Showing papers on "Spiral galaxy published in 1979"

Gas dynamics in barred spirals: Gaseous density waves and galactic shocks

Abstract: Steady-state gasdynamical studies, previously limited to tightly wound normal spiral galaxies, are extended to models of barred spirals with a 5% to 10% perturbing potential. The models show that a strong wave manifestation is an important constituent of the bar structure in many barred spirals and that a density-wave shock wave can form a bar structure as pronounced as the narrow bars often evident in optical photographs of barred spirals. The dark narrow dust lanes often observed along the leading edges of bar structures are identified as tracers of shocks, and it is found that strong shocks along a bar structure during even a small part of a galaxy's lifetime might easily deplete a large enough proportion of the gas to cause a lack of gas in the inner annuli encompassing the bar by the time of the present epoch. It is emphasized that even moderate-amplitude barlike perturbations in the disk can drive large noncircular gas motions, typically 50 to 150 km/s.

The infrared luminosity/H I velocity-width relation and its application to the distance scale.

Abstract: The Tully-Fisher relation between the luminosity of late-type galaxies and their global 21 cm velocity widths has been investigated with infrared photometry. Multiaperature magnitudes at H (1.6..mu..m) have been obtained for 11 spiral galaxies in Virgo and 18 in the Ursa Major cluster. A tighter empirical correlation between luminosity and 21 cm line width has been found for H magnitudes uncorrected for inclination than for photographic magnitudes with their individually uncertain, prescribed corrections for internal absorption. The similarity of the infrared magnitude/velocity-width correlation for Virgo, for Ursa Major, and for six nearby galaxies also measured suggests that the slope of the relationship is universal in nature.An important uncertainty in previous applications of the Tully-Fisher method was an incomplete understanding of its physical basis. In the infrared the slope of the magnitude/velocity-width relation is found to be considerably steeper than that obtained optically. From this result, we demonstrate that the dynamical origin of the relationship is similar to the well-known LproportionalV/sup 4/ power law for elliptical galaxies. We further suggest that this has not been seen in the blue, because the mass-to-luminosity ratio M/L/sub B/ varies markedly in spirals of differing mass, whereas M/L/sub H/ is more nearly constant.By using amore » provisional calibration based on the distances of Sandage and Tammann for local galaxies, we derive distance moduli to Virgo and Ursa Major of 31.07 +- 0.20 and 31.08 +- 0.16. A mean value for the Hubble constant of H/sub 0/=61 +- 4 km s/sup -1/ Mpc/sup -1/ follows from these preliminary results.We believe that the infrared magnitude/velocity-width relation is now the most powerful tool available for determining redshift-independent distances to the adjacent greater clusters.« less

Density Wave Theory of Spiral Structure of Galaxies

Abstract: Summary The density-wave theory of spiral structure in galaxies is studied as a dynamical problem: the gravitational instability of a galactic disk with respect to spiral modes. The asymptotic theory for tightly wound spirals is presented in detail, although the problem has been formulated in a more general form to open the way to future research work in other directions. The gas dynamical model has been adopted; reference to the stellar dynamical model will be made when necessary. It is hoped that this article will further stimulate workers in continuum mechanics to play their important role in future research work in the astrophysics of galaxies.

Gravitational collapse in dust lanes and the appearance of spiral structure in galaxies

Abstract: The possibility is considered that there are two types of spiral for each spiral galaxy: an underlying spiral of relatively old stars seen in red and IR photographs and a spiral outlined by H II regions and young bright blue stars. The origin of the first spiral is attributed to large-scale spiral density waves, and the second spiral is identified as a stochastic spiral. It is suggested that there is a physical or causal connection between the two spirals and that dust lanes play the role of intermediary. An analysis is performed which shows that such a model can account for the formation and destruction of giant molecular cloud complexes and for observed spiral structure in one simple unified theory.

Type I supernovae come from short-lived stars

Abstract: Using a sample of 178 supernovae in external galaxies, we find three pieces of evidence that Type I supernovae are associated with a young stellar population: (1) The number of Type I supernovae per unit luminosity is much higher in I0 galaxies than in any other galaxy type; this result cannot be ascribed to small-number statistics, and it implies that the supernovae are related to the peculiarities defining the I0 class, namely signs of intense star formation in an otherwise old population. (2) The number of Type I supernovae per unit luminosity increases from early through late Hubble types; if such supernovae arose in the very old (bulge or old-disk) population, the opposite trend would be expected. (3) The Type I supernova rates in spiral galaxies are proportional to their present star formation rates, as estimated from colors; this result again implies that fairly short-lived stars become Type I supernovae. We discuss the star formation rate that is implied for elliptical galaxies if their supernovae have the same origin as those in spirals and irregulars; signs of such star formation could have escaped attention so far, but not by a wide margin, so it is not clear whether a distinctmore » type of supernova need be postulated. In any case, most Type I supernovae must come from short-lived stars.« less

