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A High-Resolution Mosaic of Molecular Gas in Stephan's Quintet
We present high-resolution 12CO J=1-0 observations of themolecular gas in the Hickson compact group Stephan's Quintet (HCG 92).Our observations consist of multiple pointings and mosaics covering allthe regions where CO and star formation have been detected. Within the100" field of view centered on the easternmost tidal tail, we detectthree clumps of emission that may be partially resolved at ourresolution of 8" two of these are new detections not previously seen inISM studies of this region. Two of these clumps lie in the optical tidaltail, while the third lies to the southeast and is coincident with alarge H I feature, but it does not correspond to any features at otherwavelengths. We also tentatively detect CO emission from thestar-forming regions in the ``old tail'' corresponding to recent starformation activity detected in recent UV and Hα observations.Observations of the rest of the compact group do not show detections,even though strong emission was detected with single-dish telescopes,which suggests that the CO emission originates from a diffuse moleculargas cloud or from at least three separate clumps with separationsgreater than around 3 kpc.

Ultraviolet Emission and Star Formation in Stephan's Quintet
We present the first Galaxy Evolution Explorer (GALEX) UV images of thewell-known interacting group of galaxies, Stephan's Quintet (SQ). Wedetect widespread UV emission throughout the group. However, there is noconsistent coincidence between UV structure and emission in the optical,Hα, or H I. Excluding the foreground galaxy NGC 7320 (Sd), most ofthe UV emission is found in regions associated with the two spiralmembers of the group, NGC 7319 and NGC 7318b, and the intragroup mediumstarburst SQ-A. The extinction-corrected UV data are analyzed toinvestigate the overall star formation activity in SQ. We find that thetotal star formation rate (SFR) of SQ is 6.69+/-0.65 Msolaryr-1. Of this, 1.34+/-0.16 Msolar yr-1is due to SQ-A. This is in excellent agreement with that derived fromthe extinction-corrected Hα luminosity of SQ-A. The SFR in regionsrelated to NGC 7319 is 1.98+/-0.58 Msolar yr-1,most of which (68%) is contributed by the disk. The contribution fromthe ``young tail'' is only 15%. In the UV, the young tail is moreextended (~100 kpc) and shows a looplike structure, including theoptical tail, the extragalactic H II regions recently discovered inHα, and other UV emission regions discovered for the first time.The UV and optical colors of the ``old tail'' are consistent with asingle stellar population of age t~=108.5+/-0.4 yr. The UVemission associated with NGC 7318b is found in a very large (~80 kpc)disk, with a net SFR of 3.37+/-0.25 Msolar yr-1.Several large UV emission regions are 30-40 kpc away from the nucleus ofNGC 7318b. Although both NGC 7319 and NGC 7318b show peculiar UVmorphology, their SFR is consistent with that of normal Sbc galaxies,indicating that the strength of star formation activity is not enhancedby interactions.

Stephan's Quintet with XMM-Newton
The prototype compact group known as Stephan's Quintet (SQ) was observedwith XMM-Newton in order to complement the excellent resolution ofChandra with high sensitivity to extended emission. SQ is a dynamicenvironment whose main effect, at both X-ray and optical wavelengths,appears to be ISM stripping. This is manifested by: 1) secular evolutionof morphological types towards earlier types and 2) growth of diffuseemission. Virtually all cold, warm, and hot gas in SQ is found outsideof the member galaxies. XMM-Newton offers the opportunity to study thehot gas with unprecedented sensitivity. We find two main components: 1)extended high surface brightness emission from shocked gas associatedwith an ongoing collision and 2) even more extended and unrelaxeddiffuse emission that follows the stripped stellar envelope of thegroup.

