arXiv subjects

Takashi Murayama

Publications and source records attributed to Takashi Murayama.

At least 55 records · Page 3Linked to original sources

arXiv2001

Seyfert-Type Dependences of Narrow Emission-Line Ratios and Physical Properties of High-Ionization Nuclear Emission-Line Regions in Seyfert Galaxies

In order to examine how narrow emission-line flux ratios depend on the Seyfert type, we compiled various narrow emission-line flux ratios of 355 Seyfert galaxies from the literature. We present in this paper that the intensity of the high-ionization emission lines, [Fe VII]6087, [Fe X]6374 and [Ne V]3426, tend to be stronger in Seyfert 1 galaxies than in Seyfert 2 galaxies. In addition to these lines, [O III]4363 and [Ne III]3869, whose ionization potentials are not high (< 100 eV), but whose critical densities are significantly high (> 10^7 cm^-3), also exhibit the same tendency. On the other hand, the emission-line flux ratios among low-ionization emission lines do not show such a tendency. We point out that the most plausible interpretation of these results is that the high-ionization emission lines arise mainly from highly-ionized, dense gas clouds, which are located very close to nuclei, and thus can be hidden by dusty tori. To examine the physical properties of these highly-ionized dense gas clouds, photoionization model calculations were performed. As a result, we find that the hydrogen density and the ionization parameter of these highly-ionized dense gas clouds are constrained to be n_H > 10^6 cm^-3 and U > 10^-2, respectively. These lower limits are almost independent both from the metallicity of gas clouds and from the spectral energy distribution of the nuclear ionizing radiation.

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arXiv2001

The [O III]4363 Emitting Region Obscured by Dusty Tori

The emission-line flux ratio of [O III]4363/[O III]5007 is a useful diagnostic for the ionization mechanism and physical properties of narrow-line regions in active galactic nuclei. However, it is known that simple photoionization models underpredict this ratio. In this contribution, we report on some pieces of evidence that a large fraction of the [O III]4363 emission arises from the dense gas, which can be obscured by dusty tori. Taking this dense component into account, we show that the flux ratio of [O III]4363/[O III]5007 can be explained by two-component photoionization models.

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arXiv2000

Where is the [O III]4363 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.

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arXiv2000

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 ring-like 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 occur at least $\sim 10^7$ 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 ring-like 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.

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arXiv2000

Poststarburst Models of LINERs

Since the discovery of low-ionization nuclear emission-line regions in many galaxies (LINERs), it has been recognized that they constitute a class of active galactic nuclei (AGNs) which are thought to be powered by gas accretion onto a central, supermassive black hole. LINERs are observed in approximately one third of galaxies in the local universe and it has been often thought that they harbor an AGN-like central engine with moderate activity. However, some LINERs show no direct evidence for AGNs such as broad emission lines, radio jets, hard X-ray emission, spectral energy distributions which are inconsistent with starlight, and so on. For such LINERs (a subset of type 2 LINERs), we present new poststarburst models which explain some of their most important optical narrow emission-line ratios. In these models, the ionization sources are planetary nebula nuclei (PNNs) with temperature of ~ 10^5 K which appear in the late-phase evolution of intermediate-mass stars with mass between ~ 3 M_solar and ~ 6 M_solar. Such PNNs left in a typical starburst nucleus can produce an Halpha luminosity of L(Halpha) ~ 10^38 ergs s^-1 for typical poststarburst LINERs and ~ 10^39 ergs s^-1 only in exceptionally bright cases. The PNN phase lasts until the death of the lowest-mass stars formed in the starburst, which is ~ 5 x 10^8 yr for an assumed lower limit of the initial mass function of 3 M_solar. This long duration appears consistent with the observed higher frequency of occurrence of LINERs if every galaxy could experience the starburst activity several times in its life. We therefore propose that some LINERs which show no direct evidence for AGNs may be poststarburst nuclei powered by a cluster of PNNs.

