Yoshiaki Taniguchi

The X-ray Baldwin effect

Using the X-ray data of active galactic nuclei (type 1 Seyfert galaxies and quasars) obtained by the X-ray satellite Ginga, we have found that the equivalent width of the Fe K emission line at 6.4 keV, EW(Fe K), is inversely correlated with the X-ray luminosity in the 2-10 keV band. This inverse correlation is basically similar to the well-known relation known as the Baldwin effect, which means that the equivalent width of the C IV λ1550 emission line is inversely correlated with the UV continuum luminosity (Baldwin 1977). Hence, we may call our finding the X-ray Baldwin effect. The X-ray Baldwin effect is pronounced for type 1 Seyfert galaxies

Where is the [O III] λ4363 Emitting Region in Active Galactic Nuclei?

The emission-line flux ratio of [O III]4363/[O III]5007 (R_OIII)is a useful diagnostic for the ionization mechanism and physical properties of emission-line regions in active galactic nuclei (AGNs). However, it is known that simple photoionization models underpredict the [O III]4363 intensity, being inconsistent with observations. In this paper, we report on several pieces of evidence that a large fraction of the [O III]4363 emission arises from the dense gas obscured by putative tori: (1) the visibility of high-R_OIII regions is correlated to that of broad-line regions, (2) higher-R_OIII objects show hotter mid-infrared colors, (3) higher-R_OIII objects show stronger highly-ionized emission lines such as [Fe VII]6087 and [Fe X]6374, and (4) higher-R_OIII objects have broader line width of [O III]4363 normalized by that of [O III]5007. To estimate how such a dense component contributes to the total emission-line flux, dual-component photoionization model calculations are performed. It is shown that the observed values of R_OIII of type 1 AGNs may be explained by introducing a 5% - 20% contribution from the dense component while those of type 2 AGNs may be explained by introducing a 0% - 2% contribution. We also discuss the [O III]4363 emitting regions in LINERs in the framework of our dual-component model.

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.

On the Hidden Nuclear Starburst in Arp 220

We construct a starburst model for the hidden starbursts in Arp 220 based on the new Starburst99 models of Leitherer and coworkers. Comparing these stellar population synthesis models with observations, we show that the hidden power source must be due to star formation (as opposed to an active galactic nucleus) at the 50% level or more in order to avoid an ionizing photon excess problem and this starburst must be young ( 30 mag for our line of sight to this hidden starburst.

NEW NEAR-INFRARED SPECTROSCOPY OF THE HIGH-REDSHIFT QUASAR B1422)231 AT z \ 3.62

We present new near-infrared (rest-frame UV-to-optical) spectra of the high-redshift, gravitationally lensed quasar B1422+231 (z = 3.62). Diagnostic emission lines of Fe II, O III λ5007, and Hβ, commonly used to determine the excitation, ionization, and chemical abundances of radio-quiet and radio-loud quasars, were detected. Our new data show that the ratio Fe II (UV)/Hβ = 18.1 ± 4.6 and Fe II (optical)/Hβ = 2.3 ± 0.6 are higher than those reported by Kawara et al. by factors of 1.6 and 3.3, respectively, although the ratio [O III] λ5007/Hβ = 0.19 ± 0.02 is nearly the same between the two measurements. The discrepancy of the line flux ratios between the measurements is likely due to improved data and fitting procedures rather that to intrinsic variability. While approximately half of the high-z quasars observed to date have much more extreme Fe II (optical)/Hβ ratios, the line ratio measured for B1422+231 are consistent with the observed range of Fe II (optical) ratios of low-z quasars.

Near-Infrared Spectroscopy of the High-Redshift Quasar S4 0636+68 at z = 3.2

?????We present near-infrared (observed frame) spectra of the high-redshift quasar S4 0636+68 at z = 3.2, which was previously thought to be one of a group of strong Fe II emitters [i.e., F(Fe II ??4434?4684)/F(H?) > 1]. Our K-band spectrum clearly shows emission lines of H? and [O III] ??4959, 5007, as well as optical Fe II emission. Our computed value of F(Fe II ??4434?4684)/F(H?) 0.8 for S4 0636+68 is less than previously thought and, in fact, is comparable to values found for radio-loud, flat-spectrum, low-z quasars. Therefore S4 0636+68 appears not to be a strong optical Fe II emitter. Although more than half () of the high-z quasars observed to date are still classified as strong optical Fe II emitters, their Fe II/H? ratios, for the most part, follow the same trend as do those of low-z quasars, i.e., an anticorrelation in EW(Fe II)/EW(H?) versus EW([O III])/EW(H?), with radio-loud quasars having a mean value of EW(Fe II)/EW(H?) approximately half that of radio-quiet quasars at comparable values of EW([O III])/EW(H?).

