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arxiv: 1906.08187 · v1 · pith:TB3ON5WYnew · submitted 2019-06-19 · 🌌 astro-ph.GA

The Fornax3D project: Tracing the assembly history of the cluster from the kinematic and line-strength maps

E. Iodice , M. Sarzi , A. Bittner , L. Coccato , L. Costantin , E. M. Corsini , G. van de Ven , P. T. de Zeeuw
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J.Falc\`on-Barroso D. A. Gadotti M. Lyubenova I. Mart\`in-Navarro R. M. McDermid B. Nedelchev F. Pinna A. Pizzella M. Spavone S. Viaene
This is my paper

Pith reviewed 2026-05-25 20:06 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords Fornax clustergalaxy kinematicsstellar populationscluster assemblyprojected phase spaceMUSE spectroscopyearly-type galaxies
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The pith

The Fornax cluster assembled inside its virial radius from an old core plus two groups accreted at later times.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper maps line-of-sight stellar kinematics and absorption-line strengths across the faces of 31 bright galaxies in the Fornax cluster, reaching two to three effective radii for most early-type systems. Placing each galaxy in projected phase space and comparing central versus outer ages, metallicities, and rotation patterns reveals three distinct populations. One population forms an old, relaxed core; a second population lies along the local large-scale structure and joined shortly afterward; the third population shows signs of more recent infall. A reader would care because the result supplies a direct, observationally ordered timeline for how a nearby cluster grew by accreting subgroups rather than a smooth, continuous process.

Core claim

The Fornax cluster appears to consist of three main groups of galaxies inside the virial radius: the old core; a clump of galaxies aligned with the local large-scale structure and accreted soon after the formation of the core; and a group of galaxies that fell in more recently. This grouping follows from the galaxies' locations in projected phase space together with their measured stellar ages, metallicities, and kinematic regularity derived from MUSE maps.

What carries the argument

Projected phase-space position combined with spatially resolved stellar kinematic maps and line-strength indices, used to assign galaxies to accretion epochs.

If this is right

  • Galaxies assigned to the old core must show uniformly old stellar populations and little ongoing star formation even in their outskirts.
  • The intermediate clump must exhibit intermediate ages and retain some alignment with the surrounding large-scale structure.
  • Recently accreted galaxies must display younger outskirts, more gas, and less regular rotation than the core population.
  • The eight galaxies with kinematically distinct cores should belong predominantly to one or two of the three groups rather than being evenly distributed.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If the three-group pattern holds, repeating the same phase-space plus stellar-population analysis in other nearby clusters should recover comparable accretion timelines.
  • The alignment of the intermediate clump with large-scale structure implies that future wide-field surveys could predict which subgroups will join a cluster next by tracing filament directions.
  • Dynamical models that ignore the three distinct orbital families may underestimate the time required for full virialization of the cluster.

Load-bearing premise

Projected phase-space location plus observed stellar ages and kinematics directly indicate the order in which galaxies joined the cluster.

What would settle it

Discovery that several galaxies placed in the recent-infall group have older stellar populations or more relaxed kinematics than those in the core group would falsify the three-epoch assembly sequence.

Figures

Figures reproduced from arXiv: 1906.08187 by A. Bittner, A. Pizzella, B. Nedelchev, D. A. Gadotti, E. Iodice, E. M. Corsini, F. Pinna, G. van de Ven, I. Mart\`in-Navarro, J.Falc\`on-Barroso, L. Coccato, L. Costantin, M. Lyubenova, M. Sarzi, M. Spavone, P. T. de Zeeuw, R. M. McDermid, S. Viaene.

