Theoretical 21-cm line profiles: Comparison with observations

In order to make a direct comparison with observations of the 21-cm line of neutral hydrogen, theoretical profiles based on the ideas of the density-wave theory are constructed for a modified Schmidt model of the Galaxy and its theoretical spiral pattern. The comparison has covered galactic longitudes lII = 30° −330° with 10° intervals in the galactic plane. Good agreement is found in most of the above directions.

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Current Problems in 21 cm Line Studies of the Large-Scale Structure of the Galaxy

Symposium - International Astronomical Union ◽

10.1017/s0074180900026875 ◽

1974 ◽

Vol 60 ◽

pp. 551-572 ◽

Cited By ~ 1

Author(s):

W. B. Burton

Keyword(s):

Shock Front ◽

Large Scale ◽

Spiral Structure ◽

Density Wave ◽

Model Fitting ◽

Neutral Hydrogen ◽

Wave Theory ◽

Large Scale Structure ◽

The Galaxy ◽

Density Wave Theory

A number of current problems in 21 cm line studies of the Galaxy as a whole are discussed. Because of the difficulties involved with straightforward mapping, it is important to isolate integrated and other properties of the hydrogen profiles, the interpretation of which does not require accurate distance determinations. In addition, methods of analysis are necessary which either account for or exploit the sensitivity of hydrogen profiles to velocity irregularities and to geometrical configurations. The model-fitting approach to the interpretation of the hydrogen profiles is useful in this respect. Extragalactic hydrogen studies which show the relative ordering of the various components of spiral structure can inspire research in our own Galaxy. Such investigations are necessary for an understanding of the forces governing the spiral structure. It seems that the neutral hydrogen is primarily a tracer of locations where the overall distribution of stars is producing a gravitational sink. Other spiral tracers, in particular the molecules, are better considered as tracers of regions where the gas has been compressed, perhaps (at least on a large scale) by the shock front predicted by the density-wave theory.

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Gas Response to a Rotating Stellar Bar

Publications of the Astronomical Society of Australia ◽

10.1017/s1323358000024656 ◽

1978 ◽

Vol 3 (3) ◽

pp. 234-236

Author(s):

M. P. Schwarz

Keyword(s):

Spiral Galaxies ◽

Fast Rate ◽

Density Wave ◽

Wave Theory ◽

Finite Amplitude ◽

Spiral Waves ◽

Galactic Rotation ◽

Spiral Pattern ◽

Density Wave Theory ◽

Gas Response

The arms in spiral galaxies cannot be material arms for then they would wind up on a time scale of one galactic rotation, or a few times 108 years. The large number of spirals suggests that the spiral pattern must persist for about 1010 years (or be continually rejuvenated). The density wave theory treats the spiral pattern as a wave phenomenon, thus overcoming this problem. Much work has been done studying small amplitude oscillations in flat stellar discs. Self-consistent spiral modes have been found, but they are not stable and grow at a fast rate. Numerical simulations of thin stellar discs, such as those of Hohl (1971), which can handle finite amplitude waves, have been more successful. Spiral waves form initially but evolve into a steady state rotating bar. It seems therefore, that a long-lived spiral cannot be formed in stars alone.

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Signatures of long-lived spiral patterns: The color gradient trend

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314005870 ◽

2012 ◽

Vol 10 (H16) ◽

pp. 323-323

Author(s):

Eric E. Martínez-García ◽

Rosa Amelia González-Lópezlira

Keyword(s):

Stellar Population ◽

Density Wave ◽

Wave Theory ◽

Disk Galaxies ◽

Population Synthesis ◽

Spiral Pattern ◽

Stellar Population Synthesis ◽

Color Gradient ◽

Pattern Speeds ◽

Density Wave Theory

AbstractBased both on observations and simulations, recent works propose that the speed of the spiral pattern in disk galaxies may decrease with increasing radius; the implications are that patterns are actually short-lived, and that the azimuthal color/age gradients across spiral arms predicted by density wave theory could not be produced. We, however, have consistently found such gradients, and measured spiral pattern speeds by comparing the observations with stellar population synthesis models (González & Graham 1996; Martínez-García et al. 2009a, b; Martínez-García & González-Lópezlira 2011). Here, we summarize our previous results in non-barred and weakly barred spirals, together with six new, as yet unpublished, objects. On the other hand, we have indeed found a trend whereby pattern speeds at smaller radii are larger than expected from a model that assumes purely circular orbits (cf. Figure 1), likely due to the effect of spiral shocks on the orbits of newborn stars. The results suggest that spirals may behave as steady long-lived patterns.

