A&A 415, 95-102 (2004)
DOI: 10.1051/0004-6361:20034314
M. Stickel1 - J. M. van der Hulst2 - J. H. van Gorkom3 - D. Schiminovich4 - C. L. Carilli5
1 - Max - Planck - Institut für Astronomie,
Königstuhl 17,
69117 Heidelberg,
Germany
2 -
Kapteyn Astronomical Institute,
Postbus 800,
9700 AV Groningen,
The Netherlands
3 -
Department of Astronomy,
Columbia University,
New York, NY 10027, USA
4 -
Division of Physics, Mathematics, and Astronomy,
California Institute of Technology,
MS 405-47,
Pasadena, CA 91125, USA
5 -
National Radio Astronomy Observatory,
PO Box 0,
Socorro, NM 87801, USA
Received 12 September 2003 / Accepted 3 November 2003
Abstract
Deep far-infrared (FIR) imaging data obtained with ISOPHOT at
,
,
and
detected the thermal emission from cold dust in the northern shell
region of NGC 5128 (Centaurus A), where previously neutral
hydrogen and molecular gas has been found. A somewhat extended FIR
emission region is present in both the
and
map, while only an upper flux limit could be derived
from the
data. The FIR spectral energy
distribution can be reconciled with a modified blackbody spectrum with
very cold dust color temperatures and emissivity indices in the range
K and
,
respectively, where the data favor the low temperature
end. A representative value for the associated dust mass
is
,
which together
with the HI gas mass gives a gas-to-dust ratio of
300,
close the average values of normal inactive spiral galaxies. This
value, in conjunction with the atomic to molecular gas mass ratio
typical for a spiral galaxy, indicates that the interstellar medium
(ISM) from the inner part of a captured disk galaxy is likely the
origin of the outlying gas and dust. These observations are in
agreement with recent theoretical considerations that in galaxy
interactions leading to stellar shell structures the less dissipative
clumpy component of the ISM from the captured galaxy can lead to
gaseous shells. Alternatively, the outlying gas and dust could be a
rotating ring structure resulting from an interaction or even late
infall of tidal material of a merger in the distant past. With all
three components (atomic gas, molecular gas, dust) of the ISM present
in the northern shell region, local star formation may account for the
chains of young blue stars surrounding the region to the east and
north. The dust cloud may also be involved in the disruption of the
large scale radio jet before entering the brighter region of the
northern radio lobe.
Key words: galaxies: individual: NGC 5128 - galaxies: elliptical & lenticular, cD - galaxies: intergalactic medium - infrared: general - infrared: galaxies
Gravitational interaction is a major process rearranging the stars and the interstellar medium (ISM) of the involved progenitor galaxies. The more violently and supposedly young cases of gravitational interactions are easily discernible by their optically highly disturbed appearance, particularly in the case of disk galaxies. The presumably older, more settled, and perhaps virialized later stages can be ascribed to the rarer optical features of shells and polar rings, which consist mostly of stars.
The fate of the ISM can be traced by the structure of the atomic hydrogen (HI) gas, which is relatively easy to observe and often found far off the galaxy centers in irregular structures, even when the optical images no longer show obvious signs of interactions (Hibbard et al. 2001; Hibbard 2000). However, the HI gas in the progenitor galaxies is usually much more extended than the stars, the molecular gas, and the dust. Its distribution in the interacting system is therefore not necessarily representative of these other components of the ISM. Moreover, a detection of molecular gas and dust in the interacting system provides evidence from where the observed ISM has been torn away and thereby can constrain models of the interaction process.
While molecular hydrogen as traced by the CO emission is now regularly found in the inner regions of disk galaxies (Helfer et al. 2003, and references therein), the detection of molecular gas far from the disk is rather rare. Besides its occurrence in Tidal Dwarf Galaxies, possibly gravitationally bound systems in the tails of violently interacting galaxies where it is likely condensing out of the HI (Braine et al. 2001), only a few additional cases with outlying molecular CO emission have been found (Braine et al. 2001; Aalto et al. 2001). Similarly, the FIR or sub-mm thermal emission of compact extra-nuclear dust structures is detected in strongly interacting systems such as NGC 4038/39 ("the Antennae'', Haas et al. 2000) and HCG 92 ("Stephans Quintett'', Sulentic et al. 2001), whereas in weakly interacting or less disturbed systems none has been reported so far. A possible exception might be a cold intergalactic dust cloud in the outskirts of the Virgo elliptical M86 (Stickel et al. 2003), which is likely undergoing tidal interactions during its infall into the Virgo cluster.