Figure 1
Figure 1. Figure 1: Left: Distribution of the spatial coverage of the F3D MUSE data for ETGs (solid line) and LTGs (dashed line) in units of effective radius, as measured along the major axis. Right: Spatial coverage of the F3D ETGs (circles) and LTGs (crosses) as a function of the total stellar mass. The long-dashed and dash-dotted lines correspond to the average spatial coverage from the long-slit data of Bedregal et al. (2… view at source ↗
Figure 2
Figure 2. Figure 2: Distribution of the seeing FWHM measured on the F3D MUSE pointings. The vertical thick segment marks the median value. The stellar kinematic maps of v, σ, h3, and h4 derived from S/N = 40 Voronoi-binned MUSE data and the radial profiles of PAkin and PAphot for the F3D galaxies are shown in Appendix A. 4.2. Ionised-gas distribution and kinematics The derivation of the ionised-gas distribution and kinematics… view at source ↗
Figure 3
Figure 3. Figure 3: Left: Specific stellar angular momentum of the F3D ETGs (circles) and LTGs (crosses) as a function of the ellipticity measured at the effective radius. Galaxies classified as regular rotators (RRs) are shown with red symbols. The light-blue symbols correspond to galaxies with a distinct core (DC). The blue circle corresponds to the only non-rotating ETG of the sample (NR). Galaxies with a kinematically dec… view at source ↗
Figure 4
Figure 4. Figure 4: Difference between the mean kinematic and photometric posi￾tion angles for the F3D galaxies as function of the projected radius from the centre of the Fornax cluster in units of virial radius. Symbols are as in the left panel of [PITH_FULL_IMAGE:figures/full_fig_p010_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Left: Mean age (top), total metallicity [M/H] (middle), and [Mg/Fe] abundance ratio (bottom) in the central parts of the F3D ETGs located in the high (red circles) and low-density region (blue circles) of the cluster as a function of the total stellar mass. The red and blue lines in the middle and bottom panels are the least-square fits of the values for the central parts of the ETGs in the high and low-de… view at source ↗
Figure 6
Figure 6. Figure 6: Projected phase-space diagram of the F3D ETGs (filled circles) and LTGs (crosses). Symbols are as in the left panel of [PITH_FULL_IMAGE:figures/full_fig_p012_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Distribution of the F3D ETGs (circles) and LTGs (crosses) onto the sky plane. The right ascension and declination (J2000.0) are given in degrees on the horizontal and vertical axes of the field of view, respectively. The background image and contours map the X-ray emission in the energy range 0.4–1.3 KeV as measured by XMM-Newton (Frank et al. 2013). The dashed circle indicates the transition from the high… view at source ↗
Figure 8
Figure 8. Figure 8: Mass-size distribution of the F3D ETGs (circles) and LTGs (crosses). Orange, green, and light-green symbols represent the ancient, intermediate, and recent infallers, respectively, according to the analy￾sis of the PPS diagram shown in [PITH_FULL_IMAGE:figures/full_fig_p015_8.png] view at source ↗
read the original abstract

The 31 brightest galaxies (m_B < 15 mag) inside the virial radius of the Fornax cluster were observed from the centres to the outskirts with the Multi Unit Spectroscopic Explorer on the Very Large Telescope. These observations provide detailed high-resolution maps of the line-of-sight kinematics and line strengths of the stars and ionised gas reaching 2-3 Re for 21 early-type galaxies and 1-2 Re for 10 late-type galaxies. The majority of the galaxies are regular rotators, with eight hosting a kinematically distinct core. Only two galaxies are slow rotators. The mean age, total metallicity, and [Mg/Fe] abundance ratio in the bright central region inside 0.5 Re and in the galaxy outskirts are presented. Extended emission-line gas is detected in 13 galaxies, most of them are late-type objects with wide-spread star formation. The measured structural properties are analysed in relation to the galaxies' position in the projected phase space of the cluster. This shows that the Fornax cluster appears to consist of three main groups of galaxies inside the virial radius: the old core; a clump of galaxies, which is aligned with the local large-scale structure and was accreted soon after the formation of the core; and a group of galaxies that fell in more recently.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 1 minor

Summary. The manuscript reports MUSE/VLT observations of the 31 brightest galaxies (m_B < 15) inside the virial radius of the Fornax cluster, delivering high-resolution maps of line-of-sight kinematics and line strengths reaching 2-3 R_e for early-types and 1-2 R_e for late-types. The majority are regular rotators with eight kinematically distinct cores; only two are slow rotators. Extended emission-line gas is found in 13 galaxies. The structural properties (ages, metallicities, [Mg/Fe]) are then placed in the projected phase-space diagram, leading to the conclusion that Fornax consists of three groups inside the virial radius: an old core, an early-accreted clump aligned with the local large-scale structure, and a more recently accreted group.

