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Astronomical Applications Department, U.S. Naval Observatory - thesis (Page 21)

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Astronomical Applications Department, U.S. Naval Observatory - thesis
7
1.2.1 Holmberg II
The apparently non-interacting Im-type galaxy Holmberg II HoII, DDO 50,
UGC 4305 is a member of the M81-NGC 2403 group of galaxies. IRAS observa-
tions suggest a minimal presence of dust, so little hidden star formation is taking
place Hunter
et
al.
1989. Hunter & Gallagher 1985 estimate the overall star
formation rate in HoII to be only 0.00084 M yr
,
1
and relatively constant over
a Hubble time. The galaxy has a low metal abundance, Z Z
0.4 Hunter &
Gallagher 1985, similar to that of the Large Magellanic Cloud LMC. Hodge
et
al.
1994 nd that the sizes of HII regions as well as their size distribution
are statistically normal in comparison to galaxies of the same type, distance, and
luminosity. In the HI, HoII has an envelope that extends to 1.5 D
Ho
where the
Holmberg diameter D
Ho
is the diameter de ned by the B isophote at 26.6 mag
arcsec
,
2
. The HI morphology of HoII is intriguing due to the various shells and
holes present throughout the galaxy. In a comprehensive study of the H I gas,
Puche
et
al.
1992 catalog these expanding features assumed to be the result of
massive star formation. Rhode
et
al.
1998 doubt that this scenario is responsi-
ble for all these features since they fail to identify candidate exciting stars in deep
optical searches. However, their results depend on the IMF, which is not well
known. Hoessel
et
al.
1998 estimate a distance of 3.05 Mpc using 28 Cepheid
candidates. This is di erent than the distance estimate of 3.63 Mpc determined
by Freedman
et
al.
1994 for M81 using Hubble Space Telescope observations of
Cepheid variables. Since HoII is likely the closest member of the M81-NGC 2403
group, 3.05 Mpc is adopted for the distance of HoII Hoessel
et
al.
1998. Table
1.1 summarizes the various parameters assumed for the galaxy in this study.

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