Introduction Data and Models Methods Results Summary

The VANDELS survey: Dust attenuation instar-forming galaxies at z = 3− 4

Cullen, F., et al.arXiv:1712.01292, accepted by MNRAS

March 7, 2018

Introduction Data and Models Methods Results Summary

Introduction

• Extinction: individual sight-lines, absorption + scattering outof the line-of-sight, related to the size distribution andcomposition of the dust grains

• Attenuation: extended sources, extinction + scattering backinto the line-of-sight, a strong dependence on the dustgeometry

• Main complication: the intrinsic shape of a galaxy SED is lesswell constrained

Some key questions on attenuation curve:

• changes with galaxy properties?

• redshift evolution?

• the presence/absence of the 2175 A UV bump?

Introduction Data and Models Methods Results Summary

Method 1

• Use an independent indicator to identify galaxies that aremore, or less, affected by dust, compare their observed SEDs

• E.g., Balmer decrement Hα/Hβ (Calzetti et al. 1994, 2000;Reddy et al. 2015, Battisti et al. 2016, 2017a, 2017b)

• Advantage: not having to assume the shape of a dust-freeSED

• Disadvantage: sensitive to the total-to-selective attenuationlaw, rather than the attenuation law directly

Introduction Data and Models Methods Results Summary

Method 2

• Assume/construct the underlying intrinsic SED of all sources,compare the observed SEDs with the intrinsic SED

• E.g., Scoville et al. 2015

• Advantage: directly probe the wavelength dependence of theattenuation curve

• Disadvantage: suffers from an increasing systematic errorsrelated to the choice of the intrinsic SED

• The problems will be mitigated somewhat at high redshiftswhere star-formation histories are predicted to become selfsimilar across the galaxy population

Introduction Data and Models Methods Results Summary

Sample selection

VANDELS Survey:

• ESO spectroscopic survey

• N ≈ 2000 galaxies at 1.0 < z < 7.0 inCDFS and UDS

• Instrument: VLT/VIMOS

• R ≈ 600, λ = 4800− 10000 A

• To be completed in January 2018

Selecting from 1502 VANDELS spectra:

• IAB ≤ 27.5 and HAB ≤ 27 (originalVANDELS selection criteria)

• the most secure redshift of3.0 ≤ z ≤ 4.0 (242)

• log(M∗/M�) ≤ 10.6 (236)

Final sample:

• 1.6× 108 ≤M∗/M� < 4.0× 1010,〈M∗〉 = 4.5× 109 M�

• 〈z〉 = 3.49

8.5 9.0 9.5 10.0 10.5 11.0log10(M? / M�)

−1.0

−0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

log 1

0(S

FR

/M�

yr−

1)

Speagle et al. 2014

Schreiber et al. 2015

Introduction Data and Models Methods Results Summary

Photometry

Four samples: CDFS-HST, CDFS-GROUND, UDS-HST, andUDS-GROUND

• CDFS-HST and UDS-HST: ∼ 50%, within CANDELS regions(CDFS and UDS), deep optical to NIR HST imaging,Spitzer/IRAC observations

• λ = 0.5− 8.0 µm, rest-frame UV to NIR SED out toλrest ∼ 2.0 µm

• CDFS-GROUND and UDS-GROUND: ∼ 50%, relies onpredominantly ground-based imaging

• λ only extends out the K-band (∼ 2.2 µm), rest-frame UV tooptical SED out to λrest ∼ 0.6 µm

Introduction Data and Models Methods Results Summary

Observed SED

Two publicly-available SED-fitting codes:

• EAZY (Brammer et al. 2008)

• linear combinations of a limitedstandard template set

• fixing z to spectroscopic redshiftmeasured from the VANDELS spectra

• LePhare (Ilbert et al. 2008)

• determine M∗

• Setting: constant SFH, BC03, Calzettiet al. (2000) attenuation curve

• Final observed SEDs: averaged thebest-fitting LePhare and EAZY SEDs

10−20

10−19

10−18

log 1

0(fλ

/er

g/s/

cm2/A

)

CDFS-HSTz=3.704 χ2

r = 0.35

10−20

10−19

10−18

log 1

0(fλ

/er

g/s/

cm2/A

)

CDFS-GROUNDz=3.403 χ2

r = 0.90

−0.2 0.0 0.2 0.4 0.6log10(λobs/µm)

10−20

10−19

10−18

log 1

0(fλ

/er

g/s/

cm2/A

)

UDS-HSTz=3.969 χ2

r = 2.33

Introduction Data and Models Methods Results Summary

A homogeneous SED shape

Assumption: all galaxies in the sample have the same intrinsic UVto optical SED shape

• First Billion Years (FiBY) simulation

• Sample: 628 galaxies with8.2 ≤ log(M/M�) ≤ 10.6 at z = 4

• Based on the similarity of SFHs, andthe narrow range in metallicity, it isreasonable to assume that theunderlying SED shape will be similaracross all masses.

