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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.
| 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.
| The Discovery of a High-Redshift X-Ray-Emitting QSO Very Close to the Nucleus of NGC 7319
A strong X-ray source only 8" from the nucleus of the Seyfert 2 galaxyNGC 7319 in Stephan's Quintet has been discovered by Chandra. We haveidentified the optical counterpart and show that it is a QSO withze=2.114. It is also an ultraluminous X-ray source withLX=1.5×1040 ergs s-1. From theoptical spectra of the QSO and the interstellar gas of NGC 7319together, we show that it is very likely that the QSO is interactingwith the interstellar gas.
| 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.
| BeppoSAX/PDS serendipitous detections at high galactic latitudes
At a flux limit of 10-11 erg cm-2 s-1in the 20-100 keV band, the PDS instrument on-board BeppoSAX offers theopportunity to study the extragalactic sky with an unprecedentedsensitivity. In this work we report on the results of a search in theBeppoSAX archive for serendipitous high energy sources at high galacticlatitudes (\vert b \vert≥13°). We have defined a set of twelveregions in which the PDS/MECS cross-calibration constant is higher thanthe nominal value. We attribute this mismatch to the presence of aserendipitous source in the PDS field of view. In four cases the likelyhigh energy emitter is also present in the MECS field of view. In thesecases, we have performed a broad band spectral analysis (1.5-100 keV) tounderstand the source spectral behaviour and compare it with previousBeppoSAX observations when available. In eight cases the identificationof the source likely to provide the PDS spectrum is based on indirectevidence (extrapolation to lower energies and/or comparison withprevious observations). This approach led to the discovery of six newhard X-ray emitting objects (PKS 2356-611, 2MASX J14585116-1652223, NGC1566, NGC 7319, PKS 0101-649 and ESO 025-G002) and to the presentationthe PDS spectrum of NGC 3227 for the first time. In the remaining fivecases we provide extra BeppoSAX observations that can be compared withmeasurements already published and/or in the archive.
| Galaxy Train Wrecks
Not Available
| 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.
| Circumnuclear Structure and Black Hole Fueling: Hubble Space Telescope NICMOS Imaging of 250 Active and Normal Galaxies
Why are the nuclei of some galaxies more active than others? If mostgalaxies harbor a central massive black hole, the main difference isprobably in how well it is fueled by its surroundings. We investigatethe hypothesis that such a difference can be seen in the detailedcircumnuclear morphologies of galaxies using several quantitativelydefined features, including bars, isophotal twists, boxy and diskyisophotes, and strong nonaxisymmetric features in unsharp-masked images.These diagnostics are applied to 250 high-resolution images of galaxycenters obtained in the near-infrared with NICMOS on the Hubble SpaceTelescope. To guard against the influence of possible biases andselection effects, we have carefully matched samples of Seyfert 1,Seyfert 2, LINER, starburst, and normal galaxies in their basicproperties, taking particular care to ensure that each was observed witha similar average scale (10-15 pc pixel-1). Severalmorphological differences among our five different spectroscopicclassifications emerge from the analysis. The H II/starburst galaxiesshow the strongest deviations from smooth elliptical isophotes, whilethe normal galaxies and LINERs have the least disturbed morphology. TheSeyfert 2s have significantly more twisted isophotes than any othercategory, and the early-type Seyfert 2s are significantly more disturbedthan the early-type Seyfert 1s. The morphological differences betweenSeyfert 1s and Seyfert 2s suggest that more is at work than simply theviewing angle of the central engine. They may correspond to differentevolutionary stages.
| 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).
| The Discovery of a High Redshift X-Ray Emitting QSO Very Close to the Nucleus of NGC 7319 in Stephan's Quintet
A strong X-ray source only 8 ″ from the nucleus of the Seyfert 2galaxy NGC 7319 in Stephan's Quintet has been discovered by Chandra. Wehave identified the optical counterpart and observed its spectrum withthe Keck I 10-m telescope. It is a QSO with ze = 2.114. It isalso a ULX with Lx = 1.5 x 1040 ergsec-1. Our Keck spectra show that it is very likely that theQSO is interacting with the interstellar gas in NGC 7319.
| 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.
