  Ultraviolet (UV) observations of Type Ia supernovae (SNe Ia) are crucial for
constraining the properties of their progenitor systems. Theoretical studies
predicted that the UV spectra, which probe the outermost layers of a SN, should
be sensitive to the metal content of the progenitor. Using the largest SN Ia UV
(lambda<2900 A) spectroscopic sample obtained from Neil Gehrels Swift
Observatory, we investigate the dependence of UV spectra on metallicity. For
the first time, our results reveal a correlation (~2 sigma) between SN Ia UV
flux and host-galaxy metallicities, with SNe in more metal-rich galaxies (which
are likely to have higher progenitor metallicities) having lower UV flux level.
We find that this metallicity effect is only significant at short wavelengths
(lambda<2700 A), which agrees well with the theoretical predictions. We produce
UV spectral templates for SNe Ia at peak brightness. With our sample, we could
disentangle the effect of light-curve shape and metallicity on the UV spectra.
We also examine the correlation between the UV spectra and SN luminosities as
parameterised by Hubble residuals. However, we do not see a significant trend
with Hubble residuals. This is probably due to the large uncertainties in SN
distances, as the majority of our sample members are extremely nearby (redshift
z<0.01). Future work with SNe discovered in the Hubble flow will be necessary
to constrain a potential metallicity bias on SN Ia cosmology.
