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Relating optical attenuation, dust & gas

at sub-kpc scales

Evangelos Paspaliaris

Simone Bianchi, Edvige Corbelli and Alice Concas

6th Workshop on Millimetre Astronomy in Italy

Italian ALMA Regional Center, CNR Bologna, June 2025

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2

dust continuum

emission

optical

attenuation

molecular and atomic

line emission

H2

CO

H2

HI

Our purpose

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Explore the correlations among these properties, in a single sample.

Salvestrini et al. (2025)

Casasola et al. (2022)

Concas and Popesso (2019)

Barrera-Ballesteros et al. (2020)

Boquien et al. (2013)

Kreckel et al. (2013)

Farley et al. (2020)

Draine (2024)

© NASA, ESA, R. O’ Connel, B. Whitemore, M. Dopita

and the WFC3 Science Oversight Committee

Abduro’uf et al. (2022)

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NGC 0628

NGC 3351

NGC 3627

NGC 4254

NGC 4321

Sample

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FUV

NUV

u

g

r

i

z

J

H

Ks

3.4μm

12μm

22μm

3.6μm

4.5μm

5.8μm

8.0μm

100μm

160μm

250μm

70μm

CO

HI

Hα

Hβ

ustPedia

FUV - FIR

HERACLES

THINGS

VIVA

MUSE IFU data

Clark et al. (2018)

Davies et al. (20019)

(Walter et al. 2008)

(Chung et al. 2009)

VLA data

IRAM-30m data

Leroy et al. (2009)

NGC 0628

Emsellem et al. (2009)

Data

18” resolution

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Our methods

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i. atomic gas surface density

ii. molecular gas surface density

iii. optical attenuation (dust in absorption)

iv. SED-fitting

Leroy et al. (2012)

Leroy et al. (2012)

Calzetti et al. (2012)

Cardelli et al. (1989)

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¹²CO(2-1) [1.3 mm] - IRAM

HI [21 cm] - VLA

Hα 6562Å

Hβ 4861Å

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Our methods

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i. atomic gas surface density

ii. molecular gas surface density

iii. optical attenuation (dust in absorption)

iv. SED-fitting

Leroy et al. (2012)

Leroy et al. (2012)

Calzetti et al. (2012)

Cardelli et al. (1989)

pixel-by-pixel

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¹²CO(2-1) [1.3 mm] - IRAM

HI [21 cm] - VLA

Hα 6562Å

Hβ 4861Å

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Our methods - SED fitting

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iv. SED-fitting

pixel-by-pixel

CIGALE

Boquien et al. (2019)

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FUV-FIR photometry of each pixel

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Our methods - SED fitting

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iv. SED-fitting

pixel-by-pixel

CIGALE

Boquien et al. (2019)

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FUV-FIR photometry of each pixel

Spectral energy distribution (SED)

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Dust as gas tracer

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atomic

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Dust as gas tracer

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0.0033

(lowest)

0.0041

0.0048

0.0054

0.0069

(highest)

“ΣHI decreases with increasing dust-to-gas ratio, indicating the shielding layers.”

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atomic

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Dust as gas tracer

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dust

HI

H2

CO

H2

0.0033

(lowest)

0.0041

0.0048

0.0054

0.0069

(highest)

“ΣHI decreases with increasing dust-to-gas ratio, indicating the shielding layers.”

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atomic

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Dust as gas tracer

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dust

HI

H2

CO

H2

0.0033

(lowest)

0.0041

0.0048

0.0054

0.0069

(highest)

“ΣHI decreases with increasing dust-to-gas ratio, indicating the shielding layers.”

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H2

CO

H2

HI

atomic

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Dust as gas tracer

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atomic

molecular

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Dust as gas tracer

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“Σdust good tracer of molecular and total gas.”

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atomic

molecular

total

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AV, BD - gas relation

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atomic

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AV, BD - gas relation

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atomic

molecular

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AV, BD - gas relation

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“ΑV, BD better tracer of total and molecular gas.”