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arXiv2000

The Nuclear Activity of the 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 which belong to 31 Hickson Compact Groups (HCGs) of Galaxies. Among them, three galaxies have discordant redshifts. Further, spectral quality is too poor to classify other three galaxies. Therefore, we describe our results for the remaining 63 galaxies. Our main results are summarized below. (1) We have found in our sample; 28 AGN, 16 HII nuclei, and 19 normal galaxies which show 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 HII nuclei in the HCGs is significantly less than that in the field. However, this difference may be due to selection bias that our HCG sample contains more early-type galaxies than the field, because it is known that HII nuclei are rarer in early-type galaxies than in later ones. (3) Applying correction 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 triggering both the AGN activity and the nuclear starburst. We discuss some implications on the nuclear activity in the HCG galaxies.

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arXiv2000

The High-Ionization Nuclear Emission-Line Region of Seyfert Galaxies

Recently Murayama & Taniguchi proposed that a significant part of the high-ionization nuclear emission-line region (HINER) in Seyfert nuclei arises from the inner wall of dusty tori because type 1 Seyfert nuclei (S1s) show the excess HINER emission with respect to type 2 Seyfert ones (S2s). This means that the radiation from the HINER has the anisotropic property and thus statistical properties of the HINER emission can be used to investigate the viewing angle toward dusty tori for various types of Seyfert nuclei. In order to investigate viewing angles toward various types of Seyferys, we apply this HINER test to them. Our main results are as follows. (1) The NLS1s are viewed from a more face-on view toward dusty tori than the S2s. However, the HINER properties of the NLS1s are indistinguishable from those of the S1s. (2) The type 1.5 Seyferts appear to comprise heterogeneous populations. (3) The type 1.8 and 1.9 Seyferts and the objects showing either a broad Pa_beta line or polarized broad Balmer lines are seen from a large inclination angle and the emission from the BLRs of such objects reaches us through optically-thin parts of dusty tori. These three results support strongly the current unified model of Seyfert nuclei. And, (4) the line ratios of [Fe X]6374 to the low-ionization emission-lines have rather isotropic property than those of [Fe VII]6087. Therefore it is suggested that the [Fe X] emission is not useful investigating the viewing angle toward the dusty torus in Seyfert nuclei. The most plausible reason seems that the [Fe X] emission is spatially extended and thus its strength tends to show less viewing angle dependence.

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arXiv2000

Dusty Tori of Seyfert Nuclei

Dusty tori around active galactic nuclei (AGNs) play an important role in the classification of Seyfert galaxies. Therefore, physical properties of dusty tori are of great interest. We briefly introduce three statistical studies investigating properties of dusty tori; 1) physical sizes of dusty tori based on water-vapor maser emission, 2) ionization condition of the inner wall of tori based on high-ionization emission lines, and 3) viewing angle toward dusty tori based on mid-infrared color.

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arXiv2000

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, Seyfert's Sextet, in VR and I bands. The LSB object is located 2.3 arcmin southwest from the group center. Its surface brightness within the angular effective radii of r_e(VR)=4.8 arcsec and r_e(I)=4.8 arcsec is very low --- mu_e(VR)=25.28 mag arcsec-2 and mu_e(I)=24.47 mag arcsec-2, respectively. The apparent magnitudes are m_AB(VR)=19.87 mag and m(I)=19.06 mag. The object is most likely a LSB dwarf galaxy, but other possibilities are also discussed.

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arXiv1999

Extended High-Ionization Nuclear Emission-Line Region in the Seyfert Galaxy NGC 4051

We present an optical spectroscopic analysis of the well-known Seyfert galaxy NGC 4051. The high-ionization nuclear emission-line region (HINER) traced by [Fe X]6374 is found to be spatially extended to a radius of 3a rcseconds (150 pc) west and southwest from the nucleus; NGC 4051 is the third example which has an extended HINER. The nuclear spectrum shows that the flux of [Fe X]6374 is stronger than that of [Fe VII] 6087 in our observation. This property cannot be interpreted in terms of a simple one-zone photoionization model. In order to understand what happens in the nuclear region in NGC 4051, we investigate the physical condition of the nuclear emission-line region in detail using new photoionization models in which the following three emission-line components are taken into account; (1) optically thick, ionization-bounded clouds; (2) optically thin, matter-bounded clouds; and (3) a contamination component which emits H$α$ and H$β$ lines. Here the observed extended HINER is considered to be associated with the low-density, matter-bounded clouds. Candidates of the contamination component are either the broad-line region (BLR) or nuclear star forming regions or both. The complexity of the excitation condition found in NGC 4051 can be consistently understood if we take account of these contamination components.

