2019天文年會

5月17日至19日

論文摘要

A young galaxy cluster in the old universe

[ Oral ]

Tetsuya Hashimoto; Tomotsugu Goto (NTHU); Rieko Momose (Univ. of Tokyo); Chien-Chang Ho (NTHU); Ryu Makiya (Kavli IPMU); Chia-ying Chiang; Seong Jin Kim (NTHU)

Galaxies evolve from the blue star-forming phase into red quiescent one by quenching their star-forming activity. In high density environments, this galaxy evolution proceeds more efficiently and rapidly. Therefore, local galaxy clusters are dominated by well-evolved red, elliptical galaxies. The fraction of blue galaxies in clusters monotonically decline with decreasing redshift, known as the Butcher-Oemler effect. In the local universe, observed blue fractions of massive clusters are only $\lesssim$ 0.2. Here we report a discovery of a \lq \lq blue cluster\rq \rq that is a local galaxy cluster with an unprecedentedly high fraction of blue star-forming galaxies yet hosted by a massive dark matter halo. The blue fraction is 0.57 that is 4.0 $\sigma$ higher than those of the other comparison clusters. The velocity dispersion of the member galaxies is 510 km s$^{-1}$, which corresponds to a dark matter halo mass of 2.0$^{+2.0}_{-1.4}\times 10^{14}$ M$_{\odot}$. The blue fraction of the cluster is beyond the standard theoretical predictions, including semi-analytic models and simulations of galaxy formation by more than 4.7 $\sigma$. The probability to find such a high blue fraction is only 0.003\%, which leads to great challenge to the current standard frameworks of the galaxy formation and evolution in the $\Lambda$CDM Universe. The galaxy distribution suggests the existence of filamentary cold gas streams in the massive halo to exist even in the local universe, which has already disappeared in the theoretically simulated local universes.