The recently established correlation between radio continuum and far infrared emission in galaxies was further investigated by comparing normal spiral and blue compact dwarf galaxies. The puzzling result is that the ratio of radio to far infrared luminosity and its dispersion is the same for both samples, although their ratios of blue to far infrared luminosity, their radio spectral indices and their dust temperatures exhibit markedly different mean values and dispersions. This suggests that the amount of energy radiated in the two regimes is enhanced in the same way although the mechanisms responsible for the two components are rather different and complex. The fact that the blue light does not increase at the same proportion shows that both the radio and the far infrared emission are connected with the recent star formation history

Klein, U.

Wunderlich, E.

English

NASA Technical Reports Server

THE RADIO-FAR INFRARED CORRELATION: N 8 7 - 2 4 3 5 1
SPIRAL AND BLUE COMPACT DWARF GALAXIES OPPOSED
U. Klein
Radioastronomisches Institut der Unversit_t Bonn
F.R.G.
E. Wunderlich
Max-Planck-Institut f_r Radioastronomie
F.R.6.
Abstract: The recently established correlation between radio continuum and
far infrared emission in galaxies has been further investigated by comparing
normal spiral and blue compact dwarf galaxies. The puzzling result is that the
ratio of radio-to-far infrared luminosity _ its dispersion is the _ame for
both samples, although their ratios of blue-to-far infrared luminosity, their
radio spectral indices and their dust temperatures exhibit markedly different
mean values and dispersions. This suggests that the amount of energy radiated in
the two regimes is enhanced in the same way although the mechanisms responsible
for the two components are rather different and complex. The fact that the blue
light does not increase at the same proportion shows that both the radio and the
far infrared emission are connected with the recent star formation history.
1: INTRODUCTION
Soon after the successful IRAS mission, several groups of investigators
reported a close correlation between the integrated radio continuum and far
infrared (FIR) emission from galaxies, with the radio continuum radiation being
predominantly of nonthermal origin (de _ong et al., 1985, Helou et al., 1985,
Sanders and Mirabel, 1985). The most straight-forward interpretation was in
terms of a close connection between dust heating and cosmic ray production via
recent star-fo_ning activity. Hummel (1986), investigating this correlation for
a sample of Sbc galaxies, considered this relation as evidence for the validity
of energy equipartition between cosmic rays and magnetic fields in Sbc spiral
galaxies, with a uniform star formation rate during the past ~ 10 9 yrs.
A similar investigation was carried out by Kunth and Sevre (1986) for a
sample of blue compact dwarf galaxies (BCDGs), and again drawing the attention
to the amazingly tight correlation between radio and FIR emission, holding even
for this outstanding class of galaxies.
In order to investigate this intriguing and puzzling correlation further,
we have undertaken a comparison of the total radio and FIR emission from normal
spiral galaxies (NSGs) and BCDGs.
2. THE SAMPLES AND THE DATA
Radio continuum measurements of BCDGs have been accumulating during the
past decade, obtained primarily with single dishes (see the compilation of
Klein, 1986). In some cases measurements at two or more frequencies exist so
that (more or less accurate) spectral indices can be derived. Recently high-
resolution VLA observations have been carried out by several groups of
Carol J. Lonsdale Persson (Editor)
Star Formation in Galaxies 583
https://ntrs.nasa.gov/search.jsp?R=19870014918 2020-03-20T11:07:39+00:00Z
U. KLEINANDE.WUNDERLICH
investigators (e.g. Brinks and Klein, 1986, Wynn-Williams and Becklin, 1986,
Sramek and Weedman, 1986) which, besides allowing more detailed studies, are
indispensable as a check on the confusion problems naturally inherent to the
single-dish measurements. Some 40 BCDGs have so far been detected in the radio
continuum (mainly at 5 GHz), and for a few of them upper limits have been
obtained. For nearly 30 of them spectral indices could be derived, but nearly
half of these are still rather unreliable. All of the flux densities
incorporated in the present work have been taken from the above-mentioned
literature, supplemented by more recent measurements carried out with the Bonn
100-m telescope (Klein, in prep.). FIR flux densities of the BCDGs have been
compiled by Kunth and Sevre (1986), in addition the work of Gondhalekar et al.
(19861 and of Wynn-Williams and Becklin (1986) provided some more data. Blue
optical magnitudes have been adopted from the list of Thuan and Martin (1981).
All of the BCDGs for which radio continuum measurements exist have been used for
the present analysis. This 'sample selection' may therefore bear the danger of
being biased, but selection effects would probably have been more serious if the
sample had been chosen using FIR detections.
