arXiv subjects

Michael Kretschmer

Publications and source records attributed to Michael Kretschmer.

8 recordsLinked to original sources

arXiv2023

Wet Compaction to a Blue Nugget: a Critical Phase in Galaxy Evolution

We utilize high-resolution cosmological simulations to reveal that high-redshift galaxies tend to undergo a robust `wet compaction' event when near a `golden' stellar mass of $\sim 10^{10} M_{\odot}$. This is a gaseous shrinkage to a compact star-forming phase, a `blue nugget' (BN), followed by central quenching of star formation to a compact passive stellar bulge, a `red nugget' (RN), and a buildup of an extended gaseous disc and ring. Such nuggets are observed at cosmic noon and seed today's early-type galaxies. The compaction is triggered by a drastic loss of angular momentum due to, e.g., wet mergers, counter-rotating cold streams, or violent disc instability. The BN phase marks drastic transitions in the galaxy structural, compositional and kinematic properties. The transitions are from star-forming to quenched inside-out, from diffuse to compact with an extended disc-ring and a stellar envelope, from dark matter to baryon central dominance, from prolate to oblate stellar shape, from pressure to rotation support, from low to high metallicity, and from supernova to AGN feedback. The central black hole growth, first suppressed by supernova feedback when below the golden mass, is boosted by the compaction, and the black hole keeps growing once the halo is massive enough to lock in the supernova ejecta.

astro-ph.GA↗

arXiv2022

A Subgrid Turbulent Mean Field Dynamo Model for Cosmological Galaxy Formation Simulations

Magnetic fields have been included in cosmological simulations of galaxy formation only recently, due to advances in numerical techniques and a better understanding of the galaxy formation physics. In this paper, we develop a new subgrid model for the turbulent dynamo that takes place in the supersonic interstellar medium in star-forming galaxies. It is based on a mean-field approach that computes the turbulent kinetic energy at unresolved scales (the so-called subgrid scales) and modifies the induction equation to account for the corresponding $α$ dynamo. Our subgrid model depends on one free parameter, the quenching parameter, that controls the saturation of the subgrid dynamo. Thanks to this mean-field approach, we can now model the fast amplification of the magnetic field inside turbulent star-forming galaxies, without relying on artificially strong initial fields or without using prohibitively expensive high-resolution simulations. We show that the evolution of the magnetic field in our zoom-in Milky Way-like galaxy is consistent with a simple picture, in which the field is in equipartition with the turbulent kinetic energy inside the star-forming disc, with a field strength around 10 $μ$G at low redshift, while at the same time strong galactic outflows fill the halo with a slightly weaker magnetic field, whose strength (10 nG) is consistent will the ideal MHD dilution factor. Our results are in good agreement with recent theoretical and numerical predictions. We also compare our simulation with Faraday depth observations at both low and high redshift, seeing overall good agreement with some caveats. Our model naturally predicts stronger magnetic fields at high redshift (around 100 $μ$G in the galaxy and 1 $μ$G in the halo), but also stronger depolarisation effects due to stronger turbulence at early time.

astro-ph.GA↗

arXiv2022

On the origin of surprisingly cold gas discs in galaxies at high redshift

We address the puzzling observational indications for very "cold" galactic discs at redshifts $z \gtrsim 3$, an epoch when discs are expected to be highly perturbed. Using a high-resolution cosmological zoom-in simulation, we identify such a cold disc at $z\sim 3.5$, with a rotation velocity to velocity dispersion ratio of $v_ϕ/σ_r \simeq 5$ for the total gas. It forms as a result of a period of intense accretion of co-planar, co-rotating gas via cold cosmic-web streams. This thin disc survives for $\sim 5$ orbital periods, after which it is disrupted by mergers and counter-rotating streams, longer but consistent with our estimate that a galaxy of this mass ($M_\star\sim10^{10}\mathrm{M_\odot}$) typically survives merger-driven spin flips for $\sim 2-3$ orbital periods. We find that $v_ϕ/σ_r$ is highly sensitive to the tracer used to perform the kinematic analysis. While it is $v_ϕ/σ_r \simeq 3.5$ for atomic HI gas, it is $v_ϕ/σ_r \simeq 8$ for molecular CO and H$_2$. This reflects the confinement of molecular gas to cold, dense clouds that reside near the disc mid-plane, while the atomic gas is spread into a turbulent and more extended thicker disc. The proposed mechanisms is a theoretical proposal that has not been validated yet with proper statistical measurements and it remains unclear whether it occurs frequently enough to explain the multiple discoveries of cold gas disks in high-z galaxies.

astro-ph.GA↗

arXiv2021

Evaluating Galaxy Dynamical Masses From Kinematics and Jeans Equilibrium in Simulations

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 hydrostatic 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 $v_ϕ$ and of the radial velocity dispersion $σ_r$ are available, the dynamical mass $M_{\rm dyn} \!\simeq\! G^{-1} V_c^2 r$ can be evaluated from the Jeans equation $V_c^2= v_ϕ^2 + ασ_r^2$ assuming cylindrical symmetry and a constant, isotropic $σ_r$. For spheroids, $α$ is inversely proportional to the Sérsic index $n$ and $α\simeq 2.5$ within $R_e$ for the simulated galaxies. The prediction for a self-gravitating exponential disc, $α= 3.36(r/R_e)$, is invalid in the simulations, where the dominant spheroid causes a weaker gradient from $α\!\simeq\! 1$ at $R_e$ to 4 at $5R_e$. The correction in $α$ for the stars due to the gradient in $σ_r(r)$ is roughly balanced by the effect of the aspherical potential, while the effect of anisotropy is negligible. When only the effective projected velocity dispersion $σ_l$ is available, the dynamical mass can be evaluated as $M_{\rm dyn} = K G^{-1} R_e σ_l^2$, where the virial factor $K$ is derived from $α$ given the inclination and $v_ϕ/σ_r$. 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.

