arXiv subjects

Tetsuya Hashimoto

Publications and source records attributed to Tetsuya Hashimoto.

At least 19 recordsLinked to original sources

arXiv2026

Statistical inference of fast radio burst environments using galaxy number density

Fast radio bursts (FRBs) are bright, millisecond-duration radio transients of unknown origin. They are categorized as repeaters and non-repeaters, possibly indicating distinct progenitor types. However, validating this distinction is difficult because of the limited number of localized FRBs. Large-scale galactic environments can provide insight into the nature of the host galaxies of FRBs and their progenitors. High-number-density regions are typically associated with old galaxies, whereas low-number-density regions are linked to young star-forming or less massive quiescent galaxies. In this study, we use galaxy number density to statistically assess the environments of 19 repeaters and 253 non-repeaters from CHIME Catalog 1, using galaxies from the WISE x PS1 catalog. A Kolmogorov-Smirnov (KS) test showed no significant difference between the two populations ($p_{\rm KS} = 0.673$). This result indicates that the statistical significance of the difference could depend on small-number statistics, highlighting the necessity of future FRB samples. Intriguingly, a comparison of FRBs with random galaxy fields suggests that FRBs may preferentially occur in underdense galactic environments, with a median $p$-value ($p_{\rm KS}$) of $2.84 \times 10^{-2}$ compared to random galaxy apertures.

astro-ph.GA↗

arXiv2026

Tracing Obscured AGN Contribution and Number Fraction Across 0 < z < 6 with JWST

Active galactic nuclei (AGN) are key drivers of galaxy evolution, yet many remain undetected in ultraviolet and optical surveys due to heavy dust obscuration. In these systems, absorbed emission is re-radiated at infrared (IR) wavelengths, making IR observations essential for identifying the full AGN population. Tracking the AGN IR contribution and number fraction provides insight into both the dominance and prevalence of AGN activity across cosmic time. Using the JWST Systematic Mid-infrared Instrument Legacy Extragalactic Survey (SMILES) and the JWST Advanced Deep Extragalactic Survey (JADES), we leverage continuous optical-to-mid-IR coverage (0.4-25 $\mu$m) in the GOODS-S field to identify obscured AGN via multi-wavelength SED fitting with CIGALE. Our sample includes 278 AGN across 0 < z < 6, representing a seven-fold increase in sample size relative to previous studies utilizing the Cosmic Evolution Early Release Science (CEERS) survey due to the larger 15-pointing SMILES MIRI footprint. We find that both AGN IR contribution and number fraction increase with redshift, with AGN fractions rising from $\lesssim$ 5% at z < 2 to ~ 30% at higher redshifts, while the median AGN contribution increases by up to ~ 0.15. In contrast, as a function of total IR luminosity over $\log(L/L_{\odot}) \approx$ 8 - 12, the AGN contribution and AGN number fraction remain fundamentally static. These trends suggest that the prevalence of obscured AGN activity is dependent on redshift while showing little to no dependence on total infrared luminosity. Our results highlight JWST's ability to uncover previously hidden AGN populations and provide new constraints on AGN-galaxy co-evolution.

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arXiv2026

A search for Fast Radio Bursts from globular clusters in M49 with FAST

The origins of fast radio bursts (FRBs) remain uncertain, although magnetars are a leading progenitor candidate. Because magnetars are thought to form primarily through core-collapse supernovae in young stellar populations, the discovery of FRB 20200120E in a globular cluster (GC) in the nearby galaxy M81 was unexpected given the ancient stellar populations of GCs. Expanding the sample of FRBs localised to nearby galaxies is therefore essential for testing FRB formation channels in old stellar environments. M49 (NGC 4472) is a nearby (~17 Mpc), radio-quiet giant elliptical galaxy in the Virgo cluster hosting about 7000 GCs, making it an ideal target for GC FRB searches. We conducted a 9-hour SnapShotCal observation of M49 using the Five-hundred-meter Aperture Spherical Telescope (FAST) 19-beam receiver, covering approximately 4230 GCs (2.1 hr per GC), and performed a comprehensive single-pulse search over a dispersion measure range of 0-5000 pc cm^-3. No unambiguous astrophysical FRBs were detected. The most significant trigger reached a post-processed signal-to-noise ratio of 8.6 sigma at a dispersion measure of 412.2 pc cm^-3, but is statistically consistent with thermal noise after accounting for the false-alarm rate. We derive a beam-averaged peak flux-density sensitivity of about 16.5 mJy (corresponding to a fluence limit of about 16.5 mJy ms for a 1 ms burst) and place an upper limit on the FRB occurrence rate of 4.7 x 10^-4 FRB GC^-1 hr^-1. Our non-detection constrains only bright bursts above this fluence threshold during the observing window.

