arXiv subjects

Youichi Ohyama

Publications and source records attributed to Youichi Ohyama.

At least 19 recordsLinked to original sources

arXiv2025

The Potential of the SPHEREx Mission for Characterizing Polycyclic Aromatic Hydrocarbon 3.3 μm Emission in Nearby Galaxies

Together with gas, stars, and supermassive black holes, dust is crucial in stellar and galaxy evolution. Hence, understanding galaxies' dust properties across cosmic time is critical to studying their evolution. In addition to photometric constraints on the absorption of blue light and its reemission at infrared wavelengths, dust grain properties can be explored spectroscopically via polycyclic aromatic hydrocarbon (PAH) emission bands in the mid-IR. The new SPHEREx space telescope conducts an all-sky spectrophotometric survey of stars and galaxies at wavelengths of 0.75-5$\,μ$m, making it ideal for studying the widespread presence of the 3.3$\,μ$m PAH emission across galaxy populations out to z ~ 0.4. In this paper, we simulated galaxy spectra to investigate SPHEREx's capability to study PAH emission in such galaxies. We find that for the all-sky survey the PAH 3.3$\,μ$m emission band flux can be measured to 30% accuracy at $\log(\rm M/{\rm M_\odot})>9.5$ and star formation rate (SFR) $> 1\,{\rm M_\odot\,yr^{-1}}$ at $z=0.1$, $\log(\rm M/{\rm M_\odot}) > 10.5$ and ${\rm SFR} > 10\,{\rm M_\odot\,yr^{-1}}$ at $z=0.2-0.3$, and $\log(\rm M/{\rm M_\odot})>11$ and ${\rm SFR} > 100\,{\rm M_\odot\,yr^{-1}}$ at $z=0.4$. For deep SPHEREx fields, a factor of ~10 deeper sensitivity limits can be reached. Overall, SPHEREx will enable the measurement of the 3.3$\,μ$m PAH band emission in several hundred thousand galaxies across the sky, providing a population study of the smallest dust grains ("nano grains") and radiation properties in massive galaxies in the nearby Universe.

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arXiv2025

Tidal Disruption of Super Star Clusters as the origin of Bluish Light at the inner region of the Perseus Cluster Central Galaxy

Relatively blue light extends beyond a spiral disk from a radius of ~5 kpc out to ~14 kpc from the center of NGC 1275. Analyses of its spectrum and broadband colors reveal a population of young stars having sub-solar metallicities superposed on a dominant population of old stars having super-solar metallicities. The young stars have a characteristic age of ~160 Myr and may span ages of a few hundred Myr, similar to that of stars comprising the central spiral disk, and a total mass about one-third that of this disk for a combined stellar mass (at birth) of ~4 $\times$ 10$^9$ $\rm M_\odot$. A multitude of arc-like features embedded in the extended blue light have brightnesses comparable to the somewhat older (~500 Myr) super star clusters (SSCs) projected against the central spiral disk. The SSCs have a relatively shallow mass function, suggesting that the tidal disruption of an initially larger population that we estimate could have had an initial total mass (far) exceeding ~1 $\times$ 10$^9$ $\rm M_\odot$ gave rise to the extended blue light -- the arc-like features corresponding to stellar streams tracing disrupted star clusters -- and perhaps also the central spiral disk. We speculate that, beginning about 500 Myr ago, an enhanced episode of AGN activity in NGC 1275, leaving still visible X-ray bubbles, induced vigorous cooling of the intracluster medium to fuel the formation of numerous star clusters: many were tidally disrupted to leave bluish light at the inner region of this galaxy, with the survivors being the SSCs projected against the central spiral disk.

