arXiv subjects

M. Umemura

Publications and source records attributed to M. Umemura.

At least 19 recordsLinked to original sources

arXiv2015

Galaxy-scale AGN Feedback - Theory

Powerful relativistic jets in radio galaxies are capable of driving strong outflows but also inducing star-formation by pressure-triggering collapse of dense clouds. We review theoretical work on negative and positive active galactic nuclei feedback, discussing insights gained from recent hydrodynamical simulations of jet-driven feedback on galaxy scales that are applicable to compact radio sources. The simulations show that the efficiency of feedback and the relative importance of negative and positive feedback depends strongly on interstellar medium properties, especially the column depth and spatial distribution of clouds. Negative feedback is most effective if clouds are distributed spherically and individual clouds have small column depths, while positive feedback is most effective if clouds are predominantly in a disc-like configuration.

astro-ph.GA↗

arXiv2013

Ultra Fast Outflows: Galaxy-Scale Active Galactic Nucleus Feedback

We show, using global 3D grid-based hydrodynamical simulations, that Ultra Fast Outflows (UFOs) from Active Galactic Nuclei (AGN) result in considerable feedback of energy and momentum into the interstellar medium (ISM) of the host galaxy. The AGN wind interacts strongly with the inhomogeneous, two-phase ISM consisting of dense clouds embedded in a tenuous hot hydrostatic medium. The outflow floods through the inter-cloud channels, sweeps up the hot ISM, and ablates and disperses the dense clouds. The momentum of the UFO is primarily transferred to the dense clouds via the ram pressure in the channel flow, and the wind-blown bubble evolves in the energy-driven regime. Any dependence on UFO opening angle disappears after the first interaction with obstructing clouds. On kpc scales, therefore, feedback by UFOs operates similarly to feedback by relativistic AGN jets. Negative feedback is significantly stronger if clouds are distributed spherically, rather than in a disc. In the latter case the turbulent backflow of the wind drives mass inflow toward the central black hole. Considering the common occurrence of UFOs in AGN, they are likely to be important in the cosmological feedback cycles of galaxy formation.

astro-ph.CO↗

arXiv2012

Driving Outflows with Relativistic Jets and the Dependence of AGN Feedback Efficiency on ISM Inhomogeneity

We examine the detailed physics of the feedback mechanism by relativistic AGN jets interacting with a two-phase fractal interstellar medium in the kpc-scale core of galaxies using 29 3D grid-based hydrodynamical simulations. The feedback efficiency, as measured by the amount of cloud-dispersal generated by the jet-ISM interactions, is sensitive to the maximum size of clouds in the fractal cloud distribution but not to their volume filling factor. Feedback ceases to be efficient for Eddington ratios P_jet/L_edd<10^-4, although systems with large cloud complexes ~50 pc require jets of Eddington ratio in excess of 10^-2 to disperse the clouds appreciably. Based on measurements of the bubble expansion rates in our simulations we argue that sub-grid AGN prescriptions resulting in negative feedback in cosmological simulations without a multi-phase treatment of the ISM are good approximations if the volume filling factor of warm phase material is less than 0.1 and the cloud complexes are smaller than ~25 pc. We find that the acceleration of the dense embedded clouds is provided by the ram pressure of the high velocity flow through the porous channels of the warm phase, flow that has fully entrained the shocked hot-phase gas it has swept up, and is additionally mass-loaded by ablated cloud material. This mechanism transfers 10% to 40% of the jet energy to the cold and warm gas, accelerating it within 10 to 100 Myr to velocities that match those observed in a range of high and low redshift radio galaxies hosting powerful radio jets.

astro-ph.CO↗

arXiv2012

Diffuse Lyman Alpha Haloes around Lyman Alpha Emitters at z=3: Do Dark Matter Distributions Determine the Lyman Alpha Spatial Extents?

