A&A 433, 73-77 (2005)
DOI: 10.1051/0004-6361:20042106
B. Rocca-Volmerange 1,2 - M. Remazeilles 1
1 - Institut d'Astrophysique de Paris,
98bis Bd Arago, 75014 Paris, France
2 - Université de Paris-Sud XI, I.A.S., 91405 Orsay Cedex, France
Received 1 October 2004 / Accepted 17 November 2004
Abstract
The spectra of the powerful 3CR
radio galaxies present a typical distribution in the far-infrared
(FIR). From the observed radio to X-ray spectral energy distribution (SED)
templates, we propose to
subtract the typical energy distributions of, respectively, the
elliptical galaxy host and the synchrotron radiation. The resulting
SED reveals that the main dust emission
is well fitted by the sum of two blackbody components at the respective
temperatures 340 K
50 K and 40 K
16 K. When the AGN is active,
the energy rate released by hot dust is much more dissipative
than cold dust and stellar emission, even when the elliptical galaxy
emission is maximum at the age of
90 Myr. Hot dust appears
as a huge cooling source which implies an extremely short time-scale
,
on balance with the short gravitational time-scale
of massive galaxies. The dissipative self-gravitational
models (Rees & Ostriker 1977) are favoured for radio sources. They
justify the existence of massive radio galaxies discovered
at z = 4
(Rocca-Volmerange et al. 2004). The synchrotron emission is emitted
up to the X-ray wavelength range, so that Extreme X-ray Objects (EXO) could
be identified with 3CR radio sources. To confirm these results in the infrared, an analysis of larger data samples from ISO and SPITZER is needed.
Key words: galaxies: evolution - infrared: galaxies - galaxies: active - galaxies: formation
In Rocca-Volmerange et al. (2004), hereafter RV04, we showed that
powerful radio sources are hosted by the most massive galaxies. Based
on measurements of stellar masses with robust evolution models, the
maximum mass is limited by the fragmentation limit at
10
(Rees & Ostriker 1977; Silk 1977)
clarifying the interpretation of the so-called K-z relation in the
K-band Hubble diagram. Moreover from this diagram interpretation,
RV04 puts a constraint on galaxy types: only host galaxies of
elliptical type fit the radio galaxy distribution from z = 0 to 4.
However at z = 4, the time-scale of mass accumulation becomes so short
that to form massive galaxies requires short dissipative time scales,
of the same order as gravitational time scales.
Typical UV to radio SEDs of 3CR galaxies were compiled from observations by ISO, IRAS and IRAM in Andréani et al. (2002). From the FIR emission, they conclude that there is a double emission respectively from a dusty torus and a larger-scale (cooler) dust distribution in the host galaxy. We propose a more detailed analysis of the dust temperatures by a multi-component approach disentangling the star and jet spectral contributions.
Statistically well identified by their high radio power, 3CR galaxies
are also hosts of massive stellar populations, which contribute to the
optical and infrared emission. In the near-infrared, the stellar
emission of the radio sources is often similar to that of the
populations of massive elliptical galaxies. In the mid-infrared,
the recent analysis of a significant sample of early-type galaxies
observed with ISOCAM (Xilouris et al. 2004) shows that the emission
is dominated by the presence of the Polycyclic Aromatic
Hydrocarbon (PAH) feature at 6.7 m, an
excess of hot dust at 15
m and a cold thermal component at 30-40 K
peaking between 70 and 100
m. A
comparison of the FIR emission of elliptical galaxies with that of
radio sources should give a specific information on the AGN contribution. Using templates from the code PÉGASE (www.iap.fr/pegase) we are able to predict the stellar emission at all galaxy ages while the synchrotron radiation contributes to SEDs
when the AGN is active.
Another objective is to estimate the cooling time-scale at the earliest phases of galaxy formation. Instead of using the classical cooling function of only helium and hydrogen clouds, each dissipative source (stars, gas, dust and AGN) has to be individually considered during galaxy evolution. In Sect. 2, we compare the various components to the averaged observed SED of 3CR radio galaxies. Section 3 presents the best fit of dust emission with the sum of two main blackbody laws, of which the hot component is an intense source of dissipation. Section 4 predicts the dissipation rate from various sources (synchrotron, stars, gas and dust) defining a cooling time scale to be compared with the dynamical time scale. The last section gives the discussion and conclusion.
