High-redshift rotation curves and MOND
  Genzel et al. have recently published the rotation curves of six
high-redshift disc galaxies ($z\sim 0.9-2.4$), which they find to be `baryon
dominated' within the studies radii. While not up to the standard afforded by
data available for analysis in the nearby Universe, these data are valuable in
constraining cosmological evolution of either DM scenarios, or -- as I discuss
here -- $z$-dependence of MOND. Indeed, these results, if taken at face value,
teach us useful lessons in connection with MOND. a. The dynamical accelerations
at the half-light radii, found by Genzel et al., are rather high compared with
the MOND acceleration constant, as measured in the nearby Universe:
$g(R_{1/2})= (3-11)a_0$. MOND then predicts fractions of `phantom matter' at
$R_{1/2}$ of at most a few tens of percents, which, galaxy by galaxy, agree
well with what Genzel et al. find. b. The asymptotic rotational speeds
predicted by MOND from the baryonic-mass estimates of Genzel et al. are
substantially lower ($0.55-0.75$) than the maximal speeds of the RCs. MOND thus
predicts a substantial decline of the RCs beyond the maximum. This too is in
line with what Genzel et al. find. c. Arguably, the most important lesson is
that the findings of Genzel et al. cast very meaningful constraints on possible
variation of $a_0$ with cosmic time. For example, they all but exclude a value
of the MOND constant of $\sim 4a_0$ at $z\sim 2$, excluding, e.g., $a_0\propto
(1+z)^{3/2}$.