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arXiv2025

Near-infrared Integral-field Spectroscopy of the Wind Forming Region of CW Leo

The circumstellar envelope of the carbon star CW Leo exhibited various unexpected changes in recent optical imaging observations. We have performed a follow-up observation using the Near-infrared Integral-Field Spectrograph (NIFS) equipped on the Gemini-North telescope. We report the near-infrared counterparts of a local brightness peak in the optical at the stellar position of CW Leo. On the other hand, a second peak detected at short wavelengths in the J band coincides with the brightest, bluest position in the optical images. The absorption features in the K band are minimized at a radius of 0.2 arcsec from the predicted stellar position. The reduction of the absorption depths likely indicates dilution of the absorption features by thermal emission of dust grains newly formed at such a radius and heated by radiation from the central star. The broad absorption feature at 1.53 um is significantly deeper than in template carbon stars, consistent with the presence of a substantial amount of circumstellar material around CW Leo. Its northeastern quadrant lacks circumstellar absorption features and scattered light in the near-infrared regime, which are possibly manifestations of its conical cavity in both gas and dust. In addition, a cross correlation of CO overtone bands indicates that the average expansion velocity of dust grains is smaller to the northern direction, likewise the velocity of transverse wind components derived using the differential proper motion of a circumstellar whirled pattern. The gradual brightening of CW Leo and the changes in its innermost circumstellar envelope need further continuous monitoring observations to properly understand its transitional phase toward the post-asymptotic-giant-branch stage.

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arXiv2024

Star Formation, Nebulae, and Active Galactic Nuclei in CLASH Brightest Cluster Galaxies. I. Dependence on Core Entropy of Intracluster Medium

We set the stage for reassessing how star formation, emission-line nebulae, and active galactic nuclei (AGNs) in brightest cluster galaxies (BCGs) depend on the thermodynamics of the intracluster medium (ICM). Our work is based on the 25 clusters observed in the CLASH program for which the aforementioned attributes in their BCGs can be well scrutinized, as has the thermodynamics of their ICM. Nine of these BCGs display complex UV morphologies tracing recent star formation, whereas the remaining 16 are characterized by a relatively compact central UV enhancement. Here, we show definitively that three of the latter BCGs also display star formation, whereas the diffuse UV of the remaining 13 is entirely consistent with old low-mass stars. The overall results support the previously established dependence of star formation and nebulae in BCGs on an "excess core entropy," K$_{0}$, for the ICM: all 11 clusters with K$_{0}$ $\leq$ 24 keV cm$^{2}$ (but only one of 14 clusters with K$_{0}$ $\geq$ 42 keV cm$^{2}$) host star-forming BCGs that almost if not always possess nebulae. Instead of an entropy floor, we show that K$_{0}$ reflects the degree to which the radial entropy profile decreases inward within $\sim$100 kpc rather than (except perhaps at large K$_{0}$) actually flattening: clusters with lower ICM entropies and hence shorter cooling times at their cores preferentially host BCGs displaying star formation, nebulae, and more radio-luminous AGNs. Nearly all BCGs possess detectable AGNs, however, indicating multiple pathways for fuelling their AGNs.

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arXiv2023

What are the Radial Distributions of Density, Outflow Rates, and Cloud Structures in the M 82 Wind?

Galactic winds play essential roles in the evolution of galaxies through the feedback they provide. Despite intensive studies of winds, the radial distributions of their properties and feedback are rarely observable. Here we present such measurements for the prototypical starburst galaxy, M 82, based on observations by Subaru telescope. We determine the radial distribution of outflow densities ($n_e$) from the spatially-resolved [S II] $λλ$ 6717, 6731 emission-lines. We find $n_e$ drops from 200 to 40 cm$^{-3}$ with radius ($r$) between 0.5 and 2.2 kpc with a best-fit power-law index of $r^{-1.2}$. Combined with resolved H$α$ lines, we derive mass, momentum, and energy outflow rates, which drop quite slowly (almost unchanged within error bars) over this range of $r$. This suggests that the galactic wind in M 82 can carry mass, momentum, and energy from the central regions to a few kpc with minimal losses. We further derive outflow cloud properties, including size and column densities. The clouds we measure have pressures and densities that are too high to match those from recent theoretical models and numerical simulations of winds. By comparing with a sample of outflows in local star-forming galaxies studied with UV absorption-lines, the above-derived properties for M 82 outflows match well with the published scaling relationships. These matches suggest that the ionized gas clouds traced in emission and absorption are strongly related. Our measurements motivate future spatially resolved studies of galactic winds, which is the only way to map the structure of their feedback effects.

