arXiv subjects

Yoshiaki Taniguchi

Publications and source records attributed to Yoshiaki Taniguchi.

At least 109 records · Page 6Linked to original sources

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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arXiv2001

Submillimeter CO emission from shock-heated gas in the L1157 outflow

We present the CO J=6-5, 4-3, and 3-2 spectra from the blueshifted gas of the outflow driven by the low-mass class 0 protostar in the L1157 dark cloud. Strong submillimeter CO emission lines with T_mb > 30 K have been detected at 63" (~0.13 pc) south from the protostar. It is remarkable that the blue wings in the submillimeter lines are stronger by a factor of 3-4 than that of the CO J=1-0 emission line. The CO line ratios suggest that the blueshifted lobe of this outflow consists of moderately dense gas of n(H_2) = (1-3)x10^4 cm^-3 heated to T_kin = 50-170 K.It is also suggested that the kinetic temperature of the outflowing gas increases from ~80 K near the protostar to ~170 K at the shocked region in the lobe center, toward which the largest velocity dispersion of the CO emission is observed. A remarkable correlation between the kinetic temperature and velocity dispersion of the CO emission along the lobe provides us with direct evidence that the molecular gas at the head of the jet-driven bow shock is indeed heated kinematically. The lower temperature of ~80 K measured at the other shocked region near the end of the lobe is explained if this shock is in a later evolutionary stage, in which the gas has been cooled mainly through radiation of the CO rotational lines.

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arXiv2000

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 et al. 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 AGN) at the 50% level or more in order to avoid an ionizing photon excess problem, and this starburst must be young ($< 7 \times 10^7$ yr). We derive a current star formation rate of $270 M_{\odot}$ yr$^{-1}$, and an extinction $A_V > 30$ mag for our line of sight to this hidden starburst.

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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

SiO Emission in the Multi-Lobe Outflow associated with IRAS 16293-2422

We have mapped the thermal emission line of SiO (v = 0; J = 2-1) associated with the quadrupolar molecular outflow driven by the very cold far-infrared source IRAS 16293-2422. The SiO emission is significantly enhanced in the northeastern red lobe and at the position ~50" east of the IRAS source. Strong SiO emission observed at ~50" east of the IRAS source presents evidence for a dynamical interaction between a part of the eastern blue lobe and the dense ambient gas condensation, however, such an interaction is unlikely to be responsible for producing the quadrupolar morphology. The SiO emission in the northeastern red lobe shows the spatial and velocity structure similar to those of the CO outflow, suggesting that the SiO emission comes from the molecular outflow in the northeastern red lobe itself. The observed velocity structure is reproduced by a simple spatio-kinematic model of bow shock with a shock velocity of 19-24 km/s inclined by 30-45 deg from the plane of the sky. This implies that the northeastern red lobe is independent of the eastern blue lobe and that the quadrupolar structure is due to two separate bipolar outflows. The SiO emission observed in the western red lobe has a broad pedestal shape with low intensity. Unlike the SiO emission in the northeastern red lobe, the spatial extent of the SiO emission in the western red lobe is restricted to its central region. The spatial and velocity structures and the line profiles suggest that three different types of the SiO emission are observed in this outflow; the SiO emission arises from the interface between the outflowing gas and the dense ambient gas clump, the SiO emission coming from the outflow lobe itself, and the broad SiO emission with low intensity observed at the central region of the outflow lobe.

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arXiv2000

A New interpretation of chain galaxies at high redshift

We propose a possible new model for the formation of chain galaxies at high redshift. Our model is summarized as follows. Step I: Successive merging of subgalactic gas clumps results in the formation of a galaxy with a mass of $10^{11-12} M_\odot$ at redshift $z \sim 5$. Step II: Subsequently, supernova explosions occur inside the galaxy and then blow out as a galactic wind (or a superwind). This wind expands into the intergalactic space and then causes a large-scale shell with a radius of several hundreds of kpc. Since this radius may be smaller than the typical separation between galaxies, interactions of shells may also occur, resulting in the formation of a large-scale gaseous slab. Step III: Since the shell or the slab can be regarded as a gaseous sheet, filament-like gravitational instability is expected to occur. Step IV: Further gravitational instability occurs in each filament, leading to intense star formation along the filament. This is the chain galaxy phase. Step V: The filament collapses gravitationally into one spheroidal system like an elliptical galaxy within one dynamical timescale of the filament ($\sim 10^8$ yr). Therefore, it seems quite difficult to find remnants of chain galaxies. We also discuss that shocked shells driven by superwinds may be responsible for some Lyman limit systems and damped Ly$α$ systems because their H {\sc i} column densities are expected to be $N_{\rm HI} \gtrsim 10^{19}$ cm$^{-2}$.

