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# NGC 2997

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 HI content in galaxies in loose groupsGas deficiency in cluster spirals is well known and ram-pressurestripping is considered the main gas removal mechanism. In some compactgroups too gas deficiency is reported. However, gas deficiency in loosegroups is not yet well established. Lower dispersion of the membervelocities and the lower density of the intragroup medium in small loosegroups favour tidal stripping as the main gas removal process in them.Recent releases of data from the HI Parkes All-Sky Survey (HIPASS) andcatalogues of nearby loose groups with associated diffuse X-ray emissionhave allowed us to test this notion. In this paper, we address thefollowing questions: (i) do galaxies in groups with diffuse X-rayemission statistically have lower gas content compared to the ones ingroups without diffuse X-ray emission? (ii) does HI deficiency vary withthe X-ray luminosity, LX, of the loose group in a systematicway? We find that (i) galaxies in groups with diffuse X-ray emission, onaverage, are HI deficient, and have lost more gas compared to those ingroups without X-ray emission; the latter are found not to havesignificant HI deficiency; (ii) no systematic dependence of the HIdeficiency with LX is found. Ram-pressure-assisted tidalstripping and evaporation by thermal conduction are the two possiblemechanisms to account for this excess gas loss. Oxygen abundances in the most oxygen-rich spiral galaxiesOxygen abundances in the spiral galaxies expected to be richest inoxygen are estimated. The new abundance determinations are based on therecently discovered ff relation between auroral and nebular oxygen-linefluxes in high-metallicity HII regions. We find that the maximumgas-phase oxygen abundance in the central regions of spiral galaxies is12+log(O/H) ~ 8.75. This value is significantly lower (by a factor of>~5) than the previously accepted value. The central oxygen abundancein the Milky Way is similar to that in other large spirals. Hubble Space Telescope Observations of Star Clusters in M101Hubble Space Telescope Advanced Camera for Surveys (ACS) images are usedto identify and study star cluster candidates in the nearby spiralgalaxy M101. About 3000 round, slightly resolved cluster candidates areidentified in 10 ACS pointings covering an area of 106arcmin2. The cluster candidates' color and size distributionsare consistent with those of star clusters in other nearby spirals. Themajority of the M101 candidates are blue and more likely to beassociated with the galaxy's spiral arms, implying that they are young.The galaxy-luminosity-normalized number of young massive clusters inM101 is similar to that found in other spirals, as is the clusterdensity at a fiducial absolute magnitude. We confirm a previous findingthat M101 has a large number of faint red star clusters: if these areold globular clusters, then this galaxy has a very large globularcluster population. More plausible is that the faint red clusters arereddened young clusters; their colors and luminosities are alsoconsistent with this explanation.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555. These observations are associated withprograms 8640 and 9490. Star-forming knots and density wave in NGC 2997Context.Many grand design spiral galaxies show strings of bright knotsalong their arms on near-infrared K-band images. The alignment of suchknots suggests a relation to the spiral pattern and possibly to alarge-scale, star-forming front associated with a density wave. Aims.Bright knots in the southern arm of NGC 2997 were studied todetermine their nature and evolutionary state.Methods.Lowresolution near-infrared K- and J-band spectra of the knots wereobserved with ISAAC on the VLT. Results.Most of the knots showstrong H I Brγ emission with some also having He I and H2emission. A few knots show indications of 12CO absorption.This suggests that the knots are very young stellar clusters with massesup to 5×104 Mȯ.Conclusions.Theknots azimuthal distance from the K-band spiral correlates well withtheir Brγ strength, indicating that they are located inside theco-rotation of the density wave, which triggered them through alarge-scale, star-forming front. These relative azimuthal distancessuggest an age spread of more than 1.6 Myr, which is incompatible withstandard models for an instantaneous star burst. This indicates a morecomplex star-formation history, such as several bursts or continuousformation. Astrophysical magnetic fields and nonlinear dynamo theoryElectronic Article Available from Elsevier Science. An atlas of calcium triplet spectra of active galaxiesWe present a spectroscopic atlas of active galactic nuclei covering theregion around the λλ8498, 8542, 8662 calcium triplet(CaT). The sample comprises 