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Why
areNuclear
Star
Clusters
interesting?
1)
Many
--
if
not
all
­
nuclear
star
clusters
appear
to
be
young,
or
at
least
have
a
young
population
of
stars.
This
means
that
they
probably
form
in
situ,
as
opposed
to
infall
of
an
old
globular
cluster.
2)
The
formation
mechanism
of
such
clusters
is
puzzling,
especially
in
late­type
spirals
without
prominent
bulges.
In
these
cases,
the
gravitational
potential
is
very
shallow,
and
it
is
not
obvious
how
the
gas
could
fall
all
the
way
to
the
center
to
form
a
dense
star
cluster.
3)
If
nuclear
clusters
indeed
form
in
the
galaxy
center,
their
impact
on
the
dynamics
of
stars
and
gas
in
the
central
region
of
the
host
galaxy
needs
to
be
understood,
because
it
can
potentially
influence
the
galaxy
morphology.
For
example,
it
has
been
suggested
that
nuclear
clusters
can
destroy
stellar
bars
and
thus
form
galaxy
bulges.
4)
If
many
galaxies
are
found
to
harbor
young
nuclear
clusters,
their
formation
is
plausibly
a
recurrent
process.
However,
there
is
no
observational
evidence
for
an
underlying
older
generation
of
stars
in
extragalactic
nuclear
clusters.
This
might
be
explained
by
the
observational
difficulty
to
detect
a
faint
older
population
in
the
presence
of
a
bright
younger
one.
On
the
other
hand,
little
is
known
about
the
fate
of
star
clusters
in
the
unique
environment
of
galactic
nuclei.
Is
it
possible
that
they
evaporate
on
relatively
short
timescales,
despite
the
fact
that
they
are
fairly
massive
(10 5
--
10 6
M#
).
###############################################
#
###
#####
T.
BÆ
ker,
S.
Laine,
R.
P.
van
derMarel
(STScI),
L.
C.
Ho
(Carnegie),
H.­W.
Rix
(Heidelberg),
&
J.
C.
Shields
(Ohio
Univ.)
Abstract
Wepresent
first
results
from
anHSTWFPC2
snapshot
survey
to
investigate
the
nature
of
nuclear
star
clusters
in
late­type
spiral
galaxies.
Wedescribe
the
scientificmotivation
of
the
program,
summarize
the
data
analysis
plans,
and
give
first
results
of
some
of
the
objects
observed
so
far.
In
particular,
our
images
reveal
the
cluster
sizes,
luminosities,
and
locations
relative
to
the
photocenter
of
the
galaxy.
These
will
yield
new
insight
into
the
formationmechanism
of
these
intriguing
objects
and
their
possible
impact
on
the
evolution
of
their
host
galaxies.
1.
Introduction
Recent
HST
observations
have
revealed
that
many
spiral
galaxies
have
a
prominent
star
cluster
in
their
dynamical
center.
However,
statistics
for
frequency,
size,
and
luminosity
of
nuclear
star
clusters
remain
incomplete.
To
fill
this
gap,
we
are
undertaking
a
Cycle
9WFPC2
snapshot
survey
of
a
well­
defined
sample
of
late­type
galaxies.
In
combination
with
follow­up
spectroscopy,
this
survey
aims
to
measure
the
age
distribution
of
nuclear
clusters
and
to
clarify
whether
their
formation
is
a
one­time
event
or
a
recurrent
process.
This
question
is
important
because
a
central
mass
concentration
is
required
for
some
scenarios
of
bulge
formation.
2.
The
Sample
We
have
defined
our
sample
according
to
the
following
criteria:
#Hubble­type between Scd and Sm (6
T
9) #
absence of a prominent
bulge makes it easier to identify any nuclear source #
Orientation close to face­on, i.e. b/a > 0.6 #
minimizes dust extinction
#
Line­of­sight velocity v z
< 2000 km/s #
clusters are more easily resolved
in nearby galaxies
A total of 122 galaxies were found to match these criteria, only
9 of which
have previously been observed with HST. 3. Data Analysis For each galaxy, we obtain a 600s CR­SPLIT exposure in the WFPC2 F814W filter (similar to I­band). The galaxy nucleus is centered on the PC chip to assure highest spatial resolution. This is crucial in order to measure the cluster sizes. For each galaxy image, we use elliptical isophote fitting
algorithms to
derive the surface brightness profile (SBP) of the galaxy. The presence of a nuclear
cluster
is
indicated
by
a
distinct
``kink''
in
the
SBP,
which
indicates
that
the
central
light
concentration
is
not
simply
an
inward
extension
of
the
bulge/disk
profile.
For
quantitative
analysis,
we
fit
an
analytical
function
to
the
SBP.
By
extrapolating
a
separate
fit
to
the
SBP
of
the
disk
inwards,
we
can
estimate
the
magnitude
of
the
nuclear
cluster
with
high
accuracy,
as
shown
in
the
Figures
below.
Results
so
far
As
of
today,
15
galaxies
have
been
observed,
10
of
which
have
been
analyzed.
Out
of
those
10,
7
clearly
show
a
nuclear
star
cluster.
One
galaxy
has
an
ambigous
detection,
and
only
two
show
no
evidence
for
a
central
source.
Although
preliminary,
and
based
on
only
a
small
subset
of
our
full
sample,
this
result
already
suggests
that
nuclear
star
clusters
are
indeed
ubiquitous
in
late­type
spirals.
In
order
to
answer
some
of
the
above
questions,
follow­up
spectroscopy
is
required.We
have
been
awarded
a
Cycle
10
snapshot
program
to
obtain
STIS
medium­resolution
spectra
of
the
40
brightest
nuclear
clusters.
The
spectra,
centered
around
the
4000
Angstrom
break,
will
allow
us
to
age­date
the
stellar
population
of
the
brightest
clusters
via
comparison
to
population
synthesis
models.
The
cluster
age
distribution,
in
turn,
will
help
to
unravel
their
formation
history.
An
example
for
this
kind
of
analysis
is
shown
below
for
the
nuclear
cluster
in
NGC
4449.
In
this
case,
the
spectrum
was
obtained
from
ground­based
observations
(BÆ
ker
et
al.,
AJ,
March
2001
issue).
The
solid
line
is
a
synthesized
spectrum
of
a
10
Myr
old
population
with
a
Salpeter
IMF,
based
on
Bruzual
&
Charlot
models.
NGC
6509
NGC
4540
NGC
2805
NGC
4540
NGC
2805
NGC
6509
mI
=19.11
mI
=18.93
mI
=19.56
Targets
analyzed
so
far
Galaxy
Nuc.
Cluster?
m I
NGC
853
Yes
tbd
UGC
3826
Yes
tbd
NGC
2805
Yes
18.93
UGC
6931
Yes
tbd
NGC
4540
Yes
19.11
NGC
4904
Amb.
?
NGC
5477
No
­
NGC
6509
Yes
19.56
A2231+32
No
­
ESO
290­39
Yes
tbd
Support
for
this
work
was
provided
by
NASA
through
grant
number
GO­08599.04­A,
awarded
by
STScI
which
is
operated
by
AURA,
Inc.,
under
NASA
contract
NAS5­26555.