Showing papers on "Spiral galaxy published in 1991"

Extended rotation curves of spiral galaxies: dark haloes and modified dynamics

Abstract: Strict criteria are applied to the sample of spiral galaxies with measured rotation curves in order to select those objects for which the observed rotation curve is an accurate tracer of the radial force law. The resulting sub-sample of 10 galaxies is then considered in view of two suggested explanations for the discrepancy between the luminous mass and the conventional dynamical mass of galaxies: dark haloes and the modified Newtonian dynamics (MOND). This is done by means of least-squares fits to the rotation curves. Three-parameter dark-halo models (M/L for the visible disc, the core radius and the asymptotic circular velocity of the halo) work well in reproducing the observed rotation curves, and it is found that, for the higher luminosity galaxies, the visible matter dominates the mass distribution within the optically bright disc. However, in the low-luminosity gas-rich dwarfs the dark component is everywhere dominant. MOND, with one free parameter, (M/L for the visible disc) generally works well in predicting the form of the rotation curves, in some cases better than multi-parameter dark-halo fits. If the distance to the galaxy is also taken as a free parameter, then the MOND fits are as good as three parameter dark-halo models and, with one exception, the implied distances are consistent with the adopted distances within the probable uncertainty in the distance estimates. Restricting the number of parameter in dark-halo models by making use of the disc-halo coupling does not produce satisfactory fits to the rotation curves. The overall conclusion is that MOND is currently the best phenomenological description of the systematics of the discrepancy in galaxies.

Molecular gas in galaxies

Abstract: Molecular gas is critical in determining both the morphology and evolution of galactic disks. It is within the giant molecular clouds that interstellar gas is cycled into the next generation of stars, and the most massive of these young stars produce a major part of the galactic luminosity. In addition, the dense interstellar medium, as it is highly dissipative, probably plays a fundamental part in determining the outcome of galactic interactions. Over the last two decades, CO observations have been used to probe the molecular component of hundreds of galaxies from the Local Group to the Virgo cluster, and in luminous galaxies with recession velocities up to cz = 45,000 km s^(-1).

The 158 micron forbidden C II line - A measure of global star formation activity in galaxies

Abstract: Some 158 micron (CII) fine structure line observations from a sample of fourteen gas rich galaxies are reported. These measurements confirm and generalize previous basic results that the (CII) line is bright amounting to approximately 0.1 to 1 percent of the Far Infra Red (FIR) luminosity of the nuclear regions of galaxies; the (CII) line is formed in the warm (temperature of the gas is greater than 200 K), dense (n sub H greater than 1000/cu cm) photodissociated gas at the interfaces between giant molecular clouds and ionized gas regions and is therefore associated with the molecular gas component in spiral galaxies; the (CII) line tracks the FIR continuum in a manner consistent with the PDR models; the integrated (CII) to isotope (12)CO (transition 1 to 0) line ratio is large (greater than or equal to 1000) in all galaxies studied, and is similarly large for galactic molecular clouds; the (CII) line is therefore energetically very important for the study of giant molecular clouds. Conclusions obtained from these results are given.

Correlations between the far-infrared, radio, and blue luminosities of spiral galaxies

Abstract: FIR-radio luminosity correlations are presently derived for two statistically complete samples spanning a wide luminosity range. While observed correlations are tight, they are not linear; the observed scattering is mostly intrinsic. An empirical correction to the radio luminosities, based on observed blue luminosities, makes the corrected FIR-radio correlation linear and eliminates most of the residual variance. A two-component model that is consistent with the improved correlation is proposed for the heating luminosity within galaxies. The first, radio-loud component contains young massive stars that heat the dust, while contributing to the cool cirrus dust-heating interstellar radiation field. The second, radio-quiet component is made up of less massive older stars.

New Cepheid distances to nearby galaxies based on BVRI CCD photometry. II, The Local Group galaxy M33

Abstract: A true distance modulus to the nearby spiral galaxy M33 has been determined based on CCD photometry obtained at BVRI wavelengths. M33 is presently one of five nearby galaxies used in the calibration of the IR Tully-Fisher relation, and thereby in the determination of the Hubble constant. Using period-luminosity relations at several wavelengths offers the advantage that the distance moduli derived can be corrected for the effects of interstellar extinction. These data indicate that there is internal reddening affecting the Cepheid photometry in M33 which must be accounted for if a true distance modulus is to be obtained for this galaxy. Adopting a true distance modulus to the LMC of 18.5 mag, the new CCD data yield a true distance to M33 of 24.64 + or - 0.09 mag, corresponding to a linear distance of 840 kpc. A mean value of the total color excess (foreground and internal) for the Cepheids in M33 is estimated to be E(B - V) = 0.10 + or - 0.09 mag, assuming a value for the total mean LMC Cepheid color excess of 0.10 mag.

