Fangzhou Jiang | Yale University (original) (raw)
Papers by Fangzhou Jiang
Research in Astronomy and Astrophysics, 2011
We make use of the images from the Sloan Digital Sky Survey Stripe 82 (Stripe 82) to present an a... more We make use of the images from the Sloan Digital Sky Survey Stripe 82 (Stripe 82) to present an analysis of r band surface brightness profiles and radial color gradients (g−r, u−r) in our sample of 111 nearby early-type galaxies (ETGs). Thanks to the Stripe 82 images, each of which is co-added from about 50 single frames, we are able to pay special attention to the low-surface-brightness areas (LSB areas) of the galaxies. The LSB areas affect the Sérsic fittings and concentration indices by making both of the indices less than the typical values for ETGs. In the Sérsic fits to all the surface brightness profiles, we found some Sérsic indices that range from 1.5 to 2.5, much smaller than those of typical de Vaucouleur profiles and relatively close to those of exponential disks, and some others much larger than four but still with accurate fitting. Two galaxies cannot be fitted with a single Sérsic profile, but once we try double Sérsic profiles, the fittings are improved: one with a profile relatively close to the de Vaucouleur law in the inner area and a profile relatively close to an exponential law in the LSB area, the other with a nice fitting in the inner area but still having a failed fitting in the outer area. About 60% of the sample has negative color gradients (red-core) within 1.5R e , much more than the approximately 10% positive ones (bluecore) within the same radius. However, taking into account the LSB areas, we find that the color gradients are not necessarily monotonic: about one third of the red-core (or blue-core) galaxies have positive (or negative) color gradients in the outer areas. So LSB areas not only make ETGs' Sérsic profiles deviate from de Vaucouleur ones and shift to the disk end, but also reveal that quite a number of ETGs have opposite color gradients in inner and outer areas. These outcomes remind us of the necessity of double-Sérsic fitting. These LSB phenomena may be interpreted by mergers and thus have different metallicity in the outer areas. Isophotal parameters are also discussed briefly in this paper with the following conclusion: there are more disky nearby ETGs that are identified than boxy ones.
Recent studies have identified a population of compact quiescent galaxies at z ∼ 2. These galaxie... more Recent studies have identified a population of compact quiescent galaxies at z ∼ 2. These galaxies are very rare today and establishing the existence of a nearby analog could allow us to study its structure in greater detail than is possible at high redshift. Here we present such a local analog, NGC 5845, which has a dynamical mass of M dyn = 4.3 ± 0.6 × 10 10 M and an effective radius of only r e = 0.45 ± 0.05 kpc. We study the structure and kinematics with HST/WFPC2 data and previously published spatially resolved kinematics. We find that NGC 5845 is similar to compact quiescent galaxies at z ∼ 2 in terms of size versus dynamical mass (r e −M dyn), effective velocity dispersion versus size (σ e −r e), and effective velocity dispersion versus dynamical mass (σ e −M dyn). The galaxy has a prominent rotating disk evident in both the photometry and the kinematics: it extends to well beyond 1/3 effective radius and contribute to 1/4 of the total light of the galaxy. Our results lend support to the idea that a fraction of z ∼ 2 compact galaxies have prominent disks and positive mass-to-light ratio gradients, although we caution that NGC 5845 may have had a different formation history than the more massive compact quiescent galaxies at z ∼ 2.
We compare subhalo mass and velocity functions obtained from different simulations with different... more We compare subhalo mass and velocity functions obtained from different simulations with different subhalo finders among each other, and with predictions from the new semi-analytical model presented in Paper I. We find that subhalo mass functions (SHMFs) obtained using different subhalo finders agree with each other at the level of ∼20 per cent, but only at the low-mass end. At the massive end, subhalo finders that identify subhaloes based purely on density in configuration space dramatically underpredict the subhalo abundances by more than an order of magnitude. These problems are much less severe for subhalo velocity functions (SHVFs), indicating that they arise from issues related to assigning masses to the subhaloes, rather than from detecting them. Overall the predictions from the semi-analytical model are in excellent agreement with simulation results obtained using the more advanced subhalo finders that use information in six-dimensional phase-space. In particular, the model accurately reproduces the slope and host-mass-dependent normalization of both the subhalo mass and velocity functions. We find that the SHMFs and SHVFs have power-law slopes of 0.86 and 2.77, respectively, significantly shallower than what has been claimed in several studies in the literature.
