A&A 443, 373-381 (2005)
DOI: 10.1051/0004-6361:20053227
M. Stickel1 - D. Barnes2 - O. Krause1,3
1 - Max-Planck-Institut für Astronomie,
Königstuhl 17,
69117 Heidelberg, Germany
2 -
School of Physics, University of Melbourne,
Victoria 3010, Australia
3 -
Steward Observatory, University of Arizona,
Tucson, AZ 85721, USA
Received 12 April 2005 / Accepted 4 July 2005
Abstract
The LGG 138 galaxy group members NGC 2292 and NGC 2293 were imaged
with ISOPHOT in the far-infrared (FIR) at
,
,
and
.
While no FIR emission is seen at
,
and only very low level
emission is present at
,
compact FIR emission
from both NGC 2292 and NGC 2293 galaxy centres and extended emission
likely associated with tidally removed dust and the HI ring
surrounding NGC 2292 / 2293 is strongly detected at
.
Additionally, a compact FIR source
associated with the neighbouring galaxy NGC 2295 is strongly
detected at
.
Remarkably, none of these three
galaxies have been detected individually in 21 cm HI emission. The
steeply rising far-infrared spectral energy distribution of the
apparently interacting NGC 2292 / 2293 pair towards longer
wavelengths indicates the thermal emission of very cold dust with a
temperature of 13 K, much lower than typical values of interacting
systems or even quiescent spiral galaxies. The FIR data of this
galaxy group clearly shows for the first time that there could be FIR
dust emission not accompanied by HI, that dust even in an interacting
system can have a very low dust temperature, and furthermore that
gravitational interaction can give rise to an extended diffuse dust
distribution.
Key words: galaxies: individual: NGC 2292 - galaxies: individual: NGC 2293 - galaxies: interactions - intergalactic medium - infrared: general - infrared: galaxies
Investigation of the atomic hydrogen (HI) emission in a wide variety of galaxies has uncovered, in addition to a smooth distribution, a wealth of different morphologies such as off-centre emission, tails, or ring-like structures. This points in most cases unambiguously to gravitational interactions shaping the distribution of the interstellar medium (ISM) in quite a number of galaxies, even if optical morphologies still appear undisturbed or only weakly disturbed (Hibbard et al. 2001; Hibbard 2000). A particularly rare HI structure is a ring around a single galaxy or a small group of galaxies. Only a handful of such structures are currently recognized (Barnes 1999; Ryan-Weber et al. 2003, and references therein), although more might be hidden among the irregular HI structures collected by Hibbard et al. (2001) and in the HI distributions around S0 galaxies (van Driel & van Woerden 1991).
The HI ring around the NGC 2293 / 2292 galaxy pair is particularly interesting since it is a complete ring (Barnes 1999) in a loose galaxy group (LGG 138, Garcia 1995) rather than only a partial or broken ring structure, such as that surrounding e.g. the M 96 galaxy group (Schneider 1985). Furthermore, the two galaxies inside are not detected as individual HI emitters (Barnes 1999; Rupen et al. 2001). The extended HI had been detected earlier by Huchtmeier et al. (1995) and interpreted as a common halo of the NGC 2293 / 2292 galaxy pair. Additionally, no HI is detected in the nearby distorted dust-lane spiral NGC 2295 (Barnes 1999; Rupen et al. 2001).
The morphological classification of NGC 2292
and NGC 2293 in the literature is somewhat controversial, but both
optical brightness profiles clearly follow an exponential disk rather
than an elliptical de Vaucouleurs law (Penereiro et al. 1994), which
supports the Hubble type S0 rather than E, as given e.g. in NED. The
distance to NGC 2293 (the eastern component) derived from surface
brightness fluctuations is 17.1 Mpc (Tonry et al. 2001), which will
be adopted in the following. It is significantly lower than the distance
of
derived from the radial velocity and a Hubble
constant of
.
The apparently interacting galaxy pair NGC 2292 / 2293
(Corwin et al. 1985) shows a break in the B-R color distribution,
indicating a younger stellar population inside the HI ring
(Barnes 1999). Nevertheless, the two galaxies do not show
emission lines in their centres (Dahari 1985),
and neither is detected in X-rays (O'Sullivan et al. 1999).
