arXiv subjects

Yasuhiro Hashimoto

Publications and source records attributed to Yasuhiro Hashimoto.

At least 19 recordsLinked to original sources

arXiv2026

Uncovering Non-Normality in Information Flow: Network Structure and Dynamics of Social Media Cascades

Information cascades on social media are conventionally conceptualized as directed, feedforward branching processes. However, real-world diffusion pathways frequently deviate from pure hierarchical trees due to localized clustering, reciprocal commentary, and multi-wave temporal surges. In this work, we quantify the directional asymmetry and hierarchical structure of empirical information cascades on X (formerly Twitter) using spectral non-normality via Henrici's departure from normality. Analyzing approximately 58,000 cascade networks across diverse topics (including politics, entertainment, natural disasters, etc.), we investigate (1) how non-normality relates to temporal dynamics such as endogenous-like versus exogenous-like patterns and burstiness, (2) whether non-normality is correlated with the peak concentration or overall size of a cascade, (3) whether the overall non-normality of a cascade's network structure can be predicted from its early stages. We find that non-normality strongly aligns with peak concentration (peak/N) rather than overall cascade size, characterizing cascades governed by rapid, asymmetric forwarding. Furthermore, while early-stage structural forecasting (<= 30% of nodes observed) exhibits expected baseline uncertainty (51%-72% accuracy at a +/- 20% error tolerance), predictability consolidates rapidly during intermediate growth, exceeding 80% across all dynamic clusters once 50%-60% of the network is observed. By identifying the topological and dynamic correlates of cascade structures, this study advances our understanding of information flow and establishes a quantifiable benchmark for forecasting directional diffusion architectures.

cs.SI↗

arXiv2026

Attentional DoS: Repeat Reposting, Collective Attention, and Information Diffusion on X

Collective attention is a finite shared resource, and social media posts compete for limited opportunities to be seen. On X, users can undo a repost and repost it again. By repeating this cycle, a user can put the same post back into followers' timelines any number of times without making new content. We call this procedure repeat reposting, and we read it as placing repeated demand on this shared resource (Attentional DoS). We formalize this idea and explore repeat reposting in a large-scale dataset of cascades with at least 1,000 reactions, originating from posts classified as Japanese on X, covering April 2025 to March 2026. Repeat reposts are rare, appearing in a small share of all (user, post) pairs. Even so, close to a million posts have at least one repeater, and most repeats come from a small group of habitual accounts. The central result is that subsequent audience growth is associated less with the number of repeats than with the estimated reach of the repeating accounts. Through the lens of seriality, how habitually the same groups of accounts repeat reposts across many posts, a distinct distributed form emerges: several serial amplifiers converge on the same post (Attentional DDoS). Its synchrony, how closely their actions are timed together, forms a continuum from bursts on the scale of minutes to a daily clock. A check of the content of 99.7% of amplified posts shows that most ADDoS repeat events are directed at Chinese-script content, which our vocabulary matching and sample inspection indicate is predominantly commercial spam. Repeat reposting thus lets a post re-enter the competition for visibility. The observed pattern is better characterized as repeated temporal coverage of an existing audience and the self-reinforcement of posts that are already growing, rather than as evidence that repeat reposting takes reach from other content.

cs.SI↗

arXiv2026

Crude, Commercial, and Self-Referential: Chinese-Language Coordinated Activity in Japanese-Language X

Malicious coordination has long been regarded as a principal source of information ecosystem pollution. Here, we focus on crude, text-repetition-based coordination. As the demand for mitigating its dissemination has grown, scholars have studied such coordination, focusing especially on bot detection. Few studies, however, have characterized malicious coordination per se or examined how it elicits reactions from general users. Leveraging a dataset of 734,173 Chinese-language coordinated accounts and around 495 million coordinated posts published between May 2024 and March 2026, this study analyzes the characteristics of coordinated behavior and how general users react to coordinated posts. We report three findings: (1) most coordinated accounts are crude and retain the classic marks of automation, and the same criterion applied to Japanese-language accounts over the same month yields a share six times lower; (2) their content is overwhelmingly non-political; (3) regarding their reach, most reactions within large observable cascades originate from coordinated accounts themselves, while posts classified as potentially harmful or illegal material receive a comparatively high proportion of reactions from outside the Chinese-dominant population. We provide a longitudinal quantitative map of crude Chinese-language coordination appearing in X's Japanese-classified stream.