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atomic

total

molecular

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AV, BD - AV, SED

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“For SF regions AV, SED / AV, BD = 0.59±0.05, slightly higher than previous studies”

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all stars

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AV, BD - AV, SED

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all stars

old stars

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AV, BD - AV, SED

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± 0.5 dex

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all stars

old stars

young stars

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AV, BD - AV, SED

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88.6%

± 0.5 dex

“AV, SED could serve as a tracer of the attenuation in larger samples”

young

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all stars

old stars

young stars

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The correlation of dust in absorption and dust in emission

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*models estimated with TRADING RT code (Bianchi 2008)

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BD

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The correlation of dust in absorption and dust in emission

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screen

Disney et al. (1989)

Calzetti et al. (1994)

*models estimated with TRADING RT code (Bianchi 2008)

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BD

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The correlation of dust in absorption and dust in emission

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screen

slab

Disney et al. (1989)

Calzetti et al. (1994)

*models estimated with TRADING RT code (Bianchi 2008)

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BD

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The correlation of dust in absorption and dust in emission

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sandwich

screen

slab

Disney et al. (1989)

Calzetti et al. (1994)

*models estimated with TRADING RT code (Bianchi 2008)

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BD

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The correlation of dust in absorption and dust in emission

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BD

old stars

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The correlation of dust in absorption and dust in emission

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BD

old stars

young stars

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Calzetti (2001)

Dust and stars distribution

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Calzetti (2001)

Dust and stars distribution

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Calzetti (2001)

Dust and stars distribution

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  • Σdust and AV,BD : better tracers of molecular and total gas rather—————— than of atomic gas (in regions traced, with fH2 > 0.4).
  • ΣHI varies from galaxy to galaxy and relates to the column density that gas needs to shield the molecular phase.
  • We find the AV, SED/AV, BD = 0.59±0.05 slightly higher than previous works.
  • AV, BD and AV, SED being in a good agreement, are affected by a combination of screen+mixed models.
  • AV, SED is explained by the assumption of a mixed model (slab, sandwich)

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Conclusions

young

old

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​

Thank you!

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extra slides

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Observational data

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NGC 0628

NGC 3351

NGC 3627

MUSE coverage

NGC 0628

NGC 3351

NGC 3627

NGC 4254

NGC 4321

MUSE

MUSE

MUSE

MUSE

MUSE

MUSE

NGC 0628

5 kpc

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  • 3σ filtering
  • At least 2 Herschel bands
  • At least eleven bands in total
  • 1/5 < best/bayes < 5

Pixels selection

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Resolution selection

GALEX FUV

SPIRE 250

SPIRE 350

SPIRE 500

18”

25"

36"

4.3”

18"

25"

36"

NGC 0628

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Minimal consequence

Resolution selection

NGC 0628

We selected the lowest resolution (18")

See also Aniano et al. (2012), Chastenet et al. (2024)

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CIGALE parameter space

Nersesian et al. (2019)

+

Our grid 99 792 000 to 299 376 000 models

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Our methods - SED fitting

200Myr

Delayed star-formation history

Modified starburst Attenuation Law

Metallicity: free parameter

Brazzini et al. (2024)

Ciesla et al. (2018)

Calzetti et al. (2000); Leitherer et al. (2002)

The modules

Nersesian et al. (2019)

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R21 & αco

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Σdust - ΣHI

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Individual galaxies

scaling relations

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  • dust - catalyst: atomic to molecular gas ———————> star formation
  • dust - absorbs stellar light —> re-emit in infrared
  • young stars - ionise gas —> SFR indicator
  • gas: fuel for star formation

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Dust-gas interplay

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Previous studies

(dust-gas)

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ustPedia

In global scale

“Dust correlates better with H2 and total gas”

Salvestrini et al. (2025)

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Previous studies

(dust-gas)

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ustPedia

In resolved scale

“Dust traces better the total gas”

Casasola et al. (2022)

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Previous studies

(BD-gas)

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In global scale

“BD correlates better with the molecular gas”

Concas and Popesso (2019)

xCOLD GASS

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Previous studies

(Av-gas)

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In various scales

“AV,BD is a good tracer of total gas in all scales”

¡ but with a theoretical HI !

Barrera-Ballesteros et al. (2020)

CALIFA

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Previous studies

(AFUV-gas)

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“AFUV mildly traces the molecular and better the total gas”

Boquien et al. (2013)

HRS

VNGS

HI

H2

Total gas

In resolved scale

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Previous studies

(Av-dust)

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“BD correlates with dust, but stellar continuum does not sample well the dust distribution”

Kreckel et al. (2013)

In resolved scale

Balmer decrement

Stellar continuum

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