GMC Populations of Nearby Galaxies - Home | Astron · 2015. 9. 7. · e.g. Koda ea (2009), Egusa ea...

Andreas Schruba, MPE for Annie Hughes, IRAP PAWS: Adam Leroy, Dario Colombo, Eva Schinnerer, Sharon Meidt, Jerome Pety, Clare Dobbs, Gaelle Dumas, Todd Thompson, Santi Garcia-Burillo, Carsten Kramer, Karl Schuster CANON: Jin Koda, Jen Donovan-Meyer, M31: Andreas Schruba MAGMA: Tony Wong, Juergen Ott

GMC Populations of Nearby Galaxies

Milky Way’s molecular mass is ~ 2 x109 M⊙

n ~ 100 cm-3 ➡ tff ~ 4 x 106 yr ➡ SFR ~ 500 M⊙ yr-1! Observed SFR is 0.7 to 1.5 M⊙ yr-1

• bound + long-lived e.g. Krumholz & McKee (2005)

• unbound + short-lived e.g. Dobbs et al (2011) • collapsing + short-lived e.g. (Ballesteros-Paredes et al. (2011)

Explanation depends on whether GMCs are:

Motivation

Annie Hughes, IRAP ● Local Gas, ASTRON ● Sept 2015

A Resolved Extragalactic SF ‘Law’

log(Molecular Gas Surface Density)

log

(SFR

Sur

face

Den

sity)

stars form from molecular gas with a constant depletion time of ~2 Gyr in nearby disk galaxies

nearby galaxies

starburst, high z

compilation by H

odge et al. (2014)

slope = -1.5

Solomon et al (1987)

slope = 0.5

dN/d

Mve

locit

y di

sper

sion

log(Molecular Gas Surface Density)

mass

size

This Talk

CARMANGC6946

MAGMA LMC

NGC253: Leroy M31: Schruba NGC4526: Utomo CANON: Donovan-Meyer M33: Druard, Gratier, Braine NGC6822: Gratier IC342: Hirota M83: Hirota

ALMA NGC628

PdBI M51

Do the properties of a GMC population depend on host galaxy properties?

Do galaxy properties & processes influence GMC formation and evolution?

Do GMC properties matter for star formation (on galactic scales)?

Are GMC Properties Universal?

“GMCs identified on the basis of their CO emission exhibit remarkably uniform properties from galaxy to galaxy…”

Confirmation from Local Group GMCs?

log(

CO L

umino

sity)

10 100Radius [pc]

2

6

4

1

Bolatto et al 2008

Radius [pc]10 100

vel.

disp

ersio

n [k

m/s

]

1

10


But...

LMC CARMA-STING

log(Σstars) log(Radius)

log(σ v

)

log(Σ H

2)

Wong ea (2013)Hughes ea (2010)

+ clear evidence for mass evolution through spiral armse.g. Koda ea (2009), Egusa ea (2011)

IC342Hirota ea (2011)

~1.5 kpc

Some observational evidence that GMC dynamical properties (e.g. mass, linewidth) may depend on galactic environment


this ‘correlation’ largely due to resolution effects

log(σ/

[km

/s])

log(Radius/[pc])

intrinsic

Size-linewidth relation: M51, M33, LMC

interarm

M33

LMC

arm Decomposition applied to CO data cubes with their original spatial and spectral resolution

Data points cluster around the resolution limits

Scaling relations obtained from composite samples must be interpreted with caution: beware observational bias


at fixed size, M51 clouds still have higher σ

Hughes et al (2013b)lo

g(σ/

[km

/s])

log(Radius/[pc]) log(Radius/[pc])

log(σ/

[km

/s])

matched

Size-linewidth relation: M51, M33, LMC

interarm

M33

LMC

arm

this ‘correlation’ largely due to resolution effects

intrinsic

R-Lco relation: Nearby Galaxies

Within M51, slope and normalisation of size-luminosity relationship varies

<Σ> at fixed scale also varies between galaxies

difference in <Σ> between spiral and dwarf galaxies (factor of ~10) exceeds XCO variations (factor of ~2 to 3)

10 100

2

4

6

log(LCO/[K km/s pc2])

M51 interarm

M33

LMC

size-luminosity

M51 arm

N6946

N4826

Radius [pc]

Annie Hughes, IRAP ● Local Gas, ASTRON ● Sept 2015Hughes et al (2013b)

Extragalactic Larson Relations at 50pc

Radius [pc]

log(σ/[km/s])

10 100

1

10

size-linewidth

<σ>/R and Σ of GMCs vary with galactic environment

10 100Radius [pc]

2

4

6

log(LCO/[K km/s pc2])

M51 interarm

M33

LMC

M51 arm

N6946

N4826

100


size-luminosity

A consequence of external pressure?

e.g. Elmegreen (1989)

➡ if Pint~Pext, environment more likely to influence cloud stability, dense gas mass function, star formation activity, GMC evolution...

PGMC=P

ISM

PGMC=1

0PISM

PGMC=0

.1PISM

M33LMCM51

M64NGC6946

dwarfs

GMC vs ISM pressure

4 65

4

6

5

log(Pext/kB[K cm-3])

log(Pint/k B[K cm-3])


How is Molecular Gas in Galaxies Distributed?

