A&A 401, 173-183 (2003)
DOI: 10.1051/0004-6361:20030108
G. Trinchieri1 - J. Sulentic2 - D. Breitschwerdt3 - W. Pietsch3
1 - INAF-Osservatorio Astronomico di Brera, via Brera 28, 20121
Milano Italy
2 -
Physics & Astronomy, University of Alabama, USA
3 -
Max-Planck-Institut für extraterrestrische Physik,
Giessenbachstraße, 85740 Garching,
Germany
Received 10 October 2002 / Accepted 17 January 2003
Abstract
Chandra observations of the compact galaxy group known as
Stephan's Quintet (SQ) are presented. The major morphological features
that were discovered with the ROSAT HRI are now imaged with higher
resolution and S/N. The large scale shock (1
5,
40 kpc if at
85 Mpc) is resolved into a narrow NS feature embedded in more extended
diffuse emission (
). The NS structure is somewhat clumpy,
more sharply bounded on the W side and prominent only in the soft band
(energies below
2 keV). Its observational properties are best
explained as a shock produced by a high velocity encounter between
NGC 7318b, a "new intruder'', and the intergalactic medium in SQ. The
shock conditions near the high speed intruder suggest that a bow shock
is propagating into a pre-existing H I cloud and heating the gas to a
temperature of 0.5 keV. The low temperature in the shock is a
problem unless we postulate an oblique shock.
One member, NGC 7319, hosts a Seyfert 2
nucleus, with an intrinsic luminosity
erg s-1,
embedded in a region of more diffuse emission with 10'' radius
extent. The nuclear spectrum can be modeled with a strongly absorbed
power-law typical of this class of sources. Several additional compact
sources are detected including three in foreground NGC 7320. Some of
these sources are very luminous and could be related to the
ultraluminous X-ray sources found in nearby galaxies.
Key words: ISM: general - X-rays: galaxies: clusters - galaxies: ISM - X-rays: ISM
Stephan's Quintet (HCG92, Hickson 1982, hereafter SQ) is the most studied example of the compact group phenomenon. It is composed of six galaxies (Sulentic et al. 2001, S01 hereafter) including a core of three (NGC 7317, NGC 7318a and NGC 7319) with essentially zero velocity dispersion and an unrelated foreground object (NGC 7320). Multiwavelength observations of SQ give strong evidence of multiple episodes of past and recent (current) interaction, most likely caused by acquisition of new members/passage of intruders from the associated larger scale galaxy population near SQ. Both NGC 7320c and NGC 7319 show spiral morphology without detectable H I while the other two core members are ellipticals. The last and presumably ongoing event involves the collision of the gas-rich spiral NGC 7318b with the debris field produced by past interactions. About half of that galaxy's ISM has been stripped/shocked. SQ is certainly the best local example of a compact group caught in flagrante delicto with multiple manifestations of interaction events. It is this kind of event that presumably accounts for general compact group characteristics including H I deficiency, quenched star formation and an excess of early-type members (e.g. Hickson & Rood 1988; Sulentic & de Mello Rabaca 1993; Verdes-Montenegro et al. 2001; Vilchez & Iglesias-Paramo 1998). SQ is thus an ideal laboratory for studying interactions per se, as well as, an excellent local analog to the processes thought to be much more common at high redshift.
We present CHANDRA observations of SQ where we confirm the complex
nature of the X-ray emission already reported (Pietsch et al. 1997, S01).
Most, if not all, of the basic types of
X-ray emission observed from extragalactic sources are found in SQ
(AGN, shocks, jets, "normal'' galactic emission, diffuse emission), and
can be studied in this context.
![]() |
Figure 1:
Left: Full field Chandra image (ACIS-S: back-illuminated chip
S3 only) of SQ in the 0.5-8 keV energy range with X-ray
contours and source numbers (see Sect. 2.2) superimposed.
The map is binned with
![]() ![]() |
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![]() |
Figure 2:
A zoomed X-ray contour map (0.5-3 keV) of the main concentration of
X-ray photons. Contours are superimposed on a CFHT
B-band image of the field. The X-ray data are
smoothed with an adaptive filter and 2.5![]() |
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A 19.7 ks observation of SQ was obtained with Chandra in July 2000
using the back-illuminated CCD (ACIS-S in imaging configuration). The
data were reprocessed using the new calibration files as described in
the CIAO documentation. Following the "CIAO Science Threads" at the
CXC home page, we verified that
the data obtained with standard processing had been properly cleaned
(e.g. for high background events). We produced several images with
pixel resolution, covering different portions of the field
and in different energy bands. We applied a Gaussian filter or an
adaptive smoothing filter using the csmooth routine in the CIAO
package (FFT method, and using 2.5
as the minimal
significance of the signal within the kernel). The left panel of
Figure 1 presents an X-ray image of the entire
back-illuminated CCD chip (S3) with X-ray contours superimposed. The
right panel shows X-ray contours superposed on an optical B-band image
from the DSS2. Considerable extended emission is concentrated within
the core of the compact group. Additional sources are associated with
the galaxies (members of SQ and the foreground galaxy NGC 7320) or, in
some cases, may be unrelated background sources (see
Table 1).