Any FIR emission from extra-nuclear dust associated with the atomic and molecular ISM components in a weakly interacting system is likely quite faint, even if galactic gas-to-dust ratios are assumed. Thus, a search for outlying dust from a captured system is promising only in nearby galaxies. A prime candidate in this respect is the radio galaxy NGC 5128 (Centaurus A), an optically highly disturbed system with a central dust lane, which is considered to result from at least one major merger of an elliptical system with a spiral galaxy. It is also particularly interesting because of its system of optical shells (Malin et al. 1983; Peng et al. 2002), which is possibly the result from one or even more earlier minor encounters. The morphology seen in deep optical images bears some resemblance to a polar ring galaxy (Richter et al. 1994) or even to a spheroid intersected by a large disk oriented along a north-east south-west direction (Haynes et al. 1983; Malin 1978; Peng et al. 2002; Cannon 1981). Other remarkable features include a highly complex radio structure (Morganti et al. 1999) and emission line regions roughly aligned with the radio jet (Morganti et al. 1991; Blanco et al. 1975; Graham 1998). A more detailed review of the current observational status of NGC 5128 can be found in Israel (1998), while schematic diagrams of the various optical, radio, and cold gas structures can be found in Dufour & van den Bergh (1978), Morganti et al. (1999) and Charmandaris et al. (2000), respectively.
The detection of HI emission in several regions near the outer shells (Schiminovich et al. 1994) and the subsequent detection of molecular gas traced by CO emission in the two regions with the strongest HI emission (Charmandaris et al. 2000) indicated that the ISM in the tidal remnant has not yet completely settled in the center of the potential (cf. Weil & Hernquist 1993). This is possibly due to a relatively high velocity of the encounter, as indicated by the rotation of the HI gas (Schiminovich et al. 1994) and the azimuthally distributed shell system (Peng et al. 2002). Alternatively, tidal material might be falling back onto the main galaxy long after the encounter, as suggested by Hibbard & van Gorkom (1996) and recently confirmed by numerical simulations (Barnes 2002). The HI morphology and velocity field of NGC 5128 (Schiminovich et al. 1994) strikingly resembles a ring structure, similar to a few rare cases of HI rings (Ryan-Weber et al. 2003; Barnes 1999, and references therein), in which case the positional association with the stellar shells might only be a projection effect. In fact, locating the three HI emission regions (Schiminovich et al. 1994) in the optical image showing the interleaved shell system (Peng et al. 2002) reveals that only the northern S1 region lies outside the outermost shell whereas the other two are situated between shells. The CO detection in the northern shell region S1 was confirmed by Curran (2001), although a different interpretation in terms of an molecular outflow associated with the radio jet was suggested.
NGC 5128 had been the target of previous FIR studies based on IRAS
data (Israel 1998, and references therein), as well as more
recently, in the near-IR (Mirabel et al. 1999) and sub-mm
(Leeuw et al. 2002). These studies were aimed
at the investigation of the dust emission of the already optically
visible dust disk in the central part of the disturbed galaxy. Up to
now, no evidence has shown up of any localized off-center FIR emission, particularly none at the position of the optical shells or
associated with the outlying off-center HI and CO emission. In the
case the IRAS data
(Marston & Dickens 1988; Marston 1992; LeWinter et al. 1993; Eckart et al. 1990), this
negative result might not only be due to sensitivity but also due to
the wavelength limit of
,
which made it
insensitive to a cold dust component with temperatures below 20 K.
The ISOPHOT detector (Lemke & Klaas 1999; Lemke et al. 1996)
aboard the Infrared Space Observatory
(ISO; Kessler et al. 1996) not only provided a higher sensitivity
and a higher angular resolution, its increased wavelength coverage out
to
also allows the detection of cold dust with
temperatures well below 20 K. Such cold dust temperatures might be
expected since there is no strong radiation field from a large number
of young stars which could heat any outlying dust. NGC 5128 was
therefore observed with ISOPHOT to search for extra-nuclear dust in
the northern part of the stellar halo, particularly dust associated with
extra-nuclear HI and CO in the northern shell S1, the optical
filaments, and the radio jet.
An area of
north of
NGC 5128 was imaged at
and
with the ISOPHOT (Lemke & Klaas 1999; Lemke et al. 1996) in August 1996.
