Mass Accretion History Research Articles

Reversing arrested development: a new method to address halo assembly bias

ABSTRACT Halo assembly bias is a phenomenon whereby the clustering of dark matter haloes is dependent on halo properties, such as age, at fixed mass. Understanding the origin of assembly bias is important for interpreting the clustering of galaxies and constraining cosmological models. One proposed explanation for the origin of assembly bias is the truncation of mass accretion in low-mass haloes in the presence of more massive haloes, called ‘arrested development’. Haloes undergoing arrested development would have older measured ages and exhibit stronger clustering than equal mass haloes that have not undergone arrested development. We propose a new method to test the validity of this explanation for assembly bias and correct for it in cosmological N-body simulations. The method is based on the idea that the early mass accretion history of a halo, before arrested development takes effect, can be used to predict the late-time evolution of the halo in the absence of arrested development. We implement this idea by fitting a model to the early portion of halo accretion histories and extrapolating to late times. We then calculate ‘corrected’ masses and ages for haloes based on this extrapolation and investigate how this impacts the assembly bias signal. We find that correcting for arrested development this way leads to a factor of two reduction in the strength of the assembly bias signal across a range of low-halo masses. This result provides new evidence that arrested development is a cause of assembly bias and validates our approach to mitigating the effect.

Self-similar mass accretion history in scale-free simulations

ABSTRACT Using a scale-free N-body simulation generated with the abacusN-body code, we test the robustness of halo mass accretion histories via their convergence to self-similarity. We compare two halo finders, rockstar and compaso. We find superior self-similarity in halo mass accretion histories determined using rockstar, with convergence to 5 per cent or better between $\sim\!\! 10^2$ and $10^5$ particles. For compaso, we find weaker convergence over a similar region, with at least 10 per cent between $\sim\!\! 10^2$ and $10^4$ particles. Furthermore, we find that the convergence to self-similarity improves as the simulation evolves, with the largest and deepest regions of convergence appearing after the scale factor quadrupled from the time at which non-linear structures begin to form. With sufficient time evolution, halo mass accretion histories are converged to self-similarity within 5 per cent with as few as $\sim\!\! 70$ particles for compaso and within 2 per cent for as few as $\sim\!\! 30$ particles for rockstar.

Surveying the Onset and Evolution of Supermassive Black Holes at High-z with AXIS

The nature and origin of supermassive black holes (SMBHs) remain an open matter of debate within the scientific community. While various theoretical scenarios have been proposed, each with specific observational signatures, the lack of sufficiently sensitive X-ray observations hinders the progress of observational tests. In this white paper, we present how AXIS will contribute to solving this issue. With an angular resolution of 1.5″ on-axis and minimal off-axis degradation, we designed a deep survey capable of reaching flux limits in the [0.5–2] keV range of approximately 2 × 10−18 erg s−1 cm−2 over an area of 0.13 deg2 in approximately 7 million seconds (7 Ms). Furthermore, we planned an intermediate depth survey covering approximately 2 deg2 and reaching flux limits of about 2 × 10−17 erg s−1 cm−2 in order to detect a significant number of SMBHs with X-ray luminosities (LX) of approximately 1042 erg s−1 up to z∼10. These observations will enable AXIS to detect SMBHs with masses smaller than 105 M⊙, assuming Eddington-limited accretion and a typical bolometric correction for Type II AGN. AXIS will provide valuable information on the seeding and population synthesis models of SMBHs, allowing for more accurate constraints on their initial mass function (IMF) and accretion history from z∼0–10. To accomplish this, AXIS will leverage the unique synergy of survey telescopes such as the JWST, Roman, Euclid, Vera Rubin Telescope, and the new generation of 30 m class telescopes. These instruments will provide optical identification and redshift measurements, while AXIS will discover the smoking gun of nuclear activity, particularly in the case of highly obscured AGN or peculiar UV spectra as predicted and recently observed by the JWST in the early Universe.

