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|Gas in early-type galaxies: cross-fuelling in late-type-early-type pairs?|
We present 12CO (J= 1-0) and 12CO (J= 2-1)observations of eight early-type galaxies, forming part of a sample ofinteracting galaxies, each consisting of one late- and one early-typesystem. All of the early-type galaxies observed are undetected in CO tolow levels, allowing us to place tight constraints on their moleculargas content. Additionally, we present HI absorption data for one system.The implications for possible gas transfer from the late- to theearly-type galaxy during the interaction are discussed.
|First Detection of PAHs and Warm Molecular Hydrogen in Tidal Dwarf Galaxies|
We observed two faint tidal dwarf galaxies (TDGs), NGC 5291 N and NGC5291 S, with the Infrared Spectrograph on the Spitzer Space Telescope.We detect strong polycyclic aromatic hydrocarbon (PAH) emission at 6.2,7.7, 8.6, 11.3, 12.6, and 16.5 μm, which match models of groups of~100 carbon atoms with an equal mixture of neutral and ionized PAHs. TheTDGs have a dominant warm (~140 K) dust component in marked contrast tothe cooler (40-60 K) dust found in starburst galaxies. For the firsttime we detect the low-J rotational lines from molecular hydrogen.Adopting LTE, there is ~105 Msolar of ~400 K gas,which is <0.1% of the cold gas mass inferred from 12CO(1-0) measurements. The combination of one-third solar metallicity witha recent (<5 million year) episode of star formation is reflected inthe S and Ne ratios. The excitation is higher than typical values forstarburst galaxies and similar to that found in BCDs. Using the InfraredArray Camera, we identify an additional 13 PAH-rich candidate TDGs.These sources occupy a distinct region of IRAC color space with[3.6]-[4.5]<0.4 and [4.5]-[8.0]>3.2. Their disturbed morphologiessuggest past merger events between companions; for example, NGC 5291 Shas a projected 11 kpc tail. NGC 5291 N and S have stellar masses of(1.5 and 3.0)×108 Msolar, which iscomparable to BCDs, although still roughly 10% of the LMC's stellarmass. The candidate TDGs are an order of magnitude less massive. Thissystem appears to be a remarkable TDG nursery.
|Orientation and size of the `Z' in X-shaped radio galaxies|
Some X-shaped radio galaxies show a Z-symmetric morphology in the lessluminous secondary lobes. Within the scenario of a merger between twogalaxies, each hosting a supermassive black hole in its centre, thisstructure has been explained before. As the smaller galaxy spiralstowards the common centre, it releases gas to the interstellar medium ofthe larger active galaxy. The ram pressure of this streaming gas willbend the lobes of the pre-merger jet into a Z-shape. After the blackholes have merged, the jet propagates in a new direction that is alignedwith the angular momentum of the binary black hole. In this paper wedeproject the pre- and post-merger jets. Taking into account theexpected angles between the jet pairs and with the assumption that theirdirections are uncorrelated, we show that one of three possibleorientations of the jets with respect to the line of sight is morelikely than the others. This actually depends on the distance where thebending occurs. Another result of our deprojection is that the streaminggas bends the jet into a Z-shape in a range between about 30 and 100 kpcdistance to the centre of the primary galaxy. We confirm this finding bycomparing our predictions for the properties of the rotational velocityfield and its radius with observations and numerical simulations ofmerging galaxies. Thus, our results support the merger scenario asexplanation for X- and Z-shaped radio galaxies with the jet pointingalong the former axis of orbital angular momentum of the binary.
|Hα Kinematics of Tidal Tails in Interacting Systems: Projection Effects and Dark Matter in TDGs|
Several interacting systems exhibit at the tip of their long tidal tailsmassive condensations of atomic hydrogen, which may be the progenitorsof Tidal Dwarf Galaxies. Because, quite often, these tails are observededge-on, projection effects have been claimed to account for the largeHI column densities measured there. Here we show that determining thevelocity field all along the tidal features, one may disentangleprojection effects along the line of view from real bound structures.Due to its large field of view, high spectral and 2D spatialresolutions, Fabry-Perot observations of the ionized gas are welladapted to detect a kinematical signature of either streaming motionsalong a bent tidal tail or of infalling/rotating material associatedwith a forming TDG. Spectroscopic observations also allow to measure thedynamical masses of the TDGs that are already relaxed and check theirdark matter content.
