談話会
Colloquium

福島 肇 (筑波大学)
低金属量の大質量星形成
大質量星質量は質量降着期に星へ降着するガス質量によって決まる。この際、原始星質量の増加に伴い星光度も大きく増加するため、この輻射により星へのガス降着が大きく抑制される。主たる輻射フィードバック機構は、初代星ではUV光によるHII領域形成であるが、太陽金属量星ではダスト粒子への輻射圧と異なる。そこで、我々は２次元輻射流体シミュレーションを実行し、星質量および輻射フィードバック機構の金属量依存性について調べた。輻射フィードバック機構については、0.1太陽金属量を境界に切り替わることを明らかにした。また、質量降着率が4.e-3 Msun/yrを超えない場合は、輻射フィードバックの金属量依存性は小さくなり、星質量については大きく変化しないことを明らかにした。

Taiki Kawamuro (NAOJ)
An X-ray and submm/mm spatially-resolved spectroscopy study of an AGN effect on host galaxies
Since the findings of good correlations between the SMBH mass and properties of stars in the galaxy bulge (i.e., mass and velocity dispersion), much effort has been made towards the study of active galactic nuclei (AGNs) to understand the close connection between the SMBH and galaxy. The AGN is certainly a phase, where an SMBH is fed by an accretion disk, and also its harsh radiation followed by the mass accretion is expected to have a strong impact on the host galaxy.

I recently focus on X-ray irradiation of inter stellar media as an AGN effect on host galaxies. The AGN is usually X-ray luminous, and therefore the irradiation is unavoidable for the host galaxy and is probably ubiquitous one. In this colloquium, I will talk mainly about our recent work that studied X-ray-irradiated gas in the central ~100 pc of the Circinus galaxy, which hosts a Compton-thick AGN (TK+19a), at 10 pc resolution using Chandra and ALMA. Based on Chandra data, we created an image of the Fe-Kalpha line at 6.4 keV, tracing X-ray-irradiated dense gas. Further, we used the ALMA data, which cover five molecular lines (CO(J = 3–2), HCN(J = 3–2), HCN(J = 4–3), HCO+(J = 3–2), and HCO+(J = 4–3)), to investigate chemical condition in the X-ray irradiated regions. The detailed spatial distribution of dense molecular gas was revealed, and compared to the iron line image. The molecular gas emission appeared faint in regions with bright iron emission. Motivated by this, we quantitatively discuss the possibility that the molecular gas is efficiently dissociated by AGN X-ray irradiation (i.e., creating an X-ray-dominated region). Based on a non-local thermodynamic equilibrium model, we constrained the molecular gas densities and found that they are as low as interpreted by X-ray dissociation. Furthermore, judging from inactive star formation reported in the literature, we suggest that the X-ray emission has the potential to suppress star formation, particularly in the proximity of the AGN. If time permits, I will further present another preliminary case study.