Characterizing the assembly of dark matter haloes with protohalo size histories: I. Redshift evolution, relation to descendant haloes, and halo assembly bias

ABSTRACT We propose a novel method to quantify the assembly histories of dark matter haloes with the redshift evolution of the mass-weighted spatial variance of their progenitor haloes, that is, the protohalo size history. We find that the protohalo size history for each individual halo at z ∼ 0 can be described by a double power-law function. The amplitude of the fitting function strongly correlates to the central-to-total stellar mass ratios of descendant haloes. The variation of the amplitude of the protohalo size history can induce a strong halo assembly bias effect for massive haloes. This effect is detectable in observation using the central-to-total stellar mass ratio as a proxy of the protohalo size. The correlation to the descendant central-to-total stellar mass ratio and the halo assembly bias effect seen in the protohalo size are much stronger than that seen in the commonly adopted half-mass formation time derived from the mass accretion history. This indicates that the information loss caused by the compression of halo merger trees to mass accretion histories can be captured by the protohalo size history. Protohalo size thus provides a useful quantity to connect protoclusters across cosmic time and to link protoclusters with their descendant clusters in observations.

The mass accretion history of dark matter haloes down to Earth mass

ABSTRACT We take advantage of the unprecedented dynamical range provided by the ‘Cosmic-Zoom’ project to study the mass accretion history (MAH) of present-day dark matter haloes over the entire mass range present in the Lambda cold dark matter paradigm when the dark matter is made of weakly interacting massive particles of mass 100 GeV. In particular, we complement previous studies by exploring the MAHs of haloes with mass from $10^8\ h^{-1}\,\mathrm{{\rm M}_{\odot }}$ down to Earth mass, $10^{-6}\ h^{-1}\,\mathrm{{\rm M}_{\odot }}$. The formation redshift of low-mass haloes anticorrelates weakly with mass, peaking at z = 3 for haloes of mass $10^{-4}\ h^{-1}\,\mathrm{{\rm M}_{\odot }}$. Even lower masses are affected by the free-streaming cut-off in the primordial spectrum of density fluctuations and form at lower redshift. We compare MAHs in our simulations with predictions from two analytical models based on the extended Press–Schechter theory (EPS), and three empirical models derived by fitting and extrapolating either results from cosmological N-body simulations or Monte Carlo realizations of halo growth. All models fit our simulations reasonably well over the mass range for which they were calibrated. While the empirical models match better for more massive haloes, $M\gt 10^{10}\ h^{-1}\,\mathrm{{\rm M}_{\odot }}$, the analytical models do better when extrapolated down to Earth mass. At the higher masses, we explore the correlation between local environment density and MAH, finding that biases are relatively weak, with typical MAHs for haloes in extremely low-density and in typical regions differing by less than 20 per cent at high redshift. If this result can be extrapolated to lower halo masses, we conclude that EPS theory predicts the hierarchical build up of dark matter haloes quite well over the entire halo mass range.

The influence of subhaloes on host halo properties

ABSTRACT Within the ΛCDM cosmology, dark matter haloes are composed of both a smooth component and a population of smaller gravitationally bound subhaloes. These components are often treated as a single halo when properties, such as density profiles, are extracted from simulations. Recent work has shown that density profiles change substantially when subhalo mass is excluded. In this paper, we expand on this result by analysing three specific host halo properties – concentration (cNFW), spin (λB), and shape (c/a) – when calculated only from the smooth component of the halo. This analysis is performed on both Milky Way-mass haloes and cluster-mass haloes in high-resolution zoom-in N-body simulations. We find that when subhaloes are excluded, the median value of (1) cNFW is enhanced by $\approx 30\pm 11$ and $\approx 77\pm 8.1~{{\ \rm per\ cent}}$ for Milky Way-mass ($10^{12.1}\, \text{M}_\odot$) and cluster-mass ($10^{14.8}\, \text{M}_\odot$) haloes, respectively, (2) λB is reduced for Milky Way-mass by $\approx 11\pm 4.9~{{\ \rm per\ cent}}$ and cluster-mass haloes by $\approx 27\pm 3.5~{{\ \rm per\ cent}}$. Additionally, with the removal of subhaloes, cluster-mass haloes tend to become more spherical as the ratio of minor-to-major axis, c/a, increases by $\approx 11\pm 3.6~{{\ \rm per\ cent}}$, whereas Milky Way-mass haloes remain approximately the same shape with c/a changed by $\approx 1.0\pm 5.8~{{\ \rm per\ cent}}$. Fractional changes of each of these properties depend primarily on the amount of mass in subhaloes and, to a lesser extent, mass accretion history. Our findings demonstrate that the properties of the smooth components of dark matter haloes are biased relative to the total halo mass.

