92. A new theory of galaxy formation needed

Posted by Dmitry

November 12, 2008

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It is well known that an ordinary galaxy can be described by seven physical parameters: total mass, baryon fraction, age, specific angular momentum, specific heat energy (related to random motion within the galaxy), its radius and concentration. According to the virial theorem, only six of them can be independent. This gives quite a bit of freedom for the model building and a large part of the astrophysical community is quite busy with constructing models of galaxy formation.

Now, a recent paper by Disney et al. claims that only one physical paramter is actually independent among those remaining six. The collaboration has analyzed parameters of about 200 neutral-hydrogene-free galaxies (their interest was motivated by the fact that those galaxies were studied in both Parkes radio telescope 21-cm survey and SDSS in optical resolution). They have measured HI (neutral hydrogen) mass and line width $\Delta V$ , redshift, inclination, two radii $R_{50}$ and $R_{90}$ respectively containing 50 and 90% of the emitted light, the luminocity L_g and 4 colors. Three of the colors are actually degenerate leaving dynamical mass $M_d=(\Delta V)^2 R_{90} / G$ , HI mass $M_{HI}$ , luminocity, radius, concentration $R_{50}/R_{90}$ and one color (SDSS (g-r)) – i.e., as many independent observables as physical quantities listed above.

There are four known correlations among the observables – the one between luminocity and color (luminous galaxies are generically redder), the one between dynamical mass and luminocity $M_d\propto L_g$ , that between $R_{50}$ and $R_{90}$ since HI galaxies have exponential profiles and finally the one between $M_{HI}$ and $R_{50}$ – all galaxies have the same HI surface density $M_{HI}/R^2_{50}$ .

The groups claims to have found a new correlation – the one between the surface brightness $L_g/R^2_{50}$ and $R_{50}$ .

What does it mean? Most probably, it means that the model building in galaxy formation studies has to end at some point, while it is better for theorists to start thinking how to develop a new theory of galaxy formation. For example, standard dark matter model of hierarchical galaxy formation provides 4 parameters per galaxy – mass, spin, halo concentration index and epoch of formation, etc., and is therefore inconsistent with the survey.

If generic galaxies come from a six-parameter set (at most), it seems rather interesting that gaseous galaxies come from a one-parametric set, and it is hard to imagine a theory (apart from correct one) that will provide 5 independent constraint equations to constrain the observations.

It would be interesting to hear what people from astrophysical community think about this paper…

Please see the observed correlations below:

Correlations in gaseous galaxies

Via ANKA project by S. Popov (in Russian).

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