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arXiv1999

What Are Narrow-Line Seyfert 1 Galaxies ?: Toward A Viewing-Angle-Dependent Unified Model for Seyfert Galaxies

Recently growing evidence has been accumulated that the NLS1s are viewed from nearly pole-on viewing angles. However, there has been no theoretical model which explains the following important correlations; 1) weaker optical Fe II emitters always have wider emission-line widths but stronger optical Fe II emitters have either wider or narrower line widths, and 2) S1s with wider line widths always have flatter soft X-ray spectra but S1s with narrower line widths have either steeper or flatter soft X-ray spectra. Photoionization models predict that the partly-ionized zone in the broad line region (BLR) with a disk-like configuration, in which the optical Fe II emission is thought to arise, is very optically thick. If we observe this disk-like BLR from an inclined viewing angle, the Fe II emitting region located in the far-side half disk cannot be seen entirely because of the large optical depth while almost all the Hbeta emission can be seen because the ionized hydrogen is located in the outer surfaces of BLR clouds. This explains that the Fe II/Hbeta ratio depends on viewing angle. Recent discovery of relativistic outflows in some NLS1s suggests that the nuclear radio jet interacts with the dense ambient gas very close to the central engine. The kinetic Doppler effect increases the soft X-ray luminosities if we observe S1s from a nearly pole-on view given the above range of beta. These explain why NLS1s tend to have steeper soft X-ray spectra together with higher soft X-ray luminosities. Furthermore, the extinction of soft X-ray photons due to dust grain above the dusty tori are expected to more serious for the BLS1s than for the NLS1s. We thus suggest that these orientation effects are responsible for the second important correlation.

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arXiv1999

New Mid-Infrared Diagnostic of the Dusty Torus Model for Seyfert Nuclei

We propose a new diagnostic of the ``dusty torus'' model for Seyfert nuclei. Dust grains in the torus are heated by the nuclear continuum, and reradiate mostly in the mid-infrared wavelengths. From the torus geometry, it is predicted that the emission at lambda < 10 micron has strong dependence on the viewing angle. Since the dependence is predicted to be insignificant at lambda > 10 micron, we study the flux ratio between 3.5 micron (L band) and 25 micron; R(L,25) = log [(nu_3.5 um S_nu_3.5 um)/(nu_25 um S_nu_25 um)]. In three different samples (optically selected, X-ray selected, and infrared selected samples) of Seyfert galaxies, the observed values of R(L,25) between type 1 Seyferts (S1s) and type 2 Seyferts (S2s) are found to be clearly separated; R(L,25) > -0.6 for S1s while R(L,25) < -0.6 for S2s. This implies universality of their torus properties. With this result and the other observational characteristics, we investigate the most plausible torus model among those presented in Pier & Krolik (1992, 1993).

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arXiv1998

A New Dual-Component Photoionization Model for the Narrow Emission-Line Regions in Active Galactic Nuclei

Having found that type 1 Seyfert nuclei have excess [FeVII]6087 emission with respect to type 2s, Murayama & Taniguchi have proposed that the high-ionization nuclear emission-line region (HINER) traced by the [FeVII]6087 emission resides in the inner wall of dusty tori. The covering factor of the torus is usually large (e.g., ~ 0.9). Further, electron density in the tori (e.g., ~ 10^7-10^8 cm-3) is considered to be higher significantly than that (e.g., ~ 10^3-10^4 cm^-3) in the narrow-line region (NLR). Therefore it is expected that the torus emission contributes to the majority of the higher-ionization emission lines. Taking this HINER component into account, we have constructed new dual-component (i.e., a typical NLR with a HINER torus) photoionization models. Comparison of our model with the observations show that that if the torus emission contributes ~ 10 % of the NLR emission, our dual-component model can explain the observed high [FeVII]6087/[OIII]5007 intensity ratios of the Seyfert 1s without invoking any unusual assumptions (e.g., the overabundance of iron).

