arXiv subjects

Vicente Rodriguez-Gomez

Publications and source records attributed to Vicente Rodriguez-Gomez.

At least 73 records · Page 4Linked to original sources

arXiv2019

Automated Distant Galaxy Merger Classifications from Space Telescope Images using the Illustris Simulation

We present image-based evolution of galaxy mergers from the Illustris cosmological simulation at 12 time-steps over 0.5 < z < 5. To do so, we created approximately one million synthetic deep Hubble Space Telescope and James Webb Space Telescope images and measured common morphological indicators. Using the merger tree, we assess methods to observationally select mergers with stellar mass ratios as low as 10:1 completing within +/- 250 Myr of the mock observation. We confirm that common one- or two-dimensional statistics select mergers so defined with low purity and completeness, leading to high statistical errors. As an alternative, we train redshift-dependent random forests (RFs) based on 5-10 inputs. Cross-validation shows the RFs yield superior, yet still imperfect, measurements of the late-stage merger fraction, and they select more mergers in bulge-dominated galaxies. When applied to CANDELS morphology catalogs, the RFs estimate a merger rate increasing to at least z = 3, albeit two times higher than expected by theory. This suggests possible mismatches in the feedback-determined morphologies, but affirms the basic understanding of galaxy merger evolution. The RFs achieve completeness of roughly 70% at 0.5 < z < 3, and purity increasing from 10% at z = 0.5 to 60% at z = 3. At earlier times, the training sets are insufficient, motivating larger simulations and smaller time sampling. By blending large surveys and large simulations, such machine learning techniques offer a promising opportunity to teach us the strengths and weaknesses of inferences about galaxy evolution.

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arXiv2019

Studying the Physical Properties of Tidal Features I. Extracting Morphological Substructure in CANDELS Observations and VELA Simulations

The role of major mergers in galaxy evolution remains a key open question. Existing empirical merger identification methods use non-parametric and subjective visual classifications which can pose systematic challenges to constraining merger histories. As a first step towards overcoming these challenges, we develop and share publicly a new Python-based software tool that identifies and extracts the flux-wise and area-wise significant contiguous regions from the model-subtracted "residual" images produced by popular parametric light-profile fitting tools (e.g., GALFIT). Using Hubble Space Telescope ($HST$) $H$-band single-Sérsic residual images of $17$ CANDELS galaxies, we demonstrate the tool's ability to measure the surface brightness and improve the qualitative identification of a variety of common residual features (disk structures, spiral substructures, plausible tidal features, and strong gravitational arcs). We test our method on synthetic $HST$ observations of a $z\sim 1.5$ major merger from the VELA hydrodynamic simulations. We extract $H$-band residual features corresponding to the birth, growth, and fading of tidal features during different stages and viewing orientations at CANDELS depths and resolution. We find that the extracted features at shallow depths have noisy visual appearance and are susceptible to viewing angle effects. For a VELA $z\sim 3$ major merger, we find that James Webb Space Telescope NIRCam observations can probe high-redshift tidal features with considerable advantage over existing $HST$ capabilities. Further quantitative analysis of plausible tidal features extracted with our new software hold promise for the robust identification of hallmark merger signatures and corresponding improvements to merger rate constraints.

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arXiv2019

Subhalo Demographics in the Illustris Simulation: Effects of Baryons and Halo-to-Halo Variation

We study the abundance of subhaloes in the hydrodynamical cosmological simulation Illustris, which includes both baryons and dark matter in a $Λ$CDM volume 106.5 Mpc a side. We compare Illustris to its dark matter-only (DMO) analogue, Illustris-Dark, and quantify the effects of baryonic processes on the demographics of subhaloes in the host mass range $10^{11}$ to $3 \times 10^{14} \msun$. We focus on both the evolved ($z=0$) subhalo cumulative mass functions (SHMF) and the statistics of subhaloes ever accreted, i.e. infall subhalo mass function. We quantify the variance in subhalo abundance at fixed host mass and investigate the physical reasons responsible for such scatter. We find that in Illustris, baryonic physics impacts both the infall and $z=0$ subhalo abundance by tilting the DMO function and suppressing the abundance of low-mass subhaloes. The breaking of self-similarity in the subhalo abundance at $z=0$ is enhanced by the inclusion of baryonic physics. The non-monotonic alteration of the evolved subhalo abundances can be explained by the modification of the concentration--mass relation of Illustris hosts compared to Illustris-Dark.Interestingly, the baryonic implementation in Illustris does not lead to an increase in the halo-to-halo variation compared to Illustris-Dark. In both cases, the normalized intrinsic scatter today is larger for Milky Way-like haloes than for cluster-sized objects. For Milky Way-like haloes, it increases from about eight per cent at infall to about 25 per cent at the current epoch. In both runs, haloes of fixed mass formed later host more subhaloes than early formers.

