The (co-)evolution of galactic structures as revealed by deep, highly �spatially-resolved spectroscopy.
Amelia Fraser-McKelvie, ESO Fellow, Garching
Outline:�- Why study galaxies in high-resolution?�- Summary of the GECKOS survey�- Kinematic signatures of galactic central structures�- Slightly speculative detailed analysis of the SFH of a GECKOS galaxy�- A realisation that not all galaxies follow simple evolutionairy trends�- Mocking GECKOS – 2 approaches
Polished
Off piste
Central gaactic structures such as bars and nuclear discs predominantly form from disc material, implying a close evolutionary link between galaxy discs and their inner components. Observationally however, disentangling the formation histories of individual structures is challenging due to the superposition of their light along most lines of sight. Edge-on systems provide a powerful means of overcoming this limitation as they allow us to unambiguously define a ‘bulge’ region. I will present results from the GECKOS Survey, an ESO/MUSE Large Programme targeting 36 edge-on, Milky Way-mass disc galaxies. This statistically significant sample enables a census of central structures in galaxies analogous to the Milky Way. We find a notable lack of evidence for dispersion-supported “classical” bulges at this stellar mass. Instead, the stellar kinematics reveal signatures consistent with disc-built structures such as bars and nuclear discs. Complementary ALMA observations allow further characterisation of bars and inner discs via measures of their resonance rings. I will highlight one galaxy as a detailed case study in which a careful photometric decomposition into a boxy/peanut bulge, nuclear disc, and main disc maps cleanly into components separated by their stellar kinematics and hosting discrete star formation histories. From this, we construct a coherent evolutionary narrative. However, extending this approach to the full GECKOS sample reveals that such straightforward stories are the exception rather than the rule. GECKOS central structures exhibit a wide diversity of star formation histories, pointing to complex and varied formation and assembly pathways. These data demonstrate that we now possess the required depth and spatial and spectroscopic resolution to robustly disentangle the evolutionary histories of individual galactic components in nearby galaxies.
Discussion points
-Diagnostics for central structures embedded within discs. What tools do we have at our disposal to differentiate between classical bulges and nuclear discs, for example? What fraction of galaxies host both? What future facility/survey do we need to place robust limits on the number of classical bulges in the local Universe? How can we age-date bars (and hence decide whether they are long-lived structures)? How linked are the evolutionary histories of discs and central structures?
The closer you look at a galaxy, the more complex it becomes…
Why study galaxies at high resolution?
Amelia Fraser-McKelvie
The closer you look at a galaxy, the more complex it becomes…
10492-3704
Why study galaxies at high resolution?
Amelia Fraser-McKelvie
The closer you look at a galaxy, the more complex it becomes…
V★ [km/s]
σ★ [km/s]
10492-3704
~kpc scale, �D = 200 Mpc
Why study galaxies at high resolution?
Amelia Fraser-McKelvie
The closer you look at a galaxy, the more complex it becomes…
V★ [km/s]
σ★ [km/s]
10492-3704
~kpc scale, �D = 200 Mpc
~200pc scale, �D = 25 Mpc
Fraser-McKelvie et al. 2025
Why study galaxies at high resolution?
Amelia Fraser-McKelvie
100s of pc scales required to resolve central strcutures
~200pc scale, �D = 25 Mpc
Fraser-McKelvie et al. 2025
de Sá Freitas et al., 2023; TIMER
Face-on views from TIMER & PHANGS
Edge-on complement for a full, 3D view of galaxies
Why study galaxies at high resolution?
Amelia Fraser-McKelvie
100s of pc scales required to resolve central strcutures
~200pc scale, �D = 25 Mpc
Fraser-McKelvie et al. 2025
de Sá Freitas et al., 2023; TIMER
Your redshift zero
Anchor
Face-on views from TIMER & PHANGS
Why study galaxies at high resolution?
Amelia Fraser-McKelvie
van de Sande, F-M, et al., in prep.
Stellar velocity, V* [km/s]
Stellar velocity dispersion, σ* [km/s]
Stellar skew, h3
Stellar kurtosis, h4
Mean , LW stellar age [Gyr]
Mean LW stellar metallicity
MUSE 3-colour images
Why study edge-on galaxies?
Common kinematic structures? – σ★
Focussing on the boxy-peanut bulges
Fraser-McKelvie et al., 2025, A&A
Clear boxy-peanut bulge visible in unsharp-masked image
Signatures of bars and nuclear discs in stellar kinematics
PGC 044931
Legacy r-band
Unsharp masked
PGC 044931
Fraser-McKelvie et al., 2025, A&A
Clear boxy-peanut bulge visible in unsharp-masked image
Signatures of bars and nuclear discs in stellar kinematics
Regions of V*-h3 sign matching and mismatching predicted by theory (and seen in 1D profiles from observations) to correspond to signature of nuclear discs and bars (e.g. Bureau & Athanassoula 2005, Chung & Bureau 2004, Iannuzzi & Athanassoula 2015)
PGC 044931
2-dimensional information required to ensure full kinematic structure is recovered!
