Low-surface-brightness (LSB) structures serve as evidence of the intricate mass assembly of galaxies, and dedicated studies of these structures promise profound insights into the evolutionary history of galaxies. Furthermore, delving into the properties of star formation (SF) in the LSB regime can broaden our understanding of SF activity in regions characterized by low surface gas density, thereby shedding light on fundamental cosmic processes. However, systematic uncertainties may hamper the exploration of the LSB universe by limiting detectable SB levels. Indeed, despite dedicated advancements in telescope and observing techniques over decades, achieving ultra-deep photometric depths in optical wavelengths remains a formidable challenge. To overcome this challenge and explore the LSB universe that we have yet to see, we have been developing a novel telescope called K-DRIFT. This paper outlines the telescope's specifications and describes various LSB features we aim for, explicitly focusing on nearby individual galaxies. To further advance the capabilities of the K-DRIFT survey, focused on LSB detection, we present several feasible research topics that utilize other survey data together and discuss the role of LSB observation in understanding the evolution of galaxies.

The KASI Deep Rolling Imaging Fast Telescope (K-DRIFT) is a pioneering instrument designed to explore low-surface-brightness (LSB) phenomena. This white paper presents a compelling set of science cases that showcase K-DRIFT's unique capabilities in unraveling the mysteries of intracluster light (ICL) and other LSB components within galaxy clusters. Exploring the origin of ICL in galaxy clusters and comparing the spatial distributions of ICL and dark matter will offer new insights into galaxy cluster dynamics. Moreover, investigating LSB objects in galaxy clusters, such as LSB structures in the brightest cluster galaxies, ultra-diffuse galaxies, and tidal features, will enhance our understanding of galaxy evolution within the cluster environment. We present our strategies for addressing scientific queries, encompassing LSB observation and analysis techniques, specialized simulations, and machine-learning approaches. Additionally, we examine the potential synergies between K-DRIFT and other ongoing and forthcoming multi-wavelength surveys. This white paper advocates for the recognition and support of K-DRIFT as a dedicated tool for advancing our understanding of the universe's subtlest phenomena.

The C IV λλ1548,1551 resonance doublet is a key tracer of warm gas (T∼10^5 K) within and around galaxies. Recent observations have detected this line in both absorption and emission, revealing asymmetric profiles in galaxies and spatially extended haloes around active galactic nuclei (AGNs). Resonance scattering can strongly modify the emergent spectra and spatial distributions, complicating their interpretation. Using 3D Monte Carlo radiative transfer simulations, we study C IV resonance scattering over a broad range of column densities, intrinsic emission-line widths, and outflow velocities. We find that multiple scattering broadens the line profile and, in outflowing media, modifies the doublet ratio, R_ CIV, defined as the flux ratio of the K and H components at 1548 and 1551 A, respectively. When the outflow velocity approaches or exceeds the doublet separation (≃500 km s^-1), K-line photons are redistributed around the H component, driving R_ CIV below its intrinsic value and, in optically thick fast outflows, even below unity. We also combine photoionization models with resonance scattering to investigate extended C IV haloes around AGNs and compare them with He II λ1640 emission. Simple photoionization models do not produce C IV emission more extended than He II, whereas resonance scattering redistributes locally produced and central-source C IV photons to larger radii. These results demonstrate that the C IV doublet ratio and spatial distribution provide complementary diagnostics of warm gas.

