Large Area Survey Research Articles

The Spherical Fast Multipole Method (sFMM) for Gravitational Lensing Simulation

In this paper, we present a spherical Fast Multipole Method (sFMM) for ray-tracing simulation of gravitational lensing on a curved sky. The sFMM is a nontrivial extension of the Fast Multiple Method to sphere , and it can accurately solve the Poisson equation with time complexity of , where N is the number of particles. It is found that the time complexity of the sFMM is near O(N) and the computational accuracy can reach 10−10 in our test. In addition, compared with the Fast Spherical Harmonic Transform, the sFMM is not only faster but also more accurate, as it has the ability to reserve high-frequency components of the density field. These merits make the sFMM an optimum method to simulate the gravitational lensing on a curved sky, which is the case for upcoming large-area sky surveys, such as the Vera Rubin Observatory and the China Space Station Telescope.

The Stellar Mass–Black Hole Mass Relation at z ∼ 2 down to Determined by HETDEX

We investigate the stellar mass–black hole mass () relation with type 1 active galactic nuclei (AGNs) down to , corresponding to a ≃ −21 absolute magnitude in rest-frame ultraviolet, at z = 2–2.5. Exploiting the deep and large-area spectroscopic survey of the Hobby–Eberly Telescope Dark Energy Experiment (HETDEX), we identify 66 type 1 AGNs with ranging from 107–1010 M ⊙ that are measured with single-epoch virial method using C iv emission lines detected in the HETDEX spectra. of the host galaxies are estimated from optical to near-infrared photometric data taken with Spitzer, the Wide-field Infrared Survey Explorer, and ground-based 4–8 m class telescopes by CIGALE spectral energy distribution (SED) fitting. We further assess the validity of SED fitting in two cases by host-nuclear decomposition performed through surface brightness profile fitting on spatially resolved host galaxies with the James Webb Space Telescope/NIRCam CEERS data. We obtain the relation covering the unexplored low-mass ranges of , and conduct forward modeling to fully account for the selection biases and observational uncertainties. The intrinsic relation at z ∼ 2 has a moderate positive offset of 0.52 ± 0.14 dex from the local relation, suggestive of more efficient black hole growth at higher redshift even in the low-mass regime of . Our relation is inconsistent with the suppression at the low- regime predicted by recent hydrodynamic simulations at a 98% confidence level, suggesting that feedback in the low-mass systems may be weaker than those produced in hydrodynamic simulations.

Radio AGN selection and characterization in three Deep-Drilling Fields of the Vera C. Rubin Observatory Legacy Survey of Space and Time

ABSTRACT The Australia Telescope Large Area Survey (ATLAS) and the VLA survey in the XMM-LSS/VIDEO deep field provide deep (≈15 $\mu$ Jy beam−1) and high-resolution (≈4.5–8 arcsec) radio coverage of the three XMM-SERVS fields (W-CDF-S, ELAIS-S1, and XMM-LSS). These data cover a total sky area of 11.3 deg2 and contain ≈11 000 radio components. Furthermore, about 3 deg2 of the XMM-LSS field also has deeper MIGHTEE data that achieve a median RMS of 5.6 $\mu$ Jy beam−1 and detect more than 20 000 radio sources. We analyse all these radio data and find source counterparts at other wavebands utilizing deep optical and infrared (IR) surveys. The nature of these radio sources is studied using radio-band properties (spectral slope and morphology) and the IR–radio correlation. Radio AGNs are selected and compared with those selected using other methods (e.g. X-ray). We found 1656 new AGNs that were not selected using X-ray and/or MIR methods. We constrain the FIR-to-UV SEDs of radio AGNs using cigale and investigate the dependence of radio AGN fraction upon galaxy stellar mass and star formation rate.

