Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
Stellar Kinematics for the Central Spheroid in the Polar Disk Galaxy NGC 4650A
We have obtained high angular resolution, high signal-to-noise ratiospectra of the calcium triplet absorption lines on the photometric axesof the stellar spheroid in the polar disk galaxy NGC 4650A. Along themajor axis, the observed rotation and velocity dispersion measurementsshow the presence of a kinematically decoupled nucleus and a flatvelocity dispersion profile. The minor-axis kinematics is determined forthe first time: along this direction some rotation is measured, and thevelocity dispersion is nearly constant and slightly increases at largerdistances from the center. The new high-resolution kinematic datasuggest that the stellar component in NGC 4650A resembles a nearlyexponential oblate spheroid supported by rotation. The main implicationsof these results on the previous mass models for NGC 4650A arediscussed. Moreover, the new kinematic data set constraints on currentmodels for the formation scenarios of polar ring galaxies, supporting aslow accretion rather than a secondary strong dissipative event.Based on data collected with the FORS2 spectrograph, mounted at the UT4of the Very Large Telescope at Cerro Paranal, Chile, operated by ESO,during observing run 70.B-0277.
| The Origin of Polar Ring Galaxies: Evidence for Galaxy Formation by Cold Accretion
Polar ring galaxies are flattened stellar systems with an extended ringof gas and stars rotating in a plane almost perpendicular to the centralgalaxy. We show that their formation can occur naturally in ahierarchical universe where most low-mass galaxies are assembled throughthe accretion of cold gas infalling along megaparsec-scale filamentarystructures. Within a large cosmological hydrodynamical simulation, wefind a system that closely resembles the classic polar ring galaxy NGC4650A. How galaxies acquire their gas is a major uncertainty in modelsof galaxy formation, and recent theoretical work has argued that coldaccretion plays a major role. This idea is supported by our numericalsimulations and the fact that polar ring galaxies are typically low-masssystems.
| Absolute Magnitude Distributions and Light Curves of Stripped-Envelope Supernovae
The absolute visual magnitudes of three Type IIb, 11 Type Ib, and 13Type Ic supernovae (collectively known as stripped-envelope supernovae)are studied by collecting data on the apparent magnitude, distance, andinterstellar extinction of each event. Weighted and unweighted meanabsolute magnitudes of the combined sample, as well as various subsetsof the sample, are reported. The limited sample size and theconsiderable uncertainties, especially those associated with extinctionin the host galaxies, prevent firm conclusions regarding differencesbetween the absolute magnitudes of supernovae of Types Ib and Ic, andregarding the existence of separate groups of overluminous andnormal-luminosity stripped-envelope supernovae. The spectroscopiccharacteristics of the events of the sample are considered. Three of thefour overluminous events are known to have had unusual spectra. Most butnot all of the normal-luminosity events have had typical spectra. Thelight curves of stripped-envelope supernovae are collected and compared.Because SN 1994I in M51 was very well observed, it often is regarded asthe prototypical Type Ic supernova, but it has the fastest light curvein the sample. Light curves are modeled by means of a simple analyticaltechnique that, combined with a constraint on E/M from spectroscopy,yields internally consistent values of ejected mass, kinetic energy, andnickel mass.
| Neutral Hydrogen in the Polar Ring Galaxy UGC 9796
We have used the Very Large Array to observe the polar ring galaxy UGC9796 (II Zw 73; PRC A-06) in the 21 cm line of neutral hydrogen. All theneutral gas in this galaxy is associated with the polar ring. Ourobservations show that ~=5×109 Msolar of H Irotates in a plane projected about 25° from perpendicular to theequatorial plane of the early-type host galaxy, consistent with earlierobservations in the Hα line. The outermost gas appears to bendaway from the minor axis of the host system, in the same sense as thestellar ring. UGC 9796 is in a very gas-rich environment: our 21 cmimages show five gas-rich companion galaxies in the same field within100 km s-1, with a total of ~=2×1010Msolar between them. However, we see no H I streamersconnecting the systems or any other evidence for an ongoing interactionbetween UGC 9796 and its companions.
