Atsushi Kawai

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.

Integral Field Spectroscopy of the Quadruply Lensed Quasar 1RXS J1131–1231: New Light on Lens Substructures*

We have observed the quadruply lensed quasar 1RXS J1131-1231 with the integral field spectrograph mode of the Kyoto Tridimensional Spectrograph II mounted on the Subaru telescope. Its field of view has simultaneously covered the three brighter lensed images A, B, and C, which are known to exhibit anomalous flux ratios in their continuum emission. We have found that the [O III] line flux ratios among these lensed images are consistent with those predicted by smooth-lens models. The absence of both microlensing and millilensing effects on this [O III] narrow-line region sets important limits on the mass of any substructures along the line of sight, which is expressed as ME < 105 M☉ for the mass inside an Einstein radius. In contrast, the Hβ line emission, which originates from the broad-line region, shows an anomaly in the flux ratio between images B and C, i.e., a factor 2 smaller C/B ratio than predicted by smooth-lens models. The ratio of A/B in the Hβ line is well reproduced. We show that the anomalous C/B ratio for the Hβ line is most likely caused by micro-/millilensing of image C. This is because other effects, such as the differential dust extinction and/or arrival time difference between images B and C, or the simultaneous lensing of another pair of images A and B, are all unlikely. In addition, we have found that the broad Hβ line of image A shows a slight asymmetry in its profile compared with those in the other images, which suggests the presence of a small microlensing effect on this line emitting region of image A.

Integrated field spectroscopy of E+A (post-starburst) galaxies with the Kyoto tridimensional spectrograph II

We have performed two-dimensional spectroscopy of three nearby E+A (post-starburst) galaxies with the Kyoto tridimensional spectrograph II (Kyoto3DII) integral field spectrograph. In all the cases, Hδ absorption is stronger at the centre of the galaxies, but significantly extended at the scale of a few kiloparsec. For one galaxy (J1656), we found a close companion galaxy at the same redshift. The galaxy turned out to be a star-forming galaxy with a strong emission in Hy. For the other two galaxies, we have found that the central post-starburst regions possibly extend toward the direction of the tidal tails. Our results are consistent with the merger/interaction origin of E+A galaxies, where the infalling-gas possibly caused by galaxy-galaxy merging creates a central starburst, succeeded by a post-starburst (E+A) phase once the gas is depleted.

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.

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.

Spatially Resolved Spectroscopic Observations of a Possible E+A Progenitor SDSS J160241.00+521426.9

In order to investigate the evolution of E+A galaxies, we observed a galaxy, SDSS J160241.00+521426.9, a possible E+A progenitor which shows both emission and strong Balmer absorptions, and its neighbor galaxy. We used the integral field spectroscopic mode of the Kyoto Tridimensional Spectrograph (Kyoto3DII), mounted on the University of Hawaii 88 inch telescope located on Mauna Kea, and the slit-spectroscopic mode of the Faint Object Camera and Spectrograph on the Subaru Telescope. We found a strong Balmer absorption region at the center of the galaxy and an emission-line region located 2?kpc from the center, in the direction of its neighbor galaxy. The recession velocities of the galaxy and its neighbor galaxy differ only by 100?km?s?1, which suggests that they are a physical pair and would have been interacting. Comparing observed Lick indices of Balmer lines and color indices with those predicted from stellar population synthesis models, we find that a suddenly quenched star formation scenario is plausible for the star formation history of the central region. We consider that star formation started in the galaxy due to galaxy interactions and was quenched in the central region, whereas star formation in a region offset from the center still continues or has begun recently. This work is the first study of a possible E+A progenitor using spatially resolved spectroscopy.

Nearby Galaxies Observed with the Kyoto Tridimensional Spectrograph II

The integral field spectrograph mode of the Kyoto tridimensional spectrograph II is optimized for a high spatial resolution with the ~ 0″.1 sampling when mounted on the Subaru Telescope, while a wider field of view is emphasized on the University of Hawaii 88-inch Telescope (UH88). One of our main targets is to resolve activities in nearby galaxies. Examples are shown: one is the low ionization emission line region in the galaxy NGC 1052 observed with the Subaru. The spatial resolution of ~ 0”.4 has revealed the structures, both in space and velocity, of the AGN outflow. An example observed with the UH88 is an interacting galaxy system NGC 6090. Through analyses of the spatial variations of emission line ratios, we have found rapid large-scale metal/abundance enhancement in starbursts.