Spiral Galaxies (spiral + galaxy)

Distribution by Scientific Domains

Terms modified by Spiral Galaxies

  • spiral galaxy ngc

  • Selected Abstracts

    Chiral asymmetry in spiral galaxies?

    CHIRALITY, Issue 7 2001
    Dilip K. Kondepudi
    Abstract Spiral galaxies are chiral entities when coupled with the direction of their recession velocity. As viewed from the Earth, the S-shaped and Z-shaped spiral galaxies are two chiral forms. What is the nature of chiral symmetry in spiral galaxies? In the Carnegie Atlas of Galaxies that lists photographs of a total of 1,168 galaxies, we found 540 galaxies, classified as normal or barred spirals, that are clearly identifiable as S- or Z- type. The recession velocities for 538 of these galaxies could be obtained from this atlas and other sources. A statistical analysis of this sample reveals no overall asymmetry but there is a significant asymmetry in certain subclasses: dominance of S-type galaxies in the Sb class of normal spiral galaxies and a dominance of Z-type in the SBb class of barred spiral galaxies. Both S- and Z-type galaxies seem to have similar velocity distribution, indicating no spatial segregation of the two chiral forms. Chirality 13:351,356, 2001. © 2001 Wiley-Liss, Inc. [source]

    Structures in the fundamental plane of early-type galaxies

    D. Fraix-Burnet
    ABSTRACT The fundamental plane of early-type galaxies is a rather tight three-parameter correlation discovered more than 20 yr ago. It has resisted both a global and precise physical interpretation despite a consequent number of works, observational, theoretical or using numerical simulations. It appears that its precise properties depend on the population of galaxies in study. Instead of selecting a priori these populations, we propose to objectively construct homologous populations from multivariate analyses. We have undertaken multivariate cluster and cladistic analyses of a sample of 56 low-redshift galaxy clusters containing 699 early-type galaxies, using four parameters: effective radius, velocity dispersion, surface brightness averaged over effective radius and Mg2 index. All our analyses are consistent with seven groups that define separate regions on the global fundamental plane, not across its thickness. In fact, each group shows its own fundamental plane, which is more loosely defined for less diversified groups. We conclude that the global fundamental plane is not a bent surface, but made of a collection of several groups characterizing several fundamental planes with different thicknesses and orientations in the parameter space. Our diversification scenario probably indicates that the level of diversity is linked to the number and the nature of transforming events and that the fundamental plane is the result of several transforming events. We also show that our classification, not the fundamental planes, is universal within our redshift range (0.007,0.053). We find that the three groups with the thinnest fundamental planes presumably formed through dissipative (wet) mergers. In one of them, this(ese) merger(s) must have been quite ancient because of the relatively low metallicity of its galaxies, Two of these groups have subsequently undergone dry mergers to increase their masses. In the k-space, the third one clearly occupies the region where bulges (of lenticular or spiral galaxies) lie and might also have formed through minor mergers and accretions. The two least diversified groups probably did not form by major mergers and must have been strongly affected by interactions, some of the gas in the objects of one of these groups having possibly been swept out. The interpretation, based on specific assembly histories of galaxies of our seven groups, shows that they are truly homologous. They were obtained directly from several observables, thus independently of any a priori classification. The diversification scenario relating these groups does not depend on models or numerical simulations, but is objectively provided by the cladistic analysis. Consequently, our classification is more easily compared to models and numerical simulations, and our work can be readily repeated with additional observables. [source]

    Radiative transfer in disc galaxies , IV.

    The effects of dust attenuation on bulge, disc structural parameters
    ABSTRACT Combining Monte Carlo radiative transfer simulations and accurate 2D bulge/disc decompositions, we present a new study to investigate the effects of dust attenuation on the apparent structural properties of the disc and bulge of spiral galaxies. We find that dust affects the results from such decompositions in ways which cannot be identified when one studies dust effects on bulge and disc components separately. In particular, the effects of dust in galaxies hosting pseudo-bulges might be different from those in galaxies hosting classical bulges, even if their dust content is identical. Confirming previous results, we find that disc scalelengths are overestimated when dust effects are important. In addition, we also find that bulge effective radii and Sérsic indices are underestimated. Furthermore, the apparent attenuation of the integrated disc light is underestimated, whereas the corresponding attenuation of bulge light is overestimated. Dust effects are more significant for the bulge parameters, and, combined, they lead to a strong underestimation of the bulge-to-disc ratio, which can reach a factor of 2 in the V band, even at relatively low galaxy inclinations and dust opacities. Nevertheless, it never reaches factors larger than about 3, which corresponds to a factor of 2 in bulge-to-total ratio. Such effect can have an impact on studies of the black hole/bulge scaling relations. [source]

