Post-starburst (or E+A) galaxies are characterized by low H$alpha$ emission and strong Balmer absorption, suggesting a recent starburst, but little current star formation. Although many of these galaxies show evidence of recent mergers, the mechanism
for ending the starburst is not yet understood. To study the fate of the molecular gas, we search for CO (1-0) and (2-1) emission with the IRAM 30m and SMT 10m telescopes in 32 nearby ($0.01<z<0.12$) post-starburst galaxies drawn from the Sloan Digital Sky Survey. We detect CO in 17 (53%). Using CO as a tracer for molecular hydrogen, and a Galactic conversion factor, we obtain molecular gas masses of $M(H_2)=10^{8.6}$-$10^{9.8} M_odot$ and molecular gas mass to stellar mass fractions of $sim10^{-2}$-$10^{-0.5}$, comparable to those of star-forming galaxies. The large amounts of molecular gas rule out complete gas consumption, expulsion, or starvation as the primary mechanism that ends the starburst in these galaxies. The upper limits on $M(H_2)$ for the 15 undetected galaxies range from $10^{7.7} M_odot$ to $10^{9.7} M_odot$, with the median more consistent with early-type galaxies than with star-forming galaxies. Upper limits on the post-starburst star formation rates (SFRs) are lower by $sim10times$ than for star-forming galaxies with the same $M(H_2)$. We also compare the molecular gas surface densities ($Sigma_{rm H_2}$) to upper limits on the SFR surface densities ($Sigma_{rm SFR}$), finding a significant offset, with lower $Sigma_{rm SFR}$ for a given $Sigma_{rm H_2}$ than is typical for star-forming galaxies. This offset from the Kennicutt-Schmidt relation suggests that post-starbursts have lower star formation efficiency, a low CO-to-H$_2$ conversion factor characteristic of ULIRGs, and/or a bottom-heavy initial mass function, although uncertainties in the rate and distribution of current star formation remain.
Certain configurations of massive structures projected along the line of sight maximize the number of detections of gravitationally lensed $zsim10$ galaxies. We characterize such lines of sight with the etendue $sigma_mu$, the area in the source plan
e magnified over some threshold $mu$. We use the Millennium I and Millennium XXL cosmological simulations to determine the frequency of high $sigma_mu$ beams on the sky, their properties, and efficient selection criteria. We define the best beams as having $sigma_{mu>3} >2000$ arcsec$^2$, for a $zsim10$ source plane, and predict $477 pm 21$ such beams on the sky. The total mass in the beam and $sigma_{mu>3}$ are strongly correlated. After controlling for total mass, we find a significant residual correlation between $sigma_{mu>3}$ and the number of cluster-scale halos ($>10^{14} M_odot h^{-1}$) in the beam. Beams with $sigma_{mu>3} >2000$ arcsec$^2$, which should be best at lensing $zsim10$ galaxies, are ten times more likely to contain multiple cluster-scale halos than a single cluster-scale halo. Beams containing an Abell 1689-like massive cluster halo often have additional structures along the line of sight, including at least one additional cluster-scale ($M_{200}>10^{14}M_odot h^{-1}$) halo 28% of the time. Selecting beams with multiple, massive structures will lead to enhanced detection of the most distant and intrinsically faint galaxies.