MERLIN observations of Stephan's Quintet
We present MERLIN L-band images of the compact galaxy group, Stephan'sQuintet (SQ). The Seyfert 2 galaxy, NGC 7319, the brightest member ofthe compact group, is seen to have a triple radio structure typical ofmany extra-galactic radio sources that have a flat spectrum core and twosteep spectrum lobes with hot spots. The two lobes are asymmetricallydistributed on opposite sides of the core along the minor axis of thegalaxy. Ultraviolet (UV) emission revealed in a high-resolution channel(HRC)/ACS Hubble Space Telescope (HST) image is strongly aligned withthe radio plasma and we interpret the intense star formation in the coreand north lobe as an event induced by the collision of the north radiojet with over-dense ambient material. In addition, a remapping ofarchive Very Large Array (VLA) L-band observations reveals more extendedemission along the major axis of the galaxy, which is aligned with theoptical axis. Images formed from the combined MERLIN and archive VLAdata reveal more detailed structure of the two lobes and hot spots.

A Nursery of Young Objects: Intergalactic H II Regions in Stephan's Quintet
We have discovered four intergalactic H II regions in Stephan's quintet,which is more than a 25 kpc projected distance from the center of thenearest group galaxy, with no apparent optical connection to it. Theyhave MB ranging from -11.9 to -12.5 mag, colors B-R=0.7-1.1mag, radial velocities from 6565 to 6651 km s-1, and they aresuperposed onto the H I tail east of NGC 7319, with a mean radialvelocity of 6610 km s-1. In addition, they have metallicitiesof the order of 12+log(O/H)=8.58+/-0.25, which suggests that they wereformed from preenriched material. We derive a mean age of 4.6+/-0.6 Myrand a mean stellar mass of (2.9+/-1.4)×104Msolar for the four objects. The masses, ages, colors,velocities, metallicities, and location of the objects suggest that theyare H II regions that were formed far away from the galaxies throughcompression of the intergalactic H I gas by galaxy collisions.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the NSF on behalf of the Geminipartnership: the National Science Foundation (US), the Particle Physicsand Astronomy Research Council (UK), the National Research Council(Canada), CONICYT (Chile), the Australian Research Council (Australia),CNPq (Brazil), and CONICET (Argentina).

Molecular and ionized gas in the tidal tail in Stephan's Quintet
We have mapped with the IRAM interferometer at Plateau de Bure (PdBI)the 12CO emission towards intergalactic star forming regionslocated in the tidal tail stemming from NGC 7319, in the Stephan'sQuintet compact group of galaxies. The 13CO emission of thesame region was observed with the IRAM 30 m telescope and opticalspectroscopy of several HII regions in the area were obtained with theCalar Alto 3.5 m telescope.We recovered with the interferometer about50% of the 12CO(1-0) total emission that had been earliermeasured with single dish observations (Lisenfeld 2002, A&A, 394,823), indicating that about half of the molecular gas is distributed onspatial scales larger than about 10-15 arcsec (corresponding to 4-6 kpc)to which PdBI is not sensitive. We find two main areas of CO emission:(i) an elongated region towards the area known as SQ B where a TidalDwarf Galaxy could currently be forming and (ii) a barely resolved areaat the tip of the optical tidal arm. Both regions follow dust lanesvisible on HST images and their CO peak coincides spatially exactly withthe maximum of the Hα line emission. In SQ B, there isfurthermore very good kinematical agreement between the CO, Hα andHI components. We conclude from these coincidences that the gaseousmatter found in quantities in the area is physically associated to theoptical tidal tail and thus that the intergalactic atomic hydrogen therewas expelled from NGC 7319. Its origin had previously been much debated.Furthermore, the relatively high oxygen abundances (about solar)estimated from the optical spectra of the HII regions imply that thegas feeding the star formation originated from the inner regions of theparent galaxy. In SQ B, we derive from different tracers a starformation rate, corrected for dust extinction - which is important inthe area - of 0.5 Mȯ/yr, i.e. one of the highest valuesso far measured outside galaxies. The inferred molecular gas consumptiontime of 0.5 Gyr lies in the range of values found for spiral andstarburst galaxies. On the other hand, the ratio of12CO/13CO > 25 is much higher than the valuesfound in disks of spiral galaxies. A relatively low opacity for the12CO gas is the most likely reason.

