We address the nature of the giant clumps in high-z galaxies that undergo Violent Disc Instability, attempting to distinguish between long-lived clumps that migrate inward and short-lived clumps that disrupt by feedback. We study the evolution of clu
mps as they migrate through the disc using an analytic model tested by simulations and confront theory with CANDELS observations. The clump ``bathtub model, which considers gas and stellar gain and loss, is characterized by four parameters: the accretion efficiency, the star-formation-rate (SFR) efficiency, and the outflow mass-loading factors for gas and stars. The relevant timescales are all comparable to the migration time, two-three orbital times. A clump differs from a galaxy by the internal dependence of the accretion rate on the varying clump mass. The analytic solution, involving exponential growing and decaying modes, reveals a main evolution phase during the migration, where the SFR and gas mass are constant and the stellar mass is rising linearly with time. This makes the inverse of the specific SFR an observable proxy for clump age. Later, the masses and SFR approach an exponential growth with a constant specific SFR, but this phase is hypothetical as the clump disappears in the galaxy center. The model matches simulations with different, moderate feedback, both in isolated and cosmological settings. The observed clumps agree with our theoretical predictions, indicating that the massive clumps are long-lived and migrating. A non-trivial challenge is to model feedback that is non-disruptive in massive clumps but suppresses SFR to match the galactic stellar-to-halo mass ratio.
Observed rotation curves in star-forming galaxies indicate a puzzling dearth of dark matter in extended flat cores within haloes of mass $geq! 10^{12}M_odot$ at $z!sim! 2$. This is not reproduced by current cosmological simulations, and supernova-dri
ven outflows are not effective in such massive haloes. We address a hybrid scenario where post-compaction merging satellites heat up the dark-matter cusps by dynamical friction, allowing AGN-driven outflows to generate cores. Using analytic and semi-analytic models (SatGen), we estimate the dynamical-friction heating as a function of satellite compactness for a cosmological sequence of mergers. Cosmological simulations (VELA) demonstrate that satellites of initial virial masses $>!10^{11.3}M_odot$, that undergo wet compactions, become sufficiently compact for significant heating. Constituting a major fraction of the accretion onto haloes $geq!10^{12}M_odot$, these satellites heat-up the cusps in half a virial time at $z!sim! 2$. Using a model for outflow-driven core formation (CuspCore), we demonstrate that the heated dark-matter cusps develop extended cores in response to removal of half the gas mass, while the more compact stellar systems remain intact. The mergers keep the dark matter hot, while the gas supply, fresh and recycled, is sufficient for the AGN outflows. AGN indeed become effective in haloes $geq!10^{12}M_odot$, where the black-hole growth is no longer suppressed by supernovae and its compaction-driven rapid growth is maintained by a hot CGM. For simulations to reproduce the dynamical-friction effects, they should resolve the compaction of the massive satellites and avoid artificial tidal disruption. AGN feedback could be boosted by clumpy black-hole accretion and clumpy response to AGN.
We provide prescriptions to evaluate the dynamical mass ($M_{rm dyn}$) of galaxies from kinematic measurements of stars or gas using analytic considerations and the VELA suite of cosmological zoom-in simulations at $z=1-5$. We find that Jeans or hydr
ostatic equilibrium is approximately valid for galaxies of stellar masses above $M_star !sim! 10^{9.5}M_odot$ out to $5$ effective radii ($R_e$). When both measurements of the rotation velocity $v_phi$ and of the radial velocity dispersion $sigma_r$ are available, the dynamical mass $M_{rm dyn} !simeq! G^{-1} V_c^2 r$ can be evaluated from the Jeans equation $V_c^2= v_phi^2 + alpha sigma_r^2$ assuming cylindrical symmetry and a constant, isotropic $sigma_r$. For spheroids, $alpha$ is inversely proportional to the Sersic index $n$ and $alpha simeq 2.5$ within $R_e$ for the simulated galaxies. The prediction for a self-gravitating exponential disc, $alpha = 3.36(r/R_e)$, is invalid in the simulations, where the dominant spheroid causes a weaker gradient from $alpha !simeq! 1$ at $R_e$ to 4 at $5R_e$. The correction in $alpha$ for the stars due to the gradient in $sigma_r(r)$ is roughly balanced by the effect of the aspherical potential, while the effect of anisotropy is negligible. When only the effective projected velocity dispersion $sigma_l$ is available, the dynamical mass can be evaluated as $M_{rm dyn} = K G^{-1} R_e sigma_l^2$, where the virial factor $K$ is derived from $alpha$ given the inclination and $v_phi/sigma_r$. We find that the standard value $K=5$ is approximately valid only when averaged over inclinations and for compact and thick discs, as it ranges from 4.5 to above 10 between edge-on and face-on projections.
