arXiv subjects

Avishai Dekel

Publications and source records attributed to Avishai Dekel.

At least 145 records · Page 8Linked to original sources

arXiv2016

Velocities of warm galactic outflows from synthetic Hα observations of star-forming galaxies

The velocity structure imprinted in the Hα emission line profiles contains valuable information about galactic outflows. Using a set of high-resolution zoom-in cosmological simulations of galaxies at z=2, we generate Hα emission line profiles, taking into account the temperature-dependent Hα emissivity, as well as dust extinction. The Hα line can be described as a sum of two gaussians, as typically done with observations. In general, its properties are in good agreement with those observed in local isolated galaxies with similar masses and star formation rates, assuming a spatially constant clumping factor of c=24. Blueshifted outflows are very common in the sample. They extend several kpc above the galaxy discs. They are also spread over the full extent of the discs. However, at small radii, the material with high velocities tends to remain confined within a thick disc, as part of galactic fountains or a turbulent medium, most probably due to the deeper gravitational potential at the galaxy centre.

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arXiv2016

Suppression of star formation in dwarf galaxies by grain photoelectric feedback

Photoelectric heating has long been recognized as the primary source of heating for the neutral interstellar medium. Simulations of spiral galaxies found some indication that photoelectric heating could suppress star formation. However, simulations that include photoelectric heating have typically found that it has little effect on the rate of star formation in either spiral galaxies or dwarfs suggesting that supernovae and not photoelectric heating are responsible for setting the star formation law in galaxies. This result is in tension with recent work indicating that a star formation law that depends on galaxy metallicity, as expected for photoelectric heating but not for supernovae, reproduces the present-day galaxy population better than a metallicity-independent one. Here we report a series of simulations of dwarf galaxies, where the effects of both photoelectric heating and supernovae are expected to be strongest. We simultaneously include space- and time-dependent photoelectric heating, and we resolve the Sedov phase of every supernova blast wave, allowing us to make a direct measurement of the relative importance of momentum injection by supernovae and dust heating by far ultraviolet (FUV) photons in suppressing star formation. We find that supernovae are unable to account for the long observed gas depletion times in dwarf galaxies. Instead, ordinary photoelectric heating is the dominant means by which dwarf galaxies regulate their star formation rate at any given time, suppressing the star formation rate by more than an order of magnitude relative to simulations with only supernovae.

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arXiv2016

Stellar Mass--Gas-phase Metallicity Relation at $0.5\leq z\leq0.7$: A Power Law with Increasing Scatter toward the Low-mass Regime

We present the stellar mass ($M_{*}$)--gas-phase metallicity relation (MZR) and its scatter at intermediate redshifts ($0.5\leq z\leq0.7$) for 1381 field galaxies collected from deep spectroscopic surveys. The star formation rate (SFR) and color at a given $M_{*}$ of this magnitude-limited ($R\lesssim24$ AB) sample are representative of normal star-forming galaxies. For masses below $10^9 M_\odot$, our sample of 237 galaxies is $\sim$10 times larger than those in previous studies beyond the local universe. This huge gain in sample size enables superior constraints on the MZR and its scatter in the low-mass regime. We find a power-law MZR at $10^{8} M_\odot < M_{*} < 10^{11} M_\odot$: ${12+log(O/H) = (5.83\pm0.19) + (0.30\pm0.02)log(M_{*}/M_\odot)}$. Our MZR shows good agreement with others measured at similar redshifts in the literature in the intermediate and massive regimes, but is shallower than the extrapolation of the MZRs of others to masses below $10^{9} M_\odot$. The SFR dependence of the MZR in our sample is weaker than that found for local galaxies (known as the Fundamental Metallicity Relation). Compared to a variety of theoretical models, the slope of our MZR for low-mass galaxies agrees well with predictions incorporating supernova energy-driven winds. Being robust against currently uncertain metallicity calibrations, the scatter of the MZR serves as a powerful diagnostic of the stochastic history of gas accretion, gas recycling, and star formation of low-mass galaxies. Our major result is that the scatter of our MZR increases as $M_{*}$ decreases. Our result implies that either the scatter of the baryonic accretion rate or the scatter of the $M_{*}$--$M_{halo}$ relation increases as $M_{*}$ decreases. Moreover, our measures of scatter at $z=0.7$ appears consistent with that found for local galaxies.

