Halo Properties Research Articles

Decoding Galactic Merger Histories

Galaxy mergers are expected to influence galaxy properties, yet measurements of individual merger histories are lacking. Models predict that merger histories can be measured using stellar halos and that these halos can be quantified using observations of resolved stars along their minor axis. Such observations reveal that Milky Way-mass galaxies have a wide range of stellar halo properties and show a correlation between their stellar halo masses and metallicities. This correlation agrees with merger-driven models where stellar halos are formed by satellite galaxy disruption. In these models, the largest accreted satellite dominates the stellar halo properties. Consequently, the observed diversity in the stellar halos of Milky Way-mass galaxies implies a large range in the masses of their largest merger partners. In particular, the Milky Way’s low mass halo implies an unusually quiet merger history. We used these measurements to seek predicted correlations between the bulge and central black hole (BH) mass and the mass of the largest merger partner. We found no significant correlations: while some galaxies with large bulges and BHs have large stellar halos and thus experienced a major or minor merger, half have small stellar halos and never experienced a significant merger event. These results indicate that bulge and BH growth is not solely driven by merger-related processes.

Beyond assembly bias: exploring secondary halo biases for cluster-size haloes

Secondary halo bias, commonly known as 'assembly bias,' is the dependence of halo clustering on a halo property other than mass. This prediction of the Lambda-Cold Dark Matter cosmology is essential to modelling the galaxy distribution to high precision and interpreting clustering measurements. As the name suggests, different manifestations of secondary halo bias have been thought to originate from halo assembly histories. We show conclusively that this is incorrect for cluster-size haloes. We present an up-to-date summary of secondary halo biases of high-mass haloes due to various halo properties including concentration, spin, several proxies of assembly history, and subhalo properties. While concentration, spin, and the abundance and radial distribution of subhaloes exhibit significant secondary biases, properties that directly quantify halo assembly history do not. In fact, the entire assembly histories of haloes in pairs are nearly identical to those of isolated haloes. In general, a global correlation between two halo properties does not predict whether or not these two properties exhibit similar secondary biases. For example, assembly history and concentration (or subhalo abundance) are correlated for both paired and isolated haloes, but follow slightly different conditional distributions in these two cases. This results in a secondary halo bias due to concentration (or subhalo abundance), despite the lack of assembly bias in the strict sense for cluster-size haloes. Due to this complexity, caution must be exercised in using any one halo property as a proxy to study the secondary bias due to another property.

Modeling 237 Lyman-αspectra of the MUSE-Wide survey

We compare $237$ Lyman-$\alpha$ (Ly$\alpha$) spectra of the "MUSE-Wide survey" (Herenz et al. 2017) to a suite of radiative transfer simulations consisting of a central luminous source within a concentric, moving shell of neutral gas, and dust. This six parameter shell-model has been used numerously in previous studies, however, on significantly smaller data-sets. We find that the shell-model can reproduce the observed spectral shape very well - better than the also common `Gaussian-minus-Gaussian' model which we also fitted to the dataset. Specifically, we find that $\sim 94\%$ of the fits possess a goodness-of-fit value of $p(\chi^2)>0.1$. The large number of spectra allows us to robustly characterize the shell-model parameter range, and consequently, the spectral shapes typical for realistic spectra. We find that the vast majority of the Ly$\alpha$ spectral shapes require an outflow and only $\sim 5\%$ are well-fitted through an inflowing shell. In addition, we find $\sim 46\%$ of the spectra to be consistent with a neutral hydrogen column density $<10^{17}\,\mathrm{cm}^{-2}$ - suggestive of a non-negligible fraction of continuum leakers in the MUSE-Wide sample. Furthermore, we correlate the spectral against the Ly$\alpha$ halo properties against each other but do not find any strong correlation.

The Illustris Simulation: Supermassive Black Hole − Galaxy Connection Beyond the Bulge

We study the spiral arm morphology of a sample of the local spiral galaxies in the Illustris simulation and explore the supermassive black hole - galaxy connection beyond the bulge (e.g., spiral arm pitch angle, total stellar mass, dark matter mass, and total halo mass), finding good agreement with other theoretical studies and observational constraints. It is important to study the properties of supermassive black holes and their host galaxies through both observations and simulations and compare their results in order to understand their physics and formative histories.We find that Illustris prediction for supermassive black hole mass relative to pitch angle is in rather good agreement with observations and that barred and non-barred galaxies follow similar scaling relations. Our work shows that Illustris presents very tight correlations between supermassive black hole mass and large-scale properties of the host galaxy, not only for early-type galaxies but also low-mass, blue and star-forming galaxies. These tight relations beyond the bulge suggest that halo properties determine those of a disc galaxy and its supermassive black hole.