    The disc-dominated host galaxy of FR-I radio source B2 0722+30

    B. H. C. Emonts
    ABSTRACT We present new observational results that conclude that the nearby radio galaxy B2 0722+30 is one of the very few known disc galaxies in the low-redshift Universe that host a classical double-lobed radio source. In this paper, we use H i observations, deep optical imaging, stellar population synthesis modelling and emission-line diagnostics to study the host galaxy, classify the active galactic nucleus (AGN) and investigate environmental properties under which a radio-loud AGN can occur in this system. Typical for spiral galaxies, B2 0722+30 has a regularly rotating gaseous disc throughout which star formation occurs. Dust heating by the ongoing star formation is likely responsible for the high infrared luminosity of the system. The optical emission-line properties of the central region identify a Low Ionization Nuclear Emission-line Region (LINER)-type nucleus with a relatively low [O iii] luminosity, in particular when compared with the total power of the Fanaroff & Riley type-I radio source that is present in this system. This classifies B2 0722+30 as a classical radio galaxy rather than a typical Seyfert galaxy. The environment of B2 0722+30 is extremely H i -rich, with several nearby interacting galaxies. We argue that a gas-rich interaction involving B2 0722+30 is a likely cause for the triggering of the radio AGN and/or the fact that the radio source managed to escape the optical boundaries of the host galaxy. [source]

    Dynamical response to supernova-induced gas removal in spiral galaxies with dark matter halo

    Hiroko Koyama
    ABSTRACT We investigate the dynamical response, in terms of disc size and rotation velocity, to mass loss by supernovae in the evolution of spiral galaxies. A thin baryonic disc having the Kuzmin density profile embedded in a spherical dark matter halo having a density profile proposed by Navarro, Frenk & White is considered. For the purpose of comparison, we also consider the homogeneous and r,1 profiles for dark matter in a truncated spherical halo. Assuming for simplicity that the dark matter distribution is not affected by mass-loss from discs and the change of baryonic disc matter distribution is homologous, we evaluate the effects of dynamical response in the resulting discs. We found that the dynamical response only for an adiabatic approximation of mass-loss can simultaneously account for the rotation velocity and disc size as observed particularly in dwarf spiral galaxies, thus reproducing the Tully,Fisher relation and the size versus magnitude relation over the full range of magnitude. Furthermore, we found that the mean specific angular momentum in discs after the mass-loss becomes larger than that before the mass-loss, suggesting that the mass-loss would most likely occur from the central disc region where the specific angular momentum is low. [source]

    The disc mass of spiral galaxies

    Paolo Salucci
    ABSTRACT We derive the disc masses of 18 spiral galaxies of different luminosity and Hubble type, both by mass modelling their rotation curves and by fitting their spectral energy distribution with spectrophotometric models. The good agreement of the estimates obtained from these two different methods allows us to quantify the reliability of their performance and to derive very accurate stellar mass-to-light ratio versus colour (and stellar mass) relationships. [source]

    Global m= 1 instabilities and lopsidedness in disc galaxies

    V. Dury
    ABSTRACT Lopsidedness is common in spiral galaxies. Often, there is no obvious external cause, such as an interaction with a nearby galaxy, for such features. Alternatively, the lopsidedness may have an internal cause, such as a dynamical instability. In order to explore this idea, we have developed a computer code that searches for self-consistent perturbations in razor-thin disc galaxies and performed a thorough mode-analysis of a suite of dynamical models for disc galaxies embedded in an inert dark matter halo with varying amounts of rotation and radial anisotropy. Models with two equal-mass counter-rotating discs and fully rotating models both show growing lopsided modes. For the counter-rotating models, this is the well-known counter-rotating instability, becoming weaker as the net rotation increases. The m= 1 mode of the maximally rotating models, on the other hand, becomes stronger with increasing net rotation. This rotating m= 1 mode is reminiscent of the eccentricity instability in near-Keplerian discs. To unravel the physical origin of these two different m= 1 instabilities, we studied the individual stellar orbits in the perturbed potential and found that the presence of the perturbation gives rise to a very rich orbital behaviour. In the linear regime, both instabilities are supported by aligned loop orbits. In the non-linear regime, other orbit families exist that can help support the modes. In terms of density waves, the counter-rotating m= 1 mode is due to a purely growing Jeans-type instability. The rotating m= 1 mode, on the other hand, grows as a result of the swing amplifier working inside the resonance cavity that extends from the disc centre out to the radius where non-rotating waves are stabilized by the model's outwardly rising Q profile. [source]