In this paper, we review the extended halo material and the circumgalactic medium (CGM), including both dust and gas, and discuss promising science cases that could be realized using the KASI Deep Rolling Imaging Fast Telescope (K-DRIFT). Scattered starlight from cirrus clouds in our Galaxy poses one of the major challenges to studying the low surface brightness features of extragalactic sources. Therefore, it is essential to investigate how to discriminate extragalactic sources from the cirrus cloud features. At the same time, interstellar dust clouds themselves are fundamental to understanding dust properties and the interstellar radiation field, both of which are essential for studies of chemical evolution and star formation in our Galaxy. Measuring the reddening of background sources, such as quasars, with K-DRIFT, which benefits from its broad field of view and accurate background subtraction, allows for effective detection of extended dust in galactic halos, the CGM, and intracluster space. Observations of the H-alpha emission lines can be used to identify signatures of star formation activity within galaxies, as well as the environmental effects acting on them. Galactic winds driven by active galactic nuclei and starbursts can be traced through H-alpha emission. Strong ram pressure stripping effectively removes the interstellar medium (ISM) from galaxies. The stripped ISM becomes ionized or dissociated through mixing with the hot intracluster medium (ICM), forming H-alpha tails. The surface brightness of these H-alpha tails correlates not only with the presence of star formation in the tails but also the mixing stage of the stripped ISM and ICM. The H-alpha survey with K-DRIFT will enable the investigation of the evolutionary stages of ram pressure stripped galaxies in cluster environments, as well as the multiphase gas reservoir around galaxies and in the CGM.

Low-surface-brightness (LSB) structures play a crucial role in understanding galaxy evolution by providing significant insights into galaxy interactions, the histories of mass assembly, and the distribution of dark matter. Nevertheless, their inherently faint nature, coupled with observational difficulties such as stray light interference and variations in the sky background, has significantly impeded comprehensive studies of LSB features. The KASI Deep Rolling Imaging Fast Telescope (K-DRIFT) project aims to address these observational challenges by developing off-axis freeform three-mirror telescopes and observational strategies specifically designed for LSB imaging surveys. The first generation of K-DRIFT (G1) has been successfully completed, and the forthcoming survey, scheduled to commence shortly, is expected to yield novel insights into the LSB universe. This paper outlines the scientific motivations of the project, discusses the technical challenges encountered, highlights the innovative solutions devised, and describes the future trajectory of the K-DRIFT.

Low-surface-brightness (LSB) structures provide critical insights into the hierarchical formation of galaxies and galaxy clusters. The KASI Deep Rolling Imaging Fast Telescope (K-DRIFT) is designed to detect such diffuse features through deep, wide-field optical imaging with a surface brightness reaching similar to 30 mag arcsec-2. To interpret the observational data expected from KDRIFT, we have developed the Galaxy Replacement Technique (GRT), an N-body simulation framework optimized for tracing the gravitational evolution of stellar components. The GRT works by inserting high-resolution galaxy models, including a dark matter (DM) halo and stellar disk, in place of multiple low-resolution DM halos in the base N-body cosmological simulation. It allows us to achieve very high mass (mstar = 5.4 & times;104 M circle dot h-1) and spatial resolution (10 pc h-1) with shorter computation time compared to full hydrodynamic cosmological simulations. Therefore, this technique is particularly well-suited for studying LSB structures, with a surface brightness reaching similar to 31 mag arcsec-2. In this paper, we present the motivation and methodology of the GRT, summarize key results from previous studies, and highlight its synergy with K-DRIFT observations. We further discuss planned science cases using the GRT, aiming to build a theoretical basis for interpreting LSB features in various environments.

Hydrogen Lyman-alpha (Lyα) is a prominent emission line from the circumgalactic medium (CGM). Due to its resonant nature, Lyα carries imprints of the physical properties and kinematics of the cold CGM. In particular, CGM rotation can modify the Lyα peak separation, which is often interpreted as a tracer of H I column density. We present 3D Monte Carlo Lyα radiative-transfer simulations in a CGM-like rotating medium and examine how the emergent spectra depend on rotational velocity (V_ rot), H I column density (N_ HI), viewing angle, clumpiness, and intrinsic source width. We find that rotation broadens the integrated spectra and increases the peak separation, with the strongest viewing-angle dependence when rotational Doppler shifts dominate over frequency diffusion. At high N_ HI, numerous scatterings reduce the sensitivity of integrated spectra to rotation, producing a degeneracy between V_ rot and N_ HI. Consequently, Lyα peak separation alone can overestimate N_ HI in a rotating medium. Spatially resolved halo spectra provide a clearer diagnostic: opposite sides of the rotating medium show systematic redshifted and blueshifted asymmetries associated with the line-of-sight velocity of the last-scattering gas. Such rotation-driven signatures can also contribute to velocity-map patterns often interpreted in terms of inflow or outflow, highlighting the need to consider rotation in spatially resolved Lyα observations. We further show that the main signatures persist in simple clumpy media, while the halo signatures are largely insensitive to the intrinsic source width. Our results demonstrate that spatially resolved Lyα observation is essential for disentangling CGM rotation from radiative-transfer effects.

