The first results are presented of a multiwavelength study of Infrared Astronomy Satellite (IRAS) galaxies with excess radio emission. The sample was selected by cross correlating the IRAS Faint Source Survey, and the Point Source Catalogue with the Texas radio survey. Recent optical (imaging and spectroscopic) and radio (VLA) observations are discussed. These observations will be used to investigate possible connections between radio galaxy activity, star formation and galaxy interactions

Dey, Arjun

Shields, Joseph C.

Vanbreugel, Wil

English

NASA Technical Reports Server

RADIO LOUD FAR-INFRARED GALAXIES 
Arjun Dey 
Astronomy Department, University of California at Berkeley 
Wil van Breugel 
Institute of Geophysics and Planetary Physics, LLNL, Livermore 
Joseph C. Shields 
Astronomy Department, University of California at Berkeley 
Abstract 
We present the first results of a multiwavelength study of IRAS galaxies with excess radio 
emission. The sample was selected by cross-correlating the IRAS Faint Source Survey (for lbl 2 50') 
and the Point Source Catalogue (for 10' < lbl < 50') with the Texas radio survey. Recent optical 
(imaging and spectroscopic) and radio (VLA) observations are discussed. These observations will 
be used to investigate possible connections between radio galaxy activity, star formation and galaxy 
interactions. 
Introduction 
Increasing evidence exists for a possible evolutionary connection between galaxy interactions, 
central starbursts and activity of galaxy nuclei (e.g. Heckman 1987; Norman and Scoville 1988). A 
popular scenario is that tidal interactions between galaxies trigger a central starburst in one of them. 
This starburst then evolves into a black hole and/or increases its fuelling rate, resulting in enhanced 
activity of the galaxy nucleus. To a large extent such models have been motivated by studies of 
galaxies discovered by the ZRAS survey. 
Optical imaging of ultraluminous far-infrared (FIR) galaxies has shown that many are 
interacting with nearby companions. Interacting systems are also frequently found associated with 
Seyferts, powerful radio galaxies and quasars. These galaxies are often FIR emitters and can be 
selected from IRAS catalogs by their 'warm" colors. Large populations of relatively young stars, 
inferred from stellar absorption lines, exist in many ultraluminous FIR galaxies (e.g. Armus et 
al. 1990). Similar evidence for the other classes of galaxies ia more difficult to obtain, but some 
good examples are known such as the radio galaxy 3C 459 (Miller 1981), and the quasar 3C 48 
(Boroson and Oke 1982; Neugebauer et al. 1982). The interpretation of emission-line data is more 
ambiguous and different sources of ionization (H I1 regions, LINERS and AGNs) might contribute 
(Armus et al. 1990). Bulk outflow, either in the form of hot winds or collimated jets, occurs in all 
above mentioned classes of objects. Taken together one may think that there is some observational 
support for a "grand unified theory of active galaxies", but better evidence is needed. In order to 
study various FIR/radio/optical relationships we have defined a sample of sources from the IRAS 
Point Source Catalog (PSC) and Faint Source Survey (FSS). We have not used any additional IR 
color, luminosity or optical selection criteria. 
Sample Selection and Observations 
Our radio sources were taken from the Texas catalogue at 365 MHz (Douglas et al. 1980). 
This catalogue is - 90% complete to a flux density limit of 250 mJy for sources of angular sizes 5 1 
arcminute , and the version that we used covered the region -35' < S < 40°, lbl > 10'. Sources 
from this catalogue were compared with the I U S  FSS (for lbl > 50'; an overlap of - 1.9 steradians) 
and the lRAS PSC (for lb] 50'; an overlap of - 4.7 steradians) for 60p sources with positional 
309 
https://ntrs.nasa.gov/search.jsp?R=19910007589 2020-03-19T19:50:41+00:00Z
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r 2  
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0 
-0.5 
-1.0 
-1.5 
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3C273 3C48 
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3C446 
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3c345 
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36273 
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Texas-IRAS sources 
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163 
A Seyfert 2 
Seyfert 1 
0 Quasar 
A unknown 
AArp 220 
A 
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310 
coincidence within 60". This resulted in 67 sources from the PSC and 32 sources from the FSS. The 
Texas radio source positions are accurate to about l", allowing optical identifications to  be found on 
Palomar Sky Survey prints. A total of 60 IRAS/radio/optical identifications were found, of which 34 
had log[F60P(Jy)] < 0.5. The remaining 26 objects are mostly well studied nearby bright galaxies. 
All sources with log[F6oP(Jy)] < 0.5 were mapped at 21 cm using the VLA in the A-array 
configuration. We obtained optical images of these sources at Lick Observatory using the l-metre 
Nickel and 3-metre Shane telescopes. Optical spectra were also obtained at a resolution of N 15A. 
Preliminary Results 
Figure la shows the 365 MHz flux density (from the Texas catalogue) plotted against the 60@ 
flux density (from IRAS ) for the sources in our sample. Figure lb shows the same plot, but using the 
6 cm flux densities for the sources as listed in the catalogue constructed by Becker et al. 1990 from the 
Condon et al. (1989) 6 cm Green Bank survey, and the Parkes catalogue. The diagonal lines in Figure 