We present the first comprehensive analysis of the segregation of dark matter subhaloes in their ... more We present the first comprehensive analysis of the segregation of dark matter subhaloes in their host haloes. Using three different numerical simulations, and two different segregation strength indicators, we examine the segregation of 12 different subhalo properties with respect to both orbital energy and halocentric 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 centre 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. It is well established that galaxy groups and clusters reveal segregation of their member galaxies. In particular, early-type galaxies (red, passive, elliptical) follow a more centrally concentrated radial distribution (and have a narrower line-of-sight velocity distribution) than their late-type (blue, active, spiral) counterparts (e.g. Postman). In addition, numerous studies have detected luminosity and/or stellar mass segregation in the sense that more luminous and massive galaxies are located at smaller group-or cluster-centric distances (e.g. Rood
We use a semi-analytical model for the substructure of dark matter haloes to assess the too big t... more 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 nine known MW satellite galaxies with V max > 15 km s −1 , demonstrate that 1.4 +3.3 −1.1 per cent 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 max = 8 km s −1 , this MW consistent fraction plummets to <5 × 10 −4 (at 68 per cent confidence level). Hence, if it turns out that the inventory of MW satellite galaxies is complete down to 8 km s −1 , then the maximum circular velocities of MW satellites are utterly inconsistent with cold dark matter predictions, unless baryonic effects can drastically increase the spread in V max values of satellite galaxies compared to that of their subhaloes.
We present a new, semi-analytical model describing the evolution of dark matter subhaloes. The mo... more 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. 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, uses a simple recipe to convert subhalo mass to maximum circular velocity, and considers subhalo disruption. The model is calibrated to accurately reproduce the average subhalo mass and velocity functions in numerical simulations. We demonstrate that, on average, the 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 a broad range in host halo mass, redshift and cold dark matter cosmology.
We present a detailed study of how dark matter haloes assemble their mass and grow their (central... more 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, V max , 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 per cent 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 cold dark matter cosmology, without having to run a numerical simulation or a set of halo merger trees.
Halo merger trees describe the hierarchical assembly of dark matter haloes, and are the backbone ... more Halo merger trees describe the hierarchical assembly of dark matter haloes, and are the backbone for modelling galaxy formation and evolution. Merger trees constructed using Monte Carlo algorithms based on the extended Press–Schechter (EPS) formalism are complementary to using N-body simulations and have the advantage that they are not trammelled by limited numerical resolution and uncertainties in identifying and linking (sub)haloes. This paper compares multiple EPS-based merger tree algorithms to simulation results using four diagnostics: progenitor mass function, mass assembly history (MAH), merger rate per descendant halo and the unevolved subhalo mass function. Spherical collapse-based methods typically overpredict major-merger rates, whereas ellipsoidal collapse dramatically overpredicts the minor-merger rate for massive haloes. The only algorithm in our comparison that yields results in good agreement with simulations is that by Parkinson et al. (P08). We emphasize, though, that the simulation results used as benchmarks in testing the merger trees are hampered by significant uncertainties themselves: MAHs and merger rates from different studies easily disagree by 50 per cent, even when based on the same simulation. Given this status quo, the P08 merger trees can be considered as accurate as those extracted from simulations.
Research in Astronomy and Astrophysics, 2011
We make use of the images from the Sloan Digital Sky Survey Stripe 82 (Stripe 82) to present an a... more We make use of the images from the Sloan Digital Sky Survey Stripe 82 (Stripe 82) to present an analysis of r band surface brightness profiles and radial color gradients (g−r, u−r) in our sample of 111 nearby early-type galaxies (ETGs). Thanks to the Stripe 82 images, each of which is co-added from about 50 single frames, we are able to pay special attention to the low-surface-brightness areas (LSB areas) of the galaxies. The LSB areas affect the Sérsic fittings and concentration indices by making both of the indices less than the typical values for ETGs. In the Sérsic fits to all the surface brightness profiles, we found some Sérsic indices that range from 1.5 to 2.5, much smaller than those of typical de Vaucouleur profiles and relatively close to those of exponential disks, and some others much larger than four but still with accurate fitting. Two galaxies cannot be fitted with a single Sérsic profile, but once we try double Sérsic profiles, the fittings are improved: one with a profile relatively close to the de Vaucouleur law in the inner area and a profile relatively close to an exponential law in the LSB area, the other with a nice fitting in the inner area but still having a failed fitting in the outer area. About 60% of the sample has negative color gradients (red-core) within 1.5R e , much more than the approximately 10% positive ones (bluecore) within the same radius. However, taking into account the LSB areas, we find that the color gradients are not necessarily monotonic: about one third of the red-core (or blue-core) galaxies have positive (or negative) color gradients in the outer areas. So LSB areas not only make ETGs' Sérsic profiles deviate from de Vaucouleur ones and shift to the disk end, but also reveal that quite a number of ETGs have opposite color gradients in inner and outer areas. These outcomes remind us of the necessity of double-Sérsic fitting. These LSB phenomena may be interpreted by mergers and thus have different metallicity in the outer areas. Isophotal parameters are also discussed briefly in this paper with the following conclusion: there are more disky nearby ETGs that are identified than boxy ones.