    Spatial distribution of luminous X-ray binaries in spiral galaxies

    Zhao-yu Zuo
    ABSTRACT We have modelled the spatial distribution of luminous X-ray binaries (XRBs) in spiral galaxies that are like the Milky Way using an evolutionary population synthesis code. In agreement with previous theoretical expectations and observations, we find that both high- and low-mass XRBs show clear concentrations towards the galactic plane and bulge. We also compare XRB distributions under the galactic potential with a dark matter halo and the modified Newtonian dynamics potential, and we suggest that the difference may serve as potential evidence to discriminate between these two types of model. [source]

    GHASP: an H, kinematic survey of spiral and irregular galaxies , V. Dark matter distribution in 36 nearby spiral galaxies

    M. Spano
    ABSTRACT The results obtained from a study of the mass distribution of 36 spiral galaxies are presented. The galaxies were observed using Fabry,Perot interferometry as part of the GHASP survey. The main aim of obtaining high-resolution H, 2D velocity fields is to define more accurately the rising part of the rotation curves which should allow to better constrain the parameters of the mass distribution. The H, velocities were combined with low resolution H i data from the literature, when available. Combining the kinematical data with photometric data, mass models were derived from these rotation curves using two different functional forms for the halo: an isothermal sphere (ISO) and a Navarro,Frenk,White (NFW) profile. For the galaxies already modelled by other authors, the results tend to agree. Our results point at the existence of a constant density core in the centre of the dark matter haloes rather than a cuspy core, whatever the type of the galaxy from Sab to Im. This extends to all types the result already obtained by other authors studying dwarf and low surface brightness galaxies but would necessitate a larger sample of galaxies to conclude more strongly. Whatever model is used (ISO or NFW), small core radius haloes have higher central densities, again for all morphological types. We confirm different halo scaling laws, such as the correlations between the core radius and the central density of the halo with the absolute magnitude of a galaxy: low-luminosity galaxies have small core radius and high central density. We find that the product of the central density with the core radius of the dark matter halo is nearly constant, whatever the model and whatever the absolute magnitude of the galaxy. This suggests that the halo surface density is independent from the galaxy type. [source]

    The baryonic and dark matter properties of high-redshift gravitationally lensed disc galaxies

    P. Salucci
    ABSTRACT We present a detailed study of the structural properties of four gravitationally lensed disc galaxies at z= 1. Modelling the rotation curves on sub-kpc scales, we derive the values for the disc mass, the reference dark matter density and core radius, and the angular momentum per unit mass. The derived models suggest that the rotation curve profile and amplitude are best fitted with a dark matter component similar to those of local spiral galaxies. The stellar component also has a similar length-scale, but with substantially smaller masses than similarly luminous disc galaxies in the local Universe. Comparing the average dark matter density inside the optical radius, we find that the disc galaxies at z= 1 have larger densities (by up to a factor of ,7) than similar disc galaxies in the local Universe. Furthermore, the angular momentum per unit mass versus reference velocity is well matched to the local relation, suggesting that the angular momentum of the disc remains constant between high redshifts and the present day. Though statistically limited, these observations point towards a spirals' formation scenario in which stellar discs are slowly grown by the accretion of angular momentum conserving material. [source]

    Global lopsided instability in a purely stellar galactic disc

    Kanak Saha
    ABSTRACT It is shown that pure exponential discs in spiral galaxies are capable of supporting slowly varying discrete global lopsided modes, which can explain the observed features of lopsidedness in the stellar discs. Using linearized fluid dynamical equations with the softened self-gravity and pressure of the perturbation as the collective effect, we derive self-consistently a quadratic eigenvalue equation for the lopsided perturbation in the galactic disc. On solving this, we find that the ground-state mode shows the observed characteristics of the lopsidedness in a galactic disc, namely the fractional Fourier amplitude A1, increases smoothly with the radius. These lopsided patterns precess in the disc with a very slow pattern speed with no preferred sense of precession. We show that the lopsided modes in the stellar disc are long-lived because of a substantial reduction (approximately a factor of 10 compared to the local free precession rate) in the differential precession. The numerical solution of the equations shows that the ground-state lopsided modes are either very slowly precessing stationary normal mode oscillations of the disc or growing modes with a slow growth rate depending on the relative importance of the collective effect of the self-gravity. N -body simulations are performed to test the spontaneous growth of lopsidedness in a pure stellar disc. Both approaches are then compared and interpreted in terms of long-lived global m= 1 instabilities, with almost zero pattern speed. [source]