Mid-Infrared Identification of Faint Submillimeter Sources*

Faint submillimeter sources detected with the Submillimeter Common-User Bolometer Array on the James Clerk Maxwell Telescope have faced an identification problem due to the telescopes broad beam profile. Here we propose a new method to identify such submillimeter sources with a mid-infrared image having a finer point-spread function. The Infrared Space Observatory has provided a very deep 6.7 ?m image of the Hawaii Deep Field SSA13. All three faint 850 ?m sources in this field have their 6.7 ?m counterparts. They have been identified with interacting galaxy pairs in optical images. These pairs are also detected in the radio. Two of them are optically faint and very red (I > 24, I-K > 4), one of which has a hard X-ray detection with the Chandra satellite. As these observing properties are similar to those of local ultraluminous infrared galaxies, their photometric redshifts are derived based on submillimeter-to-mid-infrared flux ratios assuming a spectral energy distribution (SED) of Arp 220. Other photometric redshifts are obtained via ?2-minimization between the available photometry data and template SEDs. Both estimates are in the range z = 1-2, in good agreement with a spectroscopic redshift and a millimetric one. The reconstructed Arp 220 SEDs with these redshift estimates are consistent with all the photometry data except Chandras hard X-ray detection. The sources would be a few times more luminous than Arp 220. With an assumption that contributions from active galactic nuclei are negligible, it appears that extremely high star formation rates are occurring in galaxies at high redshifts with massive stellar contents already in place.

LRG J0239?0134: A Ring Galaxy or a Pair of Superbubbles at z = 1?

The unusual morphology of LRG J0239-0134 at z = 1.062 has been interpreted as a ring galaxy. Here we propose an alternative idea that the ringlike morphology is attributed to a pair of superbubbles driven by the intense starburst in the central region of this galaxy. Supporting evidence for a superbubble model is (1) the poststarburst nature in the central body suggests that a burst of supernova explosions could have occurred at least ~107 yr ago, (2) the dark lane seen in the central body suggests that we observe this object from a nearly edge-on view, and (3) the ringlike morphology is not inconsistent with an idea that it is a pair of superbubbles. All these pieces of evidence for the superbubble model seem circumstantial. However, if this is the case, this galaxy provides an important example of the superwind activity at high redshift.

Discovery of a Low Surface Brightness Object near Seyfert’s Sextet

We report the discovery of a low surface brightness (LSB) object serendipitously found during deep CCD imaging of a compact group of galaxies, Seyferts Sextet, in the VR and I bands. The LSB object is located 23 southwest from the groups center. Its surface brightness within the angular effective radii of reff(VR) = 48 and reff(I) = 48 is very low: μeff(VR) = 25.28 mag arcsec-2 and μeff(I) = 24.47 mag arcsec-2, respectively. The apparent magnitudes are mAB(VR) = 19.87 and m(I) = 19.06. The object is most likely an LSB dwarf galaxy, but other possibilities are also discussed.

A Multi-Band Photometric Study of Tidal Debris in a Compact Group of Galaxies: Seyfert's Sextet

In order to investigate the properties of the prominent tidal debris feature extending to the northeast of a compact group of galaxies, Seyfert’s Sextet, we analyzed multi-band (U, B, V, VR , R, I, J, H ,a ndK � ) photometric imaging data and obtained the following results: 1) The radial surface brightness distribution of this tidal debris in Seyfert’s Sextet (TDSS) in each band appears to be well approximated by an exponential profile. 2) The observed B − V color of TDSS is similar to those of dwarf elliptical galaxies in nearby clusters. 3) Comparing the spectral energy distribution (SED) of TDSS with theoretical photometric evolution models and with the SED of the stars in the outer part of HCG 79b, we find that its SED is comparable to that of a ∼ 10Gyr-old stellar population with solar metallicity, similar to the stellar population in the outer part of HCG 79b. This suggests that TDSS consists of stars that may have been liberated from HCG 79b by strong galaxy interactions, not a pre-existing dwarf galaxy as previously thought.