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arXiv2016

Evolution of Galaxy Shapes from Prolate to Oblate through Compaction Events

We study the evolution of global shapes of galaxies using cosmological simulations. The shapes refer to the components of dark matter (DM), stars and gas at the stellar half-mass radius. Most galaxies undergo a characteristic compaction event into a blue nugget at $z\sim2-4$, which marks the transition from a DM-dominated central body to a self-gravitating baryonic core. We find that in the high-$z$, DM-dominated phase, the stellar and DM systems tend to be triaxial, preferentially prolate and mutually aligned. The elongation is supported by an anisotropic velocity dispersion that originates from the assembly of the galaxy along a dominant large-scale filament. We estimate that torques by the dominant halo are capable of inducing the elongation of the stellar system and its alignment with the halo. Then, in association with the transition to self-gravity, small-pericenter orbits puff up and the DM and stellar systems evolve into a more spherical and oblate configuration, aligned with the gas disc and associated with rotation. This transition typically occurs when the stellar mass is $\sim 10^9$ M$_\odot$ and the escape velocity in the core is $\sim 100$ km s$^{-1}$, indicating that supernova feedback may be effective in keeping the core DM-dominated and the system prolate. The early elongated phase itself may be responsible for the compaction event, and the transition to the oblate phase may be associated with the subsequent quenching in the core.

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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.

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arXiv2016

Gas inflow and metallicity drops in star-forming galaxies

Gas inflow feeds galaxies with low metallicity gas from the cosmic web, sustaining star formation across the Hubble time. We make a connection between these inflows and metallicity inhomogeneities in star-forming galaxies, by using synthetic narrow-band images of the Halpha emission line from zoom-in AMR cosmological simulations of galaxies with stellar masses of $M \simeq 10^9 $Msun at redshifts z=2-7. In $\sim$50\% of the cases at redshifts lower than 4, the gas inflow gives rise to star-forming, Halpha-bright, off-centre clumps. Most of these clumps have gas metallicities, weighted by Halpha luminosity, lower than the metallicity in the surrounding interstellar medium by $\sim$0.3 dex, consistent with observations of chemical inhomogeneities at high and low redshifts. Due to metal mixing by shear and turbulence, these metallicity drops are dissolved in a few disc dynamical times. Therefore, they can be considered as evidence for rapid gas accretion coming from cosmological inflow of pristine gas.

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arXiv2016

Evolution of Intrinsic Scatter in the SFR-Stellar Mass Correlation at 0.5<z<3

We present estimates of intrinsic scatter in the Star Formation Rate (SFR) - Stellar Mass (M*) correlation in the redshift range 0.5 < z < 3.0 and in the mass range 10^7 < M* < 10^11 Msun. We utilize photometry in the Hubble Ultradeep Field (HUDF12), Ultraviolet Ultra Deep Field (UVUDF) campaigns and CANDELS/GOODS-S. We estimate SFR, M* from broadband Spectral Energy Distributions (SEDs) and the best available redshifts. The maximum depth of the HUDF photometry (F160W 29.9 AB, 5 sigma depth) probes the SFR-M* correlation down to M* ~ 10 ^7 Msun, a factor of 10-100X lower in M* than previous studies, and comparable to dwarf galaxies in the local universe. We find the slope of the SFR-M* relationship to be near unity at all redshifts and the normalization to decrease with cosmic time. We find a moderate increase in intrinsic scatter with cosmic time from 0.2 to 0.4 dex across the epoch of peak cosmic star formation. None of our redshift bins show a statistically significant increase in intrinsic scatter at low mass. However, it remains possible that intrinsic scatter increases at low mass on timescales shorter than ~ 100 Myr. Our results are consistent with a picture of gradual and self-similar assembly of galaxies across more than three orders of magnitude in stellar mass from as low as 10^7 Msun.