Using stacks of Ly-a images of 2128 Ly-a emitters (LAEs) and 24 protocluster UV-selected galaxies (LBGs) at z=3.1, we examine the surface brightness profiles of Ly-a haloes around high-z galaxies as a function of environment and UV luminosity. We find that the slopes of the Ly-a radial profiles become flatter as the Mpc-scale LAE surface densities increase, but they are almost independent of the central UV luminosities. The characteristic exponential scale lengths of the Ly-a haloes appear to be proportional to the square of the LAE surface densities (r(Lya) \propto Sigma(LAE)^2). Including the diffuse, extended Ly-a haloes, the rest-frame Ly-a equivalent width of the LAEs in the densest regions approaches EW_0(Lya) ~ 200 A, the maximum value expected for young (< 10^7 yr) galaxies. This suggests that Ly-a photons formed via shock compression by gas outflows or cooling radiation by gravitational gas inflows may partly contribute to illuminate the Ly-a haloes; however, most of their Ly-a luminosity can be explained by photo-ionisation by ionising photons or scattering of Ly-a photons produced in HII regions in and around the central galaxies. Regardless of the source of Ly-a photons, if the Ly-a haloes trace the overall gaseous structure following the dark matter distributions, it is not surprising that the Ly-a spatial extents depend more strongly on the surrounding Mpc-scale environment than on the activities of the central galaxies.

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arXiv2012

Profiles of Lymanα Emission Lines

We present the results of the observations of the Lyα line profiles of 91 emission-line galaxies at z=3.1 with the spectral resolution of λ/δλ(FWHM) = 1700, or 180 km/s. A significant fraction, ~50% of the observed objects show the characteristic double peaks in their Lyαprofile. The red peak is much stronger than the blue one for most of the cases. The red peaks themselves also show weak but significant asymmetry and their widths are correlated with the velocity separation of the red and the blue peaks, which implies that the peaks are not isolated multiple components with different velocities but the parts of the single line which is modified by the absorption and/or scattering by the associated neutral hydrogen gas. The characteristic profile can be naturally explained by the scattering in the expanding shell of neutral hydrogen surrounding the Lyα emitting region while the attenuation by the inter-galactic medium should also be considered. Our results suggest that the star-formation in these Lyα emitters are dominated by the young burst-like events which produce the intrinsic Lyα emission as well as the gas outflow.

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arXiv2012

Panoramic Survey of Lyman α Emitters at z=3.1

We present the results of the extensive narrow-band survey of Lyα emission-line objects at z=3.1 in the 1.38 deg^2 area surrounding the high density region of star-forming galaxies at z=3.09 in the SSA22 field, as well as in the 1.04 deg^2 area of the three separated general blank fields. In total of 2161 Lyα emitters, 1394 in the SSA22 fields and 767 in the general fields, respectively, are detected to the narrow-band AB magnitude limit of 25.73, which corresponds to the line flux of 1.8 x 10^{-17} erg s^{-1} cm^{-2} or luminosity of 1.5 x 10^{42} erg s^{-1} at z=3.1, above the observed equivalent width threshold, 190Å. The average surface number density of the emitters at z=3.1 in the whole general fields above the thresholds is 0.20+-0.01 arcmin^{-2}. The SSA22 high-density region at z=3.09 whose peak local density is 6 times the average is found to be the most prominent outstanding structure in the whole surveyed area and is firmly identified as a robust `protocluster' with the enough large sample. We also compared the overdensity of the 100 arcmin^2 and 700 arcmin^2 areas which contain the protocluster with the expected fluctuation of the dark matter as well as those of the model galaxies in cosmological simulations. We found that the peak height values of the overdensity correspond to be 8-10 times and 3-4 times of the expected standard deviations of the counts of Lyαemitters at z=3.1 in the corresponding volume, respectively. We conclude that the structure at z=3.09 in the SSA22 field is a very significant and rare density peak up to the scale of 60 Mpc.

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arXiv2010

The Subaru Ly-alpha blob survey: A sample of 100 kpc Ly-alpha blobs at z=3

We present results of a survey for giant Ly-alpha nebulae (LABs) at z=3 with Subaru/Suprime-Cam. We obtained Ly-alpha imaging at z=3.09+-0.03 around the SSA22 protocluster and in several blank fields. The total survey area is 2.1 square degrees, corresponding to a comoving volume of 1.6 x 10^6 Mpc^3. Using a uniform detection threshold of 1.4 x 10^{-18} erg s^{-1} cm^{-2} arcsec^{-2} for the Ly-alpha images, we construct a sample of 14 LAB candidates with major-axis diameters larger than 100 kpc, including five previously known blobs and two known quasars. This survey triples the number of known LABs over 100 kpc. The giant LAB sample shows a possible "morphology-density relation": filamentary LABs reside in average density environments as derived from compact Ly-alpha emitters, while circular LABs reside in both average density and overdense environments. Although it is hard to examine the formation mechanisms of LABs only from the Ly-alpha morphologies, more filamentary LABs may relate to cold gas accretion from the surrounding inter-galactic medium (IGM) and more circular LABs may relate to large-scale gas outflows, which are driven by intense starbursts and/or by AGN activities. Our survey highlights the potential usefulness of giant LABs to investigate the interactions between galaxies and the surrounding IGM from the field to overdense environments at high-redshift.