Figure 2 presents a closer look at the X-ray emission most unambiguously associated with SQ. The soft (0.5-3.0 keV) X-ray contours are shown overlayed on an average of CFHT B-band images kindly provided by C. Mendes de Oliveira (see Plana et al. 1999; Mendes de Oliveira et al. 2001; S01 for discussion of the images). The complex X-ray emission that was detected in previous ROSAT observations (Pietsch et al. 1997; S01) is clearly resolved into two main components almost certainly associated with SQ: 1) complex clumpy and extended emission centered on a radio continuum/optical emission-line emitting shock zone and 2) emission from the Seyfert nucleus in NGC 7319. Additional sources detected do not always have obvious optical counterparts, although they appear to be associated with the galaxies in several cases (the central region of NGC 7318a, and associations in Table 1).
Figure 3 shows Chandra images in different energy ranges.
Comparison of the images shows that the extended X-ray emission is
found only below 2 keV, and more compact sources coincident with the
Seyfert 2 nucleus of NGC 7319 and an unresolved source SW of the
NGC 7318a nucleus and apparently coincident with one of the new intruder
emission regions (#14 in Fig. 7 of S01) are prominent at higher
energies.
![]() |
Figure 3: Smoothed images of the ACIS-S data in SQ, in different energy bands. An adaptive filtering technique (FFT convolution method) is applied to the data. Crosses in the rightmost panel indicate the galaxy's centers. |
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Diffuse emission in SQ extends from NE of
the Seyfert 2 nucleus of NGC 7319 to SW of
the nucleus of NGC 7318a. Elongated clumpy
NS structure lies near the center of more
diffuse emission. We derived azimuthally averaged
radial profiles of the extended emission
in several energy bands and in several ranges
of position angle. The goal of the radial profile
analysis was to determine the extent of diffuse
emission for comparison with images at other
wavelengths. We centered the radial profiles at
,
and excluded all discrete
sources (Sect. 2.2). Figure 4
shows the azimuthally averaged
raw count profiles in two broad energy bands.
Extended emission is only detected
below
1.8 keV and out to a radius of
.
![]() |
Figure 4:
Left: Azimuthally averaged radial profiles of the total emission
in the 0.5-2.0 and 3.0-10.0 keV energy ranges.
Right: Radial profile of the total emission in different
azimuthal quadrants, as indicated
(0.5-2 keV band). The ![]() ![]() ![]() ![]() |
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![]() |
Figure 5:
Total counts in a EW oriented strip
centered at 22![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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Profiles derived for different azimuthal quadrants
indicate that the extent of the emission is similar
in all directions but that the intensity distribution
is not. Detailed comparison of azimuthal averages is
not straightforward because the extended emission is
complex. The NS distribution is dominated by the shock.
The E profile is similar to the NS
distribution although less intense. It shows a smoother
decline and a higher intensity level than towards the W.
A larger and more abrupt drop in intensity is seen
towards the W followed by an apparent rebound (Fig. 4).
The E-W difference is also evident in
the EW cut taken across the middle intensity peak of the
NS feature (Fig. 5). This plot indicates a sharp decrease to a
very low count level in a narrow and elongated region
(
)
along the W edge of the NS
feature. The emission increases again W of this minimum
and extends outwards for an additional
50''-70''.
The low intensity strip is not aligned with the pixels
on the CCD chip which leads us to conclude that it is
not an instrumental artifact.
An additional weak extended X-ray component is seen near
,
.
This 15'' radius
region (
source) coincides with structure in the older
tidal tails (Arp & Lorre 1976; S01). However, since several optical
condensations are found in this same region we cannot rule out a line
of sight coincidence with background sources. We will explore this
feature in more detail with recently obtained XMM-Newton data
(Trinchieri et al., in prep).
A considerable number of discrete sources is found in and near SQ.
Some are resolved and other are not, while several show associated
(i.e. concentric) extended emission. Table 1 lists
discrete sources with positions, count statistics, fluxes, X-ray
luminosities and possible optical counterparts (see Fig. 1).
The source list is derived from the wavdetect and celldetect programs that gave virtually identical results. The
background is derived locally around each source and is negligible for
sources outside the region where complex emission is detected.
Count rates in the total energy band are
converted into unabsorbed fluxes and luminosities assuming a constant
conversion rate of 1 c
erg cm-2 s-1(corresponding to either a power law with
,
or a
bremsstrahlung with kT=10 keV, and line-of-sight absorption), and a
distance of 85.6 Mpc for SQ sources, and 12 Mpc for
NGC 7320 (see Table 1). Only fluxes are quoted for
sources outside or not unambiguously associated with SQ.