cs.SI↗

arXiv2024

Revisiting Disparity from Dual-Pixel Images: Physics-Informed Lightweight Depth Estimation

In this study, we propose a high-performance disparity (depth) estimation method using dual-pixel (DP) images with few parameters. Conventional end-to-end deep-learning methods have many parameters but do not fully exploit disparity constraints, which limits their performance. Therefore, we propose a lightweight disparity estimation method based on a completion-based network that explicitly constrains disparity and learns the physical and systemic disparity properties of DP. By modeling the DP-specific disparity error parametrically and using it for sampling during training, the network acquires the unique properties of DP and enhances robustness. This learning also allows us to use a common RGB-D dataset for training without a DP dataset, which is labor-intensive to acquire. Furthermore, we propose a non-learning-based refinement framework that efficiently handles inherent disparity expansion errors by appropriately refining the confidence map of the network output. As a result, the proposed method achieved state-of-the-art results while reducing the overall system size to 1/5 of that of the conventional method, even without using the DP dataset for training, thereby demonstrating its effectiveness. The code and dataset are available on our project site.

cs.CV↗

arXiv2022

MUSUBI (MegaCam Ultra-deep Survey: $u^\ast$-Band Imaging)-Data for the COSMOS and SXDS Fields

The Subaru Hyper Suprime-Cam (HSC) Strategic Survey is the latest-generation multi-band optical imaging survey for galaxy evolution and structure formation. The "Ultra-Deep" component of the HSC survey provides $grizy$ broad-band images over $\sim3.4$ deg$^2$ to detection limits of $\sim26$-28 AB, along with narrow-band images, in the COSMOS and the SXDS fields. These images provide an unprecedented combination of depths and area coverage, for the studies galaxies up to $z\sim7$. However, the lack of coverage at $<4000$ Ang implies incomplete sampling of the rest-frame UV at $z\lesssim 3$, which is critically needed for understanding the buildup of stellar mass in the later cosmic time. We conducted a multi-year CFHT $u^\ast$-band imaging campaign in the two HSC Ultra-Deep fields with CFHT MegaCam. By including shallower archival data, we reach 5-$σ$ depths of $u^\ast=28.1$ and 28.4 (AB) at the centers of the COSMOS and SXDS fields, respectively, and $u^\ast=27.7$ and 27.8 in the central 1 deg$^2$ fields. The image quality is $\gtrsim0.90$ arcsec, fairly good for the $u^\ast$ band. Both the photometric and astrometric quality of our data are excellent. We show that the combination of our $u^\ast$-band and HSC data can lead to high-quality photometric redshifts at $z=0$-3, and robust measurements of rest-frame UV on galaxies at $0.4<z<0.6$ for distinguishing green-valley galaxies from star-forming and quiescent galaxies. We publicly release our reduced $u^\ast$-band images and reference catalogs that can be used readily for scientific studies.

astro-ph.GA↗

arXiv2021

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) XII. Extended [C II] Structure (Merger or Outflow) in a z = 6.72 Red Quasar

