Kouichiro Nakanishi

Discovery of a Type 2 Quasar at z = 0.9

We report the discovery of a narrow-line, X-ray-luminous galaxy at z = 0.9, which is the counterpart of an X-ray source detected in a deep pointing observation with the Advanced Satellite for Cosmology and Astrophysics. The object has a hard X-ray spectrum; it is significantly detected only in the 2-10 keV band, with a flux of ~1 × 10-13 ergs s-1 cm-2. Its rest X-ray luminosity in the 2-10 keV band is estimated to be ~1 × 1044 ergs s-1 (q0 = 0.5, H0 = 50 km s-1 Mpc-1). The optical spectrum shows high-excitation and high-ionization lines but no significant broad emission line component. All these properties imply that this object is a type 2 QSO.

Search and Redshift Survey for IRAS Galaxies behind the Milky Way and Structure of the Local Void

This is the third and final paper of our systematic visual search for IRAS galaxies behind the Milky Way at |b| ≤ 15°. This paper presents a catalog of 950 IRAS galaxies with 60 μm flux densities larger than 0.6 Jy located between l = 0° and 150°, of which 293 are newly identified by this search. We made a redshift survey for the identified galaxies and obtained new redshift data of 171 galaxies. We also present newly measured redshifts of 27 IRAS galaxies between l = 150° and 225° at |b| ≤ 15°. In this paper we studied the structure of the Local void using IRAS galaxies and galaxies from the Third Reference Catalogue of Bright Galaxies in the region l = 30°-120° and b = -50° to +30°. The center of the Local void turned out to be located at l ~ 60°, b ~ -15°, and cz ~ 2500 km s-1, and the size is about 2500 km s-1 along the direction toward the center.

Hα Velocity Fields of H II Regions in Nearby Dwarf Irregular Galaxies

We present H-alpha velocity fields of thirteen giant H II regions in four nearby dwarf irregular galaxies, NGC 2366, Holmberg II, IC 2574, and WLM. We classify the velocity features as well as the morphologies of the H II regions. The H-alpha velocity features are divided into three categories; three H II regions show chaotic feature with a typical scale of variation of a few 100 pc in size and a few 10 km s^{-1} in velocity, one shows expanding-bubble feature, and the remaining nine have calm velocity fields. There is a correlation between the H-alpha velocity feature and the morphology of H II regions. We measured bulk motion of the H II regions relative to the ambient H I velocity for the H II regions with calm velocity fields and found a typical velocity difference of about 5 km s^{-1}. We discuss a model for origin of star-forming regions based on the presence of the velocity difference between H-alpha and H I gas as well as the H I characteristics.

H-Alpha Velocity Fields of H II Regions in Nearby Dwarf Irregular Galaxies

We present H-alpha velocity fields of thirteen giant H II regions in four nearby dwarf irregular galaxies, NGC 2366, Holmberg II, IC 2574, and WLM. We classify the velocity features as well as the morphologies of the H II regions. The H-alpha velocity features are divided into three categories; three H II regions show chaotic feature with a typical scale of variation of a few 100 pc in size and a few 10 km s^{-1} in velocity, one shows expanding-bubble feature, and the remaining nine have calm velocity fields. There is a correlation between the H-alpha velocity feature and the morphology of H II regions. We measured bulk motion of the H II regions relative to the ambient H I velocity for the H II regions with calm velocity fields and found a typical velocity difference of about 5 km s^{-1}. We discuss a model for origin of star-forming regions based on the presence of the velocity difference between H-alpha and H I gas as well as the H I characteristics.

Optical Identification of the ASCA Lynx Deep Survey: An Association of Quasi-Stellar Objects and a Supercluster at z=1.3?*

Results of optical identification of the ASCA Lynx Deep Survey are presented. Six X-ray sources are detected in the 2-7 keV band using the Solid-State Imaging Spectrometer in a ~20 × 20 field of view with fluxes larger than ~4 × 10-14 ergs s-1 cm-2 in the band. Follow-up optical spectroscopic observations were made, and five out of six sources are identified with active galactic nuclei/quasi-stellar objects (AGNs/QSOs) at redshifts of 0.5-1.3. We also identify two more additional X-ray sources detected in a soft X-ray band with AGNs/QSOs. We find that three QSOs identified are located at z ~ 1.3. Two rich clusters and several groups of galaxies are also placed at the same redshift in the surveyed field, and projected separations between the QSOs and the clusters are 3-8 Mpc at the redshift.

