Contents
Images
Upload your image
DSS Images Other Images
Related articles
The Pattern Speeds of 38 Barred Galaxies
We estimate the pattern speeds of 38 barred galaxies by simulationmodeling. We construct the gravitational potentials of the galaxies fromnear-IR photometry by assuming that the mass-to-light ratio (M/L) isconstant in the H band and a single pattern speed dominates in thestellar disk. We use the response of gaseous and stellar particle disksto a rigidly rotating potential to determine the pattern speed. If ourassumptions are correct, then the pattern speed depends on themorphological type: the average value of the ratio of the corotationresonance radius to the bar radius, ℛ, increases from about 1.1 intype SB0/a to 1.4 in SBb and 1.7 in SBc. Within the error estimates, allthe bars in galaxies of type SBab or earlier are fast rotators, havingℛ<=1.4, whereas late-type galaxies include both fast and slowrotators.
| Bar-induced perturbation strengths of the galaxies in the Ohio State University Bright Galaxy Survey - I
Bar-induced perturbation strengths are calculated for a well-definedmagnitude-limited sample of 180 spiral galaxies, based on the Ohio StateUniversity Bright Galaxy Survey. We use a gravitational torque method,the ratio of the maximal tangential force to the mean axisymmetricradial force, as a quantitative measure of the bar strength. Thegravitational potential is inferred from an H-band light distribution byassuming that the M/L ratio is constant throughout the disc. Galaxiesare deprojected using orientation parameters based on B-band images. Inorder to eliminate artificial stretching of the bulge, two-dimensionalbar-bulge-disc decomposition has been used to derive a reliable bulgemodel. This bulge model is subtracted from an image, the disc isdeprojected assuming it is thin, and then the bulge is added back byassuming that its mass distribution is spherically symmetric. We findthat removing the artificial bulge stretch is important especially forgalaxies having bars inside large bulges. We also find that the massesof the bulges can be significantly overestimated if bars are not takeninto account in the decomposition.Bars are identified using Fourier methods by requiring that the phasesof the main modes (m= 2, m= 4) are maintained nearly constant in the barregion. With such methods, bars are found in 65 per cent of the galaxiesin our sample, most of them being classified as SB-type systems in thenear-infrared by Eskridge and co-workers. We also suggest that as muchas ~70 per cent of the galaxies classified as SAB-types in thenear-infrared might actually be non-barred systems, many of them havingcentral ovals. It is also possible that a small fraction of the SAB-typegalaxies have weak non-classical bars with spiral-like morphologies.
| Dynamical modelling of the remarkable four-armed barred spiral galaxy ESO 566-24
ESO 566-24 is an extraordinary barred galaxy that has four regularlyspaced spiral arms in blue light images. This type of spiral structure,which is rare among the spiral population, is also clearly seen innear-infrared (NIR) images, and thus is present in the old stellarpopulation. We have constructed dynamical models of ESO 566-24. Thegravitational potential is determined using NIR photometry, and the gasdynamics is modelled as inelastically colliding particles. The resultingmorphology and kinematics with different assumed pattern speeds, discvertical thicknesses and dark halo contributions are compared withobservations. Our models reproduce the main morphological features ofthis galaxy: the four-armed spiral, and the inner and nuclear rings. Thepattern speed of the bar is such that the corotation resonance is welloutside the bar radius, rCR/rb= 1.6 +/- 0.3. Thefour-armed spiral resides in the region between inner and outer 4/1resonances. In addition, the main kinematical features, includingbar-induced deviations from circular rotation, are explained by ourmodels. The best fit is obtained when the dark halo contribution is justenough to make the modelled rotation curve match the observed one. Thus,luminous matter dominates the rotation curve within the disc region.
| Redshift-Distance Survey of Early-Type Galaxies: Spectroscopic Data
We present central velocity dispersions and Mg2 line indicesfor an all-sky sample of ~1178 elliptical and S0 galaxies, of which 984had no previous measures. This sample contains the largest set ofhomogeneous spectroscopic data for a uniform sample of ellipticalgalaxies in the nearby universe. These galaxies were observed as part ofthe ENEAR project, designed to study the peculiar motions and internalproperties of the local early-type galaxies. Using 523 repeatedobservations of 317 galaxies obtained during different runs, the dataare brought to a common zero point. These multiple observations, takenduring the many runs and different instrumental setups employed for thisproject, are used to derive statistical corrections to the data and arefound to be relatively small, typically <~5% of the velocitydispersion and 0.01 mag in the Mg2 line strength. Typicalerrors are about 8% in velocity dispersion and 0.01 mag inMg2, in good agreement with values published elsewhere.
