14 One of the reasons that initiated the present study on

In document Photometry and kinematics of pure disk galaxies (Page 34-46)


14 One of the reasons that initiated the present study on

PDG’s is that, if it is possible to detect optical tracers of a massive halo component, disk galaxies with very little or no bulge at all are obviously the best candidates. It would be easier to interpret the light profiles (e.g. minor axis profiles of edge-on systems) if we had to deal with just one component. This would save us from the uncertainties of profile decomposition into a bulge and a disk contribution. Also, it should be easier to detect a low surface brightness halo if its light is not superposed onto a bright bulge (assuming that optical tracers of the halo do exist).

Recent results of numerically simulated me rge rs of galaxies are another incentive for a thorough study of PDG's. Quinn (1982) showed that the merger of two disks could produce an elliptical-like system. Roos (1981) went a step further and proposed that all bright (M ^ -l6) galaxies were initially formed as disk galaxies resembling late-type spirals. These galaxies would then evolve along the sequence from Sd to E as they merge with other

smaller galaxies.

If the merger scenario is right, it is important to know the fundamental properties of PDG (e.g. angular momentum, equilibrium structure, importance of dark halo) if we want to be able to model realistically such mergers.

Another reason comes from the work of Van der Kruit and Searle (VdKS: 1901a, 198lb). They showed that to get a complete understanding of spirals of all types, one must start with as simple a system as possible: the pure disk galaxies are such objects. The interpretation of the light distribution both in R and in z is much easier if we have to deal with only one component. The success of VdKS in interpreting the data on the two Sb galaxies NGCU565 and

NGC 891, making use of the disk properties derived from the late type spirals NGCU2H1+ and NGC5907» illustrates this point well.

Finally, if massive halos have no optical tracers, the

only alternative is to try to identify them kinematically. But as

for the light distribution, the situation for early-type spirals is

complicated. Bulges tend to make V(r) flat in the inner parts and

it is difficult to determine reliably the contribution of each component to the potential field (different M/L, different mass

distribution). For those reasons, kinematical (dark) halos will be

easier to identify in PDG. Having only one optical component will reduce considerably the number of free parameters in the modelling of those systems.


In recent years, studies of the light distribution and

kinematical properties of spiral galaxies have concentrated on

morphological types going from Sa to Sc. This is understandable

since they account for ~ 70% of all spirals (in a sample with

limiting magnitude M ^ m < 15-l6 J ) and are at the bright end of

the sequence with (total magnitude corrected for galactic

absorption) between -I9 and -20 (figure l.l). They are also, by

far, the predominant spiral types in clusters. Being massive and

luminous, they have been the prime candidates for detailed study of spirals.

Pure disk galaxies are fainter, ranging from M° of

-l8 to -l6. For samples with > 18 J , they rapidly become

the more numerous type (figure 2; Ellis, 1979)» The difference in

16 Figure 1.1

Figure 1.

Mean corrected total B magnitude VS morphological type from de Vaucouleurs (1977)»

Apparent galaxy proportions at various limiting J magnitudes from Ellis (1979)«


bulge. While the disk mean absolute magnitudes remains fairly

constant with morphological types, the bulge mean absolute

magnitudes drop from -19 for SO/a to -l4 for Sd (Simien and de

Vauc ouleurs, 19 82).

PDG's are nearly absent from great clusters of galaxies (Dressier, 1980) and are usually found as isolated systems or in small groups of which the Sculptor group is a typical example. The fact that they are intrinsically smaller and nearly absent from regions of high galaxy density is surely telling us something on the

importance of the environment on their formation and further

evolution. It is, for example, an argument in favour of the mergers scenario.

The morphological appearance of PDG is more or less diffuse and they exhibit a very chaotic and ill defined multiple spiral arms pattern (NGC7793), in contrast to the well defined and

symmetric two arms pattern of earlier type systems. Even when two

main spiral arms are still clearly marked, one of these can be stronger than the other and this will result in an asymmetry of the

isophotes (NGC 247). Sd's are also the flattest systems (IC5250)

with a mean intrinsic flattening of 0.12 (Bottinelli et al, 1982)

which implies a ratio of major to minor axis of more than 8.

Pure disk galaxies have very blue colors with mean

< (B-V) ° > = 0.43 for Sd and < (U-B) ° > = -0.23 (de

Vaucouleurs, 1977) which implies an important star formation

activity. In fact, in all nearby PDG's, a large number of bright

HII regions and young star associations can easily be resolved. This star formation activity results in a lower M/L ratio (Tinsley, 1981) since those young stars contribute a large fraction of the light but account only for a small fraction of the mass.

