Galactic Plane Survey Research Articles

The G332 molecular cloud ring: I. Morphology and physical characteristics

We present a morphological and physical analysis of a Giant Molecular Cloud (GMC) using the carbon monoxide isotopologues ($^{12}$CO, $^{13}$CO, C$^{18}$O $^{3}P_{2}\rightarrow$ $^{3}P_{1}$) survey of the Galactic Plane (Mopra CO Southern Galactic Plane Survey), supplemented with neutral carbon maps from the HEAT telescope in Antarctica. The giant molecular cloud structure (hereinafter the ring) covers the sky region $332^\circ$ < $\ell$ < $333^\circ$ and $\mathit{b}$ = $\pm 0.5^\circ$ (hereinafter the G332 region). The mass of the ring and its distance are determined to be respectively ~2$\times10^{5}\mathrm{M_{\odot}}$ and ~3.7 kpc from Sun. The dark molecular gas fraction, estimated from the $^{13}$CO and [CI] lines, is $\sim17\%$ for a CO T$_{\mathrm{ex}}$ between [10,20 K]. Comparing the [CI] integrated intensity and N(H$_{2}$) traced by $^{13}$CO and $^{12}$CO, we define an X$\mathrm{_{CI}^{809}}$ factor, analogous to the usual X$_{\mathrm{co}}$, through the [CI] line. X$\mathrm{_{CI}^{809}}$ ranges between [1.8,2.0]$\times10^{21}\mathrm{cm}^{-2}\mathrm{K}^{-1}\mathrm{km}^{-1}\mathrm{s}$. We examined local variation in X$_{\mathrm{co}}$ and T$_{\mathrm{ex}}$ across the cloud, and find in regions where the star formation activity is not in an advanced state, an increase in the mean and dispersion of the X$_{\mathrm{co}}$ factor as the excitation temperature decreases. We present a catalogue of C$^{18}$O clumps within the cloud. The star formation (SF) activity ongoing in the cloud shows a correlation with T$_{\mathrm{ex}}$, [CI] and CO emissions, and anti-correlation with X$_{\mathrm{co}}$, suggesting a North-South spatial gradient in the SF activity. We propose a method to disentangle dust emission across the Galaxy, using HI and $^{13}$CO data. We describe Virtual Reality (VR) and Augmented Reality (AR) data visualisation techniques for the analysis of radio astronomy data.

Cosmic ray origin: Supernova remnants through the electromagnetic spectrum

Supernova Remnants have long been considered as unique candidates for cosmic-ray sources. Recent observations of several SNRs in X-rays and TeV gamma-rays will help in solving the problem of the origin of cosmic rays and are key to understanding the mechanism of particle acceleration at a propagating shock wave. The observation results of Galactic shell-type supernova remnants at different evolution stages Cas A, Tycho's SNR, γCygni SNR, IC 443 and G166.0+4.3 by the SHALON mirror Cherenkov telescope are presented. For each SNR the SHALON observation results are given with its spectral energy distribution compared with other experimental data and images by SHALON together with data from X-ray by Chandra and radio-data by Canadian Galactic Plane Survey DRAO (CGPS). The comparison of the source's morphology in different energy bands could reveal its essential features as a forward and reverse shock or the location of swept out dense molecular cloud. The experimental data presented here have confirmed the prediction of the theory about the hadronic generation mechanism of very high energy 800 GeV-100 TeV gamma-rays in Tycho's SNR, Cas A and IC 443. Also the collected experimental data help to make clear the origin of TeV gamma-ray emission in the SNRs like γCygni SNR and G166.0+4.3.

