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Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much ofmodern astrophysics. However, a quantitative prediction of the starformation rate and the initial distribution of stellar masses remainselusive. For several decades it has been thought that the star formationprocess is primarily controlled by the interplay between gravity andmagnetostatic support, modulated by neutral-ion drift (known asambipolar diffusion in astrophysics). Recently, however, bothobservational and numerical work has begun to suggest that supersonicturbulent flows rather than static magnetic fields control starformation. To some extent, this represents a return to ideas popularbefore the importance of magnetic fields to the interstellar gas wasfully appreciated. This review gives a historical overview of thesuccesses and problems of both the classical dynamical theory and thestandard theory of magnetostatic support, from both observational andtheoretical perspectives. The outline of a new theory relying on controlby driven supersonic turbulence is then presented. Numerical modelsdemonstrate that, although supersonic turbulence can provide globalsupport, it nevertheless produces density enhancements that allow localcollapse. Inefficient, isolated star formation is a hallmark ofturbulent support, while efficient, clustered star formation occurs inits absence. The consequences of this theory are then explored for bothlocal star formation and galactic-scale star formation. It suggests thatindividual star-forming cores are likely not quasistatic objects, butdynamically collapsing. Accretion onto these objects varies depending onthe properties of the surrounding turbulent flow; numerical models agreewith observations showing decreasing rates. The initial massdistribution of stars may also be determined by the turbulent flow.Molecular clouds appear to be transient objects forming and dissolvingin the larger-scale turbulent flow, or else quickly collapsing intoregions of violent star formation. Global star formation in galaxiesappears to be controlled by the same balance between gravity andturbulence as small-scale star formation, although modulated by coolingand differential rotation. The dominant driving mechanism instar-forming regions of galaxies appears to be supernovae, whileelsewhere coupling of rotation to the gas through magnetic fields orgravity may be important.
| Astrophysics in 2003
Five coherent sections appear this year, addressing solar physics,cosmology (with WMAP highlights), gamma-ray bursters (and theirassociation with Type Ia supernovae), extra-solar-system planets, andthe formation and evolution of galaxies (from reionization to assemblageof Local Group galaxies). There are also eight incoherent sections thatdeal with other topics in stellar, galactic, and planetary astronomy andthe people who study them.
| BUDDA: A New Two-dimensional Bulge/Disk Decomposition Code for Detailed Structural Analysis of Galaxies
We present BUDDA (Bulge/Disk Decomposition Analysis), a new code devotedto perform a two-dimensional bulge/disk decomposition directly from theimages of galaxies. The bulge component is fitted with a generalizedSérsic profile, whereas disks have an exponential profile. Noother components are included. Bars and other substructures, likelenses, rings, inner bars, and inner disks, are studied with theresidual images obtained through the subtraction of bulges and disksfrom the original images. This means that a detailed structural analysisof galaxies may be performed with a small number of parameters, andsubstructures may be directly studied with no a priori assumptions. Ashas been already shown by several studies, two-dimensional fitting ismuch more reliable than one-dimensional profile fitting. Moreover, ourcode has been thoroughly tested with artificial data, and we demonstrateit to be an accurate tool for determining structural parameters ofgalaxies. We also show that our code is useful in various kinds ofstudies, including galaxies of, e.g., different morphological types, andinclinations, which also may be observed at different spatialresolutions. Thus, the code has a broader range of potentialapplications than most of the previous codes, which are developed totackle specific problems. To illustrate its usefulness, we present theresults obtained with a sample of 51 mostly early-type galaxies (butcovering the whole Hubble sequence). These results show some of theapplications in which the code may be used: the determination ofparameters for fundamental plane and structural studies, quantitativemorphological classification of galaxies, and the identification andstudy of hidden substructures. We have determined the structuralparameters of the galaxies in our sample and found many examples ofhidden inner disks in ellipticals, secondary bars, nuclear rings anddust lanes in lenticulars and spirals, and also wrong morphologicalclassification cases. We now make BUDDA generally available to theastronomical community.Based on observations made at the Pico dos Dias Observatory(PDO/LNA-CNPq), Brazil.
