Invited Presentation

Early Metal Enrichment and Multiphase Interstellar Media in a Galaxy at Redshift 8.3

Author(s): Yoichi Tamura

Presenter: Yoichi Tamura (Nagoya University)

We present ALMA detections of the [OIII] 88 um, [CII] 158 um lines and 850 um dust continuum emission in a Y-dropout galaxy, MACS0416_Y1. The [OIII] and [CII] detections unambiguously confirm its spectroscopic redshift to be z = 8.312, making this object one of the furthest galaxies ever identified spectroscopically. The 850 um continuum intensity implies a large dust mass of 4 x 10^6 Msun. The ultraviolet-to-far infrared spectral energy distribution (SED) modeling where the [OIII] emissivity model is incorporated suggests the presence of a young (age ~ 4 Myr), star-forming (SFR ~ 60 Msun/yr), moderately metal-polluted (Z ~ 0.2 Zsun) stellar component with the stellar mass of 3 x 10^8 Msun. An analytic dust mass evolution model with a single episode of star formation does not reproduce the metallicity and the inferred dust mass of 4 x 10^6 Msun in the stellar age, suggesting an underlying evolved stellar component as the origin of the dust mass.

A high [O III]-to-[C II] luminosity ratio (~ 6) implies that typical H II regions are radiation-bounded, which is consistent with the inferred zero escape fraction of Lyman continuum photons obtained in the SED fits. It also suggests that the bulk of photodissociation regions are ‘truncated’ and are not terminated by molecular clouds, although this would contradict the presence of dust, which should be coupled with dense molecular gas. Thus, a possible explanation for the observed fact is patchy/porous geometry of dusty molecular clouds, photodissociation regions, and highly ionized gas at sub-kpc scales, which are all suggested by state-of-the-art hydrodynamic simulations of galaxy formation.