UOCS

Aims. NGC 6791 is one of the richest old open clusters in the Milky Way. Its position above the Galactic plane and its number density make it an interesting middle ground between Galactic open and globular clusters. We aim to detect the UV-bright population of NGC 6791 using AstroSat/UVIT images in near-UV and far-UV filters and characterise the known post-mass-transfer systems such as blue straggler stars (BSSs). Methods. We identified 20 members with large UV flux (out of 91 cluster members among 1180 detections) – which is suggestive of binarity, interactions, or stellar activity – using a multi-wavelength spectral energy distribution analysis. Results. We characterised 62 isolated cluster members, including five hot subdwarfs (sdA/sdB). Additionally, we detected ten sdA/sdB/extremely low-mass (ELM) white dwarf-type candidates hidden alongside other cluster members. Additionally, we report the discovery of four candidate blue lurkers, which are main sequence stars with mass accretion history. Conclusions. We report that this cluster has a variety of stellar (pre-)remnants, such as sdBs, sdAs, and ELM white dwarfs, which are by-products of binary evolution. These are likely to be post-mass-transfer binaries found throughout the evolutionary phases from the main sequence to the post-horizontal branch. Therefore, this dynamically old open cluster is unique, making it an ideal test bed for dynamical studies.

MultiCAM: a multivariable framework for connecting the mass accretion history of haloes with their properties

ABSTRACT Models that connect galaxy and halo properties often summarize a halo’s mass accretion history (MAH) with a single value and use this value as the basis for predictions. However, a single-value summary fails to capture the complexity of MAHs and information can be lost in the process. We present MultiCAM, a generalization of traditional abundance matching frameworks, which can simultaneously connect the full MAH of a halo with multiple halo and/or galaxy properties. As a first case study, we apply MultiCAM to the problem of connecting dark matter halo properties to their MAHs in the context of a dark matter-only simulation. While some halo properties, such as concentration, are more strongly correlated to the early-time mass growth of a halo, others, like the virial ratio, have stronger correlations with late-time mass growth. This highlights the necessity of considering the impact of the entire MAH on halo properties. For most of the halo properties we consider, we find that MultiCAM models that use the full MAH achieve higher accuracy than conditional abundance matching models that use a single epoch. We also demonstrate an extension of MultiCAM that captures the covariance between predicted halo properties. This extension provides a baseline model for applications where the covariance between predicted properties is important.

Predicting Stellar Mass Accretion: An Optimized Echo State Network Approach in Time Series Modeling

Modeling the dynamics of the formation and evolution of protostellar disks as well as the history of stellar mass accretion typically involve the numerical solution of complex systems of coupled differential equations. The resulting mass accretion history of protostars is known to be highly episodic due to recurrent instabilities and also exhibits short timescale flickering. By leveraging the strong predictive abilities of neural networks, we extract some of the critical temporal dynamics experienced during the mass accretion including periods of instability. Particularly, we utilize a novel form of the Echo-State Neural Network (ESN), which has been shown to efficiently deal with data having inherent nonlinearity. We introduce the use of Optimized-ESN (Opt-ESN) to make model-independent time series forecasting of mass accretion rate in the evolution of protostellar disks. We apply the network to multiple hydrodynamic simulations with different initial conditions and exhibiting a variety of temporal dynamics to demonstrate the predictability of the Opt-ESN model. The model is trained on simulation data of $\sim 1-2$ Myr, and achieves predictions with a low normalized mean square error ($\sim 10^{-5}$ to $10^{-3}$) for forecasts ranging between 100 and 3800 yr. This result shows the promise of the application of machine learning based models to time-domain astronomy.