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arXiv2023

The porous envelope and circumstellar wind matter of the closest carbon star, CW Leonis

Recent abrupt changes of CW Leonis may indicate that we are witnessing the moment that the central carbon star is evolving off the Asymptotic Giant Branch (AGB) and entering into the pre-planetary nebula (PPN) phase. The recent appearance of a red compact peak at the predicted stellar position is possibly an unveiling event of the star, and the radial beams emerging from the stellar position resemble the feature of the PPN Egg Nebula. The increase of light curve over two decades is also extraordinary, and it is possibly related to the phase transition. Decadal-period variations are further found in the residuals of light curves, in the relative brightness of radial beams, and in the extended halo brightness distribution. Further monitoring of the recent dramatic and decadal-scale changes of this most well-known carbon star CW Leonis at the tip of AGB is still highly essential, and will help us gain a more concrete understanding on the conditions for transition between the late stellar evolutionary phases.

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arXiv2023

Warm Molecular Gas in the Central Parsecs of the Buried Nucleus of NGC 4418 Traced with the Fundamental CO Ro-vibrational Absorptions

We investigated the inner buried nucleus of a nearby luminous infrared galaxy NGC 4418 using high-resolution spectroscopy of fundamental carbon monoxide (CO) ro-vibrational absorptions around $4.67 μ$m for the first time. This method allowed us to examine the physical and kinematical properties in the hot inner region of this nucleus. We detected a series of both very deep (partly saturated) $^{12}$CO and moderately deep (optically thin) $^{13}$CO absorption lines and inferred a large column density ($N_\mathrm{H2}=(5\pm3)\times10^{23}$ cm$^{-2}$ in front of the $5 μ$m photosphere) of warm ($T_\mathrm{ex}\simeq170$ K) molecular gas by assuming an isothermal plane-parallel slab illuminated by a compact background MIR-emitting source. We modeled that the warm CO absorber almost covers the central heating source and that it is an inner layer around the $5 μ$m photosphere (at $r=$several pc) of a compact shroud of gas and dust ($d\sim100$ pc). The width of the absorption lines ($110$ km s$^{-1}$) and their small deviation from the systemic velocity ($<10$ km s$^{-1}$) are consistent with a warm and turbulent layer with little bulk motion in the radial direction.

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arXiv2022

Time-Variable Jet Ejections from RW Aur A, RY Tau and DG Tau

We present Gemini-NIFS, VLT-SINFONI and Keck-OSIRIS observations of near-infrared [Fe II] emission associated with the well-studied jets from three active T Tauri stars; RW Aur A, RY Tau and DG Tau taken from 2012-2021. We primarily covered the redshifted jet from RW Aur A, and the blueshifted jets from RY Tau and DG Tau, to investigate long-term time variabilities potentially related to the activities of mass accretion and/or the stellar magnetic fields. All of these jets consist of several moving knots with tangential velocities of 70-240 km s-1, ejected from the star with different velocities and at irregular time intervals. Via comparison with literature, we identify significant differences in tangential velocities for the DG Tau jet between 1985-2008 and 2008-2021. The sizes of the individual knots appear to increase with time, and in turn, their peak brightnesses in the 1.644-micron emission decreased up to a factor of ~30 during the epochs of our observations. A variety of the decay timescales measured in the [Fe II] 1.644 micron emission can be attributed to different pre-shock conditions if the moving knots are unresolved shocks. However, our data do not exclude the possibility that these knots are due to non-uniform density/temperature distributions with another heating mechanism, or in some cases due to stationary shocks without proper motions. Spatially resolved observations of these knots with significantly higher angular resolutions are necessary to better understand their physical nature.

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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.

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arXiv2022

Monitoring inner regions in the RY Tau jet

We present multi-epoch observations of the RY~Tau jet for H$α$ and [\ion{Fe}{2}] 1.644 \micron~emission lines obtained with Subaru/SCExAO+VAMPIRES, Gemini/NIFS, and Keck/OSIRIS in 2019--2021. These data show a series of four knots within 1$\arcsec$ consistent with the proper motion of $\sim$0\farcs3~yr$^{-1}$, analogous to the jets associated with another few active T-Tauri stars. However, the spatial intervals between the knots suggest the time intervals of the ejections of about 1.2, 0.7, and 0.7 years, significantly shorter than those estimated for the other stars. These H$α$ images contrast with the archival VLT/SPHERE/ZIMPOL observations from 2015, which showed only a single knot-like feature at $\sim0\farcs25$. The difference between the 2015 and 2019--2021 epochs suggests an irregular ejection interval within the six-year range. Such variations of the jet ejection may be related to a short-term ($<$1 year) variability of the mass accretion rate. We compared the peaks of the H$α$ emissions with the ZIMPOL data taken in 2015, showing the brighter profile at the base ($<0\farcs3$) than the 2020--2021 VAMPIRES profiles due to time-variable mass ejection rates or the heating-cooling balance in the jet. The observed jet knot structures may be alternatively attributed to stationary shocks, but a higher angular resolution is required to confirm its detailed origin.

