Local Analogs for High-redshift Galaxies: Resembling the Physical
  Conditions of the Interstellar Medium in High-redshift Galaxies
  We present a sample of local analogs for high-redshift galaxies selected in
the Sloan Digital Sky Survey (SDSS). The physical conditions of the
interstellar medium (ISM) in these local analogs resemble those in
high-redshift galaxies. These galaxies are selected based on their positions in
the [OIII]/H$\beta$ versus [NII]/H$\alpha$ nebular emission-line diagnostic
diagram. We show that these local analogs share similar physical properties
with high-redshift galaxies, including high specific star formation rates
(sSFRs), flat UV continuums and compact galaxy sizes. In particular, the
ionization parameters and electron densities in these analogs are comparable to
those in $z\simeq2-3$ galaxies, but higher than those in normal SDSS galaxies
by $\simeq$0.6~dex and $\simeq$0.9~dex, respectively. The mass-metallicity
relation (MZR) in these local analogs shows $-0.2$~dex offset from that in SDSS
star-forming galaxies at the low mass end, which is consistent with the MZR of
the $z\sim2-3$ galaxies. We compare the local analogs in this study with those
in other studies, including Lyman break analogs (LBA) and green pea (GP)
galaxies. The analogs in this study share a similar star formation surface
density with LBAs, but the ionization parameters and electron densities in our
analogs are higher than those in LBAs by factors of 1.5 and 3, respectively.
The analogs in this study have comparable ionization parameter and electron
density to the GP galaxies, but our method can select galaxies in a wider
redshift range. We find the high sSFR and SFR surface density can increase the
electron density and ionization parameters, but still cannot fully explain the
difference in ISM condition between nearby galaxies and the local
analogs/high-redshift galaxies.