la indicate where the 6 cm - 60p correlation for spiral galaxies from de Jong et id. (1985) would fall 
when shifted to 365 MHz using a spectral index of 0.7 (lower dashed line), 1.0 or 1.3 (S, cc Y-*). All 
the objects with log~o[s60,(Jy)] < 0.5 lie above this correlation; we refer to these as 'radio loud". 
We also note that the actual 365 MHz flux densities for log10[s~0,(Jy)) > 0.5 galaxies are 
considerably below their predicted values using a typical (high frequency) spectral index of 0.7 or 
larger. This effect is clearly demonstrated by comparing Figure la with Figure lb. The diagonal line 
in Figure lb shows the de Jong et al. 6 cm - 60p correlation which, as expected, agrees with the 
IR-bright galaxies sources in our sample. Many of these sources are well studied galaxies (Arp 220, 
NGC 253, NGC 1052, NGC 1068, etc.). In the case of Arp 220 a possible explanation for the flattening 
of the radio spectrum at lower frequencies is free-free absorption by dense gas (Sopp and Alexander 
1989). This may also be the case for the other IR-bright galaxies. Further radio spectral information 
of the sources in our sample will be needed to investigate the generality of this phenomenon. 
Figure 2 shows an IRAS color-color plot for the sources in our sample. The dashed lines 
outline the regions enclosing the mean (A one standard deviation) spectral indices for normal spirals, 
starbursts, and Seyfert 2 galaxies (Sekiguchi 1987). Vertical and horizontal arrows indicate that only 
upper limits to the 25p and 1OOp flux densities were available. Although our sample was not selected 
on the basis of IR colors, most of our sources exhibit IR colors typical of active galaxies. 
We have also examined the radio - IR (60~) luminosity distribution of our sample (not shown). 
Our sample has a high median IR luminosity (Leoa - 4 x 1O1'L0, H, = 5Okm/s/Mpc), quite different 
from the L60p distribution of the Bright Galaxy Sample (Soifer et al. 1989), but more similar to 
that of Seyferts and quasars (Neugebauer et al. 1986). Sanders et al. (1988) have suggested that 
IR-ultraluminous galaxies like Arp 220 might be powered by quasar-like nuclei. We find very little 
overlap between our radio-loud sample and that of Sanders et al. Only Arp 220 is a strong enough 
radio emitter to be common to both samples over the appropriate area of sky. A more detailed analysis 
of the IR colors and optical spectra of our radio-loud FIR galaxies is needed to see whether there are 
other differing properties. 
Of the radio loud FIR galaxies, 40% have flat radio spectra ((I! 5 0.5) and very compact radio 
cores. Of the remaining 60% with relatively steep spectra, 80% have their dominant radio emission 
on small angular scales ( 5 5"). These sources are entirely embedded within their parent galaxies 
and are ideal sources in which to study the interaction between radio sources and their surrounding 
interstellar medium. One example, Arp 187, is shown in Figure 3 which includes an R-band image 
(3a), 21 cm VLA map (3b) and optical spectrum (3c). Note that HB appears in absorption, indicating 
the possible presence of a large young stellar population in the galaxy. 
31 I 
Seyfert a A 
I I 
I A -  
0 HI1 
I 
-2 - A  LINER 
n 
O A  I A  
I 
- - - -  A Seyfert 2 
B Seyfert 1 I Spiral 1 - 
@ Quasar 
A unknown 
I 
I 
- A o  
-A 0 AA 0 N520 - - - -  
AArp 220 -3 - A - 
Of the galaxies in our radio-loud sample, 7 are LINERS, 3 are Seyfert 1 galaxies, 10 are Seyfert 
2 or radio galaxies, and 7 are quasars. Thus our sample encompasses a large variety of galaxy activity, 
More detailed studies, and increasing the sample size, may help us better understand possible links 
between these classes of objects and starburst activity. 
References 
Armus, L. Heckman, T. M. and Miley, G. K., 1990, preprint 
Becker, R. H., White, R. L. and Edwards, A. L., 1990, preprint 
Boroson, T. A., and Oke, J. B., 1982, Nature, 296,397. 
Condon, J. J., Broderick, J. J. and Seielstad, G. A., 1989,Astron. J., 97,1064. 
de Jong, T., Klein, U., Wielebinski, R., and Wunderlich, E., 1985,Astron. Astrophys., 147,L6 
Douglas, J. N., Bash, F. N., Torrence, G. W., and Wolfe, C., 1980,Univ. Texas Publ. Astron. no.17 
Heckman, T. M. , 1987,in Starbursts and Galazy Evolution, T. X. Thuan, T. Montmerle, J. T. T. Van, 
Miller, J. S. 198l,Pub.A.S.P,93, 681. 
Neugebauer, G., Soifer, B. T., and Miley, G. K., 1982,Astrophys. J. Lett., 295,L27. 
Neugebauer, G., Miley, G. K., Soifer, B. T. and Clegg, P. E., 1986,Astrophys. J., 308,815 
Norman, C. and Scoville, N. Z., 1988, Astrophys. J., 332, 124. 
Sanders, D. B., Soifer, B. T., Elias, J. H., Madore, B. F., Matthews, K., Neugebauer, G., and 
Sekiguchi, K., 1987, Astrophys. J., 316,145 
Soifer, B. T., Boehmer, L., Neugebauer, G., and Sanders, D. B., 1989,Astron. J., 98,766 
Sopp, H. M. and Alexander, P., 1989, Astrophys. Space Sc., 157,287 
Eds. (Gif sur Yvette: Editions Frontieres) 
Scoville, N. Z., 1988, Astrophys. J., 325,74 
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DISCUSSION 
morris: What is t h e  d i s t r ibu t ion  of far-IR color i n  your radio- 
loud sample? I t  would be very interest ing t o  see i f  your sample 
consists only of warm ( S e y f e r t - l i k e )  objects,  or whether cooler, 
s ta rburs t  galaxies could also be radio-loud. 
Dey: So f a r  6 out of 39 galaxies have HII-region type spectra.  
The colors of our sample sca t t e r  over a large range, b u t  - on 
average are warmer than any f l u x - l i m i t e d  IRAS sample. There a r e  
some objects whose I R A S  colors are similar to those of 
s ta rburs t s .  
314 


Radio loud far-infrared galaxies

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