The authors present charge coupled device (CCD) imaging results of their sample of Infrared Astronomy Satellite (IRAS) galaxies with spectral energy distributions peaking at 60 microns (Vader et al 1988). The results support the author's suggestion that the activity in 60 micron peaking galaxies is centrally concentrated, and represents an early stage of dust-embedded nuclear activity. This activity is probably triggered by a recent interaction/merger event as indicated by their peculiar optical morphologies. The authors propose that 60 micron peakers are the precursors of SO's in the case of amorphous systems, and ellipticals in the case of interacting galaxies

Frogel, Jay A.

Heisler, Charlene A.

Vader, J. P.

English

NASA Technical Reports Server

CENTRAL A c r r v I T y  IN 60 MICRON PEAKERS 
Charlene A. Heislerl and J.P. Vader2 
Yale University, Bept. of Astronomy 
Box 6666, New Haven, CT 065 11 
and 
Jay A. Frogel' 
Ohio State University, Dept. of Astronomy 
174 W. lgth Avenue, Columbus, OH, 43210 
4i!smGt 
We present CCD imaging results of our sample of IRAS galaxies with spectral energy 
distributions peaking at 60pm (Vader et a1 1988). The results support our suggestion that the 
activity in 60p.m peaking galaxies is centrally concentrated, and represents an early stage of dust- 
embedded nuclear activity. This activity is probably triggered by a recent interactiodmerger event 
as indicated by their peculiar optical morphologies. We propose that 60pm peakers are the 
precursors of SO'S in the case of amorphous systems, and ellipticals in the case of interacting 
galaxies. 
The Far-Infrared luminosity of 6Opm Peakers spans a range of four orders of magnitude, with 
a median value of 3x1010 L,. This median value is a factor of 2 greater than the Bushouse (1986) 
interacting sample and 10 greater than the control sample of isolated spirals. (Fig.1) The lowest 
luminosity objects - IIZw40 and NGC5253- are well studied, nearby prototypes of amorphous 
galaxies. The highest luminosity sources are ultraluminous galaxies (Lm > 4x1Ol1 Lo), which 
are dust-enshrouded quasars (Frogel et al. 1989), and presumably are the result of a recent 
merger/interaction event. 
All of the 6Opm Peakm are emission line galaxies that are HII region-like (36%), Seyfert type 
2 (54%), and Seyfert type 1 (10%). A catalog of the spectra is in preparation (Vader et al1990). 
1 Guest Investigator Cerro Tololo Inter-American Observatories, N.O.A.O., operated by 
AURA Inc., under contract with the National Science Foundation 
with National Science Foundation 
Guest Investigator Very Large Array, N.R.A.O., operated by AURA Inc., under contract 
3 93 
https://ntrs.nasa.gov/search.jsp?R=19910007600 2020-03-19T19:49:57+00:00Z
Centrally concentrated activity was shown for all galaxies in our sample for which Ha imaging 
was obtained. The galaxies observed thus far have 80% of the total measured Ha flux contained 
within a region which is -0.8 kpc to 3 kpc. On the other hand we find that in the B broadband 
filters the galaxies are more diffuse - 80% of the blue flux is contained in a region 1.5 to 4 times 
the size of the Ha emission line region (Table I). We compare the Ha  flux measured from our 
CCD images to those determined from our nuclear spectra, and find agreement to within a factor of 
2. In Fig. 2 we plot the H a  flux (corrected for galactic and internal reddening) versus the far- 
infrared flux. For the 6Opm Peakers, the ratio of the FIR to the Ha luminosities is fairly constant, 
and is intermediate between that of optically selected spirals (Persson and Helou 1987), and 
interacting galaxies (Bushouse 1987). The lower values of bha for optically selected spirals 
is due to the absence of a dust obscured central source, while the higher values for the strongly 
interacting sample may be understood in terms of a cirrus component which is insignificant in the 
60pm Peakers. 
We can now use the far-infrared and H a  luminosities to calculate the covering factor of the 