The cosmic UV background and the beginning and end of star formation in simulated field dwarf galaxies

ABSTRACT We use the APOSTLE cosmological simulations to examine the role of the cosmic UV background in regulating star formation (SF) in low-mass Lambda cold dark matter (ΛCDM) haloes. In agreement with earlier work, we find that after reionization SF proceeds mainly in haloes whose mass exceeds a redshift-dependent ‘critical’ mass, Mcrit, set by the structure of the haloes and by the thermal pressure of UV-heated gas. Mcrit increases from $\sim 10^{8}\, \mathrm{M}_\odot$ at z ∼ 10 to $M_{\rm crit}\sim 10^{9.7}\, \mathrm{M}_\odot$ at z = 0, roughly following the average mass growth of haloes in that mass range. This implies that haloes well above or below critical at present have remained so since early times. Haloes of luminous dwarfs today were already above-critical and star forming at high redshift, explaining naturally the ubiquitous presence of ancient stellar populations in dwarfs, regardless of luminosity. The SF history of systems close to the critical boundary is more complex. SF may cease or reignite in dwarfs whose host halo falls below or climbs above the critical boundary, suggesting an attractive explanation for the episodic nature of SF in some dwarfs. Also, some subcritical haloes today may have been above critical in the past; these systems should at present make up a sizable population of faint field dwarfs lacking ongoing star formation. Although few such galaxies are currently known, the discovery of this population would provide strong support for our results. Our work indicates that, rather than stellar feedback, it is the ionizing UV background and mass accretion history what regulates SF in the faintest dwarfs.

Modelling the galaxy–halo connection with semi-recurrent neural networks

ABSTRACT We present an artificial neural network design in which past and present-day properties of dark matter haloes and their local environment are used to predict time-resolved star formation histories and stellar metallicity histories of central and satellite galaxies. Using data from the IllustrisTNG simulations, we train a tensorflow-based neural network with two inputs: a standard layer with static properties of the dark matter halo, such as halo mass and starting time; and a recurrent layer with variables such as overdensity and halo mass accretion rate, evaluated at multiple time steps from 0 ≤ z ≲ 20. The model successfully reproduces key features of the galaxy halo connection, such as the stellar-to-halo mass relation, downsizing, and colour bimodality, for both central and satellite galaxies. We identify mass accretion history as crucial in determining the geometry of the star formation history and trends with halo mass such as downsizing, while environmental variables are important indicators of chemical enrichment. We use these outputs to compute optical spectral energy distributions, and find that they are well matched to the equivalent results in IllustrisTNG, recovering observational statistics such as colour bimodality and mass–magnitude diagrams.

The Three Hundred: cluster dynamical states and relaxation period

ABSTRACT We introduce a new parameter λDS to quantify the dynamical state of galaxy clusters and test it using simulations from The Three Hundred cluster zoom suite. λDS is a combination of three previously used dynamical state measures, namely virial ratio, centre of mass offset, and substructure mass fraction, crafted to assume a double-Gaussian distribution, thereby yielding a natural division between relaxed and unrelaxed clusters where the Gaussians cross. Using dark matter-only simulations, we identify the optimal separator to be λDS = 3.424. We test this same criterion on two sets of fully hydrodynamical The Three Hundred runs (Gadget-X and GIZMO-SIMBA), and find only a weak dependence on the input baryonic physics. We correlate the evolution of λDS with the mass accretion history and find that halo mass changes of $\frac{\Delta M_{200}}{M_{200}} \lesssim 0.12$ do not typically alter the dynamical state. We examine the relaxation period, defined as the time taken to return to relaxation after becoming disturbed, and find a correlation between this relaxation period and the strength of halo mass change $\frac{\Delta M_{200}}{M_{200}}$. By fitting this correlation, we show that the relaxation period can be estimated from $\frac{\Delta M_{200}}{M_{200}}$ (even for multiple mass accretion events) with good accuracy.

