arXiv subjects

Yasuhiro Shioya

Publications and source records attributed to Yasuhiro Shioya.

At least 55 records · Page 3Linked to original sources

arXiv2002

Gas Metallicity of Narrow-Line Regions in Narrow-Line Seyfert 1 Galaxies and Broad-Line Seyfert 1 Galaxies

We investigate gas metallicity of narrow-line regions in narrow-line Seyfert 1 galaxies (NLS1s) and broad-line ones (BLS1s) in order to examine whether or not there is a difference in the gas metallicity between the two populations of Seyfert 1 galaxies. We apply two methods to study this issue. One is to use the emission-line flux ratio of [N II]6583/H_alpha in combination with some other optical emission-line flux ratios. This method, which has been often applied to Seyfert 2 galaxies, suggests that the gas metallicity of narrow-line regions is indistinguishable or possibly higher in BLS1s than in NLS1s. On the contrary, the other method in which only forbidden emission-line fluxes are used results in that NLS1s tend to possess metal-richer gas in the narrow-line regions than BLS1s. We point out that this inconsistency may be owing to the contamination of the broad component of permitted lines into the narrow component of ones in the first method. Since the results derived by using only forbidden emission-line fluxes do not suffer from any uncertainty of the fitting function for the broad component of Balmer lines, the results from this method are more reliable than those derived by using permitted lines. We thus conclude that the gas metallicity of narrow-line regions tends to be higher in NLS1s than in BLS1s.

astro-ph↗

arXiv2002

Spectrophotometric evolution of spiral galaxies with truncated star formation: An evolutionary link between spirals and S0s in distant clusters

A one-zone chemo-spectro-photometric model is used to investigate the time evolution of disk galaxies whose star formation is truncated, and to determine the dependence of this evolution on the previous star formation history and the truncation epoch. Truncated spirals show red colors $\sim 1$ Gyr after truncation, and evolve spectrally from an e(b) type, down through the e(a), a+k, and k+a classes, to finally become a k type. The exact behavior in this phase depends on the truncation epoch and the star formation history prior to truncation. For example, earlier type disks show redder colors and do not show a+k-type spectra after truncation of star formation. We also discuss a possible evolutionary link between the k-type galaxies with spiral morphology found in distant clusters, and present-day S0s, by investigating whether truncated spirals reproduce the infrared color-magnitude relation of Coma galaxies. We suggest that only less luminous, later-type disk galaxies whose star formation is truncated at intermediate and high redshifts can reproduce the red $I-K$ colors observed for S0s in the Coma cluster.

astro-ph↗

arXiv2001

Is There an Advection-Dominated Accretion Flow in Radio Galaxies with Double-Peaked Balmer Lines?

In order to examine the prediction that the broad-line radio galaxies (BLRGs) with double-peaked Balmer lines harbor an accretion disk characterized by an advection-dominated accretion flow (ADAF) in their nuclei, we investigate narrow emission-line flux ratios of the narrow-line regions which are photoionized by the nuclear continuum radiation. We compile data from the literature and confirm the pioneering work of Eracleous & Halpern that the BLRGs with the double-peaked Balmer emission exhibit larger flux ratios of both [O I]6300/[O III]5007 and [O II]3727/[O III]5007 than the BLRGs without the double-peaked Balmer emission. To examine whether or not these properties are attributed to the difference in the spectral energy distribution (SED) of the ionizing radiation between the BLRGs with and without the double-peaked Balmer emission, we perform photoionization model calculations using two types of input continuum radiation; one has the strong big blue bump which is expected for standard optically-thick accretion disks and another does not exhibit a strong big blue bump as expected for optically-thin ADAFs. We find that the data of the BLRGs with the double-peaked Balmer lines are consistent with the models adopting the SED without a strong big blue bump while the data of the BLRGs without the double-peaked emission lines are well described by the models adopting the SED with a strong big blue bump. On the other hand, the observed differences in the NLR emission is hard to be explain by the difference in the contribution of shocks. These results support the idea that the double-peaked Balmer lines arise at an outer region of an accretion disk which is illuminated by an inner, geometrically-thick ADAF.

astro-ph↗

arXiv2001

On the Origin of Lyman$α$ Blobs at High Redshift: Submillimetric Evidence for a Hyperwind Galaxy at z=3.1