| Hydrogen and helium traces in type Ib-c supernovae
Aims.To investigate the spectroscopic properties of a selected opticalphotospheric spectra of core collapse supernovae (CCSNe). Specialattention is devoted to traces of hydrogen at early phases. The impacton the physics and nature of their progenitors is emphasized.Methods: .The CCSNe-sample spectra are analyzed with the parameterizedsupernova synthetic spectrum code "SYNOW" adopting some simplifyingapproximations. Results: .The generated spectra are found to matchthe observed ones reasonably well, including a list of only 23 candidateions. Guided by SN Ib 1990I, the observed trough near 6300 Å isattributed to Hα in almost all type Ib events, although in someobjects it becomes too weak to be discernible, especially at laterphases. Alternative line identifications are discussed. Differences inthe way hydrogen manifests its presence within CCSNe are highlighted. Intype Ib SNe, the Hα contrast velocity (i.e. line velocity minusthe photospheric velocity) seems to increase with time at early epochs,reaching values as high as 8000 km s-1 around 15-20 daysafter maximum and then remains almost constant. The derived photosphericvelocities, indicate a lower velocity for type II SNe 1987A and 1999emas compared to SN Ic 1994I and SN IIb 1993J, while type Ib eventsdisplay a somewhat larger variation. The scatter, around day 20, ismeasured to be ~5000 km s-1. Following two simple approaches,rough estimates of ejecta and hydrogen masses are given. A mass ofhydrogen of approximately 0.02 M_ȯ is obtained for SN 1990I, whileSNe 1983N and 2000H ejected ~0.008 M_ȯ and ~0.08 M_ȯ ofhydrogen, respectively. SN 1993J has a higher hydrogen mass, ~0.7M_ȯ with a large uncertainty. A low mass and thin hydrogen layerwith very high ejection velocities above the helium shell, is thus themost likely scenario for type Ib SNe. Some interesting and curiousissues relating to oxygen lines suggest future investigations.
| On stability and spiral patterns in polar disks
To investigate the stability properties of polar disks we performedtwo-dimensional hydrodynamical simulations for flat polytropic gaseousself-gravitating disks which were perturbed by a central S0-likecomponent. Our disk was constructed to resemble that of theproto-typical galaxy NGC 4650A. This central perturbation inducesinitially a stationary two-armed tightly-wound leading spiral in thepolar disk. For a hot disk (Toomre parameter Q>1.7), the structuredoes not change over the simulation time of 4.5 Gyr. In case of colderdisks, the self-gravity of the spiral becomes dominant, it decouplesfrom the central perturbation and grows, until reaching a saturationstage in which an open trailing spiral is formed, rather similar to thatobserved in NGC 4650A. The timescale for developing non-linearstructures is 1-2 Gyr; saturation is reached within 2-3 Gyr. The mainparameter controlling the structure formation is the Toomre parameter.The results are surprisingly insensitive to the properties of thecentral component. If the polar disk is much less massive than that inNGC 4650A, it forms a weaker tightly-wound spiral, similar to that seenin dust absorption in the dust disk of NGC 2787. Our results are derivedfor a polytropic equation of state, but appear to be generic as theadiabatic exponent is varied between γ = 1 (isothermal) andγ = 2 (very stiff).
| AM 1934-563: a giant spiral polar-ring galaxy in a triplet
We have observed the emission-line kinematics and photometry of asouthern triplet of galaxies. The triplet contains a giant spiral galaxyAM 1934-563 whose optical structure resembles a polar-ring galaxy: adistorted spiral disk, seen almost edge-on, and a faint large-scale (45kpc in diameter) warped structure, inclined by 60°-70° withrespect to the disk major axis. The triplet shows a relatively smallvelocity dispersion (69 km s-1) and a large crossing time(0.17 in units of the Hubble time). The disk of AM 1934-563 demonstratesoptical colors typical of early-type spirals, a strong radial colorgradient, and almost exponential surface brightness distribution with anexponential scale-length value of 3.1 kpc (R passband). The galaxy showsa maximum rotation velocity of about 200 km s-1 and it liesclose to the Tully-Fisher relation for spiral galaxies. The suspectedpolar ring is faint (μ(B) ≥ 24) and strongly warped. Its totalluminosity comprises 10-15% of the total luminosity of AM 1934-563. Wemodel this system using numerical simulations, and study its possibleformation mechanisms. We find that the most robust model that reproducesthe observed characteristics of the ring and the host galaxy is thetidal transfer of mass from a massive gas-rich donor galaxy to the polarring. The physical properties of the triplet of galaxies are inagreement with this scenario.
| A Puzzling Polar-Ring Galaxy
Not Available
| 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.
| High resolution stellar kinematics for the central component of the Polar Ring Galaxy NGC 4650A.