A multiple galaxy collision in compact groups
It is shown that the dynamical paradox of the HCG 90, which is treatedas an old, and at the same time, as a young system (White et al), may besolved assuming that it is a rotating system in which multiplecollisions have occurred during Hubble time. In the frames of the samescenario a multiple collisions in SQ (HCG 92) are naturally explained.

Stephan's Quintet: The X-ray anatomy of a multiple galaxy collision
Chandra observations of the compact galaxy group known as Stephan'sQuintet (SQ) are presented. The major morphological features that werediscovered with the ROSAT HRI are now imaged with higher resolution andS/N. The large scale shock (1farcm 5, ~ 40 kpc if at 85 Mpc) isresolved into a narrow NS feature embedded in more extended diffuseemission (Dge3 '). The NS structure is somewhat clumpy, more sharplybounded on the W side and prominent only in the soft band (energiesbelow ~ 2 keV). Its observational properties are best explained as ashock produced by a high velocity encounter between NGC 7318b, a ``newintruder'', and the intergalactic medium in SQ. The shock conditionsnear the high speed intruder suggest that a bow shock is propagatinginto a pre-existing H I cloud and heating the gas to a temperature of0.5 keV. The low temperature in the shock is a problem unless wepostulate an oblique shock. One member, NGC 7319, hosts a Seyfert 2nucleus, with an intrinsic luminosity LX= 1043 ergs-1, embedded in a region of more diffuse emission with 10''radius extent. The nuclear spectrum can be modeled with a stronglyabsorbed power-law typical of this class of sources. Several additionalcompact sources are detected including three in foreground NGC 7320.Some of these sources are very luminous and could be related to theultraluminous X-ray sources found in nearby galaxies.

New Light and Shadows on Stephan's Quintet
We present deep broadband R and narrowband Hα images of Stephan'sQuintet. The observations in the R band show that the diffuse halo ofStephan's Quintet is larger than previously thought and extends out toNGC 7320C. However, we have not found emission connecting NGC 7331 andNGC 7320 to R~26.7 mag arcsec-2 (at more than a 3 σlevel), so there is no direct evidence up to this limiting magnitude ofa relation between the peculiar kinematical structure found in NGC 7331and an ongoing or past interaction between this galaxy and NGC 7320. TheHα emission at high velocity (6000-7000 km s-1) isdistributed in a diffuse structure running north-south between NGC 7319and NGC 7318B and in some other more concentrated features. Some ofthese are located in the tidal tails produced by the interaction betweenthe galaxies of the group. With the Hα images we have made atwo-dimensional velocity map that helps to identify the origin of eachstructure detected. This map does not show features at intermediatevelocities between the high- and low-redshift members of the group. Thisis in agreement with the standard scenario in which the apparentproximity of NGC 7320 to the rest of the galaxies of the Quintet ismerely a projection effect. The only point that is unclear in thisinterpretation is an Hα filament that is seen extending throughoutNGC 7320 with velocity at 6500 km s-1 instead of the 800 kms-1 expected for this galaxy. Accepted in final form 20002July 12.

Tidal dwarf galaxies in the Stephan's Quintet?
We present kinematics and photometric evidence for the presence of sevencandidate tidal dwarf galaxies in Stephan's Quintet. The central regionsof the two most probable parent galaxies, NGC 7319 and NGC 7318B,contain little or no gas whereas the intragroup medium and, inparticular, the optical tails that seem to be associated with NGC 7318Bare rich in cold and ionized gas. Two tidal dwarf candidates may belocated at the edge of a tidal tail, another located within a tail, andfor the four others there is no obvious stellar/gaseous bridge betweenthem and the parent galaxy. Two of the candidates are associated with HI clouds, one of which is, in addition, associated with a CO cloud. Allseven regions have low continuum fluxes and high Hα luminositydensities [F(Hα) = (1-60) × 10-14 ergs s-1cm-2]. Their magnitudes (MB = –16.1 to–12.6), sizes (˜ 3.5 h75 -1 kpc), colors(typically B – R = 0.7), and gas velocity gradients (˜ 8–26 h75 km s-1 kpc-1) are typicalfor tidal dwarf galaxies. In addition, the ratios between their starformation rates determined from Hα and from the B-band luminosityare typical of other tidal dwarf galaxies. The masses of the tidal dwarfgalaxies in Stephan's Quintet range from ˜ 2 × 108to 1010 Mȯ, and the median value for theirinferred mass-to-light ratios is 7 (M/L)ȯ. At least twoof the systems may survive possible ‘fallbacks’ ordisruption by the parent galaxies and may already be, or turn into,self-gravitating dwarf galaxies, new members of the group.

