The authors report the discovery of a large (15 kpc diameter) H alpha + (NII) emission-line disk in the elliptical galaxy IC 1459, showing weak spiral structure. The line flux peaks strongly at the nucleus and is more concentrated than the stellar continuum. The major axis of the disk of ionized gas coincides with that of the stellar body of the galaxy. The mass of the ionized gas is estimated to be approx. 1 times 10 (exp 5) solar mass, less than 1 percent of the total mass of gas present in IC 1459. The total gas mass of 4 times 10(exp 7) solar mass has been estimated from the dust mass derived from a broad-band color index image and the Infrared Astronomy Satellite (IRAS) data. The authors speculate that the presence of dust and gas in IC 1459 is a signature of a merger event

Dejong, T.

Goudfrooij, Paul

Hansen, L.

Jorgensen, H. E.

Norgaard-Nielsen, H. U.

English

NASA Technical Reports Server

N91-1 9 
A SPIRAL-LIKE DISK OF IONIZED GAS IN IC 1459: 
SIGNATURE OF A MERGING COLLISION? 
Paul Goudfrooijl , H.U. Ngrgaard-Nielsen’, H.E. J@rgensen3, L. Hansen3 
and T. de Jong’ 
‘Astronomical Institute “Anton Pannekoek” , University of Amsterdam, 
2Danish Space Research Institute, 61. Lundtoftevej 15, 2800 Lyngby, Denmark 
3Copenhagen University 0 bservatory, 0ster Voldgade 3, 1350 Kobenhavn K, Denmark 
Roetersstraat 15, 1018 WB Amsterdam, The Netherlands 
Abstract. We report the discovery of a large (15 kpc diameter) Ha+[NII] emission-line disk 
in the elliptical galaxy IC 1459, showing weak spiral structure. The line flux peaks strongly at the 
nucleus and is more concentrated than the stellar continuum. The major axis of the disk of ionized 
gas coincides with that of the stellar body of the galaxy. The mass of the ionized gas is estimated to 
be N 1 lo5 M a ,  less than 1% of the total mass of gas present in IC 1459. The total gas mass of 4 
lo7 M a  has been estimated from the dust mass derived from a broad-band colour index image and the 
TRAS data. We speculate that the presence of dust and gas in IC 1459 is a signature of a merger event. 
1. Introduction 
Contrary to the traditional view of elliptical galaxies as being essentially free of interstellar 
matter, advances in instrumental sensitivity have caused a recent renaissance of interest in 
dust and gas in elliptical galaxies. Many elliptical galaxies are now known to contain dust 
(e.g., Vbron-Cetty and V&on [1988] and references therein), HI gas (Knapp et  al., 1985), 
hot (- lo7 K) X-ray gas (Canizares et al., 1987) and ionized gas (Phillips et  al., 1986). 
&loreover, Jura et  ai. (1987) have shown that the Infrared Astronomiccil Satellite (IRAS) 
detected more than 50% of the elliptical galaxies with B& < 11 mag. in the Revised Shapley- 
A m e s  Catalog (RSA, Sandage and Tammann, 1981), indicating the presence of lo7 - lo8 M a  
of cold interstellar matter. Thus dust and gas in ellipticals seems to be quite common. 
The origin and evolution of the dust and gas are still not fully understood. To study the 
origin and evoltttion of interstellar matter in elliptical galaxies systematically we are currently 
undertaking a survey of all ellipticals with B$ < 12 mag. in the RSA catalog. This survey 
aims at obtaining deep CCD colour index images and narrow-band (Ha+[NII]) images as 
well as spectroscopic data. 
The CCD images of IC 1459, one of the galaxies in our sample, turned out to be so 
striking and unusual that we decided to present the results here. 
215 
https://ntrs.nasa.gov/search.jsp?R=19910007576 2020-03-19T19:50:13+00:00Z
The giant elliptical IC 1459 is a member of a group of galaxies (group no. 15 of Huchra 
and Geller (1982)), containing 10 galaxies. As already noticed by Sparks e t  al. (1985), there 
is some dust absorption near its centre. The colour index image presented here confirms this 
