 pm 
0 
-1 
'"):. 
::i. 
~ -2 
~ 
0 
~ 
-3 i 
CJ 
0 
.... 
-4 
-5 
-6 
• 
TOTAL • 
• • • NUC • • • 
• 
• 
• 
• 
• 
8 9 10 11 12 13 14 15 16 17 18 
LOG (FREQUENCY (Hz)) 
FIG. 1.-The energy distribution of 3C 390.3 over 10 decades in frequency. The emission from the compact radio nucleus is plotted separately from 
the total radio emission. The observations are taken from Kellermann. Pauliny-Toth, and Williams (1969) [1400, 2695, 5000 MHz]; Miley and van der Laan 
(1973) [1400 MHz nuc]; Harris (1972) (2695 MHz, 5000 MHz nuc]; CIT (1983, unpublished data) [1.2-2.2 µm, 10 µm]; Rieke (1983, unpublished data) [3.4 
m]; Barr et al. (1980) [Band 2-6 keV]; Ferland et al. (1979) [1500 AJ. 
III. DISCUSSION 
A crucial question is whether the 25 11m component in 3C 
390.3 is due to thermal or nonthermal radiation. If the radia-
tion is interpreted as thermal, its temperature is 180 ± 30 K. 
Assuming that the 3011m component is indeed that of a 180 K 
blackbody, the observed maximum gives, for an emissivity e, a 
blackbody radius of 1e - 112 pc. 
More stringent physical parameters can be attached to the 
region emitting the 25 11m radiation if we assume either a size 
for the emitting region or a mean emissivity. Barr et al. (1980) 
deduced a value of A,. = 1.4 ± 0.4 mag for the visual extinc-
tion in 3C 390.3 from the narrow-line Balmer decrement and 
also put an upper limit on the visual extinction to the optical 
continuum nucleus of 3C 390.3 of Av< 1.2 mag. The visual 
extinction, Av, can be related to the emissivity at 25 11m, e25 , 
using the relation derived for the Galaxy by Hildebrand 
(1983), modified to 25 11m. Taking Av to be between 200e25 
and 800e25 yields a value e25 < 0.0015, a low value, but one 
not atypical of emissivities assigned to grains in the Galaxy. If 
we assign this value to the emissivity in 3C 390.3, the size of 
the emitting region is greater than 180 pc. 
There is evidence for the existence of a similar component 
near 30 11m in the infrared spectrum of the Seyfert 2 galaxy 
NGC 1068 (Rieke and Low 1975; Simon and Dyck 1975; 
Jameson et al. 1974). In the case of NGC 1068, the low 
measured brightness temperature at 10 11m (Becklin et al. 
1973) has provided decisive evidence that the observed radia-
tion is thermal. Jones et al. (1977) have interpreted the maxi-
mum near 25 11m as being heated by an ultraviolet source of 
approximately 2 X 1011 L 0 luminosity, similar to the luminos-
ity of 3C 390.3. The size of this source was measured by 
Becklin et al. (1973) to be 0':9 corresponding to 63 pc. Thus, 
the assumed thermal source in 3C 390.3 resembles that in 
NGC 1068 in its gross properties. We should point out, 
however, that in many of its properties (e.g., presence of 
powerful extended radio emission and broad permitted lines, 
strong far-infrared emission, luminous X-ray emission), the 
properties of the activity in 3C 390.3 are almost diametrically 
opposed to those of NGC 1068. Hence, caution should be 
exercised in drawing too close a parallel between the mid-
infrared components in these two galaxies. 
If the 25 µ,m component is thermal, where does the energy 
come from to power it? One possibility is that the component 
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No. 1, 1984 25 MICRON COMPONENT IN 3C 390.3 L81 
is heated by ultraviolet emission from the nucleus. The re-
quired extinction is consistent with the limit placed by the 
optical continuum measurements of the nucleus. A second 
possible source of energy is the beam or jet which, according 
to current models of radio source propagation, powers the 
extended radio components (e.g., Rees, Begelman, and 
Blandford 1981; Norman and Miley 1984). 
The apparent increase in flux density between 60 and 100 
JLm can be indicative of a nonthermal link with the nuclear 
radio spectrum. Alternately, it can be the high-frequency end 
of colder thermal emission from the galaxy associated with 3C 
390.3. The latter possibility is unlikely since thermal emission 
is predominantly associated with spiral galaxies (see, e.g., de 
Jong et al. 1984), and 3C 390.3 type radio sources are associ-
ated with elliptical galaxies (Miley 1980). 
We cannot rule out a nonthermal origin for the 25 JLm 
component. The "break" in the continuum spectrum between 
the fiat-spectrum nuclear radio emission and the steep-spec-
trum optical N galaxy is near 25 JLm. The occurrence of a 
maximum at this wavelength would be consistent with self-
absorbed nonthermal emission from a recent burst of nuclear 
activity such as apparently occurred in the quasar 3C 345 
(Harvey, Wilking, and Joy 1982). Simple models of evolving 
relativistic plasmas (e.g., Pauliny-Toth and Kellermann 1966; 
van der Laan 1966) predict that the peak moves to longer 
wavelengths and decreases in amplitude over a period of a few 
months as the component expands. Rapid intensity variations 
might thus be expected from such a nonthermal component 
over a time scale of a few months. Although the data pre-
sented have showed no variability over a time scale of about 2 
months, 3C 390.3 has been monitored regularly for variability 
with IRAS. 
Without the painstaking efforts of Gene Kopan, Dave 
Gregorich, Carol Oken, and Linda Fullmer, the reduction of 
these data would have been impossible. 
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A 25 micron component in 3C 390.3

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