Side note: Molecular gas tracers excellent for bar and ND diagnosis
Fraser-McKelvie, T. Davis et al., in prep.
Signatures of bars and nuclear discs in cold gas
IC 1711 – an end -on bar
ALMA
Side quest:
Amelia Fraser-McKelvie
Side note: Molecular gas tracers excellent for bar and ND diagnosis
Fraser-McKelvie, T. Davis et al., in prep.
Signatures of bars and nuclear discs in cold gas
IC 1711 – an end -on bar
Amelia Fraser-McKelvie
ALMA
Side quest:
A simple experiment:
Do detailed photometric decompositions map to physical galaxy structures?
Legacy image
3-colour MUSE reconstructed
Amelia Fraser-McKelvie
μV = 23.5 mag/arcsec2
12 hours
7 hours
A simple experiment:
Legacy image
Fraser-McKelvie et al., 2026
3-colour MUSE reconstructed
Amelia Fraser-McKelvie
Photometric decomposition by Dimitri Gadotti; IMFIT
Do detailed photometric decompositions map to physical galaxy structures?
A simple experiment:
Amelia Fraser-McKelvie
Fraser-McKelvie et al., 2025
Do detailed photometric decompositions map to physical galaxy structures?
A simple experiment:
Amelia Fraser-McKelvie
Do detailed photometric decompositions map to physical galaxy structures?
Amelia Fraser-McKelvie
h3-V/σ anticorrelation = dynamically cold disc*
h3-V/σ correlation = non-axisymmertic orbits = bar*
*among other things
💿 Extended disc and nuclear disc exhibit kinematic behaviour expected for (near) circular dynamically cold discs
🧼 Boxy-peanut region exhibits kinematic behaviour of a region dominated by non-axisymmetric orbits (e.g. a bar!)
Clear separation of components in stellar kinematics parameter space!
Clear separation of components in stellar population parameter space!
Extended disc: continued star formation�Boxy/peanut bulge: formed from disc stars, no gas 🡪 no continued star formation
Nuclear disc: formed by bar. [M/H] offset from 10 Gyr 🡪 ND formed around this time, bar at least this old
Fraser-McKelvie et al., 2026
A clear story emerges..
A �long
time ago
in a galaxy
far far away…
Galaxy formed, started forming stars
Bar formed
Bar buckled, creating a boxy-peanut bulge
Nuclear disc formed and continued to enrich
Continued star formation in main disc: gas accretion, feeding from extended HI disc, minor gas-rich mergers
Metal-rich
Metal-poor
That’s nice.
So, do all galaxies follow this nice story?
Of course not!
PGC 044931
NGC 3957
Khoperskov+2025b
Can we do a closer comparison this week?
Milky Way
Work in progress – comments welcome!
Work in progress – comments welcome!
Work in progress – comments welcome!
Not properly separated
ND not resolved
Work in progress – comments welcome!
Mocking a GECKO
Matt Frosst, ICRAR/UWA 🡪 Queen’s University, CAN
Observation
Simulation
Mocking a GECKO
Create a sample of GECKOS analogues in TNG50
Frosst, F-M et al., in prep.
Observation
Simulation
Matt Frosst, ICRAR/UWA 🡪 Queen’s University, CAN
Mocking a GECKO
Create a sample of GECKOS analogues in TNG50
Forward model to look like MUSE cubes using SimSpin
Harborne et al. 2020
Frosst, F-M et al., in prep.
Observation
Simulation
Matt Frosst, ICRAR/UWA 🡪 Queen’s University, CAN
Mocking a GECKO
Run through GECKOS analysis pipeline.
Create a sample of GECKOS analogues in TNG50
Forward model to look like MUSE cubes using SimSpin
Harborne et al. 2020
Fraser-McKelvie et al. 2025
Frosst, F-M et al., in prep.
Observation
Simulation
Matt Frosst, ICRAR/UWA 🡪 Queen’s University, CAN
Frosst, F-M et al., in prep
🡪 Clear differences in stellar kinematics between barred (boxy-peanut bulge) and unbarred, bulge-dominated discs
Comparing stellar kinematics: �TNG50 vs. GECKOS
Frosst, F-M et al., in prep
🡪 Simulated barred and unbarred galaxies present similar kinematics to GECKOS BP and non-BP bulges
Comparing stellar kinematics: �TNG50 vs. GECKOS
Now, what can we infer about the assembly of barred galaxies?