The VLA H I Observations of Stephan's Quintet (HCG 92)
Using the Very Large Array, we have made spectral-line and continuumobservations of the neutral hydrogen in the direction of the compactgroup of galaxies Stephan's Quintet. The high-velocity clouds between5600 and 6600 km s-1, the disk of the foreground galaxy NGC7320 at 800 km s-1, the extended continuum ridge near thecenter of the group, and three faint dwarflike galaxies in thesurrounding field were imaged with the C, CS, and D configurations. Fourof the H I clouds previously detected are confirmed. The two largest H Ifeatures are coincident with and concentrated mainly along separatelarge tidal tails that extend eastward. The most diffuse of the fourclouds is resolved into two clumps, one coinciding with tidal featuressouth of NGC 7318A and the other devoid of any detectable stellar orHα sources. The two compact clouds along the same line of sighthave peak emission at luminous infrared and bright Hα sourcesprobably indicative of star-forming activity. The total amount of H Idetected at high redshifts is ~1010Msolar. As inprevious H I studies of the group, no detectable emission was measuredat the positions of any high-redshift galaxies, so that any H I stillbound to their disks must be less than 2.4×107Msolar. The National Radio Astronomy Observatory is afacility of the National Science Foundation operated under cooperativeagreement by Associated Universities, Inc.

Abundant molecular gas in the intergalactic medium of Stephan's Quintet
Stephan's Quintet (SQ) is a system consisting of at least fourinteracting galaxies which is well known for its complex dynamical andstar formation history. It possesses a rich intergalactic medium (IGM),where hydrogen clouds, both atomic and molecular, associated with twostarbursts (refered to as SQ A and B) have been found. In order to studythe extent, origin and fate of the intergalactic molecular gas and itsrelation to the formation of stars outside galaxies and Tidal DwarfGalaxies (TDGs), we mapped with the IRAM 30 m antenna the carbonmonoxide (CO) towards several regions of the IGM in SQ. In both SQ A andB, we detected unusually large amounts of molecular gas (3.1 x109 Msun and 7 x 108 Msun,respectively). In contrast, no significant CO detection was achievedtowards HII regions south of the pair NGC 7318a/b despite their highHα luminosities. The molecular gas is very extended in both SQ Aand SQ B, over areas of between 15 and 25 kpc. The CO clouds seem tohave otherwise different properties and may be of a different nature.The integrated CO line of SQ A is in particular much wider than in SQ B.Its CO spectrum shows emission at two velocities (6000 and 6700 kms-1) that are coincident with two HI lines. The strongestemission at 6000 km s-1 is however spatially offset from theHI emission and situated on a ridge south-east of the starburst region.In SQ B the CO emission coincides with that of tracers of star formation(Hα , 15 mu m and radio continuum). The CO peak lies slightlyoffset from the HI peak towards a steep HI gradient. This is indicatingthat the molecular gas is forming in-situ, possibly in a region ofcompressed HI, with subsequent star formation. The star forming regionat SQ B is the object in SQ that most resembles a TDG.