Not Available
| Principal component analysis of International Ultraviolet Explorer galaxy spectra
We analyse the UV spectral energy distribution of a sample of normalgalaxies listed in the International Ultraviolet Explorer (IUE) NewlyExtracted Spectra (INES) Guide No. 2 - Normal Galaxies using a principalcomponent analysis. The sample consists of the IUE short-wavelength (SW)spectra of the central regions of 118 galaxies, where the IUE apertureincluded more than 1 per cent of the galaxy size. The principalcomponents are associated with the main components observed in theultraviolet (UV) spectra of galaxies. The first component, accountingfor the largest source of diversity, may be associated with the UVcontinuum emission. The second component represents the UV contributionof an underlying evolved stellar population. The third component issensitive to the amount of activity in the central regions of galaxiesand measures the strength of star-formation events.In all the samples analysed here, the principal component representativeof star-forming activity accounts for a significant percentage of thevariance. The fractional contribution to the spectral energydistribution (SED) by the evolved stars and by the young population aresimilar.Projecting the SEDs on to their eigenspectra, we find that none of thecoefficients of the principal components can outline an internalcorrelation or can correlate with the optical morphological types. In asubsample of 43 galaxies, consisting of almost only compact and BCDgalaxies, the third principal component defines a sequence related tothe degree of starburst activity of the galaxy.
| Dynamical friction in flattened systems: a numerical test of Binney's approach
We carry out a set of self-consistent N-body calculations to investigatehow important the velocity anisotropy in non-spherical dark matterhaloes is for dynamical friction. For this purpose, we allow satellitegalaxies to orbit within flattened and live dark matter haloes (DMHs)and compare the resulting orbit evolution with a semi-analytic code.This code solves the equation of motion of the same satellite orbitswith mass loss and assumes the same DMH, but either employsChandrasekhar's dynamical friction formula, which does not incorporatethe velocity anisotropy, or Binney's description of dynamical frictionin anisotropic systems. In the numerical and the two semi-analyticmodels, the satellites are given different initial orbital inclinationsand orbital eccentricities, whereas the parent galaxy is composed of aDMH with aspect ratio qh= 0.6.We find that Binney's approach successfully describes the overallsatellite decay and orbital inclination decrease for the whole set oforbits, with an averaged discrepancy of less than 4 per cent in orbitalradius during the first three orbits. If Chandrasekhar's expression isused instead, the discrepancy increases to 20 per cent. Binney'streatment therefore appears to provide a significantly improvedtreatment of dynamical friction in anisotropic systems.The velocity anisotropy of the DMH velocity distribution function leadsto a significant decrease with time of the inclination of non-polarsatellite orbits. But, at the same time, it reduces the difference indecay times between polar and coplanar orbits evident in a flattened DMHwhen the anisotropic DMH velocity distribution function is not takeninto account explicitly. Our N-body calculations furthermore indicatethat polar orbits survive about 1.6 times longer than coplanar orbitsand that the orbital eccentricity e remains close to its initial valueif satellites decay slowly towards the galaxy centre. However, orbits ofrapidly decaying satellites modelled with the semi-analytic code show astrong orbital circularization () not present in the N-bodycomputations.
| Radio Continuum Emission in Polar Ring Galaxies
We have used the Very Large Array aperture synthesis telescope toconduct a radio continuum survey of polar ring galaxies, at 20 cm and 6cm. Forty objects were observed at 20 cm with ~=5" resolution. Twenty(50%) of the program sources were detected at 20 cm, down to our 5σ limit of 0.5 mJy beam-1. This detection rate issimilar to those in surveys with comparable sensitivity for early-typegalaxies without polar rings. Sixteen of the objects we detected at 20cm were also observed at 6 cm. We show radio continuum maps for the fiveobjects in our sample that have noticeably extended emission. Ourspatial resolution was sufficient to distinguish emission originating inthe host galaxy from that in the polar ring. The radio morphology of theextended sources, as well as the ratio of radio to far-infrared flux andthe radio spectral indices of our detected sources, indicate that starformation, not nuclear activity, is the dominant source of the radiocontinuum emission in polar ring galaxies. However, the implied starformation rates are modest, and only one of our sample galaxies willconsume its supply of cool gas within 500 Myr.
| Optical Imaging and Spectroscopy of the Edge-on Sbc Galaxy UGC 10043: Evidence for a Galactic Wind and a Peculiar Triaxial Bulge
We present new optical imaging and spectroscopy of the peculiar, edge-onSbc galaxy UGC 10043. Using the WIYN telescope, we have obtained B, R,and Hα+[NII] images, together with DensePak integral fieldspectroscopic measurements of the stellar Ca II infrared triplet and theHα and [N II] lines from the ionized gas. The imaging observationsshow that the inner bulge of UGC 10043 (a<=7.5") is elongatedperpendicular to the galaxy major axis. At larger r the bulge isophotestwist to become oblate and nearly circular, suggesting the bulge istriaxial. The bulge shows no clear evidence for rotation about eitherits major or minor axis. The inner, southwestern quadrant of the bulgeis girdled by a narrow dust lane parallel to the minor axis; unsharpmasking reveals that this minor-axis dust lane may be part of an innerpolar ring, although we find no unambiguous kinematic evidence oforthogonally rotating material. The stellar disk of UGC 10043 has arather low optical surface brightness [μ(0)R,i~23.2 magarcsec-2], a small scale height (hz=395 pc forD=33.4 Mpc), and a mild integral sign warp. A dusty, inner diskcomponent that appears tilted relative to the outlying disk is alsoseen. The Hα and [N II] emission lines in UGC 10043 resolve intomultiple velocity components, indicating the presence of a large-scalegalactic wind with an outflow velocity of Vout>~104 kms-1. Hα+[NII] imaging reaffirms this picture byrevealing ionized gas extended to |z|~3.5 kpc in the form of a roughlybiconical structure. The [N II]/Hα line intensity ratio increaseswith increasing distance from the plane, reaching values as high as 1.7.Unlike most galaxies with large-scale winds, UGC 10043 has only a modestglobal star formation rate (<~1 Msolar yr-1),implying the wind is powered by a rather feeble central starburst. Wediscuss evolutionary scenarios that could account for both thestructural complexities of UGC 10043 and its large-scale wind. The mostplausible scenarios require a major accretion or merger event at least afew gigayears ago.
| The Stellar Content of the Polar Rings in the Galaxies NGC 2685 and NGC 4650A
We present the results of stellar photometry of polar ring galaxies NGC2685 and NGC 4650A, using the archival data obtained with the HubbleSpace Telescope's Wide Field Planetary Camera 2. Polar rings of thesegalaxies were resolved into ~800 and ~430 stellar objects in the B, V,and IC bands, a considerable part of which are bluesupergiants located in the young stellar complexes. The stellar featuresin the CM diagrams are best represented by isochrones with metallicityZ=0.008. The process of star formation in the polar rings of bothgalaxies was continuous, and the age of the youngest detected stars isabout 9 Myr for NGC 2685 and 6.5 Myr for NGC 4650A.Based on observations made with the NASA/ESO Hubble Space Telescope,obtained from the Space Telescope Science Institute, which is operatedby the Association of Universities for Research in Astronomy, Inc.,under NASA contract NAS 5-26555.
| The stellar content of the ring in NGC 660
We present the results of stellar photometry of the polar-ring galaxyNGC 660 using the Hubble Space Telescope's archivaldata obtained with the Wide Field Planetary Camera 2. The final list ofthe resolved stars contains 550 objects, a considerable part of whichare blue and red supergiants belonging to the polar ring. The analysisof the Colour-Magnitude Diagram for polar ring stars shows that it isbest represented by the isochrones with metallicity Z = 0.008. Theprocess of star formation in the polar ring was continuous and the ageof the youngest detected stars is about 7 Myr.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Space Telescope Science Institute, which is operatedby the Association of Universities for Research in Astronomy, Inc.,under NASA contract NAS5-26555.
| Photometric structure of the peculiar galaxy ESO 235-G58
We present the near-infrared and optical properties of the peculiargalaxy ESO 235-G58, which resembles a late-type ringed barred spiralseen close to face-on. However, the apparent bar of ESO 235-G58 is inreality an edge-on disk galaxy of relatively low luminosity. We haveanalyzed the light and color distributions of ESO 235-G58 in the NIR andoptical bands and compared them with the typical properties observed forother morphological galaxy types, including polar ring galaxies. Similarproperties are observed for ESO 235-G58, polar ring galaxies, and spiralgalaxies, which leads us to conclude that this peculiar system is apolar-ring-related galaxy, characterized by a low inclined ring/diskstructure, as pointed out by Buta & Crocker in an earlier study,rather than a barred galaxy.