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arXiv1998

Dusty Tori of Seyfert Nuclei Probed by the Water Vapor Maser Emission: How Large are the Dusty Tori ?

Geometrical and physical properties of dusty tori of Seyfert nuclei probed by the water vapor maser emission at 22 GHz are discussed. We assume that the dusty torus has a simple cylindrical form and the maser emission can be detected only when we observe the torus from almost edge-on views. The observed low frequency of occurrence of the water vapor maser emission (less than 10 percent) suggests that the torus is a vertically thin cylinder whose outer radius between a few pc and ~ 10 pc. However, the observed masing regions are concentrated in the inner 1 pc regions of the torus. This property can be explained by that only the inner a few pc regions have physical conditions enough to cause the maser emission; the temperature is as high as several hundred K and the density is as high as ~ 10^(10) cm^(-3).

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arXiv1998

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(Hbeta) > 1]. Our K-band spectrum clearly shows emission lines of Hbeta and [O III]4959, 5007 as well as optical Fe II emission. Our computed value of F(Fe II 4434-4684)/F(Hbeta) ~ 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 (5/8) of the high-z quasars observed to date are still classified as strong optical Fe II emitters, their Fe II/Hbeta ratios, for the most part, follow the same trend as that of low-$z$ quasars, i.e., an anticorrelation in EW(Fe II)/EW(Hbeta) versus EW([O III])/EW(Hbeta), with radio-loud quasars having a mean value of EW(Fe II)/EW(Hbeta) approximately half that of radio-quiet quasars at comparable values of EW([O III])/EW(Hbeta).

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arXiv1998

Where is the coronal line region in active galactic nuclei?

We report the new finding that type 1 Seyfert nuclei (S1s) have excess [FeVII]6087 emission with respect to type 2s (S2s). The S1s exhibit broad emission lines which are attributed to ionized gas within 1 pc of the black hole, whereas the S2s do not show such broad lines. The current unified model of active galactic nuclei explains this difference as that the central 1 pc region in the S2s is hidden from the line of sight by a dusty torus if we observe it from a nearly edge-on view toward the torus. Therefore, our finding implies that the coronal line region (CLR) traced by the [FeVII]6087 emission resides in the inner wall of such dusty tori. On the other hand, the frequency of occurrence of the CLR in the optical spectra is nearly the same between the S1s and the S2s. Moreover, some Seyfert nuclei exhibit a very extended (~ 1 kpc) CLR. All these observational results can be unified if we introduce a three-component model for the CLR; 1) the inner wall of the dusty torus, 2) the clumpy ionized region associated with the narrow line region at distance from ~ 10 to ~ 100 pc, and 3) the extended ionized region at distance ~ 1 kpc.

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arXiv1997

The Epoch of Major Star Formation in High-z Quasar Hosts

We present the results of our observing program on near infrared spectroscopy of high-redshift quasars which have been undertaken both at Kitt Peak National Observatory and at Mauna Kea Observatory, University of Hawaii. These data are utilized for studying the epoch of major star formation in high-redshift quasar hosts.

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arXiv1997

The most Distant [OIII]-emitting Quasar PKS 1937-101 at redshift 3.8

We report the discovery of a high-z quasar with unambiguous [OIII]5007 emission; PKS 1937-101 at redshift 3.8. This quasar, however, shows little evidence for rest-frame ultraviolet and optical FeII emission. It is thus shown that PKS 1937-101 does not belong to a class of super iron-rich high-z quasars reported by Elston, Thompson, & Hill (1994). The epoch of major star formation in the host galaxy is discussed briefly.

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