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# NGC 7320

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 Powerful High-Velocity Dispersion Molecular Hydrogen Associated with an Intergalactic Shock Wave in Stephan's QuintetWe 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. GHASP: an Hα kinematic survey of spiral and irregular galaxies - IV. 44 new velocity fields. Extension, shape and asymmetry of Hα rotation curvesWe present Fabry-Perot observations obtained in the frame of the GHASPsurvey (Gassendi HAlpha survey of SPirals). We have derived the Hαmap, the velocity field and the rotation curve for a new set of 44galaxies. The data presented in this paper are combined with the datapublished in the three previous papers providing a total number of 85 ofthe 96 galaxies observed up to now. This sample of kinematical data hasbeen divided into two groups: isolated (ISO) and softly interacting(SOFT) galaxies. In this paper, the extension of the Hα discs, theshape of the rotation curves, the kinematical asymmetry and theTully-Fisher relation have been investigated for both ISO and SOFTgalaxies. The Hα extension is roughly proportional toR25 for ISO as well as for SOFT galaxies. The smallestextensions of the ionized disc are found for ISO galaxies. The innerslope of the rotation curves is found to be correlated with the centralconcentration of light more clearly than with the type or thekinematical asymmetry, for ISO as well as for SOFT galaxies. The outerslope of the rotation curves increases with the type and with thekinematical asymmetry for ISO galaxies but shows no special trend forSOFT galaxies. No decreasing rotation curve is found for SOFT galaxies.The asymmetry of the rotation curves is correlated with themorphological type, the luminosity, the (B-V) colour and the maximalrotational velocity of galaxies. Our results show that the brightest,the most massive and the reddest galaxies, which are fast rotators, arethe least asymmetric, meaning that they are the most efficient withwhich to average the mass distribution on the whole disc. Asymmetry inthe rotation curves seems to be linked with local star formation,betraying disturbances of the gravitational potential. The Tully-Fisherrelation has a smaller slope for ISO than for SOFT galaxies. Ultraviolet Emission and Star Formation in Stephan's QuintetWe 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-NewtonThe 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. Galaxy Train WrecksNot Available MERLIN observations of Stephan's QuintetWe 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. The Ultraluminous X-Ray Source Population from the Chandra Archive of GalaxiesOne hundred fifty-four discrete non-nuclear ultraluminous X-ray (ULX)sources, with spectroscopically determined intrinsic X-ray luminositiesgreater than 1039 ergs s-1, are identified in 82galaxies observed with Chandra's Advanced CCD Imaging Spectrometer.Source positions, X-ray luminosities, and spectral and timingcharacteristics are tabulated. Statistical comparisons between theseX-ray properties and those of the weaker discrete sources in the samefields (mainly neutron star and stellar-mass black hole binaries) aremade. Sources above ~1038 ergs s-1 display similarspatial, spectral, color, and variability distributions. In particular,there is no compelling evidence in the sample for a new and distinctclass of X-ray object such as the intermediate-mass black holes.Eighty-three percent of ULX candidates have spectra that can bedescribed as absorbed power laws with index <Γ>=1.74 andcolumn density =2.24×1021cm-2, or ~5 times the average Galactic column. About 20% ofthe ULXs have much steeper indices indicative of a soft, and likelythermal, spectrum. The locations of ULXs in their host galaxies arestrongly peaked toward their galaxy centers. The deprojected radialdistribution of the ULX candidates is somewhat steeper than anexponential disk, indistinguishable from that of the weaker sources.About 5%-15% of ULX candidates are variable during the Chandraobservations (which average 39.5 ks). Comparison of the cumulative X-rayluminosity functions of the ULXs to Chandra Deep Field results suggests~25% of the sources may be background objects, including 14% of the ULXcandidates in the sample of spiral galaxies and 44% of those inelliptical galaxies, implying the elliptical galaxy ULX population isseverely compromised by background active galactic nuclei. Correlationswith host galaxy properties confirm the number and total X-rayluminosity of the ULXs are associated with recent star formation andwith galaxy merging and