| The Ringed Spiral Galaxy NGC 4622. I. Photometry, Kinematics, and the Case for Two Strong Leading Outer Spiral Arms
The intriguing nearly face-on southern ringed spiral galaxy NGC 4622,the first galaxy definitively shown to have leading spiral structure, isrevisited in this paper with new images from the Hubble SpaceTelescope's (HST) WFPC2, together with ground-based optical and near-IRimaging, and a Fabry-Perot Hα velocity field. The data provide newinformation on the disk/bulge/halo mix, rotation curve, star formationin the galaxy, and the sense of winding of its prominent spiral arms.Previously, we suggested that the weaker, inner single arm most likelyhas the leading sense, based on a numerical simulation. Now, takingadvantage of HST resolution and using de Vaucouleurs' standardextinction and reddening technique to determine the near side of thegalaxy's slightly tilted disk, we come to the more surprising conclusionthat the two strong outer arms have the leading sense. We suggest thatthis highly unusual configuration may be the result of a past minormerger or mild tidal encounter. Possible evidence for a minor merger isfound in a short, central dust lane, although this is purelycircumstantial and an unrelated interaction with a different companioncould also be relevant. The leading arms may be allowed to persistbecause NGC 4622 is dark halo dominated (i.e., not ``maximum disk'' inthe inner regions) and displays a significantly rising rotation curve.The new HST observations also reveal a rich globular cluster system inthe galaxy. The mean color of these clusters is (V-I)0=1.04,and the specific frequency is 3.4+/-0.6. The luminosity function ofthese clusters confirms the membership of NGC 4622 in the CentaurusCluster.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., undercontract NAS 5-26555.
| A new catalogue of ISM content of normal galaxies
We have compiled a catalogue of the gas content for a sample of 1916galaxies, considered to be a fair representation of ``normality''. Thedefinition of a ``normal'' galaxy adopted in this work implies that wehave purposely excluded from the catalogue galaxies having distortedmorphology (such as interaction bridges, tails or lopsidedness) and/orany signature of peculiar kinematics (such as polar rings,counterrotating disks or other decoupled components). In contrast, wehave included systems hosting active galactic nuclei (AGN) in thecatalogue. This catalogue revises previous compendia on the ISM contentof galaxies published by \citet{bregman} and \citet{casoli}, andcompiles data available in the literature from several small samples ofgalaxies. Masses for warm dust, atomic and molecular gas, as well asX-ray luminosities have been converted to a uniform distance scale takenfrom the Catalogue of Principal Galaxies (PGC). We have used twodifferent normalization factors to explore the variation of the gascontent along the Hubble sequence: the blue luminosity (LB)and the square of linear diameter (D225). Ourcatalogue significantly improves the statistics of previous referencecatalogues and can be used in future studies to define a template ISMcontent for ``normal'' galaxies along the Hubble sequence. The cataloguecan be accessed on-line and is also available at the Centre desDonnées Stellaires (CDS).The catalogue is available in electronic form athttp://dipastro.pd.astro.it/galletta/ismcat and at the CDS via anonymousftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/5
| The life cycle of galaxies.
Not Available
| Supernova 2001jx in NGC 4622
IAUC 7833 available at Central Bureau for Astronomical Telegrams.
| Leading Pairs and Trailing Single Spiral Arms: Observations and Simulations of NGC4622
NGC4622 has a most unusual spiral arm pattern. Besides a pair of outerarms which wind outward clockwise (CW), Byrd et al. (1989) pointed out aweaker, single, inner arm winding outward counterclockwise. Byrd et al.suspected the single arm must lead, a rare configuration. Buta, Crocker,and Byrd's (1992) BVI photometry showed the inner arm is a real stellardisk feature. Assuming a flat rotation curve, Byrd, Freeman, and Howard(1993) simulated a single leading and outer trailing pair of arms forNGC4622 via a plunging low-mass perturber along the plane of a disk in amassive halo. However, new HST data (Byrd, Buta, and Freeman 2001)indicate via dust cloud silhouettes that the east edge of the galaxy isnearer. Additional Hα spectra indicate that the north edge alongthe disk line of nodes is receding relative to the center. The diskmaterial thus orbits CW and the arm pair leads, winding out in thedirection of disk orbital motion ``counter" to common theoreticalexpectations. Conversely, the inner single arm trails. The spectraindicate that the rotation curve is rising and getting steeper withincreasing radius in the outer disk where the pair of arms is found.There is some indication that it is flat in the single arm region(although there is little ionized gas there). This unusual curve appearsto be a key to understanding the NGC4622 arm pattern. An encounter muchlike our previous work but with the new rotation curve successfullyproduces an inner single trailing, outer pair leading spiral armpattern. See http://bama.ua.edu/ ~rbuta/ngc4622. We find support for theencounter in the lack of a systematic BVI azimuthal color displacementin the outer pair, indicating a young impulsive perturbation which iswinding up. We find evidence in the HST images that the inner patterncontains a one-sided periodic orbit ring whose pattern speed results ina clear angular displacement between the B, V, and I maximum radialdistances. Grant support: NASA/STScI GO 8707 and NSF AST-0206177.
