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This paper presents some statistical correlations of 72 northern spiral galaxies. The results show that early-type spirals that are brighter, and thicker, and the axis ratios ($H_{z}/H_r$) of the disk tend to be smaller along the Hubble sequence. We also find that $H_{z}/H_r$ correlates strongly with the galaxy's color, and early-type spirals have...
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... this section, we will present some statistical correlations between H z /H r and morphological type, luminosity and color for spiral galaxies. Ma et al. (1997Ma et al. ( , 1998) derived the flatnesses (H z /D 0 ) for 72 spiral galaxies. When we take D 0 /H r ≈ 5 (H r is defined as 2h r ), we can derive the values of H z /H r for these spirals. Fig. 2 shows H z /H r as a function of different Hubble types. It can be seen that H z /H r of spiral galaxies decreases smoothly an average along the Hubble type, but the dispersion in H z /H r among galaxies of the same Hub- ble sequence is very large. This, perhaps, reflects that the intrinsic flattening of spiral disks is smaller for later type galaxies. de Grijs (1998) also shows that galaxies become systematically thinner when going from type S0 to Sc. Fig. 3 plots H z /H r as a function of total ("face-on") absolute magnitude in the B band. In Fig. 3, we can see a clear trend reflecting an increase in the values of H z /H r as the galaxies become brighter for all but a few galaxies. Combining this data with the luminosity function Φ(L, T ) of the Virgo cluster ( Binggeli et al., 1988;Shu et al., 1995), we find that early-type spirals, which are brighter on av- erage than late-type ones, have larger values of H z /H r . Fig. 4 plots H z /H r as a function of total galactic color index ((B − V ) 0 T ) in the RC3 and shows a strong trend which suggests that a bluer galaxy has a smaller value of H z /H r . Roberts & Haynes (1994) studied the physical parameters along the Hubble sequence systematically by making use of two primary catalogues, the RC3 and a pri- vate catalogue maintained by R. Giovanclli & M. Haynes. He presented the well-established trend between morphol- ogy and mean color which reveals that the E and S0 galax- ies are clearly redder than spirals and that late-type spi- rals are bluer on average than early-type ones. So, Fig. 4 also implies that later-type galaxies have smaller values of H z /H r . (2) and Ma et al. (1997Ma et al. ( , ...
Context 2
... this section, we will present some statistical correlations between H z /H r and morphological type, luminosity and color for spiral galaxies. Ma et al. (1997Ma et al. ( , 1998) derived the flatnesses (H z /D 0 ) for 72 spiral galaxies. When we take D 0 /H r ≈ 5 (H r is defined as 2h r ), we can derive the values of H z /H r for these spirals. Fig. 2 shows H z /H r as a function of different Hubble types. It can be seen that H z /H r of spiral galaxies decreases smoothly an average along the Hubble type, but the dispersion in H z /H r among galaxies of the same Hub- ble sequence is very large. This, perhaps, reflects that the intrinsic flattening of spiral disks is smaller for later type galaxies. de Grijs (1998) also shows that galaxies become systematically thinner when going from type S0 to Sc. Fig. 3 plots H z /H r as a function of total ("face-on") absolute magnitude in the B band. In Fig. 3, we can see a clear trend reflecting an increase in the values of H z /H r as the galaxies become brighter for all but a few galaxies. Combining this data with the luminosity function Φ(L, T ) of the Virgo cluster ( Binggeli et al., 1988;Shu et al., 1995), we find that early-type spirals, which are brighter on av- erage than late-type ones, have larger values of H z /H r . Fig. 4 plots H z /H r as a function of total galactic color index ((B − V ) 0 T ) in the RC3 and shows a strong trend which suggests that a bluer galaxy has a smaller value of H z /H r . Roberts & Haynes (1994) studied the physical parameters along the Hubble sequence systematically by making use of two primary catalogues, the RC3 and a pri- vate catalogue maintained by R. Giovanclli & M. Haynes. He presented the well-established trend between morphol- ogy and mean color which reveals that the E and S0 galax- ies are clearly redder than spirals and that late-type spi- rals are bluer on average than early-type ones. So, Fig. 4 also implies that later-type galaxies have smaller values of H z /H r . (2) and Ma et al. (1997Ma et al. ( , ...