interactions. The preponderance of ULXs instar-forming galaxies as well as their similarities to less-luminoussources suggest they originate in a young but short-lived populationsuch as the high-mass X-ray binaries with a smaller contribution (basedon spectral slope) from recent supernovae. The number of ULXs inelliptical galaxies scales with host galaxy mass and can be explainedmost simply as the high-luminosity end of the low-mass X-ray binarypopulation. Star Formation Properties of a Large Sample of Irregular GalaxiesWe present the results of Hα imaging of a large sample ofirregular galaxies. Our sample includes 94 galaxies with morphologicalclassifications of Im, 26 blue compact dwarfs (BCDs), and 20 Sm systems.The sample spans a large range in galactic parameters, includingintegrated absolute magnitude (MV of -9 to -19), averagesurface brightness (20-27 mag arcsec-2), current starformation activity (0-1.3 Msolar yr-1kpc-2), and relative gas content(0.02-5Msolar/LB). The Hα images were usedto measure the integrated star formation rates, determine the extents ofstar formation in the disks, and compare azimuthally averaged radialprofiles of current star formation to older starlight. The integratedstar formation rates of Im galaxies normalized to the physical size ofthe galaxy span a range of a factor of 104 with 10% Imgalaxies and one Sm system having no measurable star formation at thepresent time. The BCDs fall, on average, at the high star formation rateend of the range. We find no correlation between star formation activityand proximity to other cataloged galaxies. Two galaxies located in voidsare similar in properties to the Sm group in our sample. The H IIregions in these galaxies are most often found within the Holmbergradius RH, although in a few systems H II regions are tracedas far as 1.7RH. Similarly, most of the star formation isfound within three disk scale lengths RD, but in somegalaxies H II regions are traced as far as 6RD. A comparisonof Hα surface photometry with V-band surface photometry shows thatthe two approximately follow each other with radius in Sm galaxies, butin most BCDs there is an excess of Hα emission in the centers thatdrops with radius. In approximately half of the Im galaxies Hα andV correspond well, and in the rest there are small to large differencesin the relative rate of falloff with radius. The cases with stronggradients in the LHα/LV ratios and with highcentral star formation rate densities, which include most of the BCDs,require a significant fraction of their gas to migrate to the center inthe last gigayear. We discuss possible torques that could have causedthis without leaving an obvious signature, including dark matter barsand past interactions or mergers with small galaxies or H I clouds.There is now a substantial amount of evidence for these processes amongmany surveys of BCDs. We note that such gas migration will also increasethe local pressure and possibly enhance the formation of massive denseclusters but conclude that the star formation process itself does notappear to differ much among BCD, Im, and Sm types. In particular, thereis evidence in the distribution function for Hα surface brightnessthat the turbulent Mach numbers are all about the same in these systems.This follows from the Hα distribution functions corrected forexponential disk gradients, which are log-normal with a nearly constantdispersion. Thus, the influence of shock-triggered star formation isapparently no greater in BCDs than in Im and Sm types. Molecular and ionized gas in the tidal tail in Stephan's QuintetWe 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. The field surrounding NGC 7603: Cosmological or non-cosmological redshifts?We present new observations of the field surrounding the Seyfert galaxyNGC 7603, where four galaxies with different redshifts - NGC 7603(z=0.029), NGC 7603B (z=0.057) and two fainter emission line galaxies(z=0.245 and z=0.394) - are apparently connected by a narrow filament,leading to a possible case of anomalous redshift. The observationscomprise broad and narrow band imaging and intermediate resolutionspectroscopy of some of the objects in the field. The new data confirmthe redshift of the two emission-line objects found within the filamentconnecting NGC 7603 and NGC 7603B, and settles their type with betteraccuracy. Although both objects are point-like in ground based images,using HST archive images we show that the objects have structure with aFWHM = 0.3-0.4 arcsec. The photometry in the R-band obtained duringthree different campaigns spread over two years does not show any signsof variability