We present new Hubble Space Telescope (HST) imaging of a stream-like systemassociated with the dwarf galaxy DDO 68, located in the Lynx-Cancer Void at adistance of D$\sim$12.65 Mpc from us. The stream, previously identified in deepLarge Binocular Telescope images as a diffuse low surface brightness structure,is resolved into individual stars in the F606W (broad V) and F814W ($\sim$I)images acquired with the Wide Field Camera 3. The resulting V, Icolor-magnitude diagram (CMD) of the resolved stars is dominated by old(age$\gtrsim$1-2 Gyr) red giant branch (RGB) stars. From the observed RGB tip,we conclude that the stream is at the same distance as DDO 68, confirming thephysical association with it. A synthetic CMD analysis indicates that the largemajority of the star formation activity in the stream occurred at epochsearlier than $\sim$1 Gyr ago, and that the star formation at epochs more recentthan $\sim$500 Myr ago is compatible with zero. The total stellar mass of thestream is $\sim10^{6} M_{\odot}$, about 1/100 of that of DDO~68. This is astriking example of hierarchical merging in action at the dwarf galaxy scales.

We present the results of CO(1-0) and CO(4-3) observations of the host galaxyof a long-duration gamma-ray burst GRB080207 at z = 2.0858 by using the Karl G.Jansky Very Large Array and the Atacama Large Millimeter/submillimeter Array.The host is detected in CO(1-0) and CO(4-3), becoming the first case for a GRBhost with more than two CO transitions detected combined with CO(2-1) andCO(3-2) in the literature. Adopting a metallicity-dependent CO-to-H2 conversionfactor, we derive a molecular gas mass of Mgas = 8.7 x 10^10 Modot, whichplaces the host in a sequence of normal star-forming galaxies in aMgas-star-formation rate (SFR) plane. A modified blackbody fit to thefar-infrared--millimeter photometry results in a dust temperature of 37 K and adust mass of Mdust = 1.5 x 10^8 Modot. The spatially-resolving CO(4-3)observations allow us to examine the kinematics of the host. The CO velocityfield shows a clear rotation and is reproduced by a rotation-dominated diskmodel with a rotation velocity of 350 km/s and a half-light radius of 2.4 kpc.The CO spectral line energy distribution derived from the four CO transitionsis similar to that of starburst galaxies, suggesting a high excitationcondition. Comparison of molecular gas properties between the host and normal(main-sequence) galaxies at similar redshifts shows that they share commonproperties such as gas mass fraction, gas depletion timescale, gas-to-dustratio, location in the Mgas-SFR (or surface density) relation, and kinematics,suggesting that long-duration GRBs can occur in normal star-formingenvironments at z ~ 2.

At $z\sim2$, star-formation activity is thought to be high even inhigh-density environments such as galaxy clusters and proto-clusters. One ofthe critical but outstanding issues is if structural growth of star-forminggalaxies can differ depending on their surrounding environments. In order toinvestigate how galaxies grow their structures and what physical processes areinvolved in the evolution of galaxies, one requires spatially resolved imagesof not only stellar components but also star-forming regions within galaxies.We conducted the Adaptive Optics(AO)-assisted imaging observations forstar-forming galaxies in a dense proto-cluster core at $z=2.53$ with IRCS andAO188 mounted on the Subaru Telescope. A combination of AO and narrow-bandfilters allows us to obtain resolved maps of H$\alpha$-emitting regions with anangular resolution of 0.1--0.2~arcsec, which corresponds to $\sim1$ kpc at$z\sim2.5$. Based on stacking analyses, we compare radial profiles ofstar-forming regions and stellar components and find that the star-formingregion of a sub-sample with $\rm log(M_*/M_\odot)\sim10-11$ is more extendedthan the stellar component, indicating the inside-out growth of the structure.This trend is similar to the one for star-forming galaxies in general fields at$z=2-2.5$ obtained with the same observational technique. Our results suggestthat the structural evolution of star-forming galaxies at $z=2-2.5$ is mainlydriven by internal secular processes irrespective of surrounding environments.

We used deep MUSE observations to perform a stellar-kinematic and populationanalysis of FCC 153 and FCC 177, two edge-on S0 galaxies in the Fornax cluster.The geometrical definition of the different structural components of these twogalaxies allows us to describe the nature of their thick disks. These are bothold, relatively metal poor and [Mg/Fe]-enhanced, and their star formationhistory (SFH) reveals a minor younger component whose chemical propertiessuggest its later accretion. Moreover, the outer regions of these geometricallydefined thick disks show higher values of metallicity and lower values of[Mg/Fe]. These stars probably formed in the thin-disk region and they weredynamically heated to form the flares present in these two galaxies. We proposedifferent formation scenarios for the three populations of these thick disks:in-situ formation, accretion and disk heating. A clear distinction in age isfound between the metal poor and [Mg/Fe]-enhanced thick disks (old, $\sim12-13$ Gyr), and the metal rich and less [Mg/Fe]-enhanced thin disks (young,$\sim 4-5$ Gyr). These two galaxies show signs of relatively recent starformation in their thin disks and nuclear regions. While the thin disks showmore continuous SFHs, the nuclei display a rather bursty SFH. These twogalaxies are located outside of the densest region of the Fornax cluster whereFCC 170 resides. This other edge-on S0 galaxy was studied by \citet{Pinna2019}.We compare and discuss our results with this previous study. The differencesbetween these three galaxies, at different distances from the cluster center,suggest that the environment can have a strong effect on the galaxyevolutionary path.