Using analytic modeling and simulations, we address the origin of an abundance of star-forming, clumpy, extended gas rings about massive central bodies in massive galaxies at $z !<! 4$. Rings form by high-angular-momentum streams and survive in galax
ies of $M_{rm star} !>! 10^{9.5-10} M_odot$ where merger-driven spin flips and supernova feedback are ineffective. The rings survive after events of compaction to central nuggets. Ring longevity was unexpected based on inward mass transport driven by torques from violent disc instability. However, evaluating the torques from a tightly wound spiral structure, we find that the timescale for transport per orbital time is long and $propto! delta_{rm d}^{-3}$, with $delta_{rm d}$ the cold-to-total mass ratio interior to the ring. A long-lived ring forms when the ring transport is slower than its replenishment by accretion and the interior depletion by SFR, both valid for $delta_{rm d} !<! 0.3$. The central mass that lowers $delta_{rm d}$ is a compaction-driven bulge and/or dark matter, aided by the lower gas fraction at $z !<! 4$, provided that it is not too low. The ring is Toomre unstable for clump and star formation. The high-$z$ dynamic rings are not likely to arise form secular resonances or collisions. AGN feedback is not expected to affect the rings. Mock images of simulated rings through dust indicate qualitative consistency with observed rings about bulges in massive $z!sim!0.5!-!3$ galaxies, in $H_{alpha}$ and deep HST imaging. ALMA mock images indicate that $z!sim!0.5!-!1$ rings should be detectable. We quote expected observable properties of rings and their central nuggets.
We address a simple model where the Kennicutt-Schmidt (KS) relation between the macroscopic densities of star-formation rate (SFR, $rho_{rm sfr}$) and gas ($n$) in galactic discs emerges from self-regulation of the SFR via supernova feedback. It aris
es from the physics of supernova bubbles, insensitive to the microscopic SFR recipe and not explicitly dependent on gravity. The key is that the filling factor of SFR-suppressed supernova bubbles self-regulates to a constant, $fsim 0.5$. Expressing the bubble fading radius and time in terms of $n$, the filling factor is $f propto S,n^{-s}$ with $ssim 1.5$, where $S$ is the supernova rate density. A constant $f$ thus refers to $rho_{rm sfr} propto n^{1.5}$, with a density-independent SFR efficiency per free-fall time $sim 0.01$. The self-regulation to $f sim 0.5$ and the convergence to a KS relation independent of the local SFR recipe are demonstrated in cosmological and isolated-galaxy simulations using different codes and recipes. In parallel, the spherical analysis of bubble evolution is generalized to clustered supernovae, analytically and via simulations, yielding $s simeq 1.5 pm 0.5$. An analysis of photo-ionized bubbles about pre-supernova stars yields a range of KS slopes but the KS relation is dominated by the supernova bubbles. Superbubble blowouts may lead to an alternative self-regulation by outflows and recycling. While the model is over-simplified, its simplicity and validity in the simulations may argue that it captures the origin of the KS relation.
We study ultra-diffuse galaxies (UDGs) in zoom in cosmological simulations, seeking the origin of UDGs in the field versus galaxy groups. We find that while field UDGs arise from dwarfs in a characteristic mass range by multiple episodes of supernova
feedback (Di Cintio et al. 2017), group UDGs may also form by tidal puffing up and they become quiescent by ram-pressure stripping. The field and group UDGs share similar properties, independent of distance from the group centre. Their dark-matter haloes have ordinary spin parameters and centrally dominant dark-matter cores. Their stellar components tend to have a prolate shape with a Sersic index n~1 but no significant rotation. Ram pressure removes the gas from the group UDGs when they are at pericentre, quenching star formation in them and making them redder. This generates a colour/star-formation-rate gradient with distance from the centre, as observed in clusters. We find that ~20 per cent of the field UDGs that fall into a massive halo survive as satellite UDGs. In addition, normal field dwarfs on highly eccentric orbits can become UDGs near pericentre due to tidal puffing up, contributing about half of the group-UDG population. We interpret our findings using simple toy models, showing that gas stripping is mostly due to ram pressure rather than tides. We estimate that the energy deposited by tides in the bound component of a satellite over one orbit can cause significant puffing up provided that the orbit is sufficiently eccentric.