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arXiv2000

Photometric Properties of the Arp 220 Super Star Clusters

We investigate the photometric properties of six super stellar clusters (SSCs) seen in both the optical and near-infrared Hubble Space Telescope images of the local ultraluminous starburst galaxy Arp 220. Three of the SSCs are located in the central 0.5 kpc region. The remaining three are in the circumnuclear region between 0.5 kpc and 2.5 kpc from the centre. Comparing the observed spectral energy distributions (SEDs) of the SSCs with the Starburst99 models of Leitherer et al., we confirm that all the three nuclear SSCs are heavily obscured ($A_V \sim 10$ mag). Considering the results from this comparison in conjunction with measurements of the near-infrared CO absorption index and of millimetre CO line widths and luminosities, we estimate the ages of the nuclear SSCs to be $10^7 - 10^8$ yrs. The bolometric luminosity of the three nuclear SSCs is at most one-fifth of the total bolometric luminosity of Arp 220. On the other hand, the circumnuclear SSCs have little internal extinction ($A_V \leq$ 1 mag). These contribute negligibly to the total bolometric luminosity.

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arXiv2000

X-ray emission from Seyfert 2 galaxies with optical polarized broad lines

We analyze the 0.5-10 keV spectra of six Seyfert 2 galaxies observed with the X-ray satellite ASCA: Mrk 3, Mrk 348, Mrk 1210, Mrk 477, NGC 7212, and Was 49b. These galaxies were selected based on their possession of optical polarized broad lines. In the 2-10 keV band, their spectra are heavily absorbed, with 2-10 keV absorption-corrected X-ray luminosities ranging from 10^{42} to 10^{43} erg/s. The observed X-ray emission is generally only about one tenth that predicted based on their known infrared and H_beta luminosities. This apparent X-ray weakness can be understood if a considerable fraction of the nuclear activity is completely blocked from view by thick matter along our line of sight to the nucleus. All of these galaxies possess significant soft X-ray emission whose origin appears to be scattered light from their nuclear emission. Based on this hypothesis, we estimate a typical scattering efficiency for X-rays to be about 10%. This efficiency is larger than the few % found for ordinary Seyfert 2 galaxies with no report of optical polarized broad lines. A large scattering efficiency is best explained by an apparent weakness of the hard X-ray luminosity rather than unusually strong scattered light in the soft band. The difference of the apparent weakness is probably associated with viewing angle.

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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

Formation of intermediate-mass black holes in circumnuclear regions of galaxies

Recent high-resolution X-ray imaging studies have discovered possible candidates of intermediate-mass black holes with masses of $M_\bullet \sim 10^{2-4} \MO$ in circumnuclear regions of many (disk) galaxies. It is known that a large number of massive stars are formed in a circumnuclear giant H {\sc ii} region. Therefore, we propose that continual merger of compact remnants left from these massive stars is responsible for the formation of such an intermediate-mass black hole within a timescale of $\sim 10^9$ years. A necessary condition is that several hundreds of massive stars are formed in a compact region with a radius of a few pc.

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arXiv2000

Superwind model of extended Lyman alpha emitters at high redshift

We propose a new model for the extended Ly alpha blobs found recently at high redshift z ~ 3. The observational properties of these blobs are; 1) the observed Ly alpha luminosities are ~ 10^{43} h^{-2} ergs s^{-1}, 2) they appear elongated morphologically, 3) their sizes amount to ~ 100 kpc, 4) the observed line widths amount to ~ 1000 km s^{-1}, and 5) they are not associated with strong radio-continuum sources. All these observational properties seem to be explained in terms of galactic winds driven by successive supernova explosions shortly after the initial burst of massive star formation in the galactic centers. The observed number density of Ly alpha blobs (~ 3.4 x 10^{-5} h^3 Mpc^{-3}) may be explained if their present-day counterparts are elliptical galaxies with a luminosity above ~ 1 L^*.

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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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