arXiv subjects

Colin DeGraf

Publications and source records attributed to Colin DeGraf.

At least 19 recordsLinked to original sources

arXiv2026

The LISA Astrophysics MBHcatalogues Project: A comparison of predictions of simulated massive black hole binaries

In the hierarchical paradigm of galaxy formation, central massive black holes (MBHs) are expected to coalesce after the merger of their host galaxies. One of the main goals of the Laser Interferometer Space Antenna (LISA) is to constrain the origin and growth of MBHs through their merger rates and mass distribution. Predicting MBH merger rates requires not only tracing their statistical population from large to small physical scales (kpc to sub-pc) but also modelling their formation, accretion, dynamics, mergers, and their galactic physical processes across cosmic time. This project is the result of a large collaborative effort undertaken by the LISA Astrophysics Working Group, bringing together its collective expertise on MBH formation, evolution, and modelling, to build a comprehensive understanding of MBH merger rates across cosmic time. The project compares various theoretical predictions of MBH merger rates, quantifies the spread, and evaluates the global astrophysical uncertainties of the LISA event rates. To build a unique and complete view, our work is based on about 20 semi-analytical models and cosmological simulations from the literature, all employing distinct approaches to modelling MBH and galaxy physics. To compute the merger rates, we also incorporate delays arising from the dynamical phase of MBH hardening to coalescence. We present the expected LISA merger rates given current galaxy formation models and discuss how the merger rate depends on model assumptions, such as the seeding model and the resolution of cosmological simulations.

astro-ph.GA↗

arXiv2023

High-redshift supermassive black hole mergers in simulations with dynamical friction modelling

In the near future, projects like LISA and Pulsar Timing Arrays are expected to detect gravitational waves from mergers between supermassive black holes, and it is crucial to precisely model the underlying merger populations now to maximize what we can learn from this new data. Here we characterize expected high-redshift (z > 2) black hole mergers using the very large volume Astrid cosmological simulation, which uses a range of seed masses to probe down to low-mass BHs, and directly incorporates dynamical friction so as to accurately model the dynamical processes which bring black holes to the galaxy center where binary formation and coalescence will occur. The black hole populations in Astrid include black holes down to 10$^{4.5}$ M$_\odot$, and remain broadly consistent with the TNG simulations at scales > 10$^6$ M$_\odot$ (the seed mass used in TNG). By resolving lower-mass black holes, the overall merger rate is ~5x higher than in TNG. However, incorporating dynamical friction delays mergers compared to a recentering scheme, reducing the high-z merger rate mass-matched mergers by a factor of ~2x. We also calculate the expected LISA Signal-to-Noise values, and show that the distribution peaks at high SNR (>100), emphasizing the importance of implementing a seed mass well below LISA's peak sensitivity (10$^6$ M$_\odot$) to resolve the majority of LISA's GW detections.

astro-ph.GA↗

arXiv2022

Properties and Evolution of Dual and Offset AGN in the ASTRID Simulation at $z \sim 2$

We examine the dual (both BHs active) and offset (one BH active) AGN population (comprising $\sim$ 2000 pairs at $0.5\,\text{kpc}\lesssim\Delta r<30\,\text{kpc}$) at $z=2\sim3$ in the ASTRID simulation covering (360 cMpc)${^3}$. The dual (offset) AGN make up $3.0(2.2)\%$ of all AGN at $z=2$. The dual fraction is roughly constant while the offset fraction increases by a factor of ten from $z=4\sim2$. Compared with the full AGN population, duals are characterized by a low $M_\text{BH}/M_*$ ratio, a high specific star-formation rate (sSFR) of $\sim 1\,\text{Gyr}^{-1}$, and a high Eddington ratio ($\sim 0.05$, double that of single AGN). The dual AGN are formed in major galaxy mergers (typically involving $M_\text{halo}<10^{13}\,M_\odot$), with BHs that have similar masses. At small separations (when their host galaxies are in the late phase of the merger) duals become $2\sim8$ times brighter (albeit more obscured) than at larger separations. $80\%$ of these bright, close duals merge in the simulation within $\sim500\,\text{Myrs}$. Notably, the initially less-massive BH in duals frequently becomes the brighter AGN during the galaxy merger. In offset AGN, the active BH is typically $\gtrsim 10$ times more massive than its non-active counterpart and than most BHs in duals. Offsets are predominantly formed in minor galaxy mergers with the active BH residing in the center of massive halos ($ M_\text{ halo}\sim 10^{13-14}\,M_\odot$). In these deep potentials, gas stripping is common and the secondary quickly deactivates. The stripping also leads to inefficient orbital decay amongst offsets, which stall at $\Delta r\sim5\,\text{kpc}$ for a few hundred Myrs.