A longstanding prediction in interstellar theory posits that significant quantities of molecular gas, crucial for star formation, may be undetected due to being 'dark' in commonly used molecular gas tracers, such as carbon monoxide. We report the discovery of Eos, a dark molecular cloud located just 94 pc from the Sun. This cloud is identified using H2 far-ultraviolet fluorescent line emission, which traces molecular gas at the boundary layers of star-forming and supernova remnant regions. The cloud edge is outlined along the high-latitude side of the North Polar Spur, a prominent X-ray/radio structure. Our distance estimate utilizes three-dimensional dust maps, the absorption of the soft-X-ray background, and hot gas tracers such as O vi; these place the cloud at a distance consistent with the Local Bubble's surface. Using high-latitude CO maps we note a small amount ( M H 2 ≈ 20 - 40 M ⊙ ) of CO-bright cold molecular gas, in contrast with the much larger estimate of the cloud's true molecular mass ( M H 2 ≈ 3.4 × 1 0 3 M ⊙ ), indicating that most of the cloud is CO dark. Combining observational data with novel analytical models and simulations, we predict that this cloud will photoevaporate in 5.7 Myr, placing key constraints on the role of stellar feedback in shaping the closest star-forming regions to the Sun.

Despite its scientific importance, the low-surface-brightness universe has yet to be fully explored due to various systematic uncertainties that affect the achievable surface-brightness limit. Reducing these uncertainties requires very accurate data processing. The dark-sky flat is a widely used calibration frame for accurate flat-field correction, generated by combining the sky background from science images. However, the night sky will likely contain complex local fluctuations, thus may still lead to photometric errors in data calibrated with dark-sky flats. To address this concern, we conduct mock observations with semi-realistic sky simulation data and evaluate observation strategies to mitigate the impact of the fluctuating sky background. Our experiments consider two representative sky conditions (clear and dirty) and perform intensive comparative analysis on two observation methods (offset and rolling). Our findings suggest that the rolling dithering method, which incorporates the operation of camera rotation into conventional dithering, can provide more accurate dark-sky flats. Finally, we discuss the broader implications of this method through additional experiments examining several factors that may affect the imaging quality of observational data.

We utilized Ly α radiative transfer calculations from H. Song et al. to investigate the properties of extended Ly α halos around star-forming galaxies in the Hubble Ultra Deep Field, observed by the Multi-Unit Spectroscopic Explorer. Expanding on the work of H. Song et al., which was limited to eight galaxies, we derived best-fit models for a significantly larger sample of 163 galaxies, which successfully reproduced both their Ly α spectra and surface brightness profiles (SBPs). These best-fit models suggest a broad medium distribution surrounding each galaxy, with low expanding velocities at large radii. This conclusion could not have been drawn from modeling either the spectrum or SBP alone, but only through simultaneous modeling of both. Our correlation analysis between observables and model parameters reveals that the spatial extent of Ly α halos is primarily determined by the extents of the medium and the source, while the spectral peak shift and full width at half maximum are governed mainly by optical depth, with the velocity structure of the medium playing a secondary yet nonnegligible role. The fact that various correlations derived from the full set of models and those from the best-fit subset can differ significantly highlights the complex and interdependent nature of Ly α radiative transfer. All model parameters interact to shape the observed Ly α features in a nontrivial way.