78 objects, divided into 43 Seyfert 2s, 26Seyfert 1s, three starburst and six normal galaxies. The spectra pertainto the inner ~300 pc in radius, and thus sample the central kinematicsand stellar populations of active galaxies. The data are used to measurestellar velocity dispersions (σ*) with bothcross-correlation and direct fitting methods. These measurements arefound to be in good agreement with each other and with those in previousstudies for objects in common. The CaT equivalent width is alsomeasured. We find average values and sample dispersions ofWCaT of 4.6 +/- 2.0, 7.0 +/- 1.0 and 7.7 +/- 1.0 Å forSeyfert 1s, Seyfert 2s and normal galaxies, respectively. We furtherpresent an atlas of [SIII]λ9069 emission-line profiles for asubset of 40 galaxies. These data are analysed in a companion paperwhich addresses the connection between stellar and narrow-line regionkinematics, the behaviour of the CaT equivalent width as a function ofσ*, activity type and stellar population properties. New giant HII regions in the southern skyWe present results of a search for giant HII regions in southerngalaxies. Using high-resolution spectra, obtained with the MagellanInamori Kyocera Echelle (MIKE) at the Las Campanas Magellan IItelescope, we were able to resolve the emission-line profiles anddetermine the intrinsic velocity dispersion of the ionized gas. Out offour observed regions, selected from previous CCD narrow-bandphotometry, we detected three HII regions showing supersonic velocitydispersion, characteristic of giant HII regions, and their location indiagnostic diagrams suggests that a powerful starburst is the source ofionization energy. Images at 20cm and the Spectrum Index of NGC2997We present total intensity maps of the galaxy NGC 2997 at frequencies1435.1 MHz (λ21 cm) and 1652.4 MHz (λ18 cm) observed withthe Very Large Array (VLA). The high spatial resolution allows us todistinguish two dominant arms. Using multi-frequency data, we separatethe thermal and non-thermal contributions to the radio emission. Thethermal emission is about 35% of the total emission at λ3 cm. Weuse it to estimate a space-averaged thermal electron density, langlenerangle~0.049 cm-3 . The spectral index map showsthat the optical arms and the central part of the galaxy have flatterspectra. We also determine the flux densities of 22 radio sources in thefield-of-view of 25'. A VLT study of metal-rich extragalactic H II regions. I. Observations and empirical abundancesWe have obtained spectroscopic observations from 3600 Åto 9200Åwith FORS at the Very Large Telescope for approximately 70 Hiiregions located in the spiral galaxies NGC 1232, NGC 1365, NGC 2903,NGC 2997 and NGC 5236. These data are part of a project to measure thechemical abundances and characterize the massive stellar content ofmetal-rich extragalactic H iiregions. In this paper we describe ourdataset, and present emission line fluxes for the whole sample. In 32 Hiiregions we measure at least one of the following auroral lines: [S ii]λ4072, [N ii] λ5755, [S iii] λ6312 and [O ii]λ7325. From these we derive electron temperatures, as well asoxygen, nitrogen and sulphur abundances, using classical empiricalmethods (both so-called “Te-based methods” and“strong line methods”). Under the assumption that thetemperature does not introduce severe biases, we find that the mostmetal-rich nebulae with detected auroral lines are found at 12 +log(O/H) ≃ 8.9, i.e. about 60% larger than the adopted solarvalue. However, classical abundance determinations in metal-rich Hiiregions may be severely biased and must be tested with realisticphotoionization models. The spectroscopic observations presented in thispaper will serve as a homogeneous and high-quality database for suchpurposes. Cosmic magnetic fields - as observed in the Universe, in galactic dynamos, and in the Milky WayCosmic magnetism has that exotic Je ne sais quoi''! Magnetism has beenobserved in various objects, located near the edge of the Universe andall the way down to the Milky Way's center. The observed magnetic fieldcan take the cell-type shape in randomly-oriented large blobs found inintracluster gas or outside of clusters of galaxies, the helix shape insynchrotron jets, the longitudinal shape in ram-pressured shocks inradio lobes near elliptical galaxies, the spiral shape of logarithmicarms in spiral galaxies, or the egg shape of an enlarged interstellarbubble. In strength, the magnetic field varies from 0.1 nG(cosmological), to 20 μG (galaxies, jets, superbubbles), and to 1 mGin the Milky Way filaments.Magnetism plays a small physical role in the formation of largestructures. It acts as a tracer of the dynamical histories ofcosmological and intracluster events, it guides the motion of theinterstellar ionised gas, and it aligns the charged dust particles.Batteries and dynamos are often employed in