D. J. Sand, J. Strader, B. Willman, D. Zaritsky, B. McLeod, N. Caldwell, A. Seth, E. Olszewski
We present deep wide-field photometry of three recently discovered faint
Milky Way satellites: Leo V, Pisces II, and Canes Venatici II. Our main goals
are to study the structure and star formation history of these dwarfs; we also
search for signs of tidal disturbance. The three satellites have similar
half-light radii ($\sim 60-90$ pc) but a wide range of ellipticities, extending
up to $\sim 0.5$ for Leo V. Both Leo V and CVn II show hints of stream-like
overdensities at large radii, and the blue horizontal branch stars in Leo V are
more spatially extended than its other stars at high statistical significance.
An analysis of the satellite color-magnitude diagrams shows that all three
objects are old ($>$ 10 Gyr) and metal-poor ([Fe/H] $\sim -2$), though neither
the models nor the data have sufficient precision to assess when the satellites
formed with respect to cosmic reionization. The lack of an observed younger
stellar population ($\la 10$ Gyr) possibly sets them apart from the other
satellites at Galactocentric distances $\ga 150$ kpc. We present a new
compilation of structural data for all Milky Way satellite galaxies and use it
to compare the properties of classical dwarfs to the ultra-faints. There is not
a significant difference between the ellipticity distributions of the two
groups. However, the faintest satellites tend to be more aligned toward the
Galactic center, and those satellites with the highest ellipticity ($\ga 0.4$)
have orientations ($\Delta \theta_{GC}$) in the range $20^{\circ} \lesssim
\Delta \theta_{GC} \lesssim 40^{\circ}$. This latter observation is in rough
agreement with predictions from simulations of dwarf galaxies that have lost a
significant fraction of their dark matter halos and are being tidally stripped.
View original:
http://arxiv.org/abs/1111.6608
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