The Properties of the Galactic Hot Gaseous Halo from X-Ray Emission

Abstract The extended hot X-ray emitting gaseous halo of the Milky Way has an optical depth ∼1 for the dominant emission lines of O vii and O viii, which are used to infer the halo properties. To improve on halo gas properties, we treat optical depth effects with a Monte Carlo radiative transfer model, which leads to slightly steeper density profiles ( β ≈ 0.5 ) than if optical depths effects were ignored. For the preferred model where the halo is rotating on cylinders at 180 km s−1, independent fits to both lines lead to identical results, where the core radius is 2.5 kpc and the turbulent component of the Doppler-b parameter is 100–120 km s−1; the turbulent pressure is 20% of the thermal pressure. The fit is improved when emission from a disk is included, with a radial scale length of 3 kpc (assumed) and a fitted vertical scale height of approximately 1.3 kpc. The disk component is a minor mass constituent and has low optical depth, except at low latitudes. The gaseous mass is 3 – 4 × 10 10 M ⊙ within 250 kpc , similar to our previous determinations and significantly lower than the mass of the missing baryons, which is 1.7 × 10 11 M ⊙ .

The descendants of the first quasars in the BlueTides simulation

Supermassive blackholes with masses of a billion solar masses or more are known to exist up to $z=7$. However, the present-day environments of the descendants of first quasars is not well understood and it is not known if they live in massive galaxy clusters or more isolated galaxies at $z=0$. We use a dark matter-only realization (BTMassTracer) of the BlueTides cosmological hydrodynamic simulation to study the halo properties of the descendants of the most massive black holes at $z=8$. We find that the descendants of the quasars with most massive black holes are not amongst the most massive halos. They reside in halos of with group-like ($\sim 10^{14}M_{\odot}$) masses, while the most massive halos in the simulations are rich clusters with masses $\sim 10^{15} M_{\odot}$. The distribution of halo masses at low redshift is similar to that of the descendants of least massive black holes, for a similar range of halo masses at $z=8$, which indicates that they are likely to exist in similar environments. By tracing back to the $z = 8$ progenitors of the most massive (cluster sized) halos at $z=0$; we find that their most likely black hole mass is less than $10^7 M_{\odot}$; they are clearly not amongst the most massive black holes. We also provide estimates for the likelihood of finding a high redshift quasar hosting a black hole with masses above $10^{7} M_{\odot}$ for a given halo mass at $z=0$. For halos above $10^{15} M_{\odot}$, there is only $20 \%$ probability that their $z=8$ progenitors hosted a black hole with mass above $10^{7} M_{\odot}$.

Revealing the Cosmic Web-dependent Halo Bias

Abstract Halo bias is the one of the key ingredients of the halo models. It was shown at a given redshift to be only dependent, to the first order, on the halo mass. In this study, four types of cosmic web environments—clusters, filaments, sheets, and voids—are defined within a state-of-the-art high-resolution N-body simulation. Within these environments, we use both halo-dark matter cross correlation and halo-halo autocorrelation functions to probe the clustering properties of halos. The nature of the halo bias differs strongly between the four different cosmic web environments described here. With respect to the overall population, halos in clusters have significantly lower biases in the mass range. In other environments, however, halos show extremely enhanced biases up to a factor 10 in voids for halos of mass . Such a strong cosmic web environment dependence in the halo bias may play an important role in future cosmological and galaxy formation studies. Within this cosmic web framework, the age dependency of halo bias is found to be only significant in clusters and filaments for relatively small halos .

How large are the globular cluster systems of early-type galaxies and do they scale with galaxy halo properties?