| Low signal-to-noise spectroscopy and surface photometry of two faint galaxies in the field of NGC 7479
We present low signal-to-noise spectroscopy and surface photometry oftwo small galaxies, detected in the field of the nearby asymmetricbarred spiral NGC 7479, near a prolongation of the western arm thatforms a faint tail in the disc, and selected as possible companions ofthis galaxy. The analysis was based on data obtained with the PrimeFocus Camera (PFC) at the 0.8 m telescope of McDonald Observatory, andthe Low Resolution Spectroscope (LRS) at the 9.2 m Hobby-EberlyTelescope (HET). We determined radial velocities using cross correlationwith 15 different templates, and derived integrated magnitudes, colorsand brightness profiles for each.The derived radial velocities are around 35 500 km s-1 forone galaxy and 38 000 km s-1 for the other, which clearlyindicates that both are far more distant than the nearby NGC 7479. Theabsolute magnitudes and diameters of these two background galaxies aretypical of normal galaxies, and their colors are similar to othergalaxies with similar redshifs. NGC 7479 continues to exist in isolationwith no detected companion.
| Bar strengths in spiral galaxies estimated from 2MASS images
Non-axisymmetric forces are presented for a sample of 107 spiralgalaxies, of which 31 are barred (SB) and 53 show nuclear activity. As adata base we use JHK images from the 2 Micron All-sky Survey, and thenon-axisymmetries are characterized by the ratio of the tangential forceto the mean axisymmetric radial force field, following Buta & Block.Bar strengths have an important role in many extragalactic problems andtherefore it is important to verify that the different numerical methodsapplied for calculating the forces give mutually consistent results. Weapply both direct Cartesian integration and a polar grid integrationutilizing a limited number of azimuthal Fourier components of density.We find that the bar strength is independent of the method used toevaluate the gravitational potential. However, because of thedistance-dependent smoothing by Fourier decomposition, the polar methodis more suitable for weak and noisy images. The largest source ofuncertainty in the derived bar strength appears to be the uncertainty inthe vertical scaleheight, which is difficult to measure directly formost galaxies. On the other hand, the derived bar strength is ratherinsensitive to the possible gradient in the vertical scaleheight of thedisc or to the exact model of the vertical density distribution,provided that the same effective vertical dispersion is assumed in allmodels. In comparison with the pioneering study by Buta & Block, thebar strength estimate is improved here by taking into account thedependence of the vertical scaleheight on the Hubble type: we find thatfor thin discs bar strengths are stronger than for thick discs by anamount that may correspond to as much as one bar strength class. Weconfirm the previous result by Buta and co-workers showing that thedispersion in bar strength is large among all the de Vaucouleurs opticalbar classes. In the near-infrared 40 per cent of the galaxies in oursample have bars (showing constant phases in the m= 2 Fourier amplitudesin the bar region), while in the optical band one-third of these barsare obscured by dust. Significant non-axisymmetric forces can also beinduced by the spiral arms, generally in the outer parts of the galacticdiscs, which may have important implications on galaxy evolution.Possible biases of the selected sample are also studied: we find thatthe number of bars identified drops rapidly when the inclination of thegalactic disc is larger than 50°. A similar bias is found in theThird Reference Catalogue of Bright Galaxies, which might be of interestwhen comparing bar frequencies at high and low redshifts.
| Local velocity field from sosie galaxies. I. The Peebles' model
Pratton et al. (1997) showed that the velocity field around clusterscould generate an apparent distortion that appears as tangentialstructures or radial filaments. In the present paper we determine theparameters of the Peebles' model (1976) describing infall of galaxiesonto clusters with the aim of testing quantitatively the amplitude ofthis distortion. The distances are determined from the concept of sosiegalaxies (Paturel 1984) using 21 calibrators for which the distanceswere recently calculated from two independent Cepheid calibrations. Weuse both B and I-band magnitudes. The Spaenhauer diagram method is usedto correct for the Malmquist bias. We give the equations for theconstruction of this diagram. We analyze the apparent Hubble constant indifferent regions around Virgo and obtain simultaneously the Local Groupinfall and the unperturbed Hubble constant. We found:[VLG-infall = 208 ± 9 km s-1] [\log H =1.82 ± 0.04 (H ≈ 66 ± 6 km s-1Mpc-1).] The front side and backside infalls can be seenaround Virgo and Fornax. In the direction of Virgo the comparison ismade with the Peebles' model. We obtain: [vinfall} =CVirgo/r0.9 ± 0.2] withCVirgo=2800 for Virgo and CFornax=1350 for Fornax,with the adopted units (km s-1 and Mpc). We obtain thefollowing mean distance moduli: [μVirgo=31.3 ± 0.2(r=18 Mpc )] [μFornax=31.7 ± 0.3 (r=22 Mpc). ] Allthese quantities form an accurate and coherent system. Full Table 2 isonly available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/57
| A Dust-penetrated Classification Scheme for Bars as Inferred from Their Gravitational Force Fields
The division of galaxies into ``barred'' (SB) and ``normal'' (S) spiralsis a fundamental aspect of the Hubble galaxy classification system. This``tuning fork'' view was revised by de Vaucouleurs, whose classificationvolume recognized apparent ``bar strength'' (SA, SAB, SB) as acontinuous property of galaxies called the ``family.'' However, the SA,SAB, and SB families are purely visual judgments that can have littlebearing on the actual bar strength in a given galaxy. Until veryrecently, published bar judgments were based exclusively on blue lightimages, where internal extinction or star formation can either mask abar completely or give the false impression of a bar in a nonbarredgalaxy. Near-infrared camera arrays, which principally trace the oldstellar population in both normal and barred galaxies, now facilitate aquantification of bar strength in terms of their gravitationalpotentials and force fields. In this paper, we show that the maximumvalue, Qb, of the ratio of the tangential force to the meanaxisymmetric radial force in a barred disk galaxy is a quantitativemeasure of the strength of a bar. Qb does not measure barellipticity or bar shape but rather depends on the actual forcing due tothe bar embedded in its disk. We show that a wide range of true barstrengths characterizes the category ``SB,'' while the de Vaucouleurscategory ``SAB'' corresponds to a narrower range of bar strengths. Wepresent Qb values for 36 galaxies, and we incorporate our barclasses into a dust-penetrated classification system for spiralgalaxies.