The striking similarity of the radio-to-UV SEDs of 3CR radio galaxies and
quasars (see Figs. 1 to 4 in Andréani et al. 2002) indicates that the various components of these complex systems have similar
properties. All the spectra present
a specific gap from the radio emission for
107 Å (1 mm), presuming
similar properties of the dust in 3CR galaxies.
Radio galaxies are embedded in massive elliptical galaxies, even at
high redshifts (van Breugel et al. 1998; Lacy et al. 2000; Pentericci
et al. 2001), of high luminosities
3 to 7 L* (Papovich
et al. 2001) which may reach 10 L* (Mc Lure et al. 2004).
The 3CR radio galaxies are the most powerful galaxies in the K-band
Hubble diagram, they are located at the brightest limit of the galaxy
distribution, the so-called
K-z sequence. In RV04, we checked with the evolution model PÉGASE
that only scenarios for elliptical galaxies of 10
baryonic
masses are able to explain the K-z sequence. However very
strong emission lines are typical of massive radio galaxies
while elliptical galaxies show no emission lines. To confirm our mass
estimates, we also included in our modeling
the nebular emission of gas ionized by the AGN component
(Moy & Rocca-Volmerange 2002), computed with the code CLOUDY (Ferland 1996).
In the present paper
(Fig. 1), the observed radio galaxies in the
Hubble K diagram are well fitted by 10
elliptical
models and AGN emission lines. The emission line widths are assumed to be
10 Å at z = 0. The redshift of
elliptical galaxy formation is
= 30, instead of
= 10 in RV04. Our conclusions on the galaxy formation theory which is
characterized by the fragmentation limit of
10
remain unchanged, allowing us to adopt this
mass for 3CR powerful radio galaxy hosts.
The NIR predictions
from 1 to 5
104 Å (1 to 5
m) are strongly dependent on the modeling
of cold star populations. The effective temperatures of giant branch
and asymptotic giant branch stars, as well as the mass population density
are crucial but are not
accurately estimated. We checked with Fig. 2 in
Fioc & Rocca-Volmerange (1996) that the PÉGASE elliptical
model predicting stellar populations on the two branches is
in agreement with the observational template.
However we still admit that the separation of the stellar and hot dust component
will be less robust from
= 1 to 5
104 Å than for
5
104 Å.
Most scenarios of galaxy evolution take into account extinction by dust, computed with a transfer model in ellipsoidal or slab geometries (Fioc & Rocca-Volmerange 1997). However in the elliptical scenario, galactic winds expel gas and dust at 1 Gyr, so that at z = 0 our modelling only simulates the stellar emission. In the following, we shall adopt the SED model of elliptical galaxies to predict the underlying populations of radio sources.
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Figure 1:
The distribution of radio (squares) and field (diamonds, crosses)
galaxies in the K-band Hubble diagram, compared to the predicted sequence of
elliptical galaxies of masses 10
![]() ![]() ![]() ![]() ![]() ![]() |
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The synchrotron radiation in radio galaxies follows a power law
=
.
Observations in the radio domain
are well fitted by
= -1.04 (Andréani et al. 2002).
The energy distribution and intensity of the dust emission can be derived
from the radio-to-UV SED template of 3CR radio galaxies:
by comparing the stellar and synchrotron emission with the
composite spectrum of averaged 3CR galaxy SEDs, we find a large
unresolved bump from = 1 to 500
104 Å (1 to 500
m and log(
)
12 to 15). The average redshift z = 0.5 of the observational sample has been
corrected to the rest frame. The comparison is presented in Fig. 2.
![]() |
Figure 2:
The observed energy distribution of 3CR galaxies by Andréani et al. (2002) (full blue line) in a large wavelength range (![]() ![]() |
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The various grains found in the interstellar medium (Desert et al. 1990) contribute at different levels to the total FIR emission of
radio sources. The global FIR emission is described as the superposition
of blackbody (BB) laws depending on the distribution of grain temperature, size and species.
