A chronicle of galaxy mass assembly in the EAGLE simulation
Summary (5 min read)
Introduction
- Key words: galaxies: evolution – galaxies: formation – galaxies: high-redshift – galaxies: interactions – galaxies: stellar content.
- To evaluate the relative importance of mergers to galaxy assembly, the authors need to know their merging histories.
- The approach may miss physical correlations between the merging objects.
- The authors investigate the assembly histories and merger histories of galaxies and discuss the impact of feedback on galaxy mass buildup in Section 3.
2.1 EAGLE simulation
- The galaxy samples for this study are selected from the EAGLE simulation suite (Crain et al. 2015; Schaye et al. 2015).
- The largest EAGLE simulation, hereafter referred to as Ref-L100N1504, employs 15043 dark matter particles and an initially equal number of gas particles in a periodic cube with side-length 100 comoving Mpc (cMpc) on each side.
- The uncertainty in these models introduces parameters whose values must be calibrated by comparison to observational data (Vernon, Goldstein & Bower 2010).
- The subgrid parameters calibrated by requiring that the model fits three key properties of local galaxies well: the galaxy stellar mass function, the galaxy size – mass relation and the normalization of the black hole mass – galaxy mass relation and that variations of the parameters alter the simulation outcome in predictable ways (Crain et al. 2015).
- The authors find that it describes many aspects of the observed universe well (i.e. within the plausible observational uncertainties), including the evolution of the galaxy stellar mass function and star formation rates (Furlong et al. 2015b), evolution of galaxy colours and luminosity functions (Trayford et al. 2015).
2.2.1 Halo identification
- Dark matter structures in the EAGLE simulations are initially identified using the ‘Friends-of-Friends’ (FoF) algorithm with a linking length of 0.2 times the mean inter-particle spacing (Davis et al. 1985).
- The gravitationally bound substructures within the FoF groups are then identified by the SUBFIND algorithm (Springel et al. 2001; Dolag et al. 2009).
- Briefly, the algorithm assigns a mass density at the position of every particle through a kernel interpolation over a certain number of its nearest neighbours.
- The local minima in the gravitational potential field are the centres of subhalo candidates.
- Particles are assigned to at most one subhalo.
2.2.2 Subhalo merger tree
- Subhaloes survive as distinct objects for an extended period of time.
- The authors use the D-Trees algorithm (Jiang et al. 2014) to locate the whereabouts of the Nlink = min(Nlinkmax, max(ftraceN, Nlinkmin)) most bound particles of the subhalo, where N is the total particle number in the subhalo.
- When two subhaloes are close to each other, their volumes of influence become intertwined and the definition of the main halo may become unclear.
- The main progenitor is then the progenitor that has the maximum branch mass among its contemporaries.
- The subhalo merger trees derived by the method described above are publicly available through an SQL data base1 similar to that used for the Millennium simulations (see McAlpine et al. 2016, for more details).
2.3 Galaxy sample, galaxy merger tree, and merger type
- Galaxies are identified as the stellar components of the subhaloes.
- Previous studies based on the EAGLE simulations adopt an aperture of 30 pkpc to measure galaxy stellar mass (e.g. Furlong et al. 2015b; Schaye et al. 2015).
- Nevertheless, subhaloes do contain a significant population of diffuse stars, particularly in more massive haloes (Furlong et al. 2015b).
- Using the actual stellar mass complicates interpretation of the relative mass contribution from different types of merger events since it depends on the age of the stellar population that is accreted.
- The authors therefore use the stellar mass initially formed (‘initial mass’), not the actual stellar mass, to evaluate the contributions from internal and external processes to galaxy assembly.
2.3.1 Galaxy sample
- In order to test the robustness of their results to resolution, the authors also extract 1381 galaxies within the same mass range, as a comparison sample, from the EAGLE simulation Recal-L025N0752 (2 × 7523 dark matter and gas particles in a 25 cMpc box), which has eight times better mass resolution and the same snapshot frequency as Ref-L100N1504.
- The authors use subgrid physical models with parameters recalibrated to the present-day observations, as this provides the best match to the observed galaxy population (see Schaye et al. 2015).
2.3.2 Galaxy merger tree
- The authors construct galaxy merger trees by focusing on the stellar component of the subhalo merger trees.
- The main branch of the tree is marked by the thick black line.
- It is important to bear in mind that the identification of the main branch is always based on the branch mass; at any particular epoch, the most massive galaxy progenitor may not lie on the main branch.
- For the reasons described in Section 2.2.2, using the branch mass yields more stable and intuitive results.