models to create and amplifyseed magnetic fields. Starting soon after the Big Bang (redshiftz>2000), this review covers the cosmological background surface(z~1100, distance ~4.3 Gpc), the epoch of first stars (z~20 distance~4.1 Gpc), the currently observable Universe (z~10, distance ~3.9 Gpc),superclusters of galaxies (size ~50 Mpc), intracluster gas (size ~10Mpc), galaxies (~30 kpc), spiral arms (~10 kpc), interstellarsuperbubbles (~100 pc), synchrotron filaments (~10 pc), and the MilkyWay's center. A plausible Galactic spiral pattern and its rotation speed We report calculations of the stellar and gaseous response to a MilkyWay mass distribution model including a spiral pattern with a locus astraced by K-band observations, superimposed on the axisymmetriccomponents in the plane of the disc. The stellar study extendscalculations from previous work concerning the self-consistency of thepattern. The stellar response to the imposed spiral mass is studied viacomputations of the central family of periodic and nearby orbits as afunction of the pattern rotation speed, Ωp, among otherparameters. A fine grid of values of Ωp was explored,ranging from 12 to 25 km s-1 kpc-1. Dynamicalself-consistency is highly sensitive to Ωp, with thebest fit appearing at 20 km s-1 kpc-1. We give anaccount of recent independent pieces of theoretical and observationalwork that are dependent on the value of Ωp, all ofwhich are consistent with the value found here: the recent starformation history of the Milky Way, local inferences of cosmic ray fluxvariations and Galactic abundance patterns. The gaseous response, whichis also a function of Ωp, was calculated via 2Dhydrodynamic simulations with the ZEUS code. For Ωp= 20km s-1 kpc-1, the response to a two-armed patternis a structured pattern of four arms, with bifurcations along the armsand interarm features. The pattern qualitatively resembles the opticalarms observed in our Galaxy and other galaxies. The complex gaseouspattern appears to be linked to resonances in stellar orbits. Amongthese, the 4:1 resonance plays an important role, as it determines theextent of the stellar spiral pattern in the self-consistency studypresented here. Our findings seemingly confirm predictions by Drimmel& Spergel (2001), based on K-band data. Amplitude of Spiral Arms and Dark MatterRelative amplitudes and pitch angles for the main two-armed spiralstructure in a sample of 54 non-barred spiral galaxies were estimatedfrom K band photometry. A lack of strong, tight patterns was observedwhich may be caused by non-linear dynamic effects. Assuming that theupper envelope corresponds to a relative radial forcing of 5%, onecan estimate the perturbation of the total potential. This suggeststhat a maximum disk solution is adequate to account for the forceperturbation and that no additional dark matter is required. A Unified Theory for the Formation of Galactic StructuresA new theory for the formation of the main structures of galaxies isproposed: these structures are viewed as low-frequency normal modes indisks consisting of precessing stellar orbits. Mathematically, thetheory is based on an integral equation in the form of a classicaleigenvalue problem, with the eigenvalues being equal to the angularvelocities Ωp of the modes. Analysis of the general properties ofthe master integral equation (without finding concrete solutions) showsthat it admits two types of solutions: barlike and spiral. The numericalalgorithms are discussed and particular solutions of the integralequation are presented. If resonance interaction can be neglected, thebar mode represents a neutral perturbation of the disk. This mode can beamplified by the effect of the long-range gravitational field of themode on stars located in the vicinity of the corotation andouter-Lindblad resonances. Spiral perturbations are waves with zerototal angular momentum, and spiral modes are excited at theinner-Lindblad resonance. The approach proposed is compared to currentlyaccepted mechanisms for the formation of galactic structures. Inparticular, Toomre's application of the swing amplification mechanism toexplain the formation of global modes is critically discussed. Inaddition, we show that it is not correct to simulate the real stellarvelocity dispersion in a galaxy using softened gravity. Three-Dimensional Magnetohydrodynamic Modeling of the Gaseous Structure of the Galaxy: Description of the SimulationsAs we have discussed previously, the extra stiffness that a magneticfield adds to the interstellar medium (ISM) changes the way the ISMreacts to the presence of a spiral perturbation. At intermediate to highz, the gas shoots up before the arm, then flows over and falls behindthe arm as it approaches the next arm. This generates a multicellcirculation pattern, within each of which the net radial mass flux ispositive near the midplane