Abstract The globular cluster (GC) systems of galaxies are well known to extend to large galactocentric radii. Here, we quantify the size of GC systems using the half number radius of 22 GC systems around early-type galaxies (ETGs) from the literature. We compare GC system sizes to the sizes and masses of their host galaxies. We find that GC systems typically extend to 4 times that of the host galaxy size; however, this factor varies with galaxy stellar mass from about 3 times for M* galaxies to 5 times for the most massive galaxies in the universe. The size of a GC system scales approximately linearly with the virial radius (R200) and with the halo mass (M200) to the 1/3 power. The GC system of the Milky Way follows the same relations as for ETGs. For ultra diffuse galaxies (UDGs), their GC system size scales with halo mass and virial radius as for more massive, larger galaxies. UDGs indicate that the linear scaling of GC system size with stellar mass for massive galaxies flattens out for low stellar mass galaxies. Our scalings are different to those reported recently by Hudson &amp; Robison.

Testing baryon-induced core formation in ΛCDM: A comparison of the DC14 and coreNFW dark matter halo models on galaxy rotation curves

Recent cosmological hydrodynamical simulations suggest that baryonic processes, and in particular supernova feedback after bursts of star formation, can alter the structure of dark matter haloes and transform primordial cusps into shallower cores. To assess whether this mechanism offers a solution to the cusp-core controversy, simulated haloes must be compared to real dark matter haloes inferred from galaxy rotation curves. For this purpose, two new dark matter density profiles were recently derived from simulations of galaxies in complementary mass ranges: the DC14 halo ($10^{10} < M_{\text{halo}}/M_{\odot} < 8 \times 10^{11}$) and the coreNFW halo ($10^{7} < M_{\text{halo}}/M_{\odot} < 10^{9}$). Both models have individually been found to give good fits to observed rotation curves. For the DC14 model, however, the agreement of the predicted halo properties with cosmological scaling relations was confirmed by one study, but strongly refuted by another. A next question is whether the two models converge to the same solution in the mass range where both should be appropriate. To investigate this, we tested the DC14 and cNFW halo models on the rotation curves of a selection of galaxies with halo masses in the range $4 \times 10^{9}$ - $7 \times 10^{10}$ $M_{\odot}$. We further applied the DC14 model to a set of rotation curves at higher halo masses, up to $9 \times 10^{11}$ $M_{\odot}$, to verify the agreement with the cosmological scaling relations. We find that both models are generally able to reproduce the observed rotation curves, in line with earlier results, and the predicted dark matter haloes are consistent with the cosmological $c-M_{\text{halo}}$ and $M_{*}-M_{\text{halo}}$ relations. The DC14 and cNFW models are also in fairly good agreement with each other, even though DC14 tends to predict slightly less extended cores and somewhat more concentrated haloes than cNFW.

The immitigable nature of assembly bias: the impact of halo definition on assembly bias

Abstract Dark matter halo clustering depends not only on halo mass, but also on other properties such as concentration and shape. This phenomenon is known broadly as assembly bias. We explore the dependence of assembly bias on halo definition, parametrized by spherical overdensity parameter, Δ. We summarize the strength of concentration-, shape-, and spin-dependent halo clustering as a function of halo mass and halo definition. Concentration-dependent clustering depends strongly on mass at all Δ. For conventional halo definitions (Δ ∼ 200 − 600 m), concentration-dependent clustering at low mass is driven by a population of haloes that is altered through interactions with neighbouring haloes. Concentration-dependent clustering can be greatly reduced through a mass-dependent halo definition with Δ ∼ 20 − 40 m for haloes with M200 m ≲ 1012 h−1M⊙. Smaller Δ implies larger radii and mitigates assembly bias at low mass by subsuming altered, so-called backsplash haloes into now larger host haloes. At higher masses (M200 m ≳ 1013 h−1M⊙) larger overdensities, Δ ≳ 600 m, are necessary. Shape- and spin-dependent clustering are significant for all halo definitions that we explore and exhibit a relatively weaker mass dependence. Generally, both the strength and the sense of assembly bias depend on halo definition, varying significantly even among common definitions. We identify no halo definition that mitigates all manifestations of assembly bias. A halo definition that mitigates assembly bias based on one halo property (e.g. concentration) must be mass dependent. The halo definitions that best mitigate concentration-dependent halo clustering do not coincide with the expected average splashback radii at fixed halo mass.