|Double Nuclei and ``TDGs": Colliding or Activity of Nucleus Monster?|
It is known that among active galaxies (AG) with strong emission lines (UV-galaxies, Sy 1 and Sy2, Markarian and Kazarian galaxies,Radio-galaxies, QSO's host galaxies and so on) there are large per centof objects with double and multiple nucleus. The common sizes andvolumes of these nuclei are on the order of a few hundred parsecs orkilo-parsecs. In fact these are not double galaxies or clusters ofgalaxies as many of astronomers believe, but just the complicatednucleus of AG. The problem is: what are the nature and the birth ofthese objects? There are in fact two basic suppositions in the subject:(a) The complicated nuclei are the result of merging or colliding of twoor more galaxies, or: (b) They are the results of nuclear activity. Theresults of detailed spectroscopic observations of a number of "tidalgalaxies", carried out with the 5m Palomar telescope, 2.6m telescope ofAmbartsumian Byurakan astrophysical observatory (multi-pupilspectroscopy with Tiger receiver) and 6m telescope of SpecialAstrophysical observatory of Russia are presented. It is shown that inmany cases the "tidal dwarf galaxies "(or actually complicatednucleus) are the result of galactic nuclear activity.
|A Green Bank Telescope Search for Water Masers in Nearby Active Galactic Nuclei|
Using the Green Bank Telescope, we have conducted a survey for 1.3 cmwater maser emission toward the nuclei of nearby active galaxies, themost sensitive large survey for H2O masers to date. Among 145galaxies observed, maser emission was newly detected in 11 sources andconfirmed in one other. Our survey targeted nearby (v<12,000 kms-1), mainly type 2 active galactic nuclei (AGNs) north ofδ=-20deg and includes a few additional sources as well.We find that more than one-third of Seyfert 2 galaxies have strong maseremission, although the detection rate declines beyond v~5000 kms-1 because of sensitivity limits. Two of the masersdiscovered during this survey are found in unexpected hosts: NGC 4151(Seyfert 1.5) and NGC 2782 (starburst). We discuss the possiblerelations between the large X-ray column to NGC 4151 and a possiblehidden AGN in NGC 2782 to the detected masers. Four of the masersdiscovered here, NGC 591, NGC 4388, NGC 5728, and NGC 6323, havehigh-velocity lines symmetrically spaced about the systemic velocity, alikely signature of molecular gas in a nuclear accretion disk. The masersource in NGC 6323, in particular, reveals the classic spectrum of a``disk maser'' represented by three distinct groups of Dopplercomponents. Future single-dish and VLBI observations of these fourgalaxies could provide a measurement of the distance to each galaxy andof the Hubble constant, independent of standard candle calibrations.
|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).
|Radio emission from AGN detected by the VLA FIRST survey|
Using the most recent (April 2003) version of the VLA FIRST survey radiocatalog, we have searched for radio emission from >2800 AGN takenfrom the most recent (2001) version of the Veron-Cetty and Veron AGNcatalog. These AGN lie in the 9033 square degrees of sky alreadycovered by the VLA FIRST survey. Our work has resulted in positivedetection of radio emission from 775 AGN of which 214 are new detectionsat radio wavelengths.Tables 3 and 4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/35
|The Redshift Distribution of Flat-Spectrum Radio Sources|
The redshift distribution of flat-spectrum radio sources with 5 GHz fluxdensities S5>~5 mJy is a key component in using currentradio lens surveys to probe the cosmological model. We have constructedthe first flat-spectrum radio sample in the flux density range 3-20 mJy.Our new sample has 33 sources; we have determined the redshifts of 14 ofthese (42% complete). The low mean redshift, ~=0.75, of ourfaintest sample needs to be confirmed by further observations to improvethe sample completeness. We also increased the redshift completeness ofseveral surveys of brighter flat-spectrum sources. While the meanredshift ~=1.1 of flat-spectrum samples fainter than 1 Jy isnearly constant, the fraction of the sources identifiable as quasarssteadily drops from ~80% to ~10% as the flux density of the sourcesdecreases.