and negative at higher z. The flow distortsthe magnetic field lines. In the arm region, the gas flows nearlyparallel to the arm, and therefore the magnetic field adopts a similarpitch angle. Between the arms, the gas flows out in radius, generating anegative pitch angle in the magnetic field. The intensity and directionof the field yield synthetic synchrotron maps that reproduce somefeatures of the synchrotron maps of external galaxies, such as theislands of emission and the displacement between the gaseous andsynchrotron arms. When comparing the magnitude of the field with thelocal gas density, two distinctive relations appear, depending onwhether the magnetic pressure is dominant. Above the plane, the densitystructure develops a shape resembling a breaking wave. This structurecollapses and rises again with a period of about 60 Myr, similar to thatof a vertical oscillation mode. The falling gas plays an important partin the overall hydrostatics, since its deceleration compresses the low-zgas, raising the average midplane pressure in the interarm region abovethat provided by the weight of the material above. Dynamical mass estimates for two luminous young stellar clusters in Messier 83Using new data from the UVES spectrograph on the ESO Very LargeTelescope and archive images from the Hubble Space Telescope, we havemeasured projected velocity dispersions and structural parameters fortwo bright young star clusters in the nearby spiral galaxy NGC5236. One cluster is located near the nuclear starburst of NGC5236, at a projected distance of 440 pc from the centre, while the otheris located in the disk of the galaxy at a projected galactocentricdistance of 2.3 kpc. We estimate virial masses for the two clusters of(4.2±0.7)×105 Mȯ and(5.2±0.8)×105 Mȯ and ages (frombroad-band photometry) of 107.1±0.2 years and108.0±0.1 years, respectively. Comparing the observedmass-to-light (M/L) ratios with simple stellar population models, wefind that the data for both clusters are consistent with a Kroupa-typestellar mass function (MF). In particular, we rule out any MF with asignificantly lower M/L ratio than the Kroupa MF, such as aSalpeter-like MF truncated at a mass of 1 Mȯ or higher.These clusters provide a good illustration of the fact that massive,globular cluster-like objects (super star clusters'') can form at thepresent epoch even in the disks of seemingly normal, undisturbed spiralgalaxies.Based on observations collected at the European Southern Observatory,Chile under programme 71.B-0303A, and on observations obtained with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555. Oxygen and nitrogen abundances in nearby galaxies. Correlations between oxygen abundance and macroscopic propertiesWe performed a compilation of more than 1000 published spectra of H IIregions in spiral galaxies. The oxygen and nitrogen abundances in each HII region were recomputed in a homogeneous way, using the P-method. Theradial distributions of oxygen and nitrogen abundances were derived. Thecorrelations between oxygen abundance and macroscopic properties areexamined. We found that the oxygen abundance in spiral galaxiescorrelates with its luminosity, rotation velocity, and morphologicaltype: the correlation with the rotation velocity may be slightlytighter. There is a significant difference between theluminosity-metallicity relationship obtained here and that based on theoxygen abundances determined through the R23-calibrations.The oxygen abundance of NGC 5457 recently determined using directmeasurements of Te (Kennicutt et al. \cite{Kennicutt2003})agrees with the luminosity-metallicity relationship derived in thispaper, but is in conflict with the luminosity-metallicity relationshipderived with the R23-based oxygen abundances. The obtainedluminosity-metallicity relation for spiral galaxies is compared to thatfor irregular galaxies. Our sample of galaxies shows evidence that theslope of the O/H - MB relationship for spirals (-0.079± 0.018) is slightly more shallow than that for irregulargalaxies (-0.139 ± 0.011). The effective oxygen yields wereestimated for spiral and irregular galaxies. The effective oxygen yieldincreases with increasing luminosity from MB  -11 toMB  -18 (or with increasing rotation velocity fromVrot  10 km s-1 to Vrot  100km s-1) and then remains approximately constant. Irregulargalaxies from our sample have effective oxygen yields lowered by afactor of 3 at maximum, i.e. irregular galaxies usually keep at least1/3 of the oxygen they manufactured during their evolution.Appendix, Tables \ref{table:refero}, \ref{table:referV}, and Figs.