The University of Tokyo · Physics and Astronomy
Professor Hiroya Umeda's research lab specializes in high-redshift astrophysics, focusing on the epoch of reionization and the early universe. The lab investigates the ionization state of the intergalactic medium using Lyα damping wing absorption in galaxies observed with JWST, aiming to measure neutral hydrogen fractions and ionized bubble sizes. They also study extreme high-ionization emission lines in young and metal-poor galaxies to probe the nature of ionizing sources, including nonthermal processes. Additionally, the lab explores the physical properties of newly discovered Little Red Dots, proposing and testing models such as the BH envelope to explain their peculiar, dust-reddened, X-ray-quiet characteristics.
Figures are computed from collected data and may differ slightly.
Abstract We present volume-averaged neutral hydrogen fractions x H I and ionized bubble radii R b measured with Ly α damping wing absorption of galaxies at the epoch of reionization. We combine JWST/NIRSpec spectra taken by the CEERS, GO-1433, DDT-2750, and JADES programs and obtain a sample containing 27 bright UV-continuum ( M UV < −18.5 mag) galaxies at 7 < z < 12. We construct four composite spectra binned by redshift and find the clear evolution of the softening break toward high r
Abstract Strong high-ionization lines such as He ii of young galaxies are puzzling at high and low redshift. Although recent studies suggest the existence of nonthermal sources, whether their ionizing spectra can consistently explain multiple major emission lines remains a question. Here we derive the general shapes of the ionizing spectra for three local extremely metal-poor galaxies (EMPGs) that show strong He ii λ 4686. We parameterize the ionizing spectra composed of a blackbody and power-la
We present volume-averaged neutral hydrogen fractions $x_{\rm \HI}$ and ionized bubble radii $R_{\rm b}$ measured with Ly$α$ damping wing absorption of galaxies at the epoch of reionization. We combine JWST/NIRSpec spectra taken by CEERS, GO-1433, DDT-2750, and JADES programs, and obtain a sample containing 27 bright UV-continuum ($M_{\rm UV}<-18.5~{\rm mag}$) galaxies at $7
Abstract We present luminosity functions (LFs) and angular correlation functions (ACFs) derived from 18,960 Ly α emitters (LAEs) at z = 2.2−7.3 over a wide survey area of ≲24 deg 2 that are identified in the narrowband data of the HSC-SSP and CHORUS surveys. Confirming the large sample with 241 spectroscopically identified LAEs, we determine Ly α LFs and ACFs in the brighter luminosity range down to 0.5 L ⋆ , and confirm that our measurements are consistent with previous studies but offer signif
Abstract Little Red Dots (LRDs) newly discovered with JWST are active galactic nuclei (AGNs) that may represent black hole (BH) growth at the earliest cosmic epochs. These sources show puzzling features unlike typical AGNs, including red optical continua, weak hot-dust emission, and a lack of detectable X-rays. Previously, LRDs have often been interpreted as dust-reddened AGNs, leading to severe inconsistencies with the luminosity and BH mass densities inferred for previously known AGNs over 0 &
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