Takao Nakagawa

This paper presents a 610 MHz radio survey covering 1.94 deg around the North Ecliptic Pole (NEP), which includes parts of the (ADF-N) and Euclid, Deep Fields North. The median 5 sensitivity is 28 Jy beam, reaching as low as 19 Jy beam, with a synthesized beam of 3.6 arcsec 4.1 arcsec. The catalogue contains 1675 radio components, with 339 grouped into multicomponent sources and 284 'isolated' components likely part of double radio sources. Imaging, cataloguing, and source identification are presented, along with preliminary scientific results. From a non-statistical sub-set of 169 objects with multiwavelength AKARI and other detections, luminous infrared galaxies (LIRGs) represent 66 of the sample, ultra-luminous infrared galaxies (ULIRGs) 4, and sources with L < 10 30. In total, 56 of sources show some AGN presence, though only seven are AGN-dominated. ULIRGs require three times higher AGN contribution to produce high-quality SED fits compared to lower luminosity galaxies, and AGN presence increases with AGN fraction. The PAH mass fraction is not significant, although ULIRGs have about half the PAH strength of lower IR-luminosity galaxies. Higher luminosity galaxies show gas and stellar masses an order of magnitude larger, suggesting higher star formation rates. For LIRGs, AGN presence increases with redshift, indicating that part of the total luminosity could be contributed by AGN activity rather than star formation. Simple cross-matching revealed 13 ROSAT QSOs, 45 X-ray sources, and 61 sub-mm galaxies coincident with GMRT radio sources.

Determining the inner structure of the molecular torus around an active galactic nucleus is essential for understanding its formation mechanism. However, spatially resolving the torus is difficult because of its small size. To probe the clump conditions in the torus, we therefore perform the systematic velocity-decomposition analyses of the gaseous 12CO rovibrational absorption lines (v = 0 → 1, ΔJ = ±1) at λ ∼ 4.67 μm observed toward four (ultra)luminous infrared galaxies using the high-resolution (R ∼ 5000-10,000) spectroscopy from the Subaru Telescope. We find that each transition has two to five distinct velocity components with different line-of-sight (LOS) velocities (V LOS ∼ −240 to +100 km s−1) and dispersions (σ V ∼ 15-190 km s−1), i.e., the components (a), (b), ⋯, beginning with the broadest one in each target, indicating that the tori have clumpy structures. By assuming a hydrostatic disk ( σ V ∝ R rot − 0.5 ), we find that the tori has dynamic inner structures, with the innermost component (a) outflowing with velocity ∣V LOS∣ ∼ 160-240 km s−1, and the outer components (b) and (c) outflowing more slowly or infalling with ∣V LOS∣ ≲ 100 km s−1. In addition, we find that the innermost component (a) can be attributed to collisionally excited hot (≳530 K) and dense ( log ( n H 2 / cm − 3 ) ≳ 6 ) clumps, based on the level populations. Conversely, the outer component (b) can be attributed to cold (∼30-140 K) clumps radiatively excited by a far-infrared-to-submillimeter background with a brightness temperature higher than ∼20-400 K. These observational results demonstrate the clumpy and dynamic structure of tori in the presence of background radiation.

Abstract We investigate the physical origins of the Balmer decrement anomalies in GS-NDG-9422 and RXCJ2248-ID galaxies at z ∼ 6 whose Hα/Hβ values are significantly smaller than 2.7, the latter of which also shows anomalous Hγ/Hβ and Hδ/Hβ values beyond the errors. Because the anomalous Balmer decrements are not reproduced under the Case B recombination, we explore the nebulae with optical depths smaller and larger than the Case B recombination by physical modeling. We find two cases quantitatively explaining the anomalies: (1) density-bounded nebulae that are opaque only up to around Lyγ–Ly8 transitions and (2) ionization-bounded nebulae partly/fully surrounded by optically thick excited H i clouds. The case of (1) produces more Hβ photons via Lyγ absorption in the nebulae, requiring fine tuning in optical depth values, while this case helps ionizing photon escape for cosmic reionization. The case of (2) needs the optically thick excited Hi clouds with N ₂ ≃ 10¹²−10¹³ cm⁻², where N ₂ is the column density of the hydrogen atom with the principal quantum number of n = 2. Interestingly, the high N ₂ values qualitatively agree with the recent claims for GS-NDG-9422 with the strong nebular continuum requiring a number of 2s-state electrons and for RXCJ2248-ID with the dense ionized regions likely coexisting with the optically thick clouds. While the physical origin of the optically thick excited H i clouds is unclear, these results may suggest gas clouds with excessive collisional excitation caused by an amount of accretion and supernovae in the high-z galaxies.

Theoretical calculations predict that high-resolution spectroscopy of H2O gas lines in the mid-infrared region is the most promising method to observationally identify the snow-line, which has been proposed as the critical factor separating gas giants from solid planets in the planetary formation process. This requires the spectroscopic observations from space with R = λ/∆λ ≥ 30, 000. For this purpose, we propose a mid-infrared (10-18 µm) high-resolution spectrometer to be onboard the GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer) mission. We are developing”immersion grating” spectroscopy technology for high-resolution spectroscopy in space. We have chosen CdZnTe as a candidate for the optical material. We report the current status of the development of the CdZnTe immersion grating, including evaluation of its optical properties (absorption coefficient and refractive index) at cryogenic temperatures, development of an anti-reflection coating with a moth-eye structure for wide-wavelength coverage, and verification of machinability for grating production. We plan to make a prototype spectrometer to demonstrate the capability of the immersion grating with ground-based observations in the N-band (λ = 8–13 µm) and beyond.

Extragalactic Background Light (EBL), the cumulative light from outside the galaxy, is a crucial observational target for understanding the history of the universe. We are developing a CubeSat; VERTECS (Visible Extragalactic background RadiaTion Exploration by CubeSat) with a 6U size (approximately 10 × 20 × 30 cm), equipped with Solar Array Wings (SAW). Our mission is to conduct extensive observations of the visible EBL. The satellite is designed to operate in a sun-synchronous orbit at an altitude of 500-680 km (approximately 15 orbits per day) and observe the EBL on the shadow side to avoid stray light from the Sun and Earth. To observe EBL, a high-performance CMOS sensor, attitude control devices, and high-speed communication equipment X-band are essential. We should note that these components these components consume a significant amount of power. Therefore, some strategic operational plans are necessary to operate this CubeSat within the limited power resources. In addition, VERTECS needs to meet its mission requirements, conducting 10 observations, 4 data downlinks, and 1 command uplink within a day. We have constructed some operational scenarios utilizing attitude control and SAW to meet these requirements, and we also constructed a power budget simulation for VERTECS. In this presentation, we describe how we plan to operate VERTECS utilizing the subsystems and the results of the power simulation during the operation.