Extragalactic Globular Clusters. II. The M31 Globular Cluster System

Abstract: Metallicities for 150 globular clusters associated with M31 are derived using the strengths of six absorption features in the cluster-integrated spectra. The properties of the M31 cluster system are very similar to those of the Galactic system. No dependence of metallicity on cluster luminosity is found. The number distribution as a function of metallicity is consistent with that of Galactic clusters. There is evidence for a weak metallicity gradient as a function of projected radius. The mean metallicity Fe/H is equal to -1.2, slightly higher than the mean for the Galactic system. 48 refs.

The Universal Galaxy Rotation Curve

Abstract: From the analysis of an extended set of selected galaxy rotation curves, it is found that for a given luminosity the rotation curves of spiral galaxies within the optical radius are a universal function This result implies strong systematic variations of both the amplitude and the profil of the circular velocity with luminosity faint (bright) galaxies having low (high) velocities and steep (shallow) velocity gradients Because luminous disk are self-similar, the observed progression of the shape of rotation curves with luminosity suggests that the dark-to-visible interplay varies with luminosity

The distribution and kinematics of neutral gas in M31

Abstract: The kinematic signature of individual spiral arm segments are used here to make a very compact yet quite complete parameterization of the global neutral hydrogen content of M31. Two continuous, trailing spiral arms can be traced from about 0.5 to 27 kpc from the nucleus which accurately account for the kinematic and spatial distribution of gas. The three-dimensional geometry of the gas distribution is derived with only limited assumptions concerning axisymmetry of the velocity field. Significant variations in inclination as a function of azimuth and radius are found, leading to departures of the local midplane from a few hundred pc in a tilted disk in the inner galaxy to in excess of a kpc in the 'warp' of the outer galaxy. The distribution of exponential scale height of the gas with radius varies between 200 and 600 pc, and the spiral pattern in the inner galaxy has an elliptical distortion which leads to significant corrections to the axisymmetric rotation velocity and implied mass distribution. 48 refs.

Galactic center gasdynamics : a one-armed spiral in a Keplerian disk

Abstract: The inner 3 × 4 pc of the Galaxy have been imaged in the [Ne II] (12.8 μm) line with 2″ and 30 km/s resolution. The morphology of much of the ionized gas can be explained by a one-armed linear (r ∞ θ) spiral. The kinematics of the gas along the spiral can be modeled by approximately circular orbits with a nearly Keplerian rotation curve, requiring a central mass of 2 ± 0.5 × 10 6 M ⊙ . Several posible origins are suggested for the spiral

Abundance analysis of giant H II regions in M51

Abstract: Spectrophotometric observations in the optical and near infrared of six giant H II regions in the high metallicity spiral galaxy M51 are presented. Five of the regions are located in the spiral arms and the sixth one is very close to the nucleus. These observations and the application of adequate photo-ionization models allow the determination of the functional parameters of the regions: ionization parameter, effective temperature of the ionizing radiation and metallicity. The results of this work confirm previously found trends of decreasing ionization parameter and radiation hardness with increasing metallicity.

The Shape of the Galaxy

Abstract: The mechanisms responsible for the deviation from circular symmetry in the emission from the outermost atomic H gas of the Galaxy, as seen in the 21-cm survey of the Galactic plane by Weaver and Williams (1974), are investigated theoretically. The analysis of this data set and the construction of a triaxial spheroid model to account for the observations are explained, and it is concluded (1) that the deviations are due to the quadrupole term in the gravitational potential, (2) that the local standard of rest is moving radially outward at velocity 14 km/sec with respect to the most distant H I gas, and (3) that the ellipticity of gas streamlines decreases with increasing distance from the Galactic center, so that (4) the outermost gas is moving in nearly circular orbits. The predictions of the triaxial spheroid model and specific observed features are compared in detail and illustrated with graphs and diagrams. 72 refs.