|A Minor-Merger Interpretation for NGC 1097's ``Jets''|
We have conducted a deep search for neutral hydrogen gas associated withthe faint optical ``jets'' of NGC 1097 using the Very Large Array.Measurable H I would have been expected if the jets were tidal in origingiven their moderately blue optical and near-infrared colors. The jetsare free of H I emission to a limiting surface density(ΣHI) of 0.06 Msolar pc-2 (3σ) over a 1102 km s-1 velocity range. We also rule outextended H I emission down to 0.02 Msolar pc-2 (3σ, ΔV=45 km s-1) within a 4' FWHM aperturecentered on the right-angle turn in jet R1. We have detected an H Isource [MHI=(5.1+/-1.0)×106Msolar] coincident with a small edge-on spiral or irregulargalaxy (NGC 1097B) 12' southwest of NGC 1097, situated between two jets.Two other ~106 Msolar H I point sources in thefield are considered marginal detections. Neither are associated withthe optical jets.The jets' radio-X-ray spectral energy distribution is most consistentwith starlight. However, from their morphology, optical/near-infraredcolors, and lack of H I, we argue that the jets are not tidal tailsdrawn out of NGC 1097's disk or stars stripped from the ellipticalcompanion NGC 1097A. We also reject in situ star formation in ancientradio jets as this requires essentially 100% conversion of gas intostars on large scales. Instead, we conclude that the jets represent thecaptured remains of a disrupted dwarf galaxy that passed through theinner few kiloparsecs of NGC 1097's disk.We present N-body simulations of such an encounter that reproduce theessential features of NGC 1097's jets: A long and narrow ``X''-shapedmorphology centered near the spiral's nucleus, right-angle bends, and nodiscernible dwarf galaxy remnant. A series of jetlike distributions areformed, with the earliest appearing ~1.4 Gyr after impact. Well-definedX shapes form only when the more massive galaxy has a strong diskcomponent. Ram-pressure stripping of the dwarf's interstellar mediumwould be expected to occur while passing through NGC 1097's disk,accounting for the jets' lack of H I and H II. The remnants' (B-V) colorwould still agree with observations even after ~3 Gyr of passiveevolution, provided the cannibalized dwarf was low-metallicity anddominated by young stars at impact.
|NGC 3256: Kinematic Anatomy of a Merger|
We have used the Australia Telescope Compact Array to image the neutralhydrogen in the merging system NGC 3256, to test the idea that globularclusters (GCs) form during the interactions and mergers of diskgalaxies. We compare our observations with hydrodynamical numericalsimulations from the literature to examine the hypothesis that the H Ifragments with masses greater than 107+/-1 Msolarare sites of GC formation. We appear to have detected detached H Ifragments in the vicinity of NGC 3256. These fragments, with masses~107 Msolar, may have little dark matter content,which is also a characteristic of globular clusters, and so ourobservations support the hypothesis that globular clusters form in thetype of interaction that resulted in NGC 3256.
