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|Powerful High-Velocity Dispersion Molecular Hydrogen Associated with an Intergalactic Shock Wave in Stephan's Quintet|
We present the discovery of strong mid-infrared emission lines ofmolecular hydrogen of apparently high-velocity dispersion (~870 kms-1) originating from a group-wide shock wave in Stephan'sQuintet. These Spitzer Space Telescope observations reveal emissionlines of molecular hydrogen and little else. This is the first time analmost pure H2 line spectrum has been seen in anextragalactic object. Along with the absence of PAH-dust features andvery low excitation ionized gas tracers, the spectra resemble shockedgas seen in Galactic supernova remnants, but on a vast scale. Themolecular emission extends over 24 kpc along the X-ray-emitting shockfront, but it has 10 times the surface luminosity as the soft X-rays andabout one-third the surface luminosity of the IR continuum. We suggestthat the powerful H2 emission is generated by the shock wavecaused when a high-velocity intruder galaxy collides with filaments ofgas in the galaxy group. Our observations suggest a close connectionbetween galaxy-scale shock waves and strong broad H2 emissionlines, like those seen in the spectra of ultraluminous infrared galaxieswhere high-speed collisions between galaxy disks are common.
|Witnessing galaxy preprocessing in the local Universe: the case of a star-bursting group falling into Abell 1367|
We present a multiwavelength analysis of a compact group of galaxiesinfalling at high speed into the dynamically young cluster Abell 1367.Peculiar morphologies and unusually high Hα emission areassociated with two giant galaxies and at least ten dwarfs/extragalacticHII regions, making this group the region with the highest density ofstar formation activity ever observed in the local clusters. MoreoverHα imaging observations reveal extraordinary complex trails ofionized gas behind the galaxies, with projected lengths exceeding 150kpc. These unique cometary trails mark the gaseous trajectory ofgalaxies, witnessing their dive into the hot cluster intergalacticmedium. Under the combined action of tidal forces among group membersand the ram-pressure by the cluster ambient medium, the group galaxieswere fragmented and the ionized gas was blown out. The properties ofthis group suggest that environmental effects within infalling groupsmay have represented a preprocessing step of the galaxy evolution duringthe high redshift cluster assembly phase.
|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.
|Physical Conditions and Star Formation Activity in the Intragroup Medium of Stephan's Quintet|
New multiband observations of the famous compact group of galaxiesStephan's Quintet (SQ) are presented and analyzed. These includefar-infrared (FIR) images at 60 and 100 μm (ISOPHOT C100 camera),radio continuum images at 1.4 GHz (VLA B configuration) and 4.86 GHz(VLA C configuration), and long-slit optical spectrographs (Palomar 200"telescope). With these new data, we aim to learn more about theX-ray/radio ridge in the middle of the intragroup medium (IGM) and theIGM starburst SQ-A, both of which are likely to be caused by thehigh-speed collision (~900 km s-1) between the intrudergalaxy NGC 7318b (v=5700 km s-1) and the IGM (v=6600 kms-1). We found that the radio ridge has a steep nonthermalspectral index (α=0.93+/-0.13) and an extremely low FIR-to-radioratio index (q<0.59). Its IR emission can be explained in terms ofcollisional heating of dust grains by shocked gas. The minimum-energymagnetic field strength is Hmin~10 μG. The long-slitspectra of sources in the ridge have typical emission-line ratios ofshock-excited gas. The very broad line widths (>=1000 kms-1) and the fact that in some cases more than two velocitysystems were detected along the same line of sight provide furtherevidence for an ongoing collision along the ridge. The IGM starburstSQ-A has a radio spectral index α=0.8+/-0.3 and an FIR-to-radioratio index q=2.0+/-0.4, consistent with those of star-forming regions.The optical spectra of two sources in this region, M1 (v=6600 kms-1) and M2 (v=6000 km s-1), have typical lineratios of H II regions. Both M1 and M2 have metallicity slightly higherthan the solar value. The star formation rate estimated from theextinction-corrected Hα luminosity of SQ-A is 1.45Msolar yr-1, of which 1.25 Msolaryr-1 is due to the v=6600 km s-1 component and0.20 Msolar yr-1 to the v=6000 km s-1component.The National Radio Astronomy Observatory is a facility of the NationalScience Foundation operated under cooperative agreement by AssociatedUniversities, Inc.