CXC name | Sou | RA / Dec | Total | Net | flux | luminosity | Notes |
# | (J2000) | counts | (0.3-10 keV) | ||||
CXOU J223553.9+335946 | 1 | 22:35:53.94 | 24 | 23.8 ![]() |
1.1
![]() |
Background? | |
33:59:46.91 | no optical counterpart | ||||||
CXOU J223555.6+335738 | 2 | 22:35:55.69 | 58 | 57.6 ![]() |
2.7
![]() |
Associated with | |
33:57:38.82 | NGC 7318b? | ||||||
CXOU J223556.6+335756 | 3 | 22:35:56.68 | 27 | 24.4 ![]() |
1.1
![]() |
9.7
![]() |
NGC 7318a 1'' radius |
33:57:56.04 | 72 | 64.4 ![]() |
3.0
![]() |
2.7
![]() |
NGC 7318a 5'' radius | ||
CXOU J223557.3+335818 | 4 | 22:35:57.36 | 8 | 7.4 ![]() |
3.4
![]() |
Near NGC 7318b | |
33:58:18.76 | emission reg | ||||||
CXOU J223557.6+335859 | 5 | 22:35:57.61 | 26 | 25.4 ![]() |
1.2
![]() |
Near NGC 7318b | |
33:58:59.72 | emission reg | ||||||
CXOU J223603.4+335653 | 6 | 22:36:03.40 | 13 | 12.5 ![]() |
5.8
![]() |
9.9
![]() |
NGC 7320 nucleus? |
33:56:53.74 | |||||||
CXOU J223603.4+335650 | 7 | 22:36:03.46 | 15 | 14.6 ![]() |
6.7
![]() |
1.2
![]() |
NGC 7320 |
33:56:50.40 | S of nucleus | ||||||
CXOU J223603.6+335833 | 8 | 22:36:03.60 | 706 | 699 ![]() |
(1.4
![]() |
(1.1
![]() |
NGC 7319 - nucleus |
33:58:33.12 | (2.6
![]() |
(2.3
![]() |
NGC 7319 - extended | ||||
CXOU J223603.6+335825 | 9 | 22:36:03.66 | 36 | 34.7 ![]() |
1.6
![]() |
1.4
![]() |
NGC 7319 |
33:58:25.01 | S of nucleus | ||||||
CXOU J223604.8+335901 | 10 | 22:36:04.82 | 13 | 12.6 ![]() |
5.8
![]() |
5.1
![]() |
NGC 7319 |
33:59:01.19 | |||||||
CXOU J223606.5+335625 | 11 | 22:36:06.50 | 25 | 24.4 ![]() |
1.1
![]() |
1.9
![]() |
NGC 7320 |
33:56:25.83 | SE edge | ||||||
CXOU J223606.7+340106 | 12 | 22:36:06.70 | 8 | 7.8 ![]() |
3.6
![]() |
Background? | |
34:01:06.40 | no optical counterpart | ||||||
CXOU J223607.0+335829 | 13 | 22:36:07.05 | 18 | 17.5 ![]() |
8.1
![]() |
7.2
![]() |
NGC 7319 |
33:58:29.52 | E source | ||||||
CXOU J223622.2+335651 | 14 | 22:36:22.29 | 19 | 13.3 ![]() |
6.1
![]() |
Background? | |
33:56:51.70 | no optical counterpart |
NOTES: a The flux is derived from the spectral analysis and refers to the nuclear component only (see Sect. 2.3). Counts refer to the total emission within 10'' radius. b The flux refers to the extended emission within a 10'' radius around the nuclear source, and is derived from the spectral analysis (see Sect. 2.3). |
Given the statistical significance of the Chandra data, we
confine our spectral analysis to the extended emission and the
central regions of NGC 7319. We used the psextract script in CIAO
to create appropriate spectral matrixes (eg. arf and rmf
files) for the analysis, which was done using XSPEC. While the script
is not appropriate for extended emission, the regions we have selected
are small and, consequently, the error introduced by the point-source
assumption is negligible (see also discussion in Trinchieri &
Goudfrooij 2002; Hicks et al. 2002). The degradation of the ACIS
Quantum efficiency, recently discussed was considered.
However, our data are not affected significantly because the observation
was done at early stages in the mission.
