We present first results from the KMOS Ultra-deep Rotation Velocity Survey
(KURVS), aimed at studying the outer rotation curves shape and dark matter
content of 22 star-forming galaxies at z∼1.5. These galaxies represent
`typical' star-forming discs at z∼1.5, being located within the
star-forming main sequence and stellar mass-size relation with stellar masses
9.5⩽log(M⋆/M⊙)⩽11.5. We extract
individual rotation curves out to 4 times the effective radius, on average, or
∼10−15 kpc. Most rotation curves are flat or rising between three- and
six-disc scale radii. Only three objects with dispersion-dominated dynamics
(vrot/σ0∼0.2) have declining outer rotation curves at more
than 5σ significance. After accounting for seeing and pressure support,
the nine rotation-dominated discs with vrot/σ0⩾1.5 have
average dark matter fractions of 50±20% at the effective radius, similar
to local discs. Together with previous observations of star-forming galaxies at
cosmic noon, our measurements suggest a trend of declining dark matter fraction
with increasing stellar mass and stellar mass surface density at the effective
radius. Simulated EAGLE galaxies are in quantitative agreement with
observations up to log(M⋆Reff−2/M⊙kpc−2)∼9.2, and over-predict the dark matter fraction of galaxies with higher
mass surface densities by a factor of ∼3. We conclude that the dynamics
of typical rotationally-supported discs at z∼1.5 is dominated by dark
matter from effective radius scales, in broad agreement with cosmological
models. The tension with observations at high stellar mass surface density
suggests that the prescriptions for baryonic processes occurring in the most
massive galaxies (such as bulge growth and quenching) need to be reassessed.Comment: 23 pages, 9 figures. Resubmitted to MNRAS after addressing the
referee's comments. Abstract slightly modified to compile with the arXiv
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