It was found that among an IR luminous subset of nearby spiral galaxies, nearly all of the systems with IRAS colors and luminosities indicative of enhanced star formation are barred. Radio continuum and IR spectroscopic results support the hypothesis that this emission originates within the central 2 kpc; possibly in a circumnuclear ring. It was also found that outer rings are over represented among these barred systems and suggest possible reasons for this phenomena

Hawarden, T. G.

Leggett, S. K.

Mountain, C. M.

Puxley, P. J.

English

NASA Technical Reports Server

N87- 24357
ENHANCED STAR FORMATION - THE IMPORTANCE OF BARS IN
SPIRAL GALAXIES
P. J. Puxley 1 T. G. Hawarden 2, C. M. Mountain 2 and
S. K. Leggett 1
1Department of Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ.
2Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ.
1. ABSTRACT
We have found that amongst an IR-luminous subset of nearby spiral galaxies, nearly
all of the systems with IRAS colours and luminosities indicative of enhanced star
formation are barred. Radio continuum and IR-spectroscopic results support the hypothesis
that this emission originates within the central 2kpc; possibly in a circumnuclear ring.
We also find that outer rings are over-represented amongst these barred systems and
suggest possible reasons for this phenomena.
Our recent investigation (Hawarden et al 1986) of the IRAS database for a large
sample of 186 spiral galaxies indicates that nearly all of the systems with excess 25#m
flux are barred. The sample includes all of the spirals in the Revised Shapley Ames
Catalogue (Sandage & Tammann 1981, 'RSA') between SO/a and Scd (inclusive) and which
were detected by IRAS in all four bands. We have excluded from our main sample those
galaxies which are extended at 100#m or with Seyfert 1,2 or LINER spectra.
Figs. l(a), (b) and (c) show the distributions of the ratios fv25/fv12 vs fv100/fv25,
for barred (SB), unbarred (SA) and mixed-type (SAB) galaxies in our sample.
Approximately one third of all the barred/mixed systems have large 25#m/12#m and
relatively small 100#m/25#m ratios which are indicative of emission from the warm dust
commonly found in starforming regions. Furthermore, these "25/_m-excess" barred galaxies
(hereafter referred to by using the prefix 'h') are considerably more luminous (typically
4.5 x 1010 Lo, with Ho = 75 kms -1 Mpc -1) than the other barred and unbarred systems
(hereafter prefixed by '1') which have typical IR luminosities of 2 x 1010 Lo.
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Figure l(a). (b) & (c)
The distribution of SB, SAB and SA galaxies in the IRAS 2-colour diagram,
fv25/fv12 against fv100/fv25.
Carol J. Lonsdale Persson (Editor)
Star Formation in Galaxies 619
PRECEDII_ PAGE BLANK NOT FILMED
https://ntrs.nasa.gov/search.jsp?R=19870014924 2020-03-20T11:06:44+00:00Z
P. J. PUXLEY ET AL. "
To support our conjecture that the extra IR emission is due to enhanced star
formation we have constructed simple models using the average IRAS fluxes of a group of
unresolved HII regions in Cygnus. Addition of about 10 _ such objects to a mean '1'
galaxy accurately reproduces the spectral shape and luminosity of an 'h' system. Only the
12#m emission is underestimated but our models so far omit the extended 12/zm flux
which is seen throughout the Cygnus region and which has been attributed to emission
from small grains (eg Wynn-Williams & Becklin 1985).
The resolution provided by IRAS was too coarse (1-2 arcmin) to enable us to
determine the exact location of the source of the extra IR emission in our barred galaxies.
However, recent radio continuum results (Puxley et al, in preparation), together with those
of Hummel (1980), show that all of the barred galaxies with enhanced star formation are
centrally concentrated (less than 20 arcsec) at 20cm. The distribution of radio properties
as a function of morphology is shown in Table I. We have adopted the classification
scheme of Hummel such mat 'C' refers to galaxies with the unresolved central component,
'E' to those with extended (ie disk) emission and 'ND' refers to non-detections. Less than
one third of T spirals have centrally concentrated radio emission.
Table 1
Distribution of radio classifications with morphological type
Galaxy type C or CE E ND
hSB & hSAB 20 (100%) 0 (_0%) 0 (_/_)
1SB & 1SAB 14 (45%) 13 (42%) 4 (13%)
1SA 11 (26%) 19 (44%) 13 (30%)
We have also obtained 2.1/zm-2.2#m spectra of a small number of galaxies in our
sample. The aperture (20 arcsec) was chosen so as to include any nuclear ring.
Preliminary reduction of the spectra indicates that Brackett gamma was detected in the
