Cold dust in the Galactic halo: first detection of dust emission in

a high-velocity cloud :

Francois Boulanger et

Marc-Antoine Miville-Deschênes

Miville Deschenes, Boulanger, Reach and Noriega-Crespo 2005, Ap. J. 631 L57.

XFLS

High Velocity Clouds

• Observed on 20% of the sky

Widespread interest in HVCs is due to their mystery ...

> Debatted origin

• Galactic fountain

• Tidal debris of local group dwarf galaxies

• Dark matter halos in the Local Group

Physical description, spatial distribution, masses and dynamics

… and their potential for being important

> Formation and evolution of galaxies

• Potential tracers of small dark matter halos

• Source of gas to fuel star formation

• Accretion of low metallicity gas for chemical evolution

• Energy, mass and angular momentum flows within galaxies

Multiphase systems

> First detection : Muller, Oort and Raimond 1963 C. R. Acad. Sc. Paris (21cm line, Dwingeloo 25 m dish)

> Today:

• Cold neutral gas (HI)

• Warm neutral and ionized gas (HI, Halpha)

• Conductive interfaces (OVI, CIV, NV)

• Embedded in a Galactic halo of coronal gas (X-rays)

Dust and Molecules?

XFLS

High Velocity Clouds

• Observed on 20% of the sky

XFLS

Spitzer Extragalactic First-Look Survey

Infrared-HI comparison of theSpitzer Extragalactic First-Look Survey

IRAS 100 micron and GBT-HI overlay

Draco nebula

HI SpectraHI Maps

Lockman and Condon 2005

GBT Observations

IRIS: a new generation of IRAS maps

• Comparison with Schlegel et al.– Better photometry (scale and brightness correction)– Better angular resolution 4 IRAS bands (Schlegel et al. is only 100 micron)

Miville-Deschenes & Lagache, 2005

Spitzer and IRAS Images

• Multivariate regression to estimate the infrared emissivity of each HI component.

First detection of dust emission in the HVC– HVC Emissivity at 100 m ~

10 times smaller than local gas, but only a factor 2 smaller at 160 m

Colder dust

Infrared-HI correlation

I (x,y) = ii NHI

i (x,y) + C(x,y)

Infrared-HI Correlation

Dust in Complex C

• Dust Temperatures and emissivities per HI atom

Gas column densities

• Dust emissivity per H gas metallicity for a fixed fraction of heavy elements in dust (fixed depletion)

• ZHVC /ZSun = 0.2 +/- 0.1 (Tripp et al. 2003)

• Dust emissivity per HI, HVC over Local ISM value > 1.6 (3 )

NH(HVC) > 1.6/0.3 * NHI (HVC)

• NHII/NH ~ 0.2 (Wakker et al. 1999)

Most of the HVC gas is in molecules

FUSE search for H2 in absorption

Richter et al. 2001

Conclusions and Perspectives• Confirmation of dust detection result on other fields

HVC in Spitzer extra-galactic fields: dedicated GBT+DRAO observations scheduled Dedicated Spitzer observing program?

HVCs could make part of the sub-mm extra-galactic background!

• Search for CO emission from molecular clumps

IRAM 30m proposal Clumps origin: self gravitating long lived structures or signatures of the dynamical interaction with the Galactic corona

New perspective on HVCs and the Galaxy evolution

New hunting grounds for molecules ?