Low-Z shore of the "island of inversion" and the reduced neutron magicity toward 28O

The two odd-even fluorine isotopes $^{27,29}\mathrm{F}$ were studied via in-beam $\ensuremath{\gamma}$-ray spectroscopy at the RIKEN Radioactive Isotope Beam Factory. A secondary beam of $^{30}\mathrm{Ne}$ was used to induce one-proton and one-proton--two-neutron removal reactions on carbon and polyethylene targets at midtarget energies of 228 MeV/$u$. Excited states were observed at 915(12) keV for $^{27}\mathrm{F}$ and at 1080(18) keV for $^{29}\mathrm{F}$. Both were assigned a $1/{2}_{1}^{+}$ spin and parity. The low transition energy for $^{29}\mathrm{F}$ largely disagrees with shell model predictions restricted to the $sd$ model space. Calculations using effective interactions that include the neutron $pf$ shell indicate that the $N=20$ gap is quenched for $^{29}\mathrm{F}$, thus extending the ``island of inversion'' to isotopes with proton number $Z=9$. Variations of the $N=20$ gap further reveal a strong correlation to the $1/{2}_{1}^{+}$ level energy in $^{29}\mathrm{F}$ and suggest a persistent reduced neutron gap for $^{28}\mathrm{O}$.