hSAB galaxies NGC4536 and in NGC3310 (whose positions are shown in Fig.l) but not in
the 1SA system NGC4736. As NGC4536 and NGC4736 have similar continuum flux
densities N3.5 Jy at 25#m) we interpret the Brackett gamma line emission in NGC4536 as
arising from star formation which dominates the IRAS fluxes of this galaxy.
The positions of the two well-studied systems NGC1097 and NGC3310 are also
indicated in Figs. l(a) and l(b). Both of these galaxies have been mapped at 10ttm by
Telesco and Gatley (1981, 1984) who find that the extra IR emission originates in a ring
(diameter about 2kpc) of HII regions around the nucleus. Observations of HI (Sancisi,
Allen & Sullivan 1979) and optical spectroscopy (Pence & Blackman 1984) implies that, in
barred systems, there is an inflow of gas/dust due to the non-axisymmetric bar potential.
Numerical models (eg Combes & Gerin 1985) suggest that the "circumnuclear" rings are
situated _.ar one of the inner Lindbladresonances where material swept inwards would be
expected to accumulate. In addition, Fig.l(a) exhibits a noticeable "split" at fv25/fv12 -2
which we speculate arises due to 'switching-on', of the enhanced star formation once some
critical density of the ISM is exceeded.
We have also found a strong correlation (see Table 2) between the occurrence of
outer rings (classified R.. or P.. in RSA) and enhanced star formation for our complete
sample (ie including 33 Seyfert's and LINER's) of 219 galaxies. Table 2 shows that,
6:0 q -
THE IMPORTANCE OF BARS
Table 2 Incidence of outer rings with morphological type
Galaxy type No. of galaxies No. with
in sample outer rings
SB/SAB with vigorous 59 9 (15%)
star formation
SB/SAB without vigorous
star formation
72 1 (1%)
SA with vigorous star
formation
2 0 (&,_)
SA without vigorous
star formation
86 lO (12%)
amongst barred systems, those with outer rings also have IRAS colours which we have
attributed to a vigorous burst of star formation. Numerical models of barred spirals (eg
Schwarz 1984) indicate that outer rings form near the outer Lindblad resonance due to the
outward transport of disk material by a spiral density wave exterior to corotation. The
existence of outer rings in these systems suggests that there is much interstellar material
available for transport. Interior to corotation, any gas and dust may be fed inwards
towards the centre by the bar-like density wave where it can form a circumnuclear ring.
Table 2 also shows that outer rings are also found in unbarred galaxies and we expect a
similar process to occur in these systems (with the inflow/outflow being driven by the
spiral density wave) except that the lack of a bar may result in a less efficient flow, and
hence we do not see vigorous star formation. This model is similar to that proposed by
Simkin, Su & Schwarz (1980) who found that outer rings are a feature of many Seyfert
galaxies.
In Fig. 2 we show the 2-colour diagram for the Seyfert galaxies which exhibit a
remarkably similar distribution to that of the normal spirals in Fig. 1. In agreement with
our assertion what the distribution in Fig. 1 is the result of circumnuclear star formation,
Rodriguez Espinosa, Rudy & Jones (1986) find that the mid- and far-IR emission from
Seyferts is extended and suggest that this is due to a vigorous burst of star formation
around the active nucleus.
Figure 2
As for Fig. 1 but showing the
galaxies whose spectra show
Seyfert 1,2 or LINER features.
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621
P. J. PUXLEY ET AL.
REFERENCES
Combes, F. & Gerin, M., 1985. Astr. Astrophys., 150, 327.
de Vaucouleurs, G., de Vaucouleurs, A. & Corwin, H.C., 1976. Second Reference
Catalogue of Bright Galaxies, University of Texas Press, Austin.
Hawarden, T.G., Mountain, C.M., Leggett, S.K. & Puxley, P.J., 1986.
Mort. Not. R. Astr. Soc., 221, 41P.
Hummel, E. 1980. Astr. Astrophys. Suppl. Ser., 41, 151.
Pence, W.D. & Blackman, C.P., 1984. Mon. Not. R. astr. Soc., 210, 547.
Rodriguez Espinosa, J.M., Rudy, R.J. & Jones, B., 1986. Preprint.
Sancisi, R., Allen, R.J. & Sullivan, W.T., 1979. Astr. Astrophys., 78, 217.
Sandage, A. & Tammann, G.A., 1981. A Revised Shapley Ames Catalogue of Bright
Galaxies, Carnegie Institution of Washington Publication No. 635.
Schwarz, M.P., 1984. Mort. Not. R. astr. Soc., 209, 93.
Simkin, S.M., Su, H.J. & Schwarz, M.P., 1980. Astrophys. J., 237, 404.
Telesco, C.M. & Gatley, I., 1981. Astrophys. J., 247, Lll.
Telesco, C.M. & Gatley, I., 1984. Astrophys. J., 284, 557.
Wynn-Williams, C.G. & Becklin, E.E., 1985. Astrophys. J., 290, 108.
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Enhanced star formation: The importance of bars in spiral galaxies

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