central active source. The expected ratios of the Lyman continuum luminosity, hv ,to Ha 
luminosity depends on the W specttai energy distribution. Values of order 30 are obtained for 
both OB-star clusters (Mezger et al. 1974), and non-thermal sources (cf. Heisler et al. 1989). 
Taking into account that about 60% of the total UV luminosity is directly absorbed by the dust one 
expects L60p=45L,a for optically thin dust, as is observed for normal HII regions. The UV 
luminosity inferred from ha represents a fraction 1-f of the total W luminosity and, given that in 
the optically thick case Lmb=45f/(l-f), we find a typical covering factor of about 80%. 
394 
'able I 
(kpa 
0.87 
3.27 
0.98 
0.85 
3.51 
0.35 
3.24 
1.54 
1.25 
1.31 
1.24 
3.39 
R(Ha)80% 
i " " i " " l *  ' " I ' " ' I " "  
0 open circles - Persson and Helou spiral sample 
crosses - 60 micron Peakers 
filled circles - Bushouse interacting sample e :  - - 
R(B)80% 
(@d 
1.54 
6.94 
3.06 
1.67 
5.12 
1.51 
6.52 
4.97 
2.38 
3.62 
3.77 
6.14 
An estimate of the location of the dust can be 
obtained by assuming that the dust is 
concentrated in a spherical shell of radius r 
centered on the nuclear source, in which case 
For a dust temperature of 50K (inferred from 
the 60 and 1OOpm fluxes for dust with emissivity 
proportional to frequency, and Lap= 1O1O to 10l2 
Lo) this yields radii in the range 0.1 to lkpc, 
which is comparable to the size of the narrow line 
region in seyfert galaxies, and suggests that the 
obscuring dust is close to the nucleus. The close 
proximity of obscuring dust to the central source 
points towards the relative youth of central activity 
in these objects. The 60pm Peakers are generally 
kiloparsec-scale radio sources, and some of them 
show dust obscured seyfert 1 nuclei in polarized light (Miller and Goodrich 1987; Kay and Miller 
1989), which further attests to the youth of the central activity. 
Firmre Two 
FIR Versus H-alpha Fluxes 
-12 
$\ 
f -13 
8 -14 
d 
8 
-15 
X 
e 
fl2.8 X 
e 
a 
X 
a 
X 0  
X 
X 
a 
111 
X 
" N O  
= o  8 
U 
0 
-18 -17 -16 -15 -14 -13 
LOG(H-alpha) W/mA2 
395 
0.3 I 
5 - 
4 0.2 
1 
0.1 
03 0.4 0.8 1.0 02 a4 0.6 1 .o 
The optical morphologies of the 60pm Peakers can be classxied as having a disturbed 
appearance (25%), or as having a smooth light distribution (70%). The remaining 5% of the 6 0 p  
Peakers are normal spirals. The amorphous, So's, and early type galaxies have all been combined 
into a class of objects possesing a smooth light distribution, because the above distinctions are not 
clear cut. For example NGC5253 is a prototype amorphous galaxy, but has also been classified as 
E and SO by others. Nearly half of the amorphous galaxies show evidence for a recent 
interactiodmerger event (shells,rings, dust patches). This classifkation issue can be further 
addressed by examining the ellipticities of the galaxies. 
We have measured the apparent ellipticity of the 60j.m Peakers at an isophotal blue magnitude 
of approximately 22 mag/arcsec2. Ellipticity distributions are shown in Fig. 3. A chi-squared test 
tells us that to a confidence level better than 99%, the whole sample of the 60pm Peakers does not 
have the same ellipticity distribution as either early or late type galaxies (Sandage et al1970), and 
similarly when one considers only the amorphous sytems. Prelhinary results indicate that the 
amorphous galaxies have surface brightness profiles consistent with So's. We suggest that the 
amorphous galaxies in our sample are the precursors of SO galaxies, and, as has been proposed by 
396 
others (eg. Toomre and Toomre 1972), the strongly interacting galaxies are precursors of 
ellipticals, some of which may develop into strong radio sources. 
Concl- 
We have presented evidence that the activity in 6opm Peakers is centrally concentrated and 
heavily obscured at optical wavelengths. We propose that nuclear activity in these galaxies is a 
recent phenomenon, and our data supports numerical simulations that show that an 
