Star formation efficiencies of molecular clouds in a galactic center environment1

...Erik Bertram

Zentrum für Astronomie der Universität Heidelberg (ZAH)Institut für Theoretische Astrophysik (ITA)

Universität Heidelberg...

1Bertram et al. (2015)Collaborators are Ralf S. Klessen, Simon C. O. Glover & Paul C. Clark

Credit: www.nasa.gov

Central region of the Milky Way(infrared image by the Spitzer space telescope)

„The Brick“

Credit: www.nasa.gov

CENTRAL REGION OF THE MW

● Morphology of the Galactic Center with its CMZ („Central Molecular Zone“):

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(Kruijssen et al. 2015)

CENTRAL REGION OF THE MW

● GC provides an accessible laboratory for studying physical processes under extreme conditions:(see, e.g. Longmore et al. 2013, Kauffmann et al. 2013, Johnston et al. 2014)

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Solar neighbourhood: Galactic Center:

Densities ~ 100cm-3 > 104cm-3

Temperatures ~ 20K ~ 70K

Velocities Several km/s > 10 km/s

Magnetic fields Several µG Several mG

Interstellar radiation field(ISRF)

G0 = 1 G

0 = 100-1000

Cosmic-ray flux(CRF)

3 x 10-17 s-1 10-14-10-15 s-1

Scientific question

SCIENTIFIC QUESTION

● How do stars form in the Galactic Center?● Problem: Observations show, that although the SFR in the CMZ is high, the specific SFR is low (lower by a factor of > 10) (Longmore et al. 2013)

● The SFR of the CMZ lies below the Kennicutt-Schmidt relation● What causes this reduced efficiency? - Turbulence? - Radiation pressure? - Feedback from stars / supernovae? - Magnetic support? ...

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Our study!

What we are doing

What we are doing

● Perform high resolution simulations of a number of GC clouds

● We use the AREPO code (Springel, 2010)

● Implementation of time-dependent chemistry with heating and cooling (Glover et al. 2010)

● Jeans refinement with self-gravity (8 cells per Jeans length)

● Self-consistent shielding (Clark et al. 2012)

● Sink particle implementation (Greif et al. 2011)

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Credit: Wikimedia

Credit: Jülich Supercomputing Center

Simulation setup

● Cloud parameters (as experienced by a typical GC cloud): - Cloud mass: ~105M

sun

- Standard solar abundance of Z = 1 - 1000 x ISRF, 1000 x CRF - decaying turbulent velocity field - periodic boundary conditions - clouds are homogeneous spheres at the beginning

● We then vary some environmental parameters:- Initial number densities: 100, 1000, 10000 cm-3

- Turbulence level: α = Ekin

/ |Egrav

| = 0.5, 1, 2, 4, 8

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What we are doing

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Simulation setup

Ekin=12

M tot σv2

Egrav=−3GM tot

2

5R

α=Ekin

|Egrav|

M tot=43

πR3ρ

t ff=√ 3π

32Gρ

ϵff=t ff⋅M s

M tot

What we are doing

● Simulation: n = 1000cm-3, box length: ~ 44pc

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What we are doing

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Star formation efficiencies against virial parameter:(compare to the canonical Milky Way average value of ɛ

ff ~ 1%)

What we are doing

εff ~ n

0

-0.5

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What we are doing

● We also perform radiative transfer post-processing with RADMC-3D and generate synthetic images of our clouds (α = 2.0):

Total column: 12CO: 13CO:

[OI] (63µm): [OI] (145µm): [CII] (158µm):

[K km/s]

[K km/s]

Summary● We find active star formation with ɛ

ff > 1% in all models!

● Our values are more comparable to Galaxy-wide SFEs (ɛ

ff ~ 1%) rather than to the low values found in the CMZ!

● The SFE decreases by a factor of ~4-10 as we increase the virial parameter from = 0.5 to = 8.0.α α

● Even in our most extreme models, we find SFEs of ~1%.● We conclude: High levels of turbulence with a strong ISRF /CRF cannot by themselves explain the low SFEs in the GC!

● We find that the atomic [OI] and [CII] fine structure lines are good tracers for MCs in a GC environment.

Summary

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Outlook for future work

● What is the impact of magnetic fields, SF feedback, etc. on the SFEs in the GC?

● Analyse the IMF of our clouds, compare to observed IMF● Perform large-scale runs of clouds in an orbit around the GC● ...

Outlook

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Thank you for your attention!

WWW: www.ita.uni-heidelberg.de/~ebertramE-Mail: Bertram@zah.uni-heidelberg.de