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