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Citations
It is unable to obtain the disk thickness of a face-on spiral galaxy by measuring its surface brightness distribution. Here, in order to obtain the disk thickness of faced-on spiral galaxies, we determine the scale heights of their disks using an analytical method, based on the asymptotic solution of Poison's equation of gravitational potential in a 3-D galactic disk under the equiangular logarithmic spiral type of density perturbation. For measuring the key parameter, namely the innermost position of spiral arms (forbidden radius r0), and free from the effect of light contamination of galactic bulge, an improved image processing method is used in this study by subtracting a two-component (disk+bulge) brightness model from the observed galaxy image. On the basis of some fitted and measured fundamental parameters of spiral structures, we obtain the disk thickness and other parameters for the two face-on spiral galaxies: one normal spiral galaxy (S) and one spiral bar galaxy (SB), and their ratio parameters (the scale ratio of bulge and disk rb/rd, and the ratio of disk scale and thickness rd/H) are also derived. By using this improved method of two-component subtraction, it is easy to obtain the forbidden radius r0, which is smaller than that obtained by measuring directly the original galaxy image, thus the obtained effective thickness of galactic disks becomes thinner.
We analyse the dependence of the stellar disc flatness on the galaxy morphological type using 2D decomposition of galaxies from the reliable subsample of the Edge-on Galaxies in SDSS catalogue. Combining these data with the retrieved models of the edge-on galaxies from the Two Micron All Sky Survey and the Spitzer Survey of Stellar Structure in Galaxies catalogue, we make the following conclusions.
The disc relative thickness z 0 / h in the near- and mid-infrared passbands correlates weakly with morphological type and does not correlate with the bulge-to-total luminosity ratio B/T in all studied bands.
Applying a 1D photometric profile analysis overestimates the disc thickness in galaxies with large bulges making an illusion of the relationship between the disc flattening and the ratio B/T.
In our sample, the early-type disc galaxies (S0/a) have both flat and ‘puffed’ discs. The early spirals and intermediate-type galaxies have a large scatter of the disc flatness, which can be caused by the presence of a bar: barred galaxies have thicker stellar discs, on average. On the other hand, the late-type spirals are mostly thin galaxies, whereas irregular galaxies have puffed stellar discs. </l
On the basis of Poisson's equation for the logarithmic perturbation of matter density, we provide improved estimates of scale heights and spiral structures for non-edge-on spiral galaxies by subtracting the surface brightness distributions from observed images. As examples, the non-edge-on spiral galaxies PGC 24996, which is face-on, and M31, which is inclined, are studied. The scale height, pitch angle and inclination angle of M31, our nearest neighbor, that are presented in this work, agree well with previous research.
We calculate the effect of finite disc thickness on the structure and stability of a differentially rotating three-dimensional spiral galaxy with stars and gases. The problem is solved exactly in terms of the rigorous mathematical method of Green function and Bessel–Fourier transform. By using the appropriate three-dimensional self-gravitational potential due to the density perturbations of the spiral arms in the galactic disc, the dispersion relation for the quasi-stationary density waves are deduced for the composite disc model of stars and gases with finite thickness. The stability criterion depicted by the Toomre Q parameter for these spiral modes are investigated in detail. In contrast to the standard Q parameter for an infinitely thin two-dimensional disc, our generalized Toomre Q parameter depends explicitly on the disc thickness and the location. In the limit of an infinitely thin disc, our result naturally reduces to Toomre's criterion for a stellar disc and to Kalnajs's result for a gaseous disc. In addition, the significant role played by the finite thickness of the galactic disc on the spatial structure of the stellar arms and the gaseous arms in the galactic plane is also briefly discussed. Application of our theory to the disc models with two stellar arms and four gaseous arms, gas content 15% and the total surface density in the solar neighbourhood σ = 52M0 pc−2 but with the thickness parameter α in the range of 3.0<α<4.0 (250 pc <hz<325 pc) appropriate to our Galaxy yields good results in general agreement with recent observations.