200 400 600 800 1000 1200Lookback Time / Myr

−2.0

−1.5

−1.0

−0.5

0.0

0.5

1.0

log 1

0(S

FR

/〈S

FR〉)

8.2 < log10(M?/M�) < 9.0

9.0 < log10(M?/M�) < 10.0

10.0 < log10(M?/M�) < 10.6

9 10log(M?/M�)

0.1

0.2

0.3

0.4

0.5

0.6

Z/Z�

Introduction Data and Models Methods Results Summary

Intrinsic SEDs:

Construct intrinsic SEDs:

• Assign each star particle an instantaneousstarburst SPS model based on age andmetallicity

• Sum up SEDs of all individual starparticles for each galaxy

• Pair each VANDELS spectrum with FiBYgalaxy based on mass

• Stack matched SEDs of FiBY galaxies

0.2 0.3 0.4 0.5 0.6λ / µm

100

101

102

log 1

0(F

λ/F

5500

)

FiBYz4-BPASSv2-100

FiBYz4-BPASSv2-100bin

FiBYz4-BPASSv2-300

FiBYz4-BPASSv2-300bin

SB99-v000-z002

SB99-v000-z008

Introduction Data and Models Methods Results Summary

Intrinsic SEDs:

Construct intrinsic SEDs:

• BPASSv2 + CLOUDY

• BPASSv2-100bin: binary star evolution +an IMF cutoff of 100 M�

• BPASSv2-100: single-star evolution + anIMF cutoff of 100 M�

• BPASSv2-300bin: binary star evolution +an IMF cutoff of 300 M�

• BPASSv2-300: single-star evolution + anIMF cutoff of 300 M�

• S99-v00-z***: constant SFH +Starburst99 + 100 Myr +Z∗ = 0.002(0.008)

0.2 0.3 0.4 0.5 0.6λ / µm

100

101

102

log 1

0(F

λ/F

5500

)

FiBYz4-BPASSv2-100

FiBYz4-BPASSv2-100bin

FiBYz4-BPASSv2-300

FiBYz4-BPASSv2-300bin

SB99-v000-z002

SB99-v000-z008

Introduction Data and Models Methods Results Summary

Fitting method

Basic formula:

fλ,obs = φfλ,inte−τλ

Aλ = 1.086τλ

1.086× ln(fλ,int/fλ,obs) = Aλ −AV

Adopting a second-order polynomial as a function of x = 1/λ forAλ:

Aλ = a0x+ a1x2

1.086× ln(fλ,int/fλ,obs) = a0x+ a1x2 −AV

RV =AV

AB −AVkλ =

AλRVAV

Introduction Data and Models Methods Results Summary

Wavelength windows for fitting

Wavelength pixels used to fit:

• an estimated nebular contribution of < 10%

• free from stellar absorption/emission features

0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60Wavelength / µm

log(

Fλ)

fesc = 0.0

fesc = 0.5

fesc = 1.0

Introduction Data and Models Methods Results Summary

An example fit

2 3 4 5 6 7 8

λ−1 / µm−1

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

1.08

6ln

(Fi/

Fo)

0.2 0.3 0.4 0.5 0.6

λ / µm

1

2

3

4

5

6

7

Aλ/A

V

Calzetti

SMC

Reddy+ 2015

Charlot+Fall 2000

Introduction Data and Models Methods Results Summary

Simple Simulation

• generated 1000 M∗ within8.2 ≤ log(M∗/M�) ≤ 10.6

• assigned AV using the AV −M∗relation for a Calzetti law fromMcLure et al. (2017a)

• FiBY-BPASSv2-100bin template +nebular component

• perturbed flux assuming a 20% error

Using the defined wavelength regions, it ispossible to recover the underlying Calzettilaw and the input AV −M∗ relation.

0.2 0.3 0.4 0.5 0.6

λ / µm

1.0

1.5

2.0

2.5

3.0

3.5

Aλ/A

V

8.5 9.0 9.5 10.0 10.5log(M∗/M�)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

AV

Introduction Data and Models Methods Results Summary

Average shape of the attenuation curve

1

2

3

4

Aλ/A

V

FiBY-BPASSv2-100: N=178/236

Calzetti

SMC

Reddy+ 2015

Charlot+Fall 2000

FiBY-BPASSv2-100bin: N=224/236

1

2

3

4

Aλ/A

V

FiBY-BPASSv2-300: N=192/236 FiBY-BPASSv2-300bin: N=229/236

0.2 0.3 0.4 0.5 0.6

Wavelength / µm

1

2

3

4

Aλ/A

V

SB99-v00-z002: N=233/236

0.2 0.3 0.4 0.5 0.6

Wavelength / µm

SB99-v00-z008: N=232/236

• Failed fit: when observedSED was bluer than theintrinsic SED

• ffailed < 5% for half ofthe templates

• The majority ofattenuation curves areshallow and Calzetti-likein shape.

