Molecular clouds in the center of M81

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Molecular clouds in the center of M81
Viviana Casasola
Observatoire de Paris-LERMA Università di
Padova, Dipartimento di Astronomia
Supervisors F. Combes (Paris) G. Galletta
(Padova)
Scuola Nazionale di Astrofisica Oggetti compatti
e pulsar - Scienza con ALMA 
Maracalagonis (CA), 20 - 26 maggio 2007
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Why and how do we study molecular gas in galaxies?

• Molecular gas is a fundamental component in
  the determining
• both the morphology and evolution of
  galaxies.
• It is within the molecular clouds that the
  interstellar gas
• is cycled into the next generation of stars
  and the most
• massive of these young stars produce a
  major part of the
• galactic luminosity.

Molecular hydrogen (H2) is the predominant
interstellar molecular specie the H2
molecule lacks a permanent electric dipole
moment and the lowest quadrupole rotational
transitions lie in the infrared.
Scuola Nazionale di Astrofisica Oggetti compatti
e pulsar - Scienza con ALMA 
Maracalagonis (CA), 20 - 26 maggio 2007
3
To study the H2 abundance indirect method
can collisionally excite
The coldest H2
CARBON MONOXIDE (CO)
tracers
The emission of lines from rotational transitions
of CO observables with the Radio-Telescopes and
Radio-Interferometers. The CO lines more
observed are J 1 ?? J 0 2.6 mm (? 115
Ghz) Tex 5.5 K J 2 ? J 1 1.3 mm (?
230 Ghz) Tex 16.6 K J 3 ? J 2 0.9 mm
(? 345 Ghz) Tex 33.2 K
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To estimate the H2 abundance
XNH2/ICO, standard conversion factorX
2.3x1020 mol cm-2 (K km/s)-1, for the Milky Way
(Strong et al. 1988)
This value, universally adopted for all galaxies
and for different regions of a same galaxy, is
still a subject of debate.The X conversion
factor depends on various factors such as, the
metallicity, the temperature, the cosmic ray
density, the UV radiation field,..
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M81 galaxy, molecular clouds in the center

• M81 is the prototype of giant spiral galaxy very
  CO-poor, especially in the central region (Combes
  et al. 77, Brouillet et al. 88, Sakamoto et al.
  2001)
• H2/HI0.1, normally for similar galaxies H2/HI1
  (e.g. Young Scoville 91)
• In general interacting galaxies, like M81,
  possess higher CO luminosity by almost an order
  of magnitude than non-interacting galaxies (e.g.
  Combes 94, Casasola et al. 04)
• M81, for this non-typical CO emission, is a
  good candidate to explore the problem of the
  varying XNH2/ICO

UV Galex
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ObservationsWe observed, with
the 30m-IRAM RadioTelescope, the central region
of M81 to investigate the molecular gas content,
its excitation physics, and fragmentation
properties.
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Clumping properties of the gas and X conversion
factor
CO(2-1)
Analyzing in detail the clumping properties of
the molecular gas, we identified very massive
GMCs, with mass of ?105 M? and dimensions of
?250 pc.
The deduced X (NH2/ICO) conversion factor
obtained for the individual resolved GMCs is a
factor ?15 larger than the standard Galactic
value.
12
Casasola, Combes et al. 07 AA submitted
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Heating of the gas
Why the molecular gas is absent or
sub-thermally excited in the M81 center?
A possible link could exist between low CO
emission and lack of UV emission that appears to
characterize the M81 central region (see GALEX
image).
FUV GALEX
For Knapen et al 06, observing in CO a region of
a spiral arm in M81, the flux of the cosmic rays
and the FUV surface brightness would be too low
to heat the molecular gas component. Probably
also for the nucleus the explanation is the same.
Casasola, Combes et al. 07 AA submitted
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Conclusions

• M81 appears not only CO-poor in the central
  region, but also with molecular clouds having
  unusual properties (T, mass and dimension), if
  compared to galaxies with similar distance and
  morphological type
• absent or very weak molecular gas emission in
  the nucleus
• low I21/I10 (0.68) ratio
• X conversion factor ?15 times larger that the X
  value
• derived for our Galaxy

The low CO emission we found in the center
suggests that the gas in the nucleus is
sub-excited. We concluded that the lack of
excitation of the gas, more than the absence of
the molecular gas, is the cause of the low CO
emission in the M81 center.
Casasola, Combes et al. 07 AA submitted