Evolution in the Dusty ISM Accross the Local Group

Large Magellanic Cloud
Warm dust
Cold dust
Hydrogen

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Evolution in the Dusty ISM
Across the Local Group
Chris Clark
with Julia Roman-Duval, Karl Gordon,
Caroline Bot, and Matthew Smith

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Chris Clark | HotSci Talk, 3rd August 2022
Eales+ (2010); Valiante+ (2016); Smith+ (2017)
Herschel observation of
~3 square degrees
(~0.2% of Herschel
extragalactic
survey area)
Importance of the Dust-to-Gas Ratio
• Dust-to-Gas ratio (D/G) dictates what
galaxies we can detect in ISM surveys.
(Majority of galaxies with detection of cold
ISM are detected only via cold dust emission.)
• Variation in D/G affects reliability of
using dust emission as gas mass tracer.
• Galaxies with higher D/G will tend to
suffer greater UV–optical extinction.

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Variation in the Dust-to-Gas Ratio (D/G)
De Vis+ (2019); Galliano+ (2018); De Cia+ (2016); Remy-Ruyer+ (2014)
“Critical Metallicity”
Transition
High metallicity → higher
D/G, due to more efficient
dust grain growth
Below “critical metallicity”,
grain growth can’t keep up
with dust destruction (shocks,
radiation), so D/G much lower
Location and behaviour of
‘critical metallicity’ transition
is very poorly constrained

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Roman-Duval+ (2022); De Vis+ (2019); Galliano+ (2018); De Cia+ (2016); Remy-Ruyer+ (2014)
⬟ D/G from FIR:
Compare dust mass from
far-IR data to gas mass
from HI & CO data
…or…
◆ D/G from UV:
Use UV absorption
spectroscopy to find
metal abundance in dust
phase vs gas phase, then
compare to H column
But, more UV-based D/G
measurements on the
way soon from Julia
Roman-Duval and
Aleksandra Hamanowicz!
Transition

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Questions That Need Answering
Roman-Duval+ (2022); De Vis+ (2019); Galliano+ (2018); De Cia+ (2016); Remy-Ruyer+ (2014)
• Where is the ‘critical metallicity’
transition in the dust-to-gas ratio?
• How much grain growth happens at
different metallicities?
• How much does the amount of grain
growth vary between environments?
• Why is there apparently dust “missing”
from some FIR estimates of D/G?
Transition

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Well-Resolved Local Group Galaxies
Clark+ (2021)
Herschel
250 μm dust
emission
Dist = 50 kpc
M
⋆
= 3 x 109 M
⊙
Z = 0.5 Z
⊙
M31
Dist = 790 kpc
M
⋆
= 1 x 1011 M
⊙
Z ~ 1.3 Z
⊙
Small Magellanic Cloud
Dist = 62 kpc
M
⋆
= 2 x 108 M
⊙
Z = 0.2 Z
⊙
M33
Dist = 840 kpc
M
⋆
= 5 x 109 M
⊙
Z = 0.5 Z
⊙

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Well-Resolved Local Group Galaxies
Clark+ (2021)
Herschel
250 μm dust
emission
H surface
density from
HI and CO

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Clark+ (2021); Meixner+ (2013); Roman-Duval+ (2014)
A Herschel extragalactic
field; approximate area of
Moon and Hubble Deep
Field shown to left for
comparison.
Portion of LMC seen
in Herschel 250 μm
Herschel Data for Local Group Galaxies
• Resolution of 7–36”, giving
physical resolution better than
15pc in LMC.
• 5 photometric filters (100, 160,
250, 350, and 500 μm),
spanning whole dust SED peak.
• Provides great constraints on
SED shape, therefore dust
mass, temperature, etc.

