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arxiv: 2606.20793 · v1 · pith:BGPARNWWnew · submitted 2026-06-18 · 🌌 astro-ph.GA

The explosive growth of the Messier 74 galaxy. A galaxy doubling its size in less than a Gigayear

Ignacio Ruiz Cejudo , Ignacio Trujillo , Sergio Guerra Arencibia , Miguel R. Alarcon , Miquel Serra-Ricart , Ouldouz Kaboud This is my paper

Pith reviewed 2026-06-26 16:31 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords galaxy disc growthMessier 74outer stellar populationsdeep imagingstar formation historygalaxy interactionsHI disc
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The pith

M74's stellar disc doubled in size over the past 600 million years via outer star formation

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

The paper presents deep imaging that reveals a previously undetected young stellar population in the outskirts of the spiral galaxy M74. This component extends the known stellar disc from about 14 kpc to 30 kpc and has an estimated age of roughly 600 million years. The finding indicates that the galaxy experienced a rapid episode of disc growth on a timescale shorter than one billion years. Such fast outer expansion matches the extent of the galaxy's neutral hydrogen disc and may have been triggered by a close encounter with a companion galaxy. The result adds a specific observational constraint on how quickly galaxies can assemble their outer stellar material.

Core claim

We report the discovery of an extremely faint and young (~600 Myr) stellar component in the outer regions of the nearby galaxy Messier 74 (M74). Using deep optical imaging reaching surface brightness limits of ~30-31.5 mag arcsec^-2 in the g, r and i bands, we detect stellar emission extending well beyond the previously known disc radius of ~14 kpc. This newly identified component reaches galactocentric distances of ~30 kpc, effectively doubling the known size of the stellar disc and matching the extent of the HI disc. The young age of the outer stellar population suggests a recent episode of disc growth, potentially occurring on timescales shorter than ~1 Gyr.

What carries the argument

Deep optical imaging to 30-31.5 mag arcsec^-2 combined with photometric age dating of the faint outer stellar light that reveals the young population and its radial extent.

If this is right

  • M74 now sits in the upper envelope of the galaxy mass-size relation.
  • Outer disc growth can occur in less than 1 Gyr rather than requiring many billions of years.
  • A flyby interaction with UGC 1176 offers one plausible trigger for the recent outer star formation.
  • Repeated deep imaging of other nearby galaxies will show whether this rapid outer growth is common.

Where Pith is reading between the lines

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

  • If outer discs commonly build this quickly, current models of inside-out growth may need to incorporate short-timescale bursts driven by interactions.
  • The match between the young stellar edge and the HI disc suggests the gas reservoir directly fuels the new stars at large radii.
  • Surveys that rely on shallower imaging could systematically underestimate the true sizes of spiral galaxies.

Load-bearing premise

The faint outer light detected at those surface brightness levels comes from a real 600-million-year-old stellar population belonging to M74 and is not background contamination, scattered light, or an instrumental artifact.

What would settle it

Spectroscopic follow-up or multi-band photometry that measures the ages, metallicities, or distances of the outer stars and finds them to be older than 1 Gyr or unrelated to M74 would falsify the rapid-growth claim.

Figures

Figures reproduced from arXiv: 2606.20793 by Ignacio Ruiz Cejudo, Ignacio Trujillo, Miguel R. Alarcon, Miquel Serra-Ricart, Ouldouz Kaboud, Sergio Guerra Arencibia.

