Molecular gas budget and characterization of intermediate-mass
  star-forming galaxies at $z\approx 2-3$
  Star-forming galaxies (SFGs) with stellar masses below $10^{10}\,M_\odot$
make up the bulk of the galaxy population at $z>2$. The properties of the cold
gas in these galaxies can only be probed in very deep ALMA observations or by
targeting strongly lensed galaxies. Here we report the results of a pilot
survey using the Atacama Compact Array (ACA) of molecular gas in the most
strongly magnified galaxies selected as giant arcs in optical data. The
selection in rest-frame UV wavelengths ensures that sources are regular star
forming galaxies, without a priori indications of intense dusty starburst
activity. We conducted Band 4 and Band 7 observations to detect mid-$J$ CO, [C
I] and thermal continuum as molecular gas tracers from four strongly lensed
systems at $z\approx2-3$: our targets are SGAS J1226651.3+215220 (A and B),
SGAS J003341.5+024217 and the Sunburst Arc. The measured molecular mass is then
projected onto the source plane with detailed lens models developed from high
resolution HST observations. Multiwavelength photometry is then used to obtain
the intrinsic stellar mass and star formation rate via SED fitting. In only one
of the sources are the three tracers robustly detected, while in the others
they are either undetected or detected in continuum only. The implied molecular
gass masses range from $4\times 10^{9}\,M_\odot$ in the detected source to an
upper limit of $\lesssim 10^9\,M_\odot$ in the most magnified source. The
inferred gas fraction and gas depletion timescale are found to lie
approximately 0.5 to 1.0 dex below the established scaling relations based on
previous studies of unlensed massive galaxies. Our results indicate that the
cold gas content of intermediate to low mass galaxies should not be
extrapolated from the trends seen in more massive high-$z$ galaxies. (Abridged
abstract)
