  SN2017egm is the closest (z=0.03) H-poor superluminous supernova (SLSN-I)
detected to date, and a rare example of an SLSN-I in a massive and metal-rich
galaxy. Here we present the HST UV & optical spectra covering (1000 - 5500)A
taken at +3 day relative to the peak. Our data reveal two sets of absorption
systems, separated by 235 km/s, at redshifts matching the host galaxy, NGC3191
and its companion galaxy 73 arcsec apart. Weakly damped Lyman-alpha absorption
lines are detected at these two redshifts, with HI column densities of
$(3.0\pm0.8)\times10^{19}$ and $(3.7\pm0.9)\times10^{19}$\,cm$^{-2}$
respectively. This is an order of magnitude smaller than HI column densities in
the disks of nearby galaxies ($>10^{10}M_\odot$) and suggests that SN2017egm is
on the near side of NGC3191 and has a low host extinction (E(B-V)=0.007). Using
unsaturated metal absorption lines and taking into account of H ionization and
dust depletion corrections, we find that the host of SN2017egm probably has a
solar or higher metallicity and is unlikely to be a dwarf companion to NGC3191.
Comparison of early-time UV spectra of SN2017egm, Gaia16apd, iPTF13ajg and
PTF12dam finds that the continuum at wavelength > 2800A is well fit by a
blackbody, whereas the continuum at wavelength < 2800A is considerably below
the model. The degree of UV suppression varies from source to source, with the
1400A to 2800A continuum flux ratio of 1.5 for Gaia16apd and 0.4 for iPTF13ajg.
This can not be explained by the differences in magnetar power or blackbody
temperature (i.e. color temperature). Finally, the UV spectra reveal a common
set of seven broad absorption features and their equivalent widths are similar
(within a factor of 2) among the four events. These seven features bode well
for future high-z SLSN-I spectral classifications.
