arXiv subjects

M. Heyer

Publications and source records attributed to M. Heyer.

At least 19 recordsLinked to original sources

arXiv2026

The Neutral Atomic Hydrogen in the solar neighborhood (NeAtHood) project I. Ghost in the shell: Neutral atomic hydrogen in the extended Orion nebula

The Orion Nebula is the nearest site of ongoing and recent high-mass star formation. It is a unique laboratory for studying the mass, energy, and momentum input from high-mass stars. We present 21-centimeter emission line observations that resolve for the first time the neutral atomic hydrogen (HI) gas in the extended Orion nebula (EON) at a resolution of one arcminute, which corresponds to a physical scale of 0.12 parsecs at the standard distance to the region. Our HI emission maps reveal an expanding shell that matches the EON contours delineated by recent observations of ionized carbon ([CII]) line emission. However, our combination of single-dish and interferometric HI observations suggests 100 solar masses of material for the front hemisphere of the shell, which is lower by roughly a factor of ten than the mass inferred from [CII] observations. This discrepancy suggests that the mass of the nearest wind-blown bubble has been overestimated, although we do not rule out the possibility that a significant amount of molecular hydrogen (H_2) in the shell may account for part of the difference. Our extended 21 cm line maps also reveal uncharted structures in and around the EON. They include a probable secondary bubble and a linear protrusion extending roughly four parsecs from the shell boundary. Our results illustrate the potential of HI interferometric observations to elucidate key aspects of the multiphase structure of star-forming regions and their connection to their surroundings.

astro-ph.GA↗

arXiv2026

SIMPLIFI-Study of Interstellar Magnetic Polarization: A Legacy Investigation of Filaments. II. Enhancement of grain alignment near embedded protostars in the DR21 Ridge

Thermal dust continuum polarimetry is a powerful indirect probe of magnetic field geometry in dense molecular clouds while at the same time providing information on the alignment of dust grains with the magnetic field. The leading theory of grain alignment, Radiative Torque Alignment (RAT), has been successful in explaining a variety of observations, including the loss of polarization fraction toward high column densities. One prediction of RAT is that an increase in grain alignment efficiency should be observed in the environments surrounding protostars, due to radiation from the embedded source. However, observational confirmation of this prediction remains scarce. In this study, we sought to test the theoretical prediction of enhanced grain alignment near protostars in the high-mass star forming region DR21 using 214 $\mu m$ SOFIA/HAWC+ observations. We investigated the correlation of the polarization fraction of dust emission, $p$, and the polarization angular dispersion, $S$, with respect to total intensity. We also probed intrinsic dust polarization properties using the product $S\times p$ as a proxy. We detected significant polarization fractions even at the highest intensities, where strong depolarization is typically expected. The polarization fraction-intensity trend flattens at $I > 1.6^{+0.3}_{-0.3} \times 10^4$ MJy/sr ($N_{H_2}$ $\sim 2\times 10^{23}$ ${cm}^{-2}$). We compared the observed trends with predictions from an analytical model of a centrally heated envelope surrounding an embedded luminous protostar. The predictions from the simple model agree well with the observed trends. Our results provide strong support for enhancement of grain alignment by local radiation from embedded sources.

astro-ph.GA↗

arXiv2025

Anisotropy in the carbon monoxide (CO) line emission across the Milky Way's disk

We present a study of the $^{12}$CO(1-0) line emission anisotropy across the Milky Way's disk to examine the effect of stellar feedback and Galactic dynamics on the distribution of the dense interstellar medium. The Hessian matrix method is used to characterize the CO line emission distribution and identify the preferential orientation across line-of-sight velocity channels in the Dame et al. 2001 composite Galactic plane survey, which covers the Galactic latitude range $|b|<5^{\circ}$. The structures sampled with this tracer are predominantly parallel to the Galactic plane toward the inner Galaxy, in clear contrast with the predominantly perpendicular orientation of the structures traced by neutral atomic hydrogen (HI) emission toward the same regions. The analysis of the Galactic plane portions sampled at higher angular resolution with other surveys reveals that the alignment with the Galactic plane is also prevalent at smaller scales. We find no preferential orientation in the CO emission toward the outer Galaxy, in contrast with the preferential alignment with the Galactic plane displayed by HI in that portion of the Milky Way. We interpret these results as the combined effect of the decrease in mid-plane pressure with increasing Galactocentric radius and SN feedback lifting diffuse gas more efficiently than dense gas off the Galactic plane.

