Speaker
Isadora Chaves Bicalho
(Observatoire de Paris)
Description
The spatially resolved star formation law has been studied
in great detail in galaxies in recent years. At high surface
density, when most of the gas is molecular, the
Kennicutt-Schmidt relation is almost linear providing a
constant gas consumption time-scale of about 3Gyr (e.g
Bigiel et al. 2011, Saintonge et al 2011). However the star
formation efficiency (SFE) falls very quickly when the
surface density drops below 10Mo/pc2, and the gas is mainly
atomic. The star formation rate (SFR) becomes a highly
non-linear function of gas density, and the depletion
time-scale is several Gyrs up to Hubble time. This is the
case for dwarf galaxies and the most external parts of disk
galaxies (Bigiel et al 2010), where the low gas density,
low temperature and low metallicity conditions resemble
early galaxies in the universe. Recent star formation within
such environments was detected in H-alpha (one of the main
star formation tracer). However, the Galaxy Evolution
Explorer (GALEX) data demonstrate that H-alpha observations,
tracing ~10Myr SF, still fail to detect a significant
population of moderate-age stars in the outermost disks of
spiral galaxies. Our aim is to detect the corresponding
molecular gas expected in these regions. One remarkable
example is M83, a nearby galaxy with an extend XUV disk
reaching 2 times the optical major radius (Gil de Paz et al.
2007). However, our progress in understanding these XUV
disks has been halted by the difficulty of detecting
molecular gas via CO emission. In particular, no highly
significant (>5sigma) CO emission was detect in ALMA maps of
the XUV disk of M83 when we expected to detect 20-30
molecular clouds with SNR > 17. We hypothesize that the
molecular clouds in the ALMA data are CO-dark, caused by the
strong UV radiation field, which dissociates CO
preferentially, keeping the H2 gas intact.
Primary author
Isadora Chaves Bicalho
(Observatoire de Paris)
Co-authors
Celia Verdugo
(ALMA Observatory)
Françoise Combes
(Observatoire de Paris)
Monica Rubio
(Universidad de Chile)
Philippe Salome
(Observatoire de Paris)