• diagnostic of GMC formation & destruction processes • potential link to core mass function and cluster IMF • in inner MW, γ = -1.5 ➡ GMCs dominate total H2 mass

GMC Mass Distributions: Motivations


slope = -1.5

Solomon et al (1987)dN

/dM

cloud mass

Kruijssen (2014)

observed mass

Toom

re M

ass

PAWS Survey of M51: Dynamical environments defined in Colombo et al (2014)

Colombo et al (2014)

log(

N(m

>M)/[

kpc2

])

-1

2

-1

2

5 76 5 76 5 76log(Mass/M⊙)

GMC Mass Spectra: M51 environments

ARM outside CR

ARM inside CR DOWN

UP

-1.3

-1.6 -1.8

-1.8

-2.4

-2.6

RING

BAR

GMC & Young Cluster PropertiesMass Function Slope Maximum (P95) Mass

log(Max GMC Mass)MF slope (GMCs)

MF

slope

(clus

ters

)

Log(

Max

Clus

ter M

ass) Hughes et al (2013a)

GMC Mass Spectra: Nearby Galaxies


log (GMC Mass/[M⦿])

LMC M31 N628

simple power law is insufficient for most galaxies GMC mass spectra are steeper in diffuse environments

log(

N(M

* >M

)/[kp

c2 ])

M51arms

GMC Mass Distributions: Trends


shape of GMC mass distribution is not universal: slope and turnover mass increase in more massive systems

M51M31LMC N628 N6946M33

Slop

eTr

unca

tion

Mas

s

Stellar Mass Σ(stars) RmolΣ(gas)

Stellar Mass Σ(stars) RmolΣ(gas)

Hughes et al (in prep)

Do GMC properties matter for star formation (on galactic scales)?

Star Formation: TheoryIn star formation theory, the SFR depends on the properties of the parent cloud

e.g. Krumholz & McKee 2005, Padoan & Nordlund 2011, Hennebelle & Chabrier 2011, Federrath & Klessen 2012,


Fede

rrath

& K

lesse

n 20

12

SFR

per f

ree-

fall

time

Padoan & Nordlund

Krumholz & McKee

Hennebelle & Chabrier

Mach Number

virial parameter

• cloud properties act as the boundary conditions for small scale star formation • turbulent cloud properties set the

density distribution • Mach number sets the width of

density distribution

Star Formation in Local Clouds

log(Visual Extinction)

Rela

tive

Freq

uenc

y

In local clouds, SFR relates to high density gas, not total H2

• YSOs found in regions of high column density and extinction

• Actively SF clouds show excess amounts of high extinction material

• SF appears closely associated with dense filamentary substructure

Kainulainen et al (2009), Lada et al (2010), Heiderman et al (2010), Arzoumanian et al (2011), Stutz et al (2015)


Kainu

laine

n et

al (2

009)

log

(SFR

Sur

face

Den

sity

)

undisturbed disks

merger + interacting

Extragalactic Star Formation

incr

easi

ng s

tella

r mas

s

log(H2 Surface Density)

SFR-per-H2 varies (systematically) on large scales in local galaxies• High SFR/CO in galaxy

centres, luminous starbursts, low-metallicity dwarfs

• Low SFR/CO in ETGs

• SFR/CO shows variations with local gas kinematics, galaxy morphology and metallicity

Schruba et al (2011), Saintonge et al (2011), Davis et al (2014), Meidt et al (2013), Koda et al (2009), Leroy et al

(2013), Gao & Solomon (2004), Huang et al (2015)


Schr

uba

et a

l (201

1),

Saint

onge

et a

l (201

1

SF closely related to

H2

GMC Properties and Star Formation


Measure TIR/CO, SFR/HCN, or HCN/CO ratio in a ~kpc aperture that is small enough to roughly isolate local conditions.

For simplicity, we write TIR/CO: overall SFE in H2 HCN/CO: dense gas fraction TIR/HCN: SFE of dense gas

Integration over a ~kpc area is needed to capture the time-cycling of gas and stars

Schinnerer et al (2013), Pety et al (2013)

GMC Properties and Star Formation


Within the ~kpc aperture, measure the typical cloud-scale properties of the molecular gas, e.g. mass-weighted surface density, velocity dispersion (Mach number), and gravitational boundedness

Underlying GMC Properties Matter

Cloud Boundedness

TIR-

to-C

O a

t 1kp

c ~

SFE


Within the PAWS field-of-view, the ability of the ISM to form stars (IR/CO) correlates with the self-gravity of the gas at cloud (50 pc) scales – gas that is more bound appears better at forming stars.

more boundLe

roy,

Hugh

es, S

chinn

erer

, Big

iel (in

pre

p)


H2 Surface Density

HC

N-to

-CO

at 1

kpc

~ de

nse

gas

fract

ion


Within the PAWS field-of-view, the average small scale surface density within a kpc region correlates with the apparent fraction of the molecular gas mass that is dense.

denserLero

y, Hu

ghes

, Sch

inner

er, B

igiel

(in p

rep)


H2 Surface Density

IR-to

-HC

N a

t 1kp

c

~SFE

in d

ense

gas


But higher cloud-scale surface densities are associated with lower ratios of star formation per unit dense gas! I.e. dense gas is worse at forming stars as overall ISM density increases

denserLe

roy,

Hugh

es, S

chinn

erer

, Big

iel (in

pre

p)

SummaryGMC physical properties and mass distributions vary with environment, both within and among galaxies

More massive systems tend to have denser, more turbulent GMCs, and tend to build more massive GMCs

In M51, relations between star formation, molecular gas and dense gas are qualitatively consistent with a “whole cloud” view in which star formation occurs in local overdensities (rather than a universal density threshold)

GMC Populations of Nearby Galaxies - Home | Astron · 2015. 9. 7. · e.g. Koda ea (2009), Egusa ea...

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