  We present radio very long baseline interferometry (VLBI) and X-ray studies
of the starburst galaxy NGC 3628. The VLBI observation at 1.5 GHz reveals seven
compact (0.7$-$7 parsec) radio sources in the central $\sim$250 parsec region
of NGC 3628. Based on their morphology, high radio brightness temperatures
($10^5-10^7$ K), and steep radio spectra, none of these seven sources can be
associated with active galactic nuclei (AGNs); instead, they can be identified
as supernova remnants (SNRs), with three of them appearing consistent with
partial shells. Notably, one of them (W2) is likely a nascent radio supernova
and appears to be consistent with the star formation rate of NGC 3628 when
assuming a canonical initial mass function. The VLBI observation provides the
first precise measurement of the diameter of the radio sources in NGC 3628,
which allow us to fit a well-constrained radio surface brightness - diameter
($\Sigma-D$) correlation by including the detected SNRs. Furthermore, future
VLBI observations can be conducted to measure the expansion velocity of the
detected SNRs. In addition to our radio VLBI study, we analyze Chandra and
XMM-Newton spectra of NGC 3628. The spectral fitting indicates that the SNR
activities could well account for the observed X-ray emissions. Along with the
Chandra X-ray image, it further reveals that the X-ray emission is likely
maintained by the galactic-scale outflow triggered by SN activities. These
results provide strong evidence that SN-triggered activities play a critical
role in generating both radio and X-ray emissions in NGC 3628 and further
suggest that the galaxy NGC 3628 is in an early stage of starbursts.
