Systematic Studies on Dwarf Irregular Galaxies via their SEDs

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Systematic Studies on Dwarf Irregular Galaxies
via their SEDs
Thanks to Dr. A. Tomita, Dr. K. Yoshikawa, and
Dr. H. Hirashita
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WHY study DIGs (Part I)
1930s to 1940s DIGs have been regarded as a minor
group. 1970s DIGs are a dominant group!
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WHY study DIGs (Part II)
DIGs have nearly the same magnitude of star
formation as Spirals. (Ellipticals have NO
present star formation.)
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WHY study DIGs (Part III)
Distant galaxies ( galaxies in the early
Universe)
Small size Irregular shape
Resemble to Local DIGs.
Study Local DIGs.
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Present Star Formation Rates from Ha Line
SFR is various!
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A mixture of various stellar populations.
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Past average SFR
Assumption Age of the galaxy is the age of the
universe.
Past average SFR
(Total stars)/age of the galaxy.
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Present-to-past SFR ratios
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Summary from present observations
Star formation rate in irregulars Changing
violently in time. Can be very high compared with
that in the past.
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Theoretical scope for galaxy formation I
Importance of the study of low metallicity
galaxies in the infrared
Dust amount of low-metallicity galaxies
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Theoretical scope for galaxy formation II
H2 formation in extremely low-metallicity
environment
Molecular hydrogen is formed on dust grains.
The H2 formation can be ineffective in primordial
galaxies because of the lack of dust.
In such case H2 can only be formed through
reaction in gas phase.
Since only H2 molecule is the coolant of gas in
primeval galaxies, star formation in such
galaxies is controlled by the formation rate of
H2.
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Dependence of H2 formation on dust
H2 is formed on dust grains.
Xe electron fraction
H2 is formed in gas phase.
Gas temperature
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Theoretical scope for galaxy formation III
H2 formation efficiency is strongly dependent on
dust temperature (e.g. Katz et al. 1999)
Flux of atoms
15 K
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H2 formation on dust grain might be suppressed in
dIrrs/BCDs because of the high dust temperature.
Dust temperature has crucial importance.
Furthermore.
Supernova rate and destruction rate of dust
grains are related.
ASTRO-F all-sky survey is a unique opportunity
for these studies.
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How about SIRTF SINGS ?
SINGS (SIRTF Nearby Galaxies Survey) Kennicutt et
al.
Scientific scope is similar to ours. 75 galaxies
with all morphological types. 17 Irregular
galaxies (only 5 is fainter than -14 mag).
Is the sample enough?
It seems too small, because generally irregulars
are divergent in their properties, especially SFR
and Z.
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Expected number of star-forming irregulars
From the Local LF, about 103 irregulars/BCDs will
be detected in the all sky.
For secure statistical studies, all-sky survey is
crucial. Homogeneous measurement including upper
limits has vital importance in this study.
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An example of observed BCD
SBS 0335-052
Size 11 Distance 53.3 Mpc mB 16.76
mag LB 5.3108 Lsolar f?(12.5µm) 35
mJy f?(67.3µm) 112 mJy
Very hot dust!
Dale et al. 2001 (astro-ph/0107108)
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MIR and other observations
BCDs have strong UV radiation field and therefore
generally hot.
MIR observation is expected to be feasible.
The IRC survey mode is too shallow, so IRC
pointing observation of known galaxies is
suitable for this purpose. But it is still not
efficient, so the sky area should be limited.
Then, spectroscopy?