astro-ph.GA↗

arXiv2022

An estimate of the stochastic gravitational wave background from the MassiveBlackII simulation

A population of super-massive black hole binaries is expected to generate a stochastic gravitational wave background (SGWB) in the pulsar timing array (PTA) frequency range of $10^{-9}$--$10^{-7}$ Hz. Detection of this signal is a current observational goal and so predictions of its characteristics are of significant interest. In this work we use super-massive black hole binary mergers from the MassiveBlackII simulation to estimate the characteristic strain of the stochastic background. We examine both a gravitational wave driven model of binary evolution and a model which also includes the effects of stellar scattering and a circumbinary gas disk. Results are consistent with PTA upper limits and similar to estimates in the literature. The characteristic strain at a reference frequency of $1 yr^{-1}$ is found to be $A_{yr^{-1}} = 6.9 \times 10^{-16}$ and $A_{yr^{-1}} = 6.4 \times 10^{-16}$ in the gravitational-wave driven and stellar scattering/gas disk cases, respectively. Using the latter approach, our models show that the SGWB is mildly suppressed compared to the purely gravitational wave driven model as frequency decreases inside the PTA frequency band.

astro-ph.GA↗

arXiv2021

Massive Black Hole Mergers with Orbital Information: Predictions from the ASTRID Simulation

We examine massive black hole (MBH) mergers and their associated gravitational wave signals from the large-volume cosmological simulation Astrid. Astrid includes galaxy formation and black hole models recently updated with a MBH seed population between $3\times 10^4M_{\odot}/h$ and $3\times 10^5M_{\odot}/h$ and a sub-grid dynamical friction (DF) model to follow the MBH dynamics down to $1.5\;\text{ckpc}/h$. We calculate initial eccentricities of MBH orbits directly from the simulation at kpc-scales, and find orbital eccentricities above $0.7$ for most MBH pairs before the numerical merger. After approximating unresolved evolution on scales below ${\sim 200\,\text{pc}}$, we find that the in-simulation DF on large scales accounts for more than half of the total orbital decay time ($\sim 500\,\text{Myrs}$) due to DF. The binary hardening time is an order of magnitude longer than the DF time, especially for the seed-mass binaries ($M_\text{BH}<2M_\text{seed}$). As a result, only $\lesssim20\%$ of seed MBH pairs merge at $z>3$ after considering both unresolved DF evolution and binary hardening. These $z>3$ seed-mass mergers are hosted in a biased population of galaxies with the highest stellar masses of $>10^9\,M_\odot$. With the higher initial eccentricity prediction from Astrid, we estimate an expected merger rate of $0.3-0.7$ per year from the $z>3$ MBH population. This is a factor of $\sim 7$ higher than the prediction using the circular orbit assumption. The LISA events are expected at a similar rate, and comprise $\gtrsim 60\%$ seed-seed mergers, $\sim 30\%$ involving only one seed-mass MBH, and $\sim 10\%$ mergers of non-seed MBHs.