| 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.
| Dust-penetrated morphology in the high-redshift universe: Clues from NGC 922
Results from the Hubble Deep Field (HDF) North and South show a largepercentage of high-redshift galaxies whose appearance falls outsidetraditional classification systems. The nature of these objects ispoorly understood, but sub-mm observations indicate that at least someof these systems are heavily obscured (Sanders \cite{sanders00}). Thisraises the intriguing possibility that a physically meaningfulclassification system for high-redshift galaxies might be more easilydevised at rest-frame infrared wavelengths, rather than in the opticalregime. Practical realization of this idea will become possible with theadvent of the Next Generation Space Telescope (NGST). In order toexplore the capability of NGST for undertaking such science, we presentNASA-IRTF and SCUBA observations of NGC 922, achaotic system in our local Universe which bears a striking resemblanceto objects such as HDF 2-86 (z=0.749) in the HDF North. If objects suchas NGC 922 are common at high-redshifts, then thisgalaxy may serve as a local morphological ``Rosetta stone'' bridging lowand high-redshift populations. In this paper we demonstrate thatquantitative measures of galactic structure are recoverable in therest-frame infrared for NGC 922 seen at highredshifts using NGST, by simulating the appearance of this galaxy atredshifts z=0.7 and z=1.2 in rest-frame K'. While this object cannot beclassified within any optical Hubble bin, simulated NGST images at theseredshifts can be readily classified using the dust penetrated z ~ 0template of Block & Puerari (\cite{blockpuerari99}) and Buta &Block (\cite{butablock01}). The near-infrared disk of NGC 922 is notpeculiar at all; rather, it is remarkably regular, even presentingspiral arm modulation, a characteristic signature of several granddesign galaxies. Our results suggest that the capability of efficientlyexploring the rest-wavelength IR morphology of high-z galaxies shouldprobably be a key factor in deciding the final choice of instruments forthe NGST.
| 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.
| Galaxy collisions.
Theories of how galaxies, the fundamental constituents of large-scalestructure, form and evolve have undergone a dramatic paradigm shift inthe last few decades. Earlier views were of rapid, early collapse andformation of basic structures, followed by slow evolution of the stellarpopulations and steady buildup of the chemical elements. Currenttheories emphasize hierarchical buildup via recurrent collisions andmergers, separated by long periods of relaxation and secularrestructuring. Thus, collisions between galaxies are now seen as aprimary process in their evolution. This article begins with a briefhistory; we then tour parts of the vast array of collisional forms thathave been discovered to date. Many examples are provided to illustratehow detailed numerical models and multiwaveband observations haveallowed the general chronological sequence of collisional morphologiesto be deciphered, and how these forms are produced by the processes oftidal kinematics, hypersonic gas dynamics, collective dynamical frictionand violent relaxation. Galaxy collisions may trigger the formation of alarge fraction of all the stars ever formed, and play a key role infueling active galactic nuclei. Current understanding of the processesinvolved is reviewed. The last decade has seen exciting new discoveriesabout how collisions are orchestrated by their environment, howcollisional processes depend on environment, and how these environmentsdepend on redshift or cosmological time.
| Spiral patterns with straight arm segments
The phenomenon of `rows', which are straight geometrical segments in thespiral arms of galaxies, is studied. The Whirlpool nebula, Messier 51(NGC 5194) in Canes Venatici, is considered to be an example of a giantgrand design galaxy. Optical photographs, Hα, ultraviolet andfar-ultraviolet images, CO, 21-cm and synchrotron emission maps, and aK_s-band mosaic of M51 are studied. With this observational material,multiple rows can be recognized in the spiral arms of the galaxy. Therows comprise a major part of the arms. The lengths of the rows increasealmost linearly with distance from the centre. They intersect oneanother at an (average) angle ~2π/3. A possible physical explanationof the phenomenon of rows is discussed on the basis of the assumptionthat the formation of straight arm segments might be due to thegas-dynamical effect of stability of flat shock fronts, and the tendencyof a slightly curved shock front to become flat. A quantitativeflattening criterion enables an explanation of the geometricalproperties of the arm patterns found in M51 and also in M101. A brieflist of spirals with rows is given.