The comoving mass density of Mg ii from z ∼ 2 to 5.5

We present the results of a survey for intervening MgII absorbers in the redshift range z $\simeq$2-6 in the foreground of four high redshift quasar spectra, 5.79$\le z_{em}\le$6.133, obtained with the ESO VLT X-shooter. We identify 24 absorbers at $\ge 5\sigma$ significance in the equivalent width range 0.117$\le W_{2796}\le 3.655\unicode{xC5}$ with the highest redshift absorber at $z=4.89031\pm4\times10^{-5}$. For weak ($W_{2796}<0.3\unicode{xC5}$) systems, we measure an incidence rate $dN/dz$=1.35$\pm$0.58 at <z>=2.34 and find that it almost doubles to $dN/dz$=2.58$\pm$0.67 by <z>=4.81. Weak absorbers exceeds the number expected from an exponential fit to stronger systems ($W_{2796}>0.3\unicode{xC5}$). We find that there must be significant evolution in the absorption halo properties of MgII absorbers with $W_{2796} >0.1\unicode{xC5}$ by <z>=4.77 and/or that they are associated with galaxies with luminosities beyond the limits of the current luminosity function at z $\sim$5. We find that the incidence rate of strong MgII absorbers ($W_{2796}>1.0\unicode{xC5}$) can be explained if they are associated with galaxies with $L\ge0.29L_{\ast}$ and/or their covering fraction increases. If they continue to only be associated with galaxies with $L\ge0.50L_{\ast}$ then their physical cross section ($\sigma_{phys}$) increases from 0.015 Mpc$^2$ at z=2.3 to 0.041 Mpc$^2$ at <z>=4.77. We measure $\Omega_{MgII}$=2.1$^{+6.3}_{-0.6}\times10^{-8}$, 1.9$^{+2.9}_{-0.2}\times10^{-8}$, 3.9$^{+7.1}_{-2.4}\times10^{-7}$ at <z>=2.48, 3.41, 4.77, respectively. At <z>=4.77, $\Omega_{MgII}$ exceeds the value expected from $\Omega_{HI}$ estimated from the global metallicity of DLAs at z $\simeq$4.85 by a factor of $\sim$44 suggesting that either MgII absorbers trace both ionised and neutral gas and/or are more metal rich than the average DLA at this redshift.

Galaxy formation with BECDM - I. Turbulence and relaxation of idealized haloes.

We present a theoretical analysis of some unexplored aspects of relaxed Bose-Einstein condensate dark matter (BECDM) haloes. This type of ultralight bosonic scalar field dark matter is a viable alternative to the standard cold dark matter (CDM) paradigm, as it makes the same large-scale predictions as CDM and potentially overcomes CDM's small-scale problems via a galaxy-scale de Broglie wavelength. We simulate BECDM halo formation through mergers, evolved under the Schrödinger-Poisson equations. The formed haloes consist of a soliton core supported against gravitational collapse by the quantum pressure tensor and an asymptotic r-3 NFW-like profile. We find a fundamental relation of the core-to-halo mass with the dimensionless invariant Ξ ≡ |E|/M3/(Gm/ħ)2 or Mc/M ≃ 2.6Ξ1/3, linking the soliton to global halo properties. For r ≥ 3.5 rc core radii, we find equipartition between potential, classical kinetic and quantum gradient energies. The haloes also exhibit a conspicuous turbulent behaviour driven by the continuous reconnection of vortex lines due to wave interference. We analyse the turbulence 1D velocity power spectrum and find a k-1.1 power law. This suggests that the vorticity in BECDM haloes is homogeneous, similar to thermally-driven counterflow BEC systems from condensed matter physics, in contrast to a k-5/3 Kolmogorov power law seen in mechanically-driven quantum systems. The mode where the power spectrum peaks is approximately the soliton width, implying that the soliton-sized granules carry most of the turbulent energy in BECDM haloes.

Universal Dark Halo Scaling Relation for the Dwarf Spheroidal Satellites

Abstract Motivated by a recently found interesting property of the dark halo surface density within a radius, r max , giving the maximum circular velocity, V max , we investigate it for dark halos of the Milky Way’s and Andromeda’s dwarf satellites based on cosmological simulations. We select and analyze the simulated subhalos associated with Milky-Way-sized dark halos and find that the values of their surface densities, Σ V max , are in good agreement with those for the observed dwarf spheroidal satellites even without employing any fitting procedures. Moreover, all subhalos on the small scales of dwarf satellites are expected to obey the universal relation, irrespective of differences in their orbital evolutions, host halo properties, and observed redshifts. Therefore, we find that the universal scaling relation for dark halos on dwarf galaxy mass scales surely exists and provides us with important clues for understanding fundamental properties of dark halos. We also investigate orbital and dynamical evolutions of subhalos to understand the origin of this universal dark halo relation and find that most subhalos evolve generally along the r max ∝ V max sequence, even though these subhalos have undergone different histories of mass assembly and tidal stripping. This sequence, therefore, should be the key feature for understanding the nature of the universality of Σ V max .