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arXiv2026

Constraints on the Physical Association between ICECAT1 Neutrinos and Fast Radio Bursts Using the Second CHIME/FRB Catalogue

We present a search for neutrino counterparts to fast radio bursts (FRBs) using temporal and spatial cross-matching between the Second CHIME/FRB catalogue and the IceCube high-energy alert-track catalogue ICECAT1. Because current FRB--neutrino models do not provide a unique consensus on emission ordering, our primary significance test adopts a two-sided, order-agnostic temporal hypothesis. The analysis accounts for declination-dependent CHIME/FRB exposure and the look-elsewhere effect across multiple trials. No statistically significant FRB--neutrino association is found. The most significant pair is FRB\,20190630C--IC\,190629A, with a post-trial probability of $p=0.076$ ($1.43\sigma$), consistent with a chance coincidence. Within our statistical framework, a detectable physical association would require a time offset shorter than $\sim256$~s at $3\sigma$ or $\sim63$~ms at $5\sigma$. Using a population-level stacking analysis, we derive 90\% upper limits on the neutrino-to-radio luminosity ratio of FRBs, $\xi \lesssim 10^{8}-10^{11}$ for neutrino power-law spectral indices $\gamma=1.0-3.0$. These limits improve upon previous constraints by approximately two orders of magnitude and represent the most stringent bounds from FRB--neutrino coincidence searches to date. Although the current limits remain above the predictions of most magnetar-based models, they begin to constrain scenarios involving exceptionally efficient hadronic energy dissipation.

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arXiv2026

Single-Pulse Correlations in PSR B0329+54: Implications for Radio Emission Zones

Individual radio pulses from a pulsar are directly linked to the underlying emission processes and the associated magnetic field geometry within its magnetosphere. Thus, single-pulse studies across frequencies can provide crucial insights into the physics of radio emission. Multiple studies have investigated single-pulse correlations in PSR B0329+54 with widely separated discrete frequencies, reporting the broadband nature of pulsar emission. However, understanding the frequency evolution of these correlations has been limited by poor frequency sampling, and the physical origin of these correlations remains unexplored. We present a detailed study of single-pulse correlations in PSR B0329+54 at low radio frequencies using the upgraded Giant Meterwave Radio Telescope (uGMRT), with well-sampled time series spanning 300-1460 MHz. We derived an inverted flux spectrum for this pulsar, with a turnover near 470 MHz. We used flux-calibrated and scintillation-corrected single pulses to study correlations across frequencies. Our results show that maximum correlations consistently occur near the longitude of the central component, with correlation strength exceeding 69\% for all frequency combinations, while outer components exhibit correlations above 46\%. These findings indicate very strong inter-frequency correlations, with no anticorrelations detected. No cross-component correlations were observed; only corresponding components correlate across frequencies. The longitudes of maximum correlation do not coincide with the intensity peaks of the average profile. We also examine how correlations vary with frequency at selected fiducial longitudes. The observations reported in this work favor curvature radiation from relativistic charge bunches in the pulsar plasma; however, reproducing the correlation curves along with spectra remains an open challenge.

astro-ph.HE↗

arXiv2026

Unveiling Hidden Clustering: An Unsupervised Machine Learning Study of Repeating FRB 20220912A

