Shingo Nishiura

The Nuclear Activity of Galaxies in the Hickson Compact Groups

In order to investigate the nuclear activity of galaxies residing in compact groups of galaxies, we present results of our optical spectroscopic program made at Okayama Astrophysical Observatory. We have performed optical spectroscopy of 69 galaxies belonging to 31 Hickson compact groups (HCGs) of galaxies. Among them, three galaxies have discordant redshifts and, moreover, spectral quality is too poor to classify another three galaxies. Therefore, we describe our results for the remaining 63 galaxies. Our main results are summarized as follows: (1) We have found in our sample 28 active galactic nuclei (AGNs), 16 H II nuclei, and 19 normal galaxies showing no emission line. We used this HCG sample for statistical analyses. (2) Comparing the frequency distributions of activity types between the HCGs and the field galaxies whose data are taken from Ho, Filippenko, & Sargent (382 field galaxies), we find that the frequency of H II nuclei in the HCGs is significantly less than that in the field. However, this difference may be due to selection bias to the effect that our HCG sample contains more early-type galaxies than the field, because it is known that H II nuclei are rarer in early-type galaxies than in later ones. (3) Applying a correction to this morphological bias to the HCG sample, we find that there is no statistically significant difference in the frequency of occurrence of emission-line galaxies between the HCGs and the field. This implies that the dense galaxy environment in the HCGs does not affect the triggering of either the AGN activity and the nuclear starburst. We discuss some implications on the nuclear activity in the HCG galaxies.

Optical versus Mid-infrared Spectroscopic Classification of Ultraluminous Infrared Galaxies

The origin of the huge infrared luminosities of ultraluminous infrared galaxies (ULIGs) is still in question. Recently, Genzel and colleagues performed a mid-infrared (MIR) spectroscopic survey of a large number of ULIGs and found the major energy source in them to be massive stars formed during recent starburst activity; ~70%-80% of the sample are predominantly powered by starbursts. However, previous optical spectroscopic observations classified the majority of ULIGs either as Seyferts or as LINERs. In order to reconcile this difference, we compare types of emission-line activity for a sample of ULIGs that have been observed in both optical and MIR. We confirm the results of previous studies that the majority of ULIGs classified as LINERs on the basis of optical emission-line diagnostics turn out to be starburst-dominated galaxies on the basis of MIR diagnostics. Since MIR spectroscopy can probe the more heavily reddened, inner parts of the ULIGs, and since it is quite unlikely that the inner parts are powered by starbursts while the outer parts are powered by nonstellar ionization sources, the most probable resolution of the dilemma is that the optical emission-line nebulae with LINER properties are powered predominantly by shock heating driven by superwind activity; i.e., blast waves driven by the collective effect of a large number of supernovae in the central region of galaxy mergers.

A Dynamical Study of Galaxies in the Hickson Compact Groups

To investigate dynamical properties of spiral galaxies in the Hickson compact groups (HCGs), we present rotation curves of 30 galaxies in 20 HCGs. We found as follows: (1) There is no significant relation between dynamical peculiarity and morphological peculiarity in HCG spiral galaxies. (2) There is no significant relation between the dynamical properties and the frequency distribution of nuclear activities in HCG spiral galaxies. (3) There are no significant correlations between the dynamical properties of HCG spiral galaxies and any group properties (i.e., size, velocity dispersion, galaxy number density, and crossing time). (4) Asymmetric and peculiar rotation curves are more frequently seen in the HCG spiral galaxies than in field spiral galaxies or in cluster ones. However, this tendency is more obviously seen in late-type HCG spiral galaxies. These results suggest that the dynamical properties of HCG spiral galaxies do not strongly correlate with the morphology, the nuclear activity, and the group properties. Our results also suggest that more frequent galaxy collisions occur in the HCGs than in the field and in the clusters.

Multicolor Shallow Decay and Chromatic Breaks in the GRB 050319 Optical Afterglow

Multiwavelength observations of the optical afterglow of GRB 050319 were performed from 1.31 to 9.92 hr after the burst. Our R-band light curves, combined with other published data, can be described by the smooth broken power-law function, with α1 = -0.84 ± 0.02 to α2 = -0.48 ± 0.03, 0.04 days after the gamma-ray burst. The optical light curves are characterized by shallow decays—as was also observed in the X-rays—which may have a similar origin, related to energy injection. However, our observations indicate that there is still a puzzle concerning the chromatic breaks in the R-band light curve (at 0.04 days) and the X-ray light curve (at 0.004 days) that remains to be solved.

A New 60-cm Radio Survey Telescope with the Sideband-Separating SIS Receiver for the 200 GHz Band

We have upgraded the 60-cm radio survey telescope located in Nobeyama, Japan. We developed a new waveguide-type sideband-separating SIS mixer for the telescope, which enables the simultaneous detection of distinct molecular emission lines both in the upper and lower sidebands. Over the RF frequency range of 205-240 GHz, the single-sideband receiver noise temperatures of the new mixer are 40-100 K for the 4.0-8.0 GHz IF frequency band. The image rejection ratios are greater than 10 dB over the same range. For the dual IF signals obtained by the receiver, we have developed two sets of acousto-optical spectrometers and a telescope control system. Using the new telescope system, we successfully detected the 12CO (J=2-1) and 13CO (J=2-1) emission lines simultaneously toward Orion KL in 2005 March. Using the waveguide-type sideband-separating SIS mixer for the 200 GHz band, we have initiated the first simultaneous 12CO (J=2-1) and 13CO (J=2-1) survey of the galactic plane as well as large-scale mapping observations of nearby molecular clouds.