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Clark+ (2021)
Emission on large angular
scales removed from
Herschel maps during the
data reduction
Herschel data therefore
missing lowest density ISM,
whilst IRAS (& Planck) data
missing highest density ISM
Extended Emission Missing from Herschel
IRAS 100 μm
300” resolution
Herschel 100 μm
10” resolution

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Combine Alllll the Data in Fourier Space…
Clark+ (2021)
COBE
Far-infrared data,
large angular scales
IRAS
Far-infrared data,
medium angular scales
Planck
Submm data, large &
COBE x IRAS
FIR data, large and
(COBE x IRAS)
+ Planck
FIR-submm data, large &
Herschel
FIR-submm data,
small angular scales
((COBE x IRAS)
+ Planck) x Herschel
FIR-submm data, large &
medium & small angular scales
x → “Feathered with”
+ → “In concert with”

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Restoring Extended Emission by Feathering
Clark+ (2021)
Herschel only
Herschel feathered with
IRAS, Planck, and COBE

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SED Fitting Our New Herschel Data
Clark+ (subm.); Gordon+ (2014)
Every pixel’s Spectral
Energy Distribution
(SED) fit using a broken-
emissivity modified
blackbody model.

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Pixel-by-Pixel Dust-to-Gas Ratio
Clark+ (subm)

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Strong Evolution in G/D versus Density
Clark+ (subm.)
Over 1 dex increases in G/D
with density suggest very
significant grain growth
H surface density values on x-axis
have been corrected for
inclination.
We can probe to 10x higher
densities in LMC and SMC,
because they are ~10x closer
than M31 and M33, hence have
~10x better density resolution.

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Chemical Evolution: Model vs Observed
Clark+ (subm.); Asano+ (2013
Asano et al. (2013) model tracks:
Parameters:
Average grain size
Dust temperature
ISM metallicity
Grain-growth episode timescale
Minimum dust-to-metals ratio
Column-to-volume density conversion

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Clark+ (2018); Clark+ (2019)
What Causes the Turnover in D/G?
Possible explanations for turnover in D/G
versus H:
• SED fitting breaks down in some way
above a certain density?
• Dark gas (atomic and/or molecular) at
intermediate densities, meaning D/G
gets overestimated (Works for M31 &
M33, but not LMC…)
• Dust destruction by hot gas?
• CO X-factor overestimated at higher
densities, driving down D/G? (Would
require very low Xco for M31, and
especially M33...)
• Dust mass absorption coefficient
falling at higher densities?
• Noise-induced anticorrelation?

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Roman-Duval+ (2022a); Clark+ (subm.); De Vis+ (2019); Galliano+ (2018); Remy-Ruyer+ (2014)
⬟ D/G from FIR:
from HI & CO data
…or…
◆ D/G from UV:
Use UV absorption
compare to H column

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D/G from FIR versus D/G from UV
Clark+ (subm.); Roman-Duval+ (2022); Roman-Duval+ (2017)
Our new FIR D/G, and
the UV spectroscopic
D/G, now agree to
within 17%

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Clark+ (subm.); Roman-Duval+ (2022); Roman-Duval+ (2017); Jenkins & Wallerstein (2017)
Discrepancy between
UV and FIR
measurements reduced
from 8x to 3x

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Clark+ (subm.); Roman-Duval+ (2022); Roman-Duval+ (2017); Jenkins & Wallerstein (2017)
x2.96
offset
If dust mass absorption
coefficient was 2.96x
smaller in SMC than
assumed for the other
galaxies, then the
disagreement
disappears
Dust at SMC
metallicities is known to
have different physical
properties to dust at
higher metallicities

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Roman-Duval+ (2022); Clark+ (subm.); De Vis+ (2019); Galliano+ (2018); Remy-Ruyer+ (2014)
⬟ D/G from previous FIR
◆ D/G from UV
⬢ D/G from our new FIR

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Warm dust
Cold dust
Hydrogen

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Conclusions & Takeaways
Clark+ (subm.)
• Our new Herschel maps of Local Group galaxies M31, M33, LMC, and SMC, restore the
previously-removed large–scale dust emission. Data publicly available!
• Combination of high resolution, and large-scale sensitivity, allows us to probe a very
wide of ISM densities (up to 2 dex), and explore dust grain growth.
• We find evidence for strong grain-growth,
causing over 1 dex of evolution in D/G ratio.
• However, D/G, and how it evolves, is not set
purely by a galaxy’s mass and metallicity.
• “Turnover” in D/G at higher densities, and
the lingering discrepancy between UV
depletion and FIR values of D/G for the
SMC, hint at possible changes in dust mass
absorption coefficient.