Fast Radio Bursts (FRBs) are millisecond-duration radio transients of extragalactic origin. Classifying repeating FRBs is essential for understanding their emission mechanisms, but remains challenging due to their short durations, high variability, and increasing data volume. Traditional methods often rely on subjective criteria and struggle with high-dimensional data. In this study, we apply an unsupervised machine learning framework that combines Uniform Manifold Approximation and Projection (UMAP) and Hierarchical Density-Based Spatial Clustering of Applications with Noise (HDBSCAN) to eight observed parameters from FRB 20220912A. Our analysis reveals three distinct clusters of bursts with varying spectral and fluence properties. Comparisons with clustering studies on other repeaters show that some of our clusters share similar features with sources such as FRB 20201124A and FRB 121102, suggesting possible common emission mechanisms. We also provide qualitative interpretations for each cluster, highlighting the spectral diversity within a single source. Notably, one cluster shows broadband emission and high fluence, which are typically seen in non-repeating FRBs. This raises the possibility that some non-repeaters may be misclassified repeaters due to observational limitations. Our results demonstrate the utility of machine learning in uncovering intrinsic diversity in FRB emission and provide a foundation for future classification studies.

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arXiv2025

Mid-IR luminosity functions: inferred dusty cosmic star-formation and black hole accretion histories from the JWST SMILES

Mid-infrared (mid-IR) observations are crucial for understanding galaxy evolution, tracing star formation, and active galactic nuclei (AGN) activity via dust emission. This work presents mid-IR galaxy luminosity functions (LFs) at $0.5 < z < 6$, derived from the JWST Systematic Mid-infrared Instrument Legacy Extragalactic Survey (SMILES) program. We combine 8 MIRI bands ($5-25$ $\mu$m) of SMILES and archival 23-band HST+JWST NIRCam photometry to construct an extensive catalog containing 2,813 galaxies with sub-$\mu$Jy level completeness in the mid-IR. We obtain monochromatic (in 5.6, 7.7, 10, 12.8, 15, 18, 21, and 25.5 $\mu$m), $L_{\rm IR}$, and AGN LFs, with a limiting luminosity down to $10^{9.5}$ $L_\odot$ at $z=0.5-1.0$, $\sim10^{10.5}$ $L_\odot$ at $z=2.0-4.0$, and to $\sim10^{11}$ $L_\odot$ at $z=4.0-6.0$. With the unprecedented sensitivity and resolution of JWST, here we better constrain the faint-end slope and its evolution of the mid-IR LFs, quantifying the dusty cosmic star formation and black hole accretion histories out to $z \sim 5$. These results provide essential insights to refine our understanding of the obscured star formation and galaxy-AGN co-evolution over cosmic time.

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arXiv2025

JWST MIRI color classification of mid-infrared selected galaxies: MIRI color classification toward cosmic noon

We investigated the James Webb Space Telescope photometric color classification of mid-infrared (MIR) selected galaxies at high redshifts, toward cosmic noon. The aim of the present work is to obtain a z-dependent mid-infrared (MIR) photometric galaxy classification tool based on broad spectral emission and absorption lines using the JWST Mid-Infrared Instrument (MIRI) and its broadband filters. We used the largest Spitzer MIR spectral database to obtain synthetic photometry in the JWST/MIRI filters. We formed MIRI filter combinations to trace the strong polycyclic aromatic hydrocarbon (PAH) emission features and the 9.7 micron silicate feature in seven redshift windows from z = 0.25-2.10. Results. We present z-dependent MIRI color-color plots that separate active galactic nuclei (AGN), star-forming galaxies (SFGs), and silicate absorption-dominated galaxies up to z$\sim$2. We applied the photometric MIR colors to the largest ($\sim$34 arcmin square) MIRI survey called the Systematic Mid-infrared Instrument Legacy Extragalactic Survey (SMILES), to identify AGN, SFGs, and Si-absorption dominated galaxies out to substantial redshifts. Our JWST/MIRI SFGs sample includes galaxies with total IR luminosities of $10^{9.2} \sim 10^{11.9} L_{\odot}$ at 0.9 $\leq$z < 1.57. The majority of them are consistent with the z$\sim$1 main sequence. We also identified the first examples of z$\sim$1 galaxies with deep silicate absorption.