Significance. The homogeneous MUSE dataset on a magnitude-limited sample constitutes a valuable observational resource for studies of cluster galaxy evolution. If the phase-space partitioning can be shown to survive projection and mixing uncertainties, the accretion chronology would provide a concrete illustration of hierarchical cluster assembly. The data products themselves enable future dynamical modeling independent of the interpretive step.

major comments (2)
  1. [projected phase-space analysis and discussion of the three groups] The central claim that the three groups correspond to distinct accretion epochs rests on the assumption that locations in the projected phase-space diagram, combined with measured ages, [Mg/Fe], and kinematics, preserve chronological order. No orbit integration, N-body realization of Fornax, or Monte-Carlo deprojection is described that would quantify the degree to which galaxies on different true 3D orbits can overlap in projection or how dynamical friction and phase mixing over several Gyr would scramble the signal (see the paragraph beginning “The measured structural properties are analysed in relation to the galaxies’ position in the projected phase space”).
  2. [abstract and results on structural properties vs. phase space] The abstract and the phase-space grouping statement supply no error budgets on the stellar-population parameters, no explicit selection-function description for the 31-galaxy sample, and no quantitative test (e.g., clustering significance or bootstrap stability) of the three-group partition. These omissions make it difficult to assess whether the reported chronological ordering is robust against measurement uncertainties.
minor comments (1)
  1. The notation for the abundance ratio is given as [Mg/Fe] throughout; a brief statement confirming the solar reference scale used would aid reproducibility.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the positive assessment of the MUSE dataset's value and for the constructive comments on the phase-space interpretation. We address each major point below with clarifications and planned revisions where appropriate.

read point-by-point responses

  1. Referee: [projected phase-space analysis and discussion of the three groups] The central claim that the three groups correspond to distinct accretion epochs rests on the assumption that locations in the projected phase-space diagram, combined with measured ages, [Mg/Fe], and kinematics, preserve chronological order. No orbit integration, N-body realization of Fornax, or Monte-Carlo deprojection is described that would quantify the degree to which galaxies on different true 3D orbits can overlap in projection or how dynamical friction and phase mixing over several Gyr would scramble the signal (see the paragraph beginning “The measured structural properties are analysed in relation to the galaxies’ position in the projected phase space”).

    Authors: We agree that a full quantification via orbit integration or N-body simulations would strengthen the chronological interpretation. The manuscript presents the three groups as an interpretive framework based on the observed correlations between stellar population properties, kinematics, and location in the projected phase-space diagram, which align with expectations from hierarchical assembly models. We will revise the relevant discussion section to explicitly state the limitations of the projected analysis, including potential overlaps from projection effects and phase mixing, and to reference existing simulations of cluster accretion without claiming a direct deprojection. This keeps the focus on the observational trends while acknowledging the assumptions. revision: partial

  2. Referee: [abstract and results on structural properties vs. phase space] The abstract and the phase-space grouping statement supply no error budgets on the stellar-population parameters, no explicit selection-function description for the 31-galaxy sample, and no quantitative test (e.g., clustering significance or bootstrap stability) of the three-group partition. These omissions make it difficult to assess whether the reported chronological ordering is robust against measurement uncertainties.