|Evolution of Star-forming and Active Galaxies in Nearby Clusters|
We have used optical spectroscopy to investigate the active galaxypopulations in a sample of 20 nearby Abell clusters. The targets wereidentified on the basis of 1.4 GHz radio emission, which identifies themas either active galactic nuclei (AGNs) or galaxies forming stars atrates comparable to or greater than that of the Milky Way. The spectrawere used to characterize the galaxies via their emission and absorptionfeatures. The spectroscopy results reveal a significant population ofstar-forming galaxies with large amounts of nuclear dust extinction.This extinction eliminates bluer emission lines such as [O II] from thespectra of these galaxies, meaning their star formation could easily beoverlooked in studies that focus on such features. Around 20% of thecluster star-forming galaxies have spectra of this type. The radialdistributions of active galaxies in clusters show a strong segregationbetween star-forming galaxies and AGNs, with star-forming galaxiesbroadly distributed and AGNs preferentially in the cluster cores. Theradial distribution of the dusty star-forming galaxies is more centrallyconcentrated than the star-forming galaxies in general, which arguesthat they are a consequence of some cluster environmental effect.Furthermore, we note that such galaxies may be identified using their4000 Å break strengths. We find that discrepancies in reportedradio luminosity functions for AGNs are likely the result ofclassification differences. There exists a large population of clustergalaxies whose radio fluxes, far-infrared fluxes, and optical magnitudessuggest their radio emission may be powered by stars yet whose spectralack emission lines. Understanding the nature of these galaxies iscritical to assessing the importance of AGNs in the radio luminosityfunction at low luminosities. We also find that regardless of thispopulation, the crossover point where the radio luminosity function iscomposed equally of star-forming galaxies and AGNs occurs at lowerluminosities in clusters than in the field. This is likely a simpleconsequence of the reduction in star formation in cluster galaxies andthe morphological mix in clusters compared with the field.Based in part on observations obtained with the Apache Point Observatory(APO) 3.5 m telescope, which is owned and operated by the AstrophysicalResearch Consortium.
|The UZC-SSRS2 Group Catalog|
We 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 Role of Mergers in Galaxy Evolution|
The evolution of galaxies has been driven by galaxy-galaxy collisions onall cosmological time-scales, from the primordial universe to thepresent. In this context, nearby mergers serve as local analogs to gaininsight into the physical processes that lead to the formation andtrans-formation of galaxies in the more distant universe. Here I reviewmultiwavelength observations -with particular emphasis on recent resultsobtained with ISO- of mergers of massive galaxies driving the formationof: 1) luminous infrared galaxies, 2) elliptical galaxy cores, 3)luminous dust-enshrouded extranuclear starbursts, 4) symbiotic galaxies,and 5) tidal dwarf galaxies. The most important implication for futurestudies on the formation of galaxies at early cosmological timescales isthat the most luminous galaxies in the local universe (z<=0.1) emitthe bulk of their energy in the mid and far-infrared, and therefore,their analogs in the more distant universe would be invisible in theultraviolet and optical wavelength rest-frames.
|Molecular Gas in Galaxies|
Knowledge of the molecular component of the ISM is fundamental tounderstand star formation. The H_2 component appears to dominate the gasmass in the inner parts of galaxies, while the HI component dominates inthe outer parts. Observation of the CO and other lines in normal andstarburst galaxies have questioned the CO-to-H_2 conversion factor, anddetection of CO in dwarfs have shown how sensitive the conversion factor is to metallicity. Our knowledge has made great progress in recentyears, because of sensitivity and spatial resolution improvements.Large-scale CO maps of nearby galaxies are now available, which extendour knowledge on global properties, radial gradients, and spiralstructure of the molecular ISM. Millimetric interferometers reveal highvelocity gradients in galaxy nuclei, and formation of embeddedstructures, like bars within bars. Galaxy interactions are veryeffective to enhance gas concentrations and trigger starbursts. Nucleardisks or rings are frequently observed, that concentrate the starformation activity. Since the density of starbursting galaxies isstrongly increasing with redshift, the CO lines and the mm dust emissionare a privileged tool to follow evolution of galaxies and observe theISM dynamics at high redshift: they could give an answer about thedebated question of the star-formation history, since many massiveremote starbursts could be dust-enshrouded.