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arXiv2009

Three-dimensional Radiative Properties of Hot Accretion Flows onto the Galactic Centre Black Hole

By solving radiative transfer equations, we examine three-dimensional radiative properties of a magnetohydrodynamic accretion flow model confronting with the observed spectrum of Sgr A*, in the vicinity of supermassive black hole at the Galactic centre. As a result, we find that the core of radio emission is larger than the size of the event horizon shadow and its peak location is shifted from the gravitational centre. We also find that the self-absorbed synchrotron emissions by the superposition of thermal electrons within a few tens of the Schwartzschild radius can account for low-frequency spectra below the critical frequency $ν_{c}\approx 10^{12}$ Hz. Above the critical frequency, the synchrotron self-Compton emission by thermal electrons can account for variable emissions in recent near-infrared observations. In contrast to the previous study by Ohsuga et al. (2005), we found that the X-ray spectra by Bremsstrahlung emission of thermal electrons for the different mass accretion rates can be consistent with both the flaring state and the quiescent state of Sgr A* observed by {\it Chandra}.

astro-ph.GA↗

arXiv2006

Imprint of Gravitational Lensing by Population III Stars in Gamma Ray Burst Light Curves

We propose a novel method to extract the imprint of gravitational lensing by Pop III stars in the light curves of Gamma Ray Bursts (GRBs). Significant portions of GRBs can originate in hypernovae of Pop III stars and be gravitationally lensed by foreground Pop III stars or their remnants. If the lens mass is on the order of $10^2-10^3M_\odot$ and the lens redshift is greater than 10, the time delay between two lensed images of a GRB is $\approx 1$s and the image separation is $\approx 10 μ$as. Although it is difficult to resolve the two lensed images spatially with current facilities, the light curves of two images are superimposed with a delay of $\approx 1$ s. GRB light curves usually exhibit noticeable variability, where each spike is less than 1s. If a GRB is lensed, all spikes are superimposed with the same time delay. Hence, if the autocorrelation of light curve with changing time interval is calculated, it should show the resonance at the time delay of lensed images. Applying this autocorrelation method to GRB light curves which are archived as the {\it BATSE} catalogue, we demonstrate that more than half light curves can show the recognizable resonance, if they are lensed. Furthermore, in 1821 GRBs we actually find one candidate of GRB lensed by a Pop III star, which may be located at redshift 20-200. The present method is quite straightforward and therefore provides an effective tool to search for Pop III stars at redshift greater than 10. Using this method, we may find more candidates of GRBs lensed by Pop III stars in the data by the {\it Swift} satellite.

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arXiv2006

The End of the Reionization Epoch Probed by Ly-alpha Emitters at z=6.5 in the Subaru Deep Field

We report an extensive search for Lyman-alpha emitters (LAEs) at z=6.5 in the Subaru Deep Field. Subsequent spectroscopy with Subaru and Keck identified eight more LAEs, giving a total of 17 spectroscopically confirmed LAEs at z=6.5. Based on this spectroscopic sample of 17, complemented by a photometric sample of 58 LAEs, we have derived a more accurate Lyman-alpha luminosity function of LAEs at z=6.5, which reveals an apparent deficit at the bright end of ~0.75 mag fainter L*, compared with that observed at z=5.7. The difference in the LAE luminosity functions between z=5.7 and 6.5 is significant at the 3-sigma level, which is reduced to 2-sigma when cosmic variance is taken into account. This result may imply that the reionization of the universe has not been completed at z=6.5. We found that the spatial distribution of LAEs at z=6.5 was homogeneous over the field. We discuss the implications of these results for the reionization of the universe.