A Multiwavelength Study of Stephan's Quintet
Stephan's Quintet (SQ) is a compact group that we find in an atypicalmoment when a high-velocity intruder is passing through it. Theintrusion is particularly interesting because a previous intruder hadstripped most of the gas from the group members. This debris field wasshocked in the ongoing collision with the new intruder. Thisevolutionary history agrees well with observations and explains how astrongly interacting system can show low levels of star formation. Wepresent new multiwavelength data including previously unpublished ROSATX-ray, Hα interference filter/Fabry-Pérot, ISO MIR/FIR, andradio line and continuum images. These observations and previouslypublished data provide new insights, as well as support for someprevious hypotheses. (1) Fabry-Pérot and H I velocities allow usto unambiguously distinguish between gas associated with SQ and the newintruder. (2) Most detected emission regions are found in the remnantinterstellar medium (ISM) of the new intruder, which allows us to inferits size and present physical state. (3) The few emission regionsassociated with the stripped ISM of SQ include the best candidate tidaldwarf galaxy. (4) Multiwavelength data suggest that strong MIR/FIRemission from the Seyfert 2 nucleus of NGC 7319 comes from dust heateddirectly by a power-law continuum rather than a starburst. (5) Thecorrespondence between extended X-ray/radio continuum/forbidden opticalemission confirms the existence of a large scale shock in SQ. (6) Weconfirm the presence of two stripped spiral members in the process oftransformation into E/S0 morphology. Finally (7) observations areconsistent with the idea that the collision in SQ is ongoing withpossible detection of H II region ablation and Rayleigh-Taylorinstabilities.

Hubble Space Telescope Images of Stephan's Quintet: Star Cluster Formation in a Compact Group Environment
Analysis of Hubble Space Telescope Wide Field Planetary Camera 2 imagesof Stephan's Quintet, Hickson Compact Group 92, yielded 115 candidatestar clusters (with V-I<1.5). Unlike in merger remnants, the clustercandidates in Stephan's Quintet are not clustered in the inner regionsof the galaxies; they are spread over the debris and surrounding area.Specifically, these sources are located in the long sweeping tail andspiral arms of NGC 7319, in the tidal debris of NGC 7318B/A, and in theintragroup starburst region north of these galaxies. Analysis of thecolors of the clusters indicate several distinct epochs of starformation that appear to trace the complex history of dynamicalinteractions in this compact group. Based on observations obtained withthe NASA/ESA Hubble Space Telescope, which is operated by the SpaceTelescope Science Institute for the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.

Candidate Tidal Dwarf Galaxies Associated with Stephan's Quintet
We present kinematic and photometric evidence for the presence of sevencandidate tidal dwarf galaxies in Stephan's Quintet. The central regionsof the two most probable parent galaxies, NGC 7319 and NGC 7318B,contain little or no gas whereas the intragroup medium and, inparticular, the optical tails that seem to be associated with NGC 7318Bare rich in cold and ionized gas. Two tidal dwarf candidates may belocated at the edge of a tidal tail, another located within a tail, andfor the four others there is no obvious stellar/gaseous bridge betweenthem and the parent galaxy. Two of the candidates are associated with HI clouds, one of which is, in addition, associated with a CO cloud. Allseven regions have low continuum fluxes and high Hα luminositydensities [F(Hα)=(1-60)×10-14 ergs s-1cm-2]. Their magnitudes (MB=-16.1 to -12.6), sizes(~3.5 h-175 kpc), colors (typically B-R=0.7), andgas velocity gradients (~8-26 h75 km s-1kpc-1) are typical for tidal dwarf galaxies. In addition, theratios between their star formation rates determined from Hα andfrom the B-band luminosity are typical of other tidal dwarf galaxies.The masses of the tidal dwarf galaxies in Stephan's Quintet range from~2×108 to 1010 Msolar, and themedian value for their inferred mass-to-light ratios is 7(M/L)solar. At least two of the systems may survive possible``fallbacks'' or disruption by the parent galaxies and may already be,or turn into, self-gravitating dwarf galaxies, new members of the group.