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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.

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arXiv2015

Non-linear violent disc instability with high Toomre's Q in high-redshift clumpy disc galaxies

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.

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arXiv2015

An Increasing Stellar Baryon Fraction in Bright Galaxies at High Redshift

Recent observations have shown that the characteristic luminosity of the rest-frame ultraviolet (UV) luminosity function does not significantly evolve at 4 < z < 7 and is approximately M*_UV ~ -21. We investigate this apparent non-evolution by examining a sample of 178 bright, M_UV < -21 galaxies at z=4 to 7, analyzing their stellar populations and host halo masses. Including deep Spitzer/IRAC imaging to constrain the rest-frame optical light, we find that M*_UV galaxies at z=4-7 have similar stellar masses of log(M/Msol)=9.6-9.9 and are thus relatively massive for these high redshifts. However, bright galaxies at z=4-7 are less massive and have younger inferred ages than similarly bright galaxies at z=2-3, even though the two populations have similar star formation rates and levels of dust attenuation. We match the abundances of these bright z=4-7 galaxies to halo mass functions from the Bolshoi Lambda-CDM simulation to estimate the halo masses. We find that the typical halo masses in ~M*_UV galaxies decrease from log(M_h/Msol)=11.9 at z=4 to log(M_h/Msol)=11.4 at z=7. Thus, although we are studying galaxies at a similar mass across multiple redshifts, these galaxies live in lower mass halos at higher redshift. The stellar baryon fraction in units of the cosmic mean Omega_b/Omega_m rises from 5.1% at z=4 to 11.7% at z=7; this evolution is significant at the ~3-sigma level. This rise does not agree with simple expectations of how galaxies grow, and implies that some effect, perhaps a diminishing efficiency of feedback, is allowing a higher fraction of available baryons to be converted into stars at high redshifts.

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arXiv2015

Satellite Quenching and Galactic Conformity at 0.3 < z < 2.5

We measure the evolution of the quiescent fraction and quenching efficiency of satellites around star-forming and quiescent central galaxies with stellar mass $\log(M_{\mathrm{cen}}/M_{\odot})>10.5$ at $0.3 9.3$. Satellites for both star-forming and quiescent central galaxies have higher quiescent fractions compared to field galaxies matched in stellar mass at all redshifts. We also observe "galactic conformity": satellites around quiescent centrals are more likely to be quenched compared to the satellites around star-forming centrals. In our sample, this conformity signal is significant at $\gtrsim3σ$ for $0.6<z<1.6$, whereas it is only weakly significant at $0.3<z<0.6$ and $1.6<z<2.5$. Therefore, conformity (and therefore satellite quenching) has been present for a significant fraction of the age of the universe. The satellite quenching efficiency increases with increasing stellar mass of the central, but does not appear to depend on the stellar mass of the satellite to the mass limit of our sample. When we compare the satellite quenching efficiency of star-forming centrals with stellar masses 0.2 dex higher than quiescent centrals (which should account for any difference in halo mass), the conformity signal decreases, but remains statistically significant at $0.6<z<0.9$. This is evidence that satellite quenching is connected to the star-formation properties of the central as well as to the mass of the halo. We discuss physical effects that may contribute to galactic conformity, and emphasize that they must allow for continued star-formation in the central galaxy even as the satellites are quenched.

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arXiv2015

Distribution of streaming rates into high-redshift galaxies

We study the accretion along streams from the cosmic web into high-redshift massive galaxies using three sets of AMR hydro-cosmological simulations. We find that the streams keep a roughly constant accretion rate as they penetrate into the halo centre. The mean accretion rate follows the mass and redshift dependence predicted for haloes by the EPS approximation, dM / dt is proportional to Mvir^{1.25} (1 + z)^{2.5}. The distribution of the accretion rates can well be described by a sum of two Gaussians, the primary corresponding to "smooth inflow" and the secondary to "mergers". The same functional form was already found for the distributions of specific star formation rates in observations. The mass fraction in the smooth component is 60 - 90 %, insensitive to redshift or halo mass. The simulations with strong feedback show clear signs of reaccretion due to recycling of galactic winds. The mean accretion rate for the mergers is a factor 2 - 3 larger than that of the smooth component. The standard deviation of the accretion rate is 0.2 - 0.3 dex, showing no trend with mass or redshift. For the smooth component it is 0.12 - 0.24 dex.