As a comparison sample to represent what one would call normal spiral
galaxies (in the sense that they have seen a uniform history of star formation
over at least the past ~ 10 9 yrs) we have chosen that investigated by Gioia et
al (19821 for which accurate flux densities over a wide frequency range and
therefore reliable spectral indices, are available. The FIR data have been taken
from the following sources: for all of the galaxies which are smaller than the
detector sizes (i.e. galaxies with angular extents smaller than about 6') the
data have been taken from the IRAS Point Source Catalogue (PSC). For those
galaxies which have sizes between about 6' and 8' the data have been taken from
the Small Scale Structure Catalogue, while galaxies larger than that have been
analyzed in the IRAS maps of the HCON1 survey. The blue magnitudes of the NSG
sample have been adopted from the list of Gioia and Gregorini (19801.
We have thus complete radio and optical data for the two samples
investigated here. FIR data (and spectral indices in case of BCDGs) are
available for only about 60_ of the galaxies.
3. INTEGRATED RADIO AND FIR PROPERTIES
In Figure la-d we present all of the properties relevant to the current
analysis for NSGs (upper row) and BCDGs (lower row). Some of the properties have
already been derived and discussed previously: it is known that the radio index
(ratio of radio-to-optical luminosity) of BCDGs is significantly higher than
that of NSGs, and that the average radio spectrum of BCDGs is much flatter than
that of NSGs (see Klein et al., 1984). Here we have supplemented the previously
existing radio data with the more recent measurements mentioned in Section 2.
High-resolution observations with the VLA (see e.g., Brinks and Klein, 1986) at
Z20 cm and ).6 cm revealed that some of the single-dish fluxes were confused by
nearby unrelated sources. The updated distributions of the radio index at Z6.3
s
cm, R6.3B , and the spectral index, a , are plotted in Figures la and d.
Hyphens indicate spectral index uncertainties in excess of 0.2. Figures lb and c
display those histograms which incorporate the FIR data from the PSC. The ratio
Bn d S 6.3dex ((mB-12.51/2.51 = 0.044 S6.3/S B, where S6. 3 and S B are inSv v a
584
THERADIO-FAR-INFRAREDCORRELATION
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R6.3FIR of radio-to-FIR luminosity is shown in Figure lb, the dust temperature
in Figure lc. The latter was derived from the ratio of the 100_-to-60_ flux
densities and by assuming an F v ~ v 1-5 • Bv(T d) dust emission law. The ratio
R6.3FIR was derived by computing the FIR luminosity using the 100_ and 60_ flux
densities which reflect the bulk of the FIR luminosity and following the
procedure of tlelou et al. (1985). This ratio is then obtained as
log R6.3FIR =-15.10 + log $6. 3 -log(2.58 $60 + $100),
where $6. 3 is in mJy and $60 and $100 are in _y. All of the mean values and
standard deviations given in each figure have been obtained by properly
weighting the individual data.
Let us first turn to the spectral indices, obtained by applying least-
squares fits to the data. As discussed by Gioia et al. (1982) the distribution
of a for NSGs is extremely narrow indicating that both the nonthermal spectral
indices (a n ) as well as the thermal/nonthermal ratios (ftnt(V)) do not varyt .
among NSGs. Thss certainly does not hold for BCDGs for which (in spite of the
large uncertainties still inherent to the data) the spectral indices show a
large scatter around a mean value, which as stated already by Klein et al.
(1984) suggests significantly flatter spectra for these than for NSGs. The
strong 'thermal' wing in Figure ld near a = 0 indicates that thermal emission
plays a major role in this galaxy type, but the spread of a down to about -0.8
implies that probably all combinations of ant and ftnt(V) may occur, in
particular nonthermal spectra which may be flatter than those derived for NSGs.
There is also a pronounced difference between the distributions of dust
temperature of NSGs and BCDGs: BCDGs appear to host much warmer dust on average
than normal spirals do. The same conclusion had already been reached by Helou
(1986) who compared samples of BCDGs and Virgo spirals. Another interpretation
would be a lack of the cooler dust component in BCDGs which according to Cox et
al. (1986) delivers about 40% of the total luminosity of the dust emission in
the Milky Way. This cooler dust component, with a temperature range of Tc ~ 15-
25 K (note that Cox et al. used an Fv ~ v 2 • Bv(T d) law), is associated with
atomic hydrogen and heated by the general interstellar radiation field (ISRF),
while the warm dust component has T c ~ 30 - 40 K and is associated with ionized
gas in extended low-density (ELD) HII regions and heated by Oil stars. According
to Figure Ic this latter component contributes a much larger fraction to the
total FIR emission in BCDGs than in NSGs, which once again emphasizes their
similarity to HII regions, as was first pointed out by Sargent and Searle
(1970). In addition the dust temperatures in BCDGs appear to occupy a wider
