A simple model was constructed to calculate the far-infrared emission of galaxies made up of a disk component containing cool dust heated by the general interstellar radiation field and of a molecular cloud component containing warm dust heated by recently formed massive stars. This model is fitted to the optical and far-infrared data of 120 Shapley-Ames galaxies and of 20 optically studied mini-survey galaxies, resulting in the determination of blue face-on extinctions and of the total luminosities of recently born massive stars and disk stars. The ratio of these two luminosities is a more reliable star formation activity index than the previously used ratio L sub IR/L sub B. The results show that infrared selected galaxies are on the average almost three times more dusty than optically selected ones. Only about 10% of the mini-survey galaxies exhibits strongly enhanced star formation

Brink, K.

Dejong, T.

English

NASA Technical Reports Server

N87- 24314
A SIMPLE TWO-COMPONENT MODEL FOR THE FAR-INFRARED EMISSION FROM GALAXIES
T. de Jong and K. Brink
Astronomical Institute "Anton Pannekoek", University of Amsterdam
Roetersstraat 15, 1018 WB Amsterdam, The Netherlands
ABSTRACT. We have constructed a simple model to calculate the far-infrared
emission of galaxies made up of a disk component containing cool dust heated by
the general interstellar radiation field and of a molecular cloud component
containing warm dust heated by recently formed massive stars. This model is
fitted to the optical and far-infrared data of 120 Shapley-Ames galaxies and of
20 optically studied mini-survey galaxies, resulting in the determination of
blue face-on extinctions and of the total luminosities of recently born massive
stars and of disk stars. The ratio of these two luminosities is a more reliable
star formation activity index than the previously often used ratio LIR/LB. The
results show that infrared selected galaxies are on the average almost three
times more dusty than optically selected ones. Only about 10% of the mini-survey
galaxies exhibits strongly enhanced star formation.
i. INTRODUCTION
A large fraction of the energy emitted by recently formed massive stars is
reradiated in the far-infrared by dust grains in molecular clouds - the dominant
site of massive star formation in a galaxy. This makes the infrared by far the
most suitable wavelength range to study the energetics of star formation and to
derive global star formation rates in galaxies.
However there are additional sources of infrared radiation in a galaxy that
complicate the picture. Dust particles in the general interstellar medium,
predominantly heated by much older field stars, also contribute to the total
infrared emission. This dust is probably cooler than the dust in molecular
clouds (cf. Mezger et al. 1982, de Jong et al. 1984). Furthermore, hot dust has
been shown to be present in the nuclei of active galaxies (de Grijp et al.
1985).
If infrared observations of galaxies with high spatial resolution would be
available the different infrared components could be separated directly.
However, because of the large detector sizes (typically several arcminutes, cf.
Neugebauer et al. 1984) the IRAS data, on which our study will be based, provide
only total energy information.
Thus to study star formation in galaxies the observed infrared emission has
to be decomposed into several components. In this paper we present some
preliminary results of fitting simple two-component models to the IRAS 60 and
i00 _m fluxes of about 140 galaxies.
Carol J. Lonsdale Persson (Editor)
Star Formation in Galaxies 323
https://ntrs.nasa.gov/search.jsp?R=19870014881 2020-03-20T10:56:33+00:00Z
T.DEJONGANDK.BRINK
2. THE MODEL
The purpose of our model calculation is to fit the observed 60 and I00 _m
fluxes and the BT magnitude of a galaxy. In addition to these three observables
the model also makes use of the observed galaxy inclination (or equivalently the
axial ratio b/a).
First we fit two infrared components to the observed infrared spectral
energy distribution. To do so we assign a dust temperature Tw to the warm dust
in molecular clouds and Tc to the cool dust in the general interstellar medium
(the disk component). We further assume that the dust emissivity is proportional
to k-I. This fit results in 60 and i00 _m fluxes for each component, that add up
to the observed fluxes. We then calculate the total luminosity of each
component, Lw and Lc, by integrating over all wavelengths.
Next we calculate the total luminosity of recently formed massive stars,
L2, by assuming that half of the luminosity of these stars is completely
converted to infrared radiation inside molecular clouds, so that L 2 = 2Lw. The
other half is emitted by stars that have moved away from the molecular clouds in
which they were born within their lifetime. The 50% estimate above has been
derived for stars born with a Salpeter initial mass function (IMF) and a cloud
residence time of 3 10 6 yrsl(cf. Garmany et al. 1982), i.e. for stars moving
with velocities of i0 km s in clouds with sizes of order 60 pc. The fact that
we do observe 0B stars is convincing evidence that an appreciable fraction of
them indeed escapes from clouds within their lifetimes.