Observations have shown that the optical colors of Galactic cirrus clouds differ significantly from those of extragalactic sources; thus, they can be used to distinguish Galactic cirrus from extragalactic low surface brightness (LSB) features. To understand these properties, we calculate radiative transfer models in dust clouds, where photons are incident from the ambient interstellar medium (ISM). Dust clouds are modeled to mimic a turbulent medium using a fractional Brownian motion algorithm, resulting in a lognormal density distribution and a power-law power spectral density that are appropriate for the ISM. The results are compared with optical observations of cirrus clouds in the Stripe 82 region. The observed color–color ( g − r , r − i , and i − z ) diagrams of the cirrus clouds can be reproduced by scattered light if the interstellar radiation field (ISRF) of Mathis et al. (as updated by Draine) is modified, either by reducing the intensities in the i and z bands or by enhancing those in the g and r bands. Similar results can also be obtained by adjusting the scattering albedos at the corresponding wavelengths. This demonstrates that the color–color diagrams are effective not only for identifying extragalactic LSB features but also for studying the ISRF and the properties of interstellar dust.

Using data from the MIRIS Pa α Galactic Plane Survey (MIPAPS), we present a Pa α 1.87 μ m line image of the entire Galactic plane within the latitude range of −3° ≲ b ≲ +3°, revealing numerous Pa α features. Based on the MIPAPS Pa α image and the Wide-field Infrared Survey Explorer (WISE) H ii region catalog, we compile a catalog of 1489 Pa α emission-line sources in the Galactic plane within 90° ≤ ℓ ≤ 330°. By comparing our Pa α images with H α images constructed from the IPHAS and VPHAS+ survey data, we demonstrate the advantages of Pa α line observations. We identify 902 Pa α sources associated with H ii regions, and newly confirm 619 H ii region candidates as definitive H ii regions through Pa α or H α detections. We also identify 261 extended and 326 pointlike Pa α sources not included in the WISE catalog, most of which have H α counterparts in the IPHAS or VPHAS+ images. A search of the SIMBAD database indicates that these sources originate from diverse object types. By measuring Pa α and H α fluxes, we estimate the E ( B − V ) color excesses derived from extended emissions for 138 Pa α sources, showing good agreement with values obtained from spectrophotometry of ionizing stars in previous studies. Furthermore, we calculate total Lyman continuum luminosities for 42 Pa α sources, providing constraints on the distances to H ii regions and the spectral types of their ionizing stars. These results highlight the scientific potential of Pa α line observations and the benefits of combining multiple hydrogen recombination lines in exploring ionized regions.

This paper presents a hydrodynamic simulation that couples detailed non-local thermodynamic equilibrium (NLTE) calculations of the hydrogen and helium level populations to model the H$\alpha$ and He 10830 transmission spectra of the hot Jupiter HAT-P-32b. A Monte Carlo simulation is applied to calculate the number of Ly$\alpha$ resonance scatterings, which is the main process for populating H(2). In the examined parameter space, only the models with H/He $\geq$ 99.5/0.5, $(0.5 \sim 3.0)$ times the fiducial value of $F_{\rm XUV}$, $\beta_m = 0.16\sim 0.3$, can explain the H$\alpha$ and He 10830 lines simultaneously. We find a mass-loss rate of $\sim (1.0\sim 3.1) \times 10^{13}$ g s$^{-1}$, consistent with previous studies. Moreover, we find that the stellar Ly$\alpha$ flux should be as high as $4 \times 10^{5}$ erg cm$^{-2}$ s$^{-1}$, indicating high stellar activity during the observation epoch of the two absorption lines. Despite the fact that the metallicity in the lower atmosphere of HAT-P-32b may be super-solar, our simulations tentatively suggest it is close to solar in the upper atmosphere. The difference in metallicity between the lower and upper atmospheres is essential for future atmospheric characterisations.