Caesar source finder: Recent developments and testing

AbstractA new era in radio astronomy will begin with the upcoming large-scale surveys planned at the Australian Square Kilometre Array Pathfinder (ASKAP). ASKAP started its Early Science programme in October 2017 and several target fields were observed during the array commissioning phase. The Scorpio field was the first observed in the Galactic Plane in Band 1 (792–1 032 MHz) using 15 commissioned antennas. The achieved sensitivity and large field of view already allow to discover new sources and survey thousands of existing ones with improved precision with respect to previous surveys. Data analysis is currently ongoing to deliver the first source catalogue. Given the increased scale of the data, source extraction and characterisation, even in this Early Science phase, have to be carried out in a mostly automated way. This process presents significant challenges due to the presence of extended objects and diffuse emission close to the Galactic Plane.In this context, we have extended and optimised a novel source finding tool, named Caesar, to allow extraction of both compact and extended sources from radio maps. A number of developments have been done driven by the analysis of the Scorpio map and in view of the future ASKAP Galactic Plane survey. The main goals are the improvement of algorithm performances and scalability as well as of software maintainability and usability within the radio community. In this paper, we present the current status of Caesar and report a first systematic characterisation of its performance for both compact and extended sources using simulated maps. Future prospects are discussed in the light of the obtained results.

A search for hypercompact H iiregions in the Galactic Plane

We have carried out the largest and most unbiased search for hypercompact (HC) HII regions. Our method combines four interferometric radio continuum surveys (THOR, CORNISH, MAGPIS and White2005) with far-infrared and sub-mm Galactic Plane surveys to identify embedded HII regions with positive spectral indices. 120 positive spectrum HII regions have been identified from a total sample of 534 positive spectral index radio sources. None of these HII regions, including the known HCHII regions recovered in our search, fulfills the canonical definition of an HCHII region at 5 GHz. We suggest that the current canonical definition of HCHII regions is not accurate and should be revised to include a hierarchical structure of ionized gas that results in an extended morphology at 5 GHz. Correlating our search with known ultracompact (UC) HII region surveys, we find that roughly half of detected UCHII regions have positive spectral indices, instead of more commonly assumed flat and optically thin spectra. This implies a mix of optically thin and thick emission and has important implications for previous analyses which have so far assumed optically thin emission for these objects. Positive spectrum HII regions are statistically more luminous and possess higher Lyman continuum fluxes than HII regions with flat or negative indices. Positive spectrum HII regions are thus more likely to be associated with more luminous and massive stars. No differences are found in clump mass, linear diameter or luminosity-to-mass ratio between positive spectrum and non-positive spectrum HII regions.

Natal molecular cloud of SNR Kes 41. Complete characterisation

Using high-resolution data of the 12CO and 13CO (J = 1–0) line emission from the Mopra Southern Galactic Plane CO Survey in conjunction with neutral hydrogen observations from the Southern Galactic Plane Survey (SGPS) and mid-infrared Spitzer data, we have explored the large-scale environment of the supernova remnant Kes 41. On the basis of these data, we identified for the first time the parent cloud of Kes 41 in its whole extension and surveyed the HII regions, masers, and the population of massive young stellar objects in the cloud. The whole unveiled giant cloud, located at the kinematic distance of 12.0 ± 3.6 kpc, whose average total mass and size are ~10–30 × 105 M⊙ and ~ 26′, also shines in γ-rays, as revealed by the Large Area Telescope on board the Fermi satellite. We determined a high average proton density ~500–1000 cm−3 in the large molecular complex, of which protons from the neutral atomic and ionised gases comprise only ~15%.

A catalogue of Galactic supernova remnants in the far-infrared: revealing ejecta dust in pulsar wind nebulae