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Neutral hydrogen in the Sagittarius and Scutum spiral arms

Symposium - International Astronomical Union ◽

10.1017/s0074180900000942 ◽

1970 ◽

Vol 38 ◽

pp. 397-414 ◽

Cited By ~ 3

Author(s):

W. B. Burton ◽

W. W. Shane

Keyword(s):

Large Scale ◽

Galactic Center ◽

Density Wave ◽

Neutral Hydrogen ◽

Wave Theory ◽

Galactic Rotation ◽

Circular Rotation ◽

Kinematic Models ◽

Density Wave Theory ◽

Galactic Longitude

Observations of the neutral hydrogen in the first quadrant of galactic longitude have been analysed. The existence of large-scale streaming motions such as the streaming associated with the Sagittarius arm makes interpretation of the observations in terms of circular galactic rotation unsatisfactory. It is shown that application of the density-wave theory formulated by Lin et al. (1969) leads to a more satisfactory interpretation. Using kinematic models based on this theory the distribution and motion of the neutral hydrogen are studied. Failures of kinematic models based on circular rotation are pointed out. A map of the distribution of neutral hydrogen is produced. The Scutum arm is composed of inner and outer arcs both of which seem to be moving outward from the galactic center with velocities of the order of 30 km s−1.

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HI and continuum structure of external galaxies compared to the situation in the galaxy

Symposium - International Astronomical Union ◽

10.1017/s0074180900014881 ◽

1979 ◽

Vol 84 ◽

pp. 417-426

Author(s):

Arnold H. Rots

Keyword(s):

Gas Flow ◽

Density Wave ◽

Wave Theory ◽

Radio Halo ◽

Ngc 6946 ◽

Total Luminosity ◽

The Galaxy ◽

External Galaxies ◽

Continuum Structure ◽

Density Wave Theory

Continuum observations of NGC 6946 strongly suggest the presence of an exponential non-thermal disk component. When applied to galactic 408 MHz observations, the scale length is found to be between 5 and 6 kpc. Continuing the parallel with NGC 6946, a total luminosity of 2 × 1010L⊙is derived. Observations of the edge-on galaxies NGC 891 and 4631 show the existence of flattened halos with a steep spectrum around these galaxies. This is consistent with the constraints that are known for the galactic radio halo. On the basis of the radial distribution of HI in the Galaxy and in M81 three regimes can be defined: a central one which is hyperdeficient in hydrogen; a middle one which is deficient in HI, but not necessarily in total hydrogen; and an outer one which is neither deficient in HI, nor in H. Density wave theory, combined with gas flow dynamics appear a powerful tool in interpreting the kinematical data. No rotation curve is yet known to become Keplerian in the outer parts. Many external galaxies are warped, like our own; one of them, NGC 5907, does not have a visible companion.

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The Cosmogonical, Significance of the Z Distribution of Stars

International Astronomical Union Colloquium ◽

10.1017/s0252921100067634 ◽

1977 ◽

Vol 45 ◽

pp. 271-274

Author(s):

L.G. Balazs

Keyword(s):

