Observations of the Impact of Starbursts on The Interstellar
Medium in Dwarf Galaxies
A.T. Marlowe, T.M. Heckman, R. F. G. Wyse (JHU) & R.Schommer (CTIO)
ABSTRACT
Dwarf galaxies play a crucial role in our understanding of the formation
and evolution of galaxies, and the concept of supernova-driven mass
outflows is a vital ingredient in theories of the structure and evolution
of dwarf galaxies. Despite the theoretical importance of these outflows,
there is a very limited amount of direct observational evidence for their
existece. We have therefore begun a detailed multi-wave-band search for
outflows in dwarf ( M(B) > = -18) galaxies with extensive recent or on-going
star-formation.
We report the first results of this search in the present paper. Observations
of the ionized gas in dwarf Amorphous galaxies with centrally concentrated
populations of massive stars provide evidence for the large-scale expansion
of their ionized interstellar media. Fabry-Perot H-alpha images reveal the
presence of kpc-scale 'superbubbles' and filaments which tend to be
oriented along the galaxy minor axis. These structures are comparable
in size to the characteristic optical sizes of the galaxies, and
dominate the morphology of the galaxies at low surface brightness in
H-alpha. Echelle spectra taken through these structures demonstrate that
the kimematics are consistent with hollow kpc-scale structures expanding
at typical velocities of about 50 km/s (ranging from about 25 and 100 km/s).
The mechanical energy output from the supernovae and stellar winds in the
central region of each galaxy appears adequate to power expansion motions
of this size and speed. Since the inferred exansion velocites are comparable
to (but probably a bit smaller than) the rather uncertain escape velocities
of these galaxies, the ultimate fate of the accelerated gas is not clear.
However, even if this cool and relatively dense H-alpha emitting gas is
bit ejected from the galaxies, is is likely that the much hotter (supernova
and stellar wind heated) gas in the interior of the superbubble will escape
the galaxies once the superbubble fragments. Since this hot gas contains
the newly sythesized metals, the chemical evolution of dwarf galaxies may
be strongly affected by these outflows, even if the galaxy is able to retain
most of its interstellar medium. Since expanding structures of this size and
velocity are not observed in all low-mass galaxies with recent or on-going
star formation, we suggest that we are witnessing transient events that likely
have a relatively low 'duty-cycle' in such galaxies. That is, we argue that
the particular galaxies in the present paper have had signifcantly elevated
star-formation rates over the past 10^7 to 10^8 years (i.e. these are
starburst or young post-starburst systems). This interpertation is consistent
with the optical colors and emission-line properties of these galaxies.
Subject Headings: Galaxies: Evolution - Galaxies: Interstellar Medium
- Galaxies: Kinematics & Dynamics - Galaxies: Starbursts