\ref{figure:sample2}-\ref{figure:sample5} are only available inelectronic form at http://www.edpsciences.org} Spiral galaxies observed in the near-infrared K band. I. Data analysis and structural parametersDeep surface photometry in the K band was obtained for 54 normal spiralgalaxies, with the aim of quantifying the percentage of faint bars andstudying the morphology of spiral arms. The sample was chosen to cover awider range of morphological types while inclination angles anddistances were limited to allow a detailed investigation of the internalstructure of their disks and future observations and studies of the diskkinematics. An additional constraint for a well defined subsample wasthat no bar structure was seen on images in the visual bands. Accuratesky projection parameters were determined from the K maps comparingseveral different methods. The surface brightness distribution wasdecomposed into axisymmetric components while bars and spiral structureswere analyzed using Fourier techniques.Bulges were best represented by a Sérsic r1/n law withan index in the typical range of 1-2. The central surface brightness ofthe exponential disk and bulge-to-disk ratio only showed weakcorrelation with Hubble type. Indications of a central point source werefound in many of the galaxies. An additional central, steep, exponentialdisk improved the fit for more than 80% of the galaxies suggesting thatmany of the bulges are oblate.Bars down to the detection level at a relative amplitude of 3% weredetected in 26 of 30 galaxies in a subsample classified as ordinary SAspirals. This would correspond to only 5% of all spiral galaxies beingnon-barred at this level. In several cases, bars are significantlyoffset compared to the starting points of the main spiral pattern whichindicates that bar and spiral have different pattern speeds. A smallfraction (10%) of the sample has complex central structuresconsisting of several sets of bars, arcs or spirals.A majority of the galaxies (60%) displays a two-armed, grand-designspiral pattern in their inner parts which often breaks up into multiplearms in their outer regions. Phase shifts between the inner and outerpatterns suggest in some cases that they belong to different spiralmodes. The pitch angles of the main two-armed symmetric spiral patternin the galaxies have a typical range of 5-30 °. The sample shows alack of strong, tight spirals which could indicate that such patternsare damped by non-linear, dynamical effects due to their high radialforce perturbations.Based on observations collected at the European Southern Observatory, LaSilla, Chile; programs: ESO 63.N-0343, 65.N-0287, 66.N-0257.Table 2 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/423/849Appendix A is only available in electronic form athttp://www.edpsciences.org The structure and environment of young stellar clusters in spiral galaxiesA search for stellar clusters has been carried out in 18 nearby spiralgalaxies, using archive images from the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope. All of the galaxies have previouslybeen imaged from the ground in UBVI. A catalogue of structuralparameters, photometry and comments based on visual inspection of theclusters is compiled and used to investigate correlations betweencluster structure, environment, age and mass. Least-squares fits to thedata suggest correlations between both the full-width at half-maximum(FWHM) and half-light radius (Reff) of the clusters and theirmasses (M) at about the 3σ level. Although both relations show alarge scatter, the fits have substantially shallower slopes than for aconstant-density relation (size ∝ M1/3). However, manyof the youngest clusters have extended halos which make theReff determinations uncertain. There is no evidence forgalaxy-to-galaxy variations in the mean cluster sizes. In particular,the mean sizes do not appear to depend on the host galaxy star formationrate surface density. Many of the youngest objects (age <107 years) are located in strongly crowded regions, and about1/3-1/2 of them are double or multiple sources. The HST images are alsoused to check the nature of cluster candidates identified in a previousground-based survey. The contamination rate in the ground-based sampleis generally less than about 20%, but some cluster identificationsremain ambiguous because of crowding even with HST imaging, especiallyfor the youngest objects.Full Tables \ref{tab:all}-\ref{tab:hstphot}, and \ref{tab:gb} are onlyavailable 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/416/537Based on observations obtained with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555. Disk-bulge decompositions of spiral galaxies in UBVRIA sample of 26 bright spiral galaxies (Btot < 12.7) withlow to medium inclination and without a bar was observed with UBVRIfilters. The CAFOS focal reducer camera at the Calar Alto 2.2 mtelescope was used. The surface-brightness distributions were fittedusing a 2-dimensional algorithm with corresponding functions for thedisk- and bulge-structure. For the disks an exponential function wasused, for the bulges a Sérsic Rβ law, was