![]() |
Figure 6:
Spectral distribution and plots of the contributions to
![]() ![]() ![]() ![]() |
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We identified a region along the NS feature that includes the three
brightest condensations. The background was obtained from adjacent
regions. The data was binned to yield a
per bin after
background subtraction. This feature was only detected in the 0.35-1.75 keV energy range. We adopted a plasma code (MEKAL model in XSPEC) to
describe the data which gives best fit parameters
keV
(0.47-0.58 at 90% confidence for one interesting parameter), low
abundance (0.12-0.30) and absorption consistent with the galactic line
of sight value of
cm-2 (see
Fig. 6). Assuming the best fit model parameters, we
derive an unabsorbed
erg cm-2 s-1,
and
erg s-1. Some residuals might be
present possibly even coincident with oxygen lines that are not
accounted for by the assumed single temperature model. They might be
indicative of more complex gas properties (see Fig. 6).
We also note that the derived best fit parameters are far from unique:
a two temperature component spectrum, with a 0.5 keV MEKAL plasma and
solar abundances plus
5 keV bremsstrahlung component yield an
equally good fit to the NS feature. Moreover, a crude examination of
the X-ray colors along the NS feature indicates that the three clumps
might have different energy distributions, suggesting spectral
variations even within this small region.
The lower surface brightness emission around the NS feature (i.e. what
we used as background) was also modeled (Fig. 6). We
selected a circle of
radius, excluding discrete sources
(e.g. NGC 7318a and NGC 7319) and the NS feature. Assuming a thermal
plasma spectrum with low abundances we find kT=0.61 keV (0.56-0.65).
The total unabsorbed flux of this component is
erg cm-2 s-1.
We point out that our
interpretation of the NS feature and surrounding area in terms of a
recent shock would likely imply different spectral characteristics
(namely a multi-temperature non-equilibrium spectrum,
cf. Breitschwerdt & Schmutzler 1999) than those derived above under
the assumptions of equilibrium conditions.
However, the relatively small number of photons in these
components does not allow us to consider more sophisticated models,
that might be possible with the higher statistics XMM-Newton
data.
![]() |
Figure 7: Comparison of the energy distribution for resolved and unresolved photons from NGC 7319. Error bars on the y-axis indicate the statistical uncertainty, on the x-axis the width of the energy bin. The larger aperture (10'' radius, plus symbol) indicates a significant increase in photons with energies below 2 keV relative to the smaller (1'' radius, circle symbols) aperture. |
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![]() |
Figure 8:
Energy distribution of the counts from
the entire 10'' radius region centered on NGC 7319 (and ![]() ![]() ![]() ![]() ![]() ![]() |
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We examined the spectral properties of both the central point source
(1'' radius circle) and a larger region (r=10'') inclusive of
it. The
nuclear X-ray spectrum is complex and requires multiple components.
Comparison with the spectral distribution for
the larger region indicates an increase in a soft contribution which
may be related to a "galactic" component (see Fig. 7).
Since the optical spectrum indicates a Seyfert 2 nucleus, we assume a
combination of absorbed and unabsorbed power law components plus a
narrow 6.4 keV emission line (see e.g. Della Ceca et al. 1999;
Moran et al. 2001; and references therein). The power law index should be
the same for both components with the unabsorbed component representing
1-10% of the absorbed one. Moran et al. report two different
power-law indices for an average X-ray spectrum involving 29 sources
although a single value is consistent with their data. Given the
limited statistical significance of our data, we model the nuclear
source using a single
for both power-law components (free)
plus a Gaussian line at 6.4 keV (at the redshift of NGC 7319).
We applied the absorbed+unabsorbed power-law model to both the nuclear
and larger regions. Data above 1.5-2 keV are well described by
such a model but excess residual emission is found at lower energies
and becomes stronger in the spectrum of the larger region. This
motivated us to exclude data below
2 keV from the fit. Adopting
such a "nuclear" model yields best fit
1.7,
cm-2. The FeK line is fit with
E=6.4 keV and
eV. This model gives a good
for 29 degrees of freedom (d.o.f.). Figure 8 shows the spectral
data and the contributions to
in individual energy bins
relative to the combined plasma + unabsorbed and absorbed power-law
model with Gaussian line. A bump is suspected at
keV.
Although introducing this additional component might improve the fit
(the best fit
value reduces by 8 for 3 additional parameters;
f-test probability = 99.99%), there are no known thermal emission lines
in this energy range (see Wilms et al. 2001 where a similar feature is
suspected in a Seyfert 1 source). The larger aperture data require an
additional component at soft energies that can be modeled with either a
MEKAL or a raymond plasma code (Raymond & Smith 1997) with kT = 0.65 keV (MEKAL) or
0.75 (raymond).
