It has been established for decades that rotation curves deviate from theNewtonian gravity expectation given baryons alone below a characteristicacceleration scale $g_{\dagger}\sim 10^{-8}\,\rm{cm\,s^{-2}}$, a scale promotedto a new fundamental constant in MOND. In recent years, theoretical andobservational studies have shown that the star formation efficiency (SFE) ofdense gas scales with surface density, SFE $\sim \Sigma/\Sigma_{\rm crit}$ with$\Sigma_{\rm crit} \sim \langle\dot{p}/m_{\ast}\rangle/(\pi\,G)\sim1000\,\rm{M_{\odot}\,pc^{-2}}$ (where $\langle \dot{p}/m_{\ast}\rangle$ is themomentum flux output by stellar feedback per unit stellar mass in a youngstellar population). We argue that the SFE, more generally, should scale withthe local gravitational acceleration, i.e. that SFE $\sim g_{\rm tot}/g_{\rmcrit} \equiv (G\,M_{\rm tot}/R^{2}) / \langle\dot{p}/m_{\ast}\rangle$, where$M_{\rm tot}$ is the total gravitating mass and $g_{\rmcrit}=\langle\dot{p}/m_{\ast}\rangle = \pi\,G\,\Sigma_{\rm crit} \approx10^{-8}\,\rm{cm\,s^{-2}} \approx g_{\dagger}$. Hence the observed $g_\dagger$may correspond to the characteristic acceleration scale above which stellarfeedback cannot prevent efficient star formation, and baryons will eventuallycome to dominate. We further show how this may give rise to the observedacceleration scaling $g_{\rm obs}\sim(g_{\rm baryon}\,g_{\dagger})^{1/2}$(where $g_{\rm baryon}$ is the acceleration due to baryons alone) and flatrotation curves. The derived characteristic acceleration $g_{\dagger}$ can beexpressed in terms of fundamental constants (gravitational constant, protonmass, and Thomson cross section): $g_{\dagger}\sim 0.1\,G\,m_{p}/\sigma_{\rmT}$.

We use high-resolution, multi-band imaging of ~16,500 galaxies in the CANDELSfields at 0 < z < 2.5 to study the evolution of color gradients and half-massradii over cosmic time. We find that galaxy color gradients at fixed massevolve rapidly between z~2.5 and z~1, but remain roughly constant below z~1.This result implies that the sizes of both star-forming and quiescent galaxiesincrease much more slowly than previous studies found using half-light radii.The half-mass radius evolution of quiescent galaxies is fully consistent with amodel which uses observed minor merger rates to predict the increase in sizesdue to the accretion of small galaxies. Progenitor bias may still contribute tothe growth of quiescent galaxies, particularly if we assume a slower timescalefor the minor merger growth model. The slower half-mass radius evolution ofstar-forming galaxies is in tension with cosmological simulations andsemi-analytic galaxy models. Further detailed, consistent comparisons withsimulations are required to place these results in context.

Based on well established scaling relation for star forming galaxies as afunction of redshift, we argue that the implied growth by a large factor oftheir angular momentum requires that the angular momentum of the inflowing gasfuelling star formation and disk growth must also secularly increase. We thenpropose that star formation in disks can cease (quench) once the accretedmaterial (mainly atomic hydrogen) comes in with excessive angular momentum forsustaining an adequate radial flow of cold, molecular gas. Existingobservational evidence supporting this scenario is mentioned, together withsome future observational studies that may validate (or invalidate) it.

Here we report the discovery with the Giant Metrewave Radio Telescope of anextremely large ($\sim$115 kpc in diameter) HI ring off-centered from a massivequenched galaxy, AGC 203001. This ring does not have any bright extendedoptical counterpart, unlike several other known ring galaxies. Our deep $g$,$r$, and $i$ optical imaging of the HI ring, using the MegaCam instrument onthe Canada-France-Hawaii Telescope, however, shows several regions with faintoptical emission at a surface brightness level of $\sim$28 mag/arcsec$^2$. Suchan extended HI structure is very rare with only one other case known so far --the Leo ring. Conventionally, off-centered rings have been explained by acollision with an "intruder" galaxy leading to expanding density waves of gasand stars in the form of a ring. However, in such a scenario the impact alsoleads to large amounts of star formation in the ring which is not observed inthe ring presented in this paper. We discuss possible scenarios for theformation of such HI dominated rings.

We present spectra of the most massive quiescent galaxy yet discovered at$z>3$, spectroscopically confirmed via the detection of Balmer absorptionfeatures in the $H-$ and $K-$bands of Keck/MOSFIRE. The spectra confirm agalaxy with no significant ongoing star formation, consistent with the lack ofrest-frame UV flux and overall photometric spectral energy distribution. With astellar mass of $3.1^{+0.1}_{-0.2} \times 10^{11} ~\rm{M}_\odot$ at $z =3.493$, this galaxy is nearly three times more massive than the highestredshift spectroscopically confirmed absorption-line identified galaxy known.The star-formation history of this quiescent galaxy implies that it formed$>1000 ~\rm{M}_\odot$/yr for almost 0.5 Gyr beginning at $z\sim7.2$, stronglysuggestive that it is the descendant of massive dusty star-forming galaxies at$5<z<7$ recently observed with ALMA. While galaxies with similarly extremestellar masses are reproduced in some simulations at early times, such a lackof ongoing star formation is not seen there. This suggests the need for a morerapid quenching process than is currently prescribed, challenging our currentunderstanding of how ultra-massive galaxies form and evolve in the earlyUniverse.