Does Stellar Feedback Create HI Holes? An HST/VLA Study of
Does Stellar Feedback Create HI Holes?
An HST/VLA Study of Holmberg II
ApJ, 2009, 704, 1538
Evan Skillman1, John Cannon2, Andy Dolphin3,
Rob Kennicutt4, Janice Lee5, Fabian Walter6
of Minnesota, 2Macalester College, 3Raytheon Company, 4IoA Cambridge, 5OCIW, 6MPIA
HI Holes in Holmberg II
Nearby Dwarf Galaxies
Dozens of holes, shells, and bubbles
have been identified in the ISM of Ho II.
Each `hole’ (green) has a measured size
and expansion velocity. Non-hole
control fields are used for comparison
(cyan). Rhode et al. (1999; red) explored
the stellar feedback connection to hole
creation, finding few bright central
clusters. We use deep HST/ACS imaging
to explore the stellar populations of the
The Local Volume provides a wealth of
diverse galaxies ideal for studying SF
through resolved stellar populations.
The ACS Nearby Galaxy Survey Treasury
(ANGST; Dalcanton et al. 2009; Weisz et
al. 2010) has multi-color photometry of
individual stars in ~60 nearby dwarf
HST/ACS Imaging of Ho II
CMDs provide a fossil record of past SF
activity in a galaxy. Model CMDs show
how we extract the ages of stars. We
can then convert these to SFHs, SFR(t,Z).
Young MS and core red and blue helium
burning stars (RHeBs, BHeBs) provide
excellent leverage for the ages and
locations of recent SF.
We observed Ho II as part of the M81
Group dwarfs program (Weisz et al.
2008) in F555W and F814W.
Ho II is one of the larger dIs in the M81
Group recent SFRs ~0.01-0.1 M/yr. It’s
proximity to M81 makes it a typical
group member, but it is not strongly
interacting – suggesting an internal
origin of the holes. Further, there is
nothing peculiar about the stellar or HI
distribution of Ho II to suggest a recent
Stars Within HI Holes
Deep HST/ACS imaging reveals that all
HI holes in Ho II contain young, mixed
age stellar populations. In many cases,
the youngest stars are tens of Myr old,
and fainter then detection limits of
Rhode et al. (1999). Control fields show
similar CMDs, suggesting small holes
(<100 pc; HI resolution limit) may be
Star Formation Histories
From the CMDs, we compute the SFHs
for stars in each HI hole. Comparing the
SFHs to the dynamical ages (HI
expansion velocity and hole size; red
dashed line), we see the majority of SF
does not always take place within the
dynamical age. This suggests multiple
SF events, and not a single episode of
SF, i.e., a cluster, is the most likely mode
of HI hole creation.
Spatially Resolved SFH
BHeBs have a unique age-luminosity
relationship. Knowing the magnitude of
a BHeB equates to knowing its age.
Combining the age and spatial
information from the BHeBs with the
CMD based SFH, we construct the
spatially resolved SFH of Ho II, showing
how SF has propagated in the galaxy in
space, time, and amplitude over ~ 100
Z = 0.2 Z
HI and Stellar Energetics
The energy necessary to evacuate an HI hole with diameter, d, depends on the
initial HI volume density, n0, and the observed HI expansion velocity, v, Chevalier
(1974) show that:
The stellar energies can then be
computed using the SFHs and
STARBURST99. We can then compare
the stellar energy profile over the last
200 Myr to the HI hole creation energy.
We find energy from SF is generally not
sufficient to have created an HI hole
within its dynamical age (grey dashed
line). However, in all cases they do have
enough energy to have created the
holes on slightly longer timescales,
suggesting multiple generations of SF
may have been present.
We also consider the effects of stellar
feedback efficiencies (red:100%,
Locations of Young Stars
We use the young MS stars to study the
spatial distribution of recent SF. The
youngest stars (<10 Myr; red) are
typically clustered on the edges of HI
holes, while the older population (< 75
Myr; blue) is slightly more distributed.
We do not see evidence for clustering
inside HI holes. SF at multiple locations
could have created small HI holes, that
later merged to form the larger ones we
MB = -16.72
We compare the HI Hole locations to
Hα, GALEX UV, and Spitzer 24μm
imaging. We find Hα around the
rims of holes, indicating triggered
secondary SF. The UV is also
clumpy, but not typically within
the HI holes. Generally it is diffuse
within these regions. These SF
indicators are not reliable tracers
of HI hole locations.
1. The timing, energetics, SFHs, and spatial distributions of stars
suggest that HI hole formation is complex and likely due to multiple
generations of SF.
2. The simple model of single epoch SF resulting in HI hole formation
appears to be an uncommon mode of HI hole formation.
3. Only deep CMDs of resolved stars are capable of detecting the
remnant young stellar populations.
Chevalier, 1974, ApJ, 188, 501; Dalcanton, 2009, ApJS, 183, 67D;
Rhode et al., 1999, AJ, 118, 323; Weisz et al., 2010, In Prep.
This work has been supported by the University of Minnesota and HST GO-10605