- Galaxy merger trees appear broadly similar to subhalo merger trees, except that the latter contain more fine branches corresponding to small subhaloes within which no stars have formed.
2.3.3 Merger type
- The effects of tidal forces and torques during a merger depend on the mass ratio of the merging systems (e.g. Barnes & Hernquist 1992).
- It is therefore useful to classify mergers into different types according to the mass ratio between the two merging systems, μ ≡ M2/M1 (M1 > M2).
- For galaxy mergers, μ is the ratio of stellar masses between two merging galaxies.
- While this is straightforward in semi-analytic models (since galaxies are uniquely defined entities), in numerical simulations (and in nature as well), merging systems experience mass-loss due to tidal stripping throughout the merging process.
- The value of Rmerge ranges from ∼20 to 200 pkpc in the stellar mass range explored in this work , and is similar to the projected separation criteria adopted in observational galaxy pair studies.
3.1 Galaxy formation and assembly time-scales
- In contrast, the most massive galaxies formed their stars relatively early, tf ∼ 11 Gyr, and have ta < tf indicating that a fraction of their stars are formed elsewhere and subsequently assembled into the final system.
- This trend agrees well with previous work (e.g. De Lucia et al.
- Many previous studies have pointed out that in a CDM cosmology, halo growth is driven by a mix of mergers and accretion of matter that has not yet collapsed into identifiable haloes (e.g. Kauffmann & White 1993; Lacey & Cole 1993; Guo & White 2008; Fakhouri & Ma 2010; Genel et al.
- This fundamental differences results in the stark contrast between Figs 3 and 4.
3.2 The redshift evolution of galaxy formation and assembly times
- In previous section, the authors have shown that the delay between formation time and assembly time can provide some useful hints on how a galaxy assembles its mass.
- The authors show results for Ref-L100N1504 (solid lines), as well as for the higher resolution (but smaller volume) simulation Recal-L025N0752 (dashed lines) in order to demonstrate the convergence of the results.
- The shaded region represents the 25th–75th percentiles of the δt distribution.
- While low-mass galaxies have median δt < 0.1 at all redshifts, high-mass galaxies have median δt decreasing with increasing redshift, showing that stellar accretion loses ground to in situ star formation.
- The same redshift dependence is also found in semi-analytic studies (e.g. Guo & White 2008).
3.3 The contribution of star formation in external galaxies
- Time-scale studies shed light on the manner in which galaxies with different masses at different redshifts aggregate their stars.
- But they do not explore quantitatively the roles of internal and external processes therein.
- The authors sum up the mass that a galaxy has acquired from mergers and accretion, and derive the fractional contribution of external processes, fext, by comparing this mass to the final galaxy mass.
- Lines show the median values, while the shaded regions represent the 25th–75th percentiles of the distribution.
- Both results of the reference Ref-L100N1504 (solid lines) and the higher resolution Recal-L025N0752 (dashed lines) simulations are shown in order to demonstrate the convergence of the results.
3.4 Galaxy merging history
- In preceding sections, the authors explored the relative roles that in situ and external star formation play in galaxy mass build-up.
- The authors continue their investigation by exploring the separate contributions of the different external processes in galaxy assembly.
3.4.1 Redshift of last major merger
- Almost all of their present-day galaxies, irrespective of their stellar mass, have experienced at least one major merger event in their lives.
- The authors use the merger trees to determine the redshift, zlast, when they experienced their last major merger.
- The most massive galaxies have a very active merging history, with 68 per cent of the population having been involved in a major merger event since z = 1.5 (a lookback time of 10 Gyr).
- For comparison, Fig. 7 also shows the cumulative distributions of zlast for the parent subhaloes of those galaxies (dashed lines).
- In sharp contrast to the active merging histories of high-mass galaxies, only 20 per cent of their host subhaloes have undergone a major merger event in the last 10 Gyr. Intermediate- and low-mass galaxies share more similarity with their parent subhaloes.
3.4.2 The contributions of major mergers, minor mergers, and accretion
- The authors continue their investigation of fractional mass contribution in Section 3.3 further to explore the respective contributions from major merger, minor merger and accretion and their dependence on galaxy mass and redshift.
- The panels from left to right in Fig. 8 show the cumulative fraction of galaxies at redshift z = 0 (solid lines), 1 (dashed lines), and 2 (dotted lines) as a function of the minimum fractional mass contribution from major mergers, minor mergers and accretion, respectively.
- Low-mass galaxies at redshift z = 0 mainly acquire their external masses through accretion, while major mergers are the main contributor for their high-mass counterparts.