in these objects above 0.3-0.4 mag. All the aboveinformation and the relative strength and width of the main spectrallines allow us to classify these as HII galaxies with very vigorous starformation, while the rest of the filament and NGC 7603B lack starformation. We delineate the halo of NGC 7603 out to 26.2mag/arcsec2 in the Sloan r band filter and find evidence forstrong internal distortions. New narrow emission line galaxies atz=0.246, 0.117 and 0.401 are also found at respectively 0.8, 1.5 and 1.7arcmin to the West of the filament within the fainter contour of thishalo. We have studied the spatial distribution of objects in the fieldwithin 1.5 arcmin of NGC 7603. We conclude that the density of QSOs isroughly within the expected value of the limiting magnitude of ourobservations. However, the configuration of the four galaxies apparentlyconnected by the filament appears highly unusual. The probability ofthree background galaxies of any type with apparent B-magnitudes up to16.6, 21.1 and 22.1 (the observed magnitudes, extinction correctionincluded) being randomly projected on the filament of the fourth galaxy(NGC 7603) is ≈ 3× 10-9. Furthermore, the possibledetection of very vigorous star formation observed in the HII galaxiesof the filament would have a low probability if they were backgroundnormal-giant galaxies; instead, the intensity of the lines is typical ofdwarf HII galaxies. Hence, a set of coincidences with a very lowprobability would be necessary to explain this as a fortuitousprojection of background sources. Several explanations in terms ofcosmological or non-cosmological redshifts are discussed. The Hα galaxy survey. I. The galaxy sample, Hα narrow-band observations and star formation parameters for 334 galaxiesWe discuss the selection and observations of a large sample of nearbygalaxies, which we are using to quantify the star formation activity inthe local Universe. The sample consists of 334 galaxies across allHubble types from S0/a to Im and with recession velocities of between 0and 3000 km s-1. The basic data for each galaxy are narrowband H\alpha +[NII] and R-band imaging, from which we derive starformation rates, H\alpha +[NII] equivalent widths and surfacebrightnesses, and R-band total magnitudes. A strong correlation is foundbetween total star formation rate and Hubble type, with the strongeststar formation in isolated galaxies occurring in Sc and Sbc types. Moresurprisingly, no significant trend is found between H\alpha +[NII]equivalent width and galaxy R-band luminosity. More detailed analyses ofthe data set presented here will be described in subsequent papers.Based on observations made with the Jacobus Kapteyn Telescope operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto deAstrofísica de Canarias.The full version of Table \ref{tab3} is available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/23 Reduced image datafor this survey can be downloaded fromhttp://www.astro.livjm.ac.uk/HaGS/ Hot Intra-group Gas in Stephan's Quintet as a Tracer of Multiple Galaxy CollisionNot Available Physical Conditions and Star Formation Activity in the Intragroup Medium of Stephan's QuintetNew 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. A multiple galaxy collision in compact groupsIt 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 collisionChandra 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. Study of the X-Ray Background Spectrum and Its Large-Scale Fluctuation with ASCAWe studied the energy spectrum and the large-scale fluctuation of theX-ray background with the ASCA GIS instrument based on the ASCA MediumSensitivity Survey and Large Sky Survey observations. A total of 91fields with Galactic latitude |b| > 10° were selected with a skycoverage of 50 deg2 and 4.2 Ms of exposure. For each field,non-X-ray events were carefully subtracted and sources brighter than ~ 2× 1013 erg cm-2 s-1 (2-10keV)were eliminated. Spectral fits with a single power-law model for theindividual 0.7-10 keV spectra showed a significant excess below ~ 2keV,which could be expressed by an additional thermal model with kT ~= 0.4keV or a steep power-law model with a photon index ofΓsoft ~= 6. The 0.5-2keV intensities of the softthermal component varied significantly from field to field by 1 σ= 52 +4-5%, and showed a maximum toward theGalactic Center. This component is considered to be entirely Galactic.As for the hard power-law component, an average photon index of 91fields was obtained to be Γhard = 1.412 +/- 0.007 +/-0.025 and the average 2-10keV intensity was calculated asFhardX = (6.38 +/- 0.04 +/- 0.64) ×10-8erg cm-2 s-1 