The Star Formation History of Low Surface Brightness Galaxies
The Star Formation History of Low Surface Brightness Galaxies
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Date
2007-11-14
Authors
Kim, Ji Hoon
Advisor
McGaugh, Stacy S
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Abstract
The star formation histories of low surface brightness galaxies are
interesting but poorly constrained.
These objects tend to be rather blue, contradicting the initial
impression that they may simply be faded remnants of higher surface brightness
galaxies whose star formation has finished.
Other scenarios span a broad range:
a young mean age, less dust, a lower metallicity, perhaps even a variable IMF.
Distinguishing between these scenarios requires sufficient information to build
stellar population synthesis models which, if not unique, at least exclude
certain possibilities.
The total stellar mass (M*) of a galaxy is
most closely traced with the Ks-band light.
Considering that this mass must form over a Hubble time, this in effect
gives a measure of the time averaged star formation rate (<SFR>
~M* Ho).
H alpha emission traces the location of star formation, and also provides
a fairly robust quantitative measure of its current rate.
We have obtained near-infrared broadband photometry and H alpha
photometry of a large sample of low surface brightness galaxies
to measure the current and the time-averaged star formation rate
in order to constrain their star formation histories.
The current star formation rates of
LSBGs generally are higher than
their past star formation rate, suggesting that
the mean age of their stellar population
is relatively young. This may stem from
either a late epoch of formation or a sluggish
evolution.
In the latter case, the star formation
efficiency may be an increasing function
of time, perhaps due in part to the slow
build up of metals and dust. Nevertheless,
star formation remains sporadic and is
generally not well organized across the disk.
We find a strong correlation between
the ratio of current to past average
star formation rate and the gas mass
fraction. Galaxies with large reservoirs
of gas have relatively high current SFRs.
There is a conspicuous absence of high
gas mass fraction,
low SFR galaxies, suggesting that
the observed trend is not driven by bursts
of star formation with short duty
cycles.