|The Radio Galaxy Populations of Nearby Northern Abell Clusters|
We report on the use of the NRAO VLA Sky Survey (NVSS) to identify radiogalaxies in 18 nearby Abell clusters. The listings extend from the coresof the clusters out to radii of 3 h-175 Mpc, whichcorresponds to 1.5 Abell radii and approximately 4 orders of magnitudein galaxy density. To create a truly useful catalog, we have collectedoptical spectra for nearly all of the galaxies lacking public velocitymeasurements. Consequently, we are able to discriminate between thoseradio galaxies seen in projection on the cluster and those that are inactuality cluster members. The resulting catalog consists of 329 clusterradio galaxies plus 138 galaxies deemed foreground or backgroundobjects, and new velocity measurements are reported for 273 of theseradio galaxies. The motivation for the catalog is the study of galaxyevolution in the cluster environment. The radio luminosity function is apowerful tool in the identification of active galaxies, as it isdominated by star-forming galaxies at intermediate luminosities andactive galactic nuclei (AGNs) at higher luminosities. The flux limit ofthe NVSS allows us to identify AGNs and star-forming galaxies down tostar formation rates less than 1 Msolar yr-1. Thissensitivity, coupled with the all-sky nature of the NVSS, allows us toproduce a catalog of considerable depth and breadth. In addition tothese data, we report detected infrared fluxes and upper limits obtainedfrom IRAS data. It is hoped that this database will prove useful in anumber of potential studies of the effect of environment on galaxyevolution. Based in part on observations obtained with the Apache PointObservatory 3.5 m telescope, which is owned and operated by theAstrophysical Research Consortium (ARC).
|Molecular Gas in Optically Selected Mergers|
We have mapped the 2.6 mm CO J=1-->0 emission in three opticallyselected ``Toomre sequence'' mergers (NGC 520, NGC 3921, NGC 4676). Themolecular gas distribution is well resolved by the observations. For NGC520 and NGC 4676A, the nuclear gas concentrations form a disklike or aringlike structure, and the gas kinematics are regular and consistentwith simple rotation. Discrete molecular gas complexes are found alongthe stellar bar in NGC 4676B, and the gas kinematics is consistent withthe disk rotation traced in Hα. The molecular gas distribution inNGC 3921 is asymmetric about the stellar remnant, and both thedistribution and kinematics suggest that the molecular gas has notsettled into the center of the remnant. Molecular gas clouds aredetected outside the central regions of NGC 3921 and NGC 4676, and theymay be associated with the tidal tails and bridges mapped in H I.Departures from the canonical scenario for a merger involving two largespiral galaxies are found in all three Toomre sequence mergers studied.Our data suggest that one of the progenitor disks in NGC 520 and NGC3921 was relatively gas-poor. A detailed comparison of these opticallyselected mergers and more luminous IR-selected mergers is deferred to aseparate paper.
|High-Resolution H I Mapping of NGC 4038/39 (``The Antennae'') and Its Tidal Dwarf Galaxy Candidates|
We present new VLA C+D array H I observations and optical andnear-infrared imaging of the well-known interacting system NGC 4038/39,``The Antennae.'' At low spatial resolution (~40"), the radio data reacha limiting column density of ~1019 cm-2 (2.5σ), providing significantly deeper mapping of the tidal featuresthan afforded by earlier H I maps. At relatively high spatial resolution(~10"), the radio data reveal a wealth of gaseous substructure bothwithin the main bodies of the galaxies and along the tidal tails. Inagreement with previous H I studies, we find that the northern tail hasH I along its outer length but none along its base. We suggest that theH I at the base of this tail has been ionized by massive stars in thedisk of NGC 4038. The gas in the southern tail has a bifurcatedstructure, with one filament lying along the optical tail and anotherrunning parallel to it but with no optical counterpart. The twofilaments join just before the location of several star-forming regionsnear the end of the tail. The H I velocity field at the end of the tailis dominated by strong velocity gradients, which suggests that at thislocation the tail is bending away from us. We delineate and examine tworegions within the tail previously identified as possible sites of aso-called tidal dwarf galaxy condensing out of the expanding tidalmaterial. The tail velocity gradients mask any clear kinematic signatureof a self-gravitating condensation in this region. A dynamical analysissuggests that there is not enough mass in gas alone for either of theseregions to be self-gravitating. Conversely, if they are bound theyrequire a significant contribution to their dynamical mass from evolvedstars or dark matter. Even if there are no distinct dynamical tidalentities, it is clear that there is a unique concentration of gas,stars, and star-forming regions within the southern tail: the H Ichannel maps show clear evidence for a significant condensation near thetail star-forming regions, with the single-channel H I column densitieshigher than anywhere else in the system, including within the maindisks. Finally, the data reveal H I emission associated with the edge-on``superthin'' Scd galaxy ESO 572-G045, which lies just beyond thesouthern tidal tail of The Antennae, showing it to be a companionsystem.