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arXiv2019

The optical morphologies of galaxies in the IllustrisTNG simulation: a comparison to Pan-STARRS observations

We have generated synthetic images of $\sim$27,000 galaxies from the IllustrisTNG and the original Illustris hydrodynamic cosmological simulations, designed to match Pan-STARRS observations of $\log_{10}(M_{\ast}/{\rm M}_{\odot}) \approx 9.8$-$11.3$ galaxies at $z \approx 0.05$. Most of our synthetic images were created with the SKIRT radiative transfer code, including the effects of dust attenuation and scattering, and performing the radiative transfer directly on the Voronoi mesh used by the simulations themselves. We have analysed both our synthetic and real Pan-STARRS images with the newly developed $\tt{statmorph}$ code, which calculates non-parametric morphological diagnostics -- including the Gini-$M_{20}$ and concentration-asymmetry-smoothness (CAS) statistics -- and performs two-dimensional Sérsic fits. Overall, we find that the optical morphologies of IllustrisTNG galaxies are in good agreement with observations, and represent a substantial improvement compared to the original Illustris simulation. In particular, the locus of the Gini-$M_{20}$ diagram is consistent with that inferred from observations, while the median trends with stellar mass of all the morphological, size and shape parameters considered in this work lie within the $\sim$1$σ$ scatter of the observational trends. However, the IllustrisTNG model has some difficulty with more stringent tests, such as producing a strong morphology-colour relation. This results in a somewhat higher fraction of red discs and blue spheroids compared to observations. Similarly, the morphology-size relation is problematic: while observations show that discs tend to be larger than spheroids at a fixed stellar mass, such a trend is not present in IllustrisTNG.

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arXiv2018

Anomalously low metallicity regions in MaNGA star-forming galaxies: Accretion Caught in Action?

We use data from 1222 late-type star-forming galaxies in the SDSS IV MaNGA survey to identify regions in which the gas-phase metallicity is anomalously-low compared to expectations from the tight empirical relation between metallicity and stellar surface mass-density at a given stellar mass. We find anomalously low metallicity (ALM) gas in 10% of the star-forming spaxels, and in 25% of the galaxies in the sample. The incidence rate of ALM gas increases strongly with both global and local measures of the specific star-formation rate, and is higher in lower mass galaxies and in the outer regions of galaxies. The incidence rate is also significantly higher in morphologically disturbed galaxies. We estimate that the lifetimes of the ALM regions are a few hundred Myr. We argue that the ALM gas has been delivered to its present location by a combination of interactions, mergers, and accretion from the halo, and that this infusion of gas stimulates star-formation. Given the estimated lifetime and duty cycle of such events, we estimate that the time-averaged accretion rate of ALM gas is similar to the star-formation rate in late type galaxies over the mass-range M$_* \sim10^9$ to 10$^{10}$ M$_{\odot}$.

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arXiv2018

Jellyfish galaxies with the IllustrisTNG simulations: I. Gas-stripping phenomena in the full cosmological context

We use IllustrisTNG, a suite of gravity and MHD simulations, to study the demographics and properties of jellyfish galaxies in the full cosmological context. By jellyfish galaxies, we mean satellites orbiting in massive groups and clusters that exhibit highly asymmetric distributions of gas and gas tails. We use the TNG100 run and select galaxies at redshifts $z\le0.6$ with stellar mass exceeding $10^{9.5}{\rm M_\odot}$ and with host halo masses of $10^{13}-10^{14.6}\,{\rm M_\odot}$. Among more than about 6000 (2600) galaxies with stars (and some gas), we identify 800 jellyfish galaxies by visually inspecting their gas and stellar mass maps in random projections. About $31\%$ of cluster satellites are found with signatures of ram-pressure stripping and gaseous tails stemming from the main luminous bodies. This is a lower limit, since the random orientation entails a loss of about $30\%$ of galaxies that in an optimal projection would otherwise be identified as jellyfish. The connection with ram-pressure stripping is further confirmed by a series of findings: jellyfish galaxies are more frequent at intermediate and large cluster-centric distances ($r/R_{\rm 200c}\gtrsim 0.25$); they move through the ICM with larger bulk velocities and Mach numbers than the general cluster population, typically orbiting supersonically and experiencing larger ram pressures. Furthermore, the gaseous tails usually extend in opposite directions to the galaxy trajectory, with no relation between tail orientation and the host's center. The frequency of jellyfish galaxies shows a very weak dependence on redshift $(0\le z\le0.6)$ but larger fractions of disturbed gaseous morphologies occur in more massive hosts and at smaller satellite masses. Finally, jellyfish galaxies are late infallers ($< 2.5-3$ Gyrs ago, at $z=0$) and the emergence of gaseous tails correlates well with the presence of bow shocks in the ICM.