We do not consider the emission signatures typical
of PAH, generally attributed to
star formation: in evolved elliptical galaxies, the star formation
activity is nil or very low. The first result of the analysis is that
it is not possible to reproduce the IR bump with one single BB temperature. We then propose to limit the decomposition to only two BB laws. The second result is the excellent
fit for two extreme temperatures: the hot component
corresponds to a BB temperature of 340 K
50 K,
the cold component to a BB temperature of 40 K
16 K.
Figure 3 presents the stellar and
synchrotron fluxes F(
)
as a function of the wavelength
and a comparison of the two black-body laws with the observed bump.
From the Wien law, the 340 K
50 K BB emission peaks at
the wavelength
8.5
1.25
104 Å (8.5
1.25
m). In 3CR radio galaxies,
this hot component is an extreme source of energy dissipation. A similar temperature
was observed in the far-infrared continua of quasars (Wilkes et al. 1998)
but in quasars it is not possible to disentangle the contribution of
other components, in particular stellar. Because it is found in
radio galaxies, this hot component may be attributed to the active galaxy nucleus (AGN);
however, the hot dust component at 260 K found in early-type normal galaxies
from ISOCAM data (Ferrari et al. 2002) could be of similar nature, even if
its luminosity is much lower.
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Figure 3:
The three main fluxes contributing to the radiative energy
of the 3CR radio galaxy sample are the elliptical SED (green line), the synchrotron
power law (red line) and the two blackbody laws (pink lines) of respectively
340 K ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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The wavelength peak for 40 K as derived from the Wien law is at 7.2
105 Å (72
m).
Fluxes at mid-height correspond to an accuracy of
16 K.This cold
component may be compared with the already known 40 K-60 K
temperatures found by the ISO and SPITZER telescopes in dusty evolved galaxies (Blain et al. 2004; Xilouris et al. 2004).
Qualitatively this result of two components is confirmed by the splendid
IRAC/SPITZER image of the nearby radio galaxy Centaurus A
(Fazio et al. 2004). A third emission peak at 3
104 Å (3
m) may be present in the data (dashed line in Fig. 3).
The disentangling of the cold stellar and very hot (
900 K) dust
emission is less robust and of minor importance for the energy balance.
The classical cooling function predicted for initial clouds of hydrogen and helium
(Rees & Ostriker 1977, and references therein) becomes insufficient,
and may be wrong, when intense cooling sources are activated during
galaxy formation. While stellar and cold dust emission, in particular
in case of rapid metal enrichment,
depends on the star formation rate, the cooling
processes due to the AGN are dissipative only
when the AGN is active. However the cooling processes efficiently
contribute to the decrease of
the cooling time-scale
.
We propose to estimate
hereafter the radiative energy balance during the star formation
evolution when the AGN is active. We integrated all the previously considered
fluxes (stellar SEDs, BB laws, power law)
over their largest wavelength domain. The dissipative energy rate
in erg s-1 is then computed for all sources. For simplicity, we separate stellar and AGN sources. Stellar sources evolve on the galaxy time-scale (
14 Gyr) with passive and active evolution while AGN, supposed to be active at an arbitrary time, have a short
lifetime (<108 years).
![]() |
Figure 4:
Various stellar (blue color) dissipative rates
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Based on SEDs of 3CR radio galaxies observed from the X-ray to radio
domain, we identify the dust emission by subtraction of the template of
elliptical galaxies and of the synchrotron power law.
Two main components of dust black body (BB) emission (and a possible
minor hotter component) are revealed in the
diagram, while the classical diagrams
(or
F(
) -
)
(Haas et al. 1998)
are less suitable for component separation.
The hot dust emission peaks at the wavelength of 8.5
1.5
104 Å,
corresponding to a blackbody temperature of 340 K
50 K. This component is in agreement with the standard AGN molecular torus model (Pier & Krolik 1993), embedded in
a cooler component. However, if we make a comparison with the IRAC/Spitzer image
of Centaurus A (Fazio et al. 2004) we see that the bright hot grain structure
is not in the inner core but within a larger structure.