One important feature, that comes from abundances studies (Goad and Roberts, 1981; Pagel et a l , 1979)» is their nitrogen deficiency which shows in their very small [N IX]/[SII] line ratio (typically one-fourth of the value seen in a normal HII region). If nitrogen is a secondary product of nucleosynthesis (sulphur is primary), those observations imply that star formation proceeds more slowly or started later in those systems than in earlier types. On the other hand, if a substantial component of nitrogen is a primary product whose yield depends on the age and/or initial mass function (Edmunds and Pagel, 1978), the interpretation of this result is not as straightforward.

With regard to their kinematical properties, the shape of the rotation curves of PDG's within the optical radius is very different from that of early-type spirals. While early-type rotation curves rise very steeply and usually reach a V around


200 km/sec well within the bulge (R ^ 2 * kpc), the rotation


curve for PDG's have a much slower rise and reach a smaller V max between 50 to 100 km/sec close to their optical radii (R > 4-5


k p c ).

To understand the meaning of this difference, it is worthwhile to look at the behaviour of some dynamical parameters with morphological types. Figure 1.3 shows how V (maximum


rotational velocity) varies with morphological type. We see that we have a fairly strong correlation with a steady decrease in maximum

rotational velocity as one goes toward later types. For R max (radius of maximum velocity), the situation is slightly more


Figure 1.3 Maximum rotational velocity as a function of

morphological type. The data is from Rubin et al,

1978, Bosnia, 1978 and our sample.

Figure l.U Radius of maximum rotational velocity (normalized by

D(o), RC2) as a function of morphological type. The

m a f< )/ D (O I


complicated because this parameter suffers from distance uncertainties. However, if we normalized R ^ by some photometric


length scale like D(o) (which is a multiple of the disk scale length), we should then have a parameter independent of distance. We see, as described qualitatively before, that this ratio R^ax/^lo) increases as one goes towards later types.

It is interesting that the product of V and


R r /D(o) (figure 1.5) is fairly constant with type. This means max

that the bulge and disk length scales are related, at least for intermediate types, as shown by the following simple argument.

First, we must ask how R /D(o) and V are

max max

determined dynamically. On the inner parts, the bulge dominates the potential field. For the earlier type systems with significant bulges, R /D(o) is small (figure l.U) and we will assume that


R is determined mainly by the bulge. Then R r is

ffliSlX D18.X

proportional to the bulges' effective radius r^. The diameter D(o) is defined mainly by the disk, so D(o) is proportional to the disk length scale h, and

Rm x /D(o) e/h (1)

[This proportionality will break down, of course, for systems with very weak bulges].

On the other hand, we would argue that V is max determined primarily by the disk. Firstly the Fisher-Tally law shows that and the luminosity of a spiral are closely related; for most spirals. With the exception of the very early types, the luminosity comes mainly from the disk (Simiens and de

Figure 1.5 Product of V(max) x R(max)/D(o) as a function of morphological type. The data is from de Vaucouleurs, 1977.

Figure 1.6 Square root of the disk effective radius plotted against the bulge effective radius. The data from Simien and de Vaucouleurs, 1932.


Vaucouleurs, 19Ö2), so V depends mainly on the disk luminosity IT18.X

and hence on the disk length scale (Freeman, 1970).

Secondly, we note that in a typical spiral of intermediate type, the rotational velocity very rarely exceeds the usually flat level of rotational velocity further out. Therefore, even though the bulge probably dominates the potential field in the inner parts, the effective rotational velocity in these inner parts appears to be tightly linked to the rotational velocity in the outer parts of the spiral, where the disk surely dominates the potential. For an exporential disk,

V max



the product V . R /D(o) is then proportional to

max max


r^h , so its constancy with type means that

r cc e



In other words, systems with small bulge scale length would be expected to have small disk scale length and vice-versa. A

relation of this form is illustrated in figure 1.6, the data give

Reff 1/2 (disk) = 1.54 + 0.74 Reff (bulge) (4)

where the zero point probably results from the breakdown of the proportionality R /D(o) e/h for systems with very small


bulges. We see that the relation holds for types from Scd (leftmost point) to type Sb. As expected, the relation fails for SO where


In document Photometry and kinematics of pure disk galaxies (Page 34-46)