Identifying Active Galactic Nuclei at z ∼ 3 from the HETDEX Survey Using Machine Learning

We used data from the Hobby–Eberly Telescope Dark Energy Experiment (HETDEX) to study the incidence of AGN in continuum-selected galaxies at z ∼ 3. From optical and infrared imaging in the 24 deg2 Spitzer HETDEX Exploratory Large Area survey, we constructed a sample of photometric-redshift selected z ∼ 3 galaxies. We extracted HETDEX spectra at the position of 716 of these sources and used machine-learning methods to identify those which exhibited AGN-like features. The dimensionality of the spectra was reduced using an autoencoder, and the latent space was visualized through t-distributed stochastic neighbor embedding. Gaussian mixture models were employed to cluster the encoded data and a labeled data set was used to label each cluster as either AGN, stars, high-redshift galaxies, or low-redshift galaxies. Our photometric redshift (photoz) sample was labeled with an estimated 92% overall accuracy, an AGN accuracy of 83%, and an AGN contamination of 5%. The number of identified AGN was used to measure an AGN fraction for different magnitude bins. The ultraviolet (UV) absolute magnitude where the AGN fraction reaches 50% is M UV = −23.8. When combined with results in the literature, our measurements of AGN fraction imply that the bright end of the galaxy luminosity function exhibits a power law rather than exponential decline, with a relatively shallow faint-end slope for the z ∼ 3 AGN luminosity function.

Detecting Long-period Variability in the SDSS Stripe 82 Standards Catalog

We report the results of a search for long-period (100 < P < 600 days) periodic variability in the SDSS Stripe 82 standards catalog. The SDSS coverage of Stripe 82 enables such a search because there are on average 20 observations per band in ugriz bands for about one million sources, collected over about 6 yr, with a faint limit of r ∼ 22 mag and precisely calibrated 1%–2% photometry. We calculated the periods of variable source candidates in this sample using the Lomb–Scargle periodogram and considered the three highest periodogram peaks in each of the gri filters as relevant. Only those sources with gri periods consistent within 0.1% were later studied. We use the Kuiper statistic to ensure uniform distribution of data points in phased light curves. We present five sources with the spectra consistent with quasar spectra and plausible periodic variability. This SDSS-based search bodes well for future sensitive large-area surveys, such as the Rubin Observatory Legacy Survey of Space and Time, which, due to its larger sky coverage (about a factor of 60) and improved sensitivity (∼2 mag), will be more powerful for finding such sources.

Identifying anomalous radio sources in the Evolutionary Map of the Universe Pilot Survey using a complexity-based approach

ABSTRACTThe Evolutionary Map of the Universe (EMU) large-area radio continuum survey will detect tens of millions of radio galaxies, giving an opportunity for the detection of previously unknown classes of objects. To maximize the scientific value and make new discoveries, the analysis of these data will need to go beyond simple visual inspection. We propose the coarse-grained complexity, a simple scalar quantity relating to the minimum description length of an image that can be used to identify unusual structures. The complexity can be computed without reference to the broader sample or existing catalogue data, making the computation efficient on new surveys at very large scales (such as the full EMU survey). We apply our coarse-grained complexity measure to data from the EMU Pilot Survey to detect and confirm anomalous objects in this data set and produce an anomaly catalogue. Rather than work with existing catalogue data using a specific source detection algorithm, we perform a blind scan of the area, computing the complexity using a sliding square aperture. The effectiveness of the complexity measure for identifying anomalous objects is evaluated using crowd-sourced labels generated via the Zooniverse.org platform. We find that the complexity scan identifies unusual sources, such as odd radio circles, by partitioning on complexity. We achieve partitions where 5 per cent of the data is estimated to be 86 per cent complete, and 0.5 per cent is estimated to be 94 per cent pure, with respect to anomalies and use this to produce an anomaly catalogue.

Microstructure Turbulence Measurement in the Northern South China Sea from a Long-Range Hybrid AUV.