The similarity between the distributions of spins for galaxies ($lambda_{rm g}$) and for dark-matter haloes ($lambda_{rm h}$), indicated both by simulations and observations, is naively interpreted as a one-to-one correlation between the spins of a g
alaxy and its host halo. This is used to predict galaxy sizes in semi-analytic models via $R_{rm e}simeqlambda_{rm h} R_{rm v}$, with $R_{rm e}$ the half-mass radius of the galaxy and $R_{rm v}$ the halo radius. Utilizing two different suites of zoom-in cosmological simulations, we find that $lambda_{rm g}$ and $lambda_{rm h}$ are in fact only barely correlated, especially at $zgeq 1$. A general smearing of this correlation is expected based on the different spin histories, where the more recently accreted baryons through streams gain and then lose significant angular momentum compared to the gradually accumulated dark matter. Expecting the spins of baryons and dark matter to be correlated at accretion into $R_{rm v}$, the null correlation at the end reflects an anti-correlation between $lambda_{rm g}/lambda_{rm h}$ and $lambda_{rm h}$, which can partly arise from mergers and a compact star-forming phase that many galaxies undergo. On the other hand, the halo and galaxy spin vectors tend to be aligned, with a median $costheta=0.6$-0.7 between galaxy and halo, consistent with instreaming within a preferred plane. The galaxy spin is better correlated with the spin of the inner halo, but this largely reflects the effect of the baryons on the halo. Following the null spin correlation, $lambda_{rm h}$ is not a useful proxy for $R_{rm e}$. While our simulations reproduce a general relation of the sort $R_{rm e}=AR_{rm vir}$, in agreement with observational estimates, the relation becomes tighter with $A=0.02(c/10)^{-0.7}$, where $c$ is the halo concentration, which in turn introduces a dependence on mass and redshift.
Cold Fronts and shocks are hallmarks of the complex intra-cluster medium (ICM) in galaxy clusters. They are thought to occur due to gas motions within the ICM and are often attributed to galaxy mergers within the cluster. Using hydro-cosmological sim
ulations of clusters of galaxies, we show that collisions of inflowing gas streams, seen to penetrate to the very centre of about half the clusters, offer an additional mechanism for the formation of shocks and cold fronts in cluster cores. Unlike episodic merger events, a gas stream inflow persists over a period of several Gyrs and it could generate a particular pattern of multiple cold fronts and shocks.
We report on the search for galaxies in the proximity of two very metal-poor gas clouds at z~3 towards the quasar Q0956+122. With a 5-hour MUSE integration in a ~500x500 kpc^2 region centred at the quasar position, we achieve a >80% complete spectros
copic survey of continuum-detected galaxies with m<25 mag and Ly{alpha} emitters with luminosity L>3e41 erg/s. We do not identify galaxies at the redshift of a z~3.2 Lyman limit system (LLS) with log Z/Zsun = -3.35 +/- 0.05, placing this gas cloud in the intergalactic medium or circumgalactic medium of a galaxy below our sensitivity limits. Conversely, we detect five Ly{alpha} emitters at the redshift of a pristine z~3.1 LLS with log Z/Zsun < -3.8, while ~0.4 sources were expected given the z~3 Ly{alpha} luminosity function. Both this high detection rate and the fact that at least three emitters appear aligned in projection with the LLS suggest that this pristine cloud is tracing a gas filament that is feeding one or multiple galaxies. Our observations uncover two different environments for metal-poor LLSs, implying a complex link between these absorbers and galaxy halos, which ongoing MUSE surveys will soon explore in detail. Moreover, in agreement with recent MUSE observations, we detected a ~90 kpc Ly{alpha} nebula at the quasar redshift and three Ly{alpha} emitters reminiscent of a dark galaxy population.
We utilize zoom-in cosmological simulations to study the nature of violent disc instability (VDI) in clumpy galaxies at high redshift, $z=1$--$5$. Our simulated galaxies are not in the ideal state assumed in Toomre instability, of linear fluctuations
in an isolated, uniform, rotating disk. There, instability is characterised by a $Q$ parameter below unity, and lower when the disk is thick. Instead, the high-redshift discs are highly perturbed. Over long periods they consist of non-linear perturbations, compact massive clumps and extended structures, with new clumps forming in inter-clump regions. This is while the galaxy is subject to frequent external perturbances. We compute the local, two-component $Q$ parameter for gas and stars, smoothed on a $sim1~{rm kpc}$ scale to capture clumps of $10^{8-9}~{rm M}_odot$. The $Q<1$ regions are confined to collapsed clumps due to the high surface density there, while the inter-clump regions show $Q$ significantly higher than unity. Tracing the clumps back to their relatively smooth Lagrangian patches, we find that $Q$ prior to clump formation typically ranges from unity to a few. This is unlike the expectations from standard Toomre instability. We discuss possible mechanisms for high-$Q$ clump formation, e.g. rapid turbulence decay leading to small clumps that grow by mergers, non-axisymmetric instability, or clump formation induced by non-linear perturbations in the disk. Alternatively, the high-$Q$ non-linear VDI may be stimulated by the external perturbations such as mergers and counter-rotating streams. The high $Q$ may represent excessive compressive modes of turbulence, possibly induced by tidal interactions.