|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.
|Abundant molecular gas in tidal dwarf galaxies: On-going galaxy formation|
We investigate the process of galaxy formation as can be observed in theonly currently forming galaxies - the so-called Tidal Dwarf Galaxies,hereafter TDGs - through observations of the molecular gas detected viaits CO (Carbon Monoxide) emission. These objects are formed of materialtorn off of the outer parts of a spiral disk due to tidal forces in acollision between two massive galaxies. Molecular gas is a key elementin the galaxy formation process, providing the link between a cloud ofgas and a bona fide galaxy. We have detected CO in 8 TDGs (two of themhave already been published in Braine et al. 2000, hereafter Paper I),with an overall detection rate of 80%, showing that molecular gas isabundant in TDGs, up to a few 108 Msun. The COemission coincides both spatially and kinematically with the HIemission, indicating that the molecular gas forms from the atomichydrogen where the HI column density is high. A possible trend of moreevolved TDGs having greater molecular gas masses is observed, in accordwith the transformation of HI into H2. Although TDGs sharemany of the properties of small irregulars, their CO luminosity is muchgreater (factor ~ 100) than that of standard dwarf galaxies ofcomparable luminosity. This is most likely a consequence of the highermetallicity (gtrsim 1/3 solar) of TDGs which makes CO a good tracer ofmolecular gas. This allows us to study star formation in environmentsordinarily inaccessible due to the extreme difficulty of measuring themolecular gas mass. The star formation efficiency, measured by the COluminosity per Hα flux, is the same in TDGs and full-sizedspirals. CO is likely the best tracer of the dynamics of these objectsbecause some fraction of the HI near the TDGs may be part of the tidaltail and not bound to the TDG. Although uncertainties are large forindividual objects, as the geometry is unknown, our sample is now ofeight detected objects and we find that the ``dynamical" masses of TDGs,estimated from the CO line widths, seem not to be greater than the``visible" masses (HI + H2 + a stellar component). Althoughhigher spatial resolution CO (and HI) observations would help reduce theuncertainties, we find that TDGs require no dark matter, which wouldmake them the only galaxy-sized systems where this is the case. Darkmatter in spirals should then be in a halo and not a rotating disk. Mostdwarf galaxies are dark matter-rich, implying that they are not of tidalorigin. We provide strong evidence that TDGs are self-gravitatingentities, implying that we are witnessing the ensemble of processes ingalaxy formation: concentration of large amounts of gas in a boundobject, condensation of the gas, which is atomic at this point, to formmolecular gas and the subsequent star formation from the dense molecularcomponent.
|Formation of molecular gas in the tidal debris of violent galaxy-galaxy interactions|
In many gravitational interactions between galaxies, gas and stars thathave been torn from the precursor galaxies can collect in tidal `tails'.Star formation begins anew in some of these regions, producing tidaldwarf galaxies. Observations of these new galaxies provides insight intoprocesses relevant to galaxy formation more generally, because thetimescale of the interaction is well defined. But tracking the starformation process has hitherto been difficult because the tidal dwarfgalaxies with young stars showed no evidence of the molecular gas out ofwhich those young stars formed. Here we report the discovery ofmolecular hydrogen (traced by carbon monoxide emission) in two tidaldwarf galaxies. In both cases, the concentration of molecular gas peaksat the same location as the maximum in atomic-hydrogen density, unlikethe situation in most gas-rich galaxies. We infer from this that themolecular gas formed from the atomic hydrogen, rather than being torn inmolecular form from the interacting galaxies. Star formation in thetidal dwarf galaxies therefore appears to mimic the process in normalspiral galaxies like our own.