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arXiv2005

The Reionization History and Early Metal Enrichment inferred from the Gamma-Ray Burst Rate

Based on the gamma-ray burst (GRB) event rate at redshifts of $4 \leq z \leq 12$, which is assessed by the spectral peak energy-to-luminosity relation recently found by Yonetoku et al., we observationally derive the star formation rate (SFR) for Pop III stars in a high redshift universe. As a result, we find that Pop III stars could form continuously at $4 \leq z \leq 12$. Using the derived Pop III SFR, we attempt to estimate the ultraviolet (UV) photon emission rate at $7 \leq z \leq 12$ in which redshift range no observational information has been hitherto obtained on ionizing radiation intensity. We find that the UV emissivity at $7 \leq z \leq 12$ can make a noticeable contribution to the early reionization. The maximal emissivity is higher than the level required to keep ionizing the intergalactic matter at $7 \leq z \leq 12$. However, if the escape fraction of ionizing photons from Pop III objects is smaller than 10%, then the IGM can be neutralized at some redshift, which may lead to the double reionization. As for the enrichment, the ejection of all metals synthesized in Pop III objects is marginally consistent with the IGM metallicity, although the confinement of metals in Pop III objects can reduce the enrichment significantly.

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arXiv2004

The Subaru Deep Field: The Optical Imaging Data

The Subaru Deep Field (SDF) project is a program of Subaru Observatory to carry out a deep galaxy survey over a blank field as large as 34'x27'. The program consists of very deep multi-band optical imaging, near infrared imaging for smaller portions of the field and follow-up optical spectroscopy. Major scientific goals of the project are to construct large samples of Lyman-break galaxies at z~4-5 and Lyman alpha emitters at z~5.7 and 6.6, and to make detailed studies these very high-redshift galaxy populations. In this paper, we describe the optical imaging observations and data reduction, presenting mosaicked images and object catalogs in seven bandpasses.The optical imaging was made through five broad-band filters, B, V, R, i', z', and two narrow-band filters, NB816 (lambda_c=8150A) and NB921 (lambda_c=9196A) with almost 10 hours long integrations for each band. The limiting magnitudes measured at 3-sigma on a 2" aperture are B=28.45, V=27.74, R=27.80, i'=27.43, z'=26.62, NB816=26.63, and NB921=26.54 in the AB system. The object catalog constructed for each of the seven bands contains more than 10^5 objects. The galaxy number counts corrected for detection incompleteness and star count contribution are found to be consistent with previous results in the literature. The mosaicked images and catalogs of all the bands have been made open to the public on Oct. 1, 2004 on the SDF project website at http://soaps.naoj.org/sdf/.

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arXiv2004

A Strong Lyman-alpha Emitter at z=6.33 in the Subaru Deep Field Selected as an i' Dropout

We report on the discovery of a star-forming galaxy at z=6.33 in the Subaru Deep Field. This object is selected as a candidate of an i'-dropout, high-redshift galaxy around z=6 because of its red i'-z' color in our deep optical imaging survey in the Subaru Deep Field. Our follow up optical spectroscopy reveals that this object is a strong Ly-alpha emitter with only very faint ultraviolet continuum. The rest-frame equivalent width of the detected Ly-alpha emission is as much as 130 A. Thus the light detected in our z' image is largely attributed to the Ly-alpha emission, i.e., ~40% of the z'-band flux is the strong Ly-alpha emission, giving a very red i'-z' color. This is consistent with the photometric property of this object because the narrow-band data obtained with the NB921 filter shows a significant depression, z'-NB921 = -0.54 mag. By using the photometric data, we show that some other objects among the 48 i'-dropout high-redshift galaxy candidates found in the Subaru Deep Field also show a significant NB921 depression. We briefly discuss the nature of these NB921-depressed objects.

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arXiv2004

The Structure and Evolution of Early Cosmological HII Regions

We study the formation and evolution of HII regions around the first stars formed at z=10-30. We use a one-dimensional Lagrangian hydrodynamics code which self-consistently incorporates radiative transfer and non-equilibrium primordial gas chemistry. The star-forming region is defined as a spherical gas cloud with a Population III star embedded at the center. We explore a large parameter space of host halo mass, gas density profile, and stellar luminosity. The formation of the HII region is characterized by initial slow expansion of a weak D-type ionization front near the center, followed by rapid propagation of an R-type front throughout the outer gas envelope. We find that the transition between the two front types is indeed a critical condition for the complete ionization of halos of cosmological interest. In small mass (< 10^6 M_sun) halos, the transition takes place within a few 10^5 yr, yielding high escape fractions (>80%) of both ionizing and photodissociating photons. In larger mass (> 10^7 M_sun) halos, the ionization front remains to be of D-type over the lifetime of the massive star, the HII region is confined well inside the virial radius, and the escape fractions are essentially zero. We derive an analytic formula that reproduces well the results of our simulations. We discuss immediate implications of the present results for the star formation history and early reionization of the Universe.