The data quality does not allow us to derive meaningful confidence
contours, nor does it guarantee that the model is unique. We can
however describe the spectral characteristics of the emission from
NGC 7319 as due to the superposition of a strong and heavily absorbed
nuclear source embedded in more diffuse softer emission. The
parameters for the nucleus are consistent with those of a typical
Seyfert 2. The intrinsic fluxes derived from the best fit models are:
erg cm-2 s-1 (0.1-2 keV) and
erg cm-2 s-1 (0.3-10 keV) for the extended soft emission;
erg cm-2 s-1 in the entire 0.3-10 keV band
and
erg cm-2 s-1 in the 2-10 keV band for the
Seyfert nucleus, corresponding to an intrinsic
(2-10 keV) =
erg s-1. The high energy component from NGC 7319
was already discovered by ASCA (Awaki et al. 1997). The lower spatial
resolution of that observation did not allow the authors to properly
separate the Sey 2 contribution from more extended emission although
all hard emission was assumed to be from the nucleus. The spectral fit
was similar to the one derived here except that we find a
significantly smaller equivalent width.
![]() |
Figure 9: A very schematic view of the presumed path of the old intruder NGC 7320c. The image of new intruder NGC 7318b is suppressed (shaded out) since it was not in SQ at the time. |
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A key element in compact group evolutionary history involves stripping events. The distribution of neutral gas in SQ is a spectacular example of these processes that must involve multiple interactions with group members and/or intruders acquired from the surroundings. Studies of the events and conditions in SQ can therefore give insights into processes that are common in such galaxy aggregates.
Moles et al. (1997) proposed a dynamical model for SQ to explain the
observational evidence. Using a larger body of multiwavelength data,
S01 recently evaluated and tested their "two intruder'' scenario, that
interprets twin tidal tails as the products of two subsequent passages
of NGC 7320c, the old intruder, through the group (cf.
Fig. 9). The new intruder, NGC 7318b, is now entering SQ
with a relative line of sight velocity of 1000 km s-1.
Some aspects of the two intruder scenario are not shared by all
investigators (e.g. Williams et al. 2002), but there is a consensus
about the evidence of past and present episodes of interactions in SQ,
that allow us to interpret much of the X-ray
evidence in a self-consistent way.
The increased sensitivity of Chandra allows us to identify several
components of extended X-ray emission. 1) The NS feature, a clumpy
structure elongated in the NS direction (with possible branches towards
NGC 7319 and the NW) closely coincident with the strongest radio
continuum and optical line emission. 2) An irregular low surface
brightness component surrounding the NS feature (
).
3) Smaller scale extended emission coincident with NGC 7319 and
NGC 7318a. Component 1) is almost certainly shock related while
component 2) could involve shock and/or underlying diffuse emission.
The most straightforward interpretation for components 3) involves an
association with the respective galaxies. We cannot rule out a
connection with the recent/ongoing collision because both galaxies,
especially NGC 7318a, may lie in the path of the new intruder.
Individual galaxies are discussed in Sect. 4.
![]() |
Figure 10:
X-ray contours (from the 0.5-2.0 keV band adaptively smoothed map)
superimposed on optical emission line and radio continuum data.
Contours are overlayed on: a) X-ray image, binned to
![]() ![]() ![]() |
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The NS feature was previously interpreted as evidence for a large scale
shock (Pietsch et al. 1997; S01). Chandra data reinforce this view
with better evidence for (see Fig. 10):
a) spatial coincidence between the NS feature
and similar radio continuum, H
and [N I I ] emission structures;
b) lack of coincidence between the X-ray emission and the galaxies or
extended stellar halo in SQ (Moles et al. 1998) and c) lack of X-ray
coincidence with HI line emission in the velocity range of the new
intruder NGC 7318b (S01).
Optical spectroscopy (Xu et al. 2001) also shows that emission line
ratios from gas coincident with the NS region imply shock conditions.
In the simplest scenario, the shock results from the collision of
NGC 7318b with previously stripped gas in SQ.
In the strong shock approximation, as expected from an
impact with
1000 km s-1, the shock temperature
This shock temperature is significantly higher than that derived in
Sect. 2.3 and cannot be interpreted as the result of
significant cooling if the collision is ongoing (or even recent, see
later). It
is more likely that the shock conditions are not as simple as assumed
in Eq. (1), and require either an oblique (small ,
cf. Appendix A.1) and/or weak shock (i.e. the upstream medium is
hot and has a sizeable counter-pressure) in which case the strong shock
assumption breaks down. The effects of a magnetic field are also
considered in Appendix A.2, but the field strength derived appears
to be rather high.
If the new intruder collides with a neutral hydrogen cloud, as
suggested by the spatial "continuity" between the NS feature and
H I clouds N and S of it with consistent velocities (see
Fig. 11), we expect strong shock conditions to prevail. To
reconcile expected and observed post-shock temperatures, the incoming
flow should cross the shock at an angle of
,
for
an upstream H I temperature of 100 K (see Appendix A.2,
Eq. (A.3)). Since the upstream Mach number,
,
is very high, the opening angle of the Mach cone,
and thus also of the bow shock, should indeed be fairly small.
Therefore we do not expect a significant amount of hotter gas to exist
(e.g. in the stagnation point region, where the temperature is at most
a factor of 4.5 higher). Thus the shock conditions look fairly
reasonable, and although they may look somewhat specific, they are
easily realized.