astro-ph.GA↗

arXiv2025

Irregularity in Active Fast Radio Burst Repeaters and Magnetar Periodic Radio Pulses: Time, Energy, and Frequency Analyses

Fast Radio Bursts (FRBs) are millisecond-duration radio pulses with largely unknown origins, with a subset exhibiting repeating behavior. Magnetars highly magnetized neutron stars and a leading progenitor candidate for FRBs also produce similar but much fainter millisecond radio pulses, suggesting a possible connection between the two phenomena. The irregularity of the time series of repeating FRBs and magnetar pulses may provide insight into the underlying progenitor activity. In this study, we analyze time-series data from three repeating FRB sources (four datasets) and the Galactic magnetar SGR J1935+2154 to investigate potential patterns in burst arrival times, energy fluctuations, and peak-frequency shifts. We quantify the degree of randomness (Pincus Index; PI) and chaos (Largest Lyapunov Exponent; LLE) for these three parameters. We find that waiting times across all repeating FRBs exhibit high PI (high randomness) and low LLE (low chaos), consistent with the behavior of magnetar radio pulses. This similarity suggests that both may share a common triggering mechanism. In contrast, the energy fluctuations of both repeating FRBs and magnetar pulses occupy the same region in PI-LLE phase space but display much larger scatter than the other two domains. We discuss the possibility that beaming effects or strong variability in radio-emission efficiency may explain their distinct behavior in the energy domain.

astro-ph.HE↗

arXiv2025

Machine learning classification of baseband data of CHIME FRBs

Fast Radio Bursts (FRBs) are bright millisecond radio pulses. Their origin is still unknown in the field of astronomy. A notable distinction among FRBs is that some sources repeat, while others appear to be non-repeating events. Interestingly, repeating FRBs tend to exhibit broader temporal widths and narrower spectral bandwidths compared to non-repeat events, suggesting they may arise from different physical mechanisms. However, current radio telescopes have limited coverage and sensitivity, which hinders a complete survey with continuous long-term monitoring. This issue makes it difficult to confirm repeat activity and potentially leads to misclassification of repeaters as non-repeaters; these are referred to as repeater candidates. To address this, machine learning techniques have emerged as a useful tool for classifying distinct FRB types in previous studies. In this study, we utilize the CHIME/FRB baseband catalog with three orders of magnitude better time resolution than the intensity catalog. Measured fluences are available in the baseband catalog, while only upper limits are reported in the intensity catalog. We apply machine learning to the baseband catalog to evaluate classification outcomes. We identify 15 repeater candidates among 122 non-repeating FRBs in the baseband catalog. Additionally, our classification identifies 31 sources previously categorized as repeater candidates as non-repeaters, highlighting a significant difference from the prior work. Of these repeater candidates, 14 overlap with previous findings, while 1 is newly identified in this work. Notably, one of our candidates was confirmed as a repeater by CHIME/FRB. Follow-up observations for the 14 candidates are highly encouraged.

astro-ph.HE↗

arXiv2025

Photometrically Selected Protocluster Candidates at z~9-10 in the JWST COSMOS-Web field

High-redshift protoclusters are crucial for understanding the formation of galaxy clusters and the evolution of galaxies in dense environments. The James Webb Space Telescope (JWST), with its unprecedented near-infrared sensitivity, enables the first exploration of protoclusters beyond $z>$10. Among JWST surveys, COSMOS-Web Data Release 0.5 offers the largest area $\sim$0.27 deg$^2$, making it an optimal field for protocluster searches. In this study, we searched for protoclusters at $z\sim$9-10 using 366 F115W dropout galaxies. We evaluated the reliability of our photometric redshift by validation tests with the JADES DR3 spectroscopic sample, obtaining the likelihood of falsely identifying interlopers as $\sim25\%$. Overdensities ($\delta$) are computed by weighting galaxy positions with their photometric redshift probability density functions (PDF), using a 2.5 cMpc aperture and a redshift slice of $\pm$0.5. We selected the most promising core galaxies of protocluster candidate galaxies with an overdensity greater than the 95th percentile of the distribution of 366 F115W dropout galaxies. The member galaxies are then linked within an angular separation of 7.5 cMpc to the core galaxies, finding seven protocluster candidates. These seven protocluster candidates have inferred halo masses of $M_{\text{halo}} \sim 10^{11} M_{\odot}$. The detection of such overdensities at these redshifts provides a critical test for current cosmological simulations. However, confirming these candidates and distinguishing them from low-redshift dusty star-forming galaxies or Balmer-break galaxies will require follow-up near-infrared spectroscopic observations.