astro-ph.GA↗

arXiv2021

The ASTRID simulation: the evolution of Supermassive Black Holes

We present the evolution of black holes (BHs) and their relationship with their host galaxies in Astrid, a large-volume cosmological hydrodynamical simulation with box size 250 $h^{-1} \rm Mpc$ containing $2\times5500^3$ particles evolved to z=3. Astrid statistically models BH gas accretion and AGN feedback to their environments, applies a power-law distribution for BH seed mass $M_{\rm sd}$, uses a dynamical friction model for BH dynamics and executes a physical treatment of BH mergers. The BH population is broadly consistent with empirical constraints on the BH mass function, the bright end of the luminosity functions, and the time evolution of BH mass and accretion rate density. The BH mass and accretion exhibit a tight correlation with host stellar mass and star formation rate. We trace BHs seeded before z>10 down to z=3, finding that BHs carry virtually no imprint of the initial $M_{\rm sd}$ except those with the smallest $M_{\rm sd}$, where less than 50\% of them have doubled in mass. Gas accretion is the dominant channel for BH growth compared to BH mergers. With dynamical friction, Astrid predicts a significant delay for BH mergers after the first encounter of a BH pair, with a typical elapse time of about 200 Myrs. There are in total $4.5 \times 10^5$ BH mergers in Astrid at z>3, $\sim 10^3$ of which have X-ray detectable EM counterparts: a bright kpc scale dual AGN with $L_X>10^{43}$ erg/s. BHs with $M_{\rm BH} \sim 10^{7-8} M_{\odot}$ experience the most frequent mergers. Galaxies that host BH mergers are unbiased tracers of the overall $M_{\rm BH} - M_{*}$ relation. Massive ($>10^{11} M_{\odot}$) galaxies have a high occupation number (>10) of BHs, and hence host the majority of BH mergers.

astro-ph.GA↗

arXiv2021

On the detectability of massive black hole merger eventsby LISA

The launch of space based gravitational wave (GW) detectors (e.g. Laser Interferometry Space Antenna; LISA) and current and upcoming Pulsar Timing Arrays (PTAs) will extend the GW window to low frequencies, opening new investigations into dynamical processes involving massive black hole binaries (MBHBs) and their mergers across cosmic time. MBHBs are expected to be among the primary sources for the upcoming low frequency ($10^{-4}-10^{-1}$ Hz) window probed by LISA. It is important to investigate the expected MBH merger rates and associated signals, to determine how potential LISA events are affected by physics included in current models. To study this, we post-process the large population of MBHBs in the Illustris simulation to account for dynamical friction time delays associated with BH infall/inspiral. We show that merger delays associated with binary evolution have the potential to decrease the expected merger rates, with $M_{\rm{BH}} > 10^6 M_\odot$ MBHBs (the lowest mass in Illustris) decreasing from $\sim 3$ yr$^{-1}$ to $\sim 0.1 $yr$^{-1}$, and shifting the merger peak from z $\sim 2$ to $\sim 1.25$. During this time, we estimate that accretion grows the total merging mass by as much as 7x from the original mass. Importantly, however, dynamical friction associated delays (which shift the mergers toward lower-redshift and higher-masses) lead to a stronger signal/strain for the emitted GWs in the LISA band, increasing mean frequency from $10^{-3.1}$ to $10^{-3.4}-10^{-4.0}$ Hz, and mean strain from $10^{-17.2}$ to $10^{-16.3}-10^{-15.3}$. Finally, we show that after including a merger delay and associated $M_{\rm{BH}}$ growth, mergers still tend to lie on the typical $M_{\rm{BH}}-M_*$ relation, but with an increased likelihood of an undermassive black hole.

astro-ph.HE↗

arXiv2021

Dynamical Friction Modeling of Massive Black Holes in Cosmological Simulations and Effects on Merger Rate Predictions

In this work we establish and test methods for implementing dynamical friction for massive black hole pairs that form in large volume cosmological hydrodynamical simulations which include galaxy formation and black hole growth. We verify our models and parameters both for individual black hole dynamics and for the black hole population in cosmological volumes. Using our model of dynamical friction (DF) from collisionless particles, black holes can effectively sink close to the galaxy center, provided that the black hole's dynamical mass is at least twice that of the lowest mass resolution particles in the simulation. Gas drag also plays a role in assisting the black holes' orbital decay, but it is typically less effective than that from collisionless particles, especially after the first billion years of the black hole's evolution. DF from gas becomes less than $1\%$ of DF from collisionless particles for BH masses $> 10^{7}$ M$_{\odot}$. Using our best DF model, we calculate the merger rate down to $z=1.1$ using an $L_{\rm box}=35$ Mpc$/h$ simulation box. We predict $\sim 2$ mergers per year for $z>1.1$ peaking at $z\sim 2$. These merger rates are within the range obtained in previous work using similar-resolution hydro-dynamical simulations. We show that the rate is enhanced by factor of $\sim 2$ when DF is taken into account in the simulations compared to the no-DF run. This is due to $>40\%$ more black holes reaching the center of their host halo when DF is added.

