Tsuyoshi Ishigaki

A Study of the Distribution of Star-forming Regions in Luminous Infrared Galaxies by Means of H? Imaging Observations

We performed Hα imaging observations of 22 luminous infrared galaxies to investigate how the distribution of star-forming regions in these galaxies is related to galaxy interactions. Based on correlation diagrams between Hα flux and continuum emission for individual galaxies, a sequence for the distribution of star-forming regions was found: very compact (~100 pc) nuclear starbursts with almost no star-forming activity in the outer regions (type 1), dominant nuclear starbursts 1 kpc in size with a negligible contribution from the outer regions (type 2), nuclear starbursts 1 kpc in size with a significant contribution from the outer regions (type 3), and extended starbursts with relatively faint nuclei (type 4). These classes of star-forming regions were found to be strongly related to global star-forming properties, such as star formation efficiency, far-infrared color, and dust extinction. There was a clear tendency for the objects with more compact distributions of star-forming regions to show a higher star formation efficiency and hotter far-infrared color. An appreciable fraction of the sample objects were dominated by extended starbursts (type 4), which is unexpected in the standard scenario of interaction-induced starburst galaxies. We also found that the distribution of star-forming regions was weakly but clearly related to galaxy morphology: severely disturbed objects had a more concentrated distribution of star-forming regions. This suggests that the properties of galaxy interactions, such as dynamical phase and orbital parameters, play a more important role than the internal properties of progenitor galaxies, such as dynamical structure or gas mass fraction. We also discuss the evolution of the distribution of star-forming regions in interacting galaxies.

Polarization interferometric stellar coronagraph

A nulling stellar coronagraph based on polarization interferometry is proposed that has achromatic characteristics. The principle and the optical setup of the nulling stellar coronagraph are described. Experimental results with monochromatic and white light show the usefulness of the proposed method. A liquid-crystal device is used as a four-quadrant polarization mask, which is the key element of a polarization interferometric stellar coronagraph.

Nulling interferometry by use of geometric phase

Nulling interferometry is a method of detecting a faint source near a bright one, in which destructive interference is realized for the light from the bright source. A nulling interferometer that makes use of geometric phase (Pancharatnam phase) is proposed. An experimental setup is constructed to simulate a stellar interferometer with geometric-phase modulation. We attained extinction of 6 x 10(-5) in white light.

Birefringent imaging spectrometer

A Fourier-transform imaging spectrometer, believed to be novel, based on the Savart polariscope is presented. There is no slit in this instrument, which means that it has a high throughput. The principle and the system configuration are described. Several preliminary experimental results are shown.

Kyoto tridimensional spectrograph II: progress

We are building the Kyoto tridimensional spectrograph II and are planning to mount it on Subaru telescope. The spectrograph has four observational modes: Fabry-Perot imager, integral field spectrograph (IFS) with a microlens array, long-slit spectrograph, and filter-imaging modes. The optics is designed to be used in wide wavelength range from 360 nm to 900 nm. The design well matches with high spatial resolution of Subaru: 0 inch .06 pixel-1 in Fabry- Perot mode, for which we actually will use binning before adaptive optics at optical wavelengths becomes available, and 0 inch .1 lens-1 in microlens array mode. These well sample image sizes obtained by Subaru, which are about 0 inch .4 in relatively good conditions. We have evaluated a point spread function of our cylindrical microlens array and found that it consists of a diffraction pattern and more extended component which probably comes from border regions between microlenses. With a suitable mask at the micro pupil position, the crosstalk between spectra will be limited down to a few percent. With a suitable mask at the micro pupil position, the crosstalk between spectra will be limited down to a few percent. We have succeeded in synchronizing frequency switching of Fabry-Perot etalons with the movement of charge on the CCD. This technique enables to average out all temporal variations between each passband.