The main body of the galaxy was placed at the southern edge of the
maps to cover the northern shell region S1 (Charmandaris et al. 2000),
where previously HI and CO had been detected
(Charmandaris et al. 2000; Schiminovich et al. 1994). These long wavelength
observations were carried out with the ISOPHOT C200 camera, a
pixel array of stressed Ge:Ga with a pixel size of
in the regular raster mapping mode.
The total integration time for the two filters was
4500 s. A somewhat larger region north of NGC 5128 was observed at
with the C100 camera of ISOPHOT, a
pixel array with a pixel size of
5 in the chopped raster
mapping mode.
The full area was mosaiced in eight separate parts in July 1996 with a
total integration time of 6600 s. For all observations, calibration
measurements with the on-board Fine Calibration Source (FCS) of
ISOPHOT were taken before and after each (sub-)map to put the observed
signals on an absolute level.
The signal derivation for each detector pixel and sky position made
use of the full distribution of pairwise ramp read-out differences
instead of ramp slope fitting. This gives a much better rejection of
cosmic ray hits, which is critical for the detection of faint FIR
sources (Stickel et al. 2003). The signals were corrected for the
dependence on ramp integration times to be consistent with calibration
observations, dark-current subtracted, and finally flux calibrated
with ISOPHOT Interactive Analysis package
PIA version 9.1 / Cal G version 6.0
(Gabriel et al. 1997).
For the conversion of detector signals to fluxes, the average
of the signals of the two FCS measurements accompanying
each (sub-)map was used.
The flux calibrated data streams of the detector pixels of all observations showed significant differences in the overall sky levels levels of up to 30%, most likely coming from inappropriately corrected pixel-to-pixel sensitivities (flatfield), which moreover appeared to be time-dependent. If uncorrected, this would lead to quite severe striping and checker board patterns in the final maps, thereby degrading the limit for detecting faint structures significantly. Therefore, the data streams were smoothed with robust filtering techniques, and each individual data stream rescaled to the common mean, while any residual time trend was removed with robust low-order polynomial fits. However, due to the very high contrast between the FIR bright central disk and the very low FIR emission in the outer stellar halo of NGC 5128, an acceptable flattening of the whole observed region could not be achieved. Since the central parts of the disk region is in any case saturated in all three filters, the flat-fielding has therefore been optimized for the outer stellar halo region of NGC 5128.
The ISOPHOT raster mapping observations contain an inherent redundancy
such that a given sky position is observed with several pixels, and
additionally, in the case of the
chopped
measurement with the C100 detector, during several chopper sweeps. For
the longer wavelengths C200 observations at
and
,
this redundancy was utilized by producing
for each detector pixel a full map with a common center using the
Drizzle mapping method (Hook & Fruchter 1997) within
IRAF
.
Similarly, full maps with a common center were created for each pixel
and chopper throw from the
data streams, again
with the Drizzle mapping method. These individual maps were
subsequently stacked, and a final map created from the stack. In the
case of the C200 detector with only 4 pixels, a robust biweighted mean
was applied to each pixel in the stack. For the
map, the stack consists of a total of 117 full images, which were
averaged after applying a min-max outlier rejection scheme. This
stack-averaging reduction scheme is similar to standard near-infrared data
processing and has been shown to suppress quite effectively small
scale detector variability and cosmic ray hits, leading to
significantly lower detection limits (Stickel et al. 2003).
For cross-checking the obtained results, full maps have additionally been created in a single drizzling step from the pairwise as well as the standard linear ramp fitted data streams. Furthermore, the total integration time per sky position was divided into two halves and the pairwise signals from each part treated as completely separate measurements, which were again fully processed to the complete maps. Except for an increased noise due to the less robust signal derivation, these four additional sets of maps showed results consistent with the above described pairwise stack maps, thereby providing an independent confirmation of the features described below.
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Figure 1:
Gray-scale representation with overlaid isocontours of the
ISOPHOT
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Figure 2:
Gray-scale representation with overlaid isocontours of the
ISOPHOT
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Figure 3:
Same as Fig. 2, but for the
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The final
(Fig. 1),
(Fig. 2) and
(Fig. 3) maps are dominated by FIR emission of
the central dusty disk of NGC 5128. The galaxy center is saturated in
all three bands, although this is not obvious from the displayed
images. Unfortunately, this precludes the derivation of integral
fluxes for the central part of the galaxy. As a consequence of the
high fluxes, long term drifts were apparent in the one-dimensional
data streams, which could not be removed completely by the above
described flat-fielding process. Any structures directly adjacent to
the central dust disk must therefore be considered unreliable.