We present ALMA [C II] 158 $μ$m line and far-infrared (FIR) continuum emission observations toward HSC J120505.09$-$000027.9 (J1205$-$0000) at $z = 6.72$ with the beam size of $\sim 0''.8 \times 0''.5$ (or 4.1 kpc $\times$ 2.6 kpc), the most distant red quasar known to date. Red quasars are modestly reddened by dust, and are thought to be in rapid transition from an obscured starburst to an unobscured normal quasar, driven by powerful active galactic nucleus (AGN) feedback which blows out a cocoon of interstellar medium (ISM). The FIR continuum of J1205$-$0000 is bright, with an estimated luminosity of $L_{\rm FIR} \sim 3 \times 10^{12}~L_\odot$. The [C II] line emission is extended on scales of $r \sim 5$ kpc, greater than the FIR continuum. The line profiles at the extended regions are complex and broad (FWHM $\sim 630-780$ km s$^{-1}$). Although it is not practical to identify the nature of this extended structure, possible explanations include (i) companion/merging galaxies and (ii) massive AGN-driven outflows. For the case of (i), the companions are modestly star-forming ($\sim 10~M_\odot$ yr$^{-1}$), but are not detected by our Subaru optical observations ($y_{\rm AB,5σ} = 24.4$ mag). For the case of (ii), our lower-limit to the cold neutral outflow rate is $\sim 100~M_\odot$ yr$^{-1}$. The outflow kinetic energy and momentum are both much smaller than what predicted in energy-conserving wind models, suggesting that the AGN feedback in this quasar is not capable of completely suppressing its star formation.

astro-ph.GA↗

arXiv2019

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) VIII. A less biased view of the early co-evolution of black holes and host galaxies

We present ALMA [CII] line and far-infrared (FIR) continuum observations of three $z > 6$ low-luminosity quasars ($M_{\rm 1450} > -25$) discovered by our Subaru Hyper Suprime-Cam (HSC) survey. The [CII] line was detected in all three targets with luminosities of $(2.4 - 9.5) \times 10^8~L_\odot$, about one order of magnitude smaller than optically luminous ($M_{\rm 1450} \lesssim -25$) quasars. The FIR continuum luminosities range from $< 9 \times 10^{10}~L_\odot$ (3$σ$ limit) to $\sim 2 \times 10^{12}~L_\odot$, indicating a wide range in star formation rates in these galaxies. Most of the HSC quasars studied thus far show [CII]/FIR luminosity ratios similar to local star-forming galaxies. Using the [CII]-based dynamical mass ($M_{\rm dyn}$) as a surrogate for bulge stellar mass ($M_{\rm bulge}$), we find that a significant fraction of low-luminosity quasars are located on or even below the local $M_{\rm BH} - M_{\rm bulge}$ relation, particularly at the massive end of the galaxy mass distribution. In contrast, previous studies of optically luminous quasars have found that black holes are overmassive relative to the local relation. Given the low luminosities of our targets, we are exploring the nature of the early co-evolution of supermassive black holes and their hosts in a less biased way. Almost all of the quasars presented in this work are growing their black hole mass at much higher pace at $z \sim 6$ than the parallel growth model, in which supermassive black holes and their hosts grow simultaneously to match the local $M_{\rm BH} - M_{\rm bulge}$ relation at all redshifts. As the low-luminosity quasars appear to realize the local co-evolutionary relation even at $z \sim 6$, they should have experienced vigorous starbursts prior to the currently observed quasar phase to catch up with the relation.

astro-ph.GA↗

arXiv2019

Open-ended Evolution and a Mechanism of Novelties in Web Services

Analogous to living ecosystems in nature, web services form an artificial ecosystem consisting of many tags and their associated media, such as photographs, movies, and web pages created by human users. Concerning biological ecosystems, we regard tag as a species and human as a hidden environmental resource. We subsequently analyze the evolution of the web services, in particular social tagging systems, with respect to the self-organization of new tags. The evolution of new combinations of tags is analyzed as the open-ended evolution (OEE) index. The tag meaning is computed by the types of associated tags; tags that vary their meanings temporally exist. We argue that such tags are the examples of OEE.

cs.SI↗

arXiv2018

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) III. Star formation properties of the host galaxies at $z \gtrsim 6$ studied with ALMA