We present disk thicknesses, some other parameters and their statistics of 108 non-edge-on spiral galaxies. The method for determining the disk thickness is based on solving Poisson's equation for a disturbance of matter density in three-dimensional spiral galaxies. From the spiral arms found we could obtain the pitch angles, the inclination of the galactic disk, and the position of the innermost point (the forbidden region with radius r 0 to the galactic center) of the spiral arm, and finally the thickness.
We analyze the R-and K
s-band photometric profiles for two independent samples of edge-on galaxies. The thickness of old stellar disks is shown to
be related to the relative masses of the spherical and disk components of galaxies. The radial-to-vertical scale length ratio
for galactic disks increases (the disks become thinner) with increasing total mass-to-light ratio of the galaxies, which reflects
the relative contribution of the dark halo to the total mass, and with decreasing central deprojected disk brightness (density).
Our results are in good agreement with numerical models of collisionless disks that evolved to a marginally stable equilibrium
state. This suggests that, in most galaxies, the vertical stellar-velocity dispersion, on which the equilibrium-disk thickness
depends, is close to a minimum value that ensures disk stability. The thinnest edge-on disks appear to be low-brightness galaxies
in which the dark-halo mass far exceeds the stellar-disk mass.
We investigate whether the behavior of the pitch angle, predicted by a local dispersion relation derived from the density
wave theory, can be applied to that of the fastest growing, globally unstable modes of flat stellar disks. We pay attention
to two-armed modes, and obtain such global modes by numerically integrating the linearized collisionless Boltzmann equation.
The results show that the pitch angle of the fastest growing modes has a tight correlation with the ratio of the square of
the radial velocity dispersion to the surface density, as indicated by the local theory, over a sufficiently wide range of
radii. The correspondence between the global modes and global properties of spiral galaxies is briefly discussed.
In this paper we calculate the effect of finite disk thickness on the structure and stability of a differentially rotating three-dimensional disk galaxy with stars and gases. The problem is solved exactly in terms of the rigorous mathematical method of Green function and Bessel–Fourier transform. In order to bring out the physical significance of the effect of finite thickness clearly, we concentrate on the axially symmetric modes with azimuthal symmetry. The corresponding reduction factor for these modes are investigated in detail. In this way we avoid the WKB approximation generally adopted for the spiral modes. Our reduction factor depends not only on the surface density and the epicyclic frequency (which depicts the effect of differential rotation) but also explicitly on the disk thickness and the location. In the limit of an infinitely thin disk, our result naturally reduces to Toomre's criterion for a stellar disk and to Kalnajs's result for a gases disk. In addition, the significant role played by the finite thickness of the galactic disk on the mass of the galactic halo required for the Milky Way is also briefly discussed in and .
According to one version of the recently proposed "manifold" theory that
explains the origin of spirals and rings in relation to chaotic orbits,
galaxies with stronger bars should have a higher spiral arms pitch angle when
compared to galaxies with weaker bars. A sub-sample of barred-spiral galaxies
in the Ohio State University Bright Galaxy Survey, was used to analyze the
spiral arms pitch angle. These were compared with bar strengths taken from the
literature. It was found that the galaxies in which the spiral arms maintain a
logarithmic shape for more than 70$\degr$ seem to corroborate the predicted
trend.
This paper presents some new statistical correlations between the properties of disks and spiral arms with some physical properties of galaxies. Our results show that the thickness of spiral disks tend to diminish along the Hubble sequence in the sense that disks of Sc galaxies are 40% thinner than disks of Sab-Sb galaxies. Moreover, the thinner disks tend also to be bluer. We also find that there exists a correlation between HI linewidths and arm pattern within each Hubble type, which suggests that the arm shape is partially determined by the mass of a galaxy. Total mass luminosity ratios and total mass surface densities also have correlations with pitch angles, i.e., for disks with lower surface densities and lower total mass luminosity ratios, the pitch angles tend to be greater. Comment: 8 pages, has been accepted for publication in A&Ap