We search for far-infrared (FIR) counterparts of known supernova remnants (SNRs) in the Galactic plane (10 deg <| l |< 60 deg) at 70-500 micron using the Herschel Infrared Galactic Plane Survey (Hi-GAL). Of 71 sources studied, we find that 29 (41 per cent) SNRs have a clear FIR detection of dust emission associated with the SNR. Dust from 8 of these is in the central region, and 4 indicate pulsar wind nebulae (PWNe) heated ejecta dust. A further 23 have dust emission in the outer shell structures which is potentially related to swept up material. Many Galactic SNe have dust signatures but we are biased towards detecting ejecta dust in young remnants and those with a heating source (shock or PWN). We estimate the dust temperature and mass contained within three PWNe, G11.2-0.3, G21.5-0.9, and G29.7-0.3 using modified blackbody fits. To more rigorously analyse the dust properties at various temperatures and dust emissivity index beta, we use point process mapping (PPMAP). We find significant quantities of cool dust (at 20-40 K) with dust masses of Md = 0.34 +/- 0.14 solar mass, Md = 0.29 +/- 0.08 solar mass, and Md = 0.51 +/- 0.13 solar mass for G11.2-0.3, G21.5-0.9, and G29.7-0.3 respectively. We derive the dust emissivity index for the PWN ejecta dust in G21.5-0.3 to be beta = 1.4 +/- 0.5 compared to dust in the surrounding medium where beta = 1.8 +/- 0.1.

An Optical Study of Two VY Sculptoris-type Cataclysmic Binary Stars: V704 And and RX J2338+431

Abstract We report observations of the known cataclysmic variable star (CV) V704 And, and also confirm that the optical counterpart of the ROSAT Galactic Plane Survey source RX J2338+431 is a heretofore-neglected CV. Photometric and spectroscopic observations from MDM Observatory show both systems to be nova-like variables that exhibit dips of 4–5 mag from their mean brightnesses, establishing them as members of the VY Scl subclass. From high-state emission-line radial velocities, we determine orbital periods of 0.151424(3) days (3.63 hr) for V704 And and 0.130400 (1) days (3.13 hr) for RX J2338+431. In V704 And, we find that the Hα emission-line measures cluster into distinct regions on a plot of equivalent width versus full width at half-maximum, which evidently correspond to high, intermediate, and low photometric states. This allows us to assign spectra to photometric states when contemporaneous photometry is not available, an apparently novel method that may be useful in studies of other novalikes. Our low-state spectra of RX J2338+431 show features of an M-type secondary star, from which we estimate a distance of 890 ± 200 pc, in good agreement with the Gaia DR2 parallax.

MIRIS Paα Galactic Plane Survey. I. Comparison with IPHAS Hα in ℓ = 96°–116°

Abstract The Multipurpose InfraRed Imaging System (MIRIS) performed the MIRIS Paα Galactic Plane Survey (MIPAPS), which covers the entire Galactic plane within the latitude range of at Paα (1.87 μm). We present the first result of the MIPAPS data extracted from the longitude range of –116.°3 and demonstrate the quality and scientific potential of the data by comparing them with Hα maps obtained from the INT Photometric Hα Survey (IPHAS) data. We newly identify 90 H ii region candidates in the WISE H ii region catalog as definite H ii regions by detecting the Paα and/or Hα recombination lines, out of which 53 H ii regions are detected at Paα. We also report the detection of an additional 29 extended and 18 point-like sources at Paα. We estimate the E(B−V) color excesses and total Lyman continuum luminosities for H ii regions by combining the MIPAPS Paα and IPHAS Hα fluxes. The E(B−V) values are found to be systematically lower than those estimated from point stars associated with H ii regions. Utilizing the MIPAPS Paα and IPHAS Hα images, we obtain an E(B−V) map for the entire region of the H ii region Sh2-131 with an angular size of ∼2.°5. The E(B−V) map shows not only numerous high-extinction filamentary features but also negative E(B−V) regions, indicating Hα excess. The Hα excess and systematic underestimation of E(B−V) are attributed to light scattered by dust.