astro-ph.GA↗

arXiv2021

Resolving Disks & Mergers in $z\sim2$ Heavily Reddened Quasars and their Companion Galaxies with ALMA

We present sub-arcsecond resolution ALMA imaging of the CO(3-2) emission in two $z\sim2.5$ heavily reddened quasars (HRQs) - ULASJ1234+0907 and ULASJ2315+0143 - and their companion galaxies. Dynamical modeling of the resolved velocity fields enables us to constrain the molecular gas morphologies and host galaxy masses. Combining the new data with extensive multi-wavelength observations, we are able to study the relative kinematics of different molecular emission lines, the molecular gas fractions and the locations of the quasars on the M$_{\rm{BH}}$-M$_{\rm{gal}}$ relation. Despite having similar black-hole properties, the two HRQs display markedly different host galaxy properties and local environments. J1234 has a very massive host, M$_{\rm{dyn}} \sim 5 \times 10^{11}$M$_\odot$ and two companion galaxies that are similarly massive located within 200 kpc of the quasar. The molecular gas fraction is low ($\sim$6%). The significant ongoing star formation in the host galaxy is entirely obscured at rest-frame UV and optical wavelengths. J2315 is resolved into a close-separation major-merger ($\Delta$r=15 kpc; $\Delta$v=170 km/s) with a $\sim$1:2 mass ratio. The total dynamical mass is estimated to be $\lesssim$10$^{11}$M$_\odot$ and the molecular gas fraction is high ($>$45%). A new HSC image of the galaxy shows unobscured UV-luminous star-forming regions co-incident with the extended reservoir of cold molecular gas in the merger. We use the outputs from the Illustris simulations to track the growth of such massive black holes from $z\sim6$ to the present day. While J1234 is consistent with the simulated $z\sim2$ relation, J2315 has a black hole that is over-massive relative to its host galaxy.

astro-ph.GA↗

arXiv2020

Morphological evolution of supermassive black hole merger hosts and multimessenger signatures

With projects such as Laser Interferometer Space Antenna (LISA) and Pulsar Timing Arrays expected to detect gravitational waves from supermassive black hole mergers in the near future, it is key that we understand what we expect those detections to be, and maximize what we can learn from them. To address this, we study the mergers of supermassive black holes in the Illustris simulation, the overall rate of mergers, and the correlation between merging black holes and their host galaxies. We find that these mergers occur in typical galaxies along the $M_{\rm{BH}}-M_*$ relation, and that between LISA and PTAs we expect to probe the full range of galaxy masses. As galaxy mergers can trigger increased star formation, we find that galaxies hosting low-mass black hole mergers tend to show a slight increase in star formation rates compared to a mass-matched sample. However, high-mass merger hosts have typical star formation rates, due to a combination of low gas fractions and powerful AGN feedback. Although minor black hole mergers do not correlate with disturbed morphologies, major mergers (especially at high-masses) tend to show morphological evidence of recent galaxy mergers which survives for ~500 Myr. This is on the same scale as the infall/hardening time of the merging black holes, suggesting that electromagnetic followups to gravitational wave signals may not be able to observe this correlation. We further find that incorporating a realistic timescale delay for the black hole mergers could shift the distribution of merger masses toward higher-masses, decreasing the rate of LISA detections while increasing the rate of PTA detections.

astro-ph.GA↗

arXiv2019

Cosmological simulations of massive black hole seeds: predictions for next generation electromagnetic and gravitational wave observations

