The spiral structure in the inner parts of the Galaxy

The spiral structure of the inner parts of the Galaxy is studied using 21 cm line data and stellar data. To study the neutral hydrogen distribution in the galactic layer a parameter =(dV/dr) proportional to the mean densities is calculated using a first approximation for the velocity gradients due to differential rotation.The obtained distribution (R, Z) shows spiral features completely consistent with the early star distribution and with the Hii regions. The corrugation effect of the galactic layer is observed in all the studied zones in neutral hydrogen and in the distribution of the OB stars in the Carina zone.The pattern obtained indicates four spiral arms for the inner parts of the Galaxy, three of which are identified also in the stellar data (arms -I, -II, and -III) and the more distant -IV in Hii regions.The local arm according to the stellar data of Kilkennyet al. forms a feature completely similar to the arms -I and -II and there are no indications that this arm is a special material branch between two main spiral arms as has been supposed in order to conciliate the neutral hydrogen pattern with the stellar distribution.The pitch angles for the spiral arms are approximately 13°–17°.The observed wave form distribution of the hydrogen cloud layer is completely consistent with the theoretical predictions of Nelson (1976) but there are no indications of such an effect in the intercloud hydrogen. The corrugated cloud layer has a width of 100 pc, a wave amplitude of 70 pc, and a wavelength which grows with the galactic center distance (approx. 2 kpc in the zones next to the galactic nucleus and 2.6–3.0 kpc in the zones next to the Sun). To each wavelength correspond two spiral arms. The spiral features in our Galaxy show characteristics quite similar to the features in the Andromeda nebula, not only in the component materials (neutral hydrogen, Hii regions and possibly also dust and stars) but also in their kinematics.     

The outermost ob associations in M33

The blink survey ofUBVR plates of the Tautenburg Schmidt telescope reveals about 20 new stellar associations. A few suspected associations are located at 8 kpc from the center of M33. The new associations fit well into the spiral structure of the galaxy.     

Outer pseudoring in the galaxy

The kinematics of the Sagittarius (R = 5.7 kpc),Carina (R = 6.5 kpc), Cygnus (R = 6.8 kpc), and Perseus (R = 8.2 kpc) arms suggests the existence of two spiral patterns in the Galaxy that rotate with different speeds. The inner spiral pattern that is represented by the Sagittarius arm rotates with the speed of the bar, Ω_b = 60 ± 5 km s⁻¹ kpc⁻¹, while the outer spiral pattern that includes the Carina, Cygnus, and Perseus arms rotates with a lower speed, Ω_s = 12–22 km s⁻¹ kpc⁻¹.The existence of an outer slow tightly wound spiral pattern and an inner fast spiral pattern can be explained by numerically simulating the dynamics of outer pseudorings. The outer Lindblad resonance of the bar must be located between the Sagittarius and Carina arms. The Cygnus arm appears as a connecting link between the fast and slow spiral patterns.     

30 Doradus as the active centre of the Large Magellanic Cloud

Evidence is presented for the hypothesis that the supergiantHii-complex 30 Doradus (NGC 2070) is the mildly active galactic nucleus of the Large Magellanic Cloud. For this purpose the general properties of galactic nuclei and the characteristics of active nuclei are reviewed (Section 2). Examination of 30 Doradus shows that it plasy the same exceptional role among allHii-regions of the LMC as Sgr A among those of our Galaxy, and has all the properties of a galactic nucleus (luminosity, emission spectrum, IR source, semistellar central object R 136, symmetry centre of an starting point of spiral structure). Evidence for the activity is given by the peculiar filamentary structure (Figure 1), the young spiral filaments superposed on old, broad and smooth near-circular arms (Figure 2), the splitting of the [Oiii] 5007 profile in two components corresponding to an expansion velocity of 50 km s^–1, and the strong non-thermal component (Section 3). The mass loss of 30 Dor is estimated at 0.05M /a. It is speculated that the nucleus of a galaxy may be wandering due to explosive events.     