Due to the high Mach number, the compression ratio should be close to
that for a perpendicular shock, e.g.
(Eq. (A.1)). To evaluate the gas densities
,
we
can assume the simplified spectral models derived in
Sect. 2.3.1, that will give a good estimate of these
quantities.
In the NS feature, with a luminosity
erg s-1 in an ellipsoidal volume with
(a, b,
c denote the semi-major axes), the density:
![]() |
(2) |
We can infer the pre-shock density
as a mean value of
cm-3 derived for the H I densities in
Arc-S and NW-HV clouds (from the data of Williams et al. 2002). We therefore
derive a compression ratio
,
consistent to the
expected value (Eq. (A.1)).
The large multiwavelength database available for SQ motivates additional considerations:
The gas involved in the NS
X-ray/H
+[NII]6583/radio continuum feature, and at least some of
the H I clouds, is a product of past tidal interactions. Ram
pressure stripping is not very efficient, given the low densities
involved and the low velocity dispersion in SQ. This suggest that
NGC 7318b can retain a significant residual ISM after the encounter
with SQ:
(Takeda et al. 1984) gives
km s-1,
using
for the IGM number
density and
for the average
galaxy's ISM density. The derived velocity dispersion
corresponds to a galaxy mass of less than
.
The NS feature is spatially (anti)correlated with
the residual H I gas N and S of it. A lot of this gas may be related to
the last passage of NGC 7320c through SQ. The central part of it has
recently been shocked by NGC 7318b. The transverse velocity component
is unknown and the direction of motion of NGC 7318b is therefore
uncertain. Striations in the HST and CFHT images (S01) suggest motion
from SW to NE or vice versa. Contour lines of the X-ray emission in NS
feature suggest a higher compression on the W side, favouring motion
from NE to SW.
![]() |
Figure 11:
Comparison of the spatial distribution of the neutral hydrogen (left;
from
Fig. 5 in S01) and X-rays (right) relative to H![]() ![]() ![]() |
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The low surface brightness emission surrounding the
sharper NS feature could be interpreted as a preexisting hot IGM.
The low velocity dispersion in SQ members requires it
to have been heated by previous collisions (e.g. NGC 7320c?). The
inferred trajectory in Fig. 9 and a projected distance of
120 kpc, allow us to estimate the projected velocity
km s-1, which is a lower limit. According to
Eq. (1) this velocity corresponds to
K, in excellent agreement with the measured X-ray temperature.
The long cooling time derived above relative to the inferred last
passage of NGC 7320c suggests that such a hot IGM component
would not have cooled significantly over this period. The rather
uniform distribution of the low surface brightness emission (in
contrast to e.g. the NS feature) would be due to the equilibration by
shock and/or sound waves that were able to propagate through the IGM
within at least the last
(sound crossing
time scale). We expect that the impact of NGC 7318b will produce only a
weak shock here, given the relatively high gas temperature. If this
component is related to the ISM of NGC 7318b, rather than residual hot
gas in SQ, we must assume that it has expanded and cooled from an
expected post-shock temperature given by Eq. (1).
The X-ray/radio continuum/optical emission line extension
or tail towards the NW is a puzzle if the collision is ongoing.
The X-ray cooling timescale is about an order of magnitude longer
than the new intruder crossing (
(NGC 7318b)
years). If the former timescale were a better estimate for the
age of the shock we could more easily account for the NW extension as
a tidal feature.
The X-ray gap shown in Fig. 5 is an additional
puzzle although similar features have been observed in richer
environments. The prominent "cold front" observed in the cluster
A3776 is interpreted as colder gas moving into a hotter ambient medium
(Vikhlinin et al. 2001). Mazzotta et al. (2002) identify two
additional features, one of which is an arc-like filamentary X-ray
depression oriented perpendicular to the direction of motion. Large
scale hydrodynamic instabilities coupled with magnetic field effects
are suggested to explain these morphological peculiarities.
The surface brightness discontinuity seen in
SQ appears sharper and more prominent than in the other examples. A
temperature discontinuity is associated with the surface brightness
edges in other examples and we cannot detect such a feature in SQ. On
the other hand, the temperatures of both the ambient gas and the NS
shock feature in SQ are significantly lower than the gas in A3667. This
could cause a more significant drop in emissivity if the temperature
falls below the energies at which the instruments are sensitive.
Forthcoming observations with XMM-Newton, that will give more spectral
information, will hopefully provide clues to its nature.
A few discrete sources can be identified with galaxies in the group.
NGC 7319:
NGC 7319 is dominated by a strong heavily absorbed nuclear point source
embedded in fainter extended emission that extends to 10''radius. Three additional sources (#9, 10, 13) are associated with
this galaxy: source #9 is near an optical point source and the other
two are projected on the spiral arms but do not appear to coincide with
individual sources or with specific condensations (see
Fig. 12).
