arXiv subjects

Fangzhou Jiang

Publications and source records attributed to Fangzhou Jiang.

At least 73 records · Page 4Linked to original sources

arXiv2021

A Search for Wandering Black Holes in the Milky Way with Gaia and DECaLS

We present a search for "hyper-compact" star clusters in the Milky Way using a combination of Gaia and the Dark Energy Camera Legacy Survey (DECaLS). Such putative clusters, with sizes of ~1 pc and containing 500-5000 stars, are expected to remain bound to intermediate-mass black holes (Mbh~10^3-10^5 M-sun) that may be accreted into the Milky Way halo within dwarf satellites. Using the semi-analytic model SatGen we find an expected ~100 wandering intermediate-mass black holes with if every infalling satellite hosts a black hole. We do not find any such clusters in our search. Our upper limits rule out 100% occupancy, but do not put stringent constraints on the occupation fraction. Of course, we need stronger constraints on the properties of the putative star clusters, including their assumed sizes as well as the fraction of stars that would be compact remnants.

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arXiv2020

The Dekel-Zhao profile: A mass-dependent dark-matter density profile with flexible inner slope and analytic potential, velocity dispersion, and lensing properties

We explore a function with two shape parameters for the dark-matter halo density profile subject to baryonic effects, which is a special case of the general Zhao family of models applied to simulated dark matter haloes by Dekel et al. This profile has variable inner slope and concentration parameter, and analytic expressions for the gravitational potential, velocity dispersion, and lensing properties. Using the NIHAO cosmological simulations, we find that it provides better fits than the Einasto profile and the generalized NFW profile with variable inner slope, in particular towards the halo centers. We show that the profile parameters are correlated with the stellar-to-halo mass ratio $M_{\rm star}/M_{\rm vir}$. This defines a mass-dependent density profile describing the average dark matter profiles in all galaxies, which can be directly applied to observed rotation curves of galaxies, gravitational lenses, and semi-analytic models of galaxy formation or satellite-galaxy evolution. The effect of baryons manifests itself by a significant flattening of the inner density slope and a 20\% decrease of the concentration parameter for $M_{\rm star}/M_{\rm vir} = 10^{-3.5}$ to $10^{-2}$, corresponding to $M_{\rm star} \sim 10^{7-10} M_\odot$. The accuracy by which this profile fits simulated galaxies is similar to certain multi-parameter, mass-dependent profiles, but its fewer parameters and analytic nature make it most desirable for many purposes.

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arXiv2020

The SFR-radius connection: data and implications for wind strength and halo concentration

This paper is one in a series that explores the importance of radius as a second parameter in galaxy evolution. The topic investigated here is the relationship between star formation rate (SFR) and galaxy radius ($R_{\rm e}$) for main-sequence star-forming galaxies. The key observational result is that, over a wide range of stellar mass and redshift in both CANDELS and SDSS, there is little trend between SFR and $R_{\rm e}$ at fixed stellar mass. The Kennicutt-Schmidt law, or any similar density-related star formation law, then implies that smaller galaxies must have lower gas fractions than larger galaxies (at fixed $M_{\ast}$), and this is supported by observations of local star-forming galaxies. We investigate the implication by adopting the equilibrium "bathtub" model: the ISM gas mass is assumed to be constant over time and the net star formation rate is the difference between the accretion rate of gas onto the galaxy from the halo and the outflow rate due to winds. To match the observed null correlation between SFR and radius, the bathtub model requires that smaller galaxies at fixed mass have weaker galactic winds. Our hypothesis is that galaxies are a 2-dimensional family whose properties are set mainly by halo mass and concentration. Galaxy radius and accretion rate plausibly both depend on halo concentration, which predicts how wind strength should vary with $R_{\rm e}$ and SFR.