Stellar associations and complexes in M33

About 460 OB associations were selected by a comparison of theUBV plates. TheUBV photographic photometry of 1944 blue stars in the associations was made. The new associations appear like cores within Humphreys and Sandage's associations. Their star content, size distribution, and mean size 80 pc confirm their identity with the OB associations in the Galaxy and in the Magellanic clouds. The boundaries of the associations are delineated mainly by the density of the resolved stars on theU plates. It is impossible to divide them into smaller areas. The genuine OB associations form groups of two or more members with a length scale of 250 pc. Their boundaries were delineated independently, but they coincide with the OB associations of Humphreys and Sandage (1980). These groups represent real concentration of blue massive stars with a large age dispersion. The star complexes unify a group of associations,Hii regions, andHi peak distribution. Their mean size is 570 pc. The extensiveHi clouds with a mean size of 1.2 kpc contain two or more star complexes. The questions related to star formation are briefly discussed.     

Peculiar features of the velocity field of OB associations and the spiral structure of the galaxy

Some of the peculiar features of the periodic velocity-field structure for OB associations can be explained using the Roberts-Hausman model, in which the behavior of a system of dense clouds is considered in a perturbed potential. The absence of statistically significant variations in the azimuthal velocity across the Carina arm probably results from its sharp increase behind the shock front, which is easily blurred by distance errors. The existence of a shock wave in the spiral arms and, at the same time, the virtually free motion of OB associations in epicycles can be reconciled in the model of particle clouds with a mean free path of 0.2–2 kpc. The velocity field of OB associations exhibits two appreciable nonrandom deviations from an ideal spiral pattern: a 0.5-kpc displacement of the Cygnus-and Carina-arm fragments from one another and a weakening of the Perseus arm in quadrant III. However, the identified fragments of the Carina, Cygnus, and Perseus arms do not belong to any of the known types of spurs.     

Origins of galactic spiral structures

Theories of galactic structure are reviewed briefly before comparing them with recent observations. Also reviewed is the evidence for an intergalactic magnetic field and its possible effects on gas concentrations and patterns of star creation, including spiral arms. It is then shown that normal spiral galaxies may be divided into the M51-type and others. The rare M51-type have Hi gas arms coincident with unusually filamentary and luminous optical arms; they also have a companion galaxy. The remaining great majority of spirals have no well-defined gas arms and their optical arms are irregular, broader and less luminous; they have no companion galaxy. It appears that without exception the half-dozen or so galaxies whose structures appear to support the density-wave theory show one or more of the characteristics of the rare type of spiral, and that the three principal confirmations of the spiral-wave idea (M51, M81, M101) have companions which may account for their arms. Toomre has rejected this idea on the grounds that his models do not agree with the observed structures. It is shown that these models are inadequate in two major respects, and when replaced by magneto-tidal models using non-uniform gas disks one might expect agreement. The original hydromagnetic model of spiral arms is now reserved for non-interacting galaxies, of which M33 might be taken as a prototype. The model predicts broad or massive optical arms and no corresponding arms of neutral hydrogen, as observed.     