Chemical Evolution of N2H+ in Six Massive Star-forming Regions

Abstract To investigate how the abundance of N2H+ varies as massive clumps evolve, here we present a multiwavelength study toward six molecular clouds. All of these clouds contain several massive clumps in different evolutionary stages of star formation. Using archival data of the Herschel infrared Galactic Plane Survey (Hi-GAL), we made H2 column density and dust temperature maps of these regions by the spectral energy distribution method. We found that all of the six clouds show distinct dust temperature gradients, ranging from ∼20 to ∼30 K. This makes them good candidates to study chemical evolution of molecules (such as N2H+) in different evolutionary stages of star formation. Our molecular line data comes from the Millimeter Astronomy Legacy Team Survey at 90 GHz (MALT90). We made column density and then abundance maps of N2H+. We found that when the dust temperature is above 27 K, the abundance of N2H+ begins to decrease or reaches a plateau. We regard that this is because in the photodissociation regions around classical H ii regions, N2H+ is heavily destroyed by free electrons. However, when the dust temperature is below 27 K, the abundance of N2H+ increases with the dust temperature. This seems to be inconsistent with previous chemical models made in low-mass star-forming regions. In order to investigate whether this inconsistency is caused by a different chemistry in high-mass star-forming clumps, higher angular resolution observations are necessary.

Detecting free-floating planets using water-dependent colour terms in the next generation of infrared space-based surveys

The next decade will see two large-scale space-based near-infrared surveys, Euclid and WFIRST. This paper shows that the subtle differences between the filters proposed for these surveys and those from ground-based photometric systems will produce a ground-space colour term that is dependent on water absorption in the spectra of astronomical objects. This colour term can be used to identify free-floating planets in star forming regions, mimicking a successful ground-based technique that uses a filter sensitive to water absorption. This paper shows that this colour term is an effective discriminant between reddened background stars and ultracool dwarfs. This represents just one science justification for a Galactic Plane survey in the event of an extension to the Euclid mission beyond its original timeframe.

Quasi-simultaneous 43 and 86 GHz SiO Maser Observations and Potential Bias in the BAaDE Survey Are Resolved

Abstract We observed the 43 GHz v = 1, 2, and 3 and 86 GHz v = 1 SiO maser transitions quasi-simultaneously for a Mira-variable-dominated sample of over 80 sources from the Bulge Asymmetries and Dynamical Evolution (BAaDE) project, using ATCA, and statistically compared the relative line strengths. On average, the 43 GHz v = 1 line is brighter than the 86 GHz v = 1 line by a factor of 1.36 − 0.14 + 0.15 . As a result, an 86 GHz v = 1 observed sample can be observed to 85.9% of the distance of a 43 GHz v = 1 observed sample using the same sensitivity. We discuss what impact this may have on the BAaDE Galactic plane survey using the VLA and ALMA. Despite fewer v = 3 detections, specific trends are discerned or strengthened when the 43 GHz v = 3 line is detected. In particular the 43 and 86 GHz v = 1 lines are on average equal for sources with no detectable 43 GHz v = 3 emission, but the 43 GHz v = 1 line strength is on average about twice as bright as the 86 GHz v = 1 line for sources with detectable 43 GHz v = 3 emission. Some weak correlations are found between line strengths and Midcourse Space Experiment flux densities and colors, which are tightened when considering only sources with detectable 43 GHz v = 3 emission. We discuss these trends in the context of a radiative pumping model to highlight how the 43 GHz v = 3 line, when coupled with the v = 1 and v = 2 lines, can further our understanding of variable conditions like density in the circumstellar envelopes.

Northern Galactic molecular cloud clumps in Hi-GAL: dense gas map and environmental trends

In the quest to understand high-mass star formation, it is necessary to understand that from which the high-mass stars will form -- the dense molecular gas clouds and clumps. The Herschel infrared GALactic plane survey (Hi-GAL) is a comprehensive survey of thermal dust emission that can be used to characterize the properties and Galactic distribution of molecular gas. We have analyzed the survey maps within the Galactic longitude range $10^\circ < \ell < 56^\circ$ and have transformed these 2D maps into a 3D dense gas map of the Galactic plane using distance probability density functions. This range corresponds to the extent of the Galactic Ring Survey (GRS), which provided the majority of our line-of-sight velocities, and thus kinematic distances. In this section of Hi-GAL, we identify 19,886 clumps, out of which 10,124 have, at minimum, a kinematic distance, and 5,405 have their distances well constrained through Bayesian techniques. Of those with well-constrained distances, clump masses tend to decrease with Galactocentric radius, whereas clump radii are independent of Galactocentric radius.