New Observations of Extra-Disk Molecular Gas in Interacting Galaxy Systems, Including a Two-Component System in Stephan's Quintet
We present new CO (1-0) observations of 11 extragalactic tails andbridges in nine interacting galaxy systems, almost doubling the numberof such features with sensitive CO measurements. Eight of these 11features were undetected in CO to very low CO/H I limits, with the mostextreme case being the NGC 7714/5 bridge. This bridge contains luminousH II regions and has a very high H I column density(1.6×1021 cm-2 in the 55" CO beam), yet wasundetected in CO to rms T*R=2.4 mK. The H I columndensity is higher than standard H2 and CO self-shieldinglimits for solar-metallicity gas, suggesting that the gas in this bridgeis metal-poor and has an enhancedNH2/ICO ratio compared with theGalactic value. Only one of the 11 features in our sample wasunambiguously detected in CO, a luminous H I-rich star formation regionnear an optical tail in the compact group Stephan's Quintet. We detectCO at two widely separated velocities in this feature, at ~6000 and~6700 km s-1. Both of these components have H I and Hαcounterparts. These velocities correspond to those of galaxies in thegroup, suggesting that this gas is material that has been removed fromtwo galaxies in the group. The CO/H I/Hα ratios for bothcomponents are similar to global values for spiral galaxies.

The Hubble Space Telescope Extragalactic Distance Scale Key Project. X. The Cepheid Distance to NGC 7331
The distance to NGC 7331 has been derived from Cepheid variablesobserved with the Hubble Space Telescope (HST) WFPC2, as part of theExtragalactic Distance Scale Key Project. Multiepoch exposures in F555W(~V) and F814W (~I), with photometry derived independently from DoPHOTand DAOPHOT/ALLFRAME programs, were used to detect a total of 13reliable Cepheids, with periods between 11 and 42 days. The relativedistance moduli between NGC 7331 and the LMC, derived from the V and Imagnitudes, imply an extinction to NGC 7331 of AV = 0.47 +/- 0.15 magand an extinction-corrected distance modulus to NGC 7331 of 30.89 +/-0.14 (random), equivalent to a distance of 15.1^{+1.0}_{-0.9} Mpc. Thereare additional systematic uncertainties in the distance modulus of+/-0.12 mag resulting from the calibration of the Cepheidperiod-luminosity relation and a systematic offset of +0.05 +/- 0.04 magif the metallicity correction inferred from the M101 results ofKennicutt et al. are applied.

Catalogue of HI maps of galaxies. I.
A catalogue is presented of galaxies having large-scale observations inthe HI line. This catalogue collects from the literature the informationthat characterizes the observations in the 21-cm line and the way thatthese data were presented by means of maps, graphics and tables, forshowing the distribution and kinematics of the gas. It containsfurthermore a measure of the HI extension that is detected at the levelof the maximum sensitivity reached in the observations. This catalogueis intended as a guide for references on the HI maps published in theliterature from 1953 to 1995 and is the basis for the analysis of thedata presented in Paper II. The catalogue is only available inelectronic form at the CDS via anonymous ftp orhttp://cdsweb.u-strasbg.fr/Abstract.html

Total magnitude, radius, colour indices, colour gradients and photometric type of galaxies
We present a catalogue of aperture photometry of galaxies, in UBVRI,assembled from three different origins: (i) an update of the catalogueof Buta et al. (1995) (ii) published photometric profiles and (iii)aperture photometry performed on CCD images. We explored different setsof growth curves to fit these data: (i) The Sersic law, (ii) The net ofgrowth curves used for the preparation of the RC3 and (iii) A linearinterpolation between the de Vaucouleurs (r(1/4) ) and exponential laws.Finally we adopted the latter solution. Fitting these growth curves, wederive (1) the total magnitude, (2) the effective radius, (3) the colourindices and (4) gradients and (5) the photometric type of 5169 galaxies.The photometric type is defined to statistically match the revisedmorphologic type and parametrizes the shape of the growth curve. It iscoded from -9, for very concentrated galaxies, to +10, for diffusegalaxies. Based in part on observations collected at the Haute-ProvenceObservatory.