Evolution of Dust Grains through a Hot Gaseous Halo

Abstract: The evolution of bare spherical dust grains in the halo of spiral galaxies is analyzed. Two different grain compounds, graphites and astronomical silicates, are considered. The detailed mass and luminosity distributions for the Milky Way and NGC 3198 (considered as representative of the Sb and Sc types, respectively) are used to evaluate the range of possible evolutionary tracks. Aside from radiative and gravitational forces, the effects of drag and sputtering from a gaseous halo are included. A simple isothermal and hydrostatic density structure, with temperatures in the range 3 × 10 5 -10 6 K, has been used for this gaseous halo

The Outer Lindblad Resonance and the Morphology of Early Type Disk Galaxies

Abstract: The morphology of outer rings and pseudorings in SB and SAB galaxies includes a number of distinct categories that bear a remarkable resemblance to the gaseous rings which have developed near the outer Lindblad resonance in n-body models of barred spirals. This paper displays these categories using CCD images and color index maps of 22 galaxies from the Catalogue of Southern Ringed Galaxies (CSRG). The prominence of the OLR morphologies in these and many other galaxies provides an important handle on pattern speeds of bars and ovals, as well as insights into internal dynamics. It also provides an important distinction to be considered when comparing early and late-type galaxies

Neutral Hydrogen Observations of NGC 4565 and NGC 891

Abstract: Neutral hydrogen distributions from two spiral galaxies, NGC 4565 and NGC 891, are mapped on the basis of high-resolution observations with the NRAO's VLA in its 'B', 'C', and 'D' arrays. The paper considers the acquisition and reduction of the VLA data and discusses the kinematics of the H I in the two galaxies. Two H I companions of NGC 4565 are described. 45 refs.

On the interpretation of the morphology-density relation for galaxies in clusters

Abstract: A reexamination of Dressler's sample of nearly 6000 galaxies in 55 clusters shows that galactic morphology correlates as well with the projected distance from the center of the cluster, R(clus), as it does with the projected local density of the 10 nearest galaxies. Most of the variation occurs within 0.5 Mpc of the cluster center. The effect is strongest in the 34 clusters with D galaxies, with the percentage of ellipticals rising from 18 + or - 3 percent at R(clus) = 0.6 Mpc to 65 + or - 7 percent at R(clus) = 0.1 Mpc. Various correlations suggest that the physical mechanisms which result in the population gradients for the spiral and S0 galaxies are different than the mechanism responsible for the preponderance of ellipticals near the cluster centers. Tentative evidence is also found that the fraction of S0 galaxies declines sharply within 0.2 Mpc of the cluster center, especially around central D galaxies. 19 refs.

Morphological differences between optical and infrared images of the spiral galaxy NGC309

Abstract: THE morphological classification of spiral galaxies into various types is suspected to be highly dependent on the wavelength of observation1, because optical images emphasize young population I stars at the expense of other stellar types, as well as ionized gas and dust. Extension of the classification of galaxies out to wavelengths of a few micrometres has had to await the development of large-format near-infrared array cameras. We present here images at 2.1 μm wavelength of NGC309, one of the largest 'grand design' (type ScI) spiral galaxies, obtained with the 256x256 array camera developed for the NICMOS (near-infrared camera and multiobject spectrograph) instrument, designed for installation on the Hubble Space Telescope. Optically, NGC309 presents a classic multi-arm morphology, but at 2.1 μm we see a two-arm spiral and the appearance of a prominent central bar; it resembles the SBa galaxy NGC1358. These studies underscore existing indications2 that the disk structure of spiral galaxies may be unrelated to the Hubble type assigned from the transient population I morphology.

A kinematic study of the Fornax dwarf spheroidal galaxy

Abstract: Precise radial velocities of 44 stars and four globular clusters located in two fields of the Fornax dwarf spheroidal galaxy are obtained on the basis of photon-counting echelle spectroscopy with a resolution of approximately 14 km/s. BV CCD photometry of the giant branch of Fornax in both fields are presented as well. A variety of kinematic and photometric criteria are used to identify 10-12 probable nonmembers in the present sample of spectroscopically observed stars. Based on the most probable members, the mean heliocentric systemic velocity of Fornax is 53.0 + or - 1.8 km/s, with no evidence of any significant rotation about the minor axis. The intrinsic velocity dispersion of the stars in Fornax's central field is 9.9 + or - 1.7 km/s, while for the outer field the velocity dispersion is 1.20 + or - 2.8 km/s. The true central velocity dispersion is not more than 1.6 km/s larger than the observed central dispersions for a number of reasonable models.

High-resolution aperture synthesis of molecular gas in NGC 1068

Abstract: High-resolution aperture synthesis imaging of NGC 1068 in the CO J = 1-0 line is presented. The major features of the molecular distribution in the central 3 kpc of the galaxy are a continuous pair of spiral arms originating from the end of the stellar bar seen in 2.2 micron images and a nuclear gas concentration of about 80 million solar masses. The total molecular gas content of the spiral arms is 5 billion solar masses; most of this gas is distributed in 38 complexes of 10 million to 700 million solar masses, with sizes ranging up to 500 pc. The large reservoir of interstellar gas seen in the spiral arms at 1.7 kpc radius may play a central role in both the abundant star formation and nuclear activity. 37 refs.