The FMOS-COSMOS Survey of Star-forming Galaxies at Z ∼ 1.6. V: Properties of Dark Matter Halos Containing Hα Emitting Galaxies

Abstract We study the properties of dark matter halos that contain star-forming galaxies at 1.43 ≤ z ≤ 1.74, using the FMOS-COSMOS survey. The sample consists of 516 objects with a detection of the Hα emission line, which represent the star forming population at this epoch, having a stellar mass range of 109.57 ≤ M */M ⊙ ≲ 1011.4 and a star-formation rate range of 15 ≲ SFR/(M ⊙ yr−1) ≲ 600. We measure the projected two-point correlation function while carefully taking into account observational biases, and find a significant clustering amplitude at scales of 0.04–10 h −1 cMpc, with a correlation length and a bias . We interpret our clustering measurement using a halo occupation distribution model. The sample galaxies appear to reside in halos with mass on average, which will likely become present-day halos of mass M h (z = 0) ∼ 2 × 1013 h −1 M ⊙, equivalent to the typical halo mass scale of galaxy groups. We then confirm the decline of the stellar-to-halo mass ratio at M h &lt; 1012 M ⊙, finding M */M h ≈ 5 × 10−3 at M h = 7. 5 × 1011 M ⊙, which is lower by a factor of 2–4 than those measured at higher masses (M h ∼ 1012–13 M ⊙). Finally, we use our results to illustrate the future capabilities of Subaru’s Prime-Focus Spectrograph, a next-generation instrument that will provide strong constraints on the galaxy-formation scenario by obtaining precise measurements of galaxy clustering at z &gt; 1.

Do You See What I See? Exploring the Consequences of Luminosity Limits in Black Hole–Galaxy Evolution Studies

Abstract In studies of the connection between active galactic nuclei (AGNs) and their host galaxies, there is widespread disagreement on some key aspects of the connection. These disagreements largely stem from a lack of understanding of the nature of the full underlying AGN population. Recent attempts to probe this connection utilize both observations and simulations to correct for a missed population, but presently are limited by intrinsic biases and complicated models. We take a simple simulation for galaxy evolution and add a new prescription for AGN activity to connect galaxy growth to dark matter halo properties and AGN activity to star formation. We explicitly model selection effects to produce an “observed” AGN population for comparison with observations and empirically motivated models of the local universe. This allows us to bypass the difficulties inherent in models that attempt to infer the AGN population by inverting selection effects. We investigate the impact of selecting AGNs based on thresholds in luminosity or Eddington ratio on the “observed” AGN population. By limiting our model AGN sample in luminosity, we are able to recreate the observed local AGN luminosity function and specific star formation-stellar mass distribution, and show that using an Eddington ratio threshold introduces less bias into the sample by selecting the full range of growing black holes, despite the challenge of selecting low-mass black holes. We find that selecting AGNs using these various thresholds yield samples with different AGN host galaxy properties.

High Angular Momentum Halo Gas: A Feedback and Code-independent Prediction of LCDM

Abstract We investigate angular momentum acquisition in Milky Way-sized galaxies by comparing five high resolution zoom-in simulations, each implementing identical cosmological initial conditions but utilizing different hydrodynamic codes: Enzo, Art, Ramses, Arepo, and Gizmo-PSPH. Each code implements a distinct set of feedback and star formation prescriptions. We find that while many galaxy and halo properties vary between the different codes (and feedback prescriptions), there is qualitative agreement on the process of angular momentum acquisition in the galaxy’s halo. In all simulations, cold filamentary gas accretion to the halo results in ∼4 times more specific angular momentum in cold halo gas (λ cold ≳ 0.1) than in the dark matter halo. At z &gt; 1, this inflow takes the form of inspiraling cold streams that are co-directional in the halo of the galaxy and are fueled, aligned, and kinematically connected to filamentary gas infall along the cosmic web. Due to the qualitative agreement among disparate simulations, we conclude that the buildup of high angular momentum halo gas and the presence of these inspiraling cold streams are robust predictions of Lambda Cold Dark Matter galaxy formation, though the detailed morphology of these streams is significantly less certain. A growing body of observational evidence suggests that this process is borne out in the real universe.