A CO measurement towards NGC 2293 failed to
detect any emission at its systemic velocity
of
(Jorgensen et al. 1995), but remarkably
a marginal detection was achieved at
(Huchtmeier & Tamman 1992), very close to the radial velocity of the
brightest south-eastern receding side of the HI ring
(Barnes 1999). This can be taken as evidence that molecular gas is
associated with the ring rather than with NGC 2293 itself, and
that possibly also dust could be present there.
Knapp et al. (1989)
listed IRAS detections for both NGC 2292 and NGC 2293 at all four
IRAS wavelengths. The
and
fluxes for the two members of the pair were similar,
and
(as listed in NED),
respectively, but the
fluxes
were considered unreliable because of source confusion. Nevertheless,
taking the
flux
ratio at face value, this is already a hint for rather low dust
temperature in this interacting system. A sub-mm measurement of
NGC 2292 by Fich & Hodge (1993) at
,
and
found it to be below the detection limits.
The aperture used, however, was only
and the chop
throw only
,
which means that for an extended dust
distribution the off-position would have been still inside the source,
thereby losing much of the source flux. Similarly, the non-detection
of NGC 2292 at
by Knapp et al. (1992) used an
even smaller aperture and chop-throw of
and
,
respectively, which is again not sensitive to
extended emission larger than the chop-throw. These restrictions
unfortunately prevent use of these measurements for a combined
Near/Mid/Far-infrared spectral energy distribution (SED).
Overall, the limited data available suggest that some kind of
interaction has taken place that removed the HI completely from or
consumed the HI near the centre of the two galaxies and created the
ring, although the sources and interaction partner(s) are not quite
clear, as discussed by Barnes (1999). While the molecular gas
might have shared the fate of the HI, as indicated by the weak CO detection (Huchtmeier & Tamman 1992), the location of the dust as the
third component of the ISM cannot be determined
from the IRAS data. Higher sensitivity, higher angular resolution,
and an increased wavelength coverage was provided by the ISOPHOT
detector aboard the Infrared Space Observatory
(ISO; Kessler et al. 1996). Three ISOPHOT FIR maps of the
NGC 2293 / 2292 galaxy pair are available in the ISO archive, which
allowed a more detailed analysis of the cold dust in this
extraordinary system. An additional ISOCAM image at
traces the distribution of the old stellar population.
The ISOPHOT observations of NGC 2293 / 2292 are also included in the
study of the FIR emission of early type galaxies by Temi et al. (2004),
but no detailed analysis of the data has been given.
Barnes (1999) noted the similarity of the HI ring in
NGC 2293 / 2292 with the HI ring around IC 2006, an apparently
undisturbed E1 galaxy. The distance to IC 2006, as derived from
surface brightness fluctuations, is 20.8 Mpc (Tonry et al. 2001), and
its radial velocity
.
For IC 2006, the
ISO archival data consist of a set of strip maps at four FIR
wavelength between
and
and a
ISOCAM image, which will be briefly
discussed along
with the NGC 2293 / 2292 data to show the wide range of FIR
properties found in these two apparently similar HI ring systems.
An area of
centred on
the NGC 2293 / 2292 galaxy pair has been mapped with chopped raster
observations at
,
,
and
with the photometer ISOPHOT
(Lemke et al. 1996; Lemke & Klaas 1999) aboard the Infrared Space Observatory
(ISO). An ISOCAM image at
covers an area of
along position angle
.
Narrow strip maps across IC 2006 were obtained
with ISOPHOT at
,
,
,
and
,
and a full map of the
inner
with ISOCAM at
.
Details of the observations are listed in
Table 1, which gives the wavelength (Col. 1), the
detector used (Col. 2), the integration time (Col. 3), the observing
date (Col. 4), and the ISO Target Dedicated Time (TDT) number
(Col. 5), a unique data set identifier of ISO observations. PI of all
observations was G. Knapp, Princeton.
Table 1: Details of ISO observations.
The ISOPHOT data reduction followed the procedures outlined by
Stickel et al. (2003), where the signal derivation made use of the
distribution of pairwise ramp read-out differences instead of ramp
slope fitting, and the final maps were derived from the averaging of a
stack of individually drizzled images. 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 were used.