The most remarkable class of high-redshift objects observed so far is extended Ly$α$ emission-line blobs found in an over-density region at redshift 3.1. They may be either a dust-enshrouded, extreme starburst galaxy with a large-scale galactic outflow (superwind) or cooling radiation from dark matter halos. Recently one of these Ly$α$ blobs has been detected at submillimeter wavelengths (450 and 850 $μ$m). Here we show that its rest-frame spectral energy distribution between optical and far-infrared is quite similar to that of Arp 220, which is a typical ultraluminous starburst/superwind galaxy in the local universe. This suggests strongly that the superwind model proposed by Taniguchi & Shioya is applicable to this Ly$α$ blob. Since the blob is more luminous in the infrared by a factor of 30 than Arp 220, it comprises a new population of hyperwind galaxies at high redshift.

astro-ph↗

arXiv2001

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 NW and SE 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 $\sim 1 \times 10^9$ yr ago. On the other hand, the age of the radio jets may be of the order of $\sim 10^6$ 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.

astro-ph↗

arXiv2001

Spectroscopic evolution of dusty starburst galaxies

By using a one-zone chemical and spectrophotometric evolution model of a disk galaxy undergoing a dusty starburst, we investigate, numerically, the optical spectroscopic properties in order to explore galaxy evolution in distant clusters. We adopt an assumption that the degree of dust extinction (represented by $A_V$) depends on the ages of starburst populations in such a way that younger stars have larger $A_V$ (originally referred to as selective dust extinction by Poggianti & Wu 2000). In particular, we investigate how the time evolution of the equivalent widths of [OII]$λ$3727 and H$δ$ is controlled by the adopted age dependence. This leads to three main results: (1) If a young stellar population (with the age of $\sim$ $10^6$ yr) is more heavily obscured by dust than an old one ($>$ $10^8$ yr), the galaxy can show an ``e(a)'' spectrum characterized by strong H$δ$ absorption and relatively modest [OII] emission. (2) A dusty starburst galaxy with an e(a) spectrum can evolve into a poststarburst galaxy with an a+k (or k+a) spectrum 0.2 Gyr after the starburst and then into a passive one with a k-type spectrum 1 Gyr after the starburst. This result clearly demonstrates an evolutionary link between galaxies with different spectral classes (i.e., e(b), e(a), a+k, k+a, and k). (3) A dusty starburst galaxy can show an a+k or k+a spectrum even in the dusty starburst phase if the age-dependence of dust extinction is rather weak, i.e., if young starburst populations with different ages ($\le$ $10^7$ yr) are uniformly obscured by dust.

astro-ph↗

arXiv2001

Tidal truncation of gas replenishment and global suppression of galactic star formation in distant clusters

Recent spectroscopic observations of galaxies in distant clusters have revealed that the rate of star formation in star-forming galaxies is significantly suppressed with respect to their counterparts in the field at a similar redshift. It is, however, highly uncertain which physical processes are responsible for this suppression. We present the results of a numerical investigation of how the global tidal field of a cluster dynamically influences the reservoir of halo gas surrounding a disk galaxy as it falls into the cluster from the surrounding field. We find that the tidal field of the cluster efficiently removes the halo gas from the galaxy, thereby halting its accretion onto the disk, and thus the fueling of star formation within. This effectively truncates the galaxy's star formation. We also find that this tidal truncation does not depend very strongly on the orbit of the disk with respect to the center of the cluster. These results suggest that the global tidal field of clusters is capable of causing a widespread and uniform suppression of star formation in galaxies accreted by the cluster. In light of these results, we discuss the importance of this tidal truncation of the gas supply in the formation of S0 galaxies in clusters.

astro-ph↗

arXiv2000

Formation and evolution of E+A galaxies in dusty starburst galaxies

The formation and evolution of the ``E+A'' (also named ``k+a'' and ``a+k'' types by Dressler et al. 1999) galaxies found in significant numbers in the cores of intermediate redshift clusters has been extensively discussed by many authors. In this paper, we model the spectral, dynamical and morphological evolution of a prime candidate for producing this spectral signature: a dusty starburst associated with a major galaxy merger. We show that as this system evolves dynamically, its spectral type changes from and ``e(a)'' type (exhibiting strong H$δ$ absorption and modest [OII] emission -- the identifying features of local dusty starburst galaxies) to a k+a type and then finally to a passive ``k'' type. This result shows that galaxies with an e(a) spectral type can be precursors to the k+a systems and that dynamical evolution greatly controls the spectral evolution in these merger cases. Our simulations also show that a merger with very high infrared luminosity ($L_{\rm IR}$ $>$ $10^{11}$ $L_{\odot}$) is more likely to show an e(a) spectrum, which implies that spectral types can be correlated with infrared fluxes in dusty starburst galaxies. Based on these results, we discuss the origin of the evolution of k+a/a+k galaxies in distant clusters and the role merging is likely to have.