astro-ph.GA↗

arXiv2025

Cold molecular gas in the hot nuclear wind of the Milky Way

Using the Large Millimeter Telescope and the SEQUOIA 3~mm focal plane array, we have searched for molecular line emission from two atomic clouds associated with the Fermi Bubble of the Milky Way. Neither 12CO nor 13CO J=1-0 emission is detected from the HI cloud, MW-C20. 12CO J=1-0 emission is detected from MW-C21 that is distributed within 11 clumps with most of the CO luminosity coming from a single clump. However, we find no 13CO emission to a 3sigma brightness temperature limit of 0.3 K. Using this limit and RADEX non local thermodynamic equilibrium (non-LTE) excitation models, we derive H2 column density upper limits of (0.4-3)x10^{21} cm-2 for a set of physical conditions and a H2 to 12CO abundance ratio of 10^4. Model CO-to-H2 conversion factors are derived for each set of physical conditions. We find the maximum value is 1.6x10^{20} cm-2/(K km/s). Increasing [H2/12CO] to 10^5 to account for photodissociation and cosmic ray ionization increases the column density and XCO upper limits by a factor of 10. Applying these XCO limits to the CO luminosities, the upper limit to the total molecular mass in MW-C21 is 132+/-2~Msun, corresponding to less than 27% of the neutral gas mass. For the three clumps that are fully resolved, lower limits to the virial ratios are 288+/-32, 68+/-28, and 157+/-39, which suggest that these structures are bound by external pressure to remain dynamically stable over the entrainment time of 2x10^6 years or are being disrupted by shear and expansion over the clump crossing times of (3-8)x10^5 years. The observations presented in this study add to the growing census of cold gas entrained within the Galactic Center wind.

astro-ph.GA↗

arXiv2023

Cold atomic gas identified by HI self-absorption. Cold atomic clouds toward giant molecular filaments

Stars form in the dense interiors of molecular clouds. The dynamics and physical properties of the atomic interstellar medium (ISM) set the conditions under which molecular clouds and eventually stars will form. It is, therefore, critical to investigate the relationship between the atomic and molecular gas phase to understand the global star formation process. Using the high angular resolution data from The HI/OH/Recombination line survey of the Milky Way (THOR), we aim to constrain the kinematic and physical properties of the cold atomic hydrogen gas phase toward the inner Galactic plane. HI self-absorption (HISA) has proven to be a viable method to detect cold atomic hydrogen clouds in the Galactic plane. With the help of a newly developed self-absorption extraction routine (astroSABER), we build upon previous case studies to identify HI self-absorption toward a sample of Giant Molecular Filaments (GMFs). We find the cold atomic gas to be spatially correlated with the molecular gas on a global scale. The column densities of the cold atomic gas traced by HISA are usually of the order of $10^{20}\rm\,cm^{-2}$ whereas those of molecular hydrogen traced by $\rm^{13}CO$ are at least an order of magnitude higher. The HISA column densities are attributed to a cold gas component that accounts for a fraction of $\sim$5% of the total atomic gas budget within the clouds. The HISA column density distributions show pronounced log-normal shapes that are broader than those traced by HI emission. The cold atomic gas is found to be moderately supersonic with Mach numbers of a $\sim$few. In contrast, highly supersonic dynamics drive the molecular gas within most filaments.

astro-ph.GA↗

arXiv2023

A panoptic view of the Taurus molecular cloud I. The cloud dynamics revealed by gas emission and 3D dust