This study describes the development of a long-range hybrid autonomous underwater vehicle (AUV) for ocean turbulence measurement. It is a unique instrument, combining the characteristics of the conventional AUV and the buoyancy-driven glider, with a variety of flexible motion modes, such as cruise mode, glider mode, drift mode, and combination of multiple motion modes. The hybrid AUV was used for continuous turbulence measurement in the continental slope of the northern South China Sea in 2020. A total of ten continuous profiles were completed covering a horizontal span of 25 Km and a depth of 200 m. The hybrid AUV was operated in the combined glider and cruise mode. The hybrid AUV's flight performance was stable and satisfied the requirement for turbulence observation. The measured velocity shears from both probes were in good agreement, and the noise-reduced shear spectra were in excellent agreement with the Nasmyth spectrum. The water column in the study area was highly stratified, with a thick thermocline. The dissipation rate (ε) varied from 1.41 × 10-10 to 4.18 × 10-7 W·kg-1. In the surface mixed layer, high values of ε (10-9∼10-8 W·kg-1) were observed toward the water surface. In the thermocline, ε was 10-9.5∼10-9 W·kg-1, which was smaller than the level of the surface mixed layer. This result was mainly because of the strong "barrier"-like thermocline, which damped the transmission of wind and heat energy from the surface mixed layer to the deep layer. Overall, this study demonstrates the utility of hybrid AUVs for collecting oceanic turbulence measurements. They are a powerful addition to traditional turbulence instruments, as they make it possible to survey large areas to obtain high-quality and high-resolution data in both vertical and horizontal directions over long durations.

CMZoom III: Spectral line data release

ABSTRACT We present an overview and data release of the spectral line component of the SMA Large Program, CMZoom. CMZoom observed 12CO (2–1), 13CO (2–1), and C18O (2–1), three transitions of H2CO, several transitions of CH3OH, two transitions of OCS, and single transitions of SiO and SO within gas above a column density of N(H2) ≥ 1023 cm−2 in the Central Molecular Zone (CMZ; inner few hundred pc of the Galaxy). We extract spectra from all compact 1.3 mm CMZoom continuum sources and fit line profiles to the spectra. We use the fit results from the H2CO 3(0, 3)–2(0, 2) transition to determine the source kinematic properties. We find ∼90 per cent of the total mass of CMZoom sources have reliable kinematics. Only four compact continuum sources are formally self-gravitating. The remainder are consistent with being in hydrostatic equilibrium assuming that they are confined by the high external pressure in the CMZ. We find only two convincing proto-stellar outflows, ruling out a previously undetected population of very massive, actively accreting YSOs with strong outflows. Finally, despite having sufficient sensitivity and resolution to detect high-velocity compact clouds (HVCCs), which have been claimed as evidence for intermediate mass black holes interacting with molecular gas clouds, we find no such objects across the large survey area.

Event Horizon and Environs (ETHER): A Curated Database for EHT and ngEHT Targets and Science

The next generation Event Horizon Telescope (ngEHT) will observe multiple supermassive black hole (SMBH) candidates down to a few tens of mJy, and profoundly transform our understanding of the local SMBH population. Given the impossibility of large-area high-resolution millimeter surveys, multi-frequency spectral energy densities (SEDs), and models are required to both identify source samples tailored to specific science goals, and to predict the feasibility of detection of individual interesting sources. Here, we present the Event Horizon and Environs (ETHER) source and SED model database whose primary use is to enable the selection and optimization of targets for EHT and ngEHT science. The living ETHER database currently consolidates 1.6 million black hole mass estimates, ∼15,500 milliarcsec-scale radio fluxes, ∼14,000 hard X-ray fluxes (expected to grow by factor ≳40 with the eROSITA data release) and SED information as obtained from catalogs and database queries, the literature, and our own new observations. Jet and accretion flow models are fit to individual SEDs in an automated way in order to predict the ngEHT observable fluxes from the jet base and accretion inflow. The database can be filtered by parameters or cross matched to a user source list, with the automated SED fitting models optionally fine tuned by the user. We have identified an initial ngEHT ‘gold sample’ for jet base studies and potentially black hole shadows; this sample will grow significantly in the coming years. While the ngEHT requires and will best exploit the ETHER database, six (eleven) ETHER sources have already been observed (scheduled) with the EHT in 2022 (2023), and the database has wide ranging applications in galaxy and black hole mass evolution studies.