|Shakhbazian compact galaxy groups. II. Photometric and spectroscopic study of ShCG 376|
The results of the redshift measurements and of the detailed surfacephotometry in BVR of the compact group ShCG 376 are presented. Theradial velocity dispersion, the virial mass, the total luminosity, theM/L ratio, and the crossing time of the group are estimated. The groupconsists of eight accordant redshift spiral galaxies. Four (or possiblyfive) of the group members have emission-line spectra. Suchmorphological content and the number of emission-line galaxies are veryatypical for compact galaxy groups. There are signs of interactionbetween some members of the group. It is suggested that the irregularshape of the brightest galaxy No. 4 is probably due to interaction withother members of the group, particularly, the emission line galaxy No. 6with a discordant redshift (Delta v = 2600 km s-1). It isspeculated that the latter galaxy may be a infalling intruder to thegroup.
|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.
|Is HCG 31 undergoing a merger or a fly-by interaction?|
We present Fabry-Perot and multi-object spectroscopy of the galaxies inHickson compact group 31 (HCG 31). Based upon our Hα data cubes,galaxies A and C are a single entity, showing no discontinuity in theirkinematics. Kinematically, galaxy E is probably a component of the A+Ccomplex; otherwise it is a recently detached fragment. Galaxy F appears,both kinematically and chemically, to have formed from material tidallyremoved from the A+C complex. Galaxies B and G are kinematicallydistinct from this complex. Galaxy Q also has a radial velocitycompatible with group membership. Galaxies A, B, C, and F have nearlyidentical oxygen abundances, despite spanning a luminosity range of 5mag. Galaxy B's oxygen abundance is normal for its luminosity, whilegalaxy F's abundance is that expected given its origin as a tidalfragment of the A+C complex. The oxygen abundances in galaxies A and Care also understandable if the A+C complex is a late-type spiralsuffering strong gas inflow and star formation as a result of a tidalinteraction. Given the kinematics of both the galaxies and the H I gas,the oxygen abundances, and the position of galaxy G, we propose that aninteraction of galaxy G with the A+C complex, rather than a merger ofgalaxies A and C, is a more complete explanation for the tidal featuresand other properties of HCG 31. In this case, the A+C complex need notbe a merger in progress, though this is not ruled out.
|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 to12.6), sizes ( 3.5 h75 -1 kpc), colors(typically B R = 0.7), and gas velocity gradients ( 826 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.
|Star-forming Objects in the Tidal Tails of Compact Groups|
A search for star-forming objects belonging to tidal tails has beencarried out in a sample of deep Hα images of 16 compact groups ofgalaxies. A total of 36 objects with Hα luminosities greater than1038 ergs s-1 has been detected in five groups.The fraction of the total Hα luminosity of their respective parentgalaxies shown by the tidal objects is always below 5% except for thetidal features of Hickson compact group 95, whose Hα luminosityamounts to 65% of the total luminosity. Out of these 36 objects, ninestar-forming tidal dwarf galaxy candidates finally have been identifiedon the basis of their projected distances to the nuclei of the parentgalaxies and their total Hα luminosities. Overall, the observedproperties of the candidates resemble those previously reported for theso-called tidal dwarf galaxies.
|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.