| Bogus dust screens from well-mixed exponential discs in galaxies
The V-K colours along the minor axes of spiral galaxies typically changefrom red to blue with increasing distance, giving the impression thatthe near side is systematically screened by dust. Such a preferredorientation for dust screens is unlikely. Here we show that commonextinction from the embedded dust layer in an exponential disc has thesame effect, making the near side systematically redder as theinclination increases. The galaxy NGC 2841 is modelled as an example,where the V-K profile is profoundly asymmetric and actually step-likeacross the centre. We predict that the minor-axis emission profile ofthe same dust in the far-infrared, at wavelength lambda ~ 200 mu m, willbe much more symmetric than the optical profiles, implying nearly equalcolumn densities of dust on both sides of the minor axis.
| 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.
| Cosmic Masks Still Dance
The Hubble classification scheme of galaxies is based on their opticalappearance or `masks'. As one goes from early to late type spirals, bothbarred and unbarred, the optical appearance will be dominated more andmore by the young Population I, i.e., blue stars and dust. Atlasesreveal the rich variety of responses of the Population I component ofgas and dust (the mask) to the underlying, older, stellar population.However, the gaseous Population I component, may only constitute 5percent of the dynamical mass of the galaxy. Masks of negligible massmay conceal the human face - and that of galaxy. In the near-infrared,the morphology of older star-dominated disk indicates a simpleclassification scheme: the dominant Fourier m-mode in the dustpenetrated regime, and the associated pitch angle. A ubiquity of low m=1and m=2 modes is confirmed. On the basis of deprojected H (1.65 μm)and K' (2.1μm) images, we propose that the evolved stellar disks maybe grouped into three principal dust penetrated archetypes: those withtightly wound stellar arms characterised by pitch angles at K' of ~10^° (the α class), an intermediate group with pitch angles of~ 25^° (the β class) and thirdly, those with open spiralsdemarcated by pitch angles at K' of ~ 40^° (the γ bin). Flator falling rotation curves give rise to the tightly wound α class;rising rotation curves are associated with the open γ class. Theobserved dust penetrated classes are inextricably related to the rate ofshear in the stellar disk, as determined by A/ω. Here A is thefirst Oort constant andω denotes the angular velocity. There is nocorrelation between our dust penetrated classes and optical Hubblebinning; the Hubble tuning fork does not constrain the morphology of theold stellar Population II disks. NGC 3223 and NGC 7083 (both SbI-II andalmost the same absolute blue magnitude) have identical Hubble types andidentical luminosity classes; the dust penetrated disk of NGC 3223 hastightly wrapped arms of class α, whereas the near-infrared disk ofNGC 7083 has open arms of class γ. This is in turn associated withtheir very different rotation curve shapes yielding different rates ofshear A/ω in their stellar disks. Any specific dust penetratedarchetype may be the resident disk of both an early or late type galaxy.The number of arms and the pitch angle of the arms at K' of theearly-type `a' spiral NGC 718 are almost identical to those for thelate-type `c' spiral NGC 309. We demonstrate that galaxies on oppositeends of the tuning fork can display remarkably similar evolved diskmorphologies and belong to the same dust penetrated class. In thissense, there is no differentiation between an early and late typegalaxy: the Hubble tuning fork becomes a circle. Furthermore, aprototypically flocculent galaxy such as NGC 5055 (Elmegreen arm class3) can have an evolved disk morphology almost identical to that of NGC5861, characterised in the optical as having one of the most regularspiral patterns known and of Elmegreen class 12. Both opticallyflocculent or grand design galaxies can reside within the same dustpenetrated morphological bin. As was suggested by Block et al. (1994a),it is the gas dominated Population I component which determines theoptical types (a, b, c). This may be partially or even fully decoupledfrom the Population II disk. Those L=lopsided galaxies (where m=1 is adominant mode) are designated Lα, Lβ and Lγ accordingto the dust penetrated pitch angle; E=evensided galaxies (where m=2 isthe dominant Fourier mode) are classified into classes Eα, Eβand Eγ, according to our three principal dust penetratedarchetypes. The L and E modes are the most common morphologies in oursample, which spans a range of Hubble types from early (a) to late(irregular).