| New optical spectra and general discussion on the nature of ULXs
We present optical spectroscopic observations of three Ultra LuminousX-ray sources (ULXs). Two of them are very close to the active galaxyNGC 720 and the other is near NGC 1073. The two around NGC 720 turn outto be quasars at z= 2.216 and z= 0.959, the one near NGC 1073 seems tobe associated to an HII region at the redshift of NGC 1073. Weconcentrate our analysis on the two quasars and analyze them inconjunction with a set of 22 additional X-ray sources close to nearbygalaxies which also fit the criteria of ULXs and which also have beenidentified as quasars of medium to high redshift. This sample shows anunusually large fraction of rare BL Lac type objects. The high redshiftsof these ULXs and their close proximity to their low redshift,supposedly parent galaxies is a surprising result in the light ofstandard models. We describe the main properties of each of theseobjects and their parent galaxy, and briefly discuss possibleinterpretations.
| Seyfert galaxies in UZC-Compact Groups
We present results concerning the occurrence of Seyfert galaxies in anew automatically selected sample of nearby Compact Groups of galaxies(UZC-CGs). Seventeen Seyferts are found, constituting 3% of theUZC-CG galaxy population. CGs hosting and non-hosting a Seyfert memberexhibit no significant differences, except that a relevant number of Sy2is found in unusual CGs, all presenting large velocity dispersion(σ>400 km s-1), many neighbours and a high number ofellipticals. We also find that the fraction of Seyferts in CGs is 3times as large as that among UZC-single-galaxies, and results from anexcess of Sy2s. CG-Seyferts are not more likely than other CG galaxiesto present major interaction patterns, nor to display a bar. Our resultsindirectly support the minor-merging fueling mechanism.
| Hot Intra-group Gas in Stephan's Quintet as a Tracer of Multiple Galaxy Collision
Not Available
| A Possible Signature of Connection between Blazars and Seyfert Galaxies
The accretion rates (dot{M}) and their correlation with cosmologicalredshifts for a sample of blazars and Seyfert galaxies are presented.The sample includes 77 blazars (28 FSRQs, 26 LBLs, and 23 HBLs) and 60Seyfert galaxies, of which the extended spectral energy distributioninformation and redshifts are available. Within the framework ofaccreting black holes, the accretion rates for these sources wereestimated based on their bolometric luminosities. The result shows thatthe accretion rates are significantly different for each subclass of theblazars and Seyfert galaxies. Their averages are, respectively, 50.2,17.0, 1.0, 0.1Modot yr-1 for the FSRQs, LBLs, HBLs, and theSeyfert galaxies, exhibiting a well descending sequence ofFSRQs-LBLs-HBLs-Seyfert galaxies. They are strongly correlated with theredshifts for both blazars and Seyfert galaxies. The linear correlationcoefficients are 0.81 and 0.68 with a chance probab ility of p <0.0001, respectively. A plot of dot{M} - z shows that the blazars andthe Seyfert galaxies distribute in a distinguishable regions with aconnection at z 0.7 and almost all the sources lie in a narrowregion of z1.40 ≤ dot{M} ≤ 250 z1.40,illustrating a strong correlation between the two quantities for thewhole sample. The regression line is dot{M} = (14.5 ± 1.2)z1.40±0.06 Modot yr-1 with a linearcoefficient of 0.93 and a chance probability of p < 0.0001,suggesting a connection between blazars and Seyfert galaxies. Thisconnection might imply that the two classes are on the same evolutionarysequence. Although the correlations of the data are formally solid, theconclusion may be affected by one source of considerable uncertainty atthe data level, which is also discussed.
| 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.
| Advanced Camera for Surveys Observations of Young Star Clusters in the Interacting Galaxy UGC 10214
We present the first Advanced Camera for Surveys (ACS) observations ofyoung star clusters in the colliding/merging galaxy UGC 10214. Theobservations were made as part of the Early Release Observation (ERO)program for the newly installed ACS during service mission SM3B for theHubble Space Telescope (HST). Many young star clusters can be identifiedin the tails of UGC 10214, with ages ranging from ~3 to 10 Myr. Theextreme blue V-I (F606W-F814W) colors of the star clusters found in thetail of UGC 10214 can only be explained if strong emission lines areincluded with a young stellar population. This has been confirmed by ourKeck spectroscopy of some of these bright blue stellar knots. The mostluminous and largest of these blue knots has an absolute magnitude ofMV=-14.45, with a half-light radius of 161 pc, and if it is asingle star cluster, it would qualify as a super star cluster (SSC).Alternatively, it could be a superposition of multiple scaled OBassociations or clusters. With an estimated age of ~4-5 Myr, its derivedmass is less than 1.3×106Msolar. Thus, theyoung stellar knot is unbound and will not evolve into a normal globularcluster. The bright blue clusters and associations are much younger thanthe dynamical age of the tail, providing strong evidence that starformation occurs in the tail long after it was ejected. UGC 10214provides a nearby example of processes that contributed to the formationof halos and intracluster media in the distant and younger universe.