Detection of strong H 2 O emission from galaxy NGC4945

Abstract: AFTER the first detection of the 616–523 transition of water vapour in an external galaxy by Churchwell et al.1 in M33, other detections were reported by Lepine and dos Santos2 in NGC253, and by Huchtmeier et al.3 in M33 and IC342. We report here the detection of H2O maser emission from NGC4945, a bright edge-on spiral galaxy in the Centaurus group. This is intrinsically the most powerful H2O maser yet detected in any astronomical source. The search for H2O emission in NGC4945 was motivated by the previous detection of OH and H2CO absorption by Whiteoak and Gardner4 and Gardner and Whiteoak5, and also by the similarity of this galaxy to NGC253. The observations were made on 20–22 September 1978, with the 13.7-m Itapetinga radiotelescope equipped with a ruby maser amplifier with an apparent SSB system temperature of about 150 K. The signal was analysed by a 46-channel, 100–kHz (1.35 km s−1) resolution filter bank, and observations were made with the filter bank centred at different frequencies to obtain greater velocity coverage. The 4′ beam was pointed towards the nucleus of the galaxy. The overall dimensions of the galaxy are 17′ × 3′. The telescope was beam switched at a rate of 100 Hz. The beams were separated by 9.4 arc min and the reference beam was displaced in azimuth, which did not correspond to a fixed direction with respect to the equatorial plane of NGC4945. Antenna temperatures were corrected for atmospheric attenuation and random attenuation using the standard procedure at Itapetinga6 and then converted to flux units, the error in the flux scale being ∼10%.

The structure and evolution of X-ray clusters.

Abstract: Observations of the structure of the X-ray emission from 12 nearby rich clusters of galaxies are presented and interpreted in terms of dynamic cluster evolution. X-ray structures revealed by the Einstein Observatory imaging proportional counter in the range 0.25 to 3.0 keV were analyzed and used to classify the clusters based on their X-ray morphologies. Four categories are observed, consisting of spiral-rich clusters with low X-ray temperatures and velocity dispersions with broad and highly clumped emission, spiral-poor clusters with high X-ray temperatures and velocity dispersions with smoothly varying emission broadly or sharply peaked around a dominant galaxy, and clusters with emission typical of a cD galaxy in a poor cluster or group. The broad, highly clumped cluster emission is interpreted as a result of an early evolutionary stage, while the cD and centrally enhanced emissions represent successive later stages in X-ray galactic cluster evolution.

Properties of spiral galaxies from a stochastic star formation model

Abstract: We have examined the properties of model galaxies produced by the stochastic, self-propagating, star-formation model and compared them with those observed in real galaxies. Observations have shown that a good correlation exists between the morphological type of a spiral galaxy and its rotational velocity, color, and gas fraction. We have shown that the stochastic, self-propagating, star-formation model directly correlates the morphological type with the rotation curve, the most important parameter being the maximum rotational velocity. The relative star-formation rates given by the model lead qualitatively to the observed variation of integrated colors and gas fraction as a function of morphological type, as well as to the observed decrease in the density of regions of recent star formation with radius. We show that this model is also capable of giving spiral arms in red plates and discuss the color profiles across spiral arms. Inclusion of the interaction of star formation and the interstellar gas shows that this model is also capable of inducing spiral ordering in the gas.

New large-scale magnetic features of the Milky Way

Abstract: A large body of newly determined rotation measures (RM) for extragalactic radio sources gives a new, much clearer picture of the magnetic structure of our Galaxy. The RM sky is dominated by a very strong zone which extends to large (down to b≈ −40°) negative galactic latitudes. Both the strength and extent of this feature are greater than appreciated from earlier data. It coincides in angular position with loop II of the non-thermal radio background radiation of the Galaxy. This, and two other strong RM features suggest that our Galaxy has very large-scale magnetic field zones, probably between the spiral arms, and that the prevailing sense of the field is opposite in the interarm regions on each side of the Sagittarius arm.

The H II region-molecular cloud complex W51.

Abstract: We have made new observations of the H II region--molecular cloud complex W51 in the spectral lines of lambda=2.6 mm CO, lambda=3.1 mm CS, lambda=2 cm H/sub 2/CO, and the recombination line H76..cap alpha... Since the line of sight to this H II region lies nearly tangent to the Sagittarius spiral arm, W51 offers an excellent opportunity to study the activity associated with the large-scale spiral structure of our Galaxy. Our molecular observations are most consistent with a picture in which three streams of cold gas flow through the complex. These streams have been placed within the structure of the hydrogen observed in this neighborhood of the galactic plane. This study shows that the H II regions are now near the outside of these molecular streams, ionizing cold gas that flows into the low-pressure regions between the clouds. Simple models of the H II regions in the W51 complex have been constructed which reproduce many of the continuum and spectral line observations that have been obtained.

Space distribution of Sc galaxies: Clues to the large-scale structure

Abstract: The redshifts (in units of 100 km s/sup -1/) of the homogeneous sample of Sc c--Sc II galaxies with m/sub p/=140--150 mag obtained by Rubin et al are placed at their galactic locations on an equal-area projection of the celestial sphere This ''redshift map'' and auxiliary maps with narrower ''redshift windows'' illustrate the existence of the Hydra/Centaurus and Hercules/A2199 super-clusters They also reveal that the Coma clusters is part of a structure which includes not only A1367 but A779 as well, and may extend more than 200 Mpc across the celestial sphere Similarly, the Perseus chain of clusters is part of a structure which fills a large fraction or all of the sky By examining the redshift maps, one could gain the impression that the universe is an irregular three-dimensional ''net'': chains and clumps of galaxies separating vast voids

Distance of M83 and of the Centaurus group.