astro-ph.GA↗

arXiv2025

No Metallicity Preference in Fast Radio Burst Host Galaxies

Fast radio bursts (FRBs) are millisecond-duration extragalactic radio transients of unknown origin, and studying their host galaxies could offer clues to constrain progenitor models. Among host properties, gas-phase metallicity is a key factor influencing stellar evolution and transient productions. We analyze the largest uniformly selected sample of FRB host galaxies, measuring oxygen abundances (12+log(O/H)=8.04-8.85) for 31 hosts at redshifts z=0.04-0.98, using consistent emission-line diagnostics. Using a volume-limited subsample, we compare the distributions of stellar mass, star formation rate (SFR), and metallicity to a control sample of star-forming galaxies selected by the same criteria. We find that FRB host galaxies span a wide metallicity range and are broadly consistent with the SFR-weighted mass-metallicity relation of star-forming galaxies. We find no clear lower metallicity bound, suggesting that FRB progenitors can form in any metallicity environment through channels largely insensitive to metal abundance. Encouragingly, this implies FRBs can arise even in low-metallicity, high-redshift galaxies, supporting their potential as probes of matter distribution across cosmic time. Additionally, we find marginal ($\sim$2$\sigma$) evidence for a -0.09\pm0.04 dex metallicity offset from the fundamental metallicity relation. Despite model uncertainties, if real, this offset likely reflects suppressed SFRs at fixed mass and metallicity rather than metal deficiency. Similar offsets are observed in local post-merger galaxies and may reflect a post-starburst phase following galaxy interactions. Such systems may host FRB progenitors formed during the starburst that produce FRBs after a 100-500 Myr delay, broadly consistent with observed delay-time distributions, although further data are needed to confirm this interpretation.

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arXiv2025

A Far-Infrared Search for Planet Nine Using AKARI All-Sky Survey

An unusual orbital element clustering of Kuiper belt objects (KBOs) has been observed. The most promising dynamic solution is the presence of a giant planet in the outer Solar system, Planet Nine. However, due to its extreme distance, intensive searches in optical have not been successful. We aim to find Planet Nine in the far-infrared, where it has the peak of the black body radiation, using the most sensitive all-sky far-infrared survey to date, AKARI. In contrast to optical searches, where the energy of reflected sunlight decreases by $d^{4}$, thermal radiation in the infrared decreases with the square of the heliocentric distance $d^{2}$. We search for moving objects in the AKARI Single Scan Detection List. We select sources from a promising region suggested by an N-body simulation from Millholland and Laughlin 2017: $30^{\circ}<$ R.A. $<50^{\circ}$ and $-20^{\circ}<$ Dec. $<20^{\circ}$. Known sources are excluded by cross-matching AKARI sources with 9 optical and infrared catalogues. Furthermore, we select sources with small background strength to avoid sources in the cirrus. Since Planet Nine is stationary in a timescale of hours but moves on a monthly scale, our primary strategy is to select slowly moving objects that are stationary in 24 hours but not in six months, using multiple single scans by AKARI. The selected slowly moving AKARI sources are scrutinised for potential contamination from cosmic rays. Our analysis reveals two possible Planet Nine candidates whose positions and flux are within the theoretical prediction ranges. These candidates warrant further investigation through follow-up observations to confirm the existence and properties of Planet Nine.

