Field:
Physics incl. High-energy and Astrophysics
Session Topic:
Virtual Galaxies
Speaker:
Masao Mori/Senshu University
1. Introduction
How a galaxy began its life is one of the greatest mysteries. The recent advances in
observational technique allow us to directly observe the universe 10 billion years ago
and find bright young objects there. Lyman α emitters1 have been recently discovered
at redshifts greater than 3 (2.1109 yr after the Big-Bang) and Lyman break galaxies2
are high-redshift star-forming galaxies with the intense ultraviolet radiation from young
stars. In the theoretical view point, galaxy formation is believed to proceed in a ‘bottom
up’ manner, starting with the formation of small clumps of gas and stars that then
merge hierarchically into giant systems. The gas loses thermal energy by radiative
cooling and falls towards the centres of the new galaxies, while supernovae explosion
blow gas out. Any realistic model therefore requires a proper treatment of these
processes, but hitherto this has been far from satisfactory. Moreover, previous
simulations do not give an answer about the relation between distant young objects
and present-day galaxies. To explore the evolution of galaxies, the coupling of the
dynamics and the chemical evolution through star formation and supernova feedback
needs to be treated properly. In particular, it is crucial to resolve accurately the
thermalization of the kinetic energy released by multiple supernovae3. Here I report a
simulation4 that follows evolution from the earliest stages of galaxy formation
through the period of dynamical relaxation, at which point the resulting galaxy is in
its final form.
2. Numerical Modeling
An ultra-high-resolution (1,0243 grids) hydrodynamic simulation pursues the early
evolution of a proto-galaxy as an assemblage of sub-galactic condensations with a
mass of 109 M (where M is the solar mass) building up a total mass of 1011 M. The
overdensity region of this mass-scale decouples from the cosmic expansion at redshift
7.8 (0.65109 yr after the Big-Bang) at a radius of 53.7 kpc (1 kpc=3,260 light yr),
where the initial conditions are set up. The mass of gaseous matter is 1.31010 M
initially. The subgalactic condensations are distributed randomly within the
galaxy-scale overdensity. Radiative cooling for the gaseous component is calculated
using the standard cooling function. Stars are assumed to form in gravitationally
unstable cooled regions with the initial mass function, at a rate that is inversely
proportional to the local free-fall time. Stars more massive than 8 M explode as
supernovae with an explosion energy of 1051 erg, and eject synthesized heavy
elements.
3. Simulation Results
Figure 1 shows the results for the time sequence of star formation, gas dynamics and
chemical enrichment. In the first 108 yr, stars form in high-density peaks within
subgalactic condensations and the burst of star formation starts. Then, massive stars
in the star forming regions explode as supernovae one after another. The gas in the
vicinity of supernovae is quickly enriched with ejected metals. At 3108 yr,
Fig.1: Spatial distributions of the stellar density,
the gas density, and the oxygen abundance [O/H] =
log10(NO/NH)–log10 (NO/NH) where the subscript 
refers to the Sun, and NO and NH are the number
densities of oxygen and hydrogen, respectively.
Each simulation box has a physical size of 134 kpc.
supernova-driven
shocks
collide with each other to
generate super-bubbles of
~50 kpc and the high density,
cooled shells. New stars are
born in the enriched gas and
subsequent
supernovae
again eject heavy elements.
The hot bubbles expand
further
by
continual
supernovae, and the shells
sweep up the partially
enriched ambient gas. The
gas density in dense shells
increases owing to the
efficient radiative cooling,
mainly through collisional
excitation
of
neutral
hydrogen. After 5108 yr, the
hot bubbles blow out into
the intergalactic space. After
this stage, the interstellar
medium
is
recycled
repeatedly and the mixing of
heavy elements is nearly completed.
The bubble structures of gas revealed in the simulation (for times < 3108 years)
resemble closely high-redshift Lyman α emitters. The Lyman α luminosity of the
simulated galaxy perfectly matches that of observed Lyman α emitters. After 109 years,
these bodies are dominated by stellar continuum radiation and then resemble the Lyman
break galaxies. At this point, the abundance of elements heavier than helium appears to
be solar. Furthermore, the long-term dynamical evolution of the model galaxy was studied with an
N-body simulation containing one million particles. We found that the assembly of sub-condensations
and the virialization of the total system are almost completed in 3109 yr, so that the system achieves a
quasi-equilibrium state. The resultant stellar system forms a virialized, spheroidal system. They have a
large central concentration that accords well with de Vaucouleurs’ r1/4 profile, which is commonly found
in nearby elliptical galaxies5. It is suggested that Lyman break galaxies evolve into elliptical galaxies
through purely collisionless dynamical evolution.
4. Conclusion
The present simulation, for the first time, revealed how distant young objects are
related to the birth of galaxies, and unravelled the missing rink between distant
young objects and present-day galaxies. By the comparisons of the simulation results
with the observations, it is concluded that Lyman α emitters and Lyman break
galaxies correspond to infants of elliptical galaxies, and the on-going, major chemical
enrichment phases of elliptical galaxies.
References
1.
2.
3.
4.
5.
Taniguchi, Y. et al. J. Korean Astron. Soc., 36, 123–144 (2003).
Giavalisco, M. Annu. Rev. Astron. Astrophys., 40, 579–641 (2002).
Mori, M., Umemura, M. and Ferrara, A. Astrophys. J., 613, L97–L100 (2004).
Mori, M. and Umemura, M. Nature, 440, 644–647 (2006).
de Vaucouleurs, G. Ann. Astrophys., 11, 247–287 (1948).