Insights into the origin of halo mass profiles from machine learning

ABSTRACT The mass distribution of dark matter haloes is the result of the hierarchical growth of initial density perturbations through mass accretion and mergers. We use an interpretable machine-learning framework to provide physical insights into the origin of the spherically-averaged mass profile of dark matter haloes. We train a gradient-boosted-trees algorithm to predict the final mass profiles of cluster-sized haloes, and measure the importance of the different inputs provided to the algorithm. We find two primary scales in the initial conditions (ICs) that impact the final mass profile: the density at approximately the scale of the haloes’ Lagrangian patch RL ($R\sim 0.7\, R_L$) and that in the large-scale environment (R ∼ 1.7 RL). The model also identifies three primary time-scales in the halo assembly history that affect the final profile: (i) the formation time of the virialized, collapsed material inside the halo, (ii) the dynamical time, which captures the dynamically unrelaxed, infalling component of the halo over its first orbit, (iii) a third, most recent time-scale, which captures the impact on the outer profile of recent massive merger events. While the inner profile retains memory of the ICs, this information alone is insufficient to yield accurate predictions for the outer profile. As we add information about the haloes’ mass accretion history, we find a significant improvement in the predicted profiles at all radii. Our machine-learning framework provides novel insights into the role of the ICs and the mass assembly history in determining the final mass profile of cluster-sized haloes.

The Universal Specific Merger Rate of Dark Matter Halos

We employ a set of high resolution N-body simulations to study the merger rate of dark matter halos. We define a specific merger rate by normalizing the average number of mergers per halo with the logarithmic mass growth change of the hosts at the time of accretion. Based on the simulation results, we find that this specific merger rate, , has a universal form, which is only a function of the mass ratio of merging halo pairs, ξ, and does not depend on the host halo mass, M, or redshift, z, over a wide range of masses (1012 ≲ M ≲ 1014 M ⊙ h −1) and merger ratios (ξ ≥ 1e − 2). We further test with simulations of different Ω m and σ 8, and get the same specific merger rate. The universality of the specific merger rate shows that halos in the universe are built up self-similarly, with a universal composition in the mass contributions and an absolute merger rate that grows in proportion to the halo mass growth. As a result, the absolute merger rate relates with redshift and cosmology only through the halo mass variable, whose evolution can be readily obtained from the universal mass accretion history (MAH) model of Zhao et al. Last, we show that this universal specific merger rate immediately predicts an universal unevolved subhalo mass function that is independent on the redshift, MAH or the final halo mass, and vice versa.

Effects of neutrino masses and asymmetries on dark matter halo assembly

Massive cosmological neutrinos suppress the Large-Scale Structure (LSS) in the Universe by smoothing the cosmic over-densities, and hence structure formation is delayed relative to that in the standard Lambda-Cold Dark Matter (ΛCDM) model.We characterize the merger and mass accretion history of dark matter halos with the halo formation time a 1/2, tree entropy s and halo leaf function ℓ(X) and measure them using neutrino-involved N-body simulations. We show that a non-zero sum of neutrino masses Mν delays the a 1/2 for halos with virial mass between 1013 M ⊙ and 3 × 1013 M ⊙, whereas a non-zero neutrino asymmetry parameter η 2 has the opposite effect. While the mean tree entropy s̅ does not depend significantly on either Mν or η 2, the halo leaf function does. Furthermore, the dependencies of ℓ on Mν and η 2 have significant evolution in redshift z, with the relative contributions of Mν and η 2 showing a sigmoid-like transition as a function of z around z ≈ 0.6. Together with the matter power spectrum, these halo parameters allow us to break the parameter degeneracy between Mν and η 2 so that they can both be constrained in principle.

The three hundred project: galaxy cluster mergers and their impact on the stellar component of brightest cluster galaxies

ABSTRACT Using the data set of the three hundred project, i.e. a suite of 324 hydrodynamical resimulations of cluster-sized haloes, we study galaxy cluster mergers and their effect on colour and luminosity changes of their brightest cluster galaxies (BCG). We track the main progenitor of each halo at z = 0 and search for merger situations based on its mass accretion history, defining mergers as very rapid increases in the halo mass. Based upon the evolution of the dynamical state of the cluster we define a pre- and post-merger phase. We create a list of all these events and statistically study their mass ratio and time-scales, with the former verifying that all instances are in fact major mergers. By comparing to a control sample of clusters without mergers, we study the effect mergers have on the stellar component of the BCG. Analysing the mass, age, and metallicity of the BCG stellar particles, we find that the stellar content of BCGs grows significantly during mergers and, even though the main growth mechanism is the accretion of older stars, there is even a burst in star formation induced by the merger. In our simulations, BCGs in mergers form in median around 70 per cent more stars than those normally growing, although this depends on the radius considered for defining the BCG. Regarding observable properties, we see an increase in SDSS-u luminosity of 20 per cent during mergers, accompanied by a slightly slower increase of the galaxy g − r colour as compared to the control sample.