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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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arXiv2020

Quenching as a Contest between Galaxy Halos and their Central Black Holes

Existing models of galaxy formation have not yet explained striking correlations between structure and star-formation activity in galaxies, notably the sloped and moving boundaries that divide star-forming from quenched galaxies in key structural diagrams. This paper uses these and other relations to ``reverse-engineer'' the quenching process for central galaxies. The basic idea is that star-forming galaxies with larger radii (at a given stellar mass) have lower black-hole masses due to lower central densities. Galaxies cross into the green valley when the cumulative effective energy radiated by their black hole equals $\sim4\times$ their halo-gas binding energy. Since larger-radii galaxies have smaller black holes, one finds they must evolve to higher stellar masses in order to meet this halo-energy criterion, which explains the sloping boundaries. A possible cause of radii differences among star-forming galaxies is halo concentration. The evolutionary tracks of star-forming galaxies are nearly parallel to the green-valley boundaries, and it is mainly the sideways motions of these boundaries with cosmic time that cause galaxies to quench. BH-scaling laws for star-forming, quenched, and green-valley galaxies are different, and most BH mass growth takes place in the green valley. Implications include: the radii of star-forming galaxies are an important second parameter in shaping their black holes; black holes are connected to their halos but in different ways for star-forming, quenched, and green-valley galaxies; and the same BH-halo quenching mechanism has been in place since $z \sim 3$. We conclude with a discussion of black hole-galaxy co-evolution, the origin and interpretation of BH scaling laws.

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arXiv2020

SatGen: a semi-analytical satellite galaxy generator -- I. The model and its application to Local-Group satellite statistics

We present a semi-analytic model of satellite galaxies, SatGen, which can generate large samples of satellite populations for a host halo of desired mass, redshift, and assembly history. The model combines dark-matter halo merger trees, empirical relations for the galaxy-halo connection, and analytic prescriptions for tidal effects, dynamical friction, and ram pressure stripping. SatGen emulates cosmological zoom-in hydro-simulations in certain aspects. Satellites can reside in cored or cuspy DM subhaloes, depending on the halo response to baryonic physics that can be formulated from hydro-simulations and physical modeling. The subhalo profile and the stellar mass and size of a satellite evolves depending on its tidal mass loss and initial structure. The host galaxy can include a baryonic disc and a stellar bulge, each described by a density profile that allows analytic orbit integration. SatGen complements simulations by propagating the effect of halo response found in simulated field galaxies to satellites (not properly resolved in simulations) and outperforms simulations by sampling the halo-to-halo variance of satellite statistics and overcoming artificial disruption due to insufficient resolution. As a first application, we use the model to study satellites of Milky Way sized hosts, making it emulate simulations of bursty star formation and of smooth star formation, respectively, and to experiment with a disc potential in the host halo. Our model reproduces the observed satellite statistics reasonably well. Different physical recipes make a difference in satellite abundance and spatial distribution at the 25% level, not large enough to be distinguished by current observations given the halo-to-halo variance. The MW disc depletes satellites by 20% and has a subtle effect of diversifying the internal structure of satellites, important for alleviating certain small-scale problems.

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arXiv2020

A mass threshold for galactic gas discs by spin flips

We predict, analytically and by simulations, that gas discs tend to survive only in haloes above a threshold mass $\sim 2 \times 10^{11} M_\odot$ (stellar mass $\sim 10^9 M_\odot$), with only a weak redshift dependence. At lower masses, the disc spins typically flip in less than an orbital time due to mergers associated with a change in the pattern of the feeding cosmic-web streams. This threshold arises from the halo merger rate when accounting for the mass dependence of the ratio of galactic baryons and halo mass. Above the threshold, wet compactions lead to massive central nuggets that allow the longevity of extended clumpy gas rings. Supernova feedback has a major role in disrupting discs below the critical mass, by driving the stellar-to-halo mass ratio that affects the merger rate, by stirring up turbulence and suppressing high-angular-momentum gas supply, and by confining major compactions to the critical mass. Our predictions seem consistent with current observed fractions of gas discs, to be explored by future observations that will resolve galaxies below $10^9 M_\odot$ at high redshifts, e.g. by JWST.