A connection between interactions and AGN activity has been difficult
to prove conclusively although recent evidence has been advanced for an
excess of Seyfert 2 nuclei in a reasonably complete sample of compact
groups (Coziol et al. 2000). NGC 7319 shows both evidence for a past
interaction event that stripped it of its ISM (tied to
the last passage of NGC 7320c and the generation of the younger
optical tidal tail) and near nuclear activity in the form of
an emission line jet (Aoki et al. 1996) and triple lobed radio (Aoki
et al. 1999) structure on a 10'' scale that cannot be so easily
related to past interaction episodes. One can argue that the
quasi-continuous nature of the tidal perturbations in compact groups
might more efficiently channel gas into nuclei giving rise to phenomena
of this kind. Interpretation of the extended X-ray emission around
NGC 7319 is complicated by the presence of this optical jet and also by
the nearby unresolved X-ray source (source #9). The most
straightforward assumption is that source #9 is unrelated to the
nuclear activity. That assumption may be challenged by the discovery of
other AGN with nearby unresolved X-ray sources (Mrk 3, Morse et al.
1995; NGC 4258, Wilson et al. 2001a): in NGC 4151 (Yang et al. 2001)
and Pictor A (Wilson et al. 2001b). Source #9 is 8'' S from
the nucleus (Fig. 12) and may coincide with a compact
optical object (#54 in Gallagher et al. 2001). Its BVI colors are
consistent with a late F to early G main sequence star but the observed
magnitude would put it at
30 kpc without taking into account any
possible extinction. It is therefore unlikely to be a galactic star.
It is embedded in a region where CO is detected (Yun et al. 1997) which
indicates that star formation may be occurring there.
Given the high X-ray luminosity of X-ray point sources assumed to
belong to NGC 7319,
erg s-1, they may be
analogs of the Ultra Luminous X-ray sources (ULX), that are found
in actively star forming galaxies (Zezas et al. 1999; Roberts &
Warwick 2000; Fabbiano et al. 2001) and some in more normal spiral galaxies
(Tennant et al. 2001; Prestwich 2002). A
definite interpretation about the origin of ULX is not yet
available. Both spectral characteristics and variability
arguments suggest that they are binary accretion sources, although the
requirement of a sizeable central collapsed object might favour beaming
or anisotropy in the accretion process (King et al. 2001; Körding et al. 2002).
Sources 10 and 13 are in a
region of NGC 7319 with colors (S01) consistent with a star formation
event that ended about the time it was stripped of its ISM (a few
times 108 years ago).
![]() |
Figure 12:
Close-up of several interesting regions in SQ. The X-ray contours
are superimposed on an HST WFPC2 B-band image
in panel a) and b) and a [S I I ]![]() |
Open with DEXTER |
NGC 7318a. A relatively hard source is coincident with the nucleus, but a broad band (0.5-5.0 keV) radial profile is inconsistent with the Chandra Point Spread Function, suggesting that the emission is extended and likely related to the entire galaxy. This elliptical galaxy appears to lie directly in the path of the ongoing collision. It is the only galaxy in SQ other than NGC 7319 that shows radio emission. All H I and H I I that are found N, W and E of this galaxy belong to new intruder NGC 7318b. One can therefore infer an "inverse-Cartwheel'' scenario where the disk of NGC 7318b has passed directly through this galaxy. While it is tempting to say "yes'' to such an hypothesis, two arguments warrant caution: 1) the X-ray (and radio) properties of NGC 7318a are not unlike more isolated galaxies of the same type and 2) the high velocity of the new intruder leaves little time for the formation of interaction induced structure or activity, unless the collision is "old", as discussed earlier. Of course in situ shocking of gas within the galaxy is not subject to the latter objection.
NGC 7317, NGC 7320c, NGC 7318b. None of these
galaxies are formally detected with Chandra. An enhancement is
visible at the position of NGC 7317. We measure
counts in the
0.3-10 keV band within an area with
4'' radius, which
corresponds to an unabsorbed flux
erg cm-2 s-1. This value can be used as an upper limit for X-ray emission
from the other two galaxies. This is particularly true for NGC 7318b
where complex emission from the NS feature precludes a direct measure.
Isolated bright sources. X-ray sources #2, 5, and possibly 4, are detected close to and possibly associated with bright emission regions SW and N of NGC 7318ab. All such emission regions show velocities consistent with the "b" galaxy (S01). Source #5 could be associated with "putative star" #127 in Gallagher et al. (2001). An enhancement is also seen coincident with ISO detected starburst A (Xu et al. 1999) this time with an SQ velocity. The inferred luminosities are relatively high and consistent with ULX sources in starforming regions.