Die Spiralstruktur der Grossen Magellanschen Wolke

The spiral structure of the Large Magellanic Cloud has been investigated using the best spiral indicators (Table I). A clear spiral pattern emerges from the distribution of the optical Hii-regions with diametersD256 pc and the blue supergiants with (U-B)_o–0.60 (i.e., O-B8 Ia-O to Iab) (Figure 1). This structure is reflected in the distribution of supernova remnants, OB-associations, young open clusters, Wolf-Rayet-Stars and X-ray sources (Figure 3), andM-supergiants. The spiral features emanate from the 30 Doradus Hii-complex as centre and are completely unrelated to the LMC Bar. The structure around 30 Dor is especially well seen in the distribution and even the form (elongation) of the dark clouds (Figure 3). In Section 3 a new interpretation of the 21 cm-line data is attempted based on an analysis of the neutral hydrogen densityN _HI integrated over the line of sight. The ridge lines of the integrated 21 cm-line intensities delineate two main features with a half-intensity width of 0.9 kpc, roughly coinciding with the two main optical features. Apart from many differing details the basic structure of the distribution of Hi and of the optical spiral tracers is similar: (1) 30 Dor is the centre of density and starting-point of the spiral features, (2) two main complex arms I and II dominate the distribution, (3) the arms are fragmented optically (Ia-c and IIa-e) as well as in the Hi, start with the same steep pitch angles in directions displaced by about 60° (instead of the usual diametral symmetry of common two-armed spirals) and wind in the same directions. In the discussion (Section 4) it is first shown why the optical Hii-regions with small diametersD<6 pc do not show the spiral structure (Figure 2) due to selection and age effects. The LMC is classified exclusively from the appearance of the spiral tracers as Scp in the Hubble-system (the peculiarity is the asymmetry of the spiral structure) and ScIII-IVp in the DDO-system. Arguments are given to justify such a classification procedure. Similar galaxies may be MCG 4-31-14 and NGC 3664 S(B)IV-V:.The tidal action of our Galaxy is most probably unimportant for the spiral structure of the LMC, certainly so in the inner parts where the features are sharpest. Evidence is presented to support the view that the enormous supergiant Hii-complex 30 Doradus (M _{H II}3×10⁵ M ,M _tot=5×10⁶ M ) is the nucleus of the spiral LMC.The LMC as a pathological spiral helps us to understand together with the disturbed also the normal function of spiral formation. A density-wave type theory of spiral structure is hardly compatible with the observations; an ejection theory appears to be more promising.     

Distribution of luminous blue stars in M33

M₃₃ luminous stars with M_V ≲ -7 and U--B < o are selected in this paper. The spatial distribution of the stars shows a good agrement with the associations and HII regions. However HII regions are absent in the three youngest associations in M₃₃ (t ≈ 5 ÷ 7 × 10yr). It is supposed that the true distance modulus of M₃₃ derived from the brightest blue stars is 24.2. An age gradient across the arms is absent. The spiral arms outlined by the luminous stars differ form those in M₃₁. They are diffuse and wide. The multiarm features of M₃₃ support the stochastic self-propagating model.     

Distance estimation of the LMC,SMC, and M33 from the apparent radius distribution ofHii-regions

The size distribution ofHii-regions in the LMC, SMC, and M33 is used to estimate their distances. The values deduced are: 42.66±2.51, 89.73±5.03, and 755.11±47.14 kpc, respectively.     

Simulation of the apparent anomalies of the galactic structure

In the maps of the galactic structure based on the kinematical method, several systematic heliocentric anomalies are found: in the northern galactic hemisphere the spiral arms are more tightly wound and the extent of neutral hydrogen is smaller than in the southern hemisphere; with separate rotation curves for the north and the south the arms become anomalously circular with a consequent discrepancy to the stellar distribution; there are straight portions in the arms pointing towards the Sun, as well as systematic strong curvatures and knee-like features; the inner arms affect the structure of the outer arms; with the northern rotation model, Hii-regions and Hi avoid the southern tangential circle; in the rear of the Galaxy, at symmetric longitudes, enhanced Hi-densities are found; the Perseus arm is displaced atl=180°. All of these anomalies can be explained with a simple model involving a non-velocity redshift field within the Galaxy, with an enhancement within the spiral arms. This is demonstrated by numerical simulations of the structural anomalies. Reducing the redshift effect from the kinematic data, the Galaxy's structure and kinematics appear symmetric. The significance of the result for the redshift problem is discussed.     