appliedwith the concentration parameter β = 1/n as another fit variable.Correlations of the resulting structural parameters of disks and bulgesin UBVRI are investigated, giving clues to the formation history of thebulges.We confirm that the large and bright bulges of early-type spirals aresimilar to elliptical galaxies. They were probably formed prior to thedisks in a monolithic collapse or via early mergers. Late-type spiralshave tiny and faint bulges with disk-like profiles. These bulges wereprobably formed after the disk in secular evolution processes, e.g. froma disk instability. The comparison of the color indices of correspondingspirals and bulges with population synthesis computations support aboveformation scenarios.Tables 2-4 are only 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/415/63 Star cluster formation and evolution in nearby starburst galaxies - II. Initial conditionsWe use the ages, masses and metallicities of the rich young star clustersystems in the nearby starburst galaxies NGC 3310 and 6745 to derivetheir cluster formation histories and subsequent evolution. We furtherexpand our analysis of the systematic uncertainties involved in the useof broad-band observations to derive these parameters (Paper I) byexamining the effects of a priori assumptions on the individual clustermetallicities. The age (and metallicity) distributions of both theclusters in the circumnuclear ring in NGC 3310 and of those outside thering are statistically indistinguishable, but there is a clear andsignificant excess of higher-mass clusters in the ring compared to thenon-ring cluster sample. It is likely that the physical conditions inthe starburst ring may be conducive for the formation of higher-massstar clusters, on average, than in the relatively more quiescentenvironment of the main galactic disc. For the NGC6745 cluster system wederive a median age of ~10 Myr. NGC6745 contains a significantpopulation of high-mass super star clusters', with masses in the range6.5 <~ log(Mcl/Msolar) <~ 8.0. Thisdetection supports the scenario that such objects form preferentially inthe extreme environments of interacting galaxies. The age of the clusterpopulations in both NGC3310 and 6745 is significantly lower than theirrespective characteristic cluster disruption time-scales, respectivelylog(tdis4/yr) = 8.05 and 7.75, for 104Msolar clusters. This allows us to obtain an independentestimate of the initial cluster mass function slope, α=2.04(+/-0.23)+0.13-0.43 for NGC3310, and1.96(+/-0.15) +/- 0.19 for NGC6745, respectively, for massesMcl>~ 105 Msolar andMcl>~ 4 × 105 Msolar. Thesemass function slopes are consistent with those of other young starcluster systems in interacting and starburst galaxies. Magnetohydrodynamic density waves in a composite disc system of interstellar medium and cosmic ray gasMultiwavelength observations from radio to soft X-ray bands oflarge-scale galactic spiral structures offer synthesized andcomprehensive views of nearby disc galaxies. In the presence of amassive dark matter halo, the density-wave dynamics on galactic scalesinvolves the stellar disc, the gas disc of the interstellar medium(ISM), the magnetic field and the cosmic ray gas (CRG). In this paper,we explore the dynamic and electromagnetic interplay between themagnetized ISM disc and the CRG disc so that structural and diagnosticfeatures of optical, infrared and synchrotron radio-continuum emissionsfrom a spiral galaxy can be understood physically. On time-scales ofgalactic density waves, cosmic rays collectively may be treated as arelativistically hot tenuous gas fluid that is tied to the large-scalemean magnetic field in transverse bulk motions but moves otherwisedifferently along the magnetic field relative to the ISM. For both fastand slow magnetohydrodynamic (MHD) density waves in a composite discsystem of magnetized ISM and CRG, the minute CRG mass-densityenhancement is phase shifted relative to the enhancement of the parallelmagnetic field. Owing to the extremely small number of cosmic rays, thelarge-scale magnetic field enhancement dominates in synchrotronradio-continuum emissions (as if the CRG is almost unperturbed) forspiral structural manifestations. In addition to the fast and slow MHDdensity waves, there also exists a suprathermal MHD wave mode by whichCRG adjusts itself with an effective suprathermal speed of sound closeto the speed of light c. Supernova 2003jg in NGC 2997IAUC 8236 available at Central Bureau for Astronomical Telegrams. Supernova 2003jg in NGC 2997IAUC 8235 available at Central Bureau for Astronomical Telegrams. The 2MASS Large Galaxy AtlasWe present the largest galaxies as seen in the near-infrared (1-2μm), imaged with the Two Micron All Sky Survey (2MASS), ranging inangular size from 1' to 1.5d. We highlight the 100 largest in thesample. The galaxies span all Hubble