| Galaxy coordinates. II. Accurate equatorial coordinates for 17298 galaxies
Using images of the Digitized Sky Survey we measured coodinates for17298 galaxies having poorly defined coordinates. As a control, wemeasured with the same method 1522 galaxies having accurate coordinates.The comparison with our own measurements shows that the accuracy of themethod is about 6 arcsec on each axis (RA and DEC).
| Toward a dust penetrated classification of the evolved stellar Population II disks of galaxies
To derive a coherent physical framework for the excitation of spiralstructure in galaxies, one must consider the co-existence of twodifferent dynamical components: a gas-dominated Population I disk (OBassociations, HII regions, cold interstellar HI gas) and an evolvedstellar Population II component. The Hubble classification scheme has asits focus, the morphology of the Population I component only. In thenear-infrared, the morphology of evolved stellar disks indicates asimple classification scheme: the dominant Fourier m-mode in the dustpenetrated regime, and the associated pitch angle. On the basis ofdeprojected K' (2.1microns ) images, we propose that the evolved stellardisks may be grouped into three principal dust penetrated archetypes:those with tightly wound stellar arms characterised by pitch angles atK' of ~ 10(deg) (the alpha class), an intermediate group with pitchangles of ~ 25(deg) (the beta class) and thirdly, those with openspirals demarcated by pitch angles at K' of ~ 40(deg) (the gamma bin).There is no correlation between our dust penetrated classes and opticalHubble binning; the Hubble tuning fork does not constrain the morphologyof the old stellar Population II disks. Any specific dust penetratedarchetype may be the resident disk of both an early or late type galaxy.The number of arms and the pitch angle of the arms at K' of theearly-type `a' spiral NGC 718 are almost identical to those for thelate-type `c' spiral NGC 309. We demonstrate that galaxies on oppositeends of the tuning fork can display remarkably similar evolved diskmorphologies and belong to the same dust penetrated class. Furthermore,a prototypically flocculent galaxy such as NGC 5055 (Elmegreen arm class3) can have an evolved disk morphology almost identical to that of NGC5861, characterised in the optical as having one of the most regularspiral patterns known and of Elmegreen class 12. Both opticallyflocculent or grand design galaxies can reside within the same dustpenetrated morphological bin. As was suggested by Block et al.(\cite{block94a}), it is the gas dominated Population I component whichdetermines the optical types (a, b, c), decoupled from the PopulationII. Those L=lopsided galaxies (where m=1 is a dominant mode) aredesignated Lalpha , Lbeta and Lgamma according to the dust penetratedpitch angle; E=evensided galaxies (where m=2 is the dominant Fouriermode) are classified into classes Ealpha , Ebeta and Egamma , accordingto our three principal dust penetrated archetypes. The L and E modes arethe most common morphologies in our sample, which spans a range ofHubble types from early (a) to late (irregular). Having formulated ourdust penetrated classification scheme here, we have tested it on anindependent sample of 45 face-on galaxies observed in the near-infraredby Seigar and James (\cite{seigar98a}, b).
| Vorontsov-Vel'yaminov's rows in giant spiral galaxies: geometrical properties and physical interpretation.
Not Available
| Total magnitude, radius, colour indices, colour gradients and photometric type of galaxies
We present a catalogue of aperture photometry of galaxies, in UBVRI,assembled from three different origins: (i) an update of the catalogueof Buta et al. (1995) (ii) published photometric profiles and (iii)aperture photometry performed on CCD images. We explored different setsof growth curves to fit these data: (i) The Sersic law, (ii) The net ofgrowth curves used for the preparation of the RC3 and (iii) A linearinterpolation between the de Vaucouleurs (r(1/4) ) and exponential laws.Finally we adopted the latter solution. Fitting these growth curves, wederive (1) the total magnitude, (2) the effective radius, (3) the colourindices and (4) gradients and (5) the photometric type of 5169 galaxies.The photometric type is defined to statistically match the revisedmorphologic type and parametrizes the shape of the growth curve. It iscoded from -9, for very concentrated galaxies, to +10, for diffusegalaxies. Based in part on observations collected at the Haute-ProvenceObservatory.