astro-ph.EP↗

arXiv2025

Do they repeat? Monitoring 36 non-repeating FRBs with FAST

The origin of fast radio bursts (FRBs), highly energetic, millisecond-duration radio pulses originating from beyond our galaxy, remains unknown. Observationally, FRBs are classified as non-repeating or repeating, however, this classification is complicated by limited observing time and sensitivity constraints, which may result in some repeating FRBs being misidentified as non-repeating. To address this issue, we adopt both empirical and machine-learning techniques from previous studies to identify candidates that may have been misclassified. We conducted follow-up observations of 36 such candidates, each observed for 10 minutes using the Five-hundred-meter Aperture Spherical Telescope (FAST). No radio bursts exceeding a signal-to-noise ratio of 7 were detected, with a typical 7 sigma fluence limit of ~0.013 Jy ms. We constrain the repetition rates of these sources using two statistical models of FRB occurrence. Combining our FAST non-detections with prior observations, we derive upper limits on the repetition rates of ~$10^{-2.6}$-$10^{-0.22}$ hr$^{-1}$ under a Poisson process, and ~$10^{-2.3}$-$10^{-0.25}$ hr$^{-1}$ under a Weibull process. This work presents one of the most stringent upper limits on FRB repetition rates to date, based on a sample size five times larger than those used in previous studies.

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arXiv2025

Inferring Obscured Cosmic Black Hole Accretion History from AGN Found by JWST/MIRI CEERS Survey

This study presents the black hole accretion history (BHAH) of obscured active galactic nuclei (AGNs) identified from the JWST CEERS survey by Chien et al. (2024) using mid-infrared (MIR) SED fitting. We compute black hole accretion rates (BHARs) to estimate the black hole accretion density (BHAD), $\rho_{L_{\mathrm{disk}}}$, across $0 < z < 4.25$. MIR luminosity functions (LFs) are also constructed for these sources, modeled with modified Schechter and double power law forms, and corresponding BHAD, $\rho_{\mathrm{LF}}$, is derived by integrating the LFs and multiplying by the luminosity. Both $\rho_{\mathrm{LF}}$ extend to luminosities as low as $10^7 \, L_{\odot}$, two orders of magnitude fainter than pre-JWST studies. Our results show that BHAD peaks between redshifts 1 and 3, with the peak varying by method and model, $z \approx 1$--2 for $\rho_{L_{\mathrm{disk}}}$ and the double power law, and $z \approx 2$--3 for the modified Schechter function. A scenario where AGN activity peaks before cosmic star formation would challenge existing black hole formation theories, but our present study, based on early JWST observations, provides an initial exploration of this possibility. At $z \sim 3$, $\rho_{\mathrm{LF}}$ appears higher than X-ray estimates, suggesting that MIR observations are more effective in detecting obscured AGNs missed by X-ray observations. However, given the overlapping error bars, this difference remains within the uncertainties and requires confirmation with larger samples. These findings highlight the potential of JWST surveys to enhance the understanding of co-evolution between galaxies and AGNs.

astro-ph.GA↗

arXiv2025

Decoding the cosmological baryonic fluctuations using localized fast radio bursts

Aims: The enigma of the missing baryons poses a prominent and unresolved problem in astronomy. Dispersion measures (DM) serve as a distinctive observable of fast radio bursts (FRBs). They quantify the electron column density along each line of sight and reveal the missing baryons that are described in the Macquart (DM-z) relation. The scatter of this relation is anticipated to be caused by the variation in the cosmic structure. This is not yet statistically confirmed, however. We present statistical evidence that the cosmological baryons fluctuate. Methods: We measured the foreground galaxy number densities around 14 and 13 localized FRBs with the WISE-PS1-STRM and WISE x SCOS photometric redshift galaxy catalog, respectively. The foreground galaxy number densities were determined through a comparison with measured random apertures with a radius of 1 Mpc. Results: We found a positive correlation between the excess of DM that is contributed by the medium outside galaxies (DM_cosmic) and the foreground galaxy number density. The correlation is strong and statistically significant, with median Pearson coefficients of 0.6 and 0.6 and median p-values of 0.012 and 0.032 for the galaxy catalogs, respectively, as calculated with Monte Carlo simulations. Conclusions: Our findings indicate that the baryonic matter density outside galaxies exceeds its cosmic average along the line of sight to regions with an excess galaxy density, but there are fewer baryons along the line of sight to low-density regions. This is statistical evidence that the ionized baryons fluctuate cosmologically on a characteristic scale of $\lesssim$6 Mpc.

