# Uncovering the multi-scale structure of dust distribution in nearby galaxies

Ella Tanchon

► To cite this version:

Ella Tanchon. Uncovering the multi-scale structure of dust distribution in nearby galaxies. 6th Complex Days, Académie Systèmes Complexes, Feb 2026, NICE, France. hal-05530466

HAL Id: hal-05530466

https://hal.science/hal-05530466v1

Submitted on 27 Feb 2026

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# Uncovering the multi-scale structure of dust distribution in nearby galaxies

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Ella Tanchon – 1st year PhD student

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# What is astronomy?

"The scientific study of the universe and of objects that exist naturally in space, such as the moon, the sun, planets, and stars."

*From the Cambridge Dictionary*

"Modern astronomers [...] study the **physical phenomena** that govern the **formation and behavior** of astronomical objects."

*From the International Astronomical Union*

<figure><img src="image_5.png" /><figcaption>Stars</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Planets</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Asteroid</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Nebulae</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Clusters</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Galaxies</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Pulsars</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Black Holes</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>CMB</figcaption></figure>

<figure><img src="image_14.png" /></figure>
# What is astronomy?

"The scientific study of the universe and of objects that exist naturally in space, such as the moon, the sun, planets, and stars."

*From the Cambridge Dictionary*

"Modern astronomers [...] study the **physical phenomena** that govern the **formation and behavior** of astronomical objects."

*From the International Astronomical Union*

<figure><img src="image_15.png" /><figcaption>Stars</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>Planets</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>Asteroid</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>Nebulae</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>Clusters</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>Galaxies</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>Pulsars</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>Black Holes</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>CMB</figcaption></figure>

<figure><img src="image_24.png" /></figure>
## How can we observe?

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From SKAO website

Different instruments and telescopes allow to observe different wavelengths, ...
# What can we observe?

<figure><img src="image_27.png" /></figure>

JWST - Infrared

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NASA, ESA, CSA, STScI, the PHANGS (Physics at High Angular resolution in Nearby GalaxieS) Team

HST - UV/Visible

Different wavelengths allow to observe different objects, features, physical processes, ...

NGC0628 in different wavelengths

<figure><img src="image_29.png" /></figure>
# What can we observe?

<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /><figcaption>IRAC HEALPix survey, color - Spitzer mission - JPL/NASA</figcaption></figure>

NGC0628 in infrared observed by
Spitzer (D = 0.85 m - top) and
JWST (D = 6 m - right)

NASA, ESA, CSA, STScI, the PHANGS (Physics at High Angular resolution in Nearby GalaxieS) Team

<figure><img src="image_32.png" /></figure>
# The PHANGS galaxy sample

Physics at High Angular resolution in Nearby GalaxieS

19 galaxies:

- nearby,

- massive star-forming galaxy,

- covering a wide range of morphology

We aim to understand the **link between dust emission processes and the different structures inside galaxies.**

<figure><img src="image_33.png" /></figure>

<figure><img src="image_34.png" /></figure>
# Dust composition in galaxies

Dust in the Interstellar Medium (ISM) is composed of **silicates** ($SiO_2$, MgO) and carbonaceous grains such as **Polycyclic Aromatic Hydrocarbons** (PAHs).

<figure><img src="image_35.png" /></figure>

B. T. Draine 2002.12.05

## Example of PAH molecules

From Draine (2003)

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NASA, ESA, CSA, STScI, T. Temim

## The Crab nebula observed by the JWST

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# The origin of dust emission

Dust is heated by the radiation emitted by stars. It reprocesses the incoming radiation in the infrared (IR). The incoming light can originate from young stars in star forming regions or from older stellar populations.

<figure><img src="image_38.png" /></figure>

Infrared emission spectrum for model with silicate and graphite/PAH grains in ISRF intensity scale factor U from 0.1 to 10⁴ (U = 1 is the local ISRF).

<figure><img src="image_39.png" /></figure>
# Dust emission in galaxies

Dust in the Interstellar Medium (ISM) is composed of **silicates** ($SiO_2$, MgO) and carbonaceous grains called **Polycyclic Aromatic Hydrocarbons** (PAHs)

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<figure><img src="image_42.png" /></figure>

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Adapted from Sutter et al. (2023)

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Two models of Mid Infrared emission compiled using the Code for Investigating GALaxy Emission (CIGALE, Boquien et al. 2019)
# Decomposition algorithm

We use the **Constrained Diffusion Method** (Li. 2022) to separate the different emission scales.

The image i contains the structures of sizes between Lᵢ and Lᵢ+1 parsec.

We build decomposed maps to understand the behavior of dust emission at different scales.

<figure><img src="image_46.png" /></figure>

From Tanchon et al. in prep
# Masking process

We use several **masks** to isolate
**morphological regions** such as:

- star forming regions;

- arms, bar, centre,...

<figure><img src="image_47.png" /></figure>

From Querejeta et al. (2021)

From Groves et al. (2023)

<figure><img src="image_48.png" /></figure>
# PAH transition

We investigate the **PAH fraction** (mass fraction of PAH with less than 1000 carbon atoms) at different scales. We observe a **break scale** around 200 pc.

<figure><img src="image_49.png" /></figure>

<figure><img src="image_50.png" /></figure>

From Tanchon et al. in prep
# PAH transition

We investigate the **PAH fraction** in different morphological environments.

<figure><img src="image_51.png" /></figure>

<figure><img src="image_52.png" /></figure>

From Tanchon et al. in prep
# Take home message

We can derive galactic properties and parameters from observation in different wavelengths;

Using novel decomposition methods, we can analyse different scales of dust emission

We observe a transition around 200 pc, showing the existence of a transition scale between stellar feedback in nebular regions and heating in the diffuse ISM.

A paper on this is under revision!

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<figure><img src="image_55.png" /></figure>
Thank you for your attention

Any questions ?

COMPLEX DAYS - 12/13 Feb 2026

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LAGRANGE

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OBSERVATOIRE
DE LA CÔTE D'AZUR
# Appendix - Constrained Diffusion algorithm

<figure><img src="image_58.png" /></figure>

Test image

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From Li (2022)

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Wavelet transform on two Gaussians

From Li (2022)

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# Appendix - Constrained Diffusion algorithm

Solve a modified version of the diffusion equation

$$I_{t_1} > 0 \qquad I_{t_1+\delta t}^{+} = \min(I_{t_1} * \mathcal{G}(\sqrt{2\delta t}), I_{t_1}),$$

$$I_{t_1} < 0 \qquad I_{t_1+\delta t}^{-} = \max(I_{t_1} * \mathcal{G}(\sqrt{2\delta t}), I_{t_1})$$

More accurate than Wavelet transform

Does not produce negative values

<figure><img src="image_62.png" /><figcaption>Wavelet transform on two Gaussians</figcaption></figure>

From Li (2022)

<figure><img src="image_63.png" /><figcaption>Constrained Diffusion on two Gaussians</figcaption></figure>