| Ultraviolet Signposts of Resonant Dynamics in the Starburst-ringed SAB Galaxy M94 (NGC 4736)
The dynamic orchestration of star-birth activity in the starburst-ringedgalaxy M94 (NGC 4736) is investigated using images from the UltravioletImaging Telescope (UIT; far-ultraviolet [FUV] band), Hubble SpaceTelescope (HST; near-ultraviolet [NUV] band), Kitt Peak 0.9 m telescope(Hα, R, and I bands), and Palomar 5 m telescope (B band), alongwith spectra from the International Ultraviolet Explorer (IUE) and theLick 1 m telescope. The wide-field UIT image shows FUV emission from (1)an elongated nucleus, (2) a diffuse inner disk, where Hα isobserved in absorption, (3) a bright inner ring of H II regions at theperimeter of the inner disk (R=48"=1.1 kpc), and (4) two 500 pc sizeknots of hot stars exterior to the ring on diametrically opposite sidesof the nucleus (R=130"=2.9 kpc). The HST Faint Object Camera imageresolves the NUV emission from the nuclear region into a bright core anda faint 20" long ``minibar'' at a position angle of 30°. Optical andIUE spectroscopy of the nucleus and diffuse inner disk indicates a~107-108 yr old stellar population from low-levelstar-birth activity blended with some LINER activity. Analysis of theHα-, FUV-, NUV-, B-, R-, and I-band emissions, along with otherobserved tracers of stars and gas in M94, indicates that most of thestar formation is being orchestrated via ring-bar dynamics, involvingthe nuclear minibar, inner ring, oval disk, and outer ring. The innerstarburst ring and bisymmetric knots at intermediate radius, inparticular, argue for bar-mediated resonances as the primary drivers ofevolution in M94 at the present epoch. Similar processes may begoverning the evolution of the ``core-dominated'' galaxies that havebeen observed at high redshift. The gravitationally lensed ``PretzelGalaxy'' (0024+1654) at a redshift of ~1.5 provides an importantprecedent in this regard.
| Dynamics of Ringed Barred Spiral Galaxies. I. Surface Photometry and Kinematics of NGC 1433 and NGC 6300
This paper presents new near-infrared images and surface photometry andHα Fabry-Perot radial velocities for NGC 1433 and NGC 6300, twolarge and previously well-studied nearby ringed barred spiral galaxies.Stellar absorption-line radial velocities and new optical surfacephotometry are also presented for NGC 1433. The data are intended to setthe stage for dynamical modeling of the two galaxies, with the principalgoals being to derive mass distributions, bar mass-to-light ratios, barpattern speeds, and resonance locations, parameters which have beenderived for very few ringed disk galaxies. The presence of strong ringsand pseudorings in the two galaxies allows the possibility to linkspecific orbital resonances with the observed rings. The new data allowus to derive a fairly complete composite rotation curve of NGC 1433,leading us to make some interesting preliminary judgments about thestructure of the galaxy. The new data also verify the previous findingthat the radial velocity of the Seyfert nucleus in NGC 6300 differs fromthe actual systemic velocity of the galaxy by nearly 100 kms-1. We demonstrate in this paper that the offset is not anartifact of significant extinction in the inner regions of the galaxy.