Recent studies have identified a population of compact quiescent galaxies at z ∼ 2. These galaxie... more Recent studies have identified a population of compact quiescent galaxies at z ∼ 2. These galaxies are very rare today and establishing the existence of a nearby analog could allow us to study its structure in greater detail than is possible at high redshift. Here we present such a local analog, NGC 5845, which has a dynamical mass of M dyn = 4.3 ± 0.6 × 10 10 M and an effective radius of only r e = 0.45 ± 0.05 kpc. We study the structure and kinematics with HST/WFPC2 data and previously published spatially resolved kinematics. We find that NGC 5845 is similar to compact quiescent galaxies at z ∼ 2 in terms of size versus dynamical mass (r e −M dyn), effective velocity dispersion versus size (σ e −r e), and effective velocity dispersion versus dynamical mass (σ e −M dyn). The galaxy has a prominent rotating disk evident in both the photometry and the kinematics: it extends to well beyond 1/3 effective radius and contribute to 1/4 of the total light of the galaxy. Our results lend support to the idea that a fraction of z ∼ 2 compact galaxies have prominent disks and positive mass-to-light ratio gradients, although we caution that NGC 5845 may have had a different formation history than the more massive compact quiescent galaxies at z ∼ 2.
We compare subhalo mass and velocity functions obtained from different simulations with different... more We compare subhalo mass and velocity functions obtained from different simulations with different subhalo finders among each other, and with predictions from the new semi-analytical model presented in Paper I. We find that subhalo mass functions (SHMFs) obtained using different subhalo finders agree with each other at the level of ∼20 per cent, but only at the low-mass end. At the massive end, subhalo finders that identify subhaloes based purely on density in configuration space dramatically underpredict the subhalo abundances by more than an order of magnitude. These problems are much less severe for subhalo velocity functions (SHVFs), indicating that they arise from issues related to assigning masses to the subhaloes, rather than from detecting them. Overall the predictions from the semi-analytical model are in excellent agreement with simulation results obtained using the more advanced subhalo finders that use information in six-dimensional phase-space. In particular, the model accurately reproduces the slope and host-mass-dependent normalization of both the subhalo mass and velocity functions. We find that the SHMFs and SHVFs have power-law slopes of 0.86 and 2.77, respectively, significantly shallower than what has been claimed in several studies in the literature.
We present the first comprehensive analysis of the segregation of dark matter subhaloes in their ... more We present the first comprehensive analysis of the segregation of dark matter subhaloes in their host haloes. Using three different numerical simulations, and two different segregation strength indicators, we examine the segregation of 12 different subhalo properties with respect to both orbital energy and halocentric 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 centre 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. It is well established that galaxy groups and clusters reveal segregation of their member galaxies. In particular, early-type galaxies (red, passive, elliptical) follow a more centrally concentrated radial distribution (and have a narrower line-of-sight velocity distribution) than their late-type (blue, active, spiral) counterparts (e.g. Postman). In addition, numerous studies have detected luminosity and/or stellar mass segregation in the sense that more luminous and massive galaxies are located at smaller group-or cluster-centric distances (e.g. Rood
We use a semi-analytical model for the substructure of dark matter haloes to assess the too big t... more 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 nine known MW satellite galaxies with V max > 15 km s −1 , demonstrate that 1.4 +3.3 −1.1 per cent 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 max = 8 km s −1 , this MW consistent fraction plummets to <5 × 10 −4 (at 68 per cent confidence level). Hence, if it turns out that the inventory of MW satellite galaxies is complete down to 8 km s −1 , then the maximum circular velocities of MW satellites are utterly inconsistent with cold dark matter predictions, unless baryonic effects can drastically increase the spread in V max values of satellite galaxies compared to that of their subhaloes.
We present a new, semi-analytical model describing the evolution of dark matter subhaloes. The mo... more 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. 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, uses a simple recipe to convert subhalo mass to maximum circular velocity, and considers subhalo disruption. The model is calibrated to accurately reproduce the average subhalo mass and velocity functions in numerical simulations. We demonstrate that, on average, the 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 a broad range in host halo mass, redshift and cold dark matter cosmology.
We present a detailed study of how dark matter haloes assemble their mass and grow their (central... more 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, V max , 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 per cent 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 cold dark matter cosmology, without having to run a numerical simulation or a set of halo merger trees.
Halo merger trees describe the hierarchical assembly of dark matter haloes, and are the backbone ... more Halo merger trees describe the hierarchical assembly of dark matter haloes, and are the backbone for modelling galaxy formation and evolution. Merger trees constructed using Monte Carlo algorithms based on the extended Press–Schechter (EPS) formalism are complementary to using N-body simulations and have the advantage that they are not trammelled by limited numerical resolution and uncertainties in identifying and linking (sub)haloes. This paper compares multiple EPS-based merger tree algorithms to simulation results using four diagnostics: progenitor mass function, mass assembly history (MAH), merger rate per descendant halo and the unevolved subhalo mass function. Spherical collapse-based methods typically overpredict major-merger rates, whereas ellipsoidal collapse dramatically overpredicts the minor-merger rate for massive haloes. The only algorithm in our comparison that yields results in good agreement with simulations is that by Parkinson et al. (P08). We emphasize, though, that the simulation results used as benchmarks in testing the merger trees are hampered by significant uncertainties themselves: MAHs and merger rates from different studies easily disagree by 50 per cent, even when based on the same simulation. Given this status quo, the P08 merger trees can be considered as accurate as those extracted from simulations.