Multiband Optical Follow-up Observations of GRB 020813 at the Kiso and Bisei Observatories

Observations were made of the optical afterglow of GRB 020813 (Fox, Blake, & Price) with the Kiso observatory 1.05 m Schmidt telescope and the Bisei astronomical observatory 1.01 m telescope. Four-band (B, V, R, and I) photometric data points were obtained on 2002 August 13 (10:52-16:46 UT), or 0.346-0.516 days after the burst. In order to investigate the early-time (<1 day) evolution of the afterglow, four-band light curves were produced by analyzing the data taken at these two astronomical observatories as well as the publicly released data taken using the Magellan Baade telescope (Gladders & Hall). The light curves can be approximated by a broken power law, of which the indices are approximately 0.46 and 1.33 before and after a break at ~0.2 days, respectively. The optical spectral index stayed approximately constant at ~0.9 over 0.17-4.07 days after the burst. Since the temporal decay index after the break and the spectral index measured at that time are both consistent with those predicted by a spherical expansion model, the early break is unlikely to be a jet break but is likely to represent the end of an early bump in the light curve, as was observed in the optical afterglow of GRB 021004.

Deep Optical Imaging of a Compact Group of Galaxies: Seyfert’s Sextet

To investigate the dynamical status of Seyferts Sextet (SS), we have obtained a deep optical (VR+I) image of this group. Our image shows that a faint envelope, down to a surface brightness ?optical(AB) 27 mag arcsec-2, surrounds the member galaxies. This envelope is irregular in shape. It is likely that this shape is attributed either to recent-past or to ongoing galaxy interactions in SS. If the member galaxies have experienced a number of mutual interactions over a long timescale, the shape of the envelope should be rounder. Therefore, the irregularly shaped morphology suggests that SS is in an early phase of dynamical interaction among the member galaxies. It is interesting to note that the soft X-ray image obtained with ROSAT (Pildis, Bregman, &Evrard) is significantly similar in morphology. We discuss the possible future evolution of SS briefly.

Dynamical Mass of Type 2 Seyfert Nuclei

We have derived the masses of central objects (MBH) of nine type 2 Seyfert nuclei using the observational properties of the hidden broad Hβ emission. We obtain the average dynamical mass log (MBH/M☉) 8.00 ± 0.51 - 0.475 log (τes/1), where τes is the optical depth for electron scattering. If τes ~ 1, this average mass is almost comparable with those of type 1 Seyfert nuclei. However, if τes 1, as is usually considered, the average mass of type 2 Seyfert nuclei may be more massive than that of type 1 Seyfert nuclei. We discuss implications for issues concerning both the current unified model of Seyfert nuclei and physical conditions of the electron scattering regions.

A Shock-induced Pair of Superbubbles in the High-Redshift Powerful Radio Galaxy MRC 0406–244

We present new optical spectroscopy of the high-redshift powerful radio galaxy MRC 0406-244 at redshift of 2.429. We find that the two extensions toward the northwest and southeast probed in the rest-frame ultraviolet image are heated mainly by the nonthermal continuum of the active galactic nucleus. However, each extension shows a shell-like morphology, suggesting that they are a pair of superbubbles induced by the superwind activity rather than by the interaction between the radio jet and the ambient gas clouds. If this is the case, the intense starburst responsible for the formation of superbubbles could occur ~1 × 109 yr ago. On the other hand, the age of the radio jets may be on the order of ~106 yr, being much shorter than the starburst age. Therefore, the two events, i.e., the starburst and the radio jet activities, are independent phenomena. However, their directions of the expanding motions could be governed by the rotational motion of the gaseous component in the host galaxy. This idea appears to explain the alignment effect of MRC 0406-244.

Unusual Emission-Line Regions in the Tidal Arm of NGC 7318B in Stephan's Quintet

We report the optical spectroscopic discovery of two unusual emission-line regions in the tidal arm of a galaxy, NGC 7318B, that belongs to Stephans Quintet. They are associated spatially with the unusual radio continuum emission discovered between the two galaxies NGC 7318B and NGC 7319. Taking account of both the large velocity widths of the emission lines, ~900 km s−1, and the strong [S II] λλ6717, 6731 emission with respect to Hα emission, we identify them as optical emission-line filaments of supernova remnants (SNRs) traced by the radio continuum and soft X-ray emission. Our narrowband Hα imaging shows that the unusual emission-line regions are indeed parts of the ionized bubble associated with one radio continuum peak. The required number of SNRs is estimated to be ~105-106 on the basis of the Hα, radio, and soft X-ray luminosities. Since there is no optical counterpart like a star cluster, it is suggested that ~105-106 supernova events occurred nearly simultaneously in this region several times 106 yr ago.