The ISOCAM data reduction of NGC 2293 / 2292 within
IRAF
followed standard procedures for ground-based near-infrared (NIR)
image stacks, which includes the derivation of a flatfield from the
raw image stack, a sliding window background subtraction, and a
min-max rejection scheme for the averaging of the drizzled
(Hook & Fruchter 1997) individual images. Only the dark level was
removed within the ISOCAM Interactive Analysis (Ott et al. 2001)
software package, while absolute flux calibration used the standard
ISOCAM conversion factor. Because of too small dither shifts, the data
reduction for IC 2006 had to make use of a flatfield from a different
observation in the same filter. The background was, in this case,
modelled by fitting a two-dimensional polynomial of second order to
the averaged drizzled stack of individual images and subtracted to
give the final map.
The IRAM 30 m telescope was used to search for CO(1-0) and CO(2-1) emission from the south-eastern extension detected in the FIR data,
which is positionally coincident with the peak of the HI emission (see
below). Both lines were observed simultaneously using SIS heterodyne
receivers and the VESPA autocorrelator during a 1.5 h integration
on Nov. 6, 2003. The spectra were centred on a redshift of
,
similar to the strongest, south-eastern
peak of the HI ring (Barnes 1999), and the weak CO detection of
NGC 2293 by Huchtmeier & Tamman (1992). The spectra covered a
velocity range of
and
for the CO(1-0) and (2-1) lines,
respectively. They were smoothed to a resolution of 13 km s-1 for
analysis, yielding rms noise levels of 8 mK and 13 mK. The
beam-width of the CO(1-0) and CO(2-1) observations are 23
and 11
,
respectively.
Since there is considerable disagreement on the radial velocities of the NGC 2293 / 2292 / 2295 galaxy group in the literature, which is reflected in the NED and Simbad database entries for these galaxies, optical spectra were obtained in October 1998 with the RGO spectrograph attached to the 3.9 m Anglo-Australian Telescope. Two 1200 s observations covered the wavelength range between 4500 Å and 6000 Å with a dispersion of 1.6 Å pixel-1, accompanied by CuAr comparison spectra and a spectrum of the star LTT 2415 for flux calibration. Data reduction followed standard procedures within IRAF.
The three ISOPHOT FIR maps show a remarkable variation of the FIR
morphology of NGC 2293 / 2292 galaxy pair with increasing
wavelength. While no emission attributable to the two galaxies is seen
at
(Fig. 1), a weak source
centred on the two galaxies is definitely present at
(Fig. 2). At
(Fig. 3), the combined two
galaxies are strongly detected, with NGC 2293 (eastern component)
being the brighter of the pair, which moreover are surrounded by
extended emission, most notably towards south-east and north. This
towards longer wavelength steeply rising FIR spectral energy distribution (SED)
already indicates that very cold dust is present in
this interacting system. The extended area of FIR emission seen in
the
(Fig. 3) map is largely
coincident with the extended ring structure seen in the 21 cm HI emission.
The very cold and complex extended FIR emission NGC 2293 / 2292
has also been noted by Temi et al. (2004).
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Figure 1:
Gray-scale representation with overlaid isocontours of the
background subtracted ISOPHOT
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Figure 2:
Gray-scale representation with overlaid isocontours of the
background subtracted ISOPHOT
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Extended FIR emission near the western edge towards NGC 2295 appears
to be present in all three maps, but the exact shape cannot be
determined from the
and
maps due to edge effects, i.e. the redundancy is lower there because
not all detector pixels contributed to the final map. However, the
somewhat larger
map
(Fig. 3) clearly shows a rather compact bright
FIR source surrounded by extended emission, which appears to be
associated with the nearby disturbed spiral galaxy
NGC 2295.
The optically highly elongated NGC 2295 galaxy is located very close
to the edge of the mapped area in each of the three wavelengths, which
unfortunately prevents derivation of its SED. Since IRAS did not
detect any strong source near NGC 2295 at
,
the
FIR emission (Fig. 3)
also indicates a strongly rising FIR SED and thus very cold dust. From
the
map, a total flux of
is derived for the integrated FIR emission in the vicinity of NGC 2295.