morphological types, includingelliptical galaxies, normal and barred spirals, and dwarf and peculiarclasses. The 2MASS Large Galaxy Atlas provides the necessary sensitivityand angular resolution to examine in detail morphologies in thenear-infrared, which may be radically different from those in theoptical. Internal structures such as spirals, bulges, warps, rings,bars, and star formation regions are resolved by 2MASS. In addition tolarge mosaic images, the atlas includes astrometric, photometric, andshape global measurements for each galaxy. A comparison of fundamentalmeasures (e.g., surface brightness, Hubble type) is carried out for thesample and compared with the Third Reference Catalogue. We furthershowcase NGC 253 and M51 (NGC 5194/5195) to demonstrate the quality anddepth of the data. The atlas represents the first uniform, all-sky,dust-penetrated view of galaxies of every type, as seen in thenear-infrared wavelength window that is most sensitive to the dominantmass component of galaxies. The images and catalogs are availablethrough the NASA/IPAC Extragalactic Database and Infrared ScienceArchive and are part of the 2MASS Extended Source Catalog. Simulations of dual morphology in spiral galaxiesGas and stars in spiral galaxies are modelled with a combination ofhydrodynamic and N-body techniques. The simulations reveal morphologicaldifferences mirroring the dual morphologies seen in B and K' bandobservations of many spiral galaxies: gaseous images have tighter pitchangles, are more asymmetric, more flocculent and more likely to havemultiple arms. Morphological decoupling increases as the stellararm-interarm contrast and the Q parameter fall. The flocculence of agalaxy is quantified by decomposing the images into logarithmic spiralsand defining a parameter closely related to the uniformity of theresulting 2D Fourier spectrum. Thus, a significant amount ofmorphological decoupling in spiral galaxies is shown to be due to thedifference in the dynamics of stars and gas, rather than dust, starformation or galaxy interactions. Astronomical image representation by the curvelet transformWe outline digital implementations of two newly developed multiscalerepresentation systems, namely, the ridgelet and curvelet transforms. Weapply these digital transforms to the problem of restoring an image fromnoisy data and compare our results with those obtained via wellestablished methods based on the thresholding of wavelet coefficients.We show that the curvelet transform allows us also to well enhanceelongated features contained in the data. Finally, we describe theMorphological Component Analysis, which consists in separating featuresin an image which do not present the same morphological characteristics.A range of examples illustrates the results. On the origin of nitrogenThe problem of the origin of nitrogen is considered within the frameworkof an empirical approach. The oxygen abundances and nitrogen to oxygenabundances ratios are derived in H II regions of a number of spiralgalaxies through the recently suggested P-method using more than sixhundred published spectra. The N/O-O/H diagram for H II regions inirregular and spiral galaxies is constructed. It is found that the N/Ovalues in H II regions of spiral galaxies of early morphological typesare higher than those in H II regions with the same metallicity inspiral galaxies of late morphological types. This suggests along-time-delayed contribution to the nitrogen production. The N/O ratioof a galaxy can then be used as an indicator of the time that haselapsed since the bulk of star formation occurred, or in other words ofthe nominal `age'' of the galaxy as suggested by Edmunds & Pagelmore than twenty years ago. The scatter in N/O values at a given O/H canbe naturally explained by differences in star formation histories ingalaxies. While low-metallicity dwarf galaxies with low N/O do notcontain an appreciable amount of old stars, low-metallicity dwarfgalaxies with an appreciable fraction of old stars have high N/O.Consideration of planetary nebulae in the Small Magellanic Cloud and inthe Milky Way Galaxy suggests that the contribution of low-mass stars tothe nitrogen production is significant, confirming the conclusion thatthere is a long-time-delayed contribution to the nitrogen production. Quarter-turn spirals just beyond the principal arms of galaxiesObservations in the optical show that grand design spirals consist of aset of principal arms and characteristic near-circular extensions thatcan be described as quarter-turn spirals. Arguments are presented infavour of the idea that the latter set of spirals is merely caused bythe response of the material of the disc to the gravitational potentialof the main spiral arms. Stationary models for fast and slow logarithmic spiral patterns in disc galaxiesA recent wavelet analysis on multiwavelength