| An Einstein X-Ray Survey of Optically Selected Galaxies. I. Data
We present the results of a complete Einstein imaging proportionalcounter X-ray survey of optically selected galaxies from theShapley-Ames Catalog, the Uppsala General Catalogue, and the EuropeanSouthern Observatory Catalog. Well-defined optical criteria are used toselect the galaxies, and X-ray fluxes are measured at the opticallydefined positions. The result is a comprehensive list of X-ray detectionand upper limit measurements for 1018 galaxies. Of these, 827 haveeither independent distance estimates or radial velocities. Associatedoptical, redshift, and distance data have been assembled for thesegalaxies, and their distances come from a combination of directlypredicted distances and those predicted from the Faber-Burstein GreatAttractor/Virgocentric infall model. The accuracy of the X-ray fluxeshas been checked in three different ways; all are consistent with thederived X-ray fluxes being of <=0.1 dex accuracy. In particular,there is agreement with previously published X-ray fluxes for galaxiesin common with a 1991 study by Roberts et al. and a 1992 study byFabbiano et al. The data presented here will be used in further studiesto characterize the X-ray output of galaxies of various morphologicaltypes and thus to enable the determination of the major sourcescontributing to the X-ray emission from galaxies.
| Galactic Rings
About one fifth of all spiral disk galaxies include a ring-shapedpattern in the light distribution, and an additional one third appear tohave broken or partial rings made up of spiral arms (pseudorings). Theserings are a special problem in galaxy morphology with a direct bearingon the internal dynamics and evolution of disk galaxies. Morphologicaldata have shown that rings are most often associated with bars or othercommon nonaxisymmetric perturbations, such as ovals. Kinematic andmetric data have provided considerable evidence for intrinsic ovalshapes and preferred alignments between ring major axes and bars.Photometric data have demonstrated that most rings are sites of currentactive star formation, and in some galaxies a ring is the only placewhere recent star formation is found. A few rings are sites of the mostspectacular "starbursts" known in non-violently interacting galaxies.Though a small fraction of observed rings may be due to collisions ormergers of galaxies, or to accretion of intergalactic gas, the vastmajority of rings are probably simple resonance phenomena, caused by theactions of a rotating bar or other nonaxisymmetic disturbance on themotions of gas clouds in the disk. The evidence in support of this ideahas accumulated steadily during the past 15 years, and our goal in thisreview is to bring together a large body of theoretical andobservational results in one place. We shall see that rings are anatural consequence of barred galaxy dynamics, and that they are moreeasily understood than the bars and ovals which undoubtedly create them.However, there are interesting problems, such as the lack of any ringsin some barred galaxies, the less common but by no means rare cases ofrings in nonbarred galaxies, the role of mild tidal interactions, wherethe gas that fuels star formation in rings actually comes from, theexistence of different ring types of very different time-scales in thesame galaxy. We will discuss these problems in some detail here, andindicated where the solutions may lie.
| 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.
| Astrophysics in 1993
After the astronomical excitements of 1991 and 1992, 1993 was frankly abit of a letdown. More papers by more people were submitted andpublished than ever before (and this review has twice as many authors asthe previous ones). Nevertheless, there seem to have been fewer large,definitive steps. As a result, we have focused on a handful of broadterrains for which the maps have improved, including stellar rotationand mass loss, dynamics of globular clusters, and quasar absorptionlines, but have also highlighted many more of the small steps by whichastonomy advances toward inventorying and and understanding theuniverse. As a consequence, the ordering of topics is less obviouslyfrom near to far than in 1991 and 1992. And the potential formisattributions and unjustifiable neglect is probably somewhat larger.
| Formation of a retrograde spiral arm induced by a Ram pressure
The response of a galactic disk to a ram pressure due to anintergalactic gas is studied; it is found that the ram pressure excitesa spiral structure in the disk. Development of a prominent spiralstructure is demonstrated by a simple numerical simulation of a gas disksubjected to a ram pressure. The spiral structure induced by a rampressure is a wave phenomenon, and, in contrast to the normal spiralwaves, it is single-armed and slowly rotates in a retrograde direction.The sense of the spiral is trailing with respect to the patternrotation; thus the spiral is apparently leading with respect to thegalatic rotation. The spiral pattern slowly winds up in a retrogradedirection.
| NGC 4378's one-armed spiral pattern: Leading or trailing?