    Testing modified Newtonian dynamic with Local Group spiral galaxies

    Edvige Corbelli
    ABSTRACT The rotation curves and the relative mass distributions of the two nearby Local Group spiral galaxies, M31 and M33, show discrepancies with modified Newtonian dynamic (MOND) predictions. In M33, the discrepancy lies in the kinematics of the outermost regions. It can be alleviated by adopting tilted ring models compatible with the 21-cm datacube but different from the one that best fits the data. In M31, MOND fails to fit the falling part of the rotation curve at intermediate radii, before the curve flattens out in the outermost regions. Newtonian dynamics in a framework of a stellar disc embedded in a dark halo can explain the complex rotation curve profiles of these two galaxies, while MOND has some difficulties. However, given the present uncertainties in the kinematics of these nearby galaxies, we cannot address the success or failure of MOND theory in a definite way. More sensitive and extended observations around the critical regions, suggested by MOND fits discussed in this paper, may lead to a definite conclusion. [source]

    The formation of molecular clouds in spiral galaxies

    C. L. Dobbs
    ABSTRACT We present smoothed particle hydrodynamics simulations of molecular cloud formation in spiral galaxies. These simulations model the response of a non-self-gravitating gaseous disc to a galactic potential. The spiral shock induces high densities in the gas, and considerable structure in the spiral arms, which we identify as molecular clouds. We regard the formation of these structures as due to the dynamics of clumpy shocks, which perturb the flow of gas through the spiral arms. In addition, the spiral shocks induce a large velocity dispersion in the spiral arms, comparable with the magnitude of the velocity dispersion observed in molecular clouds. We estimate the formation of molecular hydrogen, by post-processing our results and assuming the gas is isothermal. Provided the gas is cold (T, 100 K), the gas is compressed sufficiently in the spiral shock for molecular hydrogen formation to occur in the dense spiral arm clumps. These molecular clouds are largely confined to the spiral arms, since most molecular gas is photodissociated to atomic hydrogen upon leaving the arms. [source]

    Stellar haloes and elliptical galaxy formation: origin of dynamical properties of the planetary nebula systems

    Kenji Bekki
    ABSTRACT Recent spectroscopic observations of planetary nebulae (PNe) in several elliptical galaxies have revealed structural and kinematical properties of the outer stellar halo regions. In order to elucidate the origin of the properties of these planetary nebula systems (PNSs), we consider the merger scenario in which an elliptical galaxy is formed by merging of spiral galaxies. Using numerical simulations, we particularly investigate radial profiles of projected PN number densities, rotational velocities and velocity dispersions of PNSs extending to the outer halo regions of elliptical galaxies formed from major and unequal-mass merging. We find that the radial profiles of the project number densities can be fitted to the power law and the mean number density in the outer haloes of the ellipticals can be more than an order of magnitude higher than that of the original spiral's halo. The PNSs are found to show a significant amount of rotation (V/, > 0.5) in the outer halo regions (R > 5Re) of the ellipticals. Two-dimensional velocity fields of PNSs are derived from the simulations and their dependences on model parameters of galaxy merging are discussed in detail. We compare the simulated kinematics of PNSs with that of the PNS observed in NGC 5128 and thereby discuss advantages and disadvantages of the merger model in explaining the observed kinematics of the PNS. We also find that the kinematics of PNSs in elliptical galaxies are quite diverse depending on the orbital configurations of galaxy merging, the mass ratio of merger progenitor spirals and the viewing angle of the galaxies. This variation translates directly into possible biases by a factor of 2 in observational mass estimation. However, the biases in the total mass estimates can be even larger. The best case systems viewed edge-on can appear to have masses lower than their true mass by a factor of 5, which suggests that current observational studies on PN kinematics of elliptical galaxies can significantly underestimate their real masses. [source]

