- The M33 CO(2-1) survey - Star formation and

molecular clouds formation

Clément Druard

Jonathan Braine (LAB) and Karl Schuster (IRAM)Pierre Gratier (IRAM)

YERAC 2013 - 10/01/2013

Messier 33 : the Triangulum galaxy

• Local Group galaxy

• intermediate object

• small “floculent” spiral galaxy

• MM33 ~10% MMilky Way

• Roptical ~8kpc

Messier 33 : the Triangulum galaxy

• Why M33 ?

• half solar metallicity = chemically young

• gas rich

• analogous to younger or intermediate z galaxies

• relatively nearby (840 kpc)

• giant molecular cloud resolution (~50pc)

• ideal position (optimal 56° inclination, between edge-on and face-on)

• regions of interest : HII, arms ?

Messier 33 : the Triangulum galaxy

• HerM33es project(Herschel M33 extended survey) :

- survey [CII], [OI], [NII] ...

- dust : 100 to 500 μm

• HI @ VLA data

• UV, X, ...

• rotation curve

• > molecular gas tracer

M33 molecular gas• H2 cannot be observed directly

• tracer : CO J=2-1 @ 230,538 GHz

• IRAM 30m radiotelescope

• HERA multibeam receiver in OTF mode

• spectral resolution : 2,6km/s

• spatial resolution : 12’’ ~ 50pc

M33 molecular gas

• IRAM Large Program :

• 500h (from june 2005 to december 2012)

• 17 681 418 spectra

• ~300Go of data

• homogeneous RMS noise ~ 20mK

• total coverage of the optical disk up to R ~ 7kpc

• map 3400’’ x 2000’’

Data processing• Difficulties :

• observations during a large period (7 years)

• non homogeneous data (position, sampling of the spectra, ...)

• huge amount of data for this kind of instrument

• require a robust pipeline !

• Goal :

• best possible data reduction process (no data loss, no artificial signal created...)

• obtain a global and reliable set of data

Data reduction

DATACUBE !!!!

3rd order baselining :

- hypothesis : atomic (HI) and molecular (CO) gas cannot exist at fundamentally different velocities

- window centered on the HI peak velocity for the same position

- for HI-free regions (< 4σ detection), we use the analytic rotation curve

spectra with higher S/N ratio

datacube - 3D map (position-position-velocity)

noise filtering (theoretical noise vs. spectrum noise)

0 order baselining - “continuum fitting”(no window)

resampling of all the sepctra - homogeneous dataset

elimination of poor data (bad pixels, ...)

11% of the spectra

Rotation curve vs. HI peak velocity

Integrated intensity maps (ICO)

• building method:

- 1 spatial pixel of the cube = 1 spectrum

- for each spatial pixel, sum the intensity corresponding to the CO emission, not the noise => windows based on the HI > 4σ or rotation curve

- we assume that CO cannot be present at HI-free positions and velocities

• why these maps?

- CO signal (noise free)

- molecular gas masses derived from the CO(2-1) emission

I0 = ��

X

i

Ti

M = ICO(2�1).ICO(1�0)

ICO(2�1).XCO.D

2.2mp

Msol.fH2+He

signal noisenoise

XCO = 4.1020cm�2.K�1.km�1.s

fH2+He = 1/0, 73

ICO(1�0)

ICO(2�1)= 1/0, 73

Mmolecular gas = 3.49 108M☉

Velocity recentered cube

• method : recenter each CO spectra at the same reference corresponding to the HI peak velocity at the same position

HI peak velocity

0

Velocity recentered cube

• aims :

- stack the recentered spectra to make the signal appear at position where single spectrum contains a too weak emisison

- study large CO-free zones

• analyze the global/local dynamics

- global scale : narrow velocity dispersion (~11km/s) => CO follows the rotation curve well

- locally : CO in HI-free zones ? check if the previous assumption is true !

Velocity recentered cube

Is there CO without HI ?

• hypothesis : “there is no molecular gas (CO) where there is no atomic gas (HI)”

• average spectrum of HI-free regions (no 4σ detection)

• MHI-free = 4,77E+06 Msol et Merror beam = 5,07E+06 Msol

black : HI-free regions

CO(2-1) emissionerror beam emission

average spectra for all the HI-free regions

Giant Molecular Clouds (GMCs) detection

• detection algorithm CPROPS (Rosolowsky et al. 2007) :

• ~ 500 detected clouds (337 in previous work of Gratier et al. 2012 but for a smaller coverage)

• catalog + property list

Giant Molecular Clouds (GMCs) detection

Detections at different velocities

Giant Molecular Clouds (GMCs) detection

Mclouds = 1.79 108M☉~50% molecular mass

Outlook :• Ico maps :

- conversion factor evolution, column density maps

• CO(2-1) / CO(1-0) ratio evolution

• GMCs catalog + relation to star forming regions

- comparison with different tracers

- statistics (radial evolution of the properties ?)

- formation conditions / evolution

- what mechanism can trigger the molecular cloud formation ?

Thank you for your attention !