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Millimeter dust continuum emission as a tracer of molecular gas in galaxies:
comparison of SMC and Local GMCs

Caroline Bot a project w/ Francois Boulanger, IAS France & Monica Rubio, Universidad de Chile


Molecular clouds


Sites of star formation Most of the molecular gas is in giant molecular clouds PROBLEM:H2 is quite impossible to observe directly in cold interstellar regions Use of tracers (CO, gamma rays, dust emission)







CO(1-0) Dame et al. 2001


Molecular clouds tracers


No perfect tracer:
­ ­ ­

Different tracers in different environments Simplified assumptions Evolution with metallicity?

Zo H

CO
2

Z ­

2

CO

Lequeux et al. 1994 AndrИ et al. 2004

Grenier et al. 2005: dark clouds= H2 without CO

Compare in a consistent manner different tracers in different environments


In the SMC...


Resolve GMCs Lower metallicity In SMCB1#1
­





Mmm(H2)>> MCO(H2)
Rubio et al. 2004



General trend in the SMC? S3MC+NANTEN
­ ­
Leroy et al. (2007)



H2 larger than predicted by CO But IR dust emission biased by temperature



Is there the same difference in our Galaxy? How do we understand it?




The Data


SMC:
­

LIRS49 SMCB2N SMCB2S Hodge15

LIRS36


SEST SIMBA & CO data (M. Rubio) FIRAS CO (Dame et al. 2001)

Galaxy:
­ ­

SMCB1#1


Same linear resolution
Lupus

SMCB1#2 Cepheus TaurusPerseus Ophiucus

Chamaeleon

Orion


Molecular gas masses


From millimeter dust emission
­ ­ ­ ­ ­

corrected from free-free, CO(2-1) and dust in HI T
dust=15K

(weak assumption)
region)

NH= cst . I . (xdref/xd

/

ref H

(xdSMC/xdlocal)=0.17 Href: emissivity of dust in molecular gas




computed in the molecular ring ~2x diffuse medium value (Boulanger et grain-grain coagulation effects Virial masses: M=190V2 R (MacLaren 1988) Mdark from ray analysis
(Grenier et al. 2005)

al., in prep.)




Results
Galaxy Galaxy

SMC



M
­

mm

M

dark

+MCO
H2

M

mm

= true M

if the dark gas is H2 without CO
mm



In our Galaxy: Mvir>M In the SMC: Mvir< M



mm


Interpretation
H
2

C O



In our Galaxy: Mvir>M
­ ­

mm

Outside pressure effect Solomon et al. (1987)


Interpretation

SMC


In the SMC:
mm

Mvir < M


(~2)

M
­ ­

mm

are lower limits:

enhanced emissivity in the SMC? (harsh environment) We took high dust/gas ratios


Interpretation


In the SMC:
­ ­

Mvir << M

mm

Confirms Rubio et al. 2004 result on all SMC clouds of our sample

Galaxy SMC



Revisit virial theorem to understand the difference

Partial support of the cloud by a magnetic field


Results


Magnetic field > 15µG
­

Coherent with the densities observed if B n
(Mouschovias 1976)

1/2



New image of the GMCs in the SMC:
-CO clumps in densest regions -Large enveloppes of H2 -Magnetic + gravitational support
CO

H

2


Conclusions


Millimeter emission traces molecular gas Good comprehension of the Galactic clouds
­



coherence of different tracers



In both samples: large enveloppes of H2 and small CO clumps Support by a magnetic field
­



only in the SMC? -> different formation of GMCs? Effect of higher ionization?



astro-ph/0704.3257