Preliminary results are presented from a radio survey of galaxies detected by the IRAS minisurvey. It was found that the main difference between galaxies selected in the far infrared and those selected in the optical is that the former have higher radio luminosities and that the radio emission is more centrally concentrated. There is some evidence that the strong central radio sources in the galaxies selected in the infrared are due to star formation, the star formation rate divided by the volume in which the star formation is occuring is 100 to 1000 times greater in the galaxies selected in the infrared than in the disks of normal galaxies

Beichman, C. A.

Eagles, S. A.

Wynn-Williams, C. G.

English

NASA Technical Reports Server

N87- 24342
VLA OBSERVATIONS OF A SAMPLE OF GALAXIES WITH HIGH FAR-INFRARED LUMINOSITIES
S. A. Eales and C. G. Wynn-Willlams
University of Hawaii, Institute for Astronomy
Honolulu, Hawaii 96822 USA
C. A. Beichman
California Institute of Technology
Pasadena, California 91125 USA
ABSTRACT. We present preliminary results from a radio survey of galaxies
detected by the IRAS mlnlsurvey. We find that the main difference between
galaxies selected in the far-lnfrared and those selected in the optical is that
the former have higher radio luminosities and that the radio emission is more
centrally concentrated. There is some evidence that the strong central radio
sources in the galaxies selected in the infrared are due to star formation
rather than to active nuclei. If the radio emission is caused by star forma-
tion, the star formation rate divided by the volume in which the star formation
is occurring is I00-I000 times greater in the galaxies selected in the infrared
than in the disks of normal galaxies.
INTRODUCTION
We present preliminary results from a VLA survey of the galaxies detected
by the IRAS mlnlsurvey. The sample consists of the galaxies in the area of the
IRAS minlsurvey with Mpg _ 18 and S60_m _ 0.5 Jy (Solfer et al. 1984), together
with the mlnlsurvey sources that did not have optical counterparts on the
Palomar Sky Survey (Houck et al. 1984) but that were later found to be galax-
ies (Aaronson and Olszewskl 1984, Houck et al. 1985). Eighty-elght of the 92
galaxies in the sample have redshlfts, measured either at Mauna Kea
Observatory or at Palomar (Lonsdale, private communication). Because it was
selected in the infrared, the sample is biased towards galaxies with high far-
infrared luminosities; 76% of the galaxies have Lfl r > i0 I0 Le, compared with
5% of the spirals in an optical catalogue (Devereux, private communication).
We made 10-mlnute 'snapshot' observations of all the sources at 5 GHz with
either the B- or C-array of the VLA, or with both. The data were reduced in a
standard way, with extensive use being made of the CLEAN algorithm of Hogbom
(1974). To obtain structural information on the many sources that were just
resolved by the VLA, we found the two-dimensional Gausslan brightness distribu-
tions that best represented the sources. This procedure gave us three struc-
tural parameters for each source: the distances between the half-lntenslty
points along the major and minor axes and the position angle of the major axis.
Flux densities were obtained either from the Gausslan fits or from integration
over the maps in the vicinity of the significant components. Typical noise on
the final maps was 0.2 mJy beam -I.
Carol J. Lonsdale Persson (Editor)
Star Formation in Galaxies 531
https://ntrs.nasa.gov/search.jsp?R=19870014909 2020-03-20T11:07:14+00:00Z
S.A.EALES ET AL.
RESULTS
Of the 92 objects in the sample, 57 were detected by the VLA. An addi-
tional I0 were detected at 5 GHz with the Effelsberg telescope by the Bonn
group (Klein, private communication). Eighteen of the 57 VLA detections were
unresolved. There is one double-lobed radio source (0421+040; Belchman et al.
1985), and three of the pairs of interacting galaxies in the sample have radio
sources coincident with both galaxies.