Magnetic and optical spiral arms in the galaxy NGC 6946

The spiral pattern in the nearby spiral galaxy NGC 6946 has been studied using the wavelet transformation technique, applied to galaxy images in polarized and total non-thermal radio emission at λλ 3.5 and 6.2 cm, in broadband red light, in the λ 21.1 cm H  i line and in the optical Hα line. Well-defined, continuous spiral arms are visible in polarized radio emission and red light, where we can isolate a multi-armed pattern in the range of galactocentric distances 1.5–12 kpc, consisting of four long arms and one short spiral segment. The 'magnetic arms' (visible in polarized radio emission) are localized almost precisely between the optical arms. Each magnetic arm is similar in length and pitch angle to the preceding optical arm (in the sense of galactic rotation) and can be regarded as its phase-shifted image. Even details like a bifurcation of an optical arm have their phase-shifted counterparts in the magnetic arms. The average relative amplitude of the optical spiral arms (the stellar density excess over the azimuthal average) grows with galactocentric radius up to 0.3–0.7 at r ≃5 kpc, decreases by a factor of two at r =5–6 kpc and remains low at 0.2–0.3 in the outer parts of the galaxy. By contrast, the magnetic arms have a constant average relative amplitude (the excess in the regular magnetic field strength over the azimuthal average) of 0.3–0.6 in a wide radial range r =1.5–12 kpc. We briefly discuss implications of our findings for theories of galactic magnetic fields.     

Leading wave as a component of the spiral pattern of the galaxy

The spiral pattern of the Galaxy, identified by analyzing the kinematics of young stars within 3 kpc of the Sun, is Fourier decomposed into spiral harmonics. The spiral pattern of the Galaxy is shown to be representable as a superposition of trailing and leading waves with interarm distances of λ = 1.8 ± 0.4 and 4 ± 2 kpc, respectively. Shock waves are probably present only in the portions of the trailing spiral pattern where it crosses the crest of the leading wave. The small interarm distance of the trailing spiral wave (λ = 1.8 kpc) can be explained by its evolution—by the decrease in the interarmd istance as the wave is displaced toward the inner Lindblad resonance. The Carina arm may be part of this resonance ring.     

Differential efficiency of massive star formation in NGC 628

The efficiency ratio of massive star formation in the arms of the spiral NGC 628 to that in the interarm region has been determined from the sizes and positions of itsHii regions following the method described in Cepa and Beckman (1989a, b). The variation of this relative star formation efficiency with radius shows a very similar behaviour for each of the three arms of this galaxy with a minimum at 100 arc sec from the centre, which can be identified with the corotation radius distances at which the arms show bifurcations or sudden changes in their pitch angles. This result, together with the higher star formation efficiency in the arms than that in the interarm zones, is evidence supporting the hypothesis that the spiral arms of NGC 628 are driven by a density wave which triggers the star formation according to the spiral density wave theory.     

Weitere OB-Assoziationen in den äußersten Randzonen von M 31

In comparing a U- and a B-plate of the Tautenburg 2 m Schmidt telescope 7 scattered groupings of blue objects have been discovered in the outer regions of the large Andromeda nebula. A closer investigation showed these objects to be apparently so far not known OB associations, well fitting to a spiral structure. With distances from the centre of the Andromeda nebula between R = 22.8 kpc and R = 27.9 kpc the objects still surpass the most distant association OB 188 of VAN den BERGH (R = 25.0 kpc).     

The NGC 1275 enigma

The following arguments suggest that NGC 1275 does not consist of a giant elliptical (E) galaxy that is colliding with (or is superimposed on) a late-type spiral (L):

• 1 The total diameter of the region containing young associations is 33 (100/H) kpc. This size is characteristic of ScI galaxies. Neither the morphology nor the integrated luminosity of the L component of NGC 1275 supports such a classification.
• 2 The chaotic appearance of the L component of NGC 1275 is unlikely to be due to tidal damage. This is so because: (a) the E and L components are still approaching each other, (b) their relative velocity is ≈︁ 3000 km s⁻¹, (c) no stripped galaxy core (which would survive a catastrophic tidal encounter) is seen near NGC 1275.
• 3 The core of the Perseus cluster contains only one (anemic) spiral. The a priori probability that NGC 1275 represents a chance superimposition (or collision) of a spiral and an elliptical galaxy is therefore low.
• 4 The assumption that the L component of NGC 1275 is superimposed on, but not interacting with, the E component does not account for (a) the presence of an active SEYFERT nucleus, (b) the peculiar filamentary HII shell, discovered by LYNDS , (c) the presence of recently-formed stars, (d) the X-ray emission and the radio emission of NGC1275.

