Molecular clouds in the center of M81

Viviana Casasola

Observatoire de Paris-LERMA&

Università di Padova, Dipartimento di Astronomia

Scuola Nazionale di AstrofisicaOggetti compatti e pulsar - Scienza con ALMA  Maracalagonis (CA), 20 - 26 maggio 2007

Supervisors:F. Combes (Paris)G. Galletta (Padova)

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.

Why and how do we study molecular gas in galaxies?

Scuola Nazionale di Astrofisica

Oggetti compatti e pulsar - Scienza con ALMA  Maracalagonis (CA), 20 - 26 maggio 2007

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.

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

To study the H2 abundance…… indirect method …

The coldest H2 CARBON MONOXIDE (CO)

tracers

can collisionally excite

X=NH2/ICO, “standard” conversion factor

X = 2.3x10^20 mol cm-2 (K km/s)-1, for the Milky Way (Strong et al. 1988)

To estimate the H2 abundance…

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,…..

M81 galaxy, molecular clouds in the center

UV Galex

• 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/HI~0.1, normally for similar galaxies H2/HI~1 (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 X=NH2/ICO

Observations

We observed, with the 30m-IRAM RadioTelescope, the central region of M81 to investigate the molecular gas content, its excitation physics, and fragmentation properties.

CO10y-axis, TA [-20/70 mK]

CO21

x-axis,Vh [-600/600 km/s]

1”17.4 pc

• In the central 1.6 kpc the CO emission (both CO(1-0) and CO(2-1)) comes from different regions

• the nucleus presents some regions devoid of molecular gas

•other regions in which the CO is detected but atVERY LOW TEMPERATURE

TCO(1-0) = 33.44 mK TCO(2-1) = 21.47 mK

• I21/I10 = 0.68, quite low and non-typical for galactic nuclei

Casasola, Combes et al. 07A&A submitted

12’’

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.

Casasola, Combes et al. 07A&A submitted

The deduced X (=NH2/ICO) conversion factor obtained for the individual resolved GMCs is a factor 15 larger than the standard Galactic value.

Clumping properties of the gas and X conversion

factor

Casasola, Combes et al. 07A&A submitted

Heating of the gasWhy 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. 07A&A submitted

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.