On the galaxy–halo connection in the EAGLE simulation

Abstract Empirical models of galaxy formation require assumptions about the correlations between galaxy and halo properties. These may be calibrated against observations or inferred from physical models such as hydrodynamical simulations. In this Letter, we use the EAGLE simulation to investigate the correlation of galaxy size with halo properties. We motivate this analysis by noting that the common assumption of angular momentum partition between baryons and dark matter in rotationally supported galaxies overpredicts both the spread in the stellar mass–size relation and the anticorrelation of size and velocity residuals, indicating a problem with the galaxy–halo connection it implies. We find the EAGLE galaxy population to perform significantly better on both statistics, and trace this success to the weakness of the correlations of galaxy size with halo mass, concentration and spin at fixed stellar mass. Using these correlations in empirical models will enable fine-grained aspects of galaxy scalings to be matched.

The structural evolution of galaxies with both thin and thick discs

We perform controlled N-body simulations of disc galaxies growing within live dark matter (DM) haloes to present-day galaxies that contain both thin and thick discs. We consider two types of models: a) thick-disc initial conditions to which stars on near-circular orbits are continuously added over ~10 Gyr and b) models in which the birth velocity dispersion of stars decreases continuously over the same time-scale. We show that both schemes produce double-exponential vertical profiles similar to that of the Milky Way (MW). We indicate how the spatial age structure of galaxies can be used to discriminate between scenarios. We show that the presence of a thick disc significantly alters and delays bar formation and thus makes possible models with a realistic bar and a high baryon-to-DM mass ratio in the central regions, as required by microlensing constraints. We examine how the radial mass distribution in stars and DM is affected by disc growth and non-axisymmetries. We discuss how bar buckling shapes the vertical age distribution of thin- and thick-disc stars in the bar region. The extent to which the combination of observationally motivated inside-out growth histories and cosmologically motivated dark halo properties leads to the spontaneous formation of non-axisymmetries that steer the models towards present-day MW-like galaxies is noteworthy.

Dark matter haloes in modified gravity and dark energy: interaction rate, small- and large-scale alignment

We study the properties of dark matter haloes in a wide range of modified gravity models, namely, $f(R)$, DGP, and interacting dark energy models. We study the effects of modified gravity and dark energy on the internal properties of haloes, such as the spin and the structural parameters. We find that $f(R)$ gravity enhance the median value of the Bullock spin parameter, but could not detect such effects for DGP and coupled dark energy. $f(R)$ also yields a lower median sphericity and oblateness, while coupled dark energy has the opposite effect. However, these effects are very small. We then study the interaction rate of haloes in different gravity, and find that only strongly coupled dark energy models enhance the interaction rate. We then quantify the enhancement of the alignment of the spins of interacting halo pairs by modified gravity. Finally, we study the alignment of the major axes of haloes with the large-scale structures. The alignment of the spins of interacting pairs of haloes in DGP and coupled dark energy models show no discrepancy with GR, while $f(R)$ shows a weaker alignment. Strongly coupled dark energy shows a stronger alignment of the halo shape with the large-scale structures.

Particle tagging and its implications for stellar population dynamics

We establish a controlled comparison between the properties of galactic stellar halos obtained with hydrodynamical simulations and with `particle tagging'. Tagging is a fast way to obtain stellar population dynamics: instead of tracking gas and star formation, it `paints' stars directly onto a suitably defined subset of dark matter particles in a collisionless, dark-matter-only simulation.Our study shows that there are conditions under which particle tagging generates good fits to the hydrodynamical stellar density profiles of a central Milky-Way-like galaxy and its most prominent substructure. Phase-space diffusion processes are crucial to reshaping the distribution of stars in infalling spheroidal systems and hence the final stellar halo. We conclude that the success of any particular tagging scheme hinges on this diffusion being taken into account, at a minimum by making use of `live' tagging schemes, in which particles are regularly tagged throughout the evolution of a galaxy.