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arXiv2002

A Spectrophotometric Method to Determine the Inclination of Class I Objects

A new method which enables us to estimate the inclination of Class I young stellar objects is proposed. Since Class I objects are not spherically symmetric, it is likely that the observed feature is sensitive to the inclination of the system. Thus, we construct a protostar model by carefully treating two-dimensional (2D) radiative transfer and radiative equilibrium. We show from the present 2D numerical simulations that the emergent luminosity L_SED,which is the frequency integration of spectral energy distribution (SED), depends strongly on the inclination of the system i, whereas the peak flux is insensitive to i. Based on this result, we introduce a novel indicator f_L, which is the ratio of L_SED to the peak flux, as a good measure for the inclination. By using f_L, we can determine the inclination regardless of the other physical parameters. The inclination would be determined by f_L within the accuracy of +- 5 degree, if the opening angle of bipolar outflows is specified by any other procedure. Since this spectrophotometric method is easier than a geometrical method or a full SED fitting method, this method could be a powerful tool to investigate the feature of protostars statistically with observational data which will be provided by future missions, such as SIRTF, ASTRO-F, and ALMA.

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arXiv2001

Reionization of an Inhomogeneous Universe

Full radiative transfer in 3D space has been solved to pursue the reionization history in an inhomogeneous universe. It has been shown that the reionization of an inhomogeneous universe is not a prompt event, but a fairly slow process. Also, QSO absorption line systems are simulated with using the results of radiative transfer calculations. Ly$α$ continuum depression implies that the metagalactic UV intensity decreases rapidly with $z$ at $z>5$, and the reionization must have taken place between $z=6$ and 10. Finally, it is stressed that H$α$ forest is a more powerful tool to probe the reionization history and the density fluctuations in the universe at $z>5$.

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arXiv2001

Cosmic Reionization and Galaxy Formation

Using 3D radiative transfer calculations on the reionization of an inhomogeneous universe, QSO absorption line systems are simulated and they are compared with observations of Ly$α$ continuum depression at high redshifts. By this comparison, it is found that the metagalactic UV intensity decreases rapidly with $z$ at $z>4$ as $I_{21}=0.5\exp [3(4-z)]$, and the reionization must have taken place between $z=6$ and 10. Based on this time-dependence of UV background intensity, we explore the collapse of pregalactic clouds in the UV background, and find that the self-shielding is prominent above a mass scale as $ M_{BIF}=3.0\times 10^{11} M_\odot [(1+z_c)/5]^{-4.2}(I_{21}/0.5)^{0.6}. $ This mass scale predicts the bifurcation of galactic morphology, and by confrontation with observations it turns out that the bifurcation mass successfully discriminates between elliptical and spiral galaxies.

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arXiv2001

Theoretical Models of Multi-waveband QSO Luminosity Functions

Cosmological evolution of the QSO luminosity functions (LFs) at NIR/optical/X-ray bands for 1.3 < z < 3.5 is investigated based on the realistic QSO spectra. The accretion-disk theory predicts that although QSO luminosities only depend on mass-accretion rate, \Mdot, QSO spectra have a dependence on black-hole mass, M_{BH}, as well. The smaller M_{BH} is and/or the larger \Mdot is, the harder becomes the QSO NIR/optical/UV spectrum. We model disk spectra which can reproduce these features and calculated LFs for redshift z ~ 3 with the assumption of new-born QSOs being shining at the Eddington luminosity. The main results are: (i) the observed LFs at optical and X-rays can be simultaneously reproduced. (ii) LFs at optical and X-ray bands are not sensitive to M_{BH}, while LFs at NIR bands are; about one order of magnitude difference is expected in volume number densities at L_{I, J} ~ 10^{46} erg s^{-1} between the case that all QSOs would have the same spectral shape as that of M_{BH} = 10^{9} M_sun and the case with M_{BH} = 10^{11} M_sun. (iii) The resultant LFs at NIR are dominated by 10^{7} M_sun black-holes at L_{I, J} ~ 10^{44} erg s^{-1}, and by 10^{11} M_sun black-holes at L_{I, J} \~ 10^{46} erg s^{-1}. Future infrared observations from space(e.g.NGST) will probe cosmological evolution of black hole masses. For redshift z < 3, on the other hand, the observed optical/X-ray LFs can be fitted, if the initial QSO luminosity L_0 is below the Eddington luminosity. Interestingly, the best fitting values of l = L_0/L_{Edd} are different in B- and X-ray bands; l_B ~ 2.5 l_X. The reason for this discrepancy is briefly discussed.

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