Recent analysis of the SDSS-III/APOGEE Data Release 12 stellar catalogue hasrevealed that the Milky Way's metallicity distribution function (MDF) changesshape as a function of radius, transitioning from being negatively skewed atsmall Galactocentric radii to positively skewed at large Galactocentric radii.Using a high resolution, N-body+SPH simulation, we show that the changingskewness arises from radial migration - metal-rich stars form in the inner diskand subsequently migrate to the metal-poorer outer disk. These migrated starsrepresent a large fraction (> 50%) of the stars in the outer disk; theypopulate the high metallicity tail of the MDFs and are, in general, moremetal-rich than the surrounding outer disk gas. The simulation also reproducesanother surprising APOGEE result: the spatially invariant high-[alpha/Fe] MDFs.This arises in the simulation from the migration of a population formed withina narrow range of radii (3.2+/-1.2 kpc) and time (8.8+/-0.6 Gyr ago), ratherthan from spatially extended star formation in a homogeneous medium at earlytimes. These results point toward the crucial role radial migration has playedin shaping our Milky Way.

Atacama Large Millimeter/submillimeter Array (ALMA) 12CO(J=1-0) observationsare used to study the cold molecular ISM of the Cartwheel ring galaxy and itsrelation to HI and massive star formation (SF). CO moment maps find$(2.69\pm0.05)\times10^{9}$ M$_{\odot}$ of H$_2$ associated with the inner ring(72%) and nucleus (28%) for a Galactic I(CO)-to-N(H2) conversion factor($\alpha_{\rm CO}$). The spokes and disk are not detected. Analysis of theinner ring's CO kinematics show it to be expanding ($V_{\rm exp}=68.9\pm4.9$ kms$^{-1}$) implying an $\approx70$ Myr age. Stack averaging reveals CO emissionin the starburst outer ring for the first time, but only where HI surfacedensity ($\Sigma_{\rm HI}$) is high, representing $M_{\rmH_2}=(7.5\pm0.8)\times10^{8}$ M$_{\odot}$ for a metallicity appropriate$\alpha_{\rm CO}$, giving small $\Sigma_{\rm H_2}$ ($3.7$ M$_{\odot}$pc$^{-2}$), molecular fraction ($f_{\rm mol}=0.10$), and H$_2$ depletiontimescales ($\tau_{\rm mol} \approx50-600$ Myr). Elsewhere in the outer ring$\Sigma_{\rm H_2}\lesssim 2$ M$_{\odot}$ pc$^{-2}$, $f_{\rm mol}\lesssim 0.1$and $\tau_{\rm mol}\lesssim 140-540$ Myr (all $3\sigma$). The inner ring andnucleus are H$_2$-dominated and are consistent with local spiral SF laws.$\Sigma_{\rm SFR}$ in the outer ring appears independent of $\Sigma_{\rm H_2}$,$\Sigma_{\rm HI}$ or $\Sigma_{\rm HI+H_2}$. The ISM's long confinement in therobustly star forming rings of the Cartwheel and AM0644-741 may result ineither a large diffuse H$_2$ component or an abundance of CO-faint low columndensity molecular clouds. The H$_2$ content of evolved starburst rings maytherefore be substantially larger. Due to its lower $\Sigma_{\rm SFR}$ and agethe Cartwheel's inner ring has yet to reach this state. Alternately, the outerring may trigger efficient SF in an HI-dominated ISM.

We present an analysis of galaxies in groups and clusters at $0.8<z<1.2$,from the GCLASS and GEEC2 spectroscopic surveys. We compute a "conversionfraction" $f_{\rm convert}$ that represents the fraction of galaxies that wereprematurely quenched by their environment. For massive galaxies, $M_{\rmstar}>10^{10.3}M_\odot$, we find $f_{\rm convert}\sim 0.4$ in the groups and$\sim 0.6$ in the clusters, similar to comparable measurements at $z=0$. Thismeans the time between first accretion into a more massive halo and final starformation quenching is $t_p\sim 2$ Gyr. This is substantially longer than theestimated time required for a galaxy's star formation rate to become zero onceit starts to decline, suggesting there is a long delay time during which littledifferential evolution occurs. In contrast with local observations we findevidence that this delay timescale may depend on stellar mass, with $t_p$approaching $t_{\rm Hubble}$ for $M_{\rm star}\sim 10^{9.5}M_\odot$. The resultsuggests that the delay time must not only be much shorter than it is today,but may also depend on stellar mass in a way that is not consistent with asimple evolution in proportion to the dynamical time. Instead, we find the dataare well-matched by a model in which the decline in star formation is due to"overconsumption", the exhaustion of a gas reservoir through star formation andexpulsion via modest outflows in the absence of cosmological accretion.Dynamical gas removal processes, which are likely dominant in quenching newlyaccreted satellites today, may play only a secondary role at $z=1$.