| Photometric structure of polar-ring galaxies
The results of B, V, R surface photometry of three polar-ring galaxies(PRGs) - A 0017+2212, UGC 1198, UGC 4385 - are presented. The data wereacquired at the 6-m telescope of the Special Astrophysical Observatoryof the Russian Academy of Sciences. It was shown that all three galaxiesare peculiar late-type spirals in the state of ongoing interaction ormerging. We discuss available photometric properties of the PRGs withspiral hosts and consider the Tully-Fisher relation for different typesof PRGs. In agreement with Iodice et al. (\cite{Iodice03}), we haveshown that true PRGs demonstrate 1/3 larger maximum rotationvelocities than spiral galaxies of the same luminosity. Peculiar objectswith forming polar structures satisfy, on average, the Tully-Fisherrelation for disk galaxies but with large scatter.
| The stellar population study of the polar rings in the galaxies NGC 2685 and NGC 4650A
Polar ring galaxies (PRGs) are dynamically peculiar systems with a ringor annulus of gas, stars and dust orbiting in a plane nearlyperpendicular to the equatorial plane of the host galaxy (Withmore1990). According to the most popular point of view, PRGs are the resultof galaxy interaction, which ranges from simple gas accretion to acomplete merger. Alternatively, polar rings can represent the delayedinflows of primordial gas. Therefore, the problem of PR's age is actual(Eskridge & Podge 1997; Gallagher et al 2002). Deep single-starphotometry can directly identify stars in various evolutionary phasesthrough their positions on CMD. This can cast light on the origin andevolution of PRGs.
| Stellar Motions in the Polar Ring Galaxy NGC 4650A
We present the first measurement of the stellar kinematics in the polarring of NGC 4650A. There is well-defined rotation, with the stars andgas rotating in the same direction, and with similar amplitude. Thegaseous and stellar kinematics suggest an approximately flat rotationcurve, providing further support for the hypothesis that the polarmaterial resides in a disk rather than in a ring. The kinematics of theemission-line gas at and near the center of the S0 galaxy suggest thatthe polar disk lacks a central hole. We have not detected evidence oftwo equal-mass, counterrotating stellar polar streams, as is predictedin the resonance levitation model proposed by Tremaine & Yu. Amerger seems the most likely explanation for the structure andkinematics of NGC 4650A.
| Polar Ring Galaxies and the Tully-Fisher Relation: Implications for the Dark Halo Shape
We have investigated the Tully-Fisher relation for polar ring galaxies(PRGs), based on near-infrared, optical, and H I data available for asample of these peculiar objects. The total K-band luminosity, whichmainly comes from the central host galaxy, and the measured H I linewidth at 20% of the peak line flux density, which traces the potentialin the polar plane, place most polar rings in the sample far from theTully-Fisher relation defined for spiral galaxies, with many PRGsshowing larger H I line widths than expected for the observed K-bandluminosity. This result is confirmed by a larger sample of objects,based on B-band data. This observational evidence may be related to thedark halo shape and orientation in these systems, which we study bynumerical modeling of PRG formation and dynamics: the larger rotationvelocities observed in PRGs can be explained by a flattened polar halo,aligned with the polar ring.