    Australia Telescope Compact Array H i observations of the NGC 6845 galaxy group

    Scott Gordon
    ABSTRACT We present the results of Australia Telescope Compact Array (ATCA) H i line and 20-cm radio continuum observations of the galaxy quartet NGC 6845. The H i emission extends over all four galaxies but can only be associated clearly with the two spiral galaxies, NGC 6845A and B, which show signs of strong tidal interaction. We derive a total H i mass of at least 1.8 × 1010 M,, most of which is associated with NGC 6845A, the largest galaxy of the group. We investigate the tidal interaction between NGC 6845A and B by studying the kinematics of distinct H i components and their relation to the known H ii regions. No H i emission is detected from the two lenticular galaxies, NGC 6845C and D. A previously uncatalogued dwarf galaxy, ATCA J2001,4659, was detected 4.4 arcmin NE from NGC 6845B and has an H i mass of ,5 × 108 M,. No H i bridge is visible between the group and its newly detected companion. Extended 20-cm radio continuum emission is detected in NGC 6845A and B as well as in the tidal bridge between the two galaxies. We derive star formation rates of 15,40 M, yr,1. [source]

    The host galaxies of Type Ia supernovae at z= 0.6

    D. Farrah
    ABSTRACT We examine the host galaxies of high-redshift Type Ia supernovae (SNe Ia) using archival I - and R -band data from the Hubble Space Telescope. The SNe Ia host galaxies show a wide variety of morphologies, including undisturbed ellipticals, spirals and disturbed systems. SNe Ia are also found over a wide range of projected distances from the host galaxy centres, ranging from 3 kpc to ,30 kpc. For a sample of 22 SNe Ia at ,z,= 0.6, ,70 per cent are found in spiral galaxies and ,30 per cent are found in elliptical systems, similar to the proportions observed locally. Including data from Ellis & Sullivan (2001), we find no significant difference in the average light-curve shape-corrected MBpeak for high- z SNe Ia between spirals and ellipticals. These results are consistent with predictions based on the locally derived understanding of SNe Ia physics and the influence of progenitor mass and metallicity. We also construct colour maps for two host galaxies and find that both show a non-uniform colour structure with typical variations of rest-frame B,V, 0.5. This is most plausibly attributed to the presence of, and variation in, dust extinction in these galaxies. Moreover, we find no evidence that the SNe Ia are preferentially found in outer regions (> 10 kpc) of the host galaxies where extinction would be low. This suggests that the range of host galaxy extinctions of SNe Ia at z, 0.6 should be comparable to those of local SNe Ia. Although observational bias cannot be completely ruled out, this appears to be in conflict with the finding of low extinction for SNe Ia found in the high- z supernova search studies. [source]

    The SCUBA Local Universe Galaxy Survey , II.

    m data: evidence for cold dust in bright IRAS galaxies
    This is the second in a series of papers presenting results from the SCUBA Local Universe Galaxy Survey. In our first paper we provided 850-,m flux densities for 104 galaxies selected from the IRAS Bright Galaxy Sample and we found that the 60-, 100-,m (IRAS) and 850-,m (SCUBA) fluxes could be adequately fitted by emission from dust at a single temperature. In this paper we present 450-,m data for the galaxies. With the new data, the spectral energy distributions of the galaxies can no longer be fitted with an isothermal dust model , two temperature components are now required. Using our 450-,m data and fluxes from the literature, we find that the 450/850-,m flux ratio for the galaxies is remarkably constant, and this holds from objects in which the star formation rate is similar to our own Galaxy, to ultraluminous infrared galaxies (ULIRGs) such as Arp 220. The only possible explanation for this is if the dust emissivity index for all of the galaxies is ,2 and the cold dust component has a similar temperature in all galaxies . The 60-,m luminosities of the galaxies were found to depend on both the dust mass and the relative amount of energy in the warm component, with a tendency for the temperature effects to dominate at the highest L60. The dust masses estimated using the new temperatures are higher by a factor of ,2 than those determined previously using a single temperature. This brings the gas-to-dust ratios of the IRAS galaxies into agreement with those of the Milky Way and other spiral galaxies which have been intensively studied in the submm. [source]

    Broad-band colours of Virgo cluster low surface brightness dwarf irregular galaxies