The Rapid ASKAP Continuum Survey IV: continuum imaging at 1367.5 MHz and the first data release of RACS-mid

Abstract The Australian SKA Pathfinder (ASKAP) is being used to undertake a campaign to rapidly survey the sky in three frequency bands across its operational spectral range. The first pass of the Rapid ASKAP Continuum Survey (RACS) at 887.5 MHz in the low band has already been completed, with images, visibility datasets, and catalogues made available to the wider astronomical community through the CSIRO ASKAP Science Data Archive (CASDA). This work presents details of the second observing pass in the mid band at 1367.5 MHz, RACS-mid, and associated data release comprising images and visibility datasets covering the whole sky south of $\delta_{\text{J2000}}=+49^\circ$ . This data release incorporates selective peeling to reduce artefacts around bright sources, as well as accurately modelled primary beam responses. The Stokes I images reach a median noise of 198 $\mu$ Jy PSF $^{-1}$ with a declination-dependent angular resolution of 8.1–47.5 arcsec that fills a niche in the existing ecosystem of large-area astronomical surveys. We also supply Stokes V images after application of a widefield leakage correction, with a median noise of 165 $\mu$ Jy PSF $^{-1}$ . We find the residual leakage of Stokes I into V to be $\lesssim 0.9$ – $2.4$ % over the survey. This initial RACS-mid data release will be complemented by a future release comprising catalogues of the survey region. As with other RACS data releases, data products from this release will be made available through CASDA.

Lyman-α at cosmic noon I: Lyα spectral type selection ofz∼ 2 – 3 Lyman break galaxies with broadband imaging

AbstractHigh-redshift Lyman break galaxies (LBGs) are efficiently selected in deep images using as few as three broadband filters, and have been shown to have multiple intrinsic and small- to large-scale environmental properties related to Lyman-$\alpha$. In this paper we demonstrate a statistical relationship between net Lyman-$\alpha$equivalent width (net Ly$\alpha$EW) and the optical broadband photometric properties of LBGs at$z\sim2$. We show that LBGs with the strongest net Ly$\alpha$EW in absorption (aLBGs) and strongest net Ly$\alpha$EW in emission (eLBGs) separate into overlapping but discrete distributions in$(U_n-\mathcal{R})$colour and$\mathcal{R}$-band magnitude space, and use this segregation behaviour to determine photometric selection criteria by which sub-samples with a desired Ly$\alpha$spectral type can be selected using data from as few as three broadband optical filters. We propose application of our result to current and future large-area and all-sky photometric surveys that will select hundreds of millions of LBGs across many hundreds to thousands of Mpc, and for which spectroscopic follow-up to obtain Ly$\alpha$spectral information is prohibitive. To this end, we use spectrophotometry of composite spectra derived from a sample of 798 LBGs divided into quartiles on the basis of net Ly$\alpha$EW to calculate selection criteria for the isolation of Ly$\alpha$-absorbing and Ly$\alpha$-emitting populations of$z\sim3$LBGs usingugribroadband photometric data from the Vera Rubin Observatory Legacy Survey of Space and Time (LSST).

Geophysical and Geochemical Proxies of Neolithic Sites from Thessaly: A Comparative Study on the Potential of Soil Magnetic Susceptibility and Phosphate Analyses for Minimally Invasive Location and Interpretation of Buried Features

This paper presents the results of a study exploring the potential of magnetic-susceptibility and phosphate soil analyses to locate and characterize buried Neolithic settlements in Thessaly, Greece. Using the preliminary results of large-area magnetometer surveys, soil samples were collected at three well-known sites along exploratory lines and augers targeting the locations of possible features of interest, including habitational structures and enclosures. The results demonstrated the capability of these analyses to detect the sites, outline hotspots and better interpret the features identified in the magnetometer results.

A global gridded municipal water withdrawal estimation method using aggregated data and artificial neural network.