result. It is one of the few ellipticals which are detected by IRAS in a l l  four passbands (Jura 
et al., 1987). It is also a compact, powerfvl radio source with a brightness of 1.0 Jy at 6 cm 
(Sadler et al., 1989). Finally, it has been detected by the EINSTEIN satellite (Forman et 
al., 1985). Franx and Illingworth (1988) discovered a fast (170f20 km s-l) counter-rotating 
core, which is suggestive of a recent merging collision. 
2. Observations and Results 
Deep CCD imaging of IC 1459 has been perfdrmed using the ESO/MPI 2.2m and Danish 
1.5m telescopes on La Silla, Chile. The galaxy was observed through Johnson B and V filters 
as well as through two narrow-band filters: one centered on the redshifted Ha! line and one 
on the nearby continuum. 
The final B-V colour index image of IC 1459 was obtained by dividing the (processed) 
V image by thc B image. With the present high-quality data this technique reliably reveals 
features redder than.about 0.005 in B-V over their surroundings. 
To produce the final emission-line maps, we scaled the continuum image to the on- 
line iiriage by comparing the counts in the two frames not too close to the dusty features as 
determined from the colour index image. After scaling the continuum image it was subtracted 
from the on-line image, yielding an image containing emission only. 
A detailed account of the reduction and analysis method is given elsewhere (Goudfrooij 
e t  al., 1990). Here we only present results on the dust and ionized gas in IC 1459. 
2.1. Mass of neutral gas 
In the B-V image (Fig. la) a red, diffuse nuclear region shows up. The mean colour excess of 
the  dusty feature equals EB-v = 0.03 over an area of 1083 (“)2. Assuming Ho = 50 km s-l 
Mpc-l and N ~ I / E B - v  = 5.8 1021 mag-’ (Bohlin e t  al., 1978) we find M g a s , ~ - v  = 
3.6 107M0. This mass estimate may be compared with an estimate based on the IR4S 60 
and 100 pm data, using Mdust = FA R2/ [ I E ~  Bx(T)] where FA is the observed fluxdensity at 
distance R, I E ~  is the dust opacity and Bx(T) is the Planck function. Adopting I E ~  oc X-’, a 
uniform temperature of 35 K as derived from the IRAS S~Oo/S60 ratio (Jura et al., 1987) and 
Mgas/Mdast = 100, we find M g a s , l ~ ~ ~  = 3.9 107M0. This agrees very well with M g a s , B - V .  
The gas masses mentioned above are in agreement with the upper limit MHI < 2 lo8 M a  as 
derived by Welsh e t  al. (1990) from radio observations. 
216 
2.2. Mass of ionized gas 
In Fig. lb  the Har+[NII] emission-line map is shown. The ionized gas is distributed in an 
extended disk, showing weak spiral structure. 
Using the emission-line ratio [NII]X6583/Hcu = 1.44 as determined by Phillips et al. 
(1986) from a long-slit spectrum of IC 1459 taken in P.A. 144”, i .e. refering to the nuclear 
region only, we derive L(Ha) = 5.3 lo4’ erg s-’ from our Ha+[NII] map. Adopting an electron 
density of - lo3 ~ r n - ~  as implied by the [SII]XX6717,6731 doublet ratio in the spectrum of 
Phillips e t  al. a,nd standard (case B) recombination theory we derive a mass of ionized gas 
M H I I  = 1.2 i O S 1 % f ~ .  
Figure 1 .  (a) Grey-scale plot of the 3 - V  index in the central 1!34 X 1134 of IC 1459. North is 
up and east is left. Red features show BS black. The very nucleus is reddened by EB-v = 0.07. (b) 
Grey-scale plot of the Ha+[NII] emission in the central 1!34 X 1!34 of IC 1459. Emission shows as 
black. A spiral-like disk structure can be distinguished. Total Ha+[iVII] luminosity is 1.5 1 dl’ ergs-’ . 
3. Discussion 
9.1. O n  the  morphology of the ISM in IC 1459 
The coloiir index map and the Hcr+[NII] map show some interesting differences. First, the 
colour index map shows a smooth, diffuse colour gradient in the central region, whereas the 
Hm+[NII] intensity peaks very strongly at the centre. Secondly, the diffuse “dust patches” as 