To calculate the luminosity L I of disk stars, that are largely responsible
for the blue luminosity of a galaxy and for the heating of the cool interstellar
dust, we assume a surface black-body temperature for these stars of T I = 7000 K.
Anticipating the result that in a typical spiral galaxy L2/L I = 0.5 (see section
3) and taking T 2 = 30,000 K (see below) the mean colour temperature of the
interstellar radiation field is found to be 12,000 K, close to the canonical
value of I0,000 K.
However, OB stars outside molecular clouds also contribute to the heating
of the interstellar dust and to the observed blue light. The effective
temperature of these stars is about 30,000 K, another result of the calculation
of the fraction of the luminosity emitted outside clouds by stars born with a
Salpeter IMF inside molecular clouds. Taking account of the fact that the
interstellar absorption roughly scales with I/k so that the effective optical
thickness of the disk for 7000 K and 30,000 K radiation is different, we
iteratively solve for the dust optical thickness at k4400, _B' of the galactic
disk (equivalent to the face-on extinction) and L I. To fit the observed blue
magnitude we take into account that the blue light is attenuated by optical
thicknessn (a/b)_B" The face-on extinction is then found from the relation
A_ = 1.086
_B"D
Our model thus results in the determination of the face-on blue extinction
A_ of a galaxy and of the luminosities L I and L 2. The quantity L2/L I is a much
better measure of the ratio of present-to-past star formation rates than the
often used quantity LIR/LB (de Jong et al. 1984, Soifer et al. 1984).
324
A TWO-COMPONENTMODEL
3. RESULTS AND DISCUSSION
We have fitted our simple two-component model to infrared and optical data
of 120 galaxies in the Revised Shapley-Ames Catalog of Bright Galaxies (Sandage
and Tammann 1981) and to the data of 20 mini-survey galaxies (Soifer et al.
1984) for which morphological classifications, detailed optical and infrared
photometry and optical spectra have recently been presented by Moorwood et al.
(1986). Infrared flux densities of these galaxies were taken from the IRAS Point
Source Catalog (JISWG 1985), supplemented by flux densities from the IRAS Small
Scale Structure Catalog (JISWG 1986), if listed. Optical blue magnitudes of both
the RSA galaxies and of the mini-survey galaxies were corrected for extinction
in our own galaxy according to the prescriptions given by Sandage and Tammann
(1981).
For our model calculations we took T. = 60 K and Tc = 16 K. These are not
arbitrary choices but were selected as fo_lows. Tw was chosen to be somewhat
larger than the largest dust temperature as derived from the observed SIo0/S60
flux density ratios. Tc was chosen such that the average face-on extinction
derived for the RSA galaxies agreed with optically determined values (see fig.l
and discussion below). The low temperature of the cool component implies that a
substantial fraction of the total infrared luminosity of galaxies (more than 50%
in most cases) is emitted at wavelengths longward of i00 _m.
We note that our adopted values of Tw and Tc are remarkably close to the
ones recently derived by Chini et al. (1986) by combining sub-millimeter
observations with IRAS data for about 20 bright galaxies. This agreement may be
somewhat fortuitous because Chini et al. assumed a k _ dust emissivity law.
We have verified that the results are rather insensitive to changes in the
adopted parameters. The largest effect is observed for Tc, understandably
because Lc changes significantly if T c changes by only a few degrees. We
emphasize that the value adopted for T is found by fitting optical extinction
C
measurements of galaxies and is thus rather well-determined.
A0
0,3
01
I I I I I I I I I
RSA
L So Sob Sb Sbc Sc Scd/mIm A
Figure i. Mean values of the face-on blue extinction A_ as a function of
morphological type. The numbers of galaxies on which this mean value is based,
are listed.
325
T.DEJONGANDK.BRINK
The distribution of average face-on blue extinctions as a function of
morphological type for the RSA galaxies is shown in fig.l. Formal uncertainties
in the averages are typically of order 10% - 20%. As noted above the temserature
of the cool dust has been fixed by requiring that the weighted mean of AB over
all spirals equals that calculated using the prescriptions of Sandage and
Tammann (1981). The somewhat higher values for Sa and Sab galaxies quoted by
Sandage and Tammann, may be due to the exclusion of dust-poor galaxies in their
determination of A_ for the earliest morphological types.
The variation of A_ with m_rphological type is quite interesting and seems
intuitively rather plausible. A_ is small for early-type galaxies because they
contain relatively small amounts of interstellar matter and thus also of dust.
Proceeding to later morphological types the mass fraction of interstellar matter
increases and so does the face-on extinction. For the latest morphological types
we deal with galaxies in which evolution has been so slow or in which star
formation has started so late that their metalllcity is still relatively low,
showing up here as a low face-on extinction.