We present our ALMA Cycle 4 measurements of the [CII] emission line and the underlying far-infrared (FIR) continuum emission from four optically low-luminosity ($M_{\rm 1450} > -25$) quasars at $z \gtrsim 6$ discovered by the Subaru Hyper Suprime Cam (HSC) survey. The [CII] line and FIR continuum luminosities lie in the ranges $L_{\rm [CII]} = (3.8-10.2) \times 10^8~L_\odot$ and $L_{\rm FIR} = (1.2-2.0) \times 10^{11}~L_\odot$, which are at least one order of magnitude smaller than those of optically-luminous quasars at $z \gtrsim 6$. We estimate the star formation rates (SFR) of our targets as $\simeq 23-40~M_\odot ~{\rm yr}^{-1}$. Their line and continuum-emitting regions are marginally resolved, and found to be comparable in size to those of optically luminous quasars, indicating that their SFR or likely gas mass surface densities (key controlling parameter of mass accretion) are accordingly different. The $L_{\rm [CII]}/L_{\rm FIR}$ ratios of the hosts, $\simeq (2.2-8.7) \times 10^{-3}$, are fully consistent with local star-forming galaxies. Using the [CII] dynamics, we derived their dynamical masses within a radius of 1.5-2.5 kpc as $\simeq (1.4-8.2) \times 10^{10}~M_\odot$. By interpreting these masses as stellar ones, we suggest that these faint quasar hosts are on or even below the star-forming main sequence at $z \sim 6$, i.e., they appear to be transforming into quiescent galaxies. This is in contrast to the optically luminous quasars at those redshifts, which show starburst-like properties. Finally, we find that the ratios of black hole mass to host galaxy dynamical mass of the most of low-luminosity quasars including the HSC ones are consistent with the local value. The mass ratios of the HSC quasars can be reproduced by a semi-analytical model that assumes merger-induced black hole-host galaxy evolution.

astro-ph.GA↗

arXiv2017

Anomalous popularity growth in social tagging ecosystems

In social tagging systems, the diversity of tag vocabulary and the popularity of such tags continue to increase as they are exposed to selection pressure derived from our cognitive nature and cultural preferences. This is analogous to living ecosystems, where mutation and selection play a dominant role. Such population dynamism, which yields a scaling law, is mathematically modeled by a simple stochastic process---the Yule--Simon process, which describes how new words are introduced to the system and then grow. However, in actual web services, we have observed that a large fluctuation emerges in the popularity growth of individual tags that cannot be explained by the ordinary selection mechanism. We introduce a scaling factor to quantify the degree of the deviation in the popularity growth from the mean-field solution of the Yule--Simon process, and we discuss possible triggers of such anomalous popularity behavior.

physics.soc-ph↗

arXiv2017

BAT AGN Spectroscopic Survey I: Spectral Measurements, Derived Quantities, and AGN Demographics

We present the first catalog and data release of the Swift-BAT AGN Spectroscopic Survey (BASS). We analyze optical spectra of the majority of AGN (77%, 641/836) detected based on their 14-195 keV emission in the 70-month Swift BAT all-sky catalog. This includes redshift determination, absorption and emission line measurements, and black hole mass and accretion rate estimates for the majority of obscured and un-obscured AGN (74%, 473/641) with 340 measured for the first time. With ~90% of sources at z<0.2, the survey represents a significant census of hard-X-ray selected AGN in the local universe. In this first catalog paper, we describe the spectroscopic observations and datasets, and our initial spectral analysis. The FWHM of the emission lines show broad agreement with the X-ray obscuration (~94%), such that Sy 1-1.8 have NH<10^21.9 cm^-2, and Seyfert 2, have NH>10^21.9 cm^-2. Seyfert 1.9 show a range of column densities. Compared to narrow line AGN in the SDSS, the X-ray selected AGN have a larger fraction of dusty host galaxies suggesting these types of AGN are missed in optical surveys. Using the most sensitive [OIII]/Hbeta and [NII]/Halpha emission line diagnostic, about half of the sources are classified as Seyferts, ~15% reside in dusty galaxies that lack an Hbeta detection, but for which the line upper limits imply either a Seyfert or LINER, ~15% are in galaxies with weak or no emission lines despite high quality spectra, and a few percent each are LINERS, composite galaxies, HII regions, or in known beamed AGN.