In the
map, no FIR emission further away from the
central disk is seen, particularly not at the position of the HI
(Schiminovich et al. 1994) and CO (Charmandaris et al. 2000) in the
northern shell S1, nor is there anything discernible at the position
of the northern large scale radio jet (Morganti et al. 1999) or the
ionized diffuse clouds and filaments (Graham 1998). A few very low
level structures lying close to the edges of the map are not
significant, because their appearance is strongly dependent on the on
the data set used (see above) to create the map. They are due to
detector noise and residual baseline variations not completely removed
by the flat field correction.
In contrast, the
(Fig. 2) and
(Fig. 3) maps show in addition
to the bright FIR source due to the dusty central disk also a somewhat
extended FIR source
northeast of NGC 5128. Its
peak emission is at the same position at both wavelengths
(
),
which in turn is closely coincident with the maximum of the HI
(Schiminovich et al. 1994) and CO (Charmandaris et al. 2000) emission
in the northern shell S1. The positional offset of the peak of the FIR
emission (
south) from the HI and CO
emission, the somewhat brighter southern end as well as the
apparently faint curved extension of the FIR emission present in both
maps, which in fact bears some resemblance to the curved HI morphology
(Schiminovich et al. 1994), can not necessarily be taken as evidence
for a strongly asymmetric FIR emission region, because these features
could be a result of the asymmetric sampling of a compact dust cloud
with the large (
)
ISOPHOT C200 pixels. However,
it should be emphasized that the FIR emission in both maps lies away
from the map border and could therefore not be due to edge effects,
which result from the reduced redundancy and incomplete coverage of a
raster scan. Additionally, each of the
and
maps made from the other above described data
streams showed the same FIR emission region, albeit with minor
differences in its level and structure.
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Figure 4:
Gray-scale representation of the
ISOPHOT
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The combined
/ DSS-2 B-band image
(Fig. 4) shows that the off-center FIR
emission lies at the position where the faint optical extensions seen
in deep images (Haynes et al. 1983; Malin 1978; Cannon 1981) start, as indicated by the
departure of the optical isophotes from elliptical symmetry. Remarkably,
the diffuse
emission (Fig. 1 of Blanco et al. 1975)
has also a strong depression there.
Other known features of NGC 5128, namely dust patches seen in the optical (Dufour & van den Bergh 1978), the arc of young blue stars north-west of the central dust disk (Karovska et al. 2002; Peng et al. 2002), the northern inner radio jet and lobe (Burns et al. 1983), and the inner gaseous filaments (Morganti et al. 1991; Blanco et al. 1975) lie in all three maps too close to the very bright central dust disk to be discernible, even if compact FIR emission up to a few Jy would have been present.
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Figure 5:
FIR spectrum of the off-center source at the position of the
HI/CO detection in NGC 5128. 30% error bars are indicated
for the ISOPHOT detections at
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The spectral energy distribution (SED, Fig. 5)
shows that the FIR emission is strongly rising from
to
,
and is further increasing to
.
Apparently, its peak is not covered by observed wavelength range and likely
lies beyond
.
The steep flux increase towards
longer FIR wavelengths is indicative of the thermal emission of a cold
dust component. The dust emission is usually represented by a
modified blackbody (Planck) function
![]() |
(1) |
The dust mass associated with the FIR emission can be estimated from
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(2) |
With the detection of the FIR emission from cold dust at the position
of previously detected HI and CO in the northern shell S1 of
NGC 5128, all three components of the ISM, atomic and molecular gas
as well as dust, have now for the first time been seen in an outer
shell structure of an early type galaxy. As it is obvious from the
spectral energy distribution (Fig. 5), the
newly discovered extra-nuclear dust is rather cold having a color
temperature below 14 K, which is in fact significantly colder than
the dust color temperature of the Milky Way (Sodroski et al. 1994) and
of other normal inactive spiral galaxies (Stickel et al. 2000), but
similar to cold interstellar clouds in the Milky Way
(Ristorcelli et al. 1998; Tóth et al. 2000). This low temperature together
with the rather small inferred dust mass leads to very low
(
)
FIR fluxes at wavelengths up to
,
which in turn precluded an earlier detection
with IRAS.
