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TwitterA&A 493, 453-466 (2009)
DOI: 10.1051/0004-6361:200810566
Star formation in M 33: multiwavelength signatures across the disk
S. Verley1, E. Corbelli1, C. Giovanardi1, and L. K. Hunt21 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: [simon;edvige;giova]@arcetri.astro.it
2 INAF - Istituto di Radioastronomia-Sezione Firenze, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: hunt@arcetri.astro.it
Received 10 July 2008 / Accepted 27 September 2008
Abstract
Aims. We use different tracers, such as H
,
ultraviolet (UV), and infrared (IR) emissions at various wavelengths,
to study the dust and star formation (SF) conditions throughout the disk
of M 33.
Methods. We derive the radial distribution of dust, of the old and young stellar
population using Spitzer and GALEX data, complemented
by ground-based optical data and available surveys of atomic
and molecular gas. We separate the contribution of discrete sources
to the IR brightness from the diffuse emission.
Results. At 8 and 24 
m, discrete sources account for 
40% of the IR emission
in the innermost 3 kpc, and for 
20% further out.
We find that stochastic emission from very small grains in the diffuse
interstellar medium accounts for only ~10% of the diffuse 24 m emission,
and that dusty circumstellar shells
of unresolved, evolved AGB stars (carbon stars) are a viable alternative.
The 8 m profile suggests that PAH emission
declines faster with radius than the dust continuum.
In annular regions 0.24 kpc wide, we find a mean extinction value for the stellar
continuum
mag with a weak dependence on radius, consistent with
the shallow metallicity gradient observed. Dust opacity derived from the 160 m emission
decreases instead by a factor of 10 from the center to edge of the star forming disk.
Conclusions. Using extinction corrected UV and H maps we find the global SF rate in M 33 over the last
100 Myr to be
yr-1.
Far-IR and total-IR luminosities can trace SF even though
a high conversion factor is required to recover the effective rate.
If carbon stars are powering the diffuse 24 m emission in M 33, this
can trace star formation 1 Gyr ago and provide a more complete view of the
SF history of the galaxy. Today
the SF rate declines radially with a scale length of ~2 kpc, longer than
for the old stellar population, suggesting an inside-out growth of the disk.
Key words: galaxies: individual: M 33 -- galaxies: ISM -- galaxies: Local Group -- galaxies: spiral
© ESO 2009