Municipal water withdrawal (MWW) information is of great significance for water supply planning, including water supply pipeline networks planning, optimization and management. Currently most MWW data are reported as spatially aggregated over large-area survey regions or even lack of data, which is unable to meet the growing demand for spatially detailed data in many applications. In this paper, six different models are constructed and evaluated in estimatingglobal MWW using aggregated MWW data and gridded raster covariates. Among the models, the artificial neural network-based indirect model (NNM) shows the best accuracy with higher R2and lower NMAE and NRMSE in different spatial scales. The estimates achieved from the NNMmodel are consistent with census and survey data, and outperforms the existing global gridded MWW dataset. At last, the NNMmodel is applied to mapping global gridded MWW for the year 2015 at 0.1 × 0.1° resolution.The proposed method can be applied to a wider aggregated output learning problemand the high-resolution global gridded MWW data can be used in hydrological models and water resources management.

ATLASGAL: methanol masers at 3 mm

AbstractWe analyzed the 3 mm wavelength spectral line survey of 408 clumps from the APEX telescope large area survey of the Galaxy, focusing on the methanol maser transitions. The main goals of this study are (1) to search for new methanol masers, (2) to statistically study the relationship between class I masers and shock tracers, (3) to study the properties between methanol masers and their host clumps, also as a function of their evolutionary stages and, (4) to better constrain the physical conditions using multiple co-spatial line pairs.

H i HOD. I. The Halo Occupation Distribution of H i Galaxies

The next generation of galaxy surveys will provide more precise measurements of galaxy clustering than have previously been possible. The 21 cm radio signals that are emitted from neutral atomic hydrogen (H i) gas will be detected by large-area radio surveys such as the Widefield Australian Square Kilometre Array (SKA) Pathfinder L-band Legacy All-sky Blind Survey and SKA, and deliver galaxy positions and velocities that can be used to measure galaxy clustering statistics. However, to harness this information to improve our cosmological understanding and learn about the physics of dark matter and dark energy, we need to accurately model the manner in which galaxies detected in H i trace the underlying matter distribution of the universe. For this purpose, we develop a new H i-based halo occupation distribution (HOD) model, which makes predictions for the number of galaxies present in dark matter halos conditional on their H i mass. The parameterized HOD model is fit and validated using the Dark Sage semi-analytic model, where we show that the HOD parameters can be modeled by simple linear and quadratic functions of the H i mass. However, we also find that the clustering predicted by the HOD depends sensitively on the radial distributions of the H i galaxies within their host dark matter halos, which does not follow the Navarro–Frenk–White profile in the Dark Sage simulation. As such, this work enables—for the first time—a simple prescription for placing galaxies of different H i masses within dark matter halos in a way that is able to reproduce the H i mass-dependent galaxy clustering and H i mass function simultaneously and without requiring knowledge of the optical properties of the galaxies. Further efforts are required to demonstrate that this model can be used to produce large ensembles of mock galaxy catalogs for upcoming surveys.

Potential Signature of Population III Pair-instability Supernova Ejecta in the BLR Gas of the Most Distant Quasar at z = 7.54* *At the time of writing this paper, the highest redshift record has been updated to z = 7.642, ULAS J0313-1806, as reported by Wang et al. ().

The search for Population III stars has fascinated and eluded astrophysicists for decades. One promising place for capturing evidence of their presence must be high-redshift objects; signatures should be recorded in their characteristic chemical abundances. We deduce the Fe and Mg abundances of the broadline region (BLR) from the intensities of ultraviolet Mg ii and Fe ii emission lines in the near-infrared spectrum of UKIDSS Large Area Survey (ULAS) J1342+0928 at z = 7.54, by advancing our novel flux-to-abundance conversion method developed for quasars up to z ∼ 3. We find that the BLR of this quasar is extremely enriched, by a factor of 20 relative to the solar Fe abundance, together with a very low Mg/Fe abundance ratio: [Fe/H] = +1.36 ± 0.19 and [Mg/Fe] =−1.11 ± 0.12, only 700 million years after the Big Bang. We conclude that such an unusual abundance feature cannot be explained by the standard view of chemical evolution that considers only the contributions from canonical supernovae. While there remains uncertainty in the high-mass end of the Population III initial mass function, here we propose that the larger amount of iron in ULAS J1342+0928 was supplied by a pair-instability supernova (PISN) caused by the explosion of a massive Population III star in the high-mass end of the possible range of 150–300 M ⊙ . Chemical evolution models based on initial PISN enrichment well explain the trend in [Mg/Fe]-z all the way from z < 3 to z = 7.54. We predict that stars with very low [Mg/Fe] at all metallicities are hidden in the galaxy, and they will be efficiently discovered by ongoing new-generation photometric surveys.