astro-ph.CO↗

arXiv2025

A Search for Planet Nine with IRAS and AKARI Data

The outer solar system is theoretically predicted to harbour an undiscovered planet, often referred to as P9. Simulations suggest that its gravitational influence could explain the unusual clustering of minor bodies in the Kuiper Belt. However, no observational evidence for P9 has been found so far, as its predicted orbit lies far beyond Neptune, where it reflects only a faint amount of Sunlight. This work aims to find P9 candidates by taking advantage of two far-infrared all-sky surveys, which are IRAS and AKARI. The epochs of these two surveys were separated by 23 years, which is large enough to detect the ~3'/year orbital motion of P9. We use a dedicated AKARI Far-Infrared point source list for our P9 search - AKARI Monthly Unconfirmed Source List, which includes sources detected repeatedly only in hours timescale, but not after months. We search for objects that moved slowly between IRAS and AKARI detections given in the catalogues. First, we estimated the expected flux and orbital motion of P9 by assuming its mass, distance, and effective temperature to ensure it can be detected by IRAS and AKARI, then applied the positional and flux selection criteria to narrow down the number of sources from the catalogues. Next, we produced all possible candidate pairs whose angular separations were limited between 42' and 69.6', corresponding to the heliocentric distance range of 500 - 700 AU and the mass range of 7 - 17 Earth masses. There are 13 pairs obtained after the selection criteria. After image inspection, we found one good candidate, of which the IRAS source is absent from the same coordinate in the AKARI image after 23 years and vice versa. However, AKARI and IRAS detections are not enough to determine the full orbit of this candidate. This issue leads to the need for follow-up observations, which will determine the Keplerian motion of our candidate.

astro-ph.EP↗

arXiv2025

Brown dwarf number density in the JWST COSMOS-Web field

Brown dwarfs are failed stars with very low mass (13 to 75 $M_J$), and an effective temperature lower than 2500 K. Thus, they play a key role in understanding the gap in the mass function between stars and planets. However, due to their faint nature, previous searches are inevitably limited to the solar neighbourhood (20 pc). To improve our knowledge of the low mass part of the initial stellar mass function and the star formation history of the Milky Way, it is crucial to find more distant brown dwarfs. Using James Webb Space Telescope (JWST) COSMOS-Web data, this study seeks to enhance our comprehension of the physical characteristics of brown dwarfs situated at a distance of kpc scale. The exceptional sensitivity of the JWST enables the detection of brown dwarfs that are up to 100 times more distant than those discovered in the earlier all-sky infrared surveys. The large area coverage of the JWST COSMOS-Web survey allows us to find more distant brown dwarfs than earlier JWST studies with smaller area coverages. To capture prominent water absorption features around 2.7 $\mu$m, we apply two colour criteria, F115W-F277W+1 0.9. We then select point sources by CLASS_STAR, FLUX_RADIUS, and SPREAD_MODEL criteria. Faint sources are visually checked to exclude possibly extended sources. We conduct SED fitting and MCMC simulations to determine their physical properties and associated uncertainties. Our search reveals 25 T-dwarf and 2 Y-dwarf candidates, more than any previous JWST brown dwarf searches. They are located from 0.3 kpc to 4 kpc away from the Earth. The cumulative number count of our brown dwarf candidates is consistent with the prediction from a standard double exponential model. Three of our brown dwarf candidates were detected by HST, with transverse velocities $12\pm5$ km s$^{-1}$, $12\pm4$ km s$^{-1}$, and $17\pm6$ km s$^{-1}$.

astro-ph.SR↗