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Image credit: ESA
Herschel recap:
• ESA mission, from 2009–2013
• 3.5m primary mirror
• Photometry & spectroscopy
from 52–670 μm
• Six photometric filters, observed
using two instruments:
PACS: 70, 100, 160 μm
SPIRE: 250, 350, 500 μm
• Resolution 7–36 arcsec

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Missing Diffuse Emission in Herschel
Clark+ (2021)
These patches of background have
roughly the same brightness
These patches of background
have very different brightness
Superbubbles contain
significant emission
Superbubbles have had
almost all emission removed
100 μm observations of the Large
Magellanic Cloud, from IRAS and Herschel
IRAS 100 μm = 300 arcsec resolution
Herschel 100 μm = 10 arcsec resolution

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Power Spectra of Data
Clark+ (2021)

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Relative Change due to Feathering
Clark+ (2021)
LMC SMC
M31 M33
Plots showing how much
emission was restored.
Contours show outlines of
the galaxies.
(Regions can also have flux
“removed”, because the
zero-point in the old
Herschel maps was
somewhat arbitrary. Our
feathering also restored
large-scale MW cirrus
structure in foreground,
making some regions
become fainter.)

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Relative Difference When Feathering
Clark+ (2021)
LMC SMC
M31 M33

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Foreground Subtraction
Roman-Duval+ (2017); Clark+ (subm.)

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More Emission Restored at Shorter λ
Clark+ (2021)
More emission needed to be
restored in the shorter-
wavelength Herschel bands than
in the longer wavelengths
Therefore, In new maps,
galaxies are significantly bluer,
especially in diffuse peripheries
(20–30% bluer 500/160 colour)
The dust temperature is main
driver of FIR SED colour, so
change in colour will significantly
modelled dust mass

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D/G Turnover Caused by Falling ⍺CO
?
Clark+ (2021)

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Composition vs Density in MW, LMC, SMC
Clark+ (2021); Roman-Duval+ (2022a)

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Clark+ (2018); Clark+ (2019); Roman-Duval+ (2022a)
Dust Emissivity Changes With ISM Density?
H Surface Density
Dust Mass Absorption Coefficient
SMC Dust Composition
Recent work shows that chemical
composition of dust changes with
density – and does so differently in
SMC vs LMC vs Milky Way
(see Roman-Duval+ 2022a)
Previous work found evidence
for dust mass absorption
coefficient falling with density.
Milky Way Dust Composition

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D/G Evolution vs Ionised Gas Density
Clark+ (2021)

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D/G Evolution with Degraded Resolution
Clark+ (2021)

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Dust MAC Falls at Higher Density?
Clark+ (subm.); Clark+ (2019)
Fainter points:
Dust mass absorption
coefficient is constant
Bold points:
Dust mass absorption
coefficient falls according
to power law of -0.4 above
a transition density
(M31 & M33 transition density
= 4 M
⊙
pc-2
LMC transition density
= 40 M
⊙
pc-2)

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Dark Gas Candidates in the LMC?
Clark+ (subm.); Clark+ (2019)
4 times less H observed
from HI and CO, than
predicted from dust data
(when using overall LMC
trends for dust vs H).

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D/G of Damped Lyman-α Systems
Roman-Duval+ (subm.); De Vis+ (2019); Galliano+ (2018); De Cia+ (2016); Remy-Ruyer+ (2014)
D/G from emission:
from HI & CO data
…or…
D/G from depletions:
Use UV absorption
compare to H column

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Evolution in the Dusty ISM Accross the Local Group

Evolution in the Dusty ISM Accross the Local Group

Chris Clark

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