Is the bulge of our Galaxy triaxial

Abstract: AS in the case of other spiral galaxies, such as M31 (refs 1, 2), the rotation curve of gas in our Galaxy indicates that its nuclear bulge is triaxial3. Despite several studies4,5 of the distribution of specific objects in the galactic bulge, no observational evidence of asymmetry has emerged. Recently, however, Blitz and Spergel6 have reanalysed balloon-infrared observations at 2.4 μm, where obscuration is relatively low, to suggest that the galactic bulge is bar-like, and tilted with respect to the plane of the Galaxy. Here we report that the distribution of IRAS (the Infrared Astronomy Satellite) bulge stars shows an asymmetry with respect to the Galactic Centre, providing clear evidence for triaxiality.

The stellar populations of M 33

Abstract: content of the Triangulum nebula = M 33 = NGC 598 A distance modulus of (m - M)0_=245 ± 02 (D = 795 ± 75 kpc) and a Galactic foreground reddening EB-V_=007, from which MV_= - 1887, are adopted throughout this paper The disk of M 33 is embedded in a halo of globular clusters, ,metal- poor red giants, and RR Lyrae stars Its nuclear bulge component is weak (MV_ > -14) This suggests that the halos of galaxies are not extensions of their bulges to large radii The ages of M 33 clusters do not appear to exhibit a hiatus in their star-forming history like that which is observed in the Large Magellanic Cloud Young and intermediate-age clusters with luminosities rivaling the populous clusters in the LMC are rare in M 33 The integrated light of the semistellar nucleus of M 33, which contains the strongest X-ray source in the Local Group, is dominated by a young metal-rich population At optical wavelengths the disk scale length of M 33 is 96', which is similar to the 99' scale length of OB associations The ratio of the nova rate in M 33 to that in M 31 is approximately equal to the ratio of their luminosities This suggests that the nova rate in a galaxy is not determined entirely by the integrated luminosity of old bulge stars The gas-depletion time scale in the central region of M 33 is found to be ~17 X 109^ yr, which is significantly shorter than a Hubble time Available data do not yet allow an unambiguous choice between the density wave and self-propagating star-formation models for the two main spiral arms of M 33

Orbit Crowding of Molecular Gas at a Bar-Spiral Arm Transition Zone in M83

Abstract: The southwestern bar-spiral arm transition zone in M83 is been studied in CO, H-alpha, H I, red light, and the radio continuum. A massive molecular gas complex in the heart of the transition zone is composed or two principal components which have the morphology and kinematics expected from orbit crowding, where gas on highly elliptical orbits form the bar region converges with gas on more circular orbits from the spiral arm region. Three mechanisms for the origin of the orbit crowding are investigated, and it is proposed that the crowding is due primarily to density wave streaming motions caused by the bar and spiral arms. The inner CO component is partially coincident with a region of highly polarized radio continuum emission which precedes the H-alpha spiral arm by 15-25 arcsec, indicating that it lies on or just downstream from a shock front. This suggests that the bar gas approaching the transition zone is shocked and explains the ridge of dense gas seen upstream from the spiral arm.

The Neutral Hydrogen Distribution of NGC 3079

Abstract: Multiarray VLA H I observations of the radio lobe spiral galaxy NGC 3079 and it companions NGC 3073 and MCG9-17-9 are reported. It is found that the density and velocity distributions in this galaxy can be adequately described, to first order, by simple exponential or Gaussian functions and a Brandt curve, respectively. From residual line profiles, an anomalous redshifted emissions feature centered at a radius of roughly 3.5 kpc on the southern major axis is found. A distinct redshifted absorption component several hundred pc from the nucleus is detected from nuclear absorption profiles. The galaxy is also distinguished by a centrally peaked H I distribution. 55 refs.

