7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

On the Long-term Evolution of Our Galaxy: Importance of the Diffuse X-ray
Emitting Plasma
Jiro Shimoda1,2, & Shu-ichiro Inutsuka2
Institute of Cosmic Ray Research, The University of Tokyo
2
Department of Physics, Graduate School of Science, Nagoya University
e-mail (speaker): jshimoda@icrr.u-tokyo.ac.jp
1The evolution of galaxy over cosmic time (~1 Gyr) is
related to many astrophysical subjects such as theformation of stellar objects, the origin of cosmic particles
and radiations, and environmental evolution for life on
the planes, and has been widely studied. Nevertheless,our current understandings are far from sufficient. Ourgalaxy's long-term star formation, which continues withan almost constant rate of several Mo/yr during thecosmic age of 14 Gyr [1], is a representative puzzle. If
the constant star formation resulted from the similar
Galactic disk conditions during 14 Gyr, the gaseous
matter as the fuel of star formation with a mass of 10^9Mo was depleted within ~1 Gyr. This puzzle can betranslated by the cosmological context. Supposing thecosmic density fraction ratio of the baryon (usual matter)to dark matter (DM), ~0.1 [2], the total baryon mass of
our galaxy is estimated as ~10^11 Mo, while the total
mass of stars is 4-6x10^10 Mo [3]. Therefore, we mustfind the reason why half of the gaseous matter isconverted to the stars leaving ~1 % of mass at theGalactic disk.
The galactic wind (outflow from the disk) is
invoked to explain the observed metal absorption lines atthe circumgalactic medium (surrounding gaseous matteraround the Galaxy with a radius of ~100 kpc, [4]). Notethat the metal (elements heavier than helium) is createdinside the stars which are formed in the disk. Therefore,
the existence of the metal at ~100 kpc distant from thedisk should be explained by the outflow. We showed [5]that the diffuse X-ray emitting plasma (DXEP) canescape from the disk as the wind due to the effects ofcosmic-rays (CRs). The CRs are non-thermal, relativistic
particles whose observed energy density of ~1 eV/cc iscomparable to the other components such as the energy
density of thermal particles and the magnetic field in theGalactic disk. They can heat the background plasma via
the generation and dissipation of the Alfven waves. We
found that this heating rate is comparable to the radiative
cooling rate of the DXEP, resulting in a sufficiently hightemperature of the DXEP to escape the Galactic system.
Note that the gravitational virial temperature for ourgalaxy is ~0.3 keV, the plasma with such temperature isbright at the X-ray band. The mass transfer rate of thewind is several Mo/yr, comparable to the gas depletion
rate due to the star formation. Such comparable massloss is preferred to keep the small mass of the gaseous
matter in the disk.
To explain the long-term star formation, the gas
accretion onto the Galactic system is a must. The DM

accretion rate can be estimated from the cosmologicalN-body simulations. According to the results given by[5], our galaxy suffers the DM accretion with a rate of~70 Mo/yr at the current time. We can estimate the gas
accretion rate by multiplying the cosmic baryon fraction
as ~7 Mo/yr. Then, if a tenth of the supernovae explosion
energy is consumed to form the DXEP with atemperature of 0.3 keV, the outflow rate becomes ~4Mo/yr. To reproduce the quasi-steady state of the disk,
the remaining gaseous matter should be consumed forthe star formation with a rate of ~7 Mo/yr – 4 Mo/yr ~ 3
Mo/yr. Hence, the existence of the Galactic wind can beessential to understand the long-term evolution of theGalactic system. Constructing the time-dependent modelof the Galactic system including the accretion, wind, starformation, CRs, metal produced by stars, and stellar
dynamics, we confirm that our picture can be consistent
with the observed star formation history and other
Galactic conditions. In this talk, we review the long-term
evolution of our galaxy according to this scenario andthe results of our model.
References[1] Heywood et al. 2016, A&A 589, A66[2] Planck Collaboration 2020, A&A, 641, A6[3] Bland-Hawthron & Gerhard 2016, ARA&A, 54, 529[4] Tumlinson et al. 2017, ARA&A, 55, 839[5] Shimoda & Inutsuka 2022, ApJ 926, 1, 8

Figure xxNote: Abstract should be in (full) double-columned one page.