     

Suspected OB stars near the andromeda nebula

An investigation of suspected OB stars on plates of the Schmidt telescope of Karl Schwarzschild Observatory Tautenburg gave the following results: I. There exists a sharp boundary of the M 31 disk. For distances larger than R = 30 kpc from the centre no OB stars of the Andromeda galaxy could be found. 2. The isolated OB stars in the outer regions of M 31 fit into the spiral structure. 3. The suspected OB stars outside M 31 (R>30 kpc) in all probability belong to our own galaxy. It is to be expected that they partly are blue horizontal branch stars of extreme population II, partly white dwarfs rather far left of the black body line in the two colour diagram.     

Grand-Design Spirals in the Core of NGC 5248

We report the detection of a bisymmetric nuclear spiral structure in the spiral galaxy NGC 5248.The two red spiral arms can be followed for about 3 arcsec, before they appear to end inside the radius of the circumnuclear starburst `ring' at about 5.5 arcsec or 400 pc distance from the nucleus. We combine our near-infrared Canada–France–Hawaii Telescope adaptive optics images with traditional near-infrared and optical images and show that spiral structure is present in this galaxy at spatial scales reaching from a hundred parsecs to 15 kpc. Comparison with a Hubble Space Telescope ultraviolet image shows how the starburst ring is related to the nuclear spiral structure. We also show a two-dimensional H_α velocity field that reveals no evidence for systematic streaming motions near the nuclear spiral or the starburst ring, nor for a rapidly rising rotation curve. This revised version was published online in July 2006 with corrections to the Cover Date.     

Meta-analysis from different tracers of the small Local Arm around the Sun—extent,shape, pitch,origin

The Sun in not located in a major spiral arm, and sits in a small ‘Local Arm’ (variously called arm, armlet, blob, branch, bridge, feather, finger, segment, spur, sub-arm, swath, etc.). The diversity of names for the ‘Local Arm’ near the Sun indicates an uncertainty about its shape or pitch or its extent from the Sun in each galactic quadrant, as well as an uncertainty about its origin.Here we extract data about the small ‘Local Arm’ near the Sun, from the recent observational literature, over many arm tracers, and we use statistics in order to find the local arm’s mean extent from the Sun, its possible shape and pitch angle from the direction of galactic longitude \(90^{\circ}\). Employing all tracers, the Local Arm is about 4 kpc long by 2 kpc large. The Sun is within 1 kpc of the center of the local arm. Proposed ‘bridges’ and ‘fingers’ are assessed. These bridges to nearby spiral arms and fingers across spiral arms may not reach the nearest spiral arms, owing to kinematic and photometric distance effects.We then compare these statistical results with some predictions from recent models proposed to explain the local arm (perturbations, resonances, density wave, halo supercloud, debris trail from a dwarf galaxy).The least controversial models involve importing materials from elsewhere (halo supercloud, debris trail) as a first step, and to be later deformed in a second step (by the Galaxy’s differential rotation into become roughly parallel to spiral arms) and then subjected to ongoing forces (global density waves, local perturbations).     

On the spiral structure of M33

The spiral structure of M33 has been examined on the basis of 197 associations. The parameters of the orientation of the plane are derived. It is supposed to be a system consisting of seven logarithmic spirals. Two principal arms have been interpreted through the use of the density-wave theory. The remaining multiple arm features of M33 may be explained with the stochastic self-propagating model. The distribution of surface brightness segments speaks for the existence of different mechanisms of star formation in M33.