| 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.
| Chandra Observations of the Interacting NGC 4410 Galaxy Group
We present high-resolution X-ray imaging data from the ACIS-S instrumenton the Chandra telescope of the nearby interacting galaxy group NGC4410. Four galaxies in the inner portion of this group are clearlydetected by Chandra, including the peculiar low-luminosity radio galaxyNGC 4410A. In addition to a nuclear point source, NGC 4410A containsdiffuse X-ray emission, including an X-ray ridge extending out to about12" (6 kpc) to the northwest of the nucleus. This ridge is coincidentwith an arc of optical emission-line gas, which has previously beenshown to have optical line ratios consistent with shock ionization. Thisstructure may be due to an expanding superbubble of hot gas caused bysupernovae and stellar winds or by the active nucleus. The Chandraobservations also show four or five possible compact ultraluminous X-ray(ULX) sources (LX>=1039 ergs s-1)associated with NGC 4410A. At least one of these candidate ULXs appearsto have a radio counterpart, suggesting that it may be due to an X-raybinary with a stellar-mass black hole, rather than an intermediate-massblack hole. In addition, a faint diffuse intragroup X-ray component hasbeen detected between the galaxies (LX~1041 ergss-1). This supports the hypothesis that the NGC 4410 group isin the process of evolving via mergers from a spiral-dominated group(which typically has no X-ray-emitting intragroup gas) to anelliptical-dominated group (which often has a substantial intragroupmedium).
| 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.
| Tidal dwarf candidates in a sample of interacting galaxies. II. Properties and kinematics of the ionized gas
We present low-resolution spectroscopy of the ionized gas in a sample ofoptical knots located along the tidal features of 14 interactinggalaxies previously selected as candidate Tidal Dwarf Galaxies (TDGs).From redshift measurements, we are able to confirm their physicalassociation with the interacting system in almost all cases. For mostknots, the oxygen abundance does not depend on the blue luminosity. Theaverage, 12+log (O/H) =8.34+/-0.20, is typical of TDGs and is comparableto that measured in the outer stellar disk of spirals from which theywere formed. A few knots showing low metallicities are probablypre-existing low-mass companions. The estimated Hα luminosity ofthe TDG candidates is higher than that of typical individual H Iiregions in spiral disks and is comparable to the global Hαluminosity of dwarf galaxies. We find several instances of velocitygradients with amplitudes apparently larger than 100 km s-1in the ionized gas in the tidal knots and discuss various possibleorigins for the large velocity amplitudes. While we can exclude tidalstreaming motions and outflows, we cannot rule out projection effectswith the current resolution. The velocity gradients could be indicativeof the internal kinematic characteristic of self-gravitating objects.Higher resolution spectra are required to confirm whether the tidalknots in our sample have already acquired their dynamical independenceand are therefore genuine Tidal Dwarf Galaxies. Based on observationscollected at the European Southern Observatory, La Silla, Chile (ESO No64.N-0361).Figures \ref{fig:chart:AM0529-565} to\ref{fig:chart:AM1325-292}, Table \ref{tab:newTDGcandPhot} andAppendices A and B are only available in electronic form athttp://www.edpsciences.org
| Spectral Signatures of Jet-Driven Shocks in Active Galactic Nuclei
It is now clear that jet-driven shocks in active galaxies areresponsible for an important fraction of the emission of the extendednarrow-line regions (at all wavelengths). The remainder arises fromphotoionisation by UV photons originating at the active nucleus itself.This review examines the spectral and dynamical signatures of jet-drivenshocks in these environments, and shows how these can be used toconstrain the shock velocities, the parameters of the jet, the galacticor circumgalactic medium and the nature of the central engine.
| The formation of ultracompact dwarf galaxies
Recent spectroscopic observations of galaxies in the Fornax Clusterreveal nearly unresolved `star-like' objects with redshifts appropriateto the Fornax Cluster. These objects have intrinsic sizes of ~100pc andabsolute B -band magnitudes in the range and lower limits for thecentral surface brightness , and so appear to constitute a newpopulation of ultracompact dwarf galaxies (UCDs). Such compact dwarfswere predicted to form from the amalgamation of stellar superclusters(by Kroupa) , which are rich aggregates of young massive star clusters(YMCs) that can form in collisions between gas-rich galaxies. Here wepresent the evolution of superclusters in a tidal field. The YMCs mergeon a few supercluster crossing times. Superclusters that are initiallyas concentrated and massive as knot S in the interacting Antennaegalaxies evolve to merger objects that are long-lived and showproperties comparable to the newly discovered UCDs. Less massivesuperclusters resembling knot 430 in the Antennae may evolve to ωCen-type systems. Low-concentration superclusters are disrupted by thetidal field, dispersing their surviving star clusters while theremaining merger objects rapidly evolve into the region populated bylow-mass Milky Way dSph satellites.
| X-Ray versus Optical Observations of Active Galactic Nuclei: Evidence for Large Grains?