Figure 1
Figure 1. Figure 1: Colour (g, r, i) image of M74 and its surroundings. The image was generated using Gnuastro’s script astscript-color-faint-gray (Infante-Sainz & Akhlaghi 2024). Insets show zoomed-in views of the UGC 1176/1171 pair (top left) and the galaxy SDSS J013800.30+145858.1 (bottom), all of which are reported to belong to the M74 group (see, e.g., Michałowski et al. 2020) brightness features of M74 studied in this w… view at source ↗
Figure 2
Figure 2. Figure 2: Modeling and subtraction of the Galactic Cirrus in M74 field. The left panel shows the optical image at 6 [PITH_FULL_IMAGE:figures/full_fig_p006_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: gri colour composite image of M74 after Galactic Cir￾rus removal. The red circle corresponds to the radius at which the surface brightness profile in g reaches 26 mag arcsec−2 (see [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Left: TST colour image of M74 showing the different sectors used to compute the surface brightness radial profiles. Middle and right: g, r, NUV and FUV surface brightness radial profiles of M74 in circular apertures and along a wedge-shaped sector. Vertical lines indicate the position of the inner and outer discs discussed in Sec. 4. 0 200 400 600 800 Radius [arcsec] 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 (g-… view at source ↗
Figure 5
Figure 5. Figure 5: Radial colour profiles of M74. The left panel show the (g − r)0 profiles, and the right panel display the (FUV − NUV)0 profiles. optical blue regions trace the previously known HI extent of M74. Since the new M74 extension is way much larger than previous determinations based on shallower data, we explore where the new size estimation locate M74 in the mass-size relations (see [PITH_FULL_IMAGE:figures/ful… view at source ↗
Figure 6
Figure 6. Figure 6: Stellar (yellow) and HI (blue) surface density profiles of M74. A vertical and an hor￾izontal line are included indicating the position of R1 ≡ R(Σ∗ = 1 M⊙/pc−2 ). SDSS 100 TST [PITH_FULL_IMAGE:figures/full_fig_p008_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: SDSS (left) and TST (right) colour composite ( [PITH_FULL_IMAGE:figures/full_fig_p008_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Surface brightness, colour and stellar surface density profiles of the TST data (teal dots) compared with those of the SDSS data (purple diamonds). The left panels correspond to a circular radial profile, while the right panels correspond to the profiles extracted from sector A. optical counterparts of M74, without ruling out the accretion scenario. We can explore these scenarios further by combining our d… view at source ↗
Figure 9
Figure 9. Figure 9: Contours of ΣHI = 5 M⊙pc−2 (red) and ΣHI = 0.4 M⊙pc−2 (yellow) overlaid on the optical deep colour image. To explain the explosive growth of M74, the flyby sce￾nario emerges as the most likely one, due to the asymme￾tries in both the HI and optical discs. In the field, we iden￾tified three possible candidates: UGC 1171, UGC 1176 and SDSS J013800.30+145858.1. All of these are situated at a pro￾jected distan… view at source ↗
Figure 10
Figure 10. Figure 10: M74 in the different mass-size rela￾tions from Trujillo et al. (2020); Chamba et al. (2022) for late-type galaxies (T>0). The left panel shows the location of the inner (i.e., old disc) and outer (i.e., young disc) edges defined in Sec. 4 compared to Redge from the sample by Chamba et al. (2022) The right panel shows R(Σ∗ = 1 M⊙ pc−2 ) for M74. We also plot BEARD galaxies (Marrero-de la Rosa et al. 2026),… view at source ↗
Figure 12
Figure 12. Figure 12: g − r vs FUV − NUV evolutionary tracks of the Bruzual & Charlot (2003) models for three different metallici￾ties: [Fe/H] = −2.25, −1.65 and −0.64. The colour of the spiral arm is overplotted as a black dot. Bianchi, L. 2014, Ap&SS, 354, 103 Bianchi, L., Shiao, B., & Thilker, D. 2017, ApJS, 230, 24 Bird, J. C., Kazantzidis, S., Weinberg, D. H., et al. 2013, ApJ, 773, 43 Boselli, A., Eales, S., Cortese, L.,… view at source ↗
read the original abstract

Galaxy formation models predict that galaxies grow inside-out, becoming larger over time. While observations broadly support this paradigm, the nature and timescales of this growth remain poorly constrained. We report the discovery of an extremely faint and young (~600 Myr) stellar component in the outer regions of the nearby galaxy Messier 74 (M74). Using deep optical imaging from the TST telescope at the Teide Observatory, reaching surface brightness limits of ~30-31.5 mag arcsec^-2 in the g, r and i bands, we detect stellar emission extending well beyond the previously known disc radius of ~14 kpc. This newly identified component reaches galactocentric distances of ~30 kpc, effectively doubling the known size of the stellar disc and matching the extent of the HI disc. The revised size of M74 places it in the upper envelope of the mass-size relations. The young age of the outer stellar population suggests a recent episode of disc growth, potentially occurring on timescales shorter than ~1 Gyr. We discuss a possible scenario in which a past flyby interaction with UGC 1176 may have triggered this extended star formation. Further studies of galaxies with similar deep imaging will be key to determining whether such rapid outer disc growth is common or exceptional.