On the influence of halo mass accretion history on galaxy properties and assembly bias

ABSTRACT Halo assembly bias is the secondary dependence of the clustering of dark matter haloes on their assembly histories at fixed halo mass. This established dependence is expected to manifest itself on galaxy clustering, a potential effect commonly known as galaxy assembly bias. Using the IllustrisTNG300 magnetohydrodynamical simulation, we analyse the dependence of the properties and clustering of galaxies on the specific mass accretion history of their hosting haloes (sMAH). We first show that several halo and galaxy properties strongly correlate with the slope of the sMAH (β) at fixed halo mass. Haloes with increasingly steeper β increment their masses faster early on, and their hosted galaxies present larger stellar-to-halo mass ratios, lose their gas faster, reach the peak of their star formation histories at higher redshift, and become quenched earlier. We also demonstrate that β provides a more stable link to these key galaxy formation properties than other broadly employed halo proxies, such as formation time. Finally, we measure the secondary dependence of galaxy clustering on β at fixed halo mass. By tracing back the evolution of individual haloes, we show that the amplitude of the galaxy assembly bias signal for the progenitors of z = 0 galaxies increases with redshift, reaching a factor of 2 at z = 1 for haloes of Mhalo = 1011.5–1012 h−1 M⊙. The measurement of the evolution of assembly bias along the merger tree provides a new theoretical perspective to the study of secondary bias. Our findings have also important implications for the generation of mock catalogues for upcoming cosmological surveys.

The thermalization of massive galaxy clusters

ABSTRACT In the hierarchical scenario of structure formation, galaxy clusters are the ultimate virialized products in mass and time. Hot baryons in the intracluster medium (ICM) and cold baryons in galaxies inhabit a dark matter dominated halo. Internal processes, accretion, and mergers can perturb the equilibrium, which is established only at later times. However, the cosmic time when thermalization is effective is still to be assessed. Here, we show that massive clusters in the observed universe attained an advanced thermal equilibrium ∼1.8 Gyr ago, at redshift z = 0.14 ± 0.06, when the universe was 11.7 ± 0.7 Gyr old. Hot gas is mostly thermalized after the time when cosmic densities of matter and dark energy match. We find in a statistically nearly complete and homogeneous sample of 120 clusters from the Planck Early Sunyaev-Zel’dovich (ESZ) sample that the kinetic energy traced by the galaxy velocity dispersion is a faithful probe of the gravitational energy since a look back time of at least ∼5.4 Gyr, whereas the efficiency of hot gas in converting kinetic to thermal energy, as measured through X-ray observations in the core-excised area within r500, steadily increases with time. The evolution is detected at the ∼98 per cent probability level. Our results demonstrate that halo mass accretion history plays a larger role for cluster thermal equilibrium than radiative physics. The evolution of hot gas is strictly connected to the cosmic structure formation.

A preserved high-z compact progenitor in the heart of NGC 3311 revealed with MUSE 2D stellar population analysis

