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There are several mechanisms that in principle could account for the dust heating, among
them the evolved hot
star populations. However Binette et al. (1994) developed
a starburst evolution model and showed that Post-AGB stars are by far the
main source of UV photons at an age of 1010 years, while other sources,
such as nuclei of planetary nebulae can contribute significant amounts of
UV photons for ages up to years. There are also other
alternatives, such as hot electrons in hot emitting gas or exposed hot
cores (105 K) of post red giant stars. The problem with the latter two
mechanisms is that they will also produce X-ray emission, which is only
observed for a small number of galaxies in this sample.
In Paper I we found a strong correlation between the luminosity and the luminosity in the B band, inside the region occupied by the
line emitting gas. We also demonstrated that Post-AGB stars provide enough
ionized radiation to account for the observed luminosity.
In most cases there were considerably more UV photons available than those needed to
produce the observed (typically by a factor of 1.5
- 2). This excess of UV photons are available to heat
the dust which then reprocesses these photons reemiting them at IR wavelengths.
The correlation between the dust mass and the blue luminosity inside the emitting region,
shown in Fig. 3, is exactly as expected if the post-AGB stars are the main
source of the UV photons and also heat the dust.
A simple estimate for dust luminosity can be obtained by assuming
a model where the dust is distributed roughly uniformly around the
sources or concentrated in spherically symmetric dust clouds. The total energy
absorbed by the dust and reemited in the infrared () is related
to the total incident UV luminosity () as (Bonatto &
where is the solid angle subtended by the dust distribution and
is the effective optical depth to the incident UV/optical
continuum along the line of sight. The optical depth is defined as
and assuming a Galactic
extinction law . The visual
extinction was computed from the extinction maps (see Table 2).
The UV photons emited by post-AGB stars were calculated from
(see Table 5 of Paper I) using
recombination theory (Osterbrock 1974).
are the recombination coefficients.
From the emited by the post-AGB stars we have
computed the , using Eq. (6).
From the observed IRAS fluxes we can compute the two
components of the IR luminosities, namely the warm component
, which assumes that most of the dust emission is
produced at 25 and m, and the cold component
, derived from the 60 and m
luminosities, as in Bonatto & Pastoriza (1997).
Figure 4 shows a good agreement between the calculated and
both of the observed IR luminosities for most of the galaxies, except NGC
3311 and NGC 4473 at the lower right, which seem to have a very luminous IR
This simple calculation suggests that post-AGB stars are an ideal source of
ionizing photons for the gas and heating photons for the dust. The post-AGB
stars are known to exist and are distributed throughout the galaxy as the old stellar population
thus providing in-situ sources of ionizing photons.
This does, of course, not prove that they are the only mechanism capable of
producing the observed dust emission and indeed more than one mechanism
is likely to contribute in different amounts in each individual galaxy.
Relation between the observed and calculated infrared luminosities |
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