|Where is the neutral atomic gas in Hickson groups?|
We have analyzed the total HI contents of 72 Hickson compact groups ofgalaxies (HCGs) and the detailed spatial distributions and kinematics ofHI within a subset of 16 groups using the high angular resolutionobservations obtained with the VLA in order to investigate a possibleevolutionary scenario for these densest systems in the present daygalaxy hierarchy. For the more homogeneous subsample of 48 groups, wefound a mean HI deficiency of Def_HI = 0.40 +/- 0.07, which correspondsto 40% of the expected HI for the optical luminosities and morphologicaltypes of the member galaxies. The individual galaxies show largerdegrees of deficiency than the groups globally, Def_HI = 0.62 +/- 0.09(24% of the expected HI), due in most cases to efficient gas strippingfrom individual galaxies into the group environment visible in the VLAmaps. The degree of deficiency is found to be similar to the centralgalaxies of Virgo and Coma cluster, and Coma I group, in spite of thesignificantly different characteristics (number of galaxies, velocitydispersion) of these environments. It does not seem plausible that asignificant amount of extended HI has been missed by the observations.Hence phase transformation of the atomic gas should explain the HIdeficiency. The groups richer in early type galaxies or more compactwith larger velocity dispersions show a weak tendency to be more HIdeficient. The detection rate of HCGs at X-ray wavelengths is larger forHI deficient groups, although the hot gas distribution and hence itsorigin is only known for a few cases. In the evolutionary scenario wepropose, the amount of detected HI would decrease further withevolution, by continuous tidal stripping and/or heating. The H_2 contentalso tends to be lower than expected for the galaxies in HI deficientgroups, this may suggest that the HI stripping by frequent tidalinteraction breaks the balance between the disruption of molecularclouds by star formation and the replenishment from the ambient HI. Thiswork is partially based on observations made with the VLA operated bythe National Radio Astronomy Observatory, a facility of the NationalScience Foundation operated under cooperative agreement by AssociatedUniversities, Inc., ALFOSC, which is owned by the Instituto deAstrofísica de Andalucía (IAA, CSIC) and operated at theNordic Optical Telescope (NOT) under agreement between IAA and the NBIfAof the Astronomical Observatory of Copenhagen, and 1.5 m telescope ofthe Observatorio de Sierra Nevada, Granada, Spain, which is operated bythe IAA (CSIC).
|CO in Stephan's Quintet: First Evidence of Molecular Gas in the Intragroup Starburst|
We present the first interferometric evidence of CO (1-0) emission inthe intragroup starburst (IGS) region of HCG 92 (Stephan's Quintet).BIMA's large primary beam fully covers both the IGS region and thedominant group member Seyfert galaxy NGC 7319, as well as partiallycovering the intruder galaxy NGC 7318B. CO emission of >~4 σ isdetected in all of them. The detection of the CO emission associatedwith the IGS is further supported by comparisons with observations inother wave bands (e.g., H I, mid-IR, and Hα). Assuming thestandard conversion factor, (3.6-8.0)×108Msolar molecular gas is found to be associated with the IGS.By confining ourselves to the region of the CO emission peak, we findthat a relatively high star formation efficiency of 8.6Lsolar/Msolar is derived for the IGS, which isnearly comparable to that of local starburst galaxies. The IGStriggering mechanism is also briefly discussed.
|The Nuclear Activity of Galaxies in the Hickson Compact Groups|
In order to investigate the nuclear activity of galaxies residing incompact groups of galaxies, we present results of our opticalspectroscopic program made at Okayama Astrophysical Observatory. We haveperformed optical spectroscopy of 69 galaxies belonging to 31 Hicksoncompact groups (HCGs) of galaxies. Among them, three galaxies havediscordant redshifts and, moreover, spectral quality is too poor toclassify another three galaxies. Therefore, we describe our results forthe remaining 63 galaxies. Our main results are summarized as follows:(1) We have found in our sample 28 active galactic nuclei (AGNs), 16 HII nuclei, and 19 normal galaxies showing no emission line. We used thisHCG sample for statistical analyses. (2) Comparing the frequencydistributions of activity types between the HCGs and the field galaxieswhose data are taken from Ho, Filippenko, & Sargent (382 fieldgalaxies), we find that the frequency of H II nuclei in the HCGs issignificantly less than that in the field. However, this difference maybe due to selection bias to the effect that our HCG sample contains moreearly-type galaxies than the field, because it is known that H II nucleiare rarer in early-type galaxies than in later ones. (3) Applying acorrection to this morphological bias to the HCG sample, we find thatthere is no statistically significant difference in the frequency ofoccurrence of emission-line galaxies between the HCGs and the field.This implies that the dense galaxy environment in the HCGs does notaffect the triggering of either the AGN activity and the nuclearstarburst. We discuss some implications on the nuclear activity in theHCG galaxies.
|The Arizona-New Mexico Spectroscopic Survey of Galaxies. I. Data for the Western End of the Perseus Supercluster|
We present new optical spectroscopic data for 347 galaxies in the regionof the Perseus supercluster. The new data were obtained using theSteward Observatory 2.3 m telescope and cover the whole optical window.Included are redshifts (for 345 objects), absorption-line equivalentwidths, a continuum index measuring the 4000 Å break, andemission-line flux ratios. After 11 objects are rejected for being toofaint and redshifts for 26 objects are added from the literature, wearrive at a complete sample of 361 galaxies. The distribution ofredshifts for the whole sample is examined, and we show the relationshipof the continuum index to morphology.
