We have explored the properties of a peculiar object detected in deep opticalimaging and located at the tip of an HI tail emerging from Hickson CompactGroup 16. Using multiband photometry from infrared to ultraviolet, we were ableto constrain its stellar age to 58$^{+22}_{-9}$ Myr with a rather highmetallicity of [Fe/H] = $-$0.16$^{+0.43}_{-0.41}$ for its stellar mass ofM$_\star$ = 4.2$\times$10$^6$ M$_\odot$, a typical signature of tidal dwarfgalaxies. The structural properties of this object are similar to those ofdiffuse galaxies, with a round and featureless morphology, a large effectiveradius (r$_{eff}$ = 1.5 kpc), and a low surface brightness (<$\mu_{g}$>$_{eff}$= 25.6 mag arcsec$^{-2}$). Assuming that the object is dynamically stable andable to survive in the future, its fading in time via the aging of its stellarcomponent will make it undetectable in optical observations in just $\sim$2 Gyrof evolution, even in the deepest current or future optical surveys. Its highHI mass, M(HI) = 3.9$\times$10$^8$ M$_\odot$, and future undetectable stellarcomponent will make the object match the observational properties of darkgalaxies, that is, dark matter halos that failed to turn gas into stars. Ourwork presents further observational evidence of the feasibility of HI tidalfeatures becoming fake dark galaxies; it also shows the impact of stellarfading, particularly in high metallicity systems such as tidal dwarfs, inhiding aged stellar components beyond detection limits in optical observations.

Much of the baryons in galaxy groups are thought to have been driven out tolarge distances ($\gtrsim$$R_{500}$) by feedback, but there are fewconstraining observations of this extended gas. This work presents the resolvedSunyaev--Zel'dovich (SZ) profiles for a stacked sample of 10 nearby galaxygroups within the mass range log$_{10}(M_{500}[M_{\odot}]) = 13.6 -13.9$. Wemeasured the SZ profiles using the publicly available $y$-map from the PlanckCollaboration as well as our own $y$-maps constructed from more recent versionsof $Planck$ data. The $y$-map extracted from the latest data release yielded asignificant SZ detection out to 3 $R_{500}$. In addition, the stacked profilefrom these data were consistent with simulations that included AGN feedback.Our best-fit model using the latest $Planck$ data suggested a baryon fraction$\approx 5.6\%$ within $R_{500}$. This is significantly lower than the cosmicvalue of $\approx 16\%$, supporting the idea that baryons have been driven tolarge radii by AGN feedback. Lastly, we discovered a significant ($\sim3\sigma$) "bump" feature near $\sim 2$ $R_{500}$ that is most likely thesignature of internal accretion shocks.

In Dou et al. (2021), we introduced the Fundamental Formation Relation (FFR),a tight relation between specific SFR (sSFR), H$_2$ star formation efficiency(SFE$_{\rm H_2}$), and the ratio of H$_2$ to stellar mass. Here we show thatatomic gas HI does not follow a similar FFR as H$_2$. The relation betweenSFE$_{\rm HI}$ and sSFR shows significant scatter and strong systematicdependence on all of the key galaxy properties that we have explored. Thedramatic difference between HI and H$_2$ indicates that different processes(e.g., quenching by different mechanisms) may have very different effects onthe HI in different galaxies and hence produce different SFE$_{\rm HI}$-sSFRrelations, while the SFE$_{\rm H_2}$-sSFR relation remains unaffected. Thefacts that SFE$_{\rm H_2}$-sSFR relation is independent of other key galaxyproperties, and that sSFR is directly related to the cosmic time and acts asthe cosmic clock, make it natural and very simple to study how different galaxypopulations (with different properties and undergoing different processes)evolve on the same SFE$_{\rm H_2}$-sSFR $\sim t$ relation. In the gas regulatormodel (GRM), the evolution of a galaxy on the SFE$_{\rm H_2}$-sSFR($t$)relation is uniquely set by a single mass-loading parameter $\lambda_{\rmnet,H_2}$. This simplicity allows us to accurately derive the H$_2$ supply andremoval rates of the local galaxy populations with different stellar masses,from star-forming galaxies to the galaxies in the process of being quenched.This combination of FFR and GRM, together with the stellar metallicityrequirement, provide a new powerful tool to study galaxy formation andevolution.