    Authors: The sample selection is the magnitude-limited set of the 31 brightest galaxies (m_B < 15) inside the virial radius, as described in Section 2. Stellar-population uncertainties are derived from the pPXF and line-strength fitting and are reported in the text, figures, and tables for the central and outer regions. The three-group partition is a qualitative grouping guided by the joint distribution of ages, [Mg/Fe], and phase-space location rather than a formal clustering algorithm. We will revise the abstract and discussion to include a brief mention of the parameter uncertainties, restate the selection function explicitly, and add a sentence noting that the grouping lacks a quantitative stability test but is robust to small changes in the adopted boundaries. No bootstrap analysis will be added as it is outside the current scope. revision: partial

Circularity Check

0 steps flagged

No circularity: groups identified from direct observables and standard classifiers

full rationale

The paper's central claim partitions galaxies into three groups by inspecting their locations in the projected phase-space diagram together with directly measured ages, [Mg/Fe], kinematics, and structural properties. No equations, fitted parameters, or self-citations are invoked that would make the grouping equivalent to the input data by construction. The analysis is an empirical classification whose validity can be tested against external benchmarks (e.g., simulations of projection effects), satisfying the criteria for a self-contained, non-circular result.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The work rests on standard assumptions of integral-field spectroscopy and stellar-population fitting; no new free parameters or invented entities are introduced in the abstract.

axioms (2)
  • domain assumption Standard kinematic classifiers (regular rotator, slow rotator, kinematically distinct core) correctly separate dynamical classes from line-of-sight velocity fields.
    Invoked when the abstract states that the majority are regular rotators with eight hosting kinematically distinct cores and only two slow rotators.
  • domain assumption Projected phase-space position correlates with time since accretion into the cluster.
    Invoked in the final sentence that divides the cluster into three groups on the basis of position in projected phase space.

pith-pipeline@v0.9.0 · 5879 in / 1293 out tokens · 27089 ms · 2026-05-25T20:06:03.631060+00:00 · methodology

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Reference graph

Works this paper leans on

16 extracted references · 16 canonical work pages

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    It is a recent infaller in the low- density region of the cluster

    on the south-western side of the cluster at a distance Rproj = 1.7◦ from the centre. It is a recent infaller in the low- density region of the cluster. It is the only non-rotating galaxy of the F3D sample, so that PA kin is not well-determined. The stel- lar velocity dispersion shows a clear decrease of ∼ 30 km s−1 towards the galaxy centre ( R <∼ 10 arcs...

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    A.5 FCC 143 is one of the most compact and faint elliptical galaxies of the F3D sample

    – Fig. A.5 FCC 143 is one of the most compact and faint elliptical galaxies of the F3D sample. It is located on the western side of the cluster at Rproj = 0.8◦ from the centre and it is close in projection to FCC 147 (NGC 1374). According to Blakeslee et al. (2009), the difference in distance between FCC 143 and FCC 147 is only∼ 0.3 Mpc. Deep FDS images sh...

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    It is one of the ancient infallers in the north-south clump of galaxies

    on the western side of the cluster. It is one of the ancient infallers in the north-south clump of galaxies. Similarly to FCC 147, which is the other giant elliptical galaxy of this cluster region, also the stellar component of FCC 161 is characterised by a small maximum rotation ( vmax∼ 30 km s−1) and a high velocity dispersion in the centre ( σ∼ 150 km ...

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    A.10 FCC 167 is the brightest lenticular galaxy inside the virial ra- dius of the Fornax cluster at Rproj = 0.6◦ from the centre

    – Fig. A.10 FCC 167 is the brightest lenticular galaxy inside the virial ra- dius of the Fornax cluster at Rproj = 0.6◦ from the centre. It is an ancient infaller in the north-south clump of galaxies on the western side of the cluster. This galaxy shows an extended rotationally-supported stellar disc with a large rotation velocity (vmax∼ 300 km s−1) and a...

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    A.11 FCC 170 is an edge-on lenticular galaxy in the high-density re- gion of the cluster at Rproj = 0.4◦ west of FCC 213 (NGC 1399)

    – Fig. A.11 FCC 170 is an edge-on lenticular galaxy in the high-density re- gion of the cluster at Rproj = 0.4◦ west of FCC 213 (NGC 1399). It belongs to the north-south clump of galaxies and is an an- cient infaller. It is characterised by a boxy bulge and a thin disc, which becomes thicker at larger radii (Iodice et al. 2019; P19b). The analysis of the ...