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arXiv2019

A model for core formation in dark matter haloes and ultra diffuse galaxies by outflow episodes

We present a simple model for the response of a dissipationless spherical system to an instantaneous mass change at its center, describing the formation of flat cores in dark matter haloes and ultra-diffuse galaxies (UDGs) from feedback-driven outflow episodes in a specific mass range. This model generalizes an earlier simplified analysis of an isolated shell into a system with continuous density, velocity and potential profiles. The response is divided into an instantaneous change of potential at constant velocities due to a given mass loss or gain, followed by energy-conserving relaxation to a new Jeans equilibrium. The halo profile is modeled by a two-parameter function with a variable inner slope and an analytic potential profile (Dekel et al. 2017), which enables determining the associated kinetic energy at equilibrium. The model is tested against NIHAO cosmological zoom-in simulations, where it successfully predicts the evolution of the inner dark-matter profile between successive snapshots in about 75% of the cases, failing mainly in merger situations. This model provides a simple understanding of the formation of dark-matter halo cores and UDGs by supernova-driven outflows, and a useful analytic tool for studying such processes.

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arXiv2019

The Global Star-Formation Law by Supernova Feedback

We address a simple model where the Kennicutt-Schmidt (KS) relation between the macroscopic densities of star-formation rate (SFR, $ρ_{\rm sfr}$) and gas ($n$) in galactic discs emerges from self-regulation of the SFR via supernova feedback. It arises 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, $f\sim 0.5$. Expressing the bubble fading radius and time in terms of $n$, the filling factor is $f \propto S\,n^{-s}$ with $s\sim 1.5$, where $S$ is the supernova rate density. A constant $f$ thus refers to $ρ_{\rm sfr} \propto n^{1.5}$, 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.

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arXiv2018

Formation of ultra-diffuse galaxies in the field and in galaxy groups

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.

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arXiv2018

Is the dark-matter halo spin a predictor of galaxy spin and size?

The similarity between the distributions of spins for galaxies ($λ_{\rm g}$) and for dark-matter haloes ($λ_{\rm h}$), indicated both by simulations and observations, is naively interpreted as a one-to-one correlation between the spins of a galaxy and its host halo. This is used to predict galaxy sizes in semi-analytic models via $R_{\rm e}\simeqλ_{\rm h} R_{\rm v}$, 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 $λ_{\rm g}$ and $λ_{\rm h}$ are in fact only barely correlated, especially at $z\geq 1$. 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 $λ_{\rm g}/λ_{\rm h}$ and $λ_{\rm h}$, 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 $\cosθ=0.6$-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, $λ_{\rm h}$ 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.

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arXiv2017

The Galaxy Clustering Crisis in Abundance Matching

Galaxy clustering on small scales is significantly under-predicted by sub-halo abundance matching (SHAM) models that populate (sub-)haloes with galaxies based on peak halo mass, $M_{\rm peak}$. SHAM models based on the peak maximum circular velocity, $V_{\rm peak}$, have had much better success. The primary reason $M_{\rm peak}$ based models fail is the relatively low abundance of satellite galaxies produced in these models compared to those based on $V_{\rm peak}$. Despite success in predicting clustering, a simple $V_{\rm peak}$ based SHAM model results in predictions for galaxy growth that are at odds with observations. We evaluate three possible remedies that could "save" mass-based SHAM: (1) SHAM models require a significant population of "orphan" galaxies as a result of artificial disruption/merging of sub-haloes in modern high resolution dark matter simulations; (2) satellites must grow significantly after their accretion; and (3) stellar mass is significantly affected by halo assembly history. No solution is entirely satisfactory. However, regardless of the particulars, we show that popular SHAM models based on $M_{\rm peak}$ cannot be complete physical models as presented. Either $V_{\rm peak}$ truly is a better predictor of stellar mass at $z\sim 0$ and it remains to be seen how the correlation between stellar mass and $V_{\rm peak}$ comes about, or SHAM models are missing vital component(s) that significantly affect galaxy clustering.