range than those of NSGs.
What about the radio continuum - FIR relation? Looking at Figure lb we
immediately realize that there is n_riodifference at all between BCDGs and NSGs,
neither in the mean values nor in their standard deviations! This is a puzzling
result: it means that regardless of what is the predominant mechanism of dust
heating (OB stars in ELD HII regions or the ISRF for the diffuse component) and
regardless of what is the thermal/nonthermal ratio and the energy spectrum of
cosmic ray electrons, the ratio of radio-to-FIR luminosity is universal! Both
(thermal and nonthermal) radio as well as thermal emission from dust must
therefore be enhanced in the same way as the star formation activity increases.
This obviously applies only to the recent star formation history, since
otherwise we would expect the blue light to increase by about the same
586
THERADIO-FAR-INFRAREDCORRELATION
proportion, which according to Figure la it does not.
It is remarkable that IZw18 and IIZw40, the BCDGs with the most extreme
properties (they are particulary metal-poor and exhibit very high gas contents,
IIZw40 has the highest radio index and the highest dust temperature) are well
within the distribution of radio-to-FIR luminosity.
4. CONCLUSIONS:
It is puzzling that the two emission processes ((thermal and nonthermal)
radio and FIR) which arise from very different mechanisms apparently follow the
same relation and probably increase at the same rate as the star formation rate
increases. The predominant dust heating process apparently has no influence on
the total amount of FIR emission, and neither has the thermal/nonthermal ratio
and the nonthermal radio spectrum on the total amount of radio radiation.
The lack of scatter in the radio-to-FIR ratios of BCDGs suggests that (at
least in this galaxy type) the (thermal and nonthermal) radio emission must be
connected with the most recent star formation as already stated by de _ong et
al. (1985) because the - undoubtedly rapidly varying - star formation history in
BCDGs would otherwise have resulted in a variation of this ratio. Hence this
ratio is not very suitable for studying variations in recent star formation
histories. The situation must be different for NSGs: under normal conditions
(i.e., in the absence of tidal interactions, strong bars, etc.) their star
formation rates did not vary over the last ~ 10 9 yrs. (Larson and Tinsley,
1978). Therefore the lack of scatter in the radio-to-FIR luminosity ratio of
NSGs was to be expected. The larger scatter in the ratio of radio-to-optical
luminosity for NSGs can be explained in terms of varying star formation rates
before that epoch, since the blue light is about equally shared by stars younger
and older than that. Consistent with this, the scatter in the radio-to-optical
luminosity ratio of BCDGs is somewhat larger than that of NSGs because in these
the relative contribution of young stars to the blue light is much higher than
in NSGs so that the scatter of R6.3B partly reflects the violent recent star
formation epoch.
We can finally rule out any obvious underabundance of dust in BCDGs as
compared to NSGs because we would otherwise expect lower values of R6.3FIR for
the former, or at least a wing of lower values. Normal amounts of dust in BCDGs
would then contradict the view of them being 'young' galaxies (in the sense that
most of them are undergoing their first burst of star formation).
References:
Brinks, E., Klein, U.: 1986, in "Star-forming Dwarf Galaxies and Related
Objects". eds. D. Kunth, T.X. Thuan, and T.X. Thuan, and
7. Tran Thanh Van. Edition Frontieres, Paris, p. 281
Cox, P., Krugel, E., Mezger, P.G.: 1986, A and A 15_5_, 380
Gioia, I.M., Gregorini, L.: 1980, A and A Suppl. 41, 329
Gioia, I.M., Gregorini, L., Klein, U.: 1982, A and A 116_, 164
Gondhalekar, P.M., Morgan, D.H., Dopita, M., Ellis, R.S.: 1986 MNRAS, 219, 505
587
U. KLEINANDE.WUNDERLICH
Helou, G.: 1986, in "Star-forming Dwarf Galaxies and Related Objects",
eds. D. Kunth, T.X. Thuan, and J. Tran Thanh Van, Edition Frontiers, Paris,
p. 319
Helou, G., Soifer, B.T., Rowan-Robinson, M.: 1985, Ap.J.Lett. 2__9_8,L7
Hummel, E.: 1986, A and A I_6_0, L4
de Jong, T., Klein, U., Wielebinski, R., Wunderlich, E.: 1985, Aand A147, L6
Klein, U.: 1986, in "Star-forming Dwarf Galaxies and Related Objects",
eds. D. Kunth0 T.X. Thuan, and J. Tran Thanh Van, Edition Frontieres, Paris,
p. 371
Klein, U., Wielebinski, R., Thuan, T.X.: 1984, A and A 141, 241
Kunth, D., Sevre, F.: 1986, in "Star-forming Dwarf Galaxies and Related
Objects".eds. D. Kunth,T.X. Thuan, and $. Tran Thanh Van,
Edition Frontieres, Paris, p. 331
Larson, R.B., Tinsley, B._l.: 1978, Ap.J. 21___99,46
Sanders, D.B., Mirabel, I.F.: 1985, Ap.J. 298, L31
Sargent, W.L., Searle. L.S.: 1970, Ap._. 162, L155
Sramek, R., Weedman, D.W.: 1986, Ap._. 3_02, 640
Thuan, T.X., Martin, G.E.: 1981, Ap._. 247, 823
Wynn-Williams, C.G., Becklin, E.E.: 1986, Ap.J. in press
588


The radio-far infrared correlation: Spiral and blue compact dwarf galaxies opposed

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