We study how statistical properties of supermassive black holes depend on the frequency and conditions for massive seed formation in cosmological simulations of structure formation. We develop a novel method to recalculate detailed growth histories and merger trees of black holes within the framework of the Illustris simulation for several seed formation models, including a physically motivated model where black hole seeds only form in progenitor galaxies that conform to the conditions for direct collapse black hole formation. While all seed models considered here are in a broad agreement with present observational constraints on black hole populations from optical, UV and X-ray studies, we find that they lead to widely different black hole number densities and halo occupation fractions which are currently observationally unconstrained. In terms of future electromagnetic spectrum observations, the faint-end quasar luminosity function and the low-mass-end black hole-host galaxy scaling relations are very sensitive to the specific massive seed prescription. Specifically, the direct collapse model exhibits a seeding efficiency which decreases rapidly with cosmic time and produces much fewer black holes in low mass galaxies, in contrast to the original Illustris simulation. We further find that the total black hole merger rate varies by more than one order of magnitude for different seed models, with the redshift evolution of the chirp mass changing as well. Supermassive black hole merger detections with LISA and International Pulsar Timing Array may hence provide the most direct means of constraining massive black hole seed formation in the early Universe.

astro-ph.GA↗

arXiv2018

The AGORA High-Resolution Galaxy Simulations Comparison Project. II: Isolated Disk Test

Using an isolated Milky Way-mass galaxy simulation, we compare results from 9 state-of-the-art gravito-hydrodynamics codes widely used in the numerical community. We utilize the infrastructure we have built for the AGORA High-resolution Galaxy Simulations Comparison Project. This includes the common disk initial conditions, common physics models (e.g., radiative cooling and UV background by the standardized package Grackle) and common analysis toolkit yt, all of which are publicly available. Subgrid physics models such as Jeans pressure floor, star formation, supernova feedback energy, and metal production are carefully constrained across code platforms. With numerical accuracy that resolves the disk scale height, we find that the codes overall agree well with one another in many dimensions including: gas and stellar surface densities, rotation curves, velocity dispersions, density and temperature distribution functions, disk vertical heights, stellar clumps, star formation rates, and Kennicutt-Schmidt relations. Quantities such as velocity dispersions are very robust (agreement within a few tens of percent at all radii) while measures like newly-formed stellar clump mass functions show more significant variation (difference by up to a factor of ~3). Systematic differences exist, for example, between mesh-based and particle-based codes in the low density region, and between more diffusive and less diffusive schemes in the high density tail of the density distribution. Yet intrinsic code differences are generally small compared to the variations in numerical implementations of the common subgrid physics such as supernova feedback. Our experiment reassures that, if adequately designed in accordance with our proposed common parameters, results of a modern high-resolution galaxy formation simulation are more sensitive to input physics than to intrinsic differences in numerical schemes.

astro-ph.GA↗

arXiv2017

Black hole growth and AGN feedback under clumpy accretion

High-resolution simulations of supermassive black holes in isolated galaxies have suggested the importance of short (~10 Myr) episodes of rapid accretion caused by interactions between the black hole and massive dense clouds within the host. Accretion of such clouds could potentially provide the dominant source for black hole growth in high-z galaxies, but it remains unresolved in cosmological simulations. Using a stochastic subgrid model calibrated by high-resolution isolated galaxy simulations, we investigate the impact that variability in black hole accretion rates has on black hole growth and the evolution of the host galaxy. We find this clumpy accretion to more efficiently fuel high-redshift black hole growth. This increased mass allows for more rapid accretion even in the absence of high-density clumps, compounding the effect and resulting in substantially faster overall black hole growth. This increased growth allows the black hole to efficiently evacuate gas from the central region of the galaxy, driving strong winds up to ~2500 km/s, producing outflows ~10x stronger than the smooth accretion case, suppressing the inflow of gas onto the host galaxy, and suppressing the star formation within the galaxy by as much as a factor of two. This suggests that the proper incorporation of variability is a key factor in the co-evolution between black holes and their hosts.