High chemical abundances in Virgo spiral galaxies

Abstract: Evidence is presented that spiral galaxies in the Virgo Cluster have systematically higher interstellar abundances than comparable field galaxies This conclusion is based on spectra of H II regions in five Virgo spirals of type Sc A possible explanation is that abundances in the field spirals are strongly affected by infall of metal-poor gas or by radial inflow of gas from the outer H I disk These processes are inhibited in the cluster environment, and the Virgo spirals may have evolved more nearly in the manner of the closed box 'simple model of chemical evolution 59 refs

A dark-halo-dominated galaxy - ngc-5585

Abstract: Studies of the light distribution, the H I kinematics, and the mass distribution of NGC 5585, a late-type spiral galaxy in the M101 group, are presented. From the photometry, an extrapolated central surface brightness B(0)c = 21.94 mag arcsec-2 and a scale length alpha-1 = 1.4 kpc are derived for the stellar disk. The H I distribution reaches a diameter of 12 arcmin, more than two times the optical diameter. The velocity field reveals a strong warp. The position angle is shown to vary from 45-degrees to 55-degrees in the outer parts, while the inclination is fairly constant at 52-degrees. The rotation curve, derived from this velocity field, reaches a maximum velocity of approximately 90 km s-1, extending out to 7 optical scale lengths. Combining the optical and kinematical data, mass models for NGC 5585 were built using both the "best-fit" and the "maximum-disk" approaches. For both models, the dark-halo component is by far the main contributor to the rotation curve and its basic parameters are fairly well constrained with a core radius r(c) = 3.0 +/- 0.2 kpc and a central density rho-0 = 0.054 +/- 0.006 M . pc-3. At the last observed point (r = 9.65 kpc), approximately 90% of the total mass is dark. Even the gaseous component is dynamically more important than the stellar disk component. In fact, despite being a fairly bright system with M(T)0(B) = -17.5, NGC 5585 is similar to many fainter dwarf systems where the stellar disk is clearly not self-gravitating even in the inner parts.

Dependence of galaxy properties on viewing angle

Abstract: Galaxies are three-dimensional objects projected onto the sky at random angles of inclination. Deduction of their true structure from their appearance requires an understanding of the variation of their apparent diameter, luminosity and surface brightness with viewing angle. But these variations in turn depend on galactic structure. It has taken astronomers several decades to realize that only with a direct knowledge of galactic distances can these two effects be disentangled.

Multifrequency windows on spiral galaxies. I - UBV and H-alpha aperture photometry

Abstract: UBV and Hα aperture photometry of 78 galaxies in the Coma and Hercules superclusters and in the Cancer cluster are presented. These data are analyzed in conjunction with available measurements forming a sample composed by a variety of environments and spiral types which, although not complete, contains most of the disk galaxies in these regions previously detected in the radio continuum and many objects detected in the far-infrared by IRAS. The optical information provides an estimate of the stellar content and star formation rate. The data are consistent with the idea that young, high-mass stars are responsible for the dust heating in galaxies with hot far-infrared colors, but in cooler galaxies the contribution from the general stellar field cannot be ruled out. The existence of strong correlation between the nonthermal radio continuum and thermal far-infrared fluxes is confirmed.

Globular clusters in the halo of M31

Abstract: The CFHT was used to obtain high-resolution CCD images of 82 cluster candidates in the halo of M31. These data, combined with radial velocities which cover an additional 27 candidates, are used to compile a catalog of 51 bona fide M31 halo globulars. The other candidates are found to be background galaxies (54) and field stars (4). The cluster sample appears to be incomplete for V greater than 18. The projected distribution of globulars follows an 1/r-squared law for r(kpc) between values of 6 and 22 and then drops faster, suggesting a cutoff at about 40 kpc. These trends are similar to those for globular clusters in the Milky Way halo. The total populaton of globulars in M31 is estimated to be larger than in the Milky Way by a factor of 1.8 + or - 0.3. 30 refs.

A Statistical Study of the Correlation of Galactic Supernova Remnants and Spiral Arms

Abstract: The distribution of observed giant H II regions and supernova remnants (SNRs) in Galactic longitude is analyzed with Monte Carlo models to determine the possible correlation with spiral arms or disk populations. The novel feature of the current analysis is that it depends mainly on the angular distribution of the H II regions and SNRs and hence is independent of distance.

Polarized radio emission from the edge-on spiral galaxies NGC 891 and NGC 4565

Abstract: Results are presented, at a resolution of 20 arcsec, of observations of the distribution of radio continuum intensity and linear polarization with the VLA in two nearby edge-on spiral galaxies, NGC 891 and NGC 4565, at 6 and 20 cm, respectively. A unified model is presented to account for the main features of the radio polarization in these two galaxies. The model geometry is determined from recent observations of face-on galaxies where the polarized emission is found to be strongest in the dark inter-arm and outer parts of the disks. A substantial Z-thickness is ascribed to this polarized emission. It is shown that the exceptionally strong wavelength dependence of this type of Faraday depolarization can result in edge-on galaxies becoming rapidly 'Faraday thick' at decimeter wavelengths, thereby obliterating the polarization from regions on the dark side of the disk. The degree of polarization observed in both galaxies increases strongly with increasing Z-distance from the plane.