Due to the small angular distance between NGC 2293 and NGC 2292 of
(one pixel of the C100 detector, half a
pixel of the C200 detector), the two galaxies are not clearly
discernible as separate compact or unresolved FIR sources. Moreover,
the diffuse surrounding emission at
,
as well as
the only weak detection at
,
prevents a
separation of these three different components. Therefore, only the
total FIR flux for the combined NGC 2293 / 2292 galaxy pair was
measured with simple aperture photometry. The integrated net
flux was derived by subtracting various rectangular background boxes
from the total flux in a box containing the two galaxies with the
surrounding extended emission, while the variations of these
differences gave an indication of the
accuracy. In the
map, the flux of the very
low level emission near the map centre was taken twice as an upper limit for
the
flux of NGC 2293 / 2292. The resulting
fluxes are 720 mJy at
,
5.30 Jy at
,
while the
upper limit
region is
.
Uncertainties in the
and
fluxes are
,
as derived from several combinations of the source
and different background regions. The
upper limit
and the flux at
are a factor
lower than the IRAS derived values (Knapp et al. 1989).
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Figure 3:
Gray-scale representation with overlaid isocontours of the
background subtracted ISOPHOT
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In the ISOCAM
image
(Fig. 4), the two galaxies are clearly resolved,
showing a morphology quite similar to the NIR images from the
2MASS Large Galaxy Atlas (Jarrett et al. 2003) and the optical image
of Penereiro et al. (1994). Particularly noteworthy
is an elongated structure south-east of
NGC 2293, which is positionally coincident with an arm- or tidal
tail- like feature already seen in the B-band DSS-1/2 image.
The integrated
flux of both galaxies is
.
The comparison of the
map with the optical B-R color image from Barnes (1999) reveals
(Fig. 5) that the strongest dust emission is
located close to the galaxy centres in a region which coincides with
the bluer stellar population there (Barnes 1999). The
extended dust emission towards south-east is associated with an
optically much redder structure, which lies at the base of the arm or
tidal tail extending from there towards south-west, as seen in the
DSS-1/2 B-band images. It is positionally coincident with the
elongated feature south-east of NGC 2293 / 2292 seen in the
image (Fig. 4). A close-up
of the B-R color image of this region reveals small radially elongated
knots which are even redder than the surrounding medium. The FIR extension towards north also lies at the base of an albeit much
fainter arm-like structure seen in the DSS-1/2 B-band images, but a
detector defect leading to a few dead CCD rows does not allow checking
whether there is also a similarly red region there.
The B-R /
overlay also shows that there is
cold dust emission in the region surrounding the nearby disturbed
spiral galaxy NGC 2295. Although the peak of the compact FIR source
(Fig. 3) is not centred exactly on the optical
galaxy, this is most likely due to the coarse sampling with the ISOPHOT
C 200 pixels near the map edge. There appears to be some FIR emission of dust lying towards north outside of the disturbed disk of NGC 2295, where the B-R image shows a compact red
structure. Overall, the FIR dust emission from NGC 2295 and its
vicinity appears complex with diffuse dust emission possibly present
outside of the main body of the galaxy. This could be due to the
gravitational interaction, which also disturbed the disk.
The column density of the HI ring has its maximum at the position of
the optically red structure described above, which in turn
is coincident with the extended FIR emission
south-east of NGC 2293 / 2292 (Fig. 6).
Overall, dust emission is seen throughout the HI ring area, but
appears to be generally shifted slightly towards south. The
maximum of the FIR emission is not exactly coincident with the hole of
the HI column density, although this might also be an artifact due to
the coarse sampling of the FIR emission with the large
(
)
C200 detector pixels.
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Figure 4:
Gray-scale representation with overlaid isocontours of the
background subtracted ISOCAM
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Figure 5:
Gray-scale representation of the optical B-R color image from
Barnes (1999) with overlaid isocontours of the
ISOPHOT
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The steep flux increase of the integrated FIR SED towards longer wavelengths
(Fig. 7) with a
/
flux ratio of
is
indicative of the thermal
emission from a very cold dust component with a temperature far below
,
the average value of the Milky Way
(Sodroski et al. 1994) and other normal inactive spiral galaxies
(Stickel et al. 2000).
The dust emission is usually characterized by a
modified blackbody (Planck) function
![]() |
(1) |
Given the FIR flux
at
,
the
dust color temperature
and distance D, the dust mass
associated with the FIR emission can be derived from
![]() |
(2) |
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Figure 6:
Gray-scale representation of the ISOPHOT
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Figure 7:
Integrated FIR spectrum of the NGC 2293/92 galaxy pair.