NGC 7320 We find two sources (#6 and 7) in the
nuclear region of NGC 7320 with luminosities 6 and
erg s-1. Source #6 appears to coincide with the
nucleus (see Fig. 12) and would represent a relatively low
luminosity nuclear source comparable to a bright binary system. A
third source, #11 at the SE edge of NGC 7320 shows a luminosity of
erg s-1. Both extranuclear sources are most
likely bright accreting binary systems similar to the ones found in
other spiral galaxies. A more speculative interpretation would relate
source 11 to the old tidal tail passing behind NGC 7320, in which case
would exceed
1039 erg s-1 at the distance of SQ.
New high sensitivity and high resolution Chandra observations confirm the complexity of the X-ray emission in SQ. The most prominent sources are associated with a large scale shock that is strongest at low energies (E<1.5 keV) and a Seyfert 2 nucleus in NGC 7319 that dominates the emission above 2 keV. Additional sources are likely associated with members of SQ and with the foreground galaxy NGC 7320. Low surface brightness diffuse emission is also detected in the core of the system associated either with a large scale IGM or with the shocked ISM of the new intruder NGC 7318b.
The complex dynamical history of SQ offers the most plausible explanation for the large scale NS shock as a collision between a high velocity intruder and a gaseous debris field produced by earlier interacting events. Analytical evaluations of the shock conditions, taking into account the X-ray morphology and spectrum, require an oblique shock propagating into a pre-existing H I cloud. The alternative is to postulate that the collision is not ongoing and that the shock has cooled considerably.
Detailed X-ray analysis of compact groups can also provide us with further insights into the problem of IGM metal enrichment and, in particular, whether galaxy interactions rather than galactic winds are the primary process for entropy and heavy element input into the IGM. The spectral fits presented here are carried out under the assumption of collisional ionization equilibrium. They suggest a low IGM metallicity and a preponderance of tidal interactions over galactic outflows. More detailed investigations are not warranted by the statistics of the Chandra data. Analysis of new XMM-Newton data will provide more stringent constraints on spectral and dynamical properties of the collision scenario discussed here. Stephan's Quintet continues to be an excellent laboratory for studying dynamics and evolution in compact groups and represents one of the most useful local analogs of phenomena thought to be much more common at high redshift.
Acknowledgements
G.T. thanks G. Hasinger, J. Trümper and all colleagues at the Max-Planck-Institut für extraterrestrische Physik (MPE) for fruitful discussions and hospitality while part of this work was done. GT acknowledges support from grants from the Italian Space Agency (ASI). D.B. thanks G. Hasinger and the MPE for financial support. J.S. acknowledges financial support under NASA grant GO0-1142X.
The compressed files of the "Palomar Observatory - Space Telescope Science Institute Digital Sky Survey" of the northern sky, based on scans of the Second Palomar Sky Survey are copyright (c) 1993-2000 by the California Institute of Technology and are distributed by agreement. All Rights Reserved.
General shock physics tells
us that a supersonically moving body will have a bow shock at its
leading edge. In the rest frame of the high velocity
intruder the incoming flow
will enter the shock wave at some angle ,
allowing the downstream
material to be deflected and flow subsonically around the galaxy (e.g.
the NW star forming region).
It is known from aerodynamics that the shape of the bow shock and its
stand-off distance (gap between the nose of the obstacle and the bow
shock) cannot be determined from hydrodynamics alone. It also depends
on the geometry since the fluid equations do not exhibit any
characteristic length scale (for a perfect gas). The bow shock can be
viewed as a transition from a planar/perpendicular (near the stagnation
point region) to an oblique shock. In the following, we will
quantitatively analyze these two situations. In the planar case, in
which the maximum compression occurs, a magnetic field, parallel to the
shock surface for the maximum effect, will also be considered. In the
next subsections we discuss simple analytic expressions that we
have used for the most plausible scenarios. We will use the subscripts
"sh'' and "'' for downstream and upstream quantities,
respectively, and an adiabatic gas with ratio of specific heats
.
M denotes the Mach number, c the speed of sound, and
the Alfvén velocity.
If the pre-shock material is the "missing"
part of an H I structure connecting the Arc-S and NW-HV regions in
Williams et al. (2002, see Fig. 11), its inferred density
and upstream temperature
K.
Thus
km s-1 and the shock is indeed hypersonic since the Mach number is
.
In case of a strong shock (
)
Eq. (A.6) yields a compression ratio of r=4 for the
derived X-ray quantities.
This results
in an unshocked magnetic field strength of
G and thus, in the strong shock
approximation, of a factor 4 higher in the shocked NS feature. As
the Alfvén speed equals
km s-1, the fast
MHD shock condition is easily fulfilled. Since
the magnetic field strength decreases with the compression
ratio r; therefore very low compression ratios are disfavored
in a perpendicular MHD shock case. Even the field value for r=4seems to be on the high side.