miroslava dessauges-zavadsky

Giant molecular clouds seen 8 billion years ago

Miroslava Dessauges-ZavadskyObservatoire de Genève, Université de Genève

with Johan Richard, Françoise Combes, Daniel Schaererand others

Submitted to Science

★Morphology of high-redshift galaxies

Cowie+95Conselice+04

Elmegreen+05,07,09

HST rest-frame UV observations haveshown that galaxies at the peak of thecosmic star formation activity do not followthe Hubble classification.

About 60% of galaxies at z ~ 2−3 have aclumpy morphology whatever the stellarmass of the galaxy.

Guo+15 study done over 3239 galaxiesfrom CANDELS/GOODS-S and UDS



Shibuya+16 17’000 galaxies fromHST legacy data

The fraction of clumpy galaxies is evolvingover z ~ 0−8 and is peaking around z = 2.

About 60% of galaxies at z ~ 2−3 have aclumpy morphology whatever the stellarmass of the galaxy.



★Origin of high-redshift clumps

Clumps were initially believed to have an external origin of accretedcores of satellite galaxies following interaction/merger events.

The formation processes of these UV-bright clumps with 105.5-9.5 M¤ stellar massesand 30-300 pc radii (MDZ+17; Cava+18) are still intensely debated:

The in-situ clump origin wasfavored when near-IR IFU kinematicstudies started to report a largeproportion of z~1−2.5 galaxiesdominated by ordered disk rotation(75% at z~1 / 60% at z~2)(e.g., Förster Schreiber+09; Wisnioski+15; Rodrigues+16; Harrison+17; Swinbank+17;Girard/MDZ+18)

Turner+17

Why ???

★Origin of high-redshift clumps

Wisnioski+15

SFR

(M¤

yr-1

)

Stellar mass (M¤)

These high-redshift disks yet are very different from their local counterparts:

strongly star-forming, gas-rich, highly turbulent, and marginally stable.

Numerical simulations suggest that high-redshift galaxies are subject to violent instabilitiesthat fragment their disk and form in-situ gravitationally bound gas clouds, theprogenitors of the observed massive star clusters.(Dekel+09; Agertz+09; Bournaud+10,14; Ceverino+10,12; Tamburello+15; Behrendt+16; Mandelker+14,17; Oklopčić+17)

M★(disk) = 4.5×1010 M¤ / vmax = 200 km/s / fgas = 0.5 Tamburello+15

Counterimage

Cosmic Snakeclumpy galaxy at z=1stellar mass = 4×1010 M¤

SFR = 30 M¤ yr-1

molecular gas fraction = 0.2in rotation at 200 km/s

beam size ~ 0.18” (= HST)amplifications up to 500resolution down to 30 pc

Ideal Experiment:Detect the parent gas clouds and their link withstellar clumps observed in a prototypical galaxynear the peak of the cosmic star formation.

MACS1206-08★Concrete proof of disk fragmentation

Observational Challenge:Typical giant molecular clouds in nearby galaxieshave masses of 104-7 M¤ and radii of 5-100 pc.(Bolatto+08)

Need to combine the most powerful millimeter interferometer ALMA with the gravitational lensing.

12:06:11.0 10.9 10.8 10.7 10.6

47:58.0

-8:48:00.0

02.0

04.0

06.0

08.0

10.0

12.0

RA (J2000)

DE

C (

J20

00

)

CO(4-3) beam

RA

DEC

Velocity

CO(4-3) data cube composed of velocityslices of 10.3 km/s used to recover the 3Ddistribution of molecular clouds.

channel v1

channel v2

channel v3Velocity channels/slices: from v1 = −150 km/s to v25 = +110 km/s

Strict cloud selection criteria applied:• all clouds are detected at ≥4σ• all clouds are observed in at least two contiguous channels

★Hunting for individual molecular clouds

S1

(ch1

05) -

119.

720

km/s

S2S1

c1

(n1)

(ch1

06) -

109.

376

km/s

(s2)

(n2)

S1

(c1)

c1

N1

(ch1

07) -

99.0

332

km/s

S1

C1

(c1)

(n1)

(ch1

08) -

88.6

899

km/s

(s5)

S4

C4

c4c3

N3,4

(ch1

09) -

78.3

467

km/s

S5

s4

(c4)

c4c3

N3,4

(ch1

10) -

68.0

034

km/s

c7

S6

c6

(c6)

S5

n5

(c3)

(n3,4)

(ch1

11) -

57.6

601

km/s

(c8)

(n8)

C7

(s6)

C6

c6

S5

N5

(ch1

12) -

47.3

169

km/s

s10s9

N9,10

s8

(c8)