Optical and CO Radio Observations of Poor Cluster Zwicky 1615.8+3505

The cluster Zwicky 1615.8+3505 is considered to be a dynamically young poor cluster. To investigate the morphology and star-formation activity of galaxies under the environment of a dynamically young poor cluster, we have performed V, R, and I surface photometry, optical low-resolution spectroscopy, and 12CO (J=1-0) line observations for member galaxies. Our data show that more than 90% of the observed galaxies show regular morphologies and no star-formation activities, indicating that the environment does not affect these galaxy properties. Among sixteen galaxies observed, only NGC 6104 shows a significant star-formation activity, and shows a distorted morphology, indicating a tidal interaction. This galaxy contains double knots, and only one knot possesses Seyfert activity, though the sizes and luminosities are similar to each other; we also discuss this feature.

Photometric Properties of Kiso Ultraviolet-Excess Galaxies in the Lynx-Ursa Major Region

We have performed a systematic study of several regions in the sky where the number of galaxies exhibiting star formation (SF) activity is greater than average. We used Kiso ultraviolet-excess galaxies (KUGs) as our SF-enhanced sample. By statistically comparing the KUG and non-KUG distributions, we discovered four KUG-rich regions with sizes of ~10° × 10°. One of these regions corresponds spatially to a filament of length ~60 h-1 Mpc in the Lynx-Ursa Major region (α~9h-10h, δ~42°-48°). We call this the Lynx-Ursa Major (LUM) filament. We obtained V(RI)C surface photometry of 11 of the KUGs in the LUM filament and used these to investigate the integrated colors, the distribution of SF regions, the morphologies, and the local environments. We found that these KUGs consist of distorted spiral galaxies and compact galaxies with blue colors. Their star formation occurs in the entire disk and is not confined to just the central regions. The colors of the SF regions imply that active star formation in the spiral galaxies occurred 107-8 yr ago, while that in the compact objects occurred 106-7 yr ago. Though the photometric characteristics of these KUGs are similar to those of interacting galaxies or mergers, most of these KUGs do not show direct evidence of merger processes.

Near-Infrared Observations Of A Type-2 QSO At z = 0.9

We report the results of near-infrared observations of a type-2 QSO, AX J08494+4454 at z = 0.9 which was identified in our optical follow-up observations of the ASCA Lynx deep survey. This object has a hard X-ray spectrum with an X-ray luminosity of about 1 x 1044 erg s-1 in 2–10 keV. The optical spectrum shows high-excitation and high-ionization lines but no significant broad Hs emission. These properties strongly suggest that this object is a “type-2” QSO (Ohta et al. 1996).

Co Observations of High-z Objects

We have made a CO(J = 2–1) observations using the Nobeyama 45m telescope aimed at examining the physical properties of the molecular gas in this object. The upper limit obtained is 1.8 mK (3σ) at a velocity resolution of 100 km s-1, which leads to an upper limit on the molecular gas mass of 5.3 x 1011 M☉, if we assume a line width of 250 km s-1 obtained in the CO(J = 5 – 4) line (rest-frame) and the Galactic CO-to-H2 conversion factor of 4.5 (M☉ K km s-1 pc2). The line ratio between the 2–1 line and the 5–4 line as well as those from the 7–6 and the 4–3 lines (Omont et al. 1996, Nature, 382, 428) imply that the mean gas density is as high as 103-5 cm-3, which is comparable to that in nearby star burst galaxies (e.g., Solomon et al. 1992, ApJ, 387, L55).

Molecular Gas Excitation and Chemistry in VV 114 and NGC 1614 with ALMA

We present high resolution molecular line observations of dusty AGN and starburst in nearby luminous infrared galaxies (LIRGs), VV 114 (band 3/4/7) and NGC 1614 (band 3/6/7/9), with ALMA. Multi-frequency imaging from 4.8 GHz to 691 GHz of NGC 1614 allows us to study spatial properties of the radio-to-FIR continuum and multiple CO transitions, and we find the CO excitation up to J upp = 6 can be explained by a single ISM model powered by nuclear starbursts. Our processing line imaging survey for VV 114 detected at least 30 molecular lines which show different chemical composition from region to region. Multi-molecule imaging helps us to diagnose the chemical differences of dusty ISM, while multi-transition imaging allows us to investigate gas physical conditions affected by nuclear activities directly.