We present a study of the three-dimensional (3D) distribution of interstellar dust derived from stellar extinction observations toward the Taurus molecular cloud (MC) and its relation with the neutral atomic hydrogen (HI) emission at 21 cm wavelength and the carbon monoxide $^{12}$CO and $^{13}$CO emission in the $J=1\rightarrow0$ transition. We used the histogram of oriented gradients (HOG) method to match the morphology in a 3D reconstruction of the dust density (3D dust) and the distribution of the gas tracers' emission. The result of the HOG analysis is a map of the relationship between the distances and radial velocities. The HOG comparison between the 3D dust and the HI emission indicates a morphological match at the distance of Taurus but an anti-correlation between the dust density and the HI emission, which uncovers a significant amount of cold HI within the Taurus MC. The HOG between the 3D dust and $^{12}$CO reveals a pattern in radial velocities and distances that is consistent with converging motions of the gas in the Taurus MC, with the near side of the cloud moving at higher velocities and the far side moving at lower velocities. This convergence of flows is likely triggered by the large-scale gas compression caused by the interaction of the Local Bubble and the Per-Tau shell, with Taurus lying at the intersection of the two bubble surfaces.

astro-ph.GA↗

arXiv2022

The APEX Large CO Heterodyne Orion Legacy Survey (ALCOHOLS). I. Survey overview

The Orion molecular cloud complex harbours the nearest GMCs and site of high-mass star formation. Its YSO populations are thoroughly characterized. The region is therefore a prime target for the study of star formation. Here, we verify the performance of the SuperCAM 64 pixel heterodyne array on APEX. We give a descriptive overview of a set of wide-field CO(3-2) spectral cubes obtained towards the Orion GMC complex, aimed at characterizing the dynamics and structure of the extended molecular gas in diverse regions of the clouds, ranging from very active sites of clustered star formation in Orion B to comparatively quiet regions in southern Orion A. We present a 2.7 square degree (130pc$^2$) mapping survey in the CO(3-2) transition, obtained using SuperCAM on APEX at an angular resolution of 19'' (7600AU or 0.037pc at a distance of 400pc), covering L1622, NGC2071, NGC2068, OriB9, NGC2024, and NGC2023 in Orion B, and the southern part of the L1641 cloud in Orion A. We describe CO integrated emission and line moment maps and position-velocity diagrams and discuss a few sub-regions in some detail. Evidence for expanding bubbles is seen with lines splitting into double components, most prominently in NGC2024, where we argue that the bulk of the molecular gas is in the foreground of the HII region. High CO(3-2)/CO(1-0) line ratios reveal warm CO along the western edge of Orion B in the NGC2023/NGC2024 region facing the IC434 HII region. Multiple, well separated radial velocity components seen in L1641-S suggest that it consists of a sequence of clouds at increasingly larger distances. We find a small, spherical cloud - the 'Cow Nebula' globule - north of NGC2071. We trace high velocity line wings for the NGC2071-IR outflow and the NGC2024 CO jet. The protostellar dust core FIR4 (rather than FIR5) is the true driving source of the NGC2024 monopolar outflow.

astro-ph.GA↗

arXiv2021

The filamentary structures in the CO emission toward the Milky Way disk

We present a statistical study of the filamentary structure orientation in the CO emission observations obtained in the Milky Way Imaging Scroll Painting (MWISP) survey in the range $25.8\deg < l < 49.7\deg$, $|b| \leq 1.25\deg$, and $-100 < v_{\rm LSR} < 135$ km/s. We found that most of the filamentary structures in the $^{12}$CO and $^{13}$CO emission do not show a global preferential orientation either parallel or perpendicular to the Galactic plane. However, we found ranges in Galactic longitude and radial velocity where the $^{12}$CO and $^{13}$CO filamentary structures are parallel to the Galactic plane. These preferential orientations are different from those found for the HI emission. We consider this an indication that the molecular structures do not simply inherit these properties from parental atomic clouds. Instead, they are shaped by local physical conditions, such as stellar feedback, magnetic fields, and Galactic spiral shocks.

astro-ph.GA↗

arXiv2020

Atomic and molecular gas properties during cloud formation

Molecular clouds, which harbor the birthplaces of stars, form out of the atomic phase of the interstellar medium (ISM). We aim to characterize the atomic and molecular phases of the ISM and set their physical properties into the context of cloud formation processes. We studied the cold neutral medium (CNM) by means of $\rm HI$ self-absorption (HISA) toward the giant molecular filament GMF20.0-17.9 and compared our results with molecular gas traced