|Dwarf Galaxy Formation Induced by Galaxy Interactions|
Growing evidence is being accumulated that some gas-rich dwarf galaxiesare formed from material liberated by galaxy collisions and/or mergers.Also, gas-poor dwarf elliptical galaxies are often found in the centralregions of clusters of galaxies. These observations suggest stronglythat the formation of most dwarf galaxies is linked to galaxyinteractions. Therefore, now seems like the right time to investigatethe formation efficiency of such tidal dwarf galaxies. Adopting thegalaxy interaction scenario proposed by Silk & Norman, we find thatif only a few dwarf galaxies are formed in each galaxy collision, we areable to explain the observed morphology-density relations for both dwarfand giant galaxies in the field, groups of galaxies, and clusters ofgalaxies. It seems worthwhile noting that tidal dwarf formation may becoupled with the transformation from gas-rich disk galaxies toearly-type galaxies such as S0 and elliptical galaxies.
|3C 48: Stellar Populations and the Kinematics of Stars and Gas in the Host Galaxy|
We present deep Keck LRIS spectroscopy of the host galaxy of 3C 48. Ourobservations at various slit positions sample the different luminouscomponents near the quasar, including the apparent tidal tail to thenorthwest and several strong emission line regions. By fitting Bruzual &Charlot population synthesis models to our spectra, we obtain ages forthe most recent major episodes of star formation in various parts of thehost galaxy covered by our slits. There is vigorous current starformation in regions just northeast and southeast of the quasar andpost-starburst regions with ages up to ~108 yr in other partsof the host galaxy, but most of the northwest tidal tail shows no signof significant recent star formation. We use these model fits, togetherwith the kinematics of the stars and gas, to outline a plausibleevolutionary history for the host galaxy, its recent starburst activity,the triggering of the quasar, and the interaction of the radio jet withthe ambient gas. There is strong evidence that the 3C 48 host is anongoing merger, and that it is probably near the peak of its starburstactivity. Nevertheless, the quasar itself seems to suffer littleextinction, perhaps because we are viewing it along a particularlyfavorable line of sight.
|Formation of a Tidal Dwarf Galaxy in the Interacting System Arp 245 (NGC 2992/93)|
Among the various phenomena observed in interacting galaxies is theejection due to tidal forces of stellar and gaseous material into theintergalactic medium and its subsequent rearranging which can lead tothe formation of self-gravitating tidal dwarf galaxies (TDGs). Weinvestigate this process with a detailed multiwavelength study of theinteracting system Arp 245 and a numerical model of the collisioncomputed with a Tree-SPH code. Our observations consist ofoptical/near-infrared broadband imaging, Hα imaging, opticalspectroscopy, H I VLA cartography and CO line mapping. The system,composed of the two spiral galaxies NGC 2992 and NGC 2993, is observedat an early stage of the interaction, about 100 Myr afterperigalacticon, though at a time when tidal tails have alreadydeveloped. The VLA observations disclose a third partner to theinteraction: an edge-on, flat galaxy, FGC 0938, which looks strikinglyundisturbed and might just be falling toward the NGC 2992/93 system. OurH I map shows prominent counterparts to the optical tails. Whereas thestellar and gaseous components of the plume that originates from NGC2992 match, the stellar and H I tails emanating from NGC 2993 have adifferent morphology. In particular, the H I forms a ring, a featurethat has been successfully reproduced by our numerical simulations. TheH I emission in the system as a whole peaks at the tip of the NGC 2992tail where a gas reservoir of about 109 Msolar, about 60% ofthe H I toward NGC 2992, coincides with a star-forming opticalcondensation, A245N. The latter tidal object exhibits properties rangingbetween those of dwarf irregular galaxies (structural parameters, gascontent, star formation rate) and those of spiral disks (metallicity,star formation efficiency, stellar population). Although it is likely,based on our analysis of the H I and model data cube, that A245N mightbecome an independent dwarf galaxy, the dynamical evidence is still opento debate. Prompted by the questions raised for this particular object,we discuss some issues related to the definition and identification ofTDGs and highlight some specific conditions which seem required to formthem. Finally, we outline what is needed in terms of future numericalsimulations in order to further our understanding of these objects.