Recently, Maiolino et al. constructed a sample of active galactic nucleifor which both the reddening E(B-V) and the column density NHto the nucleus could be determined. For most of the galaxies in theirsample, they found that E(B-V)/NH is substantially smallerthan for the diffuse interstellar medium of our Galaxy. They assertedthat either the dust-to-gas ratio is lower than in the Galaxy or thegrains are so large that they do not extinct or redden efficiently inthe optical. We show that there is no systematic increase in E(B-V) withNH for the Maiolino et al. galaxies, which suggests that theX-ray absorption and optical extinction occur in distinct media. In alater paper, Maiolino et al. suggested that the observed lines of sightfor the previous Maiolino et al. galaxies pass through the ``torus''that obscures the broad-line region and nuclear continuum in Seyfert 2galaxies and argued that the torus grains are larger than Galacticgrains. There is no reason to believe that the lines of sight for thesegalaxies pass through the torus, since the observed column densities arelower than those typically observed in Seyfert 2 galaxies. We suggestinstead that the X-ray absorption occurs in material located off thetorus and/or accretion disk, while the optical extinction occurs inmaterial located beyond the torus. The X-ray absorbing material couldeither be dust-free or contain large grains that do not extinctefficiently in the optical. There is no conclusive evidence that thegrains in active galactic nuclei are systematically larger than those inthe diffuse interstellar medium of our Galaxy. We discuss an alternativeway to probe the properties of dust in Seyfert tori but find thatobservations of Seyfert 2 nuclei with higher resolution than currentlyavailable will be needed in order to place stringent limits on the dust.
| 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.
| Active Galactic Nucleus Black Hole Masses and Bolometric Luminosities
Black hole mass, along with mass accretion rate, is a fundamentalproperty of active galactic nuclei (AGNs). Black hole mass sets anapproximate upper limit to AGN energetics via the Eddington limit. Wecollect and compare all AGN black hole mass estimates from theliterature; these 177 masses are mostly based on the virial assumptionfor the broad emission lines, with the broad-line region size determinedfrom either reverberation mapping or optical luminosity. We introduce200 additional black hole mass estimates based on properties of the hostgalaxy bulges, using either the observed stellar velocity dispersion orthe fundamental plane relation to infer σ these methods assumethat AGN hosts are normal galaxies. We compare 36 cases for which blackhole mass has been generated by different methods and find, forindividual objects, a scatter as high as a couple of orders ofmagnitude. The less direct the method, the larger the discrepancy withother estimates, probably due to the large scatter in the underlyingcorrelations assumed. Using published fluxes, we calculate bolometricluminosities for 234 AGNs and investigate the relation between blackhole mass and luminosity. In contrast to other studies, we find nosignificant correlation of black hole mass with luminosity, other thanthose induced by circular reasoning in the estimation of black holemass. The Eddington limit defines an approximate upper envelope to thedistribution of luminosities, but the lower envelope depends entirely onthe sample of AGNs included. For any given black hole mass, there is arange in Eddington ratio of up to 3 orders of magnitude.
| Filaments and Ionized Gas in the Vicinity of 3C 244.1
We present results of Hubble Space Telescope observations of the radiogalaxy 3C 244.1. The broadband F702W (R) and F555W (V) images (WFPC2/PC)show an elliptical galaxy, and gaseous filaments and blobs surroundingit. In the narrowband ramp filter, dominated by [O III] λ5007,these filaments are bright and have the same morphology as the broadbandimages. To the south, the filaments have a cone-shaped structure, andthe radio jet is located at the center of this cone. To the north of thegalaxy, the structure is found near the nucleus of the galaxy within itselliptical profile. From the photometry, the two brighter structuresseem to be extended narrow-line emission regions. A comparison withdiagnostic line ratios shows that the observed emission is consistentwith interactions between the expanding radio jet and the local densermedium. Based on observations made with the NASA/ESA Hubble SpaceTelescope, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.
| 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.
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