interaction/merger event can drive gas to the center of a galaxy in a short time scale (-lo8 yrs), and 
provide fuel for an active nucleus and/or intense centrally concentrated starburst (eg. Hernquist 
private comm., Noguchi 1988). Our results suggest that 6Opm Peakers may be the precursosrs of 
So's in the case of amorphous galaxies, and ellipticals in the case of strongly interacting systems. 
2- JibhaEa 
Bushouse H. (1986), PhD. thesis, University of Illinois 
Bushouse H. (1987), Astrophys. J., 320,49 
Bushouse H.A., Lamb S.A., and Werner M.W., (1988), Astrophys. J.,335,79 
Frogel J.A., Gillett F.C., Terndrup D.M. and Vader J.P., (1989), Astrophys J.,343,672 
Heisler C.A., Vader J.P., and Frogel J.A., (1989), Astron. J.,97,986 
Kay L.E. and Miller J.S., (1989), B.A.A.S.,21,1099 
Mezger P.G., Smith L.F., Churchwell E., (1974), Astron. and Astrophys.,32,269 
Miller J.S. and G d c h  R.W., (1987), B.A.A.S.,19,695 
Noguchi, (1988), Astron. and Astrophys.,201,37 
Persson Lonsdale C.J. and Helou G., (1987), Astrophys. J.,314,1 
Sandage A., Freeman K.C. and Stokes N.R., (1970) Astrophys. J., 160,831 
Toomre A. and Toomre J., (1972), Astrophys. J., 178,623 
Vader J.P., Frogel J.A. and Gillett F.C., (19881, in Active Galactic Nuclei, conference 
proceedings held at Georgia State University, (Springer-Verlag), eds. H.R. Miller and P.J. 
Wiita., p 134 
397 
DISCUSSION 
Kennicutt: 
1. Have you compared t h e  p r o p e r t i e s  o f  your  60 p s e l e c t e d  sample 
to  o t h e r  s u r v e y s  of hot I R A S  g a l a x i e s  based  on 25 p e m i s s i o n ?  
2. Is there any c o n f i r m i n g  e v i d e n c e  from t h e  m o r p h o l o g i e s  or 
k i n e m a t i c s  of  t h e s e  g a l a x i e s  f o r  t h e  i d e a  t h a t  t h e y  are i n  e a r l y  
s t a g e s  of a c t i v i t y ?  
Heisler: 
1. The f r a c t i o n s  of S e y f e r t  and s t a r b u r s t  g a l a x i e s  i n  
60 pm p e a k e r s  are f a i r l y  t y p i c a l  f o r  I R A S  g a l a x i e s  w i t h  w a r m  
60 pm/25 pm c o l o u r s .  
2. W e  do  n o t  have k i n e m a t i c a l  data .  The morphology of t h e  
o b j e c t s  i n d i c a t e  a r e c e n t  i n t e r a c t i o n .  The e v i d e n c e  f o r  a n  e a r l y  
s t a g e  of a c t i v i t y  is t h e  c e n t r a l l y  c o n c e n t r a t e d  a c t i v i t y ,  compact 
c e n t r a l  radio s o u r c e  ( -  kpc s c a l e ) ,  and a dus t - ensh rouded  c e n t r a l  
s o u r c e  which is s t i l l  close t o  t h e  n u c l e u s  and t h u s  f a i r l y  w a r m  
and p r o d u c i n g  t h e  w a r m  F I R  colours. 
Sulentic: One g r o u p  of  60 pm p e a k e r s  is t h e  amorphous class of  
g a l a x i e s .  W e  have o b t a i n e d  spectra of  a n o t h e r  member of  t h i s  
c lass  NGC 2777, and f i n d  it t o  have  a r emarkab ly  e a r l y  (A  t y p e )  
a b s o r p t i o n  spectrum. O t h e r  g a l a x i e s  i n  t h i s  c lass  a l so  show s u c h  
a p o p u l a t i o n .  Has anyone o b s e r v e d  an old s t e l l a r  component i n  
any  of  t h i s  c lass?  
Goudfrooij: You showed from y o u r  two-colour  p l o t  t h a t  t h e  
g a l a x i e s  i n  your  sample  are reddened  by an amount of E(B-V) = 
0 .4 .  Do you also f i n d  t h i s  v a l u e  when d e t e r m i n i n g  t h e  Ha/Hf3 
ra t ios  i n  y o u r  op t i ca l  spectra? Does the  d i f f e r e n c e  of t h e  t w o  
E(B-V) v a l u e s  ( i f  p r e s e n t )  v a r y  w i t h  t h e  t y p e  of  e m i s s i o n - l i n e  
a c t i v i t y  of t h e  g a l a x y ?  
Heisler: The H a / H B  r a t io s  i n d i c a t e  an  a v e r a g e  i n t e r n a l  r e d d e n i n g  
o f  E(B-V) N 0.5.  The e m i s s i o n - l i n e  a c t i v i t y  t y p e  of t h e  g a l a x y  
d o e s  n o t  seem t o  a f f e c t  t h e  E(B-V) v a l u e .  
3 98 


Central activity in 60 micron peakers

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