Observations of metallic doublet emission lines, particularly Mg II 2796, 2803, provide crucial information for understanding galaxies and their circumgalactic medium. This study explores the effects of resonant scattering on the Mg II doublet lines and the stellar continuum in spherical and cylindrical geometries. Our findings show that under certain circumstances, resonance scattering can cause an increase in the doublet flux ratio and the escaping flux of the lines beyond what are expected in optically thin spherical media. As expected, the doublet ratio is consistently lower than the intrinsic ratio when the scattering medium is spherically symmetric and dusty. However, if the scattering medium has a disk shape, such as face-on disk galaxies, and is viewed face-on, the doublet ratio is predicted to be higher than two. These results may provide a valuable insight regarding the complexity of the shape and orientation of distant, spatially-unresolved galaxies. The importance of the continuum-pumped emission lines and expanding media is discussed to understand various observational aspects, including doublet flux ratios, which can be lower than 1.5 or higher than two, as well as symmetric or asymmetric line profiles. It is also discussed that the diffuse warm neutral medium would be an essential source of Mg II emission lines.

To understand the mechanism behind high- z Ly α nebulae, we simulate the scattering of Ly α in a H i halo about a central Ly α source. For the first time, we consider both smooth and clumpy distributions of halo gas, as well as a range of outflow speeds, total H i column densities, H i spatial concentrations, and central source galaxies (e.g., with Ly α line widths corresponding to those typical of active galactic nucleus or star-forming galaxies). We compute the spatial-frequency diffusion and the polarization of the Ly α photons scattered by atomic hydrogen. Our scattering-only model reproduces the typical size of Ly α nebulae (∼100 kpc) at total column densities N _H I ≥ 10 ^20 cm ^−2 and predicts a range of positive, flat, and negative polarization radial gradients. We also find two general classes of Ly α nebula morphologies: with and without bright cores. Cores are seen when N _H I is low, i.e., when the central source is directly visible, and are associated with a polarization jump, a steep increase in the polarization radial profile just outside the halo center. Of all the parameters tested in our smooth or clumpy medium model, N _H I dominates the trends. The radial behaviors of the Ly α surface brightness, spectral line shape, and polarization in the clumpy model with covering factor f _c ≳ 5 approach those of the smooth model at the same N _H I . A clumpy medium with high N _H I and low f _c ≲ 2 generates Ly α features via scattering that the smooth model cannot: a bright core, symmetric line profile, and polarization jump.

Description” (2022, PASP, 134, 094104) Woowon Byun , Yun-Kyeong Sheen, Kwang-Il Seon, Luis C. Ho, Joon Hyeop Lee, Hyunjin Jeong, Sang Chul Kim, Byeong-Gon Park, Yongseok Lee, Sang-Mok Cha, and Minjin Kim 1 Korea Astronomy and Space Science Institute, Daejeon 34055, Republic of Korea 2 University of Science and Technology, Korea, Daejeon 34113, Republic of Korea 3 Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, Peopleʼs Republic of China 4 Department of Astronomy, School of Physics, Peking University, Beijing 100871, Peopleʼs Republic of China 5 School of Space Research, Kyung Hee University, Yongin, Kyeonggi 17104, Republic of Korea 6 Department of Astronomy and Atmospheric Sciences, Kyungpook National University, Daegu 41566, Republic of Korea; mkim@knu.ac.kr Received 2023 March 22; published 2023 April 5

This paper investigates the number of scatterings a photon undergoes in random walks before escaping from a medium. The number of scatterings in random walk processes is commonly approximated as tau+tau(2) in the literature, where tau is the optical thickness measured from the center of the medium. However, it is found that this formula is not accurate. In this study, analytical solutions in sphere and slab geometries are derived for both optically thin and optically thick limits, assuming isotropic scattering. These solutions are verified using Monte Carlo simulations. In the optically thick limit, the number of scatterings is found to be 0.5 tau(2) and 1.5 tau(2) in a sphere and slab, respectively. In the optically thin limit, the number of scatterings is approximate to tau in a sphere and approximate to tau(1-gamma-ln tau+tau) in a slab, where gamma similar or equal to 0.57722 is the Euler-Mascheroni constant. Additionally, we present approximate formulas that reasonably reproduce the simulation results well in intermediate optical depths. These results are applicable to scattering processes that exhibit forward and backward symmetry, including both isotropic and Thomson scattering.