    Ana B. Heller
    We present UBVRI images and surface photometry of a complete sample of 29 low-luminosity dwarf irregular galaxies in the Virgo cluster, for which we derive central surface brightnesses, scalelengths, integrated magnitudes and median colours. The colour distributions are discussed in terms of radial surface brightness profiles, and colour gradients are interpreted and compared with corresponding ones for low surface-brightness (LSB) spiral galaxies. By combining broad-band and narrow-band filter observations, the past and current influences of the cluster environment on the evolution of LSB dwarf irregular galaxies is evaluated. [source]

    The constant-density region of the dark haloes of spiral galaxies

    Paolo Salucci
    We determine a crucial feature of the dark halo density distribution from the fact that the luminous matter dominates the gravitational potential at about one disc scalelength Rd, but at the optical edge the dark matter has already become the main component of the galaxy density. From the kinematics of 137 spirals we find that the dark matter halo density profiles are self-similar at least out to Ropt and show core radii much larger than the corresponding disc scalelengths. The luminous regions of spirals consist of stellar discs embedded in dark haloes with roughly constant density. This invariant dark matter profile is very difficult to reconcile with the fundamental properties of the density distribution of cold dark matter haloes. With respect to previous work, the present evidence is obtained by means of a robust method and for a large and complete sample of normal spirals. [source]

    The Tully,Fisher relation and its implications for the halo density profile and self-interacting dark matter

    H. J. Mo
    We show that the Tully,Fisher relation observed for spiral galaxies can be explained in the current scenario of galaxy formation without invoking subtle assumptions, provided that galactic-sized dark haloes have low concentrations which do not change significantly with halo circular velocity. This conclusion does not depend significantly on whether haloes have cuspy or flat profiles in the inner region. In such a system, both the disc and the halo may contribute significantly to the maximum rotation of the disc, and the gravitational interaction between the disc and halo components leads to a tight relation between the disc mass and maximum rotation velocity. The model can therefore be tested by studying the Tully,Fisher zero points for galaxies with different disc mass-to-light ratios. With model parameters (such as the ratio between disc and halo mass, the specific angular momentum of disc material, disc formation time) chosen in plausible ranges, the model can well accommodate the zero-point, slope and scatter of the observed Tully,Fisher relation, as well as the observed large range of disc surface densities and sizes. In particular, the model predicts that low surface brightness disc galaxies obey a Tully,Fisher relation very similar to that of normal discs, if the disc mass-to-light ratio is properly taken into account. About half of the gravitational force at maximum rotation comes from the disc component for normal discs, while the disc contribution is lower for galaxies with a lower surface density. The halo profile required by the Tully,Fisher relation is as concentrated as that required by the observed rotation curves of faint discs, but less concentrated than that given by current simulations of cold dark matter (CDM) models. We discuss the implication of such profiles for structure formation in the Universe and for the properties of dark matter. Our results cannot be explained by some of the recent proposals for resolving the conflict between conventional CDM models and the observed rotation-curve shapes of faint galaxies. If dark matter self-interaction (either scattering or annihilation) is responsible for the shallow profile, the observed Tully,Fisher relation requires the interaction cross-section ,X to satisfy ,,X|v|,/mX,10,16 cm3 s,1 GeV,1, where mX is the mass of a dark matter particle. [source]

    Thick gas discs in faint dwarf galaxies

    Sambit Roychowdhury
    ABSTRACT We determine the intrinsic axial ratio distribution of the gas discs of extremely faint MB < ,14.5 dwarf irregular galaxies. We start with the measured (beam corrected) distribution of apparent axial ratios in the H i 21-cm images of dwarf irregular galaxies observed as part of the Faint Irregular Galaxy GMRT Survey (FIGGS). Assuming that the discs can be approximated as oblate spheroids, the intrinsic axial ratio distribution can be obtained from the observed apparent axial ratio distribution. We use a variety of methods to do this, and our final results are based on using Lucy's deconvolution algorithm. This method is constrained to produce physically plausible distributions, and also has the added advantage of allowing for observational errors to be accounted for. While one might a priori expect that gas discs would be thin (because collisions between gas clouds would cause them to quickly settle down to a thin disc), we find that the H i discs of faint dwarf irregulars are quite thick, with mean axial ratio ,q,, 0.6. While this is substantially larger than the typical value of ,0.2 for the stellar discs of large spiral galaxies, it is consistent with the much larger ratio of velocity dispersion to rotational velocity (,/vc) in dwarf galaxy H i discs as compared to that in spiral galaxies. Our findings have implications for studies of the mass distribution and the Tully,Fisher relation for faint dwarf irregular galaxies, where it is often assumed that the gas is in a thin disc. [source]