C8

N8

C7

S5

N5

(ch1

13) -

36.9

736

km/s

c12C11

S10S9

N9,10n8

C7

S5

N5

(ch1

14) -

26.6

304

km/s

S12

C12

C12

S11

C11

C11

S10S9

(n9,10)

C7

S5

N5

(ch1

15) -

16.2

871

km/s

s12

(c12)

C12

S11

C11

C11

n11,12

s13

N13

C7

S5

N5

(ch1

16) -

5.94

38 k

m/s

s13

N13

(s11)

C11

C11

C7

S5

N5

(ch1

17) +

4.39

94 k

m/s

s13(s11)

C11

c11C7

S5

N5

(ch1

18) +

14.7

427

km/s

(s11)

C11C7

S5

N5(c

h119

) +25

.086

0 km

/s

C7

S5

N5

(ch1

20) +

35.4

292

km/s

(c7)

S5

N5

(ch1

21) +

45.7

725

km/s

N15

S14S5

N5

(ch1

22) +

56.1

158

km/s

s15

n15

S14

N14

S5

N5

(ch1

23) +

66.4

590

km/s

S15

n15

S5

N5

(ch1

24) +

76.8

023

km/s

S15

N15

s5

N5

(ch1

25) +

87.1

455

km/s

S15

(n15)N5

(ch1

27) +

107.

832

km/s

(s17)

(n17)N16

(ch1

28) +

118.

175

km/s

Channel 1 Channel 2 Channel 3 Channel 4 Channel 5 Channel 6

Channel 16 Channel 17 Channel 18 Channel 19 Channel 20 Channel 21 Channel 22 Channel 23 Channel 24

Channel 7 Channel 8 Channel 9 Channel 10 Channel 11 Channel 12 Channel 13 Channel 14 Channel 15

17 molecular clouds identifiedin 3D within the inner 1.7 kpc

0.2"/1.6kpc

Snake North

★Physical properties of GMCs at z~1

Local GMCs: Bolatto+08; Heyer+09; DonovanMeyer+13; Colombo+14; Corbelli+17 / SDP81 GMCs: Swinbank+15

They clearly differ from typical local GMCs with >100x higher masses (8x106-1x109 M¤), >10x higher moleculargas mass surface densities (~2600 M¤/pc2), and larger internal velocity dispersions.

à significant offset from the well established Larson scaling relations1st scaling relation implies a constantmolecular gas mass surface density

2nd scaling relation calibrates the surfacedensity of virialized clouds

The universality of GMCs is definitely challenged!Environment matters.

★GMC dependence on environment

higher velocity dispersions reflect larger internalpressure required for equilibrium given the highergas mass surface densities(Hughes+13)

GMCs are not decoupled from the surrounding ISM:

at their formation, they must inherit their density and turbulence from the ambient ISM conditions to ensure their survival

The observed correlation shows that GMCsare pressure confined:

the larger internal GMC pressure goes along withthe strong hydrostatic pressure of the z~1 disk,>1000x higher than the pressure in the Milky Way

★Are the high-redshift GMCs virialized?

One-to-one relationship for virialized clouds(Bolatto+08; Leroy+15)

In total 14 GMCs out of the 17 found at z~1are virialized gravitational bound entitiesregardless of the CO-to-H2 conversion factor(αCO= 1 or 4.36).

For virialized GMCs, the CO-to-H2

conversion factor of individual GMCs at z~1can be, for the first time, constrained:

αCO = 3.8±1.1 M¤/(K km s-1 pc2) Close to the Milky Way value despite thestrong photodissociating radiation, implyinga good CO shielding by the high gas density.

★Link between GMCs and stellar clumps

The high-redshift GMCs are highly supersonic (with 10x higher Mach numbers than local

GMCs) and hence suggest a possibly greater efficiency of star formation.

(Leroy+15)

For the first time, GMCs and stellar clumps are identifiedat the same spatial resolution in a prototypical galaxy atz=1 and allow a direct estimate of the efficiency of starformation.

If the identified GMCs are representative of theparent GMC population which gave rise to theobserved massive star clusters, this wouldindicate a star formation efficiency as high ashigh 35-50% !

This would explain the shorter moleculargas consumption times observed at highredshift.

★ In conclusion

The detection of the molecular clouds in the Cosmic Snake galaxy demonstrates theexistence of parent gas clouds with masses high enough to allow for in-situformation of the massive stellar clumps seen in the galaxy.

The molecular gas mass distribution of the z~1 GMCs perfectly agrees with the gasmass distribution of simulated molecular clouds resulting from disk fragmentation.

This brings observational evidence that disk fragmentation is the main mechanismexplaining the formation of massive molecular clouds in distant galaxies.

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