7 research outputs found
The Influence of Supershells and Galactic Outflows on the Escape of Ionizing Radiation from Dwarf Starburst Galaxies
We study the escape of Lyman continuum radiation from the disks of dwarf
starburst galaxies, with and without supershells, by solving the radiation
transfer problem of stellar radiation through them. We model disks with
Md=10^8-10Msun, with exponential surface density profiles as a function of
redshift, and model the of repeated supernova explosions driving supershells
out of the disks, using the hydrodynamic simulation code, ZEUS-3D. The amount
of star formation is assumed proportional to mass above some density threshold.
We vary the threshold to explore the range of star formation efficiencies,
f*=0.006, 0.06, and 0.6. We find that the interstellar gas swept up in dense
supershells can effectively trap the ionizing photons, before the supershells
blow out of the disks. The blow-outs then create galactic outflows, chimneys
which allow the photons to escape directly to the intergalactic medium. Our
results are consistent with escape fractions of less than 0.1 measured in local
dwarf starburst galaxies, because they are likely observed while the starbursts
are young, before blow-out. We suggest that high-redshift dwarf starburst
galaxies may make a substantial contribution to the UV background radiation
with total escape fractions >0.2, as expected if star formation efficiencies
>0.06.Comment: Accepted by Astrophysical Journal, 20 pages, 14 figure
Origin of Weak MgII and Higher Ionization Absorption Lines in an Outflow from an Intermediate-Redshift Dwarf Satellite Galaxy
Observations at intermediate redshifts reveal the presence of numerous,
compact, weak MgII absorbers with near to super-solar metallicities, often
surrounded by more extended regions that produce CIV and/or OVI absorption in
the circumgalactic medium at large impact parameters from luminous galaxies.
Their origin and nature remains unclear. We hypothesize that undetected,
satellite dwarf galaxies are responsible for producing some of these weak MgII
absorbers. We test our hypothesis using gas dynamical simulations of galactic
outflows from a dwarf satellite galaxy with a halo mass of
M, which could form in a larger halo at z=2, to study the gas
interaction in the halo. We find that thin, filamentary, weak MgII absorbers
are produced in two stages: 1) when shocked core collapse supernova (SNII)
enriched gas descending in a galactic fountain gets shock compressed by upward
flows driven by subsequent SNIIs and cools (phase 1), and later, 2) during an
outflow driven by Type Ia supernovae that shocks and sweeps up pervasive SNII
enriched gas, which then cools (phase 2). The width of the filaments and
fragments are pc, and the smallest ones cannot be resolved at
12.8 pc resolution. The MgII absorbers in our simulations are continuously
generated for >150 Myr by shocks and cooling, though each cloud survives for
only ~60 Myr. Their metallicity is 10-20% solar metallicity and column density
is cm. They are also surrounded by larger (0.5-1 kpc) CIV
absorbers that seem to survive longer. In addition, larger-scale (>1 kpc) CIV
and OVI clouds are produced in both expanding and shocked SNII enriched gas
which is photoionized by the UV metagalactic radiation at intermediate
redshift. Our simulation highlights the possibility of dwarf galactic outflows
producing highly enriched multiphase gas.Comment: 21 pages, 16 figure
The Origin and Kinematics of Cold Gas in Galactic Winds: Insight from Numerical Simulations
We study the origin of Na I absorbing gas in ultraluminous infrared galaxies
motivated by the recent observations by Martin of extremely superthermal
linewidths in this cool gas. We model the effects of repeated supernova
explosions driving supershells in the central regions of molecular disks with
M_d=10^10 M_\sun, using cylindrically symmetric gas dynamical simulations run
with ZEUS-3D. The shocked swept-up shells quickly cool and fragment by
Rayleigh-Taylor instability as they accelerate out of the dense, stratified
disks. The numerical resolution of the cooling and compression at the shock
fronts determines the peak shell density, and so the speed of Rayleigh-Taylor
fragmentation. We identify cooled shells and shell fragments as Na I absorbing
gas and study its kinematics. We find that simulations with a numerical
resolution of \le 0.2 pc produce multiple Rayleigh-Taylor fragmented shells in
a given line of sight. We suggest that the observed wide Na I absorption lines,
= 320 \pm 120 km s^-1 are produced by these multiple fragmented shells
traveling at different velocities. We also suggest that some shell fragments
can be accelerated above the observed average terminal velocity of 750 km s^-1
by the same energy-driven wind with an instantaneous starburst of \sim 10^9
M_\sun. The bulk of mass is traveling with the observed average shell velocity
330 \pm 100 km s^-1. Our results show that an energy-driven bubble causing
Rayleigh-Taylor instabilities can explain the kinematics of cool gas seen in
the Na I observations without invoking additional physics relying primarily on
momentum conservation, such as entrainment of gas by Kelvin-Helmholtz
instabilities, ram pressure driving of cold clouds by a hot wind, or radiation
pressure acting on dust. (abridged)Comment: 65 pages, 22 figures, accepted by Astrophys. J. Changes during
refereeing focused on context and comparison to observation
Cosmological Feedback from High-Redshift Dwarf Galaxies
We model how repeated supernova explosions in high-redshift dwarf starburst
galaxies drive superbubbles and winds out of the galaxies. We compute the
efficiencies of metal and mass ejection and energy transport from the galactic
potentials, including the effect of cosmological infall of external gas. The
starburst bubbles quickly blow out of small, high-redshift, galactic disks, but
must compete with the ram pressure of the infalling gas to escape into
intergalactic space. We show that the assumed efficiency of the star formation
rate dominates the bubble evolution and the metal, mass, and energy feedback
efficiencies. With star formation efficiency f*=0.01, the ram pressure of
infall can confine the bubbles around high-redshift dwarf galaxies with
circular velocities v_c>52 km/s. We can expect high metal and mass ejection
efficiencies, and moderate energy transport efficiencies in halos with
v_c~30-50 km/s and f*~0.01 as well as in halos with v_c~100 km/s and f*>>0.01.
Such haloes collapse successively from 1-2 sigma peaks in LambdaCDM Gaussian
density perturbations as time progresses. These dwarf galaxies can probably
enrich low and high-density regions of intergalactic space with metals to
10^-3-10^-2 Zsun as they collapse at z~8 and z<5 respectively. They also may be
able to provide adequate turbulent energy to prevent the collapse of other
nearby halos, as well as to significantly broaden Lyman-alpha absorption lines
to v_rms~20-40 km/s. We compute the timescales for the next starbursts if gas
freely falls back after a starburst, and find that, for star formation
efficiencies as low as f*<0.01, the next starburst should occur in less than
half the Hubble time at the collapse redshift. This suggests that episodic star
formation may be ubiquitous in dwarf galaxies.Comment: Accepted for ApJ v613, 60 pages, 15 figure