Observations and simulations of the galaxy NGC 4622 indicate that it hasa true leading arm winding outward opposite the sense of disk spin. Thearm is apparently a long-lived density wave leading out to a resonancering. Simulations of a plunging retrograde passage of a small companioncreates this pattern. A proper match to the observations does not resultunless self gravity is unimportant in the disk. We now investigatewhether or not NGC 4378, another single-armed disk galaxy, also has aleading density wave arm. We find via simulations that NGC 4378'spattern is not a leading density wave but is instead an impulsivetrailing arm created by a grazing passage of a small companion. Thisimpulsive arm has a short lifetime in the simulations. Nevertheless,like in NGC 4622, a nongravitating (halo dominated) disk is necessary tocreate the observed pattern in NGC 4378.
| The optical morphology of the kinematically peculiar galaxy NGC 4826
We present charge coupled device (CCD) BVI photometry of the galaxy NGC4826, the Evil- or Black-Eye galaxy, which was recently found to havetwo counter-rotating gas disks. We study the extinction in the inner gasdisk, which gives NGC 4826 its nickname, and find that this disk can becoplanar or close to coplanar with the stellar disk and still cause thestrong absorption that is seen on one side of the galaxy. We try toconstrain the orientation of the outer gas disk by looking for a smalloverall asymmetry in the light distribution which would be present ifthere is dust in this disk, and if it is significantly tilted withrespect to the main body of the galaxy. The test shows that the lightdistribution does not preclude the outer gas disk from being coplanarwith the stellar disk as well. NGC 4826 has a small bulge, with a bulgeto total light ratio of 0.17 in B. We confirm that this galaxy is indeeda spiral, with a perfect exponential disk down to 27 mag/sq arcsec in B.The close to coplanar orientation of the gas disks is one aspect whichis in good agreement with what is expected on the basis of a mergermodel for the counter-rotating gas. The rotation direction of the innergas disk with respect to the stars, however, is not. In addition, theexistence of a well defined exponential disk probably implies that if amerger did occur it must have been between a gas-rich dwarf and aspiral, not between two equal mass spirals. The stellar spiral arms ofNGC 4826 are trailing over part of the disk and leading in the outerdisk. Recent numerical calculations by Byrd et al. for NGC 4622 suggestthat long lasting leading arms could be formed by a close retrogradepassage of a small companion. In this scenario, the outercounter-rotating gas disk in NGC 4826 might be the tidally stripped gasfrom the dwarf. However, in NGC 4826 the outer arms are leading, whileit appears that in NGC 4622 the inner arms are leading. A realisticN-body/hydro simulation of a dwarf-spiral encounter is clearly needed.It may also be possible that the counter-rotating outer gas disk is dueto gradual infall of gas from the halo, rather than from a discretemerger event.
| Evolution of spiral structure in an interacting triple galactic system I. M 31-type systems
We investigate the effect of two small companion galaxies in thecreation and evolution of spiral arms in a disk galaxy. Both companionsmove in circular orbits around the main galaxy. Using a two dimensionalself-consistent computer simulation code we find that, when there isonly one companion, one leading or two trailing spiral arms are formed,depending on the rotation of the companion (retrograde or directrespectively). In the case of two companion galaxies, when thecompanions move in opposite directions, three spiral arms are formed,one leading and two trailing. The appearance of the above structuresuggests that each companion acts independently on the main disk galaxy.The created spiral arms may intersect each other, resulting in thedisappearance of the weaker arm.
| 2.1 μm images of the evolved stellar disk and the morphological classification of spiral galaxies
Near-infrared images confirm that the Hubble classification of spiralgalaxies does not constrain the morphology of their stellar PopulationII disk, since galaxies on opposite ends of the spiral sequence candisplay remarkably similar evolved disk morphologies. Thus, the gasdominated Population I component determines the types (a, b, c),decoupled from the Population II. The underlying mass distributionsobserved in the infrared are exceptionally regular, suggesting thatlarge scale spiral structure is principally intrinsic, as argued by themodal theory. Moreover, single arms, bisymmetric arms, lopsidednessand/or bars dominate the old stellar disk. The absence of infraredmultiple-armed structure is attributed to the efficiency of InnerLindblad Resonance absorption in the evolved Population II disk. Theseobservations support a coherent framework for galaxy classificationbased on three parameters: stellar disk "temperature", gas content andactive disk mass.
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