astro-ph↗

arXiv2000

Photometric evolution of dusty starburst mergers:On the nature of ultra-luminous infrared galaxies

By performing N-body simulations of chemodynamical evolution of galaxies with dusty starbursts, we investigate photometric evolution of gas-rich major mergers in order to explore the nature of ultraluminous infrared galaxies (ULIRGs) with the total infrared luminosity ($L_{\rm IR}$ for $8\sim 1000$ $μ$m) of $\sim$ $10^{12}$ $L_{\odot}$. Main results are the following three. (1) Global colors and absolute magnitudes the during dusty starburst of a major merger do not change with time significantly, because interstellar dust heavily obscures young starburst populations that could cause rapid evolution of photometric properties of the merger. (2) Dust extinction of stellar populations in a galaxy merger with large infrared luminosity ($L_{\rm IR}$ $>$ $10^{11}$ $L_{\odot}$) is selective in the sense that younger stellar populations are preferentially obscured by dust than old ones. This is because younger populations are located in the central region where a larger amount of dusty interstellar gas can be transferred from the outer gas-rich regions of the merger. (3) Both $L_{\rm IR}$ and the ratio of $L_{\rm IR}$ to $B$ band luminosity $(L_{\rm B}$) increases as the star formation rate increase during the starburst of the present merger model, resulting in the positive correlation between $L_{\rm IR}$ and $L_{\rm IR}/L_{\rm B}$.

astro-ph↗

arXiv2000

ALMA and high redshift dusty starburst mergers

By using a new numerical code for deriving the spectral energy distributions of galaxies, we have investigated the time evolution of morphological properties, the star formation rate, and the submillimeter flux at 850 $μ$m in high-redshift ($z$) dusty starburst mergers with mass ratio ($m_{2}$) of two disks ranging from 0.1 (minor merger) to 1.0 (major one). We found that the maximum star-formation rate, the degree of dust extinction, and the 850 $μ$m flux are larger for mergers with larger $m_{2}$. The 850 $μ$m flux from mergers at 1.5 $\le z \le$ 3.0 in the observer frame is found to be a few mJy for major merger cases, and at most $\sim$ 100 $μ$Jy for minor ones. This result suggests that only high-redshift major mergers are now detected by SCUBA with the current 850 $μm$ detection limit of a few mJy. These results imply that LMSA (ALMA) with the expected detection limit of the order of 10 $μ$Jy at 850 $μm$ can be used to study high-redshift mergers with variously different $m_2$, and thus provide an important clue to the formation of galaxies in a high-redshift universe.

astro-ph↗

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.

astro-ph↗

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$^{-2}$.

astro-ph↗

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.

astro-ph↗

arXiv2000

Effects of dust extinction on optical spectroscopic properties for starburst galaxies in distant clusters

Recent observational studies on galaxies in distant clusters discovered a significant fraction of possible dusty starburst galaxies with the so-called `e(a)' spectra that are characterized by strong H$δ$ absorption and relatively modest [OII] emission. We numerically investigate spectroscopic and photometric evolution of dusty starburst galaxies in order to clarify the origin of the e(a) spectra. We found that if a young starburst population is preferentially obscured by dust than an old one in a dusty starburst galaxy, the galaxy shows e(a) spectrum. It is therefore confirmed that the selective dust extinction, which is first suggested by Poggianti & Wu (2000) and means the strongest dust extinction for the youngest stellar population among stellar populations with different ages, is critically important to reproduce quantitatively the observed e(a) spectra for the first time in the present numerical study. We furthermore discuss what physical process is closely associated with this selective dust extinction in cluster environment.

astro-ph↗

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.

astro-ph↗

arXiv2000

Formation and evolution of dusty starburst galaxies I. A new method for deriving spectral energy distribution

We present a new numerical code which is designed to derive a spectral energy distribution (SED) for an arbitrary spatial distribution of stellar and gaseous components in a dusty starburst galaxy. We apply a ray tracing method to numerical simulations and thereby estimate extinction and reemission of stellar light by dusty gas in an explicitly self-consistent manner. By using this code, we can investigate simultaneously dynamical and photometric evolution of a dusty galaxy based on stellar and gaseous dynamical simulations. As an example, we demonstrate when and how a major galaxy merger with dusty starburst becomes an ultra-luminous infrared galaxy owing to strong internal dust extinction. We furthermore discuss advantages and disadvantages of the present new code in clarifying the nature and the origin of low and high redshift dusty starburst galaxies.

astro-ph↗

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.

astro-ph↗

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^*.

astro-ph↗