Introduction Data and Models Methods Results Summary

AV −M∗ relation

• McLure et al. (2017a): 8407galaxies at 2 < z < 3

• S99-v00-z008, FiBY-BPASSv2binary star templates: excellentagreement with both the McLureet al. (2017a) data and theempirical relation for a Calzettidust law at log(M∗/M�) > 9.5.

• S99-v00-z002, FiBY-BPASSv2single-star templates: incompatiblewith the McLure et al. (2017a)data

8.5 9.0 9.5 10.0 10.5 11.0log10(M?/M�)

0.0

0.5

1.0

1.5

2.0

AV

FiBY-BPASSv2-300bin

Cal

zett

i

SMC

McLure et al. 2017

Cullen et al. 2017

Introduction Data and Models Methods Results Summary

AV −M∗ relation

8.5 9.0 9.5 10.0 10.5 11.0

log10(M?/M�)

0

1

2

3

4

5

A16

00

This work

This work - Calzetti

This work - SMC

This work - Calzetti/SMC

AM+2016

Pannella+2015

Heinis+2014

Ibar+2013

Kashino+2013

Garn+Best 2010

8.5 9.0 9.5 10.0 10.5 11.0log10(M?/M�)

0.0

0.5

1.0

1.5

2.0

AV

FiBY-BPASSv2-300bin

Cal

zett

i

SMC

McLure et al. 2017

Cullen et al. 2017

• M∗ is a good proxy for attenuation.

• The AV −M∗ relation for normal star-forming galaxies with log(M∗/M�) > 9.5does not evolve between z = 0 to z ∼ 5.

Introduction Data and Models Methods Results Summary

Parameterization of the attenuation curve

Taking FiBY-BPASSv2-300bin as fiducial intrinsic SED template:

AλAV

=0.587

λ− 0.020

λ2, 0.12 < λ < 0.63 µm

kλ =2.454

λ− 0.084

λ2

RV = 4.18± 0.29

The results are fully consistentwith the Calzetti attenuationlaw.

0.2 0.3 0.4 0.5 0.6Wavelength / µm

4

6

8

10

12

14

16

18

kλ

Reddy

SMC

Calzetti

VANDELS

3.0 3.5 4.0 4.5 5.0RV

0

10

20

30

40

50

60

N

Cal

zett

iet

al.

2000

Introduction Data and Models Methods Results Summary

Attenuation curve at low masses

• constructed stacked intrinsicSED from FiBY withlog(M∗/M) < 9.0

• a steepening of the attenuationcurve at the lowest M∗

• remain consistent with theCalzetti law within 1σ, and ruleout SMC-like curve at > 1σ

0.15 0.20 0.25 0.30

Wavelength / µm

1.5

2.0

2.5

3.0

3.5

Aλ

/AV

Calzetti

SMC

FiBY-BPASSv2-300bin

S99-v00-z008

Introduction Data and Models Methods Results Summary

Evidence for a 2175 A UV bump?

Aλ =AV4.05

[k(λ)Calzetti +D(λ)]

D(λ) =Eb(λ∆λ)2

(λ2 − λ20) + (λ∆λ)2

where λ0 = 2175 A ∆λ = 350 A,AV = 0.8.

• 122 galaxies at 3.0 < z < 3.5

• robustly rule out the presence ofa strong UV bump withEb > 1.0

• a weak 2175 A UV bumpfeature with Eb ∼ 1

0.17 0.18 0.19 0.20 0.21 0.22

Wavelength / µm

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Fλ

/10−

19er

g/s/

cm2 /A

Eb = 0.0

Eb = 1.0

Eb = 2.0

Eb = 4.0

Introduction Data and Models Methods Results Summary

Summary

• The intrinsic shape of the UVoptical SED of SFGs at z = 3− 4 isapproximately constant across 8.2 < log(M∗/M�) < 10.6.

• The average attenuation curve shapes are consistent with a greyCalzetti-like attenuation law within ±1σ.

• No evidence is found for a steep SMC-like attenuation law.

• A subset of the intrinsic template sets yield results which areconsistent with the observed data, and with the AV −M∗ relationpredicted for a Calzetti-like attenuation law.

• The AV −M∗ relation does not evolve over 0 < z < 5.

• RV = 4.18± 0.29 (the original Calzetti value of RV = 4.05± 0.80).

• Tentative evidence for steeper attenuation curve shapes atlog(M∗/M�) < 9.0 is found.

• A weak 2175 A UV bump in the average attenuation curve at z ∼ 3is suggested by spectra of 122 galaxies at 3.0 < z < 3.5.