  We present the first measurements of HI galaxy scaling relations from a blind
survey at $z>0.15$. We perform spectral stacking of 9023 spectra of
star-forming galaxies undetected in HI at $0.23<z<0.49$, extracted from
MIGHTEE-HI Early Science datacubes, acquired with the MeerKAT radio telescope.
We stack galaxies in bins of galaxy properties ($M_*$, SFR, and sSFR, with
${\rm sSFR}\equiv M_*/{\rm SFR}$), obtaining $\gtrsim 5\sigma$ detections in
most cases, the strongest HI-stacking detections to date in this redshift
range. With these detections, we are able to measure scaling relations in the
probed redshift interval, finding evidence for a moderate evolution from the
median redshift of our sample $z_{\rm med}\sim 0.37$ to $z\sim 0$. In
particular, low-$M_*$ galaxies ($\log_{10}(M_*/{\rm M_\odot})\sim 9$)
experience a strong HI depletion ($\sim 0.5$ dex in $\log_{10}(M_{\rm HI}/{\rm
M}_\odot)$), while massive galaxies ($\log_{10}(M_*/{\rm M_\odot})\sim 11$)
keep their HI mass nearly unchanged. When looking at the star formation
activity, highly star-forming galaxies evolve significantly in $M_{\rm HI}$
($f_{\rm HI}$, where $f_{\rm HI}\equiv M_{\rm}/M_*$) at fixed SFR (sSFR), while
at the lowest probed SFR (sSFR) the scaling relations show no evolution. These
findings suggest a scenario in which low-$M_*$ galaxies have experienced a
strong HI depletion during the last $\sim4$ Gyr, while massive galaxies have
undergone a significant HI replenishment through some accretion mechanism,
possibly minor mergers. Interestingly, our results are in good agreement with
the predictions of the SIMBA simulation. We conclude that this work sets novel
important observational constraints on galaxy scaling relations.
