This document summarizes observations of the Andromeda satellite dwarf galaxy Cassiopeia III using the WIYN pODI imager. Preliminary analysis of the color-magnitude diagram shows stars that coincide with a 12-Gyr isochrone at the known metallicity and distance of Cas III. Structural parameters were measured including position angle, ellipticity, and half-light radius that are consistent with previous work but have smaller uncertainties. Future work will include PSF photometry, deriving the surface brightness profile and distance, and analyzing the metallicity distribution.

1. Ge#ng
to
Know
the
Neighbors:
Deep
Imaging
of
the
Andromeda
Satellite
Dwarf
Galaxy
Cassiopeia
III
with
WIYN
pODI
Madison
Smith
1,
Katherine
Rhode
2,
Steven
Janowiecki
2
(1)
New
College
of
Florida,
Maria
Mitchell
Observatory;
(2)
Indiana
University
OVERVIEW
We
present
results
from
WIYN
pODI
imaging
of
Cassiopeia
III
(And
XXXII),
an
Andromeda
satellite
galaxy
discovered
by
MarOn
et
al.
(2013)
in
Pan-­‐STARRS1
survey
data.
Cas
III
had
not
been
discovered
earlier
because
of
its
locaOon
near
the
GalacOc
plane,
in
an
area
that
had
not
been
systemaOcally
surveyed
unOl
Pan-­‐
STARRS1.
We
use
deep,
wide-­‐field
photometry
with
WIYN
to
study
the
stellar
populaOon
of
Cas
III
in
more
detail.
Here
we
present
a
color-­‐magnitude
diagram
(CMD)
and
preliminary
measurements
of
the
galaxy’s
structural
properOes.
SUMMARY
OF
RESULTS
A
12-­‐Gyr
isochrone
from
Bressan
et
al.
(2012)
with
[Fe/H]
=
-­‐1.7
(the
spectroscopic
metallicity
for
Cas
III
from
MarOn
et
al.
2014)
and
a
distance
modulus
of
m-­‐M
=
24.45
(from
the
PanSTARRS1
discovery
paper)
coincides
with
the
Red
Giant
Branch
locaOon
in
our
CMD.
We
selected
Red
Giant
Branch
stars
on
the
CMD
and
analyzed
the
structural
parameters
of
Cas
III,
following
the
method
described
in
MarOn
et
al.
2008
and
Sand
et
al.
(2012).
Preliminary
best-­‐fidng
values
for
Cas
III
are:
Our
PA,
ellipOcity,
and
rh
values
are
consistent
with
those
in
the
MarOn
et
al.
2013
discovery
paper
within
the
stated
errors,
but
our
values
have
smaller
uncertainOes.
OBSERVATIONS
&
INITIAL
REDUCTIONS
Images
were
obtained
with
the
WIYN
3.5-­‐m
telescope
and
the
parOally-­‐filled
One
Degree
Imager
(pODI),
which
provided
a
24’x24’
field-­‐of-­‐view
and
a
pixel
scale
of
0.11”
pixel-­‐1.
We
used
nine
700-­‐sec
dithered
exposures
in
g’
and
seven
600-­‐
sec
dithered
exposures
in
i’,
with
FWHM
PSF
of
0.8”-­‐0.9”.
Standard
reducOons
(e.g.,
bias,
flat-­‐field
correcOons,
etc.)
were
applied
to
the
images
with
QuickReduce
(Kotulla
2014)
in
the
ODI
Portal,
Pipeline,
&
Archive
(ODI-­‐PPA).
The
images
were
illuminaOon-­‐corrected,
projected
to
a
common
pixel
scale,
and
combined
into
a
single,
deep
image
in
each
filter.
The
high-­‐noise
edges
of
the
dither
pamern
were
clipped
to
yield
a
20’
x
20’
usable
field
(Figure
1).
ACKNOWLEDGMENTS
We
are
grateful
to
Michael
West,
Gary
Walker,
Hannah
Pagel,
Bill
Janesh,
and
Dave
Sand
for
their
support
and
assistance
during
various
stages
of
this
project.
This
research
was
supported
by
the
NSF
Research
Experiences
for
Undergraduates
program
(grant
number
AST-­‐1358980).
REFERENCES
Bressan
et
al.
2012,
MNRAS
472,
127
MarOn
et
al.
2013,
ApJ
772,
15
Kotulla,
R.
2014,
in
ADASS
XXIII,
ASP
Conf.
Ser.
485,
375
Sand
et
al.
2012,
ApJ
756,
79
MarOn,
de
Jong,
&
Rix
2008,
ApJ
684,
1075
Figure
1.
(LeP)
Combined
g’-­‐band
pODI
image
of
Cas
III.
(Right)
LocaUons
of
11,310
point
sources
that
appear
in
both
the
g’
and
i’
images.
The
overdensity
of
stars
in
Cas
III
is
much
more
apparent
here
than
in
the
image.
ANALYSIS
STEPS
We
used
DAOFIND
to
detect
sources
in
the
images
with
S/N
>
3.5.
Extended
sources
were
removed.
We
calculated
aperture
correcOons
for
each
image
and
applied
these
along
with
GalacOc
exOncOon
correcOons
and
photometric
calibraOon
coefficients
to
compute
a
final
set
of
calibrated
magnitudes
for
all
the
point
sources
appearing
in
and
around
Cas
III.
Our
CMD
(Figure
2)
reaches
~2.5
mag
deeper
than
the
CMD
shown
in
the
discovery
paper.
The
4σ
limit
on
the
brightness
of
a
point
source
in
our
images
is
g’
~
25.3
and
i’
~
24.3.
FUTURE
WORK
These
results
are
preliminary;
future
work
will
include:
performing
PSF
photometry,
to
miOgate
crowding
effects
near
the
center
of
the
stellar
distribuOon;
deriving
a
surface
brightness
profile
and
accurate
total
magnitude;
deriving
a
Tip
of
the
Red
Giant
Branch
(TRGB)
distance;
completeness
tesOng
to
quanOfy
the
depth
of
the
images;
and
analyzing
the
photometric
metallicity
distribuOon
funcOon
of
Cas
III.
N
E
5’
Figure
2.
(LeP)
CMD
of
Cas
III
for
stars
within
one
half-­‐light
radius
of
the
galaxy
center.
The
red
line
is
a
12-­‐Gyr
isochrone
with
[Fe/H]
=
-­‐1.7
and
m-­‐M=24.45
from
Bressan
et
al.
(2012).
(Right)
CMD
for
a
“field”
region
near
the
edge
of
the
image
(a
5’
x
20’
rectangle,
approximately
9’
from
the
galaxy
center).
Figure
3.
(LeP)
The
surface
density
of
stars
vs.
radius
for
the
Cas
III
images.
(Right)
The
star
locaUons
relaUve
to
the
center
of
Cas
III,
with
the
best-­‐fi#ng
ellipse
shown
in
red.