COSMOS2020: Manifold learning to estimate physical parameters in large galaxy surveys

We present a novel method for estimating galaxy physical properties from spectral energy distributions (SEDs) as an alternative to template fitting techniques and based on self-organizing maps (SOMs) to learn the high-dimensional manifold of a photometric galaxy catalog. The method has previously been tested with hydrodynamical simulations in Davidzon et al. (2019, MNRAS, 489, 4817), however, here it is applied to real data for the first time. It is crucial for its implementation to build the SOM with a high-quality panchromatic data set, thus we selected “COSMOS2020” galaxy catalog for this purpose. After the training and calibration steps with COSMOS2020, other galaxies can be processed through SOMs to obtain an estimate of their stellar mass and star formation rate (SFR). Both quantities resulted in a good agreement with independent measurements derived from more extended photometric baseline and, in addition, their combination (i.e., the SFR vs. stellar mass diagram) shows a main sequence of star-forming galaxies that is consistent with the findings of previous studies. We discuss the advantages of this method compared to traditional SED fitting, highlighting the impact of replacing the usual synthetic templates with a collection of empirical SEDs built by the SOM in a “data-driven” way. Such an approach also allows, even for extremely large data sets, for an efficient visual inspection to identify photometric errors or peculiar galaxy types. While also considering the computational speed of this new estimator, we argue that it will play a valuable role in the analysis of oncoming large-area surveys such as Euclid of the Legacy Survey of Space and Time at the Vera C. Rubin Telescope.

Searching for nascent planetary nebulae: OHPNe candidates in the SPLASH survey

ABSTRACT The evolution of asymptotic giant branch stars from the spherical symmetry into the diverse shapes of planetary nebulae (PNe) is a topic of intensive research. Young PNe provide a unique opportunity to characterize the onset of this transitional phase. In particular, OH maser-emitting PNe (OHPNe) are considered nascent PNe. In fact, only six OHPNe have been confirmed to date. In order to identify and characterize more OHPNe, we processed the unpublished continuum data of the interferometric follow-up of the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH). We then matched the interferometric positions of OH maser and radio continuum emission, considering the latter as a possible tracer of free–free emission from photoionized gas, characteristic of PNe. We report eight objects with a positive coincidence, four of which are classified as candidate OHPNe here for the first time (IRAS 16372–4808, IRAS 17494–2645, IRAS 18019–2216, and OH 341.6811+00.2634). Available evidence strongly indicates that they are evolved stars, while the comparison with confirmed OHPNe indicates that they are likely to be PNe. Their final confirmation as bona fide PNe, however, requires optical/infrared spectroscopy. The obtained spectral indices of the radio continuum emission (between ≃0.4–1.3) are consistent with partially optically thick free–free emission from photoionized gas. Also, they cluster in the same region of a WISE colour–colour diagram as that of the confirmed OHPNe ($9.5\lesssim[3.4]{-}[22]\lesssim13.5$, and $4.0\lesssim[4.6]{-}[12] \lesssim7.0$), thus this diagram could help to identify more OHPNe candidates in the future.