The observational status of Stephan's Quintet
We present new photometric and spectroscopic data for the galaxies inthe compact group known as Stephan's Quintet. We find the strongestevidence for dynamical perturbation in the spiral component NGC 7319.Most of the damage was apparently caused by nearby NGC 7320C whichpassed through the group a few x10(8) years ago. NGC 7318B is a spiralgalaxy that shows evidence consistent with being in the early stages ofa collision with the group. NGC 7317 and 18A are either ellipticalgalaxies or the stripped bulges of former spiral components. They showno evidence of past or present merger activity but are embedded in aluminous halo which suggests that they are interacting with the othermembers of the group. The low redshift galaxy NGC 7320 is most likely alate type, dwarf spiral projected along the same line of sight as theinteracting quartet. Based on data obtained at the 1.5m telescope of theEstacion de Observacion de Calar Alto (EOCA), Instituto GeograficoNacional, which is jointly operated by the Instituto Geografico Nacionaland the Consejo Superior de Investigaciones Cientificas through theInstituto de Astrofisica de Andalucia

X-Ray Imaging Spectroscopy of Stephan's Quintet and Seyfert 2 Galaxy NGC 7319
We observed Stephan's Quintet, a compact group of galaxies, with theJapanese X-ray satellite ASCA. The ASCA observation has revealed thatthe X-ray emission consists of two components. One is a soft X-rayemission characterized by thin thermal model with kT=0.76(+0.05}_{-0.10)keV and a metal abundance of 0.08+/-0.04 solar. Since the soft X-raysare extended and the centroid is located at the region between NGC 7318and NGC 7319, thin thermal emission originates from diffuse hot gasfilling the Quintet. The total mass of hot gas is about 2*E(10) MO ,which is only 10% of the galaxy mass. The low metal abundance indicatesthat most of the hot gas consist of primordial gas. The binding mass isabout 6*E(12) MO . The baryonic fraction is about 10%. The othercomponent is hard X-ray emission located at NGC 7319, a Seyfert 2galaxy. The X-ray spectrum is well fitted by the absorbed-power lawemission plus a line. The photon index is 0.8(+0.8}_{-1.1) , and theabsorption column is about 10(23) cm(-2) . The X-ray luminosity in the2--10 keV band is about 1.7*E(42) erg s(-1) . These characteristics areconsistent with those of Seyfert 2 galaxies. We have revealed anobscured nucleus in NGC 7319. Our results well explain the flux ofHα and anisotropic radiation seen in the optical band.

The Dynamical Status of Stephan's Quintet
Multiwavelength data for Stephan's Quintet (SQ) are consistent with thefollowing model for this compact galaxy group. (1) Discordant-redshiftNGC 7320 is an unrelated foreground galaxy. (2) In the past SQ was anaccordant-redshift quartet involving NGC 7317, 7318A, 7319, and 7320C.NGC 7320C collided (probably not for the first time) with the group afew times 108 yr ago and stripped the interstellar medium from NGC 7319.(3) In the present SQ is again an accordant quartet involving NGC 7317,7318A, B, and 7319. NGC 7318B is now entering the group at high velocityfor the first time, giving rise to a shock zone. If most compact groupsare like SQ, then they are frequently visited by infalling neighborsthat perturb the group and themselves. SQ represents strong evidence forsecondary infall in a small group environment. Tidal stripping reducesthe mass of the infalling galaxies, thereby increasing the timescale fortheir orbital decay. There is little evidence that these high-velocity"intruders" are rapidly captured and/or merge with the system. Instead,they are the mechanism that sustains compact groups against collapse.Efficient gas stripping may account for the low star formation rateobserved in compact groups, and infall of residual gas into galacticnuclei may also foster the onset of active galactic nucleus activity.

Observing Hickson galaxy groups.
Not Available

A Distance Limit for NGC 7318B in Stephan's Quintet
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1974ApJ...189L...1S&db_key=AST

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Right ascension:22h36m20.30s
Aparent dimensions:0.589′ × 0.49′

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NGC 2000.0NGC 7320C

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