| Formation of polar ring galaxies
Polar ring galaxies are peculiar systems in which a gas-rich, nearlypolar ring surrounds an early-type or elliptical host galaxy. Twoformation scenarios for these objects have been proposed: they arethought to form either in major galaxy mergers or by tidal accretion ofthe polar material from a gas rich donor galaxy. Both scenarios arestudied through N-body simulations including gas dynamics and starformation. Constraints on physical parameters are drawn out, in order todetermine which scenario is the most likely to occur. Polar ringgalaxies from each scenario are compared with observations and wediscuss whether the accretion scenario and the merging scenario accountfor observational properties of polar ring galaxies. The conclusion ofthis study is that the accretion scenario is both the most likely andthe most supported by observations. Even if the merging scenario israther robust, most polar ring galaxies are shown to be the result oftidal gas accretion events.Appendices A and B are only available in electronic form athttp://www.edpsciences.org
| Extended H I Spiral Structure and the Figure Rotation of Triaxial Dark Halos
The H I disk of the blue compact dwarf (BCD) galaxy NGC 2915 extends to22 optical scale lengths and shows spiral arms reaching far beyond theoptical component. None of the previous theories for spiral structureprovide likely explanations for these very extended spiral arms. Ournumerical simulations first demonstrate that such large spiral arms canform in an extended gas disk embedded in a massive triaxial dark matterhalo with slow figure rotation, through the strong gravitational torqueof the rotating halo. We then show that the detailed morphologicalproperties of the developed spirals and rings depend strongly on thepattern speed of the figure rotation, the shape of the triaxial halo,and the inclination of the disk with respect to the plane including thetriaxial halo's long and middle axes. These results strongly suggestthat the dark matter halo of NGC 2915 is triaxial and has figurerotation. Based on these results, we also suggest that dynamical effectsof triaxial halos with figure rotation are important in various aspectsof galaxy formation and evolution, such as formation of polar ringgalaxies, excitation of nonaxisymmetric structures in low surfacebrightness galaxies, and gas fueling to the central starburst regions ofBCDs.
| Wide Field Planetary Camera 2 Observations of the Polar Ring Galaxy NGC 4650A
We present multicolor Wide Field Planetary Camera 2 images of the polarring galaxy NGC 4650A (D=35 Mpc), taken with the Hubble Space Telescope.These data allow a detailed examination of the morphology of the centralS0 component and the surrounding ``polar ring'' of stars, gas, and dust,which are spatially extended and form a ``polar disk.'' The nuclearcomplex, located near the common center of the two components, consistsof an unresolved compact source less than 20 pc across, embedded in amore extended light distribution about 60 pc in radius. A complicatedpattern of dust lanes crosses the high surface brightness central regionof the S0, where dust features appear to extend inward to small radii.In areas of the S0 exterior to the projected polar disk, the lightdistribution is relatively smooth on small spatial scales, with fewcandidates for old star clusters or signs of internal dust features.Outer regions of the S0 galaxy have uniformly red colors of a stellarpopulation where major star formation ceased ~3-5 Gyr in the past. Thepolar disk appears to be warped, with a complex morphology. On smallspatial scales, features are produced mainly by dust and clumps of starformation; gas seems to be piling up in a dusty ring that encircles thecentral S0 at a radius of ~1 kpc (6"). The polar disk is inclined byabout 63° near its center, twists toward edge-on at intermediateradii, and again becomes more nearly face-on further out, where twospiral arms are marked by young stars. We do not detect any regions witha prominent ``Baade sheet'' of older red stars in the polar disk; mostof the optical light is supplied by moderately young stars, withlifetimes of ~1 Gyr. The polar material appears relatively unevolved,especially when compared to the S0 component of NGC 4650A. Either thepolar disk is a late addition to the system or the outer gas has formedvery few stars until recently. Observations obtained with the NASA/ESAHubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy (AURA), Inc., under NASA contract NAS5-26555.
| The Puzzle of the Polar Structure in NGC 4650A
This work presents new surface photometry and two-dimensional modelingof the light distribution of the polar ring galaxy NGC 4650A, based onnear-infrared (NIR) observations and high-resolution optical imagingacquired during the Hubble Heritage program. The NIR and opticalintegrated colors of the S0 galaxy and the polar ring and their scaleparameters are compared with those for standard galaxy morphologicaltypes. The polar structure appears to be a disk of a very young age,while the colors and light distribution of the host galaxy do notresemble those of a typical early-type system. We compare theseobservational results with the predictions from different formationscenarios for polar ring galaxies. The peculiarities of the central S0galaxy, the polar disk structure, and stellar population ages suggestthat the polar ring galaxy NGC 4650A may be the result of a dissipativemerger event rather than of an accretion process. Based on observationswith the NASA/ESA Hubble Space Telescope, obtained at the SpaceTelescope Science Institute, which is operated by the Association ofUniversities for Research in Astronomy, Inc., under NASA contract NAS2-6555.