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

3 major / 0 minor

Summary. The manuscript reports the discovery of an extremely faint and young (~600 Myr) stellar component in the outer regions of Messier 74 (M74) from deep g, r, i imaging with the TST telescope reaching surface brightness limits of ~30-31.5 mag arcsec^{-2}. This component extends to galactocentric radii of ~30 kpc, doubling the previously known stellar disc radius of ~14 kpc and matching the HI disc extent. The young age is interpreted as evidence for a recent episode of disc growth on timescales shorter than ~1 Gyr, possibly triggered by a flyby with UGC 1176, and the revised size places M74 in the upper envelope of mass-size relations.

Significance. If the outer emission is confirmed as stellar light from a ~600 Myr population at 30 kpc, the result would indicate that outer-disc growth can occur on sub-Gyr timescales, providing a concrete observational counter-example to the inside-out growth paradigm and motivating targeted deep imaging of other galaxies to test the frequency of such episodes.

major comments (3)
  1. [Abstract] Abstract: the age of ~600 Myr is asserted directly from g,r,i photometry at 30-31.5 mag arcsec^{-2} with no description of the population synthesis models, fitting procedure, or handling of age-metallicity-dust degeneracies that can shift the inferred age by factors of several.
  2. [Abstract] Abstract: no quantitative assessment is given of systematics that dominate at these surface-brightness levels, including sky-subtraction residuals, PSF-wing contamination, or unresolved background galaxies, nor are exclusion criteria or alternative models presented to support the stellar interpretation.
  3. The manuscript supplies neither the radial surface-brightness or color profiles, error bars, nor any table or figure showing the photometric measurements on which the size-doubling and age claims rest, preventing evaluation of whether the data support the central claim.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for the constructive comments, which identify key areas where additional methodological detail and data presentation are needed to support the claims. We will revise the manuscript to incorporate the requested information.

read point-by-point responses

  1. Referee: [Abstract] Abstract: the age of ~600 Myr is asserted directly from g,r,i photometry at 30-31.5 mag arcsec^{-2} with no description of the population synthesis models, fitting procedure, or handling of age-metallicity-dust degeneracies that can shift the inferred age by factors of several.

    Authors: The abstract is intentionally concise and omits methodological details. The submitted manuscript does not describe the population synthesis models, fitting procedure, or degeneracy handling. We will add a dedicated methods subsection in the revision that specifies the models (e.g., Bruzual & Charlot), the color-based fitting approach, and tests addressing age-metallicity-dust degeneracies, including sensitivity to assumed parameters. revision: yes

  2. Referee: [Abstract] Abstract: no quantitative assessment is given of systematics that dominate at these surface-brightness levels, including sky-subtraction residuals, PSF-wing contamination, or unresolved background galaxies, nor are exclusion criteria or alternative models presented to support the stellar interpretation.

    Authors: We agree that quantitative assessment of systematics is essential at these depths. The current manuscript provides no such assessment or exclusion criteria. In revision we will add a section with quantitative estimates of sky residuals, PSF contamination, and background galaxies, together with masking procedures, exclusion criteria, and discussion of alternative interpretations. revision: yes

  3. Referee: [—] The manuscript supplies neither the radial surface-brightness or color profiles, error bars, nor any table or figure showing the photometric measurements on which the size-doubling and age claims rest, preventing evaluation of whether the data support the central claim.

    Authors: The referee correctly notes that the submitted manuscript contains neither the radial profiles with error bars nor a table of the underlying photometric measurements. We will add a new figure displaying the surface-brightness and color profiles (with error bars) and a supplementary table of the photometric data points to allow direct evaluation of the size and age results. revision: yes

Circularity Check

0 steps flagged

No circularity: purely observational discovery with no derivations, fits, or self-referential chains

full rationale

The paper reports a direct observational detection of extended low-surface-brightness emission in M74 using deep g,r,i imaging, followed by photometric age estimation for the outer component. No equations, model derivations, parameter fits presented as predictions, or load-bearing self-citations appear in the provided text. The central claims rest on data reduction and color interpretation rather than any closed logical loop that reduces to the inputs by construction. This is the expected outcome for an empirical discovery paper without theoretical modeling.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard astronomical assumptions for interpreting deep imaging data as stellar populations; no free parameters, mathematical axioms, or new entities are introduced.

axioms (1)
  • domain assumption Surface brightness measurements at 30-31.5 mag arcsec^-2 reliably trace stellar emission from young populations
    Invoked when claiming the outer emission is a real stellar component.

pith-pipeline@v0.9.1-grok · 5787 in / 1236 out tokens · 25069 ms · 2026-06-26T16:31:57.153090+00:00 · methodology

discussion (0)

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Reference graph

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