| Magnetic fields in barred galaxies. II. Dynamo models
We study the generation and maintenance of large-scale magnetic fieldsin barred galaxies. We take a velocity field (with strong noncircularcomponents) from a published gas dynamical simulation of Athanassoula(1992), and use this as input to a galactic dynamo calculation. Our workis largely motivated by recent high quality VLA radio observations ofthe barred galaxy NGC 1097, and we compare our results in detail withthe regular magnetic fields deduced from these observations. We are ableto reproduce most of the conspicuous large-scale features of theobserved regular field, including the field structure in the centralregions, by using a simple mean-field dynamo model in which theintensity of interstellar turbulence (more precisely, the turbulentdiffusivity) is enhanced by a factor of 2-6 in the dust lanes and nearthe circumnuclear ring. We argue that magnetic fields can be dynamicallyimportant, and therefore should be included in models of gas flow inbarred galaxies.
| Spectral analysis of the nuclear stellar population and gas emission in six nearby galaxies
Optical spectra were used to study the stellar population and gasemission, when present, in the central parts of the elliptical galaxiesNGC 6958, NGC 6851 and ESO 185-G54, and the spiral galaxies NGC 5757,NGC 7412 and IC 4214. Stellar population properties were derived bycomparing the equivalent widths of the absorption lines with those ofgalaxy templates built from star clusters, as well as by applying amethod of stellar population synthesis based on the same star clusters.For the galaxies with emission lines, the underlying stellar populationwas subtracted to obtain the pure emission spectrum, and the resultingemission line intensities were measured to determine the mechanismresponsible for the gas excitation. From this analysis we concluded thatthe galaxies have solar metallicity except the elliptical ESO 185-G54that reached metallicity slightly above solar. The three ellipticals andthe Sa galaxy IC 4214 have their nuclei dominated by old stellarpopulations, although IC 4214 appears to have some contribution from theyoung generation stars. Recent star formation (t < 500 Myr) isprominent in the nucleus of the SBb galaxy NGC 5757, and is also presentto a lesser degree in the Sbc NGC 7412. In these two galaxies, therecent star formation is responsible for the observed H Ii type emissionlines, while IC 4214 has LINER emission. Among the ellipticals, only inNGC 6958 did we detect ionized gas, whose spectra is typical of LINERs.Based on observations made at the Observatório do Pico dos Dias,CNPq/Laboratório Nacional de Astrofísica, Brazil.
| Astrophysics in 1999
The year 1999 saw the arrival of a star with three planets, a universewith three parameters, and a solar corona that could be heated at leastthree ways. In addition, there were at least three papers on everyquestion that has ever been asked in astrophysics, from ``Will theUniverse expand forever?'' to ``Does mantle convection occur in one ortwo layers?'' The answers generally were, ``Yes,'' ``No,'' and ``None ofthe above,'' to each of the questions. The authors have done their bestto organize the richness around centers defined by objects, methods, andmadnesses.
| Nearby Optical Galaxies: Selection of the Sample and Identification of Groups
In this paper we describe the Nearby Optical Galaxy (NOG) sample, whichis a complete, distance-limited (cz<=6000 km s-1) andmagnitude-limited (B<=14) sample of ~7000 optical galaxies. Thesample covers 2/3 (8.27 sr) of the sky (|b|>20deg) andappears to have a good completeness in redshift (97%). We select thesample on the basis of homogenized corrected total blue magnitudes inorder to minimize systematic effects in galaxy sampling. We identify thegroups in this sample by means of both the hierarchical and thepercolation ``friends-of-friends'' methods. The resulting catalogs ofloose groups appear to be similar and are among the largest catalogs ofgroups currently available. Most of the NOG galaxies (~60%) are found tobe members of galaxy pairs (~580 pairs for a total of ~15% of objects)or groups with at least three members (~500 groups for a total of ~45%of objects). About 40% of galaxies are left ungrouped (field galaxies).We illustrate the main features of the NOG galaxy distribution. Comparedto previous optical and IRAS galaxy samples, the NOG provides a densersampling of the galaxy distribution in the nearby universe. Given itslarge sky coverage, the identification of groups, and its high-densitysampling, the NOG is suited to the analysis of the galaxy density fieldof the nearby universe, especially on small scales.
| Circumnuclear Star Formation in the Early-Type Resonance Ring Barred Spiral Galaxy NGC 1326
We present multiband WFPC2 images of the nuclear ring of NGC 1326, anearly-type southern barred spiral in the nearby Fornax galaxy cluster.The ring is a typical example of a 1 kpc-sized star-forming ring locatedin the region of the inner Lindblad resonances with the weak primarybar. The images reveal nearly 1000 point sources in and around the ring.Those sources brighter than MV=-9 are probably massive youngclusters, while the fainter sources may include a mix of extremelyluminous stars and fainter clusters. From an analysis of reddening-freeparameters and two-color plots, we find evidence for a spread in ages ofring clusters, from less than 5 Myr to at least 200 Myr. The olderclusters still lie within the nuclear ring, with no evidence ofmigration of the ring being found over this time period. The luminosityfunction reveals no clusters having an (uncorrected) absolute magnitudeMV brighter than -11, and even after correction forextinction the most luminous cluster identified has onlyM0V=-12.6. The ring seems to lack the ``super-starclusters'' (SSCs) seen in starburst systems and in other nuclear rings,and the analysis suggests that SSCs are not a universal property ofthese rings. Complex dust structure is found inside the ring, south ofthe nucleus, and extinction is especially severe on the west side of thering. An Hα image reveals hundreds of H II regions and emissioncomplexes in the nuclear ring, but there is little correlation betweenthese H II regions and the observed continuum sources, most likely owingto reddening and age differences in the ring. Based on observations withthe NASA/ESA Hubble Space Telescope, obtained at the Space TelescopeScience Institute, which is operated by the Association of Universitiesfor Research in Astronomy (AURA), Inc., under NASA Contract NAS 5-26555.
| N-body simulations of resonance rings in galactic disks
We have studied the formation of rings in the disks of galaxies by usingtwo-dimensional N-body simulations where the gas component is modelledas dissipatively colliding test particles. Our results support thestandard hypothesis that ring formation occurs when gas is driven toresonances by the gravitational torque of a rotating stellar bar. Whenthe bar is absent, a weaker oval-shaped mode or a spiral mode can havethe same effect. Typical locations of the rings are as follows: theouter rings are usually near the outer Lindblad resonances, the innerrings near the inner 4/1-resonance and the nuclear rings near the innerLindblad resonances. However, we have also found a few exceptions tothese rules. We also have studied why a significant fraction of barredgalaxies lack one, two or all ring types. Our models suggest that theabsence of rings may be related to timescales of ring formation: theinner and nuclear rings usually form faster than the outer rings. Thelack of inner and nuclear rings can be related to the strength of thebar: in high amplitude cases, one or both of these ring types areabsent. Also, bars may rotate fast enough such that they lack the innerLindblad resonance and thus cannot form nuclear rings. The potentialouter ring region is often dominated by a slower spiral mode, which inprinciple could inhibit or delay ring formation. However, we found thatwhen both the bar mode and the slower spiral mode coexist in the outerdisk, there can be almost cyclic alternation between different outerring morphologies. In addition to the outer Lindblad resonance of thebar, certain resonances of the slower mode can also exist near the ringradius. The deceleration of the bar rotation rate and the correspondingchange in the resonance positions did not inhibit ring formation ordestroy an existing ring. The presence of more than one mode could alsoaffect the region of inner or nuclear rings. This can explain part ofthe case in which the ring is misaligned with respect to the main barcomponent.
| Something about the structure of the Galaxy
We analyse a sample of 507 evolved (OH/IR) stars in the region (10deg>l>-45 deg), (|b|<3 deg). We derive average ages for subsetsof this sample, and use those sets as beacons for the evolution of theGalaxy. In the bulge, the oldest OH/IR stars in the plane are 7.5Gyr(1.3Msolar), and in the disc 2.7Gyr (2.3Msolar).The vertical distribution of almost all AGB stars in the disc is foundto be nearly exponential, with scaleheight increasing from 100pc forstars <~1Gyr old to 500pc for stars >~5Gyr old. There may be asmall, disjunct population of OH/IR stars. The radial distribution ofAGB stars is dictated by the metallicity gradient. Unequivocalmorphological evidence is presented for the existence of a central bar,but parameters can be constrained only for a given spatial-densitymodel. Using a variety of indicators, we identify the radii of the innerultraharmonic (2.5kpc) and corotation resonance (3.5kpc). We show thatthe 3-kpc arm is likely to be an inner ring, as observed in other barredgalaxies, by identifying a group of evolved stars that is connected tothe 3-kpc Hi filament. Also, using several observed features, we arguethat an inner-Lindblad resonance exists, at ~1-1.5kpc. The compositionsof OH/IR populations within 1kpc of the Galactic Centre give insightinto the bar-driven evolution of the inner regions. We suggest that thebar is ~8Gyr old, relatively weak (SAB), and may be in a final stage ofits existence.
| A dynamo model for the magnetic field of the weakly barred galaxy IC 4214
IC 4214 is a weakly barred galaxy, the observed velocity field of whichhas been well modelled by a dynamical simulation. We take the velocityfield from this simulation as being representative of that to be foundin a class of barred spiral galaxies, and use it as input into agalactic dynamo model. The strong non-circular velocities are found toplay an important role in determining the morphology of the magneticfield. The most conspicuous features are marked magnetic `arms', steadyin the frame corotating with the bar, and trailing from near the ends ofthe bar. They are generated by a mechanism associated with thecorotation resonance. Near the centre of the galaxy, the magnetic fieldis approximately axisymmetric. The computed magnetic field structure hasfeatures similar to those of the fields observed in barred galaxies.
| Resonance Rings and Galaxy Morphology
Rings of star formation are a common phenomenon of early to intermediateHubble type disk galaxies. Most rings form by gas accumulation atresonances, usually under the continuous action of gravity torques froma bar pattern, but sometimes in response to a mild tidal interactionwith a nearby companion. In either case, a resonance is a very specialplace in any galaxy where star formation can be enhanced and may proceedeither as a starburst or continuously over a long time period. Thisarticle describes the characteristic morphologies of bar-driven andtidally-driven resonance rings.
| A Hubble Space Telescope Optical and Ground-based Near-Infrared Study of the Giant Nuclear Ring in ESO 565-11
We present multiband Wide Field Planetary Camera 2 images of the centralregions of ESO 565-11, a peculiar southern barred galaxy recently shownto have the largest known example of a circumnuclear starburst ring. Wealso present ground-based near-infrared H-band imaging and photometry ofthe galaxy. The results provide an interesting picture of thestar-forming ring and its environment. Dust connected with the nuclearring lies mainly in a symmetric two-armed spiral pattern. More than 700point sources, mostly unresolved clusters, lie on a highly ellipticalring whose major axis is rotated by more than 20 deg from that of theisophotes of the background starlight. The luminosity function of theseclusters follows a power law with slope a=-2.18+/-0.06, typical of youngcluster systems. Most of the clusters lie in the age range 4-6 Myr, andmost may be metal-rich compared with the Sun. The nuclear ring is stillclearly seen in the H band, revealing a knotty appearance indicatingthat young stars continue to have a significant impact on its brightnessin this passband. Numerical simulations are used to show that thenuclear ring of ESO 565-11 has likely formed between two allowed innerLindblad resonances with the relatively weak primary bar. The resultsindicate that the excessive size of the ring may be due to an extendedhump in the variation of the parameter Ω-κ/2 with radius.The extreme elongation of the ring and its misalignment with the bar mayindicate that it is in an early phase of development. At later times,the simulations suggest that the ring could evolve to a rounder shape.The models do not account for star formation or gas recycling.
| The Structure and Dynamics of the Early-Type Resonance Ring Galaxy IC 4214. II. Models
The first paper in this series presented optical (UBVI) andnear-infrared (H) surface photometry and Hα Fabry-Perotinterferometry of IC 4214, an excellent example of a multiringed galaxywith a weak bar. In the present paper, we analyze the nonaxisymmetricgravitational potential of IC 4214, as defined by the near-infrareddata. The gas behavior is modeled with two-dimensional sticky particlesimulations that employ a rigidly rotating nonaxisymmetric potential.Our goals are to interpret the morphology and velocity field of IC 4214and to obtain a model from which the bar pattern speed, halocontribution, and relative bulge and disk masses can be deduced. We alsostudy the orbital families in the derived IC 4214 model. A fairly goodmatch, in both morphology and kinematics, is obtained forOmega_bar~40+/-5 km s^-1 kpc^-1, placing the bar corotation slightlybeyond the end of the bar (r_bar/r_cr~0.72+/-0.05). The deduced baramplitude implies that the visible mass of the disk dominates over theunseen halo (M_disk/M_halo>3-4). Moreover, our model accountsquantitatively for the large difference in the orientation of IC 4214 asderived in our first paper with photometric and kinematic methods.
| The Structure and Dynamics of the Early-Type Resonance Ring Galaxy IC 4214. I. Observations
The southern galaxy IC 4214 is a bright, nearby example of a multiplering early-type spiral galaxy with a weak bar. The galaxy shows threestrong pseudoring features, including a nuclear ring, an inner ring, anda prominent R^'_1 outer ring, each with distinctive characteristics. Thegalaxy is important because it is a classic resonance ring galaxy wherethe ring features can be linked to specific orbital resonances with thebar. We present in this paper a detailed UBVIH study of its photometricstructure and a Fabry-Perot study of its kinematics to set the stage fora dynamical model in a separate paper. Despite the early Hubble type,ionized gas is well distributed in the inner disk regions, providing awell-sampled velocity field. Rapid rotation is found in the vicinity ofthe nuclear ring, where the rotation curve reaches a maximum, and therotation velocities drop off somewhat with increasing radius in theouter regions. Noncircular motions are clearly seen in the observedvelocity field. Analysis of both the velocity field and the shapes ofisophotes give discrepant values for the inclination of the system,ranging from 47 deg to 50 deg from photometry to 55 deg to 58 deg fromkinematics. A likely value of the inclination could be 52 deg if theinner and outer rings are intrinsically elongated and alignedperpendicular to each other.
| Swing excitation of magnetic fields in trailing spiral galaxies?