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arXiv2018

The Frequency of Dwarf Galaxy Multiples at Low Redshift in SDSS vs. Cosmological Expectations

We quantify the frequency of companions of low redshift ($0.013 < z < 0.0252$), dwarf galaxies ($2 \times 10^8$ M$_\odot <$ M$_{*} < 5 \times 10^9$ M$_\odot$) that are isolated from more massive galaxies in SDSS and compare against cosmological expectations using mock observations of the Illustris simulation. Dwarf multiples are defined as 2 or more dwarfs that have angular separations > 55'', projected separations r$_p < 150$ kpc and relative line-of-sight velocities $ΔV_{\rm LOS} < 150$ km/s. While the mock catalogs predict a factor of 2 more isolated dwarfs than observed in SDSS, the mean number of observed companions per dwarf is $N_c \sim 0.04$, in good agreement with Illustris when accounting for SDSS sensitivity limits. Removing these limits in the mock catalogs predicts $N_c\sim 0.06$ for future surveys (LSST, DESI), which will be complete to M$_* = 2\times 10^8$ M$_\odot$. The 3D separations of mock dwarf multiples reveal a contamination fraction of $\sim$40% in observations from projection effects. Most isolated multiples are pairs; triples are rare and it is cosmologically improbable that bound groups of dwarfs with more than 3 members exist within the parameter range probed in this study. We find that $<$1% of LMC-analogs in the field have an SMC-analog companion. The fraction of dwarf "Major Pairs'' (stellar mass ratio $>$1:4) steadily increases with decreasing Primary stellar mass, whereas the cosmological "Major Merger rate'' (per Gyr) has the opposite behaviour. We conclude that cosmological simulations can be reliably used to constrain the fraction of dwarf mergers across cosmic time.

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arXiv2018

Formation of a Malin 1 analogue in IllustrisTNG by stimulated accretion

The galaxy Malin 1 contains the largest stellar disk known but the formation mechanism of this structure has been elusive. In this paper, we report a Malin 1 analogue in the 100 Mpc IllustrisTNG simulation and describe its formation history. At redshift zero, this massive galaxy, having a maximum circular velocity $V_{\rm max}$ of 430 ${\rm km\ s^{-1}}$, contains a 100 kpc gas/stellar disk with morphology similar to Malin 1. The simulated galaxy reproduces well many observed features of Malin 1's vast disk, including its stellar ages, metallicities, and gas rotation curve. We trace the extended disk back in time and find that a large fraction of the cold gas at redshift zero originated from the cooling of hot halo gas, triggered by the merger of a pair of intruding galaxies. Our finding provides a novel way to form large galaxy disks as extreme as Malin 1 within the current galaxy formation framework.

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arXiv2017

Massive Close Pairs Measure Rapid Galaxy Assembly in Mergers at High Redshift

We compare mass-selected close pairs at z > 1 with the intrinsic galaxy merger rate in the Illustris Simulations. To do so, we construct three 140 arcmin^2 lightcone catalogs and measure pair fractions, finding that they change little or decrease with increasing redshift at z > 1. Consistent with current surveys, this trend requires a decrease in the merger-pair observability time, roughly as (1 + z)^-2, in order to measure the merger rates of the same galaxies. This implies that major mergers are more common at high redshift than implied by the simplest arguments assuming a constant observability time. Several effects contribute to this trend: (1) The fraction of massive, major (4:1) pairs which merge by today increases weakly from ~0.5 at z=1 to ~0.8 at z=3. (2) The median time elapsed between an observed pair and final remnant decreases by a factor of two from z~1 to z~3. (3) An increasing specific star formation rate (sSFR) decreases the time during which common stellar-mass based pair selection criteria could identify the mergers. The average orbit of the pairs at observation time varies only weakly, suggesting that the dynamical time is not varying enough to account by itself for the pair fraction trends. Merging pairs reside in dense regions, having overdensity ~10 to ~100 times greater than the average massive galaxy. We forward model the pairs to reconstruct the merger remnant production rate, showing that it is consistent with a rapid increase in galaxy merger rates at z > 1.