- Around ∼61 per cent of the most massive population acquired more than half of their external mass through major merger events.
- Parry et al. (2009) arrived at the same conclusion from their analysis of semi-analytic models in the Millennium simulation (see fig. 8 in their work).
3.4.3 Evolution of the galaxy merger fraction
- Observationally, the frequencies of galaxy pairs and morphologically distorted galaxies at different redshifts are commonly used to put constraints on the role of galaxy mergers, especially major mergers, in driving galaxy formation.
- The galaxy merger fraction increases monotonically towards high redshifts before levelling off at z 1–3, depending on mass.
- Note that this comparison is qualitative since a detailed comparison would require careful reconstruction of the observational criteria.
- The merger diagnostics are also sensitive to merger mass ratios, the authors also consider the impact on their results of extending the merger mass ratio to a smaller value (μ ≥ 1/10).
3.5 The impact of feedback on galaxy mass assembly
- A very interesting question is whether this is due to the feedback from star formation and black hole growth.
- These runs differ in simulation volume but have the same resolution.
- In the strong feedback case, the analysis consistently suggests a slight decrease of fext as more of the star-forming gas within small galaxies is lost in outflows, reducing their contribution to the stellar mass.
- In the middle panel, fext is lower than the reference simulation (and is more similar to the curve in the left-hand panel).
- In the absence of effective stellar feedback, AGN feedback has a similar impact in high- and low-mass haloes (Bower et al. 2016) and the authors expect the differences between the panels to be smaller, as seen.
4 C O M PA R I S O N S TO OTH E R WO R K
- This is a topic that has been extensively studied using N-body simulations and semi-analytic galaxy formation models.
- Assuming a uniform galaxy formation efficiency to derive galaxy merging histories from halo merging histories inevitably underestimates the importance of major galaxy mergers, and overstates the role of minor mergers.
- In the high-mass galaxies, the assembly and formation times become increasingly similar with increasing redshift and the fraction of externally formed stellar mass declines (Figs 5 and 6). (iv) As in Guo & White (2008), the authors compare the stellar mass contributions from in situ star formation and external processes to galaxies of various stellar masses and redshifts.
- The authors find both agreements and discrepancies between their results and those of other recent simulations.
A P P E N D I X C : EF F E C T O F T H E A P E RTU R E
- The galaxy mass is defined as the actual (or initial) stellar mass enclosed by a spherical aperture with a galactocentric radius of 100 pkpc (proper kpc).
- By using an aperture mass, the authors may underestimate the total stellar mass of a massive galaxy, and thus overestimate the fractional mass contributions of external processes, fext.
- Galaxies are split into three stellar mass bins as indicated by colours and legends.
- Lines represent the medians of the distributions while the shaded regions (dotted lines) mark the 25th and the 75th percentiles.
- This paper has been typeset from a TEX/LATEX file prepared by the author.
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Cites methods from "A chronicle of galaxy mass assembly..."
...We create subhalo merger trees in a similar manner to Jiang et al. (2014) and Qu et al. (2017)....
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...Following Qu et al. (2017), we define major mergers as having a stellar mass ratio M2/M1 > 1/4 (where M1 > M2)....
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191 citations
Cites background or result from "A chronicle of galaxy mass assembly..."
...As will be shown by Qu et al. (2017), the number of major and minor mergers experienced by galaxies in the EAGLE simulation is a strong function of stellar mass, consistent with findings inferred from observations of close projected pairs....
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...This definition ensures that any mass lost due to stripping, before the secondary branch coalesces with the main branch, is accounted for in the accreted mass (Qu et al. 2017)....
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...A full description of the trees is presented by Qu et al. (2017), with their public release1 discussed by McAlpine et al. (2016)....
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176 citations
Cites methods from "A chronicle of galaxy mass assembly..."
...We note that this algorithm is similar, but not identical, to that used by Qu et al. (2017) to build merger trees from the EAGLE simulations....
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"A chronicle of galaxy mass assembly..." refers methods in this paper
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"A chronicle of galaxy mass assembly..." refers background in this paper
...…& Dalla Vecchia 2008), multi-element metal enrichment (Wiersma et al. 2009), black hole formation (Rosas-Guevara et al. 2015; Springel, Di Matteo & Hernquist 2005), as well as feedback from massive stars (Dalla Vecchia & Schaye 2012) and AGN (for a complete description, see Schaye et al. 2015)....
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"A chronicle of galaxy mass assembly..." refers background in this paper
...In contrast, the galaxy stellar mass function is almost flat at low mass (e.g. Fontana et al. 2006)....
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