sr-1 (1σ statistical and systematic errors). The Galactic component ismarginally detected in the hard band. The 2-10keV intensities show a 1σ deviation of 6.49+0.56-0.61%, whiledeviation due to the reproducibility of the particle background is 3.2%.The observed deviation can be explained by the Poisson noise of thesource count in the f.o.v. (~ 0.5 deg2), even assuming asingle N-logS relation on the whole s ky. Based on the observedfluctuation and the absolute intensity, an acceptable region of theN-logS relation was derived, showing a consistent feature with therecent Chandra and XMM-Newton results. The fluctuation of the spectralindex was also examined; it implied a large amount of hard sources and asubstantial variation in the intrinsic source spectra(Γs ~= 1.1 +/- 1.0). New Light and Shadows on Stephan's QuintetWe 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. GHASP: A 3-D Survey of Spiral and Irregular Galaxies at HαNot Available The UZC-SSRS2 Group CatalogWe apply a friends-of-friends algorithm to the combined Updated ZwickyCatalog and Southern Sky Redshift Survey to construct a catalog of 1168groups of galaxies; 411 of these groups have five or more members withinthe redshift survey. The group catalog covers 4.69 sr, and all groupsexceed the number density contrast threshold, δρ/ρ=80. Wedemonstrate that the groups catalog is homogeneous across the twounderlying redshift surveys; the catalog of groups and their membersthus provides a basis for other statistical studies of the large-scaledistribution of groups and their physical properties. The medianphysical properties of the groups are similar to those for groupsderived from independent surveys, including the ESO Key Programme andthe Las Campanas Redshift Survey. We include tables of groups and theirmembers. 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. Hot intragroup gas in Stephan's Quintet as a tracer of multiple galaxy collision.Not Available Abundant molecular gas in the intergalactic medium of Stephan's QuintetStephan'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. Position-velocity diagrams of ionized gas in the inner regions of disk galaxiesWe use long-slit spectroscopy along the major axis of a sample of 23nearby disk galaxies to study the kinematic properties of theionized-gas component in their inner regions. For each galaxy, we derivethe position-velocity diagram of the ionized gas from its emissionlines. We discuss the variety of shapes observed in suchposition-velocity diagrams by comparing the gas velocity gradient,velocity dispersion and integrated flux measured in the inner (r =~+/-1'') and outer regions (r =~ +/-4''). This kind of analysis allowsthe identification of galaxies which are good candidates to host acircumnuclear Keplerian gaseous disk rotating around a central massconcentration, and to follow up with Hubble Space Telescopeobservations. Based on observations carried out at European SouthernObservatory (ESO N.58, A-0564), at the Multiple Mirror Telescope, whichis a joint facility of the Smithsonian Institution and the University ofArizona, and at the Isaac Newton Telescope operated by the Isaac Newtongroup at the La Palma island at the Spanish Observatorio del Roque delos Muchachos of the Instituto de Astrofisica de Canarias. The ASCA Medium Sensitivity Survey (the GIS Catalog Project): Source CatalogWe present the first X-ray source catalog of the ASCA Medium SensitivitySurvey (AMSS, or the GIS catalog project), constructed from data atGalactic latitudes b>10deg observed between 1993 May and 1996December. The catalog utilizes 368 combined fields and contains 1343sources with the detection significance above 5 σ either in thesurvey bands of 0.7-7 keV, 2-10 keV, or 0.7-2 keV, including targetsources. For each source, the ASCA source name, position, a 90% errorradius, count rates in the three bands, detection significances, fluxes,and a hardness ratio are provided. With extensive simulations, wecarefully evaluate the data quality of the catalog. Results fromcross-correlation with other existing catalogs are briefly summarized. Star-forming Objects in the Tidal Tails of Compact GroupsA 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 QuintetStephan'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 EnvironmentAnalysis 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 QuintetWe 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 possiblefallbacks'' or disruption by the parent galaxies and may already be,or turn into, self-gravitating dwarf galaxies, new members of the group.
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