Flg. 1 shows an unrepresentative collection of radio maps; most typical is
0402+212, an undistinguished blab of emission at the position of the IRAS
source.
21 14 55
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Figure i. VLA maps of the IRAS sources 0402+212, 0409+054, and 1752+329.
Negative contours are dashed, positive contours are solid lines, and the zero-
level contour has been omitted. For the first two sources, the contour interval
is 0.2 mJy beam-l; for 1752+329, the contour interval is 7 mJy beam -I. The
ellipse in the bottom right-hand corner of each map shows the shape and size
(FWHM) of the telescope beam. The 0409+054 source has a large physical size,
_7 kpc in diameter. 1752+329 is a palr of interacting galaxies. The radio
peaks are coincident wlth the two galaxies.
532
VLAOBSERVATIONFLUMINOUSGALAXIES
DISCUSSION
Fig. 2 shows the P-D (radio luminoslty-physical size) diagram for the
minisurvey radio sources. The angular sizes used are mostly (exSy)I/2, where ex
and 8y are the distances between the half-intensity points along the major and
minor axes of the best-flttlng Gaussian brightness distribution. For sources
with complex radio structures, angular sizes were measured off the maps between
the lowest believable contours. The diagonal lines are lines of constant sur-
face brightness. The undetected galaxies may have remained undetected because
their radio luminosities are generally lower than those of the galaxies that
were detected, or they may have similar radio luminosities but lower surface
brightnesses and so fall on the P-D diagram to the right of the limiting sur-
face brightness of the VLA--roughly the upper diagonal llne in Fig. 2. The
sizes of the sources range from 0.4 to 35 kpc, but most have sizes that bunch
around the median, ~2 kpc.
How do the radio properties of these galaxies compare with those of galax-
ies selected from optical catalogues? Hummel (1980, 1981) used the Westerbork
telescope to observe a large number of galaxies in the Reference Catalogue of
Bright Galaxies. In Fig. 2 the horizontal lines show the luminosities above
which 1% and 10% of Hummel's galaxies lle. The minisurvey radio sources are
significantly more luminous (transforming between 5 GHz and 1.4 GHz, the fre-
quency used by Hummel, by assuming _ = 0.7). The difference between the radio
properties of infrared- and optlcally-selected galaxies becomes more marked
when one considers that the radio emission from a typical minisurvey galaxy is
coming from a much smaller physical region than the emission from a typical
galaxy in an optical catalogue.
Another way to look at the difference between the radio properties of
optically- and infrared-selected galaxies is to compare surface brightnesses;
the minisurvey sources have surface brightnesses similar to those of the cen-
tral sources in some well-known nearby galaxies (Fig. 2) (and much higher sur-
face brightnesses than the disks of those galaxies) but higher luminosities and
larger physical sizes. In the nine cases where we have accurate astrometry, to
within the position errors (1-2 arcsec), the radio and optical centroids coin-
cide, showing that the minisurvey radio sources occur in the centre of the
galaxies.
What is causing the intense radio emission from the central regions of the
minlsurvey galaxies? Two possibilities, considering the high far-infrared
luminosities, are star formation or active nuclei. The data are not _ood
enough to reach a definite conclusion, but there are two arguments that suggest
that the radio emission is caused by star formation.
The median physical size of a sample of Seyferts in which the radio emis-
sion is probably caused by collimated outflow from an active nucleus is only
one-sixth that of the minisurvey galaxies (Ulvestad and Wilson 1984), which
suggests that something different in the minisurvey galaxies is causing the
radio emission. The second argument is that if the radio emission is caused by
plasma beams from an active nucleus, the radio sources should be long and thin;