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    A.12 FCC 176 is one of the most luminous LTGs of the F3D sam- ple and is characterised by a prominent bar and an outer ring

    – Fig. A.12 FCC 176 is one of the most luminous LTGs of the F3D sam- ple and is characterised by a prominent bar and an outer ring. It is located at Rproj = 0.9◦ from the centre on the south-western side of the cluster. It is an intermediate infaller in the transion between the high and low-density regions of the cluster where the X-ray emission is still ...

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    A.14 FCC 179 is a Seyfert 2 early-type spiral at Rproj = 0.7◦ from the cluster centre on the south-western side

    – Fig. A.14 FCC 179 is a Seyfert 2 early-type spiral at Rproj = 0.7◦ from the cluster centre on the south-western side. Although it is close in projection to the cluster core and located in the high-density re- gion, FCC 179 is one of the recent infallers of the cluster. This is consistent with the absence of star formation in the outer parts of the disc,...

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    A.16 FCC 184 is the bright elliptical galaxy located at Rproj = 0.3◦ west of FCC 213 (NGC 1399)

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    and optical deep imaging (Iodice et al. 2019). This feature is also de- tected in the maps of stellar velocity dispersion (smaller σ val- ues) and Hβ line-strength index (larger Hβ values). The Mgb and Fe5015 indices are higher in the inner bar compared to larger radii. The ionised-gas emission shows a central bright ring with prominent star formation and...

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    and located at a distance of Rproj = 0.2◦ from it on the south- eastern side of the cluster. FCC 219 is an ancient infaller set- tled in the cluster core. The stellar kinematic maps are regular and show large rotation ( vmax∼ 100 km s−1 at R∼ 50 arcsec) along the photometric major axis. The velocity dispersion pro- file peaks in the centre (σ∼ 250 km s−1) ...

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    A.20 FCC 249 is a compact low-luminosity non-rotating elliptical galaxy at a distance of Rproj = 2.1◦ from the centre, close to the virial radius of Fornax

    – Fig. A.20 FCC 249 is a compact low-luminosity non-rotating elliptical galaxy at a distance of Rproj = 2.1◦ from the centre, close to the virial radius of Fornax. It is an intermediate infaller in the southern low-density region of the cluster. The stellar kine- matic maps reveal a nuclear ( R <∼ 5 arcsec) component with vmax ∼ 50 km s−1. The stellar vel...

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    A.23 FCC 276 is the brightest elliptical galaxy on the eastern side of the cluster at Rproj = 0.8◦ from FCC 213 (NGC 1399)

    – Fig. A.23 FCC 276 is the brightest elliptical galaxy on the eastern side of the cluster at Rproj = 0.8◦ from FCC 213 (NGC 1399). It is the only ancient infaller on this side of the transition region from high to low density. This is one of the two slow rotators of the F3D sample. The stellar kinematic maps clearly show the pres- ence of a distinct compo...

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    in the transition from high to low-density regions. FCC 277 is ∼ 2 mag fainter than FCC 276, which is the other ETG in this area, and their distance di ffers by∼ 1 Mpc (Blakeslee et al. 2009). Di fferently from FCC 276, FCC 277 is an intermediate infaller. The stellar kinematics is consistent with a bulge of σ∼ 90 km s−1 for R <∼ 10 arcsec and a disc of vma...

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    It is one of the intermediate infallers in this area

    in the south-eastern low-density region of the clus- ter. It is one of the intermediate infallers in this area. Since the spiral arms are prominent inside∼ 2 arcmin and the disc appears smooth and featureless outwards, this galaxy may evolve into a lenticular system (see also Raj et al. 2019). The spiral arms cause perturbations in the central regions of ...

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    A.30 FCC 310 is the only barred lenticular galaxy on the eastern side of the cluster

    – Fig. A.30 FCC 310 is the only barred lenticular galaxy on the eastern side of the cluster. It is located at a distance Rproj = 2.0◦ which cor- responds to about the virial radius of the Fornax cluster. It is an intermediate infaller in the south-eastern low-density region of the cluster. The prominent peanut-shaped bar is evident in the reconstructed ga...

    work page 2019