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arXiv2022

Recent Formation of a Spiral Disk Hosting Progenitor Globular Clusters at the center of the Perseus Brightest Cluster Galaxy: I. Spiral Disk

We address the nature and origin of a spiral disk at the center of NGC 1275, the giant elliptical galaxy at the center of the Perseus cluster, that spans a radius of $\sim$$5\,\rm kpc$. By comparing stellar absorption lines measured in long-slit optical spectra with synthetic spectra for single stellar populations, we find that fitting of these lines requires two stellar populations: (i) a very young population that peaks in radial velocity at $\pm 250 {\rm \, km \, s^{-1}}$ of the systemic velocity within a radius of $\sim$$720\,\rm pc$ of the nucleus, a $1\,σ$ velocity dispersion significantly lower than $140 {\rm \, km \, s^{-1}}$, and an age of $0.15 \pm 0.05 \rm\,Gyr$; and (ii) a very old population having a constant radial velocity with a radius corresponding to the systemic velocity, a much broader velocity dispersion of $\sim$$250 {\rm \, km \, s^{-1}}$, and an age of around $10\,\rm Gyr$. We attribute the former to a post-starburst population associated with the spiral disk, and the latter to the main stellar body of NGC 1275 along the same sight line. If the spiral disk is the remnant of a cannibalized galaxy, then its progenitor would have had to retain an enormous amount of gas in the face of intensive ram-pressure stripping so as to form a total initial mass in stars of $\sim 3 \times 10^9 \,M_\odot$. More likely, the central spiral originally comprised a gaseous body accreted over the distant past from a residual cooling flow, before experiencing a starburst $\sim$$0.15\,\rm Gyr$ ago to form its stellar body.

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arXiv2022

Recent Formation of a Spiral Disk Hosting Progenitor Globular Clusters at the center of the Perseus Brightest Cluster Galaxy: II. Progenitor Globular Clusters

We address the nature and origin of Super Star Clusters (SSCs) discovered by Holtzman et al. (1992) within a radius of $\sim$$5\,\rm kpc$ from the center of NGC 1275, the giant elliptical galaxy at the center of the Perseus Cluster. We show that, in contrast with the much more numerous population of SSCs subsequently discovered up to $\sim$$30\,\rm kpc$ from the center of this galaxy, the central SSC population have maximal masses an order of magnitude higher and a mass function with a shallower power-law slope. Furthermore, whereas the outer SSC population have ages spanning a few $\rm Myr$ to at least $\sim$$1\,\rm Gyr$, the central SSC population have ages strongly concentrated around $\sim$$500 \rm \, Myr$ with a $1\,σ$ dispersion of $\sim$$100\,\rm Myr$. These SSCs share a close spatial and temporal relationship with the "central spiral," which also has a radius $\sim$$5\,\rm kpc$ centered on NGC 1275 and a characteristic stellar age of $\sim$$150\,\rm Myr$ (Paper I). We argue that both the central SSC population and the central spiral formed from gas deposited by a residual cooling flow, with the SSCs forming first followed by the formation of the stellar body of the central spiral $\sim$$300$-$400\,\rm Myr$ later. The ages of the central SSC population imply that they are able to withstand very strong tidal fields near the center of NGC 1275, making them genuine progenitor globular clusters. Evidently, a spiral disk hosting progenitor globular clusters has recently formed at the center of a giant elliptical galaxy.