image data of the nearbyspiral galaxy NGC 6946 revealed a multi-arm spiral structure thatpersists well into the outer differentially rotating disc region. Theextended spiral arms in polarized radio-continuum emission and in redlight appear interlaced with each other, while the spiral arms inemissions of total radio continuum, of Hα from HII regions, and ofneutral hydrogen all trace the red-light spiral arms, although to asomewhat lesser extent. The key issue now becomes how to sustainextended slow magnetohydrodynamic (MHD) density wave features in a thinmagnetized disc with a flat rotation curve. We describe here atheoretical model to examine stationary non-axisymmetric logarithmicspiral configurations constructed from a background equilibrium of amagnetized singular isothermal disc (MSID) with a flat rotation curveand with a non-force-free azimuthal magnetic field. It is foundanalytically that two types of stationary spiral MSID configurations mayexist, physically corresponding to the two possibilities of fast andslow spiral MHD density waves. Such stationary MHD density waves arepossible only at proper MSID rotation speeds. For the fast MSIDconfiguration, logarithmic spiral enhancements of magnetic field and gasdensity are either in phase in the tight-winding regime or shifted witha spatial phase difference >~π/2 for open spiral structures. Forthe slow MSID configuration, logarithmic spiral enhancements of magneticfield and gas density are either out of phase in the tight-windingregime or shifted with a spatial phase difference >~π/2 for openspiral structures and persist in a flat rotation curve. For NGC 6946,several pertinent aspects of the slow MSID scenario with stationarylogarithmic spiral arms are discussed. The two exact solutions can bealso utilized to test relevant numerical MHD codes. Milestones in the Observations of Cosmic Magnetic FieldsMagnetic fields are observed everywhere in the universe. In this review,we concentrate on the observational aspects of the magnetic fields ofGalactic and extragalactic objects. Readers can follow the milestones inthe observations of cosmic magnetic fields obtained from the mostimportant tracers of magnetic fields, namely, the star-lightpolarization, the Zeeman effect, the rotation measures (RMs,hereafter)of extragalactic radio sources, the pulsar RMs, radio polarizationobservations, as well as the newly implemented sub-mm and mmpolarization capabilities. The magnetic field of the Galaxy was firstdiscovered in 1949 by optical polarization observations. The localmagnetic fields within one or two kpc have been well delineated bystarlight polarization data. The polarization observations of diffuseGalactic radio background emission in 1962 confirmed unequivocally theexistence of a Galactic magnetic field.The bulk of the presentinformation about the magnetic fields in the Galaxy comes from analysisof rotation measures of extragalactic radio sources and pulsars, whichcan be used to construct the 3-D magnetic field structure in theGalactic halo and Galactic disk. Radio synchrotron spurs in the Galacticcenter show a poloidal field, and the polarization mapping of dustemission and Zeeman observation in the central molecular zone reveal atoroidal magnetic field parallel to the Galactic plane. For nearbygalaxies, both optical polarization and multifrequency radiopolarization data clearly show the large-scale magnetic field followingthe spiral arms or dust lanes. For more distant objects, radiopolarization is the only approach available to show the magnetic fieldsin the jets or lobes of radio galaxies or quasars. Clusters of galaxiesalso contain widely distributed magnetic fields, which are reflected byradio halos or the RM distribution of background objects. Theintergalactic space could have been magnetized by outflows or galacticsuperwinds even in the early universe. The Zeeman effect andpolarization of sub-mm and mm emission can be used for the study ofmagnetic fields in some Galactic molecular clouds but it is observedonly at high intensity. Both approaches together can clearly show therole that magnetic fields play in star formation and cloud structure,which in principle would be analogous to galaxy formation fromprotogalactic clouds. The origin of the cosmic magnetic fields is anactive field of research. A primordial magnetic field has not been asyet directly detected, but its existence must be considered to give theseed field necessary for many amplification processes that have beendeveloped. Possibly, the magnetic fields were generated in protogalacticplasma clouds by the dynamo process, and maintained again by the dynamoafter galaxies were formed.

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 星座: 唧筒座 右阿森松: 09h45m39.30s 赤纬: -31Â°11'28.0" 明显尺寸: 9.772′ × 6.918′

 适当名称 (Edit) NGC 2000.0 NGC 2997 HYPERLEDA-I PGC 27978 → 要求更多目录从vizier