Probing Star Formadon in Nearby Galaxies: The Case of the


Probing
Star
Forma.on
in
Nearby
Galaxies:
The
Case
of
the
Garland
Region
in
NGC
3077
NGC3077
Garland
Zhong
Wang,
Andrew
Szentgyorgyi
(CfA)
and
Susan
Neff
(GSFC)
MMT
Science
Symposium,
May
19,
2010
Summary
of
Our
Program
It
is
well
known
that
gravita.onal
interac.ons
can
strongly
affect
the
star
forma.on
process
and
the
evolu.on
history
of
galaxies.
But
exactly
how
and
to
what
extent
such
processes
take
place
remains
to
be
explored.
With
the
extraordinary
capabili.es
of
the
Hectochelle
on
MMT,
we
are
able
to
probe
the
kinema'cs
of
individual
star‐forming
regions
in
NGC
3077,
and
iden.fy
a
clear
associa.on
related
to
the
.dal
effects
within
its
group
environment.
This
group,
with
M81/M82/NGC3077
as
its
most
prominent
members,
is
clearly
undergoing
ac.ve,
mul.‐body
interac.ons
with
.dal
features
seen
both
in
stellar
and
gas
components.
With
Spitzer
we
uncovered
clues
of
.dal‐induced
star
forma.on,
and
focus
on
a
par.cularly
interes.ng
region,
known
as
“the
Garland”,
about
4
kpc
to
the
east
of
NGC
3077.
This
region
has
the
special
advantage
in
that
it
is
far
outside
the
main
galaxy’s
disk,
so
the
characteris.cs
of
on‐going
star
forma.on
can
be
iden.fied
with
rela.vely
li_le
confusion.
Our
approach
is
to
measure
at
high
precision
the
emission
(Hα)
line
profiles
of
all
HII
regions
in
and
around
the
ring‐like
structure
of
the
Garland,
with
velocity
resolu.on
of
~
0.5
km
s‐1.
We
have
successfully
measured
nearly
all
of
the
34
HII
regions
in
the
area,
and
mapped
out
the
overall
kinema.c
profile
of
the
Garland
region.
Our
results
clearly
show
that
the
ring‐like
structure
is
indeed
a
physically‐related,
coherently
moving
shell
within
the
.dal‐torn
HI
gas
concentra.on.
This
is
strong
and
direct
evidence
that
the
local
star
forma.on
is
induced
by
the
.dal
force,
and
that
an
earlier
genera.on
of
star
forma.on
may
be
triggering
newer
waves
of
star
forma.on
wherever
gas
concentra.ons
remain
precipitable.
The
Spitzer
data:
traces
of
star‐forming
ac.vi.es
IRAC
4.5
–
5.8
–
8.0
μm
color
composite:
the
ring‐like
structure
encompassing
many
star‐forming
regions
Garland
The
same
structure
is
also
seen
in
MIPS
24,
70
and
160
μm
images.
This
region
coincides
with
the
large
concentra.on
of
HI
gas
torn
off
the
center
of
NGC
3077,
as
a
result
of
.dal
forces.
Op.cal
and
radio
data:
individual
HII
regions
can
be
clearly
iden.fied
NGC
3077
Garland
Hα
image
from
the
Hubble
(ACS
F658)
Radio
con.nuum
data
(VLA
NVSS
survey)
Atomic
gas,
molecular
gas,
HII
regions,
and
emission
line
profiles
measured
with
the
Hectochelle
HII
regions
and
the
interstellar
medium
Spitzer
image
In
the
upper
leh
(courtesy
F.
Walter),
atomic
(thin
contours)
and
molecular
(thick
contours)
gas
distribu.ons
in
the
Garland
region
are
shown
along
with
many
of
the
HII
regions
(numbered
dots)
which
we
measured
using
the
MMT.
200
MMT
data
150
One
can
see:
1)
The
en.re
Garland
region
is
at
the
local
HI
peak
loca.on
2)
Most
HII
regions
are
located
close
to
the
molecular
gas
3)
HII
regions
delineate
the
same
ring
as
seen
by
Spitzer
100
50
The
lower
leh
panel
shows
the
Hα
line
profiles
of
the
fainter
set
of
HII
regions
measured
with
Hectochelle.
0
-50
0
50
km/s
100
150
Results:
The
velocity
field
of
the
HII
regions
‐‐‐
based
on
the
Hectchelle
data
(Loca.on
map
centered
on
the
Garland
region.
The
sizes
of
the
symbols
are
propor.onal
to
the
velocity
rela.ve
to
the
mean
value,
red=posi.ve,
blue=nega.ve)
Dec
offset
(arc
sec)
The
Garland
RA
offset
(arc
sec)
This
diagram
shows
that
the
many
HII
regions
clustered
around
the
Garland
have
a
very
small
velocity
dispersion
(+/‐
4km
s‐1)
with
respect
to
each
other.
So
the
Garland
is
indeed
a
large,
kinema'cally
coherent
cluster
of
HII
regions.
Based
on
these
data
and
our
models,
they
appear
to
be
associated
to
each
other
as
the
consequence
of
a
.dal‐induced
burst
of
star‐forming
events.