Swing-excited dynamo-induced magnetic fields in spiral galaxies arestudied. We investigate the phenomenon of swing excitation with linearand nonlinear 3D numerical simulations of the galactic dynamo. The modelincludes differential rotation, axisymmetric and isotropic alpha -effectand uniform eddy diffusivity. The nonaxisymmetry is introduced vialarge-scale radial and azimuthal velocity components associated with aspiral density-wave perturbation. In a first step we present a linearanalysis in order to get insight in basic properties of theswing-excitation phenomenon. Both one and two-armed spiral galaxies areinvestigated. We find enlarged growth rates for the magnetic energyconnected with the parametric resonance condition $Omega ega_p \approx 2\omega_m (Omega ega_p pattern speed, \omega_m magnetic drift rate), butonly if the amplitude of the perturbation exceeds 10 km s^{-1}. Theresonance behavior also appears in the nonlinear regime. The solutionsare a mixture of several magnetic field modes. For a two-armed spiralthe even modes (m = 0, 2,...) are preferred. The contribution of highermagnetic field modes to the solution is largest if the parametricresonance condition is fulfilled. The field geometry depends strongly onthe pattern speed Omega ega_p$, the excitation is weakest at the radiuswhere the differentially rotating gas and the spiral pattern arecorotating.
| Kinematics of the local universe. VII. New 21-cm line measurements of 2112 galaxies
This paper presents 2112 new 21-cm neutral hydrogen line measurementscarried out with the meridian transit Nan\c cay radiotelescope. Amongthese data we give also 213 new radial velocities which complement thoselisted in three previous papers of this series. These new measurements,together with the HI data collected in LEDA, put to 6 700 the number ofgalaxies with 21-cm line width, radial velocity, and apparent diameterin the so-called KLUN sample. Figure 5 and Appendices A and B forcorresponding comments are available in electronic form at thehttp://www.edpsciences.com
| Hubble constant from sosie galaxies and HIPPARCOS geometrical calibration
New distances, larger than previous ones, have been obtained for M 31and M 81 based on the geometrical zero-point of the CepheidPeriod-luminosity relation provided by the HIPPARCOS satellite. Bycombining them with independent determinations we define reasonableranges for the distances of these important calibrating galaxies. Onthis basis, we determine the Hubble constant from the method of sosies(look-alike) galaxies, galaxies having the same characteristics than thecalibrators. The method is quite secure because it is purelydifferential and it does not depend on any assumption (apart from thenatural one that two sosies galaxies have similar absoluteluminosities). Nevertheless, the Malmquist bias has to be taken intoaccount. The observations behave exactly as predicted from theanalytical formulation of the bias. Thus, rejecting galaxies which areaffected by the Malmquist bias we derive the Hubble constant: H_o = 60+/- 10 (external) km.s^{-1}.Mpc^{-1} If we strictly use the calibrationobtained with HIPPARCOS and if the bias found in the Period-LuminosityRelation is considered, the Hubble constant is smaller than this (~ 55km.s(-1}.Mpc({-1)) ). This gives arguments in favour of thelong-distance scale. We briefly discuss possible improvements aiming atstill reducing the uncertainty.
| A Survey for H 2O Megamasers in Active Galactic Nuclei. II. A Comparison of Detected and Undetected Galaxies
A survey for H2O megamaser emission from 354 active galaxies hasresulted in the detection of 10 new sources, making 16 known altogether.The galaxies surveyed include a distance-limited sample (coveringSeyferts and LINERs with recession velocities less than 7000 km s-1) anda magnitude-limited sample (covering Seyferts and LINERs with mB <=14.5). In order to determine whether the H2O-detected galaxies are"typical" active galactic nuclei (AGNs) or have special properties thatfacilitate the production of powerful masers, we have accumulated adatabase of physical, morphological, and spectroscopic properties of theobserved galaxies. The most significant finding is that H2O megamasersare detected only in Seyfert 2 and LINER galaxies, not Seyfert 1's. Thislack of detection in Seyfert 1's indicates either that they do not havemolecular gas in their nuclei with physical conditions appropriate toproduce 1.3 cm H2O masers or that the masers are beamed away from Earth,presumably in the plane of the putative molecular torus that hides theSeyfert 1 nucleus in Seyfert 2's. LINERs are detected at a similar rateto Seyfert 2's, which constitutes a strong argument that at least somenuclear LINERs are AGNs rather than starbursts, since starbursts havenot been detected as H2O megamasers. We preferentially detect H2Oemission from the nearer galaxies and from those that are apparentlybrighter at mid- and far-infrared and centimeter radio wavelengths.There is also a possible trend for the H2O-detected galaxies to be moreintrinsically luminous in nuclear 6 cm radio emission than theundetected ones, though these data are incomplete. We find evidence thatSeyfert 2's with very high (NH > 1024 cm-2) X-ray--absorbing columnsof gas are more often detected as H2O maser emitters than Seyfert 2'swith lower columns. It may be that the probability of detecting H2Omaser emission in Seyfert galaxies increases with increasing column ofcool gas to the nucleus, from Seyfert 1's through narrow-line X-raygalaxies to Seyfert 2's.