Velocity Dispersion ◽

Galactic Plane ◽

Density Wave ◽

Wave Theory ◽

Space Distribution ◽

Interstellar Gas ◽

Cooperative Phenomena ◽

Small Velocity ◽

Velocity Dispersions ◽

Density Wave Theory

According to the consequences of the density wave theory of spiral structure a shock wave is triggered in the interstellar gas leading to the formation of new stars (Roberts, 1969). The space and velocity distribution of the newly born stars is similar to those of the diffuse material which they have been formed from, i.e. they are strongly concentrated to the Galactic plane and have nearly circular velocities and small velocity dispersions. The velocity dispersions could increase in course of time by different stellar dinamical processes: encounters with large clouds of stars and gas (Spitzer and Schwarzschild, 1953; Julian, 1967; Barbanis and Woltjer, 1967), cooperative phenomena (Lynden-Bell, 1967), effect of non periodic orbits (Wielen, 1975), etc. If some of this processes has a time scale comparable with the time the star streeming needs between two consecutive passages of the density wave one expects an observable effect in the space distribution of A and late B type stars perpendicular to the Galactic plane. The Δt time difference between two consecutive passages equals 2.5×108 years near the sun. If the lifetime of stars is greater than this value their space distribution is a superposition of newly born stars with small velocity dispersion and stars born earlier and having greater velocity dispersion already. (Fig. 1.)

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Possible Pattern Speeds of a Density Wave in our Galaxy

International Astronomical Union Colloquium ◽

10.1017/s0252921100067658 ◽

1977 ◽

Vol 45 ◽

pp. 279-282 ◽

Cited By ~ 1

Author(s):

Preben J. Grosbøl

Keyword(s):

Velocity Field ◽

Spiral Structure ◽

Density Wave ◽

Wave Theory ◽

Young Stars ◽

Spiral Pattern ◽

Considerable Effort ◽

Pattern Speed ◽

Pattern Speeds ◽

Density Wave Theory

Since the density wave theory was introduced by Lin and Shu (1964) to explain the spiral structure considerable effort has been made to detect this kind of wave in our galaxy and to determine its parameters. Observations of the distribution and velocity field of gas and young objects show the present shape and location of the spiral pattern in our galaxy but tell little about its angular velocity. It was proposed by Strömgren (1967) to estimate this important parameter by calculating the places of formation of moderately young stars for which accurate space velocities and ages are known. This was done assuming that the majority of stars is formed in spiral arms so that the stellar birthplaces would outline the position of the spiral pattern at different epochs. Later, Yuan (1969) and Wielen (1973) calculated stellar birthplaces in the spiral potential given by Lin et al. (1969). These investigations showed no disagreement with the assumed density wave, however, the number of stars was too small to verify the assumed pattern speed.

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Outer rotation curve of the Galaxy with VERA. III. Astrometry of IRAS 07427−2400 and test of the density-wave theory

Publications of the Astronomical Society of Japan ◽

10.1093/pasj/psv049 ◽

2015 ◽

Vol 67 (4) ◽

pp. 69 ◽

Cited By ~ 20

Author(s):

Nobuyuki Sakai ◽

Hiroyuki Nakanishi ◽

Mitsuhiro Matsuo ◽

Nagito Koide ◽

Daisuke Tezuka ◽

...

Keyword(s):

Rotation Curve ◽

Density Wave ◽

Wave Theory ◽

The Galaxy ◽

Density Wave Theory

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Interstellar gas in the central region of the Galaxy

Symposium - International Astronomical Union ◽

10.1017/s0074180900101032 ◽

1967 ◽

Vol 31 ◽

pp. 239-251 ◽

Cited By ~ 3

Author(s):

F. J. Kerr

Keyword(s):

Central Region ◽

Galactic Plane ◽

Neutral Hydrogen ◽

Continuum Emission ◽

Galactic Centre ◽

Interstellar Gas ◽

Hydrogen Recombination ◽

The Galaxy ◽

Centre Region ◽

The Relationship

A review is given of information on the galactic-centre region obtained from recent observations of the 21-cm line from neutral hydrogen, the 18-cm group of OH lines, a hydrogen recombination line at 6 cm wavelength, and the continuum emission from ionized hydrogen.Both inward and outward motions are important in this region, in addition to rotation. Several types of observation indicate the presence of material in features inclined to the galactic plane. The relationship between the H and OH concentrations is not yet clear, but a rough picture of the central region can be proposed.

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