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arXiv2015

Formation of elongated galaxies with low masses at high redshift

We report the identification of elongated (triaxial or prolate) galaxies in cosmological simulations at $z\simeq2$. These are preferentially low-mass galaxies ($M_s \le 10^{9.5} \ M_\odot$), residing in dark-matter (DM) haloes with strongly elongated inner parts, a common feature of high-redshift DM haloes in the $Λ$CDM cosmology. Feedback slows formation of stars at the centres of these halos, so that a dominant and prolate DM distribution gives rise to galaxies elongated along the DM major axis. As galaxies grow in stellar mass, stars dominate the total mass within the galaxy half-mass radius, making stars and DM rounder and more oblate. A large population of elongated galaxies produces a very asymmetric distribution of projected axis ratios, as observed in high-z galaxy surveys. This indicates that the majority of the galaxies at high redshifts are not discs or spheroids but rather galaxies with elongated morphologies.

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arXiv2015

Diverse Structural Evolution at z > 1 in Cosmologically Simulated Galaxies

From mock Hubble Space Telescope images, we quantify non-parametric statistics of galaxy morphology, thereby predicting the emergence of relationships among stellar mass, star formation, and observed rest-frame optical structure at 1 < z < 3. We measure automated diagnostics of galaxy morphology in cosmological simulations of the formation of 22 central galaxies with 9.3 < log10 M_*/M_sun < 10.7. These high-spatial-resolution zoom-in calculations enable accurate modeling of the rest-frame UV and optical morphology. Even with small numbers of galaxies, we find that structural evolution is neither universal nor monotonic: galaxy interactions can trigger either bulge or disc formation, and optically bulge-dominated galaxies at this mass may not remain so forever. Simulated galaxies with M_* > 10^10 M_sun contain relatively more disc-dominated light profiles than those with lower mass, reflecting significant disc brightening in some haloes at 1 < z < 2. By this epoch, simulated galaxies with specific star formation rates below 10^-9.7 yr^-1 are more likely than normal star-formers to have a broader mix of structural types, especially at M_* > 10^10 M_sun. We analyze a cosmological major merger at z ~ 1.5 and find that the newly proposed MID morphology diagnostics trace later merger stages while G-M20 trace earlier ones. MID is sensitive also to clumpy star-forming discs. The observability time of typical MID-enhanced events in our simulation sample is less than 100 Myr. A larger sample of cosmological assembly histories may be required to calibrate such diagnostics in the face of their sensitivity to viewing angle, segmentation algorithm, and various phenomena such as clumpy star formation and minor mergers.

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arXiv2015

Quenching and Morphological Transformation in Semi-Analytic Models and CANDELS

We examine the spheroid growth and star formation quenching experienced by galaxies from z~3 to the present by studying the evolution with redshift of the quiescent and spheroid-dominated fractions of galaxies from the CANDELS and GAMA surveys. We compare the observed fractions with predictions from a semi-analytic model which includes prescriptions for bulge growth and AGN feedback due to mergers and disk instabilities. We facilitate direct morphological comparison by converting our model bulge-to-total stellar mass ratios to Sersic indices. We then subdivide our population into the four quadrants of the sSFR-Sersic index plane and study the buildup of each of these subpopulations. We find that the fraction of star forming disks declines steadily, while the fraction of quiescent spheroids builds up over cosmic time. The fractions of star forming spheroids and quiescent disks are both non-negligible, and stay nearly constant over the period we have studied, at about 10% and 15-20% respectively. Our model is qualitatively successful at reproducing the evolution of the two "main" populations (star forming disk-dominated galaxies and quiescent spheroid-dominated galaxies), and approximately reproduces the relative fractions of all four types, but predicts a stronger decline in star forming spheroids, and increase in quiescent disks, than seen in the observations. A model with an additional channel for bulge growth via disk instabilities agrees better overall with the observations than a model in which bulges may grow only through mergers. We study evolutionary tracks of some individual galaxies as they experience morphological transformation and quenching, and examine the importance of different physical drivers of this transformation (major and minor mergers and disk instabilities). We find that complex histories with multiple transformative events are the norm.