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arXiv2021

Temporally and Spatially Extended Star Formation in the Brightest Cluster Galaxy of MACS\,J0329.7$-$0211 at $z=0.45$: Implications for Stellar Growth

Brightest cluster galaxies (BCGs), particularly those at the centers of cool-core clusters, can exhibit star formation over spatial extents of up to $\gtrsim$100\,kpc at inferred rates of up to $\gtrsim100\rm\,M_\sun\,yr^{-1}$. Is their star formation also extended over time, as might be expected if fuelled by cooling of the surrounding hot intracluster gas -- a residual cooling flow -- as demonstrated hitherto only for the BCG in the Perseus cluster? Here, to infer the formation history of relatively young stars in the BCG of MACS\,J0329.7$-$0211, we fit model single-stellar-populations to the spectral energy distributions (spanning near-UV to near-IR) measured along different sightlines towards its young stellar population. Employing a Markov Chain Monte Carlo method, we show that star formation in this BCG has persisted at a relatively constant rate of $\sim2{\rm\,M_\sun\,yr^{-1}}$ (factors of 10--40 below the rates previously inferred using simpler methods and/or ad hoc assumptions) over the past $\sim$400\,Myr, beyond which any star formation falls below the observational detection threshold. Such persistent star formation from a residual cooling flow can contribute up to $\sim$10\% of the original stellar mass of this BCG if its progenitor was among the most massive red nuggets known at $z\sim$2 having masses of $\sim1\times10^{11}\rm\,M_\sun$, but only a few percent of its overall growth in stellar mass to $\sim8\times10^{11}\rm\,M_\sun$ at $z=0.45$. Although constituting only a minor pathway for the stellar growth of this BCG, persistent star formation from a residual cooling flow can nevertheless contribute significantly to the enormous number of globular clusters found around BCGs in the local Universe.

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arXiv2021

Multi-epoch Optical Images of IRC+10216 Tell About the Central Star And the Adjacent Environment

Six images of IRC+10216 taken by the Hubble Space Telescope at three epochs in 2001, 2011, and 2016 are compared in the rest frame of the central carbon star. An accurate astrometry has been achieved with the help of Gaia Data Release 2. The positions of the carbon star in the individual epochs are determined using its known proper motion, defining the rest frame of the star. In 2016, a local brightness peak with compact and red nature is detected at the stellar position. A comparison of the color maps between 2016 and 2011 epochs reveals that the reddest spot moved along with the star, suggesting a possibility of its being the dusty material surrounding the carbon star. Relatively red, ambient region is distributed in an $Ω$ shape and well corresponds to the dusty disk previously suggested based on near-infrared polarization observations. In a larger scale, differential proper motion of multiple ring-like pattern in the rest frame of the star is used to derive the average expansion velocity of transverse wind components, resulting in $\sim$ 12.5 km s$^{-1}$ ($d$/123 pc), where $d$ is the distance to IRC+10216. Three dimensional geometry is implied from its comparison with the line-of-sight wind velocity determined from half-widths of submillimeter emission line profiles of abundant molecules. Uneven temporal variations in brightness for different searchlight beams and anisotropic distribution of extended halo are revisited in the context of the stellar light illumination through a porous envelope with postulated longer-term variations for a period of $\lesssim10$ years.

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arXiv2021

The Sharpest Ultraviolet view of the star formation in an extreme environment of the nearest Jellyfish Galaxy IC 3418

We present the far ultraviolet (FUV) imaging of the nearest Jellyfish or Fireball galaxy IC3418/VCC 1217, in the Virgo cluster of galaxies, using Ultraviolet Imaging Telescope (UVIT) onboard the ASTROSAT satellite. The young star formation observed here in the 17 kpc long turbulent wake of IC3418, due to ram pressure stripping of cold gas surrounded by hot intra-cluster medium, is a unique laboratory that is unavailable in the Milkyway. We have tried to resolve star forming clumps, seen compact to GALEX UV images, using better resolution available with the UVIT and incorporated UV-optical images from Hubble Space Telescope archive. For the first time, we resolve the compact star forming clumps (fireballs) into sub-clumps and subsequently into a possibly dozen isolated stars. We speculate that many of them could be blue supergiant stars which are cousins of SDSS J122952.66+112227.8, the farthest star (~17 Mpc) we had found earlier surrounding one of these compact clumps. We found evidence of star formation rate (4 - 7.4 x 10^-4 M_sun per yr ) in these fireballs, estimated from UVIT flux densities, to be increasing with the distance from the parent galaxy. We propose a new dynamical model in which the stripped gas may be developing vortex street where the vortices grow to compact star forming clumps due to self-gravity. Gravity winning over turbulent force with time or length along the trail can explain the puzzling trend of higher star formation rate and bluer/younger stars observed in fireballs farther away from the parent galaxy.