astro-ph.HE↗

arXiv2017

How the nature of web services drives vocabulary creation in social tagging

A social tagging system allows users to add arbitrary strings, called "tags", on a shared resource to organize and manage information. The Yule--Simon process, which has shown the ability to capture the population dynamics of social tagging behavior, does not handle the mechanism of new vocabulary creation because it assumes that new vocabulary creation is a Poisson-like random process. In this research, we focus on the mechanism of vocabulary creation from the microscopic perspective and discuss whether it also follows the random process assumed in the Yule--Simon process. To capture the microscopic mechanism of vocabulary creation, we focus on the relationship between the number of tags used in the same entry and the local vocabulary creation rate. We find that the relationship is not the result of a simple random process, and differs between services. Furthermore, these differences depend on whether the user's tagging attitudes are private or open. These results provide the potential for a new index to identify the service's intrinsic nature.

cs.SI↗

arXiv2016

Role of AGNs in the Luminous Infrared Galaxy phase since z ~ 3

In order to understand the interactions between active galactic nuclei (AGN) and star formation during the evolution of galaxies, we investigate 142 galaxies detected in both X-ray and 70μm observations in the COSMOS (Cosmic Evolution Survey) field. All of our data are obtained from the archive, X-ray point source catalogs from Chandra and XMM-Newton observations; far-infrared 70μm point source catalog from Spitzer-MIPS observations. Although the IRAC [3.6μm]-[4.5μm] vs. [5.8μm]-[8.0μm] colours of our sample indicate that only ~63% of our sources would be classified as AGN, the ratio of the rest-frame 2-10 keV luminosity to the total infrared luminosity (8-1000μm) shows that all of the sample has comparatively higher X-ray luminosity than that expected from pure star-forming galaxies, suggesting the presence of an AGN in all of our sources. From the analysis of the X-ray hardness ratio, we find that sources with both 70μm and X-ray detection tend to have a higher hardness ratio relative to the whole X-ray selected source population, suggesting the presence of more X-ray absorption in the 70μm detected sources. In addition, we find that the observed far-infrared colours of 70μm detected sources with and without X-ray emission are similar, suggesting the far-infrared emission could be mainly powered by star formation.

astro-ph.CO↗

arXiv2015

Growth fluctuation in preferential attachment dynamics

In the Yule-Simon process, selection of words follows the preferential attachment mechanism, resulting in the power-law growth in the cumulative number of individual word occurrences. This is derived using mean-field approximation, assuming a continuum limit of both the time and number of word occurrences. However, time and word occurrences are inherently discrete in the process, and it is natural to assume that the cumulative number of word occurrences has a certain fluctuation around the average behavior predicted by the mean-field approximation. We derive the exact and approximate forms of the probability distribution of such fluctuation analytically and confirm that those probability distributions are well supported by the numerical experiments.

cond-mat.stat-mech↗

arXiv2015

BAT AGN Spectroscopic Survey II: X-ray Emission and High Ionization Optical Emission Lines

We investigate the relationship between X-ray and optical line emission in 340 nearby AGN selected above 10 keV using Swift BAT. We find a weak correlation between the extinction corrected [O III] and hard X-ray luminosity (14-195 keV) with a [OIII] large scatter (R_Pear = 0.64, sigma = 0.62 dex) and a similarly large scatter with the intrinsic 2-10 keV to [O III] luminosities (RPear=0.63, sigma = 0.63 dex). Correlations of the hard X-ray fluxes with the fluxes of high-ionization narrow lines ([O III], He II, [Ne III] and [Ne V]) are not significantly better than with the low ionization lines (Halpha, [SII]). Factors like obscuration or physical slit size are not found to be a significant part of the large scatter. In contrast, the optical emission lines show much better correlations with each other (sigma = 0.3 dex) than with the X-ray flux. The inherent large scatter questions the common usage of narrow emission lines as AGN bolometric luminosity indicators and suggests that other issues such as geometrical differences in the scattering of the ionized gas or long term AGN variability are important.