astro-ph.GA↗

arXiv2017

Black hole clustering and duty cycles in the Illustris simulation

We use the high-resolution cosmological simulation Illustris to investigate the clustering of supermassive black holes across cosmic time, the link between black hole clustering and host halo masses, and the implications for black hole duty cycles. Our predicted black hole correlation length and bias match the observational data very well across the full redshift range probed. Black hole clustering is strongly luminosity-dependent on small, 1-halo scales, with some moderate dependence on larger scales of a few Mpc at intermediate redshifts. We find black hole clustering to evolve only weakly with redshift, initially following the behaviour of their hosts. However below z ~ 2 black hole clustering increases faster than that of their hosts, which leads to a significant overestimate of the clustering-predicted host halo mass. The full distribution of host halo masses is very wide, including a low-mass tail extending up to an order of magnitude below the naive prediction for minimum host mass. Our black hole duty cycles follow a power-law dependence on black hole mass and decrease with redshift, and we provide accurate analytic fits to these. The increase in clustering amplitude at late times, however, means that duty cycle estimates based on black hole clustering can overestimate duty cycles substantially, by more than two orders of magnitude. We find the best agreement when the minimum host mass is assumed to be $10^{11.2} M_\odot$ , which provides an accurate measure across all redshifts and luminosity ranges probed by our simulation.

astro-ph.GA↗

arXiv2016

Giant Clumps in Simulated High-z Galaxies: Properties, Evolution and Dependence on Feedback

We study the evolution of giant clumps in high-z disc galaxies using AMR cosmological simulations at redshifts z=6-1. Our sample consists of 34 galaxies, of halo masses 10^{11}-10^{12}M_s at z=2, run with and without radiation pressure (RP) feedback from young stars. While RP has little effect on the sizes and global stability of discs, it reduces the amount of star-forming gas by a factor of ~2, leading to a decrease in stellar mass by a similar factor by z~2. Both samples undergo violent disc instability (VDI) and form giant clumps of masses 10^7-10^9M_s at a similar rate, though RP significantly reduces the number of long-lived clumps. When RP is (not) included, clumps with circular velocity <40(20)km/s, baryonic surface density <200(100)M_s/pc^2 and baryonic mass <10^{8.2}(10^{7.3})M_s are short-lived, disrupted in a few free-fall times. The more massive and dense clumps survive and migrate toward the disc centre over a few disc orbital times. In the RP simulations, the distribution of clump masses and star-formation rates (SFRs) normalized to their host disc is very similar at all redshifts. They exhibit a truncated power-law with a slope slightly shallower than -2. Short-lived clumps preferentially have young stellar ages, low masses, high gas fractions and specific SFRs (sSFR), and they tend to populate the outer disc. The sSFR of massive, long-lived clumps declines with age as they migrate towards the disc centre, producing gradients in mass, stellar age, gas fraction, sSFR and metallicity that distinguish them from short-lived clumps. Ex situ mergers make up ~37% of the mass in clumps and ~29% of the SFR. They are more massive and with older stellar ages than the in situ clumps, especially near the disc edge. Roughly half the galaxies at redshifts z=4-1 are clumpy over a wide range of stellar mass, with clumps accounting for ~3-30% of the SFR but ~0.1-3% of the stellar mass.

astro-ph.GA↗

arXiv2016

Evolution of Density Profiles in High-z Galaxies: Compaction and Quenching Inside-Out

Using cosmological simulations, we address the interplay between structure and star formation in high-redshift galaxies via the evolution of surface density profiles. Our sample consists of 26 galaxies evolving in the redshift range $z=7-1$, spanning the stellar mass range $(0.2-6.4)\times 10^{10}M_\odot$ at $z=2$. We recover the main trends by stacking the profiles in accordance to their evolution phases. Following a wet compaction event that typically occurs when the stellar mass is $\sim10^{9.5}~M_{\odot}$ at $z\sim2-4$, the gas develops a cusp inside the effective radius, associated with a peak in star-formation rate (SFR). The SFR peak and the associated feedback, in the absence of further gas inflow to the centre, marks the onset of gas depletion from the central 1 kpc, leading to quenching of the central SFR. An extended, star-forming ring that forms by fresh gas during the central quenching process shows as a rising specific SFR (sSFR) profile, which is interpreted as inside-out quenching. Before quenching, the stellar density profile grows self-similarly, maintaining its log-log shape because the sSFR is similar at all radii. During the quenching process, the stellar density saturates to a constant value, especially in the inner 1 kpc. The stellar mass and SFR profiles deduced from observations show very similar shapes, consistent with the scenario of wet compaction leading to inside-out quenching and the subsequent saturation of a dense stellar core. We predict a cuspy gas profile during the blue nugget phase, and a gas-depleted core, sometimes surrounded by a ring, in the post-blue nugget phase.