Context. Massive early-type galaxies are believed to be the end result of an extended mass accretion history. The stars formed in situ very early on in the initial phase of the mass assembly might have originated from an extremely intense and rapid burst of star formation. These stars may still be found within the cores of such galaxies at z = 0, depending on their accretion and merger histories. Aims. We wish to investigate the presence of a surviving high-z compact progenitor component in the brightest galaxy of the Hydra I cluster, NGC 3311, by mapping its 2D kinematics and stellar population out to 2 effective radii. Our goal is to understand the formation of its several structural components and trace their mass assembly back in time. Methods. We combined MUSE observations, a customized and extended version of the state-of-the-art EMILES single stellar population models, and a newly developed parametric fully Bayesian framework to model the observed spectra using full-spectrum fitting. Results. We present 2D maps and radial profiles of the stellar velocity dispersion, age, total metallicity, α-element, sodium abundance ([Na/Fe]), and the initial mass function (IMF) slope. All properties have significant gradients, confirming the existence of multiple structural components, also including a “blue spot” characterized by younger and more metal-rich stars. We find that the component dominating the light budget of NGC 3311 within R ≲ 2.0 kpc is the surviving z = 0 analog of a high-z compact core. This concentrated structure has a relatively small velocity dispersion (σ* ≈ 180 km s−1), is very old (ages ≳ 11 Gyr), metal-rich ([Z/H] ∼ 0.2 and [Na/Fe] ∼ 0.4), and has a bottom-heavy IMF (with slope Γb ∼ 2.4). In the outer region, instead, the line-of-sight velocity distribution becomes increasingly broad, and the stars are younger. They are also more metal and sodium poor but are richer in α-elements. The low-mass end of the IMF slope becomes Chabrier-like with increasing galactocentric distance. Conclusions. The existence of multiple structural components in NGC 3311 from photometry, kinematics, and stellar populations confirms the predictions from the two-phase formation scenario for NGC 3311, according to which a first very short, high-z star-formation episode formed a compact stellar structure in its core, which then grew in size by the extended mass assembly of relatively massive satellites. Interestingly, the outer stellar population has an overabundant [α/Fe], most likely because NGC 3311, located at the center of the galaxy cluster, accreted stars from rapidly quenched satellites.

Water maser variability in a high-mass YSO outburst

Context. Clarifying the relationship between mass accretion and ejection history is one of the key issues in understanding high-mass star formation processes. Aims. We aim to investigate the possible relationship between the mass accretion burst event in mid-June 2015 and the jet ejection in the high-mass protostar S255 NIRS 3. Methods. The Very Long Baseline Interferometer (VLBI) monitoring observations of the 22 GHz H2O masers were carried out using VLBI Exploration of Radio Astrometry (VERA) to reveal the 3D velocity and spatial structure of the outflow/jet traced by the H2O masers in S255 NIRS 3. In addition, we conducted follow-up observations of the submillimeter continuum and the 321 GHz H2O masers with the Atacama Large Millimeter/submillimeter Array (ALMA) at Band 7. Results. We successfully measured the proper motions of the 22 GHz H2O masers associated with a bipolar outflow. The structure is almost the same as was observed in 2005 and 2010. The expansion velocity of the blueshifted bow shock traced by the 22 GHz H2O masers was measured to be 28 km s−1, corresponding to a dynamical timescale of 60 yr. The direction of the maser outflow is slightly tilted compared with the radio jet, which could suggest a more recent ejection episode during the accretion burst event. The total flux density of the 22 GHz H2O masers gradually increases from the beginning of the VLBI monitoring in early 2017 and becomes almost constant in subsequent single-dish monitoring in 2018. The brightening of the H2O masers is more prominent in the northeast outflow lobe. For the first time, we revealed extended H2O maser emission at 22 GHz in a star-forming region, which is partly resolved out by VERA and even by the most extended Very Large Array (VLA) configurations. We find that the flux variation of such an extended component is similar to that of the unresolved maser emission. The ALMA Band 7 continuum emission did not show significant variations compared with the previous observations performed five months before. We mapped the 321 GHz H2O masers in S255 NIRS 3 providing the fourth example, for this maser, of the spatial distribution in a high-mass star-forming region. Conclusions. We conclude that the bow shock structure traced by the 22 GHz H2O maser features is unlikely to originate at the interface between the radio jet powered by the recent accretion outburst and the surrounding medium. The brightening of the 22 GHz H2O masers could be due to radiative excitation by photons form the (declining) infrared (IR) outburst escaping along the cavity created by the newly ejected material. The lower ratio of the 22 GHz/321 GHz maser luminosity in the blueshifted bow shock suggests a temperature ( >1000 K), higher than for the other maser features in this region.