The Bosma effect revisited
I. HI and stellar disc scaling models
Institut für Astrophysik, Georg-August-Universität,
Accepted: 17 June 2011
Context. The observed proportionality between the centripetal contribution of the dynamically insignificant HI gas in the discs of spiral galaxies and the dominant contribution of dark matter (DM) – the “Bosma effect” – has been repeatedly mentioned in the literature but largely ignored. Since this phenomenology, if statistically significant, tells us something about the relationship between the visible baryonic and invisible DM, it is important to re-examine the reality of this effect using formal tests and more modern data.
Aims. We have re-examined the evidence for the Bosma effect, either by scaling the contribution of the HI gas alone or by using both the observed stellar disc and HI gas as proxies.
Methods. We have calculated Bosma effect models for 17 galaxies in The HI Nearby Galaxy Survey data set. The results are compared with two models for exotic cold DM: internally consistent cosmological Navarro-Frenk-White (NFW) models with constrained compactness parameters, and “universal rotation curve” (URC) models using fully unconstrained Burkert density profiles.
Results. Fits to spiral galaxy rotation curves computed using just HI scaling are inadequate, despite the clear proportionality seen in the outer discs. The poor performance is obviously related to the prominent decrease in the HI surface density in regions of high stellar surface density, where HI has been converted into molecules and stars. The Bosma models that partially correct for this physical effect using the stellar discs as additional proxies are statistically nearly as good as the URC models and clearly better than the NFW ones.
Conclusions. We confirm the correlation between the centripetal effects of DM and that of the interstellar medium of spiral galaxies. The efficacy of “maximal disc” models is explained as the natural consequence of “classic” Bosma models which include the stellar disc as a proxy in regions of reduced atomic gas. The perception that the Bosma effect could be due to the near-equality of the HI surface density and the projected mass density of a cold DM halo is incorrect, both theoretically and empirically. The standard explanation – that the effect reflects a statistical correlation between the visible and exotic DM – seems highly unlikely, given that the geometric forms and hence centripetal signatures of spherical halo and disc components are so different. A literal interpretation of the Bosma effect as being due to the presence of significant amounts of disc DM requires a median visible baryon to disc DM ratio of about 40%.
Key words: galaxies: ISM / galaxies: kinematics and dynamics / galaxies: spiral / galaxies: structure / dark matter
© ESO, 2011