FIR spectra and dust temperatures for dispersed ISM dust out to large
radii from the galaxy center have been computed for ellipticals by
Temi et al. (2003). This model includes heating by star light as well
as by electron - grain collisions in a hot X-ray emitting component.
An exemplary result of the grain temperature as a function of distance
was given for NGC 4636 (Fig. 2 in Temi et al. 2003), which has a
total mass and effective radius quite similar to that of NGC 5128
(Israel 1998). It shows that large dust grains can actually attain
very low temperatures (
)
in the outer
regions, only heated by the ambient starlight. This is in accord with
the low temperature of the dust in the northern shell S1 of NGC 5128
(Fig. 5), which can be taken as evidence that
neither the surrounding chains of young stars nor the radio jet (see
below) is significantly contributing to the heating of at least the large dust
grains. Unfortunately, no deep MIR/FIR data between
and
are available for the northern outskirts
of NGC 5128 to check the presence of MIR/FIR emission from smaller
grains. They are expected to have significantly higher temperatures,
partly because electron-grain collisions become important
(Temi et al. 2003). The absence of thermal emission from these smaller
grains to very low detection limits would indicate that they might
have already been destroyed by sputtering
(Tielens et al. 1994; Dwek et al. 1996,1990), leaving preferentially
behind the larger grains. The grain sputtering time in turn can be
used to set a lower limit for the lifetime of the dust cloud, which
for an external merger origin of the dust might give an indication when
this event had happened.
For the northern shell S1, Charmandaris et al. (2000) give a HI gas
mass of
,
and an only slightly
smaller amount of molecular gas. Taking
as a representative value
for the dust mass gives a HI gas-to-dust ratio of
300.
Since the total gas HI mass is
distributed over a much larger area than the detected FIR emission
(Schiminovich et al. 1994), the HI gas-to-dust ratio of the inner
region of the northern shell is likely smaller. Given the
uncertainties in the dust parameters, this value is remarkably close
to the canonical value of the Milky Way (
160,
Sodroski et al. 1994), and even closer to the median value of
250 for a large number of inactive spiral galaxies
(Stickel et al. 2000). Similarly, the ratio of the neutral to
molecular gas mass of
1.25 is also representative for
spiral galaxies (Charmandaris et al. 2000).
There is no report of a large cluster of young stars or a large evolved star formation region associated with the northern shell region S1, which makes a creation of dust from the late phases of the stellar evolution unlikely. Rather, the observed properties of the extra-nuclear gas and dust strongly suggest an external origin, most likely coming from the normal ISM of a gas rich progenitor galaxy, while a gas poor elliptical or spheroidal galaxy and a low metallicity dwarf or low surface brightness galaxy appears to be ruled out.
The presence of dust in the northern shell already excludes the outer HI rich envelope far beyond the stellar disk of a spiral galaxy as the origin of the gaseous shell component, as suggested by Carter (1999). CO has been found to be a good tracer not only for molecular hydrogen gas but also for dust (Neininger et al. 1998, and references therein). Its confinement to the stellar disk region (Helfer et al. 2003; Regan et al. 2001, and references therein) provides strong evidence for the general absence of dust at very large radii, although there have been a few hints that dust can indeed reside outside stellar disks (Melo et al. 2002; Nelson et al. 1998; Alton et al. 1998). The approximately normal properties of the shell material then indicates that the ISM of the inner part of a spiral galaxy is its likely origin.
The detection of dust is also in agreement with models for the shell formation (Charmandaris & Combes 2000; Combes & Charmandaris 2000), where the ISM of a merging companion is not only included as a smoothly distributed, homogeneous dissipative component, but also contains a clumpy component of small and dense clouds with a very low dissipation, which in turn will survive and be present in the shell structures along with the stellar component. The consistent picture of an external origin of the outlying gas and dust also makes an collimated outflow of molecular gas from the central regions along the jets (Curran 2001) for the origin of the CO emission in the northern shell S1 less likely, although some outflow of ISM components from the central actively star forming region of NGC 5128 in form of a galactic wind is to be expected.