| Near-Infrared photometry in J, H and Kn bands for polar ring galaxies. II. Global properties
We discuss the properties of the host galaxy and ring lightdistributions in the optical and near infrared bands for a sample ofPolar Ring Galaxies (PRGs), presented in Paper I (Iodice et al.\cite{paperI}). The goal of this work is to test different formationscenarios for PRGs, proposed by different authors in the last decades,by comparing their predictions with these new data. The strategy istwofold: i) the integrated colors of the main components in thesesystems are compared with those of standard morphological galaxy types,to investigate whether differences in colors are caused by dustabsorption or difference in stellar populations. We then derived anestimate of the stellar population ages in PRGs, which can be used toset constrains on the dynamical modeling and the time evolution of thesesystems; ii) we analyse the structural parameters of the host galaxy inorder to understand whether this component is a standard early-typesystem as its morphology suggests, and the light distribution in thepolar ring to measure its radial extension. These observational resultsindicate that the global properties of PRGs are better explained bydissipative merging of disks with un-equal masses as proposed by Bekki(1998), rather than the accretion-or stripping-of gas by a pre-existingearly-type galaxy.
| Near-Infrared photometry in the J, H and Kn bands for Polar Ring Galaxies. I. Data, structural parameters
We present new Near-Infrared (NIR) observations, in the J, H and Knbands, for a sample of Polar Ring Galaxies (PRGs), selected from thePolar Ring Catalogue (Whitmore et al. \cite{PRC}). Data were acquiredwith the CASPIR near-IR camera at the 2.3 m telescope of Mount Stromloand Siding Spring Observatory. We report here on the detailmorphological study for the central host galaxy and the polar structurein all PRGs of our sample. Total magnitudes, bulge-to-disk decompositionand structural parameters are computed for all objects. These data arecrucial for an accurate modeling of the stellar population and theestimate of the star formation rates in the two components.
| A neutral hydrogen survey of polar ring galaxies. IV. Parkes observations
A total of 33 polar ring galaxies and polar ring galaxy candidates wereobserved in the 21-cm H I line with the 64-m Parkes radio telescope. Theobjects, selected by their optical morphology, are all south ofdeclination -39o and in only 5 of them H I had been reportedpreviously. H I line emission was detected towards 18 objects, though in3 cases the detection may be confused by another galaxy in the telescopebeam, and one is a marginal detection. Eight objects were detected forthe first time in H I, of which 5 did not have previously knownredshifts. Table 1 is also available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/386/140
| ESO 603-G21: A strange polar-ring galaxy
We present the results of B, V, R surface photometry of ESO 603-G21 - agalaxy with a possible polar ring. The morphological and photometricfeatures of this galaxy are discussed. The central round object of thegalaxy is rather red and presents a nearly exponential surfacebrightness distribution. This central structure is surrounded by a bluewarped ring or disk. The totality of the observed characteristics(optical and NIR colors, strong color gradients, HI and H_2 content, FIRluminosity and star-formation rate, rotation-curve shape, globalmass-to-luminosity ratio, the agreement with the Tully-Fisher relation,etc.) shows that ESO 603-G21 is similar to late-type spiral galaxies. Wesuppose that morphological peculiarities and the possible existence oftwo large-scale kinematically-decoupled subsystems in ESO 603-G21 can beexplained as being a result of dissipative merging of two spiralgalaxies or as a consequence of a companion accretion onto apre-existing spiral host. Based on observations made at theObservatório do Pico dos Dias (OPD), operated by theMCT/Laboratório Nacional de Astrofísica, Brazil.
| The Morphological Evolution of Galaxies
Many galaxies have taken on their familiar appearance relativelyrecently. In the distant Universe, galaxy morphology deviatessignificantly (and systematically) from that of nearby galaxies atredshifts (z) as low as 0.3. This corresponds to a time ~3.5 ×109 years in the past, which is only ~25% of the present ageof the Universe. Beyond z = 0.5 (5 × 109 years in thepast), spiral arms are less well developed and more chaotic, and barredspiral galaxies may become rarer. At z = 1, around 30% of the galaxypopulation is sufficiently peculiar that classification on Hubble'straditional ``tuning fork'' system is meaningless. On the other hand,some characteristics of galaxies have not changed much over time. Thespace density of luminous disk galaxies has not changed significantlysince z = 1, indicating that although the general appearance of thesegalaxies has continuously changed over time, their overall numbers havebeen conserved.
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
Catálogos y designaciones:
|