The dashed curve shows a modified blackbody
spectrum with temperature
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The weak detection of CO (1-0) at the position of NGC 2293 with
roughly the radial velocity of the receding south-eastern side of the
HI ring (Huchtmeier & Tamman 1992) indicated that even the molecular
gas is in an unusual dynamical stage. Unfortunately, the newly
acquired CO spectrum at the position of the south-eastern extended FIR
emission
away from NGC 2293 failed to
detect any significant emission in either line, although a very weak
broad emission feature might be present in the CO (2-1) spectrum at
the expected velocity. Clearly, these non-detections should definitely
be improved with a much longer integration time, to check whether
NGC 2293 might be a further member of the rare group of galaxies
with off-center molecular gas (Aalto et al. 2001; Braine et al. 2001).
The optical spectra did not show any emission lines of either
or
,
corroborating the
result of Dahari (1985), even though the
galaxies in this group are apparently interacting.
The redshift was therefore derived
from stellar absorption features of
,
,
,
and
.
The resulting radial velocities are
,
,
and
for NGC 2293 NGC 2292 and NGC 2295, respectively, with
uncertainties of
,
as judged from the
velocity variations of the measured absorption features. These values
agree with those listed in Barnes & Webster (2001), which were
carefully selected from the literature. The values for NGC 2293 and
NGC 2292 listed in Simbad, which were taken from Garcia (1993),
are too small by more than
,
while the
redshift for NGC 2292 listed in NED, which was taken from
Huchtmeier et al. (1995), also is too small by more than
.
The spectral resolution was not high enough to allow
the investigation of the front-to-back ordering of
NGC 2293 relative to NGC 2292 by searching for absorption
features of one galaxy in the spectrum of the other,
an issue of great importance
for the dynamical stage of the group (Barnes 1999).
If anything, the absorption features of NGC 2293 were found to be
slightly broader
than those of NGC 2292, which might be an indication of
an overlap between two absorption systems in the sense
that the higher redshift system of NGC 2292 is in front of the
lower redshift system of NGC 2293. However, an unequivocal clarification
would require higher spectral resolution data with much longer
integration time.
Although the HI ring morphology of IC 2006 appears to be similar to
that of NGC 2293 / 2292 (Barnes 1999), comparison of the
ISOPHOT images of NGC 2293 / 2292 and IC 2006 strikingly shows a
large difference in the FIR properties. Three of the four maps
between
and
show no
reliable trace of emission from either the galaxy centre or the
surrounding HI ring structure (Fig. 8).
From the variations across the maps, upper limits of
can be derived, unless the FIR emission is diffuse and
extended over a large fraction of the available maps. Only at
,
a weak compact source centred at the
position of the galaxy appears to be detected.
Its flux is
.
Given the weakness of the FIR emission from IC 2006, these limits and
the detection at
are consistent with the IRAS values given by Knapp et al. (1989). As expected, the
ISOCAM image (Fig. 9) shows
the resolved appearance from the star light of the elliptical galaxy
with a total flux of
,
but possibly also a central unresolved core. Overall, its appearance
is quite similar to that of the S0 galaxy NGC 3998 (Knapp et al. 1996).
Unfortunately, the image is too small to allow detection of the HI ring of IC 2006 with its
radius (Schweitzer et al. 1989).
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Figure 8:
Gray-scale representation of the
ISOPHOT
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The detection at
and non-detections at the
other wavelengths might be taken as evidence that the SED of the dust
emission actually peaks around
,
which means
that the dust in IC 2006 would be rather warm, with dust temperatures
lying in the range 25 K-30 K. Using the
flux, a dust opacity of
,
and other
parameters as above, the associated dust mass lies in the range
,
much lower than the dust mass in NGC 2293 / 2292, and also
about a factor 10 lower than that of the elliptical/S0 galaxy
sample studied by Bregman et al. (1998).
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Figure 9:
Gray-scale representation with overlaid isocontours of the
ISOCAM
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The NGC 2293 / 2292 galaxy pair has several unusual properties,
which sets it apart from other galaxy groups, particularly with
interacting members, which have hitherto been studied in the FIR.