Abstract: The distances of eight probable or possible bright (B/sup 0//sub T/ =3.3 +- 0.4 Mpc; it covers an elongated area 33/sup 0/ x 18/sup 0/=1.8 x 1.0 Mpc, parallel to the supergalactic plane, and has a depth of approximately 2 Mpc (2.0< or =..delta..< or =4.0 Mpc) in the line of sight. Eight dwarf galaxies recently discussed by Webster et al. are confirmed to be members or probable members of the group. The Fourcade--Figueroa galaxy (A1332-45) is probably a background object at ..delta..approx. =5.3 Mpc; the Circinus galaxy (A1409-65) is probably a field galaxy at ..delta..approx. =4 Mpc, but additional members between it and the group may be hidden near the galactic plane. The velocity dispersion, sigma/sub v/approx. =70 km s/sup -1/, may be interpreted either as evidence for stabilitymore » (bound group) or instability (expanding association or accidental grouping). The presence of three early-type systems (NGC 5102, 5128, 5253) in a group dominated by late-type spirals (NGC 4945, 5068, 5236) and magellanic irregulars is unusual and suggestive of a chance encounter.« less

Stochastic star formation and the evolution of galaxies

Abstract: The mechanism of stochastic self-propagating star formation has previously been invoked to explain the origin of spiral arms in galaxies. In this paper we extend the application of this mechanism to account for the diversity of morphological types and the evolution of galaxies. The new property that arises from consideration of this mechanism is that the rate of star formation exhibits the critical behavior of a phase transition. This is a general property of the system and is not strongly dependent on the details of the star--interstellar gas interaction. Examination of the properties of this phase transition provides a general scenario for the evolution of galaxies and the origin of the various morphological types.

Extended rotation curves of high-luminosity spiral galaxies. V. NGC 1961, the most massive spiral known

Abstract: The rotation curve derived for NGC 1961 indicates a total mass greater than 10/sup 12/ M/sub sun/. This large value is confirmed by the correspondingly large mass, >10/sup 11/ M/sub sun/, for the interstellar H I component derived from 21 cm measurements. This supergiant spiral has normal ratios formed from values of its total mass, H I content, and blue luminosity. The optically derived spectra indicate peculiar and unexplained motions within the system.

Photometric study of faint early-type stars in the Southern Milky Way.

Abstract: We present UBV photoelectric observations for 101 faint early-type (mainly OB) stars in four regions of the Southern Milky Way: two in Vela, one in Centaurus, and one in Circinus; H..beta.. data are also given for the brightest stars. Objective prism plates showing the H..cap alpha.. region of the spectrum are used, together with published results from other authors, to recognize Be stars in our sample. We found a new young open cluster related to the H II region RCW 38 in Vela, at 1.7 kpc from the Sun, and we favor the existence of, at least, a spur in the local arm towards lapprox. =270/sup 0/; whether such a feature extends beyond that distance cannot be decided from our data. Most of the stars studied in the Centaurus region seem to belong to the Centaurus OB 1 association, at 2.4 kpc from the Sun, and lie in the Sagittarius--Carina arm. It is not clear if the stars observed in the Circinus region are physically related to each other, but their distances to the Sun also place most of them in the Sagittarius--Carina arm.

Intermediate-band photometry of late-type stars VIII. Nonvariable stars in the halo population near the sun.

Abstract: From a discussion of a complete sample of high-velocity stars, the following criteria are suggested for identification of halo population objects: (1) all stars having less than about 25% of the solar metal abundance; (2) if only the space motion is available, those giving e>0.42; (3) for e<0.42, vertical-barWvertical-bar larger than about 60 km s/sup -1/ (or Z greater than about 1 kpc). The available data for the halo stars indicate that the whole range of halo metal abundance is present in the region of 10 to 20 kpc from the galactic center, but this range narrows beyond 20 kpc, and the upper limit, beyond 30 kpc, is near 3% of the solar abundance.

Neutral hydrogen observations of 14 nearly edge-on spiral galaxies

Abstract: H I spectra were taken along the major axes of 14 large, relatively edge-on spiral galaxies with the 21 cm circular feed at Arecibo Observatory A procedure is developed to subtract out contributions from the first sidelobe of the beam, at least approximately From the corrected profiles isophotal hydrogen diameters and (for the seven more extended cases) rotation curves are derived Velocity disturbances seem to be more common for galaxies with close companions than for isolated galaxies As found by other authors, rotation curves tend to be flat at least to the Holmberg radius, suggesting the presence of dark material in the outer regions of spiral galaxies Comparison with early-type disk galaxies shows that (a) H I diameters vary much less with Hubble type than do H I masses, (b) the surface density of gravitational mass tends to be higher in the earlier systems, and (c) the ratio of gravitational mass to luminosity varies little with Hubble type