Searching for Inflow toward Massive Starless Clump Candidates Identified in the Bolocam Galactic Plane Survey

Abstract Recent Galactic plane surveys of dust continuum emission at long wavelengths have identified a population of dense, massive clumps with no evidence for ongoing star formation. These massive starless clump candidates are excellent sites to search for the initial phases of massive star formation before the feedback from massive star formation affects the clump. In this study, we search for the spectroscopic signature of inflowing gas toward starless clumps, some of which are massive enough to form a massive star. We observed 101 starless clump candidates identified in the Bolocam Galactic Plane Survey (BGPS) in J = 1−0 using the 12 m Arizona Radio Observatory telescope. We find a small blue excess of for the complete survey. We identified six clumps that are good candidates for inflow motion and used a radiative transfer model to calculate mass inflow rates that range from 500 to 2000 Myr−1. If the observed line profiles are indeed due to large-scale inflow motions, then these clumps will typically double their mass on a freefall time. Our survey finds that massive BGPS starless clump candidates with inflow signatures in J = 1−0 are rare throughout our Galaxy.

The coordinated radio and infrared survey for high-mass star formation – IV. A new radio-selected sample of compact galactic planetary nebulae

We present a new radio-selected sample of PNe from the CORNISH survey. This is a radio continuum survey of the inner Galactic plane covering Galactic longitude, $10^\circ <l< 65^\circ$ and latitude, $|b| < 1^\circ$ with a resolution of 1.5$^{"}$ and sensitivity better than 0.4 mJy/beam. The radio regime, being unbiased by dust extinction, allows for a more complete sample selection, especially towards the Galactic mid-plane. Visual inspection of the CORNISH data, in combination with data from multi-wavelength surveys of the Galactic plane, allowed the CORNISH team to identify 169 candidate PNe. Here, we explore the use of multi-wavelength diagnostic plots and analysis to verify and classify the candidate PNe. We present the multi-wavelength properties of this new PNe sample. We find 90 new PNe, of which 12 are newly discovered and 78 are newly classified as PN. A further 47 previously suspected PNe are confirmed as such from the analysis presented here and 24 known PNe are detected. Eight sources are classified as possible PNe or other source types. Our sample includes a young sub-sample, with physical diameters < 0.12 pc, brightness temperatures (> 1000 K) and located closer than 7 kpc. Within this sample is a water-maser PN with a spectral index of $-0.55\pm 0.08$, which indicates non-thermal radio emission. Such a radio-selected sample, unaffected by extinction, will be particularly useful to compare with population synthesis models and should contribute to the understanding of the formation and evolution of PNe.

Infall Signatures in a Prestellar Core Embedded in the High-mass 70 μm Dark IRDC G331.372-00.116

Abstract Using Galactic Plane surveys, we have selected a massive (1200 M ⊙), cold (14 K) 3.6–70 μm dark IRDC, G331.372-00.116. This infrared dark cloud (IRDC) has the potential to form high-mass stars, and given the absence of current star formation signatures, it seems to represent the earliest stages of high-mass star formation. We have mapped the whole IRDC with the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.1 and 1.3 mm in dust continuum and line emission. The dust continuum reveals 22 cores distributed across the IRDC. In this work, we analyze the physical properties of the most massive core, ALMA1, which has no molecular outflows detected in the CO (2–1), SiO (5–4), and H2CO (3–2) lines. This core is relatively massive (M = 17.6 M ⊙), subvirialized (virial parameter α vir = M vir/M = 0.14), and is barely affected by turbulence (transonic Mach number of 1.2). Using the HCO+ (3–2) line, we find the first detection of infall signatures in a relatively massive, prestellar core (ALMA1) with the potential to form a high-mass star. We estimate an infall speed of 1.54 km s−1 and a high accretion rate of 1.96 × 10−3 M ⊙ yr−1. ALMA1 is rapidly collapsing, out of virial equilibrium, which is more consistent with competitive accretion scenarios rather than the turbulent core accretion model. On the other hand, ALMA1 has a mass ∼6 times larger than the clumps Jeans mass, as it is in an intermediate mass regime (M J = 2.7 &lt; M ≲ 30 M ⊙), contrary to what both the competitive accretion and turbulent core accretion theories predict.