Barred galaxies build up their stellar mass earlier
TNG50 GECKOS analogues
(See also e.g. Fragkoudi+2021, 2025; Izquierdo-Villalba+2022; Reddish+2022; Khoperskov+2024)
Frosst, F-M et al., in prep
(See also Fraser-McKelvie, Merrifield, Aragón-Salamanca et al., 2020b)
GECKOS galaxies
TNG50 GECKOS analogues
Frosst, F-M et al., in prep
(See also e.g. Fragkoudi+2021, 2025; Izquierdo-Villalba+2022; Reddish+2022; Khoperskov+2024)
Barred galaxies build up their stellar mass earlier
(See also Fraser-McKelvie, Merrifield, Aragón-Salamanca et al., 2020b)
GECKOS galaxies
TNG50 GECKOS analogues
Frosst, F-M et al., in prep
(See also e.g. Fragkoudi+2021, 2025; Izquierdo-Villalba+2022; Reddish+2022; Khoperskov+2024)
Barred galaxies build up their stellar mass earlier
Sergey’s talk earlier
Barred galaxies have higher in-situ mass fraction and form more easily in stellar-dominated discs
Frosst, F-M et al., in prep
Quieter assembly history for barred gals?
Bars form more easily in stellar-dominated discs.� (e.g. Fragkoudi+2021, 2025; Izquierdo-Villalba+2022; Reddish+2022; Bland-Hawthorn+2023; Khoperskov+2024)
f* = stellar-to-dark matter mass fraction
f*, in-situ = in-situ stellar mass fraction
Linking the diversity of disc galaxy kinematics to their formation pathways
What about other central structures?
Persistent problem: central regions of galaxies are messy, almost always containing several components whose light overlaps each other.
A solution: Structurally decompose galaxies in energy space, forward model individual components, and look for their signatures in observations.
Anna Lena Schaible, PhD student,�Heidelberg University
A Milky Way-like NIHAO galaxy
Anna Lena Schaible, PhD student,�Heidelberg University
NIHAO sims
Forward modelled using RUBIX
Run through nGIST
Schaible et al., in prep.
A Milky Way-like NIHAO galaxy
Anna Lena Schaible, PhD student,�Heidelberg University
NIHAO sims
Forward modelled using RUBIX
Run through nGIST
Schaible et al., in prep.
Split in binding energy and angular momentum space using Galactic Structure Finder 2 (GSF2; Obreja+ submitted)
A Milky Way-like NIHAO galaxy
Anna Lena Schaible, PhD student,�Heidelberg University
NIHAO sims
Forward modelled using RUBIX
Run through nGIST
Schaible et al., in prep.
A Milky Way-like NIHAO galaxy
Anna Lena Schaible, PhD student,�Heidelberg University
NIHAO sims
Forward modelled using RUBIX
Run through nGIST
Schaible et al., in prep.
No bulges, �only main disc (no bulges)
Classical bulge + main disc (no discy bulge)
Disky bulge + main disc�(no classical bulge)
All components
A Milky Way-like NIHAO galaxy
Anna Lena Schaible, PhD student,�Heidelberg University
NIHAO sims
Forward modelled using RUBIX
Run through nGIST
Schaible et al., in prep.
Kinematic predictions for bulge morphologies?�See also upcoming works from BANG survey
No bulges
No classical �bulge
No discy bulge
Total galaxy
Summary & Conclusions
Added galactic complexity reveals itself with increased spatial resolution
Bars and nuclear discs leave common signatures in their 2D stellar kinematic maps
Careful photometric decompositions yield coherent stellar structures, separated in both kinematic and chemical space.
We can infer the galaxy assembly history from the mean stellar populations of these structures
Future work to derive full star formation histories and application to a range of Milky Way-like discs should help us understand differences in disc galaxy assembly histories.
Barred galaxies show distinct assembly histories, including faster build-up of stellar mass and early star formation quenching, along with a higher in-situ fraction today
Stellar populations of individual components can be inferred with the help of forward-modelled simulations
Fraser-McKelvie et al., 2025, A&A, 700, 237�Fraser-McKelvie et al., 2026, A&A, 705, 1
Amelia Fraser-McKelvie; afraser@eso.org
My Discussion Qns
Diagnostics for central structures embedded within discs. What tools do we have at our disposal to differentiate between classical bulges and nuclear discs? What fraction of galaxies host both?
What future facility/survey do we need to place robust limits on the number of classical bulges in the local Universe?
How can we age-date bars (and hence decide whether they are long-lived structures)?
How linked are the evolutionary histories of discs and central structures?
Side note: Molecular gas tracers excellent for bar and ND diagnosis
Fraser-McKelvie et al., in prep.
Signatures of bars and nuclear discs in cold gas
PGC 044931
Amelia Fraser-McKelvie
ALMA
Side quest: