Galaxy rotation

By the end of the 18 century a sufficient number of distances had been measured to deduce that the solar system was part of a disc-like galaxy. By following the relative motion of stars with respect to nebular clouds it was discovered that, like the solar system, the entire Galaxy rotates about a centre. 

An unknown event disturbed the equilibrium of the interstellar cloud, and it collapsed. This process may have been caused by shock waves from a supernova explosion, or by a density wave of a spiral arm of the galaxy. The gas molecules and the particles were compressed, and with increasing compression, both temperature and pressure increased. It is possible that the centrifugal forces due to the rotation of the system prevented a spherical contraction. The result was a relatively flat, rotating disc of matter, in the centre of which was the primeval sun. Analogues of the early solar system, i.e., protoplanetary discs, have been identified from the radiation emitted by T Tauri stars (Koerner, 1997). 

This work aims at testing the suggestion of [5] that stellar rotation is faster at lower metallicity by direct measurements, especially in the LMC and SMC, on stars with —3.34 < My < —2.17, i.e. spectral types B0-B6 or masses from 6.7 to 14 M0. This work is complementary to that of [4], which deals with slightly more massive stars. The results are shown on Fig. 1 and commented in the caption. There is an excess of slow rotators in the Galaxy relative to the MCs, but the v sin i distributions of the LMC and the SMC are surprisingly similar. 

Fig. 2. Comparison of N/O ratios for programme stars (filled circles) with the simple models of Meynet Maeder (2002), both including stellar rotation (dotted lines) and with no stellar rotation (dashed lines) and with the analytical models of Henry et al. (2000, solid lines) which were computed to fit the N/O ratios observed in extragalactic HII regions (blue compact galaxies). Empty boxes are metal-rich stars. See Israelian et al. (2004) for details.

Massive stars play a key role in the spectral evolution of galaxies, they are also the progenitors of Wo I f Rayet (WR) stars, supernovae and y-ray bursts. They are the main agents of nucleosynthesis driving the chemical evolution of galaxies. The relative numbers of the various kinds of massive stars (blue, red supergiants, WR stars), their properties and nucleosynthesis very much depend on mass loss and rotation, as well as on the interaction of these two effects. 

We present chemical evolution models for NGC 6822 computed with five fixed parameters, all constrained by observations, and only a free parameter, related with galactic winds. The fixed parameters are i) the infall history that has produced NGC 6822 is derived from its rotation curve and a cosmological model ii) the star formation history of the whole galaxy based on star formation histories for 8 zones inferred from H-R diagrams iii) the IMF, the stellar yields, and the percentage of Type la SNe progenitors are the same than those that reproduce the chemical history of the Solar Vicinity and the Galactic disk. 

Based on the ACDM concordance cosmological model and adopting the maximum circular velocity as the rotational velocity at the last measured point, we have obtained the DM mass of the galaxy (3.4 x 1010Mq). We use the recipe for the halo mass assembly history described by van den Bosch (2002) and the effect that reionization has on the infalling gas (Kravtsov et al., 2004) to derive the rate at which gas is accreted by the galaxy. 

The existence of dark matter (either baryonic or non-baryonic) is inferred from its gravitational effects on galactic rotation curves, the velocity dispersions and hydrostatic equilibrium of hot (X-ray) gas in clusters and groups of galaxies, gravitational lensing and departures from the smooth Hubble flow described by Eq. (4.1). This dark matter resides at least partly in the halos of galaxies such as our... 

A relationship also exists between abundances and luminosity of the host galaxy, whether gas-rich or otherwise (Fig. 3.44), and especially with the rotational velocity in disk galaxies (Fig. 8.13). Similarly, studies of elliptical galaxies in general suggest that the primary factor fixing the mean metallicity of their stellar... 

Fig. 8.13. H ii region oxygen abundance against rotational velocity for irregular and spiral galaxies (at a radius of 0.4/ s), after Pilyugin, Vflchez and Contini (2004).

Fig. 8.14. Surface densities of atomic and molecular hydrogen in the Galaxy as a function of Galactocentric distance the Sun is at 8.5 kpc. Beyond that distance, the deduced surface density depends on the assumed law of Galactic rotation KBH refers to Kulkarni, Blitz and Heiles (1982). Assuming their rotation curve, the total gas surface density falls by about a factor of 2 between 4.5 and 13 kpc, corresponding to an exponential fall-off with a scale length a l of about 12 kpc. After Dame (1993). Courtesy T.M. Dame.

The flattening of the Galaxy suggests that it is rotating about an axis perpendicular to the disk, and this is indeed confirmed by direct observation of the large-scale motions of the stars. In fact, they orbit about the galactic centre, making a complete revolution every 200 million years. 

Apart from rotating about their axes, the galaxies display systematic motions relative to one another. In fact, they are moving apart at speeds proportional to the distance between them. The recession speed amounts to some 100 km s for every 3 million light-years of separation. This overall motion is the clearest evidence we have for the expansion of the Universe. 

The total mass of the Galaxy is about 10 Mq. Of this 10(X) billion solar masses, only about one-tenth is actually visible. This is what is implied by the Galaxy s rotation curve, i.e. the graph of its rotation speed at different distances from the centre. All other matter is therefore classed as dark matter. The mass of stars is thus about 100 billion solar masses, and the mass of interstellar material a few billion more. 

It is thus assumed that the (rotational) speeding offences committed in the galactic periphery are due to the existence of a massive halo of invisible matter. In our own Galaxy, there must be ten times as much dark matter as visible matter, amounting to some 1000 biUion solar masses. We may deduce that the same is true of aU... 

First, it is useful to know that a model spiral galaxy can be considered to consist of a central spherical bulge component, embedded in a rotating disk of stellar and gaseous material with the whole embedded in a spherical halo of very diffuse gas and halo stars. It is conventionally believed that this latter component is much more massive than it appears, with the deficit made up of dark matter. ... 

The second is using the Tully-Fisher relationship, which provides a direct relationship between the absolute luminosity of a spiral galaxy and its maximum rotation speed. Since the maximum rotation speed can be directly estimated by observation, one can estimate the absolute luminosity since we measure directly the apparent luminosity, the inverse-square law allows us to estimate the distance to the object concerned. For the present discussion, it is important to understand that, at optical wavelengths, estimates of maximum rotation speeds are generally extrapolations from rotation curve measurements. 

The rotation curve is calculated in two steps (1) by subtracting the global redshift component (i.e., cosmological redshift + Doppler effect arising from peculiar motion) from the Doppler profile measured directly across the galaxy s disk and (2) by determining the actual dynamical centre of the galaxy. 

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