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arXiv2017

The role of mergers and halo spin in shaping galaxy morphology

Mergers and the spin of the dark matter halo are factors traditionally believed to determine the morphology of galaxies within a $Λ$CDM cosmology. We study this hypothesis by considering approximately 18,000 central galaxies at $z=0$ with stellar masses $M_{\ast} = 10^{9}-10^{12} \, {\rm M}_{\odot}$ selected from the Illustris cosmological hydrodynamic simulation. The fraction of accreted stars -- which measures the importance of massive, recent and dry mergers -- increases steeply with galaxy stellar mass, from less than 5 per cent in dwarfs to 80 per cent in the most massive objects, and the impact of mergers on galaxy morphology increases accordingly. For galaxies with $M_{\ast} \gtrsim 10^{11} \, {\rm M}_{\odot}$, mergers have the expected effect: if gas-poor they promote the formation of spheroidal galaxies, whereas gas-rich mergers favour the formation and survivability of massive discs. This trend, however, breaks at lower masses. For objects with $M_{\ast} \lesssim 10^{11} \, {\rm M}_{\odot}$, mergers do not seem to play any significant role in determining the morphology, with accreted stellar fractions and mean merger gas fractions that are indistinguishable between spheroidal and disc-dominated galaxies. On the other hand, halo spin correlates with morphology primarily in the least massive objects in the sample ($M_{\ast} \lesssim 10^{10} \, {\rm M}_{\odot}$), but only weakly for galaxies above that mass. Our results support a scenario where (1) mergers play a dominant role in shaping the morphology of massive galaxies, (2) halo spin is important for the morphology of dwarfs, and (3) the morphology of medium-sized galaxies -- including the Milky Way -- shows little dependence on galaxy assembly history or halo spin, at least when these two factors are considered individually.

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arXiv2017

The stellar mass assembly of galaxies in the Illustris simulation: growth by mergers and the spatial distribution of accreted stars

We use the Illustris simulation to study the relative contributions of in situ star formation and stellar accretion to the build-up of galaxies over an unprecedentedly wide range of masses ($M_{\ast} = 10^9-10^{12} \, {\rm M_{\odot}}$), galaxy types, environments, and assembly histories. We find that the `two-phase' picture of galaxy formation predicted by some models is a good approximation only for the most massive galaxies in our simulation -- namely, the stellar mass growth of galaxies below a few times $10^{11} \, {\rm M_{\odot}}$ is dominated by in situ star formation at all redshifts. The fraction of the total stellar mass of galaxies at $z=0$ contributed by accreted stars shows a strong dependence on galaxy stellar mass, ranging from about 10 per cent for Milky Way-sized galaxies to over 80 per cent for $M_{\ast} \approx 10^{12} \, {\rm M_{\odot}}$ objects, yet with a large galaxy-to-galaxy variation. At a fixed stellar mass, elliptical galaxies and those formed at the centres of younger haloes exhibit larger fractions of ex situ stars than disc-like galaxies and those formed in older haloes. On average, $\sim$50 per cent of the ex situ stellar mass comes from major mergers (stellar mass ratio $μ> 1/4$), $\sim$20 per cent from minor mergers ($1/10 < μ< 1/4$), $\sim$20 per cent from very minor mergers ($μ< 1/10$), and $\sim$10 per cent from stars that were stripped from surviving galaxies (e.g. flybys or ongoing mergers). These components are spatially segregated, with in situ stars dominating the innermost regions of galaxies, and ex situ stars being deposited at larger galactocentric distances in order of decreasing merger mass ratio.