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arXiv2020

Sustained formation of progenitor globular clusters in a giant elliptical galaxy

Globular clusters (GCs) are thought to be ancient relics from the early formative phase of galaxies, although their physical origin remains uncertain. GCs are most numerous around massive elliptical galaxies, where they can exhibit a broad colour dispersion, suggesting a wide metallicity spread. Here, we show that many thousands of compact and massive (~5$\times$10$^{\rm 3}-$3$\times$ 10$^{\rm 6} M_{\odot}$) star clusters have formed at an approximately steady rate over, at least, the past ~1Gyr around NGC 1275, the central giant elliptical galaxy of the Perseus cluster. Beyond ~1Gyr, these star clusters are indistinguishable in broadband optical colours from the more numerous GCs. Their number distribution exhibits a similar dependence with luminosity and mass as the GCs, whereas their spatial distribution resembles a filamentary network of multiphase gas associated with cooling of the intracluster gas. The sustained formation of these star clusters demonstrates that progenitor GCs can form over cosmic history from cooled intracluster gas, thus contributing to both the large number and broad colour dispersion$-$owing to an age spread, in addition to a spread in metallicity$-$of GCs in massive elliptical galaxies. The progenitor GCs have minimal masses well below the maximal masses of Galactic open star clusters, affirming a common formation mechanism for star clusters over all mass scales irrespective of their formative pathways.

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arXiv2020

Possible Time Correlation Between Jet Ejection and Mass Accretion for RW Aur A

For the active T-Taur star RW Aur A we have performed long-term (~10 yr) monitoring observations of (1) jet imaging in the [Fe II] 1.644-micron emission line using Gemini-NIFS and VLT-SINFONI; (2) optical high-resolution spectroscopy using CFHT-ESPaDOnS; and (3) V-band photometry using the CrAO 1.25-m telescope and AAVSO. The latter two observations confirm the correlation of time variabilities between (A) the Ca II 8542 A and O I 7772 A line profiles associated with magnetospheric accretion, and (B) optical continuum fluxes. The jet images and their proper motions show that four knot ejections occurred at the star over the past ~15 years with an irregular interval of 2-6 years. The time scale and irregularity of these intervals are similar to those of the dimming events seen in the optical photometry data. Our observations show a possible link between remarkable (Delta_V < -1 mag.) photometric rises and jet knot ejections. Observations over another few years may confirm or reject this trend. If confirmed, this would imply that the location of the jet launching region is very close to the star (r <<0.1 au) as predicted by some jet launching models. Such a conclusion would be crucial for understanding disk evolution within a few au of the star, and therefore possible ongoing planet formation at these radii.

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arXiv2020

Search for Optically Dark Infrared Galaxies without Counterparts of Subaru Hyper Suprime-Cam in the AKARI North Ecliptic Pole Wide Survey Field

We present the physical properties of AKARI sources without optical counterparts in optical images from the Hyper Suprime-Cam (HSC) on the Subaru telescope. Using the AKARI infrared (IR) source catalog and HSC optical catalog, we select 583 objects that do not have HSC counterparts in the AKARI North Ecliptic Pole (NEP) wide survey field ($\sim 5$ deg$^{2}$). Because the HSC limiting magnitude is deep ($g_{\rm AB}$ $\sim 28.6$), these are good candidates for extremely red star-forming galaxies (SFGs) and/or active galactic nuclei (AGNs), possibly at high redshifts. We compile multi-wavelength data out to 500 $μ$m and use it for Spectral Energy Distribution (SED) fitting with CIGALE to investigate the physical properties of AKARI galaxies without optical counterparts. We also compare their physical quantities with AKARI mid-IR selected galaxies with HSC counterparts. The estimated redshifts of AKARI objects without HSC counterparts range up to $z\sim 4$, significantly higher than that of AKARI objects with HSC counterparts. We find that: (i) 3.6 $-$ 4.5 $μ$m color, (ii) AGN luminosity, (iii) stellar mass, (iv) star formation rate, and (v) $V$-band dust attenuation in the interstellar medium of AKARI objects without HSC counterparts are systematically larger than those of AKARI objects with counterparts. These results suggest that our sample includes luminous, heavily dust-obscured SFGs/AGNs at $z\sim 1-4$ that are missed by previous optical surveys, providing very interesting targets for the coming James Webb Space Telescope era.

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