Only a small fraction of the stars that form in
the Milky Way are much more massive than our Sun and explode as
supernovae type II at the end of their lifetimes. Still, these high-mass
stars influence the surrounding interstellar medium (ISM) much more
than their small number might suggest, both by their intense radiation
and powerful winds (“pre-supernova feedback”) and through their violent
supernova explosions (“supernova feedback”). Scientists at the Max
Planck Institute for Astrophysics, in the framework of the SILCC
collaboration, use complex supercomputer simulations to investigate the
detailed impact of the different feedback processes on the ISM with
conditions similar to our solar neighborhood. Ionizing radiation from
young, massive stars dominates their energy output and can exceed the
energy released during supernova explosions by an order of magnitude.
Only if the simulation includes this radiative feedback and the momentum
input from stellar winds are the results consistent with observations
of the ISM and the star formation rate is reduced.
High-mass stars dominate the energy output of newly formed
stellar populations. Most of the energy is emitted in the form of
radiation, followed by supernova explosions and stellar winds. When the
radiation deposits its energy in the ISM, the photoelectric heating of
dust and photo-ionization can lead to temperatures of a few thousand
degrees and more. The photo-ionizing radiation is also a major source of
ionized hydrogen in the ISM and drives the formation of so-called “H II
regions”, which consist of hot gas with temperatures of ten thousand
degrees around young, massive clusters. Supernovae and to some degree
stellar winds are energetic enough to shock-heat the ISM to temperatures
of a few million degrees.
The emission of radiation, stellar winds and supernova explosions
therefore all have different effects in shaping the structure of the ISM
and should be considered in concert. Modern attempts to improve the
numerical modelling towards a consistent theory of the ISM and star
formation need to take all three processes into account. A successful
model should then be able to reproduce the ISM as seen in the Milky Way
and the observed relation between the amount of dense molecular gas and
star formation in galaxies.
Together with a European team of experts, scientists at MPA have used
complex supercomputer simulations to investigate the impact of stellar
radiation, stellar winds and supernova explosions on the ISM of a
galactic disk. For the first time, the simulations include all three
dominant forms of stellar feedback and follow the chemical transitions
from ionized over neutral atomic to molecular gas. In the simulations,
star clusters form dynamically out of parcels of gas collapsing under
their own gravity. The team has investigated the effects of the
different forms of feedback from the stars in these clusters on the
structure of the surrounding ISM and the resulting star formation rate
(SFR) in the simulations (see Fig. 1).
Photoionization heating is the dominant energy source in the ISM, it
exceeds the energy input from supernovae by one and from winds by two
orders of magnitude. All the different photochemical processes started
by radiation can individually impart more energy into the ISM than
supernovae as a whole. This radiation, however, is not a constant
source; the star cluster luminosities are highly variable with time
because they are dominated by extremely massive stars that shine very
brightly but have lifetimes of only a few million years.
Fig. 2: The surface density of the Star Formation Rate (SFR) measured in the
simulation (blue) and derived via various SFR calibrations of
observations (other colours) as a function of time. Please note the
logarithmic scale. The offset between the true and the observed SFR can
be up to an order of magnitude. © MPA
The time variability of the cluster luminosities has important
consequences for SFR measurements (Fig. 2). The observed SFR only
matches the true SFR when very massive stars are present in the
clusters. Less massive stars do not produce enough ionizing radiation
and measurements of the so-called “Hα-line” then underestimate the SFR
by up to an order of magnitude; and this result is independent of the
calibration used.
Observationally, the amount of star formation within a patch inside a
galaxy is closely related to the amount of molecular gas that is
present there. The ratio of these two quantities is called the depletion
time, and it is universally found to be around 2 billion years. The
simulation with radiation naturally exhibits a similar depletion time,
while the other simulations fail to do so (Fig. 3).
The “pre-supernova feedback” by both radiation and winds also
influences the third process by significantly reducing the environmental
density of supernova explosion sites. For a simulation with supernova
feedback only, 80% of all supernovae go off in gas with mean densities
below 100 particles per cubic centimeter. If winds are included in the
simulation, this density is reduced by a factor of more than 10, and
with radiation by another factor of 100. Exploding at lower
environmental densities the supernova can cause more “damage” to the ISM
and even drive gas out of the galaxy.
The presence of radiative feedback significantly affects also the
mass fractions of the different chemical states of hydrogen. The
photoionization by star clusters ionizes the gas in the ISM. This
ionized gas then cools radiatively and produces gas in the warm phase,
at the same time leading to a substantial reduction of the fraction of
gas in the hot phase compared to simulations without radiation. This
process is essential to match the observed fractions of the warm and hot
phases.
The simulations thus indicate that “pre-supernova feedback” can
regulate star formation and the abundance of molecular, neutral and warm
ionized gas. “Supernova feedback” determines large-scale turbulent
structure of the ISM, its hot gas volume filling fraction and the
driving of outflows.
To understand which physical processes produce the ISM and star formation observed in galaxies, it is crucial to run complex simulations that include all important ingredients, which are at work simultaneously in complex star forming regions. The simulations of the SILCC collaboration are therefore an important step forward in this endeavor.
Thomas Peters and Thorsten Naab for the SILCC collaboration
Notes:
The SILCC project (Simulating the Life Cycle of molecular Clouds)
is a supercomputing initiative of a group of European scientists to
investigate the formation of molecular clouds, star formation and the
impact of massive stars on parental cloud dispersal and the driving of
galactic outflows. The team consists of Stefanie Walch, Dominik Derigs,
Annika Franeck & Daniel Seifried (University of Cologne), Andrea
Gatto, Philipp Girichidis, Thorsten Naab, Anabele Pardi & Thomas
Peters (Max-Planck-Institute for Astrophysics), Simon Glover & Ralf
Klessen (University of Heidelberg), Christian Baczynski (University of
St Andrews), Richard Wunsch (Astronomical Institute of the Czech Academy
of Sciences), Paul Clark (Cardiff University). Computations are
performed at the Leibnitz Supercomputing Centre and the Max Planck Computing and Data Facility.
Authors
Peters, Thomas
Postdoc
Phone: 2195
Email: tpeters@mpa-garching.mpg.de
Scientific Staff
Phone: 2295
Email: tnaab@mpa-garching.mpg.de
Links: personal homepage (the institute is not responsible for the contents of personal homepages)
Original Publication
The SILCC project - IV. Impact of dissociating
and ionizing radiation on the interstellar medium and Hα emission as a
tracer of the star formation rate
MNRAS 2017, 466, 3293