Most outstanding is the extremely high 170/100 integral flux ratio of ,
one of the highest ratios of all single galaxies
or pairs measured in the FIR range so far, close to the coldest
galaxies found in the ISO observations of the Virgo Cluster galaxy
sample (Popescu et al. 2002; Tuffs et al. 2002). Its immediate implication is
the very cold dust color temperature of only 13 K, much lower than
the average value of the Milky Way (Sodroski et al. 1994) and other
normal inactive spiral galaxies (Stickel et al. 2000).
Deriving similar low dust temperatures in other
interacting galaxies has as yet been possible only with the use of sub-mm
data (e.g., Haas et al. 2000).
This very low dust temperature is even more unusual, since the
NGC 2293 / 2292 2295 group is an apparently interacting system,
for which much higher dust color temperatures, mostly from
IRAS
and
data, have usually been
inferred (e.g., Bushouse et al. 1998; Horellou & Booth 1997, and references therein).
This is also the first detection of cold dust with such a very low
temperature in a galaxy group based on FIR data alone. Only the
data clearly reveal the presence of the dust
in these galaxies, while the shorter wavelengths
only give a weak detection (Figs. 2 and 3).
As a consequence, galaxies or galaxy groups with similar low dust
temperatures and low
FIR fluxes of
1 Jy have most likely
up to now escaped detection because the IRAS survey only hardly
reached this detection limit, while in shorter wavelengths data such
objects are not present and longer wavelengths data are still
scarce. Moreover, given the large discrepancy of a factor 5 between
the uncertain IRAS flux and the ISOPHOT
flux,
similar cases may just not have been recognized because of the
overestimation of the IRAS fluxes and the resulting too-high derived
dust color temperatures. A first step towards finding more such very
cold galaxies might be the galaxy catalogue from the ISOPHOT
Serendipity Survey (Stickel et al. 2004b), where a number of
detections had no counterparts at the shorter
IRAS wavelengths. Clearly, follow-up observations are required to secure
the FIR emission and the implied low dust temperatures of these sources.
This is also the first unambiguous detection of extended FIR emission
outside galaxies due to cold dust in an interacting galaxy group,
thereby strengthening the concept of dust removal via gravitational
interaction (Stickel 2004). The
detection of FIR emission from an optically visible dust structure
south-east of NGC 2293 at the base of a tidal arm,
as well the diffuse FIR emission surrounding NGC 2295,
shows that gravitational interactions can spread dust across large
regions from the galaxy centres and move dust
structures out of the disks.
A break in the B-R color of NGC 2293 / 2292 distribution has been
observed (Barnes 1999), where the region inside the HI ring closer
to the two galaxy centres is bluer by 0.2 mag than outside (see also
Fig. 5). This indicates a younger stellar
population, but nevertheless the dust there has apparently not being
heated by the strong UV radiation of recently formed stars. This is
also supported by the absence of detectable
emission, which is usually at least partially
attributed to a warmer dust component from
very small grains. Gravitational interactions therefore are not
necessarily accompanied by vigorous star formation, which in turn will
significantly heat the dust. This accords with several studies of interacting
galaxies, which also did not find a significantly enhanced FIR emission or increased dust temperatures (Bergvall et al. 2003, and references
therein). In fact, in the case of the
NGC 2293 / 2292 galaxy pair, a process capable of reducing the dust
temperature below that found in normal inactive galaxies, which are
still star-forming at a low level, seems to be necessary.