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arXiv2016

Sussing Merger Trees: Stability and Convergence

Merger trees are routinely used to follow the growth and merging history of dark matter haloes and subhaloes in simulations of cosmic structure formation. Srisawat et al. (2013) compared a wide range of merger-tree-building codes. Here we test the influence of output strategies and mass resolution on tree-building. We find that, somewhat surprisingly, building the tree from more snapshots does not generally produce more complete trees; instead, it tends to short- en them. Significant improvements are seen for patching schemes which attempt to bridge over occasional dropouts in the underlying halo catalogues or schemes which combine the halo-finding and tree-building steps seamlessly. The adopted output strategy does not affec- t the average number of branches (bushiness) of the resultant merger trees. However, mass resolution has an influence on both main branch length and the bushiness. As the resolution increases, a halo with the same mass can be traced back further in time and will encounter more small progenitors during its evolutionary history. Given these results, we recommend that, for simulations intended as precursors for galaxy formation models where of order 100 or more snapshots are analysed, the tree-building routine should be integrated with the halo finder, or at the very least be able to patch over multiple adjacent snapshots.

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arXiv2015

The diverse evolutionary paths of simulated high-z massive, compact galaxies to z=0

Massive quiescent galaxies have much smaller physical sizes at high redshift than today. The strong evolution of galaxy size may be caused by progenitor bias, major and minor mergers, adiabatic expansion, and/or renewed star formation, but it is difficult to test these theories observationally. Herein, we select a sample of 35 massive, compact galaxies ($M_* = 1-3 \times 10^{11}$ M$_\odot$, $M_*/R^{1.5} > 10^{10.5}$ M$_\odot$/kpc$^{1.5}$) at $z=2$ in the cosmological hydrodynamical simulation Illustris and trace them forward to $z=0$ to uncover their evolution and identify their descendants. By $z=0$, the original factor of 3 difference in stellar mass spreads to a factor of 20. The dark matter halo masses similarly spread from a factor of 5 to 40. The galaxies' evolutionary paths are diverse: about half acquire an ex-situ envelope and are the core of a more massive descendant, a third survive undisturbed and gain very little mass, 15% are consumed in a merger with a more massive galaxy, and a small remainder are thoroughly mixed by major mergers. The galaxies grow in size as well as mass, and only $\sim$10% remain compact by $z=0$. The majority of the size growth is driven by the acquisition of ex-situ mass. The most massive galaxies at $z=0$ are the most likely to have compact progenitors, but this trend possesses significant dispersion which precludes a direct linkage to compact galaxies at $z=2$. The compact galaxies' merger rates are influenced by their $z=2$ environments, so that isolated or satellite compact galaxies (which are protected from mergers) are the most likely to survive to the present day.

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arXiv2015

On the Assembly of Dwarf Galaxies in Clusters and their Efficient Formation of Globular Clusters

Galaxy clusters contain a large population of low mass dwarf elliptical galaxies whose exact origin is unclear: their colors, structural properties and kinematics differ substantially from those of dwarf irregulars in the field. We use the Illustris cosmological simulation to study differences in the assembly paths of dwarf galaxies (3e8 < M_*/M_sun < 1e10) according to their environment. We find that cluster dwarfs achieve their maximum total and stellar mass on average ~ 8 and ~ 4.5 Gyr ago (or redshifts z = 1.0 and z = 0.4, respectively), around the time of infall into the clusters. In contrast, field dwarfs not subjected to environmental stripping, reach their maximum mass at redshift z = 0. This different assembly history naturally produces a color bimodality, with blue isolated dwarfs and redder cluster dwarfs exhibiting negligible star-formation today. The cessation of star formation happens over median times 3.5-5 Gyr depending on stellar mass, and shows a large scatter (~ 1-8 Gyr), with the lower values associated with starburst events that occur at infall through the virial radius or pericentric passages. We argue that such starbursts together with the early assembly of cluster dwarfs can provide a natural explanation for the higher specific frequency of globular clusters (GCs) in cluster dwarfs, as found observationally. We present a simple model for the formation and stripping of GCs that supports this interpretation. The origin of dwarf ellipticals in clusters is, therefore, consistent with an environmentally-driven evolution of field dwarf irregulars. However the z = 0 field analogs of cluster dwarf progenitors have today stellar masses a factor ~ 3 larger --a difference arising from the early truncation of star formation in cluster dwarfs.