|The Neutral Hydrogen Distribution in Merging Galaxies: Differences between Stellar and Gaseous Tidal Morphologies|
As part of several H I synthesis-mapping studies of merging galaxies, wehave mapped the tidal gas in the three disk-disk merger systems Arp 157(NGC 520), Arp 220, and Arp 299 (NGC 3690). These systems differ fromthe majority of the mergers mapped in H I in that their stellar andgaseous tidal features do not coincide. In particular, they exhibitlarge stellar tidal features with little if any accompanying neutral gasand large gas-rich tidal features with little if any accompanyingstarlight. On smaller scales, there are striking anticorrelations inwhich the gaseous and stellar tidal features appear to cross. We exploreseveral possible causes for these differences, including dustobscuration, ram pressure stripping, and ionization effects. No singleexplanation can account for all of the observed differences. The factthat each of these systems shows evidence for a starburst-drivensuperwind expanding in the direction of the most strikinganticorrelations leads us to suggest that the superwind is primarilyresponsible for the observed differences, either by sweeping thefeatures clear of gas via ram pressure or by excavating a clearsightline toward the starburst and allowing UV photons to ionize regionsof the tails. If this suggestion is correct, only systems hosting agalactic superwind and experiencing a high-inclination encountergeometry (such that tidal gas is lifted high above the starburstregions) should exhibit such extreme differences between their H I andoptical tidal morphologies.
|Tidal dwarf candidates in a sample of interacting galaxies|
We present deep optical B,V ,R images of a sample of 10 interactingsystems which were selected for their resemblance to disturbed galaxiesat high redshift. Photometry is performed on knots in the tidal featuresof the galaxies. We calculate a grid of evolutionary synthesis modelswith two metallicities and various burst strengths for systemsconsisting of some fraction of the stellar population of a progenitorspiral plus starburst. By comparison with two-color diagrams weinterpret the photometric data, select from a total of about 100condensations 36 star-forming objects that are located in the tidalfeatures and predict their further evolution. Being more luminous by 4mag than normal H Ii regions we argue that these objects could be tidaldwarf galaxies or their progenitors, although they differ in number andmean luminosity from the already known tidal dwarf galaxies typicallylocated at the end of tidal tails in nearby giant interacting systems.From comparison with our models we note that all objects show youngburst ages. The young stellar component formed in these tidal dwarfcandidates contributes up to 18% to the total stellar mass at the end ofthe starburst and dominates the optical luminosity. This may result infading by up to 2.5 mag in B during the next 200 Myrs after the burst.Based on observations collected at the European Southern Observatory, LaSilla, Chile (ESO No 058.A-0260). Tables 5-14 are only available inelectronic form at the CDS via anonymous ftp to 220.127.116.11 or viahttp://cds.u-strasbg.fr/Abstract.html
|The FIR-radio correlation of Wolf-Rayet galaxies and the role of star formation in LINERs|
We find that a preliminary classification of LINERs' energetics may bemade in terms of the FIR-radio correlation of Wolf-Rayet galaxies. TheAGN- or starburst-supported LINERs can be distinguished by theirFIR-to-radio ratio, Qequiv L(1.4GHz)/ L(60mum )> or <0.01. It isinteresting to note that almost all the LINERs with inner rings might bestarburst-supported, indicating reduced AGN activities compared withthose of the AGN-supported ones. We also find that a shock-heating phasefor the warm dust component might be important for some starbursts atthe burst age of >= 107 yr, with Q<0.001.
|The role of star formation in liners.|
|A different approach to cosmology.|
|Tidal Dwarf Galaxies|
It will be reviewed the idea that dwarf galaxies can be formed out ofthe tidal debries of interacting giant galaxies. This class of dwarfgalaxies are called ``Tidal Dwarf Galaxies".
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