In fig.2 we compare values of A_ and of the recent star formation activity
index log(L2/Ll) for an optically _omplete sub-set of about 50 bright RSA
spirals (BT < 12.5, VR > 500 km s- ) and for the sub-set of the infrared-
complete mini-survey galaxies studied by Moorwood et al. (1986). The most
important conclusion to be drawn from this comparison is that the minl-survey
galaxies are on the average almost three times more dusty while the present star
formation activity is only about twice as large as in the RSA spirals. The
2t,.
18
12"6 I
0
RSA
/'
t6 6
2 2
0.0 0.4 0.8 1.2 1.6
Ag(mag)
24.
18
12
6
b
RSA
mini-survey
-1.2 -0.8 -0/,
/
0.0 O.t, 0.8 1.2 1.6
tog(L2/L1)
Figure 2. The distribution of: (a) blue face-on extinction A_, and (b) star
formation activity index log(L2/Ll) for bright optically selected RSA spirals
and infrared selected mini-survey galaxies.
326
A TWO-COMPONENTMODEL
latter may be fully explained by realizing that galaxies with higher present
star formation rates are expected to be overrepresented in an infrared complete
sample of galaxies. Thus we conclude that infrared selected samples of galaxies
are apparently dominated by dusty galaxies.
This possibility was already briefly discussed in the discovery paper by
Soifer et al. (1984). Later excitement about the alternative possibility of
enhanced star formation may have resulted in the somewhat premature conclusion
that an appreciable fraction of infrared detected galaxies are "starburst"
galaxies (e.g. de Jong 1986). In fact fig.2b suggests that only 10% of the
galaxies in an infrared complete sample shows anomalously enhanced star
formation (L2/L I > i0). It is interesting to note that the two deviating
galaxies (IRAS 0413+081 and IRAS 0414+001) are the most morphologically peculiar
in the sample studied by Moorwood et al.. The one galaxy with L2/L I > i0 among
the RSA galaxies is NGC 2146, a well-known peculiar galaxy.
The fact that most of the mini-survey galaxies are probably anomalously
15
11
0.?
-J 03
.J
o -0.1
-0.5
-0.9
Figure 3. A theoretical LTD/L = versus S n /S_ diagram. Lines of equal A_
..... iv0 ,0
:u a(dotted) and L2/L I (fl .-drawn)Zare shown, cal( .ated for representative value
of b/a = 0.6. /_s indit :ed by the labelling of these lines the values of the
parameters increase by a factor two from one line to the next. Data points for
bright optical RSA spirals (triangles) and mini-survey galaxies (squares) are
plotted.
327
T.DEJONGANDK.BRINK
dusty was derived from near-infrared photometry by Moorwood et al. (1986). Our
model analysis nicely confirms their findings.
Another way to illustrate our results is shown in the (LIR/LB) versus
(SIo0/S60) diagram in fig.3 where data points for the optically complete RSA
sub-set (triangles) and for the mini-survey sub-set (squares) have been plotted.
Using our simple two-component model we have drawn lines of equal A_ and L2/L I
in this di0gram. These lines are calculated for a representative value of b/a =
0.6. For A_ > i almost all optical radiation is reemitted in the infrared so
that the ratio L2/L I fixes the infrared color ratio SIon/S= 0 independent of the
value of AB, whereas _TD/L= iS determined almost exclusively by the extinction.
For small values of A_ the quantity LIR/L B is most strongly dependent on L2/L I.
The upper right hand corner in the diagram is forbidden because L2/L I cannot
exceed infinity.
Fig.3 clearly shows that the minl-survey galaxies are much more dusty than
the optically selected bright RSA spiral galaxies while the star formation
activity is only about twice as large in the mini-survey galaxies. It, is quite
rewarding that in spite of the simplicity of our model it is so remarkably
succesful in explaining the differences between optically and infrared selected
samples of galaxies. A more extensive discussion of our results will appear
elsewhere.
The research of K.B. is supported by a grant from the Netherlands Foundation for
Astronomical Research (ASTRON) with financial aid from the Netherlands
Foundation for the Advancement of Pure Research (ZWO)
REFERENCES
Chini, R., Kreysa, E., Krugel, E., and Mezger, P.G. 1986, Astr. Ap. 157, L1
Garmany, C.D., Conti, P.S., and Chiosi, C. 1982, Ap. J. 263, 777
Joint IRAS Science Working Group 1985, IRAS Point Source Catalog, US Government
Printing Office, Washington
Joint IRAS Science Working Group 1986, IRAS Small Scale Structure Catalog, US
Government Printing Office, Washington
de Grijp, M.H.K., Miley, G.K., Lub, J., and de Jong, T. 1985, Nature 314, 240
de Jong, T. 1986, in "Spectral Evolution of Galaxies", C.Chiosl and A.Renzini,
Reidel Publ. Co., Dordrecht, p.lll
de Jong, T., et al. 1984, Ap. J.(Letters) 278, L67
Mezger, P.G., Mathis, J.S., and Panagia, N. 1982, Astr. Ap. IO5, 372
Moorwood, A.F.M., Veron-Cetty, M.-P., and Glass, I.S. 1986, Astr. Ap. 160, 39
Neugebauer, G., et al. 1984, Ap. J.(Letters) 278, LI
Sandage, A., and Tammann, G.A. 1981, A Revised Shapley-Ames Catalog of Bright
Galaxies, Carnegie Institution of Washington, Publ. 635
Solfer, B.T., et al. 1984, Ap. J.(Letters) 278, L71
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A simple two-component model for the far-infrared emission from galaxies

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