Many speculations on the origin of the hot grains are possible,
we only conclude that there is a large
amount of dust photo-heated by the AGN.
The uncertainty in the hot BB temperature at the maximum
includes errors due to
calibration and to the modeling of the stellar emission. From
Fig. 3, the stellar emission is seen to be negligible
at about 8
104 Å. The large error of
50 K means that the resolved
signatures of PAH observed at respectively
7.7
m and 8.6
m (Puget & Leger 1989)
could be included in this peak located at
8
m.
The most important uncertainty concerns the total emission at
wavelengths around 3
104 Å. This domain is highly sensitive
to the subtraction of the stellar component. Moreover observational
data around this wavelength are highly dispersed: a large variation of
emission between quasars and radio galaxies is shown by Andréani et al.
(2002). At this wavelength, the authors present an extreme
value for quasars which would indicate a strong emission peak
while for radio galaxies, the peak is significantly weaker energetic.
The main difference with previous studies of active galaxies concerns the star
formation. While starburst activity is preferentially investigated
in Ultra Luminous Infra Red Galaxies (ULIRG) (Genzel & Cesarsky 2000), star formation is of minor importance in radio
sources, which are dominated by the evolved population of elliptical galaxies.
However, a puzzling question is arised by the comparison with PAH.
A peak of emission centred at the wavelength of 8.5
1.5
104 Å peak is also found in the ULIRGs (Ultra Luminous Infra Red Galaxies). In general attributed
to the PAH, the peak is not thermal and cannot be identified to
a black body law but to an instable episodic excitation.
It is still premature to decide if there is a link
between the 8.5
m peak discovered in strong
ULIRGs and the 8.5
m peak in powerful radio galaxies.
But the similarity of the two peaks deserves further analyses
of hidden AGN in starbursts or/and of star formation in AGN environments.
The cool component at 40 K is at about the same temperature as in elliptical galaxies (Xilouris et al. 2004). Thus its origin is not necessarily linked to the presence of the AGN but probably to dusty populations of stars (low-mass AGB stars or others). Moreover, because stellar winds in elliptical galaxies must eject gas and dust, we need to justify the presence of dust. One possibility is that galactic winds expel the interstellar gas, while the denser and more embedded dust component is maintained in the galaxy centre environment. Grains, more massive than gas, could fall into the galactic center more rapidly (or be ejected with less efficiency) than the gaseous component; this process may also depend on the angular momentum. The time scale of grain infall towards the center will then be shorter than the time scale of interstellar gas heating.
Are these results for 3CR radio galaxies acceptable for all AGNs?
3CR radio galaxies are the most powerful and massive galaxies
hosting super massive black holes (McLure &
Dunlop 2002). If dust emission, in particular by hot dust, is due to
a collapsing process of dust towards the centre, the origin of the
dust has nothing to do with the presence of an AGN
which only heat grains and make them luminous.
It might be linked to a star formation
process if massive stars are totally embedded in dusty clouds since
no evidence of SED signatures due to starbursts is revealed in
the optical part of the SED.
Whatever the origin of the energetic photons
heating the hot dust, the
energy released by this dust is
considerable. An approximate dynamical time scale
= 1/(G
)1/2
600 Myr becomes
comparable to the cooling time scale during the AGN phase.
The total energy dissipated by the 340 K emission, integrated
over the AGN life time (108 yr) gives a time scale
400 Myr. Our evaluations
show that
and
are quite comparable and the dissipation may regulate the self-gravitational collapse models of galaxy formation
(Rees & Ostriker 1977). The dissipation factor may be lower at higher redshifts
where metals and dust masses are significantly
lower. However, in the case of a massive initial gas reservoir,
the collapse is extremely rapid, metal enrichment follows
and finally the grain emission is dominant.
Another source of uncertainties is the energy released by neutrinos
from supernova explosions, so that the presence of an AGN
might not be necessary to dissipate energy on a short time scale, comparable
to the gravitational time scale. More detailed
observations from the ISO archives and the rapidly increasing data set from the satellite SPITZER are required.
Acknowledgements
We thank Nick Seymour for reading the manuscript