sccii in Fig. la extend into the outer parts of the galaxy along a position angle which deviates 
217 
from thak of the major axis of the disk of ionized gas. Thus the neutral and ionized gas in 
IC 1459 are probably distributed in a different way. 
The morphology of the interstellar medium in IC 1459 suggests that the presence of dust 
and gas in this galaxy is due to a merger event. Since the disk of ionized gas coincides with 
the photometric major axis throughout its measurable size of 15 kpc, the study by Tohline 
et al. (1982) suggests that the merging collision has taken place -3 lo9 yrs ago. This 
estimate depends on the original alignment of the smaller galaxy with the giant elliptical. 
The innermost part of the emission-line region is not ellipsoidal but rather irregular, indicating 
a young phenomenon. As to the weak spiral structure of the extended disk, this is probably 
not a remnant of the intrinsic shape of the smaller gas-rich galaxy. Tidal forces during the 
merging collision will have removed any pre-existing structure. The spiral-like structure might 
reflect the response of the gas in the gravitational potential of the elliptical galaxy. 
3.2. Possible ionization mechanisms for the gas 
Heat conduction from hot X-ray gas: This possibility has recently been put forward by Sparks 
et al. (1989) and de Jong e t  al. (1990) for the dominant cluster elliptical NGC 4696 in which 
the dust and ionized gas are distributed similarly. However, this mechanism does not seem to 
be able to account for the H a  emission of IC 1459. Although IC 1459 has been detected by 
the EINSTEIN satellite, the data by Canizares et al. (1987) show that its X-ray luminosity 
is probably dominated by the contribution of discrete sources (e.g., binary X-ray sources and 
globiilar clusters). Thus the amount of hot gas in IC 1459 is presumably small. 
Ionization by shock-heating in cloud-cloud collisions: This is to be expected while the gas 
captured by the giant elliptical is settling down to a rotating disk. 
Photoionization by a central non-thermal source: Many features in the nucleus point at this 
possibility. The spectrum of the nucleus of IC 1459 by Phillips e t  al. (1986) is typical of a 
LINER and broad [NII]X6583 line (426 km s-l FWWM). Moreover, IC 1459 is identified with 
the radio source PKS 2254-367 which is a compact active radio galaxy with a spectral index 
-0.0 (Disney and Wall, 1977) and it has a large far-infmed S25/S60 ratio, characteristic of 
an active nucleus (de Grijp et al., 1985; de Grijp and Goudfrooij, 1990). 
Burst(s) of star formation, triggered by the merging collision: It seems that we may not discard 
this possibility, at least for the outer parts of the disk. n o m  the observed Ha flux we derive 
a total Hydrogen recombination rate which can be maintained by -3 lo4 BO stars. Adopting 
itdv = -4.0 and B-V = -0.29 for a BO star (Kurucz, 1979) we obtain B N 16.6 due to 
early-type stnrs. The observed B magnitude in the corresponding area equals B = 12.7 so 
that the observed amount of Ha! emission could be due to recently formed early-type stars, 
218 
which are unobservable in the broad-band photometry. 
More severe constraints on the source of ionization and the kinematics of the extended 
emission-line regions in IC 1459 cannot be put until high signal-to-noise spectroscopic data 
are available. 
Acknowledgements. This work is based on observations collected at the European Southern 
Observatory, La.Silla, Chile. P.G. acknowledges Leids Kerkhovepl Bosscha Fonds and IAU 
travel grants, and thanks Bill Sparks for useful discussions. 
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A spiral-like disk of ionized gas in IC 1459: Signature of a merging collision

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