|Peculiar properties of 2 unusually X-ray bright early type galaxies|
Two X-ray bright early type galaxies, selected for their unusually highX-ray to optical flux ratio, have been observed at high spatialresolution with the ROSAT HRI. Both sources are clearly extended (>250 kpc in radius), thus excluding a nuclear origin of the high X-rayemission. A small group of galaxies observed around IC 1262 could be themost natural explanation for the high X-ray luminosity observed. NGC6159 could be similarly explained, although there are to date nospectroscopic confirmations of a group of galaxies associated with it.The X-ray properties of these two sources are discussed in the contextof the properties of groups of galaxies. A peculiar very bright feature,in the shape of an arc, is detected at the center of the IC 1262 image.This could be the signature of a recent merger, or of peculiar highvelocities in the group.
|Arcsecond Positions of UGC Galaxies|
We present accurate B1950 and J2000 positions for all confirmed galaxiesin the Uppsala General Catalog (UGC). The positions were measuredvisually from Digitized Sky Survey images with rms uncertaintiesσ<=[(1.2")2+(θ/100)2]1/2,where θ is the major-axis diameter. We compared each galaxymeasured with the original UGC description to ensure high reliability.The full position list is available in the electronic version only.
|On the Influence of the Environment on the Star Formation Rates of a Sample of Galaxies in Nearby Compact Groups|
We present the results of a study of the star formation rates (SFRs) ofa sample of disk galaxies in nearby compact groups compared with theSFRs of a sample of field galaxies. For this purpose, Hαluminosities and equivalent widths were derived for the galaxies of oursample. A direct comparison of the equivalent widths and Hαluminosities, normalized to the B luminosities and estimated area of thegalaxies of both samples, yields the result that the median values ofthese quantities are almost identical for both samples, although thedistributions for the compact-group sample are broader around the meanvalue than was found in the field galaxy sample. This result can beexplained by assuming that although interactions between galaxies incompact groups can alter the SFRs, the median value of the normalizedSFRs is preserved, being almost indistinguishable from the correspondingvalue for field galaxies. Measuring the global L_Hα/L_B of thegroups, including early-type galaxies, we find that most of the groupsthat show the highest level of L_Hα/L_B with respect to a set ofsynthetic groups built out of field galaxies show tidal features in atleast one of their members. Finally, we have explored the relationshipbetween the ratio L_Hα/L_B and several relevant dynamicalparameters of the groups: velocity dispersion, crossing time, radius,and the mass-to-luminosity ratio, finding no clear correlation. Thissuggests that the exact dynamical state of a group does not control theSFR of the group as a whole. Our results are compatible with a scenariofor compact groups of galaxies in which the dark matter of the group isarranged in a common halo, therefore preventing a fast collapse of thegalaxies.
|Detection and Velocities of Hα Emission Regions in Stephan's Quintet|
We present observations of Hα emission of Stephan's Quintetobtained with a scanning Fabry-Perot interferometer and samplings of ~11km s^-1 and ~1". Our observations show an intragroup arclike featureformed of bright Hα knots and diffuse gas distributed along tidaltails associated with NGC 7318B and possibly also NGC 7318A. Newvelocities for 23 emitting regions detected along the Hα structureare presented. South of NGC 7318B, along a path covering ~12 h^-1_75kpc, H II regions with velocities ranging from 5540 to 6700 km s^-1 aredetected, suggesting that two or more overlapping tidal tails and/or abow-shock region may have been formed through several interaction eventswithin the group.
|Starburst in the Intragroup Medium of Stephan's Quintet|
Based on new Infrared Space Observatory midinfrared observations andground-based H_alpha and near-infrared observations, we report thedetection of a bright starburst in the intragroup medium (IGM) of thefamous compact group of galaxies known as Stephan's Quintet (source A inFig. 1). We demonstrate that this starburst is caused by a collisionbetween a high-velocity (deltaV~1000 km s^-1) intruder galaxy (NGC7318b) and the IGM of the group. While this is the only starburst knowntoday that is induced by a galaxy/cold intergalactic medium collision,it provides new constraints to the theory for interaction-inducedstarbursts and may hint at a new mechanism for the star formation excessseen in more distant clusters.
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