A simple model for star formation based on supernova (SN) feedback andgravitational heating via the collapse of perturbations in gravitationallyunstable disks reproduces the Schmidt-Kennicutt relation between the starformation rate (SFR) per unit area, $\Sigma_{SFR}$, and the gas surfacedensity, $\Sigma_g$, remarkably well. The gas velocity dispersion, $\sigma_g$,is derived self-consistently in conjunction with $\Sigma_{SFR}$ and is found tomatch the observations. Gravitational instability triggers{``gravito-turbulence"} at the scale of the least stable perturbation mode,boosting $\sigma_g$ at $\Sigma_g> \, \Sigma_g^\textrm{thr}=50\, {\rm M}_\odot\,{\rm pc}^{-2}$, and contributing to the pressure needed to carry the diskweight vertically. $\Sigma_{SFR}$ is reduced to the observed level at $\Sigma_g > \Sigma_g^\textrm{thr}$, whereas at lower surface densities, SNfeedback is the prevailing energy source. Our proposed star formation recipesrequire efficiencies of order 1\%, and the Toomre parameter, $Q$, for the jointgaseous and stellar disk is predicted to be close to the critical value formarginal stability for $\Sigma_g< \, \Sigma_g^\textrm{thr}$, spreading to lowervalues and larger gas velocity dispersion at higher $\Sigma_g$.

Using hydrodynamical $N$-body simulations, we show that the observedstructure and kinematics of the extremely metal-poor, dwarf irregular galaxyDDO 68 is compatible with a merger event with at least two smaller satellitegalaxies. We were able to obtain a self-consistent model that simultaneouslyreproduces several of its observed features, including: the very asymmetric anddisturbed shape of the stellar component, the overall HI distribution and itsvelocity field, the arc-like stellar structure to the west, the low-surfacebrightness stellar stream to the north. The model implies the interaction ofthe main progenitor of DDO 68 with two systems with dynamical masses$7\times10^8\,M_{\odot}$ and almost $10^8\,M_{\odot}$ -- 1/20 and 1/150 timesthe dynamical mass of DDO 68, respectively. We show that the merger between DDO68 and the most massive of its satellites offers a route to explain the largeoffset of DDO 68 from the mass-metallicity relation. Assuming that theinteracting galaxies have metallicities prior to the merger compatible withthose of galaxies with similar stellar masses, we provide quantitative evidencethat gas mixing alone does not suffice at diluting the gas of the twocomponents; according to our simulations, the HII regions observed along theCometary Tail trace the low metallicity of the accreted satellite rather thanthat of DDO 68's main body. In this case, the mass corresponding to the lowmetallicity is that of the secondary body and DDO 68 becomes consistent withthe mass-metallicity relation.

Recent wide-field integral field spectroscopy has revealed the detailedproperties of high redshift Lyman-alpha (Lya) nebulae, most often targeted dueto the presence of an active galactic nucleus (AGN). Here, we use VLT/MUSE toresolve the morphology and kinematics of a nebula initially identified due tostrong Lya emission at $z\sim3.2$ (LABn06; Nilsson et al. 2006). Ourobservations reveal a two-lobed Lya nebula, at least $\sim$173 pkpc indiameter, with a light-weighted centroid near a mid-infrared source (within$\approx$17.2 pkpc) that appears to host an obscured AGN. The Lya emission nearthe AGN is also coincident in velocity with the kinematic center of the nebula,suggesting that the nebula is both morphologically and kinematically centeredon the AGN. Compared to AGN-selected Lya nebulae, the surface brightnessprofile of this nebula follows a typical exponential profile at large radii($>$25 pkpc), although at small radii, the profile shows an unusual dip at thelocation of the AGN. The kinematics and asymmetry are similar to, and the CIVand HeII upper limits are consistent with other AGN-powered Lya nebulae.Double-peaked and asymmetric line profiles suggest that Lya resonant scatteringmay be important in this nebula. These results support the picture of the AGNbeing responsible for powering a Lya nebula that is oriented roughly in theplane of the sky. Further observations will explore whether the central surfacebrightness depression is indicative of either an unusual gas or dustdistribution or variation in the ionizing output of the AGN over time.