| BVRI and H(alpha) Surface Photometry of the Triple-Ringed Galaxy IC 4214
BVRI and Hα CCD surface photometry is presented for the Sab systemIC 4214, an example of a galaxy with three rings and no conventionalbar. We obtained isophote maps, luminosity profiles, and basicphotometric parameters. The photometry suggests the galaxy has threemajor structural components: the nuclear bulge, the disk, and anon-axisymmetric component that is a mixture of bar, inner ring andlens. The bar is a very weak feature that appears clearly only in the Iband. Even so, the non-axisymmetric component contributes 23% to thetotal I luminosity. The Hα emission is concentrated mostly in thenuclear ring and in the two spiral arms that form the inner ring. Boththe inner ring and nuclear ring are blue compared with theirsurroundings, and must contain regions of star formation mixed withdust. The existence of rings in non-barred and weakly barred galaxies isstill not well explained by theories of ring formation. This galaxyseems to be a case like others that have appeared in the literature,where the bar was stronger in the past, and now is dissolving.
| A Survey for H 2O Megamasers in Active Galactic Nuclei. I. Observations
We report an extensive search for 22 GHz H_2_O maser emission fromnearby active galaxies. Our sample includes all Seyfert and LINERgalaxies listed in the Huchra catalog or the Veron-Cetty & Veroncatalog with recessional velocities less than 7000 km s^-1^, and allSeyfert galaxies and LINERs in Huchra's catalog with m_b_ <= 14. Inaddition to these distance- and magnitude-limited samples, we have alsoobserved a number of active galaxies, including radio galaxies, athigher redshift; In all, some 354 galaxies have been surveyed. Ten newH_2_O megamaser sources have been detected, resulting in 16 galaxiesthat are currently known to contain H_2_O masers with isotropicluminosities greater than 20 L_sun_. Of the observed active galaxieswith cz < 7000 km s^-1^, 5.4% have detectable H_2_O megamaseremission. This fraction increases to 11% for those sources with cz <2000 km s^-1^. The newly discovered megamaser sources were monitored onsubsequent observing runs. The strength of the maser features varies forthese sources, as they do for Galactic masers. Three of the galaxieshave sufficient data to test for velocity changes of narrow masercomponents comparable in magnitude to those of the well-studied systemicfeatures in NGC 4258. The maser line in one of these galaxies-NGC2639-is found to have a systematic redward velocity drift of 6.6 +/- 0.4km s^-1^ yr^-1^. No systematic velocity drifts are found for the othertwo sources. We also report large apparent velocity changes in theunusual broad H_2_O emission feature in NGC 1052.
| Metallicity Indices for Multi-Population Models.II.Bulges of Galaxies
We report metallicity indices in the Lick system (Hβ, Fe52, Fe53,NaD, Mg_2_) for a sample of 45 spiral bulges in the southern hemisphere.The velocity dispersion σ was also derived for each object. Spiralbulges and elliptical galaxies show a continuity in diagrams like theplane Mg_2_- or σ-Mg_2_. Using calibrations derived fromchemical evolutionary models, we estimated metallicities and abundanceratios for those bulges. The sample mean metallicity is [Fe/H]= -0.19+/- 0.27(rmsd), and the mean abundance ratios are [Mg/Fe] = 0.46 +/-0.11 (rmsd) and [Na/Fe] = 0.45 +/- 0.19(rmsd). These abundances suggestthat spiral bulges (like E's) were chemically enriched by type IIsupernovae, and that the main star formation era occurred in a timescale of the order of 1-2 Gyr.
| The Catalog of Southern Ringed Galaxies
The Catalog of Southern Ringed Galaxies (CSRG) is a comprehensivecompilation of diameters, axis ratios, relative bar position angles, andmorphologies of inner and outer rings, pseudorings, and lenses in 3692galaxies south of declination -17 deg. The purpose of the catalog is toevaluate the idea that these ring phenomena are related to orbitalresonances with a bar or oval in galaxy potentials. The catalog is basedon visual inspection of most of the 606 fields of the Science ResearchCouncil (SRC) IIIa-J southern sky survey, with the ESO-B, ESO-R, andPalomar Sky surveys used as auxiliaries when needed for overexposed coreregions. The catalog is most complete for SRC fields 1-303 (mostly southof declination -42 deg). In addition to ringed galaxies, a list of 859mostly nonringed galaxies intended for comparison with other catalogs isprovided. Other findings from the CSRG that are not based on statisticsare the identification of intrinsic bar/ring misalignment; bars whichunderfill inner rings; dimpling of R'1pseudorings; pointy, rectangular, or hexagonal inner or outer ringshapes; a peculiar polar-ring-related system; and other extreme examplesof spiral structure and ring morphology.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
Catalogs and designations:
|