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arXiv2015

The Illustris Simulation: Public Data Release

We present the full public release of all data from the Illustris simulation project. Illustris is a suite of large volume, cosmological hydrodynamical simulations run with the moving-mesh code Arepo and including a comprehensive set of physical models critical for following the formation and evolution of galaxies across cosmic time. Each simulates a volume of (106.5 Mpc)^3 and self-consistently evolves five different types of resolution elements from a starting redshift of z=127 to the present day, z=0. These components are: dark matter particles, gas cells, passive gas tracers, stars and stellar wind particles, and supermassive black holes. This data release includes the snapshots at all 136 available redshifts, halo and subhalo catalogs at each snapshot, and two distinct merger trees. Six primary realizations of the Illustris volume are released, including the flagship Illustris-1 run. These include three resolution levels with the fiducial "full" baryonic physics model, and a dark matter only analog for each. In addition, we provide four distinct, high time resolution, smaller volume "subboxes". The total data volume is ~265 TB, including ~800 full volume snapshots and ~30,000 subbox snapshots. We describe the released data products as well as tools we have developed for their analysis. All data may be directly downloaded in its native HDF5 format. Additionally, we release a comprehensive, web-based API which allows programmatic access to search and data processing tasks. In both cases we provide example scripts and a getting-started guide in several languages: currently, IDL, Python, and Matlab. This paper addresses scientific issues relevant for the interpretation of the simulations, serves as a pointer to published and on-line documentation of the project, describes planned future additional data releases, and discusses technical aspects of the release.

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arXiv2015

Modeling Galactic Conformity with the Color-Halo Age Relation in the Illustris Simulation

Comparisons between observational surveys and galaxy formation models find that the mass of dark matter haloes can largely explain galaxies' stellar mass. However, it remains uncertain whether additional environmental variables, generally referred to as assembly bias, are necessary to explain other galaxy properties. We use the Illustris Simulation to investigate the role of assembly bias in producing galactic conformity by considering 18,000 galaxies with $M_{stellar}$ > $2 \times 10^9$ $M_{\odot}$. We find a significant signal of galactic conformity: out to distances of about 10 Mpc, the mean red fraction of galaxies around redder galaxies is higher than around bluer galaxies at fixed stellar mass. Dark matter haloes exhibit an analogous conformity signal, in which the fraction of haloes formed at earlier times (old haloes) is higher around old haloes than around younger ones at fixed halo mass. A plausible interpretation of galactic conformity can be given as a combination of the halo conformity signal with the galaxy color-halo age relation: at fixed stellar mass, particularly toward the low-mass end, Illustris' galaxy colors correlate with halo age, with the reddest galaxies (often satellites) being preferentially found in the oldest haloes. In fact, we can explain the galactic conformity effect with a simple semi-empirical model, by assigning stellar mass based on halo mass (abundance matching) and by assigning galaxy color based on halo age (age matching). We investigate other interpretations for the galactic conformity, particularly its dependence on the isolation criterion and on the central-satellite information. Regarding comparison to observations, we conclude that the adopted selection/isolation criteria, projection effects, and stacking techniques can have a significant impact on the measured amplitude of the conformity signal.

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arXiv2015

Sussing Merger Trees: A proposed Merger Tree data format

We propose a common terminology for use in describing both temporal merger trees and spatial structure trees for dark-matter halos. We specify a unified data format in HDF5 and provide example I/O routines in C, FORTRAN and PYTHON.

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arXiv2015

An analysis of the evolving comoving number density of galaxies in hydrodynamical simulations

The cumulative comoving number-density of galaxies as a function of stellar mass or central velocity dispersion is commonly used to link galaxy populations across different epochs. By assuming that galaxies preserve their number-density in time, one can infer the evolution of their properties, such as masses, sizes, and morphologies. However, this assumption does not hold in the presence of galaxy mergers or when rank ordering is broken owing to variable stellar growth rates. We present an analysis of the evolving comoving number density of galaxy populations found in the Illustris cosmological hydrodynamical simulation focused on the redshift range $0\leq z \leq 3$. Our primary results are as follows: 1) The inferred average stellar mass evolution obtained via a constant comoving number density assumption is systematically biased compared to the merger tree results at the factor of $\sim$2(4) level when tracking galaxies from redshift $z=0$ out to redshift $z=2(3)$; 2) The median number density evolution for galaxy populations tracked forward in time is shallower than for galaxy populations tracked backward in time; 3) A similar evolution in the median number density of tracked galaxy populations is found regardless of whether number density is assigned via stellar mass, stellar velocity dispersion, or dark matter halo mass; 4) Explicit tracking reveals a large diversity in galaxies' assembly histories that cannot be captured by constant number-density analyses; 5) The significant scatter in galaxy linking methods is only marginally reduced by considering a number of additional physical and observable galaxy properties as realized in our simulation. We provide fits for the forward and backward median evolution in stellar mass and number density and discuss implications of our analysis for interpreting multi-epoch galaxy property observations.

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