HI is detected only in a ring, where the maximum is
closely coincident in position with extended dust emission towards
the south-east. The total HI mass rescaled to a distance of 17.1 Mpc
is
(Barnes 1999),
while the upper limit to the (rescaled) mass of molecular hydrogen
is
(Huchtmeier & Tamman 1992),
which together with the dust mass of
gives a
gas-to-dust ratio of only
,
much lower than that
of the Milky Way and at the lower end of the distribution of normal
inactive spiral galaxies (Stickel et al. 2000). Since the two
galaxies are not detected individually in HI while strong FIR emission
from cold dust is clearly detected, the inferred gas-to-dust ratio for the inner
region of the NGC 2293 / 2292 pair is much smaller. Assuming
that
of the dust mass is near the galaxy centres,
and the upper limit for the HI gas in both galaxies
is
(Barnes 1999, converted to 17.1 Mpc),
then the upper limit for the gas-to-dust ratio in the inner regions of
the galaxies is
1. The situation is similar for NGC 2295 since there is cold dust emission in the area
close to and surrounding the galaxy, although no HI has been detected
(Barnes 1999; Rupen et al. 2001). Either the gas has been separated from
the dust in all three galaxies by a dynamical process, or has been
converted to stars without the usual consequence of strong dust
heating by a young stellar population. Conversely, the low overall
gas-to-dust ratio might be taken as evidence that the large spread
seen in the gas-to-dust ratio of normal inactive spiral galaxies
(Stickel et al. 2000) might at least be partially caused by
gravitational encounters and the resulting consumption by star
formation or tidal removal of neutral HI gas. This raises some doubts
about the common practice of using a standard gas-to-dust ratio for
deriving gas masses from FIR luminosities across a broad variety of
galaxy types.
Overall, the distortions seen in the NGC 2293 / 2292 / 2295 triplet
bear some resemblance to Stephan's Quintet (Arp 319), an interacting
galaxy group with optically disturbed members (Gutierrez et al. 2002),
where most of the HI is located outside of the galaxies
(Williams et al. 2002). Large radial velocity differences of several 100 km s-1 are also seen there, similar to that found in
NGC 2293 / 2292 / 2295. Extended FIR emission is also present
throughout the group (Stickel 2004; Xu et al. 2003), but the temperature of the
diffuse dust there is most likely higher since it is already detected at
.
The NGC 2293 / 2292 / 2295 group might
thus either be less violently interacting or is in a different
earlier or later stage, where star formation has not yet been started
or has already died out, which also can explain the missing emission
lines in the galaxy centres (Dahari 1985). If large scale very
cold dust distributed outside galaxies is common in such interacting
groups, it will go undetected as FIR measurements near
are necessary and only few groups have been
imaged with ISOPHOT at these long FIR wavelengths. Particularly
missing is an FIR map of Stephan's Quintet at an FIR wavelength beyond
to study the distribution of the cold dust in
this group. A further example of cold dust
spread across an interacting galaxy group may be CG 1720-67.8
(Weinberger et al. 1999), where optical spectra revealed a strong
optical extinction and significant star-forming activity, yet the group
is not detected in the IRAS bands.
The detected cold dust in the HI ring structure around NGC 2293 / 2292 clearly shows that at least this HI ring is most likely not primordial (Knapp 1999). The HI ring around IC 2006, which is not detected in the FIR, however, could in fact be a remnant from the early collapse of the central elliptical, although a tidal capture of either an unevolved HI-rich and metal-poor dwarf or of the outer halo regions of a low surface brightness galaxy also is a viable alternative. Clearly, the apparently similar HI rings around NGC 2293 / 2292 and IC 2006 can have quite different origins.
Assuming that the origin of the HI rings is a gravitational interaction in both NGC 2293 / 2292 and IC 2006, FIR observations can hint at the type of the progenitor galaxies. If apparently similar HI features have a similar history, the detected gas can nevertheless have its origin in different types of progenitor galaxies. In the case of NGC 2293 / 2292, it very likely has been gas and dust rich, while in the case of IC 2006 it is likely to have been gas rich but dust poor. The prospect of FIR measurements is then to break the degeneracy of other indicators and to provide independent clues to the history of such events (Stickel et al. 2004a).
The currently rather limited data on NGC 2293 / 2292 / 2295 presented here clearly indicate that this is a highly unusual gravitationally interacting group. Nevertheless, a more detailed understanding of this system definitely requires new high resolution FIR, HI, and CO data, which would allow a separate investigation of the galaxy centres, the surrounding bluer region, and the dust structures seen towards south-east across a large wavelengths range. Obviously equally interesting are the compact and diffuse FIR emission near NGC 2295, which is only partially covered by the ISOPHOT data.
Acknowledgements
We thank Rachel Webster for initiating the spectroscopic observations of NGC 2292 / 2293 / 2295. 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 made use of NASA's Astrophysics Data System Abstract Service, the Simbad Database, operated at the 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. It is based on photographic data obtained using The UK Schmidt Telescope 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.