VOXEL-BASED POINT CLOUD SEGMENTATION AND BUILDING DETECTION

Abstract. In the recent years, point cloud technologies, such as Unmanned Aerial Vehicles (UAV), Terrestrial Laser Scanners (TLS), Aerial Laser Scanners (ALS), let alone Mobile Mapping Systems (MMS), have come into the focus of attention and have been a subject of considerable public concern in mapping. Thanks to these new techniques, experts can survey large areas with sufficient and homogenous accuracy with high resolution. It comes from this that there are several areas where the point clouds can be used. One of the possible applications of point clouds is updating land registry maps. Many countries worldwide face the issue that a significant part of their large-scale land registry maps are outdated and inaccurate. One of these countries is Hungary, where more than eighty percent of digital cadastre maps were digitised using analogous maps in a scale range of 1:1000 – 1:4000. In this paper, a novel processing queue is presented to find the footprints of the building. Our solution is based on primarily well-known algorithms (RANSAC, DBSCAN) implemented in open-source Python packages. An automated flow was developed, composed of simple processing steps, to cut the point cloud into wall and roof segments and vectorise the wall points under roofs into building footprints. The algorithms and Python programs were tested in villages where detached houses are typical. Tests were made on three study areas in Hungary and we achieved well-promising results.

ATLASGAL-selected massive clumps in the inner Galaxy

Context. While high-mass stars are key drivers of the evolution of galaxies, how they form and interact with the surrounding gas is still not fully understood. To shed light on this overarching issue, we have been performing a multitelescope campaign to observe carbon species in ~100 massive clumps (“Top100”) identified by the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). Our targets constitute a representative sample of high-mass star-forming regions with a wide range of masses (~20−104 M⊙), bolometric luminosities (~60−106 L⊙), and evolutionary stages (70 µm weak, infrared weak, infrared bright, and H II region sources). Aims. We aim to probe the physical conditions of [C I]-traced gas in the Top100 sample based on Atacama Pathfinder Experiment (APEX) [C I] 492 GHz observations. This is the first of a series of papers presenting results from our [C II] and [C I] campaign. Methods. To determine physical properties such as the temperature, density, and column density, we combined the obtained [C I] 492 GHz spectra with APEX observations of [C I] 809 GHz and 13CO(2−1), as well as with other multiwavelength data, and employed both local thermodynamic equilibrium (LTE) and non-LTE methods. Results. Our 98 sources are clearly detected in [C I] 492 GHz emission, and the observed integrated intensities and line widths tend to increase toward evolved stages of star formation. In addition to these “main” components that are associated with the Top100 sample, 41 emission and two absorption features are identified by their velocities toward 28 and two lines of sight, respectively, as “secondary” components. The secondary components have systematically smaller integrated intensities and line widths than the main components. We found that [C I] 492 GHz and 13CO(2–1) are well correlated with the 13CO(2–1)-to-[C I] 492 GHz integrated intensity ratio varying from 0.2 to 5.3. In addition, we derived the H2-to-[C I] conversion factor, X(C I), by dividing 870 µm-based H2 column densities by the observed [C I] 492 GHz integrated intensities and found that X(C I) (in units of cm−2 (K km s−1)−1) ranges from 2.3 × 1020 to 1.3 × 1022 with a median of 1.7 × 1021. In contrast to the strong correlation with 13CO(2–1), [C I] 492GHz has a scattered relation with the 870 µm-traced molecular gas. Finally, we performed LTE and non-LTE analyses of the [C I] 492 GHz and 809 GHz data for a subset of the Top100 sample and inferred that [C I] emission likely originates from warm (kinetic temperature ≳60 K), optically thin (opacity &lt;0.5), and highly pressurized (thermal pressure ~(2–5000) × 105 K cm−3) regions. Conclusions. Our [C I] 492 GHz survey demonstrates that [C I] 492 GHz is prevalent in the inner Galaxy and traces not only massive clumps, but also non-star-forming relatively diffuse gas. The strong correlation between [C I] 492 GHz and 13CO(2–1) indicates that they probe similar conditions, and the observed variations in the intensity ratio of the two transitions likely reflect local conditions of the interstellar medium. The scattered relation between [C I] 492 GHz and the 870 µm-based molecular gas, on the other hand, implies that [C I] 492 GHz and 13CO(2–1) probe warm molecular gas that surrounds denser and colder clumps traced by 870 µm emission.