astro-ph.GA↗

arXiv2014

Self-organization on social media: endo-exo bursts and baseline fluctuations

A salient dynamic property of social media is bursting behavior. In this paper, we study bursting behavior in terms of the temporal relation between a preceding baseline fluctuation and the successive burst response using a frequency time series of 3,000 keywords on Twitter. We found that there is a fluctuation threshold up to which the burst size increases as the fluctuation increases and that above the threshold, there appears a variety of burst sizes. We call this threshold the critical threshold. Investigating this threshold in relation to endogenous bursts and exogenous bursts based on peak ratio and burst size reveals that the bursts below this threshold are endogenously caused and above this threshold, exogenous bursts emerge. Analysis of the 3,000 keywords shows that all the nouns have both endogenous and exogenous origins of bursts and that each keyword has a critical threshold in the baseline fluctuation value to distinguish between the two. Having a threshold for an input value for activating the system implies that Twitter is an excitable medium. These findings are useful for characterizing how excitable a keyword is on Twitter and could be used, for example, to predict the response to particular information on social media.

cs.SI↗

arXiv2014

Star Formation at $4 < z < 6$ From the Spitzer Large Area Survey with Hyper-Suprime-Cam (SPLASH)

Using the first 50% of data collected for the Spitzer Large Area Survey with Hyper-Suprime-Cam (SPLASH) observations on the 1.8 deg$^2$ Cosmological Evolution Survey (COSMOS) we estimate the masses and star formation rates of 3398 $M_*>10^{10}M_\odot $ star-forming galaxies at $4 < z < 6$ with a substantial population up to $M_* \gtrsim 10^{11.5} M_\odot$. We find that the strong correlation between stellar mass and star formation rate seen at lower redshift (the "main sequence" of star-forming galaxies) extends to $z\sim6$. The observed relation and scatter is consistent with a continued increase in star formation rate at fixed mass in line with extrapolations from lower-redshift observations. It is difficult to explain this continued correlation, especially for the most massive systems, unless the most massive galaxies are forming stars near their Eddington-limited rate from their first collapse. Furthermore, we find no evidence for moderate quenching at higher masses, indicating quenching either has not occurred prior to $z \sim 6$ or else occurs rapidly, so that few galaxies are visible in transition between star-forming and quenched.

astro-ph.GA↗

arXiv2014

Multiwavelength investigations of co-evolution of bright custer galaxies

We report a systematic multi-wavelength investigation of environments of the brightest cluster galaxies (BCGs), using the X-ray data from the Chandra archive, and optical images taken with 34'x 27' field-of-view Subaru Suprime-Cam. Our goal is to help understand the relationship between the BCGs and their host clusters, and between the BCGs and other galaxies, to eventually address a question of the formation and co-evolution of BCGs and the clusters. Our results include: 1) Morphological variety of BCGs, or the second or the third brightest galaxy (BCG2, BCG3), is comparable to that of other bright red sequence galaxies, suggesting that we have a continuous variation of morphology between BCGs, BCG2, and BCG3, rather than a sharp separation between the BCG and the rest of the bright galaxies. 2) The offset of the BCG position relative to the cluster centre is correlated to the degree of concentration of cluster X-ray morphology (Spearman rho = -0.79), consistent with an interpretation that BCGs tend to be off-centered inside dynamically unsettled clusters. 3) Morphologically disturbed clusters tend to harbour the brighter BCGs, implying that the "early collapse" may not be the only major mechanism to control the BCG formation and evolution.

astro-ph.CO↗