    The stellar dynamics of spiral arms in barred spiral galaxies

    P. A. Patsis
    ABSTRACT A dynamical mechanism is proposed that explains the spiral structure observed frequently as a continuation of the bars in barred spiral galaxies. It is argued that the part of the spirals attached to the bar is due to chaotic orbits. These are chaotic orbits that exhibit for long time intervals a 4:1-resonance orbital behaviour. They are of the same type of orbit as is responsible for the boxiness of the outer isophotes of the bar in cases like NGC 4314, as indicated by Patsis, Athanassoula & Quillen. The spirals formed this way are faint with respect to the bar, open as they wind out, and do not extend over an angle larger than ,/2. A possible continuation of the spiral structure towards larger angles can be due to orbits trapped around stable periodic orbits at the corotation region. We present a family of stable, banana-like periodic orbits, precessing as EJ increases, that can play this role. [source]

    H i kinematics in a massive spiral galaxy at z= 0.89

    L. V. E. Koopmans
    ABSTRACT We present a kinematic model of the neutral hydrogen in the spiral galaxy of the lens system PKS 1830,211, based on a Multi-Element Radio-Linked Interferometer Network (MERLIN) 1.4-GHz radio map and the integrated and redshifted 21-cm hydrogen absorption-line profile as measured with the Westerbork Synthesis Radio Telescope (WSRT). Degeneracies in the models do not allow a unique determination of the kinematic centre, and forthcoming deeper Hubble Space Telescope observations with the Advanced Camera for Surveys (ACS) are required to break this degeneracy. Even so, we measure the inclination of the hydrogen disc: i= 17°,32°, indicating a close to face-on spiral galaxy. The optical depth increases with radius over the extent of the Einstein ring, suggesting H i depletion towards the lens centre. The latter could be due to star formation or conversion of H i into molecular hydrogen because of a higher metalicity/dust content in the galaxy centre. The neutral hydrogen optical depth gives NH I= 2 × 1021 cm,2 at r= 5.0 h,170 kpc in the disc (Ts= 100 K), comparable to local spiral galaxies. [source]

    Numerical modelling of the vertical structure and dark halo parameters in disc galaxies

    A. Khoperskov
    Abstract The non-linear dynamics of bending instability and vertical structure of a galactic stellar disc embedded into a spherical halo are studied with N-body numerical modelling. Development of the bending instability in stellar galactic disc is considered as the main factor that increases the disc thickness. Correlation between the disc vertical scale height and the halo-to-disc mass ratio is predicted from the simulations. The method of assessment of the spherical-to-disc mass ratio for edge-on spiral galaxies with a small bulge is considered. Modelling of eight edge-on galaxies: NGC 891, NGC 4738, NGC 5170, UGC 6080, UGC 7321, UGC 8286, UGC 9422 and UGC 9556 is performed. Parameters of stellar discs, dark haloes and bulges are estimated. The lower limit of the dark-to-luminous mass ratio in our galaxies is of the order of one within the limits of their stellar discs. The dark haloes dominate by mass in the galaxies with very thin stellar discs (NGC 5170, UGC 7321 and UGC 8286) (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Dissecting galaxies with quantitative spectroscopy of the brightest stars in the Universe

    R.-P. KudritzkiArticle first published online: 20 MAY 2010
    Abstract Measuring distances to galaxies, determining their chemical composition, investigating the nature of their stellar populations and the absorbing properties of their interstellar medium are fundamental activities in modern extragalactic astronomy helping to understand the evolution of galaxies and the expanding universe. The optically brightest stars in the universe, blue supergiants of spectral A and B, are unique tools for these purposes. With absolute visual magnitudes up to MV , -9.5 they are ideal to obtain accurate quantitative information about galaxies through the powerful modern methods of quantitative stellar spectroscopy. The spectral analysis of individual blue supergiant targets provides invaluable information about chemical abundances and abundance gradients, which is more comprehensive than the one obtained from HII regions, as it includes additional atomic species, and which is also more accurate, since it avoids the systematic uncertainties inherent in the strong line studies usually applied to the HII regions of spiral galaxies beyond the Local Group. Simultaneously, the spectral analysis yields stellar parameters and interstellar extinction for each individual supergiant target, which provides an alternative very accurate way to determine extragalactic distances through a newly developed method, called the Flux-weighted Gravity,Luminosity Relationship (FGLR). With the present generation of 10 m-class telescopes these spectroscopic studies can reach out to distances of 10 Mpc. The new generation of 30 m-class telescopes will allow to extend this work out to 30 Mpc, a substantial volume of the local universe (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Ram pressure stripping of disk galaxies in galaxy clusters