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arXiv2017

Mapping substructure in the HST Frontier Fields cluster lenses and in cosmological simulations

We map the lensing-inferred substructure in the first three clusters observed by the Hubble Space Telescope Frontier Fields Initiative (HSTFF): Abell 2744 (z = 0.308), MACSJ0416, (z = 0.396) and MACSJ1149 (z = 0.543). Statistically resolving dark-matter subhaloes down to ~10^{9.5} solar masses, we compare the derived subhalo mass functions (SHMFs) to theoretical predictions from analytical models and with numerical simulations in a Lambda Cold Dark Matter (LCDM) cosmology. Mimicking our observational cluster member selection criteria in the HSTFF, we report excellent agreement in both amplitude and shape of the SHMF over four decades in subhalo mass (10^{9-13} solar masses). Projection effects do not appear to introduce significant errors in the determination of SHMFs from simulations. We do not find evidence for a substructure crisis, analogous to the missing satellite problem in the Local Group, on cluster scales, but rather excellent agreement of the count-matched HSTFF SHMF down to M_{sub halo}/M_{halo} ~ 10^{-5}. However, we do find discrepancies in the radial distribution of sub haloes inferred from HSTFF cluster lenses compared to determinations from simulated clusters. This suggests that although the selected simulated clusters match the HSTFF sample in mass, they do not adequately capture the dynamical properties and complex merging morphologies of these observed cluster lenses. Therefore, HSTFF clusters are likely observed in a transient evolutionary stage that is presently insufficiently sampled in cosmological simulations. The abundance and mass function of dark matter substructure in cluster lenses continues to offer an important test of the LCDM paradigm, and at present we find no tension between model predictions and observations.

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arXiv2016

Statistics of Dark Matter Substructure: III. Halo-to-Halo Variance

We present a study of unprecedented statistical power regarding the halo-to-halo variance of dark matter substructure. Using a combination of N-body simulations and a semi-analytical model, we investigate the variance in subhalo mass fractions and subhalo occupation numbers, with an emphasis on how these statistics scale with halo formation time. We demonstrate that the subhalo mass fraction, f_sub, is mainly a function of halo formation time, with earlier forming haloes having less substructure. At fixed formation redshift, the average f_sub is virtually independent of halo mass, and the mass dependence of f_sub is therefore mainly a manifestation of more massive haloes assembling later. We compare observational constraints on f_sub from gravitational lensing to our model predictions and simulation results. Although the inferred f_sub are substantially higher than the median LCDM predictions, they fall within the 95th percentile due to halo-to-halo variance. We show that while the halo occupation distribution of subhaloes, P(N|M), is super-Poissonian for large , a well established result, it becomes sub-Poissonian for < 2. Ignoring the non-Poissonity results in systematic errors of the clustering of galaxies of a few percent, and with a complicated scale- and luminosity-dependence. Earlier-formed haloes have P(N|M) closer to a Poisson distribution, suggesting that the dynamical evolution of subhaloes drives the statistics towards Poissonian. Contrary to a recent claim, the non-Poissonity of subhalo occupation statistics does not vanish by selecting haloes with fixed mass and fixed formation redshift. Finally, we use subhalo occupation statistics to put loose constraints on the mass and formation redshift of the Milky Way halo. Using observational constraints on the V_max of the most massive satellites, we infer that 0.25<M_vir/10^12M_sun/h<1.4 and 0.1<z_f<1.4 at 90% confidence.

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arXiv2015

On the Segregation of Dark Matter Substructure

We present the first comprehensive analysis of the segregation of dark matter subhaloes in their host haloes. Using numerical simulations, we examine the segregation of twelve different subhalo properties with respect to both orbital energy and halo-centric radius (in real space as well as in projection). Subhaloes are strongly segregated by accretion redshift, which is an outcome of the inside-out assembly of their host haloes. Since subhaloes that were accreted earlier have experienced more tidal stripping, subhaloes that have lost a larger fraction of their mass at infall are on more bound orbits. Subhaloes are also strongly segregated in their masses and maximum circular velocities at accretion. We demonstrate that part of this segregation is already imprinted in the infall conditions. For massive subhaloes it is subsequently boosted by dynamical friction, but only during their first radial orbit. The impact of these two effects is counterbalanced, though, by the fact that subhaloes with larger accretion masses are accreted later. Because of tidal stripping, subhaloes reveal little to no segregation by present-day mass or maximum circular velocity, while the corresponding torques cause subhaloes on more bound orbits to have smaller spin. There is a weak tendency for subhaloes that formed earlier to be segregated towards the center of their host halo, which is an indirect consequence of the fact that (sub)halo formation time is correlated with other, strongly segregated properties. We discuss the implications of our results for the segregation of satellite galaxies in galaxy groups and clusters.