astro-ph.GA↗

arXiv2016

The Confinement of Star-Forming Galaxies into a Main Sequence through Episodes of Gas Compaction, Depletion, and Replenishment

Using cosmological simulations, we address the properties of high-redshift star-forming galaxies (SFGs) across their main sequence (MS) in the plane of star-formation rate (SFR) versus stellar mass. We relate them to the evolution of galaxies through phases of gas compaction, depletion, possible replenishment, and eventual quenching. We find that the high-SFR galaxies in the upper envelope of the MS are compact, with high gas fractions and short depletion times ("blue nuggets"), while the lower-SFR galaxies in the lower envelope have lower central gas densities, lower gas fractions and longer depletion times, consistent with observed gradients across the MS. Stellar-structure gradients are negligible. The SFGs oscillate about the MS ridge on timescales $\sim0.4~t_{\mathrm{Hubble}}$ ($\sim1$ Gyr at $z\sim3$). The propagation upwards is due to gas compaction, triggered, e.g., by mergers, counter-rotating streams, and/or violent disc instabilities. The downturn at the upper envelope is due to central gas depletion by peak star formation and outflows while inflow from the shrunken gas disc is suppressed. An upturn at the lower envelope can occur once the extended disc has been replenished by fresh gas and a new compaction can be triggered, namely as long as the replenishment time is shorter than the depletion time. The mechanisms of gas compaction, depletion and replenishment confine the SFGs to the narrow ($\pm0.3$ dex) MS. Full quenching occurs in massive haloes ($M_{\mathrm{vir}}>10^{11.5}~M_\odot$) and/or at low redshifts ($z<3$), where the replenishment time is long compared to the depletion time, explaining the observed bending down of the MS at the massive end.

astro-ph.GA↗

arXiv2015

Compaction and Quenching of High-z Galaxies in Cosmological Simulations: Blue and Red Nuggets

We use cosmological simulations to study a characteristic evolution pattern of high redshift galaxies. Early, stream-fed, highly perturbed, gas-rich discs undergo phases of dissipative contraction into compact, star-forming systems (blue nuggets) at z~4-2. The peak of gas compaction marks the onset of central gas depletion and inside-out quenching into compact ellipticals (red nuggets) by z~2. These are sometimes surrounded by gas rings or grow extended dry stellar envelopes. The compaction occurs at a roughly constant specific star-formation rate (SFR), and the quenching occurs at a constant stellar surface density within the inner kpc ($Σ_1$). Massive galaxies quench earlier, faster, and at a higher $Σ_1$ than lower-mass galaxies, which compactify and attempt to quench more than once. This evolution pattern is consistent with the way galaxies populate the SFR-radius-mass space, and with gradients and scatter across the main sequence. The compaction is triggered by an intense inflow episode, involving (mostly minor) mergers, counter-rotating streams or recycled gas, and is commonly associated with violent disc instability. The contraction is dissipative, with the inflow rate >SFR, and the maximum $Σ_1$ anti-correlated with the initial spin parameter, as predicted by Dekel & Burkert (2014). The central quenching is triggered by the high SFR and stellar/supernova feedback (possibly also AGN feedback) due to the high central gas density, while the central inflow weakens as the disc vanishes. Suppression of fresh gas supply by a hot halo allows the long-term maintenance of quenching once above a threshold halo mass, inducing the quenching downsizing.

astro-ph.GA↗