astro-ph.GA↗

arXiv2021

Forming early-type galaxies without AGN feedback: a combination of merger-driven outflows and inefficient star formation

Regulating the available gas mass inside galaxies proceeds through a delicate balance between inflows and outflows, but also through the internal depletion of gas due to star formation. At the same time, stellar feedback is the internal engine that powers the strong outflows. Since star formation and stellar feedback are both small scale phenomena, we need a realistic and predictive subgrid model for both. We describe the implementation of supernova momentum feedback and star formation based on the turbulence of the gas in the RAMSES code. For star formation, we adopt the so-called multi-freefall model. The resulting star formation efficiencies can be significantly smaller or bigger than the traditionally chosen value of $1\%$. We apply these new numerical models to a prototype cosmological simulation of a massive halo that features a major merger which results in the formation of an early-type galaxy without using AGN feedback. We find that the feedback model provides the first order mechanism for regulating the stellar and baryonic content in our simulated galaxy. At high redshift, the merger event pushes gas to large densities and large turbulent velocity dispersions, such that efficiencies come close to $10\%$, resulting in large SFR. We find small molecular gas depletion time during the starburst, in perfect agreement with observations. Furthermore, at late times, the galaxy becomes quiescent with efficiencies significantly smaller than $1\%$, resulting in small SFR and long molecular gas depletion time.

astro-ph.GA↗

arXiv2020

Rapid Filamentary Accretion as the Origin of Extended Thin Discs

Galactic outflows driven by stellar feedback are crucial for explaining the inefficiency of star formation in galaxies. Although strong feedback can promote the formation of galactic discs by limiting star formation at early times and removing low angular momentum gas, it is not understood how the same feedback can result in diverse objects such as elliptical galaxies or razor thin spiral galaxies. We investigate this problem using cosmological zoom-in simulations of two galaxies forming within $10^{12}~\mathrm{M_\odot}$ halos with almost identical mass accretion histories and halo spin parameters. However, the two resulting galaxies end up with very different bulge-to-disc ratios at $z = 0$. At $z>1.5$, the two galaxies feature a surface density of star formation $Σ_{\rm SFR}\simeq 10~\mathrm{M_\odot}~{\rm yr}^{-1}~{\rm kpc}^{-2}$, leading to strong outflows. After the last starburst episode, both galaxies feature a dramatic gaseous disc growth from 1~kpc to 5~kpc during 1~Gyr, a decisive event we dub "the Grand Twirl". After this event, the evolutionary tracks diverge strongly, with one galaxy ending up as a bulge-dominated galaxy, whereas the other ends up as a disc-dominated galaxy. The origins of this dichotomy are the angular momentum of the accreted gas, and whether it adds constructively to the initial disc angular momentum. The build-up of this extended disc leads to a rapid lowering of $Σ_{\rm SFR}$ by over two orders of magnitude with $Σ_{\rm SFR} \lesssim 0.1~\mathrm{M_\odot}~{\rm yr}^{-1}~{\rm kpc}^{-2}$, in remarkable agreement with what is derived from Milky Way stellar populations. As a consequence, supernovae explosions are spread out and cannot launch galactic outflows anymore, allowing for the persistence of a thin, gently star forming, extended disc.

astro-ph.GA↗

arXiv2020

Origin of Star-Forming Rings around Massive Centres in Massive Galaxies at $z\!<\!4$

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 galaxies 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 $\propto\! δ_{\rm d}^{-3}$, with $δ_{\rm d}$ 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 $δ_{\rm d} \!<\! 0.3$. The central mass that lowers $δ_{\rm d}$ 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 $H_α$ 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.

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arXiv2019

EDGE: the mass-metallicity relation as a critical test of galaxy formation physics

We introduce the "Engineering Dwarfs at Galaxy formation's Edge" (EDGE) project to study the cosmological formation and evolution of the smallest galaxies in the Universe. In this first paper, we explore the effects of resolution and sub-grid physics on a single low mass halo ($M_{\rm halo}=10^{9}~M_\odot$), simulated to redshift $z=0$ at a mass and spatial resolution of $\sim 20~M_\odot$ and $\sim 3$ pc. We consider different star formation prescriptions, supernova feedback strengths and on-the-fly radiative transfer (RT). We show that RT changes the mode of galactic self-regulation at this halo mass, suppressing star formation by causing the interstellar and circumgalactic gas to remain predominantly warm ($\sim 10^4$ K) even before cosmic reionisation. By contrast, without RT, star formation regulation occurs only through starbursts and their associated vigorous galactic outflows. In spite of this difference, the entire simulation suite (with the exception of models without any feedback) matches observed dwarf galaxy sizes, velocity dispersions, $V$-band magnitudes and dynamical mass-to-light-ratios. This is because such structural scaling relations are predominantly set by the host dark matter halo, with the remaining model-to-model variation being smaller than the observational scatter. We find that only the stellar mass-metallicity relation differentiates the galaxy formation models. Explosive feedback ejects more metals from the dwarf, leading to a lower metallicity at a fixed stellar mass. We conclude that the stellar mass-metallicity relation of the very smallest galaxies provides a unique constraint on galaxy formation physics.

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