Investigating Galactic Supernova Remnant Candidates Using LOFAR

Abstract We investigate six supernova remnant (SNR) candidates—G51.21+0.11, G52.37–0.70, G53.07+0.49, G53.41+0.03, G53.84–0.75, and the possible shell around G54.1+0.3—in the Galactic plane using newly acquired Low-Frequency Array High-band Antenna observations, as well as archival Westerbork Synthesis Radio Telescope and Very Large Array Galactic Plane Survey mosaics. We find that G52.37–0.70, G53.84–0.75, and the possible shell around pulsar wind nebula G54.1+0.3 are unlikely to be SNRs, while G53.07+0.49 remains a candidate SNR. G51.21+0.11 has a spectral index of α = − 0.7 ± 0.21, but lacks X-ray observations and as such requires further investigation to confirm its nature. We confirm one candidate, G53.41+0.03, as a new SNR because it has a shell-like morphology, a radio spectral index of α = − 0.6 ± 0.2 and it has the X-ray spectral characteristics of a 1000–8000 year old SNR. The X-ray analysis was performed using archival XMM-Newton observations, which show that G53.41+0.03 has strong emission lines and is best characterized by a nonequilibrium ionization model, consistent with an SNR interpretation. Deep Arecibo radio telescope searches for a pulsar associated with G53.41+0.03 resulted in no detection, but placed stringent upper limits on the flux density of such a source if it was beamed toward Earth.

Revised Distances to 21 Supernova Remnants

Abstract We carry out a comprehensive study of H i 21 cm line observations and 13CO line observations of 21 supernova remnants (SNRs). The aim of the study is to search for H i absorption features to obtain kinematic distances in a consistent manner. The 21 SNRs are in the region of sky covered by the Very Large Array Galactic Plane Survey (H i 21 cm observations) and Galactic Ring Survey (13CO line observations). We obtain revised distances for 10 SNRs based on new evidence in the H i and 13CO observations. We revise distances for the other 11 SNRs based on an updated rotation curve and new error analysis. The mean change in distance for the 21 SNRs is ≃25%, i.e., a change of 1.5 kpc compared to a mean distance for the sample of 6.4 kpc. This has a significant impact on interpretation of the physical state of these SNRs. For example, using a Sedov model, age and explosion energy scale as the square of distance, and inferred ISM density scales as distance.

