科研之友
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摘要
The velocity function derived from large-scale surveys can be compared with the predictions of similar to cold dark matter (Lambda CDM) cosmology, by matching the measured rotation velocities V-rot of galaxies to the maximum circular velocity of dark matter (DM) haloes V-max. For V-rot < 50 kms(-1), a major discrepancy arises between the observed and Lambda CDM velocity functions. However, the manner in which different observational measures of V-rot are associated with V-max is not straightforward in dwarf galaxies. We instead relate galaxies to DM haloes using the empirical baryon-mass to halo-mass relation, and show that different observational measures of V-rot result in very different velocity functions. We show how the W50 velocity function, i. e. using the H (Iota) profile linewidth at 50 per cent of peak H (Iota) flux to measure V-rot, can be reconciled with a Lambda CDM cosmology. Our semi-empirical methodology allows us to determine the region of rotation curves that are probed by H (Iota) measurements (R-HI), and shows that the V-rot of dwarfs are generally measured at a fraction of R-max, explaining their tendency to have rising rotation curves. We provide fitting formulae for relating R-HI and R-eff (the effective radius) to the virial radius of DM haloes. To continue to use velocity functions as a probe of Lambda CDM cosmology, it is necessary to be precise about how the different measures of rotation velocity are probing the mass of the DM haloes, dropping the assumption that any measure of rotational velocity can be equally used as a proxy for V-max.
- 出版日期2016-2-1