SUBARCSECOND STRUCTURE AND VELOCITY FIELD OF OPTICAL LINE-EMITTING GAS IN NGC 1052

We have obtained integral field spectra of the low-ionization emission-line region in the galaxy NGC 1052 by using the Kyoto Tridimensional Spectrograph II mounted on the Subaru Telescope. Our high signal-to-noise ratio data with precise template subtraction have revealed weaker features at the nucleus, including the [Fe III] and He II emission lines, as well as a broad component of the Hβ emission. The broad Hβ component suggests the existence of a broad-line region. The spatial structure and velocity field derived from the data cube suggest the existence of three main components: a high-velocity bipolar outflow, low-velocity disk rotation, and a spatially unresolved nuclear component. The outflow axis does not coincide with the disk rotation axis. The opening angle of the outflow decreases with velocity shift from the systemic velocity both in bluer and redder velocity channels. This is explained only if the outflow has intrinsically higher velocity components inside, i.e., in regions closer to the outflow axis. At both sides of the bipolar outflow, we find that the highest velocity components are detached from the nucleus. This gap can be explained by an acceleration of at least a part of the flow or the surrounding matter, or by bow shocks that may be produced by even higher velocity outflow components that are not yet detected. Along the edges of the outflow and extending east-northeast and west-southwest, there exist strong [O III] emission ridges. These are closely related to the radio jet-counterjet structure. The abrupt change in the velocity field of the ionized gas and a large [O ]/Hβ line flux ratio in this region suggest a strong interaction of the jets, and possibly also of some ridge components of the line-emitting gas, with the interstellar matter.

Nulling interferometer with geometric phase modulator

Geometric phase modulation is used to realize achromatic phase shift in nulling interferometer, where Fresnel rhombs are key components of the geometric phase modulator. Experimental results show high extinction ratio in visible region. Extension of our scheme to the infrared region is also discussed.

Rapid large-scale metal enrichment in the starbursts of an interacting Galaxy system

By obtaining integral field spectra of the interacting galaxy system NGC 6090, we have found a kiloparsec-scale region where active star formation is currently increasing the fraction of heavy elements. Young massive starbursts are occurring in regions offset from the galactic centers, highlighting the present epoch of metal enrichment over the previous ones usually seen at galactic centers. The short timescale of ~107 yr for the starbursts, inferred from the galactic rotation, provides a strong constraint on the origins of the metallicity/abundance enhancements. While oxygen is considered to originate in supernovae, the observed nitrogen enhancement is likely to be caused by winds/mass loss from massive stars rather than being a product of intermediate-mass stars.

Static Fourier-transform spectrometer based on Savart polariscope

To avoid the drawbacks of conventional Fourier transform spectrometer (CFTS) based on the Michelson interferometer which needs a scanning system to acquire the interferogram in the temporal domain, the static Fourier transform (StFT) spectrometer has been developed. A new static Fourier-transform imaging spectrometer based on the Sagnac polariscope is proposed. The novelty of this work comes from slitless in this instrument, which means high throughput. The throughput advantage of this instrument over StFT imaging spectrometer based on Sagnac interferometer is presented. The principle and the system configuration are described. Several preliminary experimental results are shown.

Two-channel polarization interferometric coronagraph to improve the detectability of extrasolar planets

A dual-channel nulling coronagraph to improve the detectability of extrasolar planets is demonstrated. We have been developing a nulling coronagraph with a four-quadrant polarization mask, and confirmed its performance with monochromatic and polychromatic light sources. However, the imperfections of the mask cause the leakage of the starlight, which is obstructive to the detection of faint companions. Here, we propose a two-channel nulling coronagraph, where s- and p-polarized components of the incident light are separated. The light scattered and reflected from the atmosphere of an extrasolar planet is expected to be partially polarized, while the light from the parent star is usually unpolarized. Thus, the differential method, in which subtraction is taken between coronagraphic images of s- and p-polarized lights, is very useful for direct detection of extrasolar planets. In this approach, cancellation of the residual unpolarized starlight is realized when the coronagraphic performance of the two channels is identical. We constructed the two-channel instrument. The experimental results confirm that the two-channel coronagraph can suppress the residual stellar noise, and improve the detectability of faint companions. The effects of the difference between two channels on the detectability are also discussed.