The population of TeV pulsar wind nebulae in the H.E.S.S. Galactic Plane Survey

The nine-year H.E.S.S. Galactic Plane Survey (HGPS) has yielded the most uniform observation scan of the inner Milky Way in the TeV gamma-ray band to date. The sky maps and source catalogue of the HGPS allow for a systematic study of the population of TeV pulsar wind nebulae found throughout the last decade. To investigate the nature and evolution of pulsar wind nebulae, for the first time we also present several upper limits for regions around pulsars without a detected TeV wind nebula. Our data exhibit a correlation of TeV surface brightness with pulsar spin-down power Ė. This seems to be caused both by an increase of extension with decreasing Ė, and hence with time, compatible with a power law RPWN(Ė) ~Ė−0.65±0.20, and by a mild decrease of TeV gamma-ray luminosity with decreasing Ė, compatible with L1−10 TeV ~Ė0.59±0.21. We also find that the offsets of pulsars with respect to the wind nebula centre with ages around 10 kyr are frequently larger than can be plausibly explained by pulsar proper motion and could be due to an asymmetric environment. In the present data, it seems that a large pulsar offset is correlated with a high apparent TeV efficiency L1−10 TeV∕Ė. In addition to 14 HGPS sources considered firmly identified pulsar wind nebulae and 5 additional pulsar wind nebulae taken from literature, we find 10 HGPS sources that are likely TeV pulsar wind nebula candidates. Using a model that subsumes the present common understanding of the very high-energy radiative evolution of pulsar wind nebulae, we find that the trends and variations of the TeV observables and limits can be reproduced to a good level, drawing a consistent picture of present-day TeV data and theory.

Population study of Galactic supernova remnants at very high γ-ray energies with H.E.S.S.

Shell-type supernova remnants (SNRs) are considered prime candidates for the acceleration of Galactic cosmic rays (CRs) up to the knee of the CR spectrum at E ≈ 3 × 1015 eV. Our Milky Way galaxy hosts more than 350 SNRs discovered at radio wavelengths and at high energies, of which 220 fall into the H.E.S.S. Galactic Plane Survey (HGPS) region. Of those, only 50 SNRs are coincident with a H.E.S.S source and in 8 cases the very high-energy (VHE) emission is firmly identified as an SNR. The H.E.S.S. GPS provides us with a legacy for SNR population study in VHE γ-rays and we use this rich data set to extract VHE flux upper limits from all undetected SNRs. Overall, the derived flux upper limits are not in contradiction with the canonical CR paradigm. Assuming this paradigm holds true, we can constrain typical ambient density values around shell-type SNRs to n ≤ 7 cm−3 and electron-to-proton energy fractions above 10 TeV to ϵep ≤ 5 × 10−3. Furthermore, comparisons of VHE with radio luminosities in non-interacting SNRs reveal a behaviour that is in agreement with the theory of magnetic field amplification at shell-type SNRs.

The H.E.S.S. Galactic plane survey

We present the results of the most comprehensive survey of the Galactic plane in very high-energy (VHE)γ-rays, including a public release of Galactic sky maps, a catalog of VHE sources, and the discovery of 16 new sources of VHEγ-rays. The High Energy Spectroscopic System (H.E.S.S.) Galactic plane survey (HGPS) was a decade-long observation program carried out by the H.E.S.S. I array of Cherenkov telescopes in Namibia from 2004 to 2013. The observations amount to nearly 2700 h of quality-selected data, covering the Galactic plane at longitudes fromℓ= 250° to 65° and latitudes |b|≤ 3°. In addition to the unprecedented spatial coverage, the HGPS also features a relatively high angular resolution (0.08° ≈ 5 arcmin mean point spread function 68% containment radius), sensitivity (≲1.5% Crab flux for point-like sources), and energy range (0.2–100 TeV). We constructed a catalog of VHEγ-ray sources from the HGPS data set with a systematic procedure for both source detection and characterization of morphology and spectrum. We present this likelihood-based method in detail, including the introduction of a model component to account for unresolved, large-scale emission along the Galactic plane. In total, the resulting HGPS catalog contains 78 VHE sources, of which 14 are not reanalyzed here, for example, due to their complex morphology, namely shell-like sources and the Galactic center region. Where possible, we provide a firm identification of the VHE source or plausible associations with sources in other astronomical catalogs. We also studied the characteristics of the VHE sources with source parameter distributions. 16 new sources were previously unknown or unpublished, and we individually discuss their identifications or possible associations. We firmly identified 31 sources as pulsar wind nebulae (PWNe), supernova remnants (SNRs), composite SNRs, or gamma-ray binaries. Among the 47 sources not yet identified, most of them (36) have possible associations with cataloged objects, notably PWNe and energetic pulsars that could power VHE PWNe.