which is not the case, as is shown in Fig. 3. There we plot for the minisurvey
sources a histogram of eccentricity, defined as the distance between the half-
intensity points along the major axis divided by the same distance along the
533
S.A. EALES ET AL.
10:;3
%
I0 z,
"1-
I0 Is
C
"_ 0 =r
- I
I I + I I/
/
÷+ +
/ v
The / Disks
M8, n
Goloxy / /
-
I /'_ I I
0.1 I I0 I00
size/kpc
Figure 2. The lumlnosity-physical size diagram for the minisurvey sources. A
Hubble constant of 75 km s-I Mpc -I has been assumed. A indicates an upper limit
to the physical size. The squares show the positions of the central sources in
some well-known nearby galaxies (Ekers 1975). The diagonal lines are lines of
constant surface brightness. Galaxy disks have surface brightnesses that lle
between these lines. The horizontal lines show the luminosities above which i%
and 10% of Hummel's galaxies lie.
minor axis. The median eccentricity is not much larger than the median
expected from the effect of projection, if the sources consisted of thin circu-
lar disks.
A measure of the intensity of a starburst is the star formation rate
divided by the volume in which the star formation is occurring, call this e. If
the radio emission is caused indirectly by star formation and directly by
supernova remnants, e = radio surface brightness and is i00-I000 times greater
than in the disks of normal galaxies.
Although star formation may be the cause of the radio emission in most of
the mlnisurvey galaxies, there are a few galaxies in which the data point to an
active nucleus. M82 is at once the prototype starburst and also the prototype
534
VLAOBSERVATIONOFLUMINOUSGALAXIES
t_
e'-
tD
2D
1:7'
tlm
B m
6- 7
4- i
iiiiiiiiii!
z-i
i!_iiiiiiii
iiiiiii_i_i
o
m
_!iiiiiiilp m
i iiiii l lii:iiii
l
L
1.0 2.0 3.0 4.0
eccentricity
Figure 3. A histogram of eccentricity for the mlnlsurvey galaxies. The hatch-
ing indicates lower limits. The median eccentricity is shown for those sources
that have exact measurements. For comparison, the median expected from the
effect of projection if the sources were just thin circular disks is 1.3.
exploding galaxy (Lynds and Sandage 1963, Bland and Tully, in preparation).
Chevalier and Clegg (1985) have suggested that this is not a coincidence: that
the explosion is caused by the large number of supernovae occurring in the
starburst at the centre of the galaxy. We speculate that for a starburst,
cannot exceed its value for M82, otherwise an explosion occurs. Three minlsur-
vey sources have, however, higher values of _, so these may be active nuclei
rather than starbursts. One mlnlsurvey source is certainly caused by outflow
from an active nucleus--the double-lobed radio source 0421+040 (Belchman et al.
1985).
535
S.A. EALES ET AL.
ACKNOWLEDGMENTS
We thank Dr. U. Klein and Dr. C. Lonsdale for communicating data prior to
publication. We thank Drs. J. Heasley and E. Becklin and Mr. G. Hill for
measuring redshifts for the minisurvey galaxies. This work was supported by NSF
grant AST 84-18197. The National Radio Astronomy Observatory is operated by
Associated Universities, Inc., under contract with the National Science
Foundation.
REFERENCES
Aaronson, M., and Olszewski, E. W. 1984, Nature, 309, 414.
Beichman, C., et al. 1985, Ap. J., 293, 148.
Chevalier, R. A., and Clegg, A. W. 1985, Nature, 317, 44.
Ekers, R. D. 1975, in Structure and Evolution of Galaxies, ed. G. Setti
(Dordrecht: Reidel), p. 217.
Hogbom, J.A. 1974, Astr. Ap. Suppl., 15, 417.
Houck, J. R. et al., 1984. Ap. J. (Letters), 278, L63.
Houck, J. R., Schneider, D. P., Danielson, G. E., Beichman, C. A., Lonsdale,
C. J., Neugebauer, G., and Soifer, B. T. 1985, Ap. J. (Letters), 290, L5.
Hummel, E. 1980, Astr. Ap. Suppl., 41, 151.
Hummel, E. 1981, Astr. Ap., 93, 93.
Lynds, C. R., and Sandage, A. 1963, Ap. J., 137, 1005.
Soifer, B. T., et al. 1984, Ap. J. (Letters), 278, LTI.
Ulvestad, J. E., and Wilson, A. S. 1984, Ap. J., 278, 544.
536


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