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arXiv2015

Comprehensive Assessment of the Too-Big-to-Fail Problem

We use a semi-analytical model for the substructure of dark matter haloes to assess the too-big-to-fail (TBTF) problem. The model accurately reproduces the average subhalo mass and velocity functions, as well as their halo-to-halo variance, in N-body simulations. We construct thousands of realizations of Milky Way (MW) size host haloes, allowing us to investigate the TBTF problem with unprecedented statistical power. We examine the dependence on host halo mass and cosmology, and explicitly demonstrate that a reliable assessment of TBTF requires large samples of hundreds of host haloes. We argue that previous statistics used to address TBTF suffer from the look-elsewhere effect and/or disregard certain aspects of the data on the MW satellite population. We devise a new statistic that is not hampered by these shortcomings, and, using only data on the 9 known MW satellite galaxies with $V_{\rm max}>15{\rm kms}^{-1}$, demonstrate that $1.4^{+3.3}_{-1.1}\%$ of MW-size host haloes have a subhalo population in statistical agreement with that of the MW. However, when using data on the MW satellite galaxies down to $V_{\rm max}=8{\rm kms}^{-1}$, this MW consistent fraction plummets to $<5\times10^{-4}$ (at 68% CL). Hence, if it turns out that the inventory of MW satellite galaxies is complete down to 8km/s, then the maximum circular velocities of MW satellites are utterly inconsistent with $Λ$CDM predictions, unless baryonic effects can drastically increase the spread in $V_{\rm max}$ values of satellite galaxies compared to that of their subhaloes.

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arXiv2014

Coming of Age in the Dark Sector: How Dark Matter Haloes grow their Gravitational Potential Wells

We present a detailed study of how dark matter haloes assemble their mass and grow their (central) potential well. We characterize these via their mass accretion histories (MAHs) and potential well growth histories (PWGHs), which we extract from the Bolshoi simulation and from semi-analytical merger trees supplemented with a method to compute the maximum circular velocity, Vmax, of progenitor haloes. The results of both methods are in excellent agreement, both in terms of the average and the scatter. We show that the MAH and PWGH are tightly correlated, and that growth of the central potential precedes the assembly of mass; the maximum circular velocity is already half the present day value by the time the halo has accreted only 2 percent of its final mass. Finally, we demonstrate that MAHs have a universal form, which we use to develop a new and improved universal model that can be used to compute the average or median MAH and PWGH for a halo of any mass in any LCDM cosmology, without having to run a numerical simulation or a set of halo merger trees.

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arXiv2014

Statistics of Dark Matter Substructure: I. Model and Universal Fitting Functions

We present a new, semi-analytical model describing the evolution of dark matter subhaloes. The model uses merger trees constructed using the method of Parkinson et al. (2008) to describe the masses and redshifts of subhaloes at accretion, which are subsequently evolved using a simple model for the orbit-averaged mass loss rates. The model is extremely fast, treats subhaloes of all orders, accounts for scatter in orbital properties and halo concentrations, and uses a simple recipe to convert subhalo mass to maximum circular velocity. The model accurately reproduces the average subhalo mass and velocity functions in numerical simulations. The inferred subhalo mass loss rates imply that an average dark matter subhalo loses in excess of 80 percent of its infall mass during its first radial orbit within the host halo. We demonstrate that the total mass fraction in subhaloes is tightly correlated with the `dynamical age' of the host halo, defined as the number of halo dynamical times that have elapsed since its formation. Using this relation, we present universal fitting functions for the evolved and unevolved subhalo mass and velocity functions that are valid for any host halo mass, at any redshift, and for any ΛCDM cosmology.

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