    E. Roediger
    Abstract While galaxies move through the intracluster medium of their host cluster, they experience a ram pressure which removes at least a significant part of their interstellar medium. This ram pressure stripping appears to be especially important for spiral galaxies: this scenario is a good candidate to explain the differences observed between cluster spirals in the nearby universe and their field counterparts. Thus, ram pressure stripping of disk galaxies in clusters has been studied intensively during the last decade. I review advances made in this area, concentrating on theoretical work, but continuously comparing to observations (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Post-starburst galaxies and the transformation of blue into red galaxies

    S. De Rijcke
    Abstract We present deep single-dish radio observations of a sample of nearby post-starburst galaxies (0.05 < z < 0.1). About 50% of these post-starburst galaxies are detected at 21 cm, with HI masses of ,109 M,, up to ,1010 M,. These post-starburst galaxies are as gas-rich as spiral galaxies with comparable luminosities. There appears to exist no direct correlation between the amount of H I present in a post-starburst galaxy and its star formation rate as traced by radio continuum emission. Moreover, the end of the starburst clearly does not necessarily require the complete exhaustion of the neutral gas reservoir. High-resolution radio observations of one post-starburst binary system suggest that most of the neutral gas resides outside the stellar bodies of the galaxies. Most likely, the gas was expelled by supernova and/or AGN feedback. This effectively stops star formation, even though copious amounts of diffuse neutral gas remain in the immediate vicinity. This remaining H I reservoir may eventually lead to further episodes of star formation. This may indicate that some post-starbursts are observed in the inactive phase ofthe star formation duty cycle (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Ordered and chaotic spiral arms

    P.A. Patsis
    Abstract The stellar flow at the arms of spiral galaxies is qualitatively different among different morphological types. The stars that reinforce the spiral arms can be either participating in an ordered or in a chaotic flow. Ordered flows are associated with normal (non-barred) spiral galaxies. Typically they are described with precessing ellipses corresponding to stable periodic orbits at successive energies (Jacobi constants). On the contrary, the spiral arms in barred-spiral systems may be supported by stars in chaotic motion. The trajectories of these stars are associated with the invariant manifolds of the unstable Lagrangian points (L1,2). Response and orbital models indicate that this kind of spirals either stop at an azimuth smaller than , /2, or present large gaps at about this angle. Chaotic spirals appear in strong bars having (L1,2) close to the ends of the bar. The arms of barred-spiral systems with corotation away from the end of the bar can be either as in the case of normal spirals, or supported by banana-like orbits surrounding the stable Lagrangian points (L4,5). We find also models combining ordered and chaotic flows. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Magnetic fields in halos of spiral galaxies

    R.-J. Dettmar
    Abstract Observations of magnetic fields in halos of edge-on disk galaxies are discussed in relation to the interstellar disk-halo interface in disk galaxies. The distribution of extra-planar diffuse ionized gas correlates on local and global scales with cosmic rays and magnetic fields as inferred from observations of the non-thermal radio continuum radiation and its polarisation. From the polarisation a large-scale and well-ordered magnetic field in these gaseous halos can be deduced. For several objects a significant poloidal component of the halo field is likely. These observations indicate the presence of physical processes which generate and maintain magnetic fields on galactic scales. The importance of differential rotation of the gaseous halos for such processes is briefly discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    The large scale magnetic field configuration in the Sombrero galaxy , persistence during galaxy evolution?

    M. Krause
    Abstract Radio polarization observations at 4.86 and 8.35 GHz of the nearby edge-on galaxy M 104 revealed a large-scale magnetic field in this early-type spiral. This is to our knowledge the first detection of a regular magntic field in an Sa galaxy in the radio range. The magnetic field orientation in M 104 is predominantly parallel to the disk but has also vertical components at larger z-distances from the disk, i.e. a field configuration typical for normal edge-on spiral galaxies. Bolometer observations at 345 GHz data pertain to the cold dust content of the galaxy. Despite the optical appearance of the object with the huge dust lane, its dust content is smaller than that of more late-type spirals. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]