Since dust is a necessary requisite for molecular gas formation, its presence in the northern shell S1 can be taken as evidence that at least a fraction of the molecular gas might have been created in situ from the neutral gas, rather than brought in from the captured system. This situation would then be similar to a few tidal dwarf galaxies, where the formation of molecular gas out of HI and dust has been proposed (Braine et al. 2001). The presence of atomic and molecular gas together with dust in a compact cold region is reminiscent of Giant Molecular Clouds in the Milky Way, which often harbor the sites of star formation. There is currently no evidence for large scale star formation going on at the center of the northern gas and dust cloud. However, there is a chain of young blue stars not only along the eastern edge of the cloud (Rejkuba et al. 2002,2001) adjacent to the emission line region likely excited by the radio jet (Morganti et al. 1991; Graham 1998), but remarkably also along its northern edge (Fassett & Graham 2000). This can be interpreted as recent star formation at the surface of the cloud, which initially might have been triggered by its interaction with the jet, but apparently has now spread even to the far side of the supposed jet-cloud interface. Even earlier phases of the stellar evolution are characterized by compact cores with gas and dust temperatures well below 15 K, which usually show up as intensity peaks in cold gas and dust clouds at the longest FIR and sub-mm wavelengths. Whether such pre-stellar cores are present and star formation might eventually independently start within the gas and dust cloud can be tested by high resolution sub-mm mapping.
A remarkable feature has shown up in recent 20 cm mapping of the brighter northern radio lobe region, namely a large scale jet connecting the inner radio lobe and the much more diffuse radio emitting region of the northern middle lobe (Morganti et al. 1999). As already noted by Morganti et al. (1999), the position where the confined jet emission becomes diffuse is positionally coincident with the HI and CO emission. The newly detected dust at the same position can be taken as supporting evidence that there is indeed some kind of dynamical interaction going on which disrupts the collimated jet.
Although a few other cases of shell galaxies with associated HI
emission are known
(Schiminovich 2001; Balcells et al. 2001; Petric et al. 1997; Schiminovich et al. 1995),
no other clear case of CO emission associated with the HI is currently
published. An uncertain case is NGC 3656, where
Charmandaris & Combes (2000) note a possible detection of the southern
shell. Unfortunately, this particular galaxy has not been observed
with ISO at FIR wavelengths. A cross-correlation of the FIR mapping
observations in the ISO data archive with the lists of shell galaxies
from Malin & Carter (1980), Wilkinson et al. (1987), and
Thronson et al. (1989) revealed that there are several additional
shell galaxies with useful mapping data at wavelengths beyond
.
Among them are the two well known cases NGC 474
and Arp 230, which have in addition to stellar shells also a highly
structured HI morphology. Again, no CO has been detected associated
with the optical shells in these galaxies
(Combes et al., priv. commun.).
The ISOPHOT observations showed in both cases no outlying compact
dust structures, and did detect the central FIR emission only from
Arp 230 but not from NGC 474.
However, the less well known object NGC 5813 from the list of
Thronson et al. (1989) clearly does show two outlying compact FIR
emission regions not associated with anything obvious in the optical
DSS-2 R band image of the galaxy, although its center is only very
weakly detected. NGC 5813 is a rather unconspicious E1 elliptical,
for which a merger origin has been suggested by Kormendy (1984) on
the basis of a non-isothermal core-witin-a-core structure, but has
not yet been investigated in detail with HI
observations. A detailed description of these archival FIR data from
the ISO database will be given elsewhere (Stickel et al., in
preparation). Since these three objects and all other shell galaxies
with FIR mapping data are at much larger distances than NGC 5128, a
similarly small amount of dust can currently not be detected. For the
time being, the detection of dust in the northern shell system of
NGC 5128 is therefore unique.
Acknowledgements
The development and operation of ISOPHOT were supported by MPIA and funds from Deutsches Zentrum für Luft- und Raumfahrt (DLR, formerly DARA). The ISOPHOT Data Centre at MPIA is supported by Deutsches Zentrum für Luft- und Raumfahrt (DLR) with funds of Bundesministerium für Bildung und Forschung, grant. no. 50 QI0201.
This research has made use of NASA's Astrophysics Data System Abstract Service, the Simbad Database, operated at CDS, Strasbourg, France, and data from the Infrared Processing and Analysis Center (IPAC) and the NASA/IPAC Extragalactic Database (NED), which are operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
Based on photographic data obtained using The UK Schmidt Telescope. The UK Schmidt Telescope was operated by the Royal Observatory Edinburgh, with funding from the UK Science and Engineering Research Council, until 1988 June, and thereafter by the Anglo-Australian Observatory. Original plate material is copyright © the Royal Observatory Edinburgh and the Anglo-Australian Observatory. The plates were processed into the present compressed digital form with their permission. The Digitized Sky Survey was produced at the Space Telescope Science Institute under US Government grant NAG W-2166.