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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.

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arXiv2015

Clumpy Galaxies in CANDELS. I. The Definition of UV Clumps and the Fraction of Clumpy Galaxies at 0.5<z<3

Although giant clumps of stars are crucial to galaxy formation and evolution, the most basic demographics of clumps are still uncertain, mainly because the definition of clumps has not been thoroughly discussed. In this paper, we study the basic demographics of clumps in star-forming galaxies (SFGs) at 0.5 0.5. The redshift evolution of Fclumpy changes with the stellar mass (M*) of the galaxies. Low-mass (log(M*/Msun)<9.8) galaxies keep an almost constant Fclumpy of about 60% from z~3.0 to z~0.5. Intermediate-mass and massive galaxies drop their Fclumpy from 55% at z~3.0 to 40% and 15%, respectively, at z~0.5. We find that (1) the trend of disk stabilization predicted by violent disk instability matches the Fclumpy trend of massive galaxies; (2) minor mergers are a viable explanation of the Fclumpy trend of intermediate-mass galaxies at z<1.5, given a realistic observability timescale; and (3) major mergers are unlikely responsible for the Fclumpy trend in all masses at z<1.5. The clump contribution to the rest-frame UV light of SFGs shows a broad peak around galaxies with log(M*/Msun)~10.5 at all redshifts, possibly linked to the molecular gas fraction of the galaxies. (Abridged)

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arXiv2015

Two Conditions for Galaxy Quenching: Compact Centres and Massive Haloes

We investigate the roles of two classes of quenching mechanisms for central and satellite galaxies in the SDSS ($z<0.075$): those involving the halo and those involving the formation of a compact centre. For central galaxies with inner compactness $Σ_{\rm 1kpc} \sim 10^{9-9.4}M_{\odot} {\rm kpc}^{-2}$, the quenched fraction $f_{q}$ is strongly correlated with $Σ_{\rm 1kpc}$ with only weak halo mass $M_{\rm h}$ dependence. However, at higher and lower $Σ_{\rm 1kpc}$, sSFR is a strong function of $M_{\rm h}$ and mostly independent of $Σ_{\rm 1kpc}$. In other words, $Σ_{\rm 1kpc} \sim 10^{9-9.4} M_{\odot} {\rm kpc}^{-2}$ divides galaxies into those with high sSFR below and low sSFR above this range. In both the upper and lower regimes, increasing $M_{\rm h}$ shifts the entire sSFR distribtuion to lower sSFR without a qualitative change in shape. This is true even at fixed $M_{*}$, but varying $M_{*}$ at fixed $M_{\rm h}$ adds no quenching information. Most of the quenched centrals with $M_{\rm h} > 10^{11.8}M_{\odot}$ are dense ($Σ_{\rm 1kpc} > 10^{9}~ M_{\odot} {\rm kpc}^{-2}$), suggesting compaction-related quenching maintained by halo-related quenching. However, 21% are diffuse, indicating only halo quenching. For satellite galaxies in the outskirts of halos, quenching is a strong function of compactness and a weak function of host $M_{\rm h}$. In the inner halo, $M_{\rm h}$ dominates quenching, with $\sim 90\%$ of the satellites being quenched once $M_{\rm h} > 10^{13}M_{\odot}$. This regional effect is greatest for the least massive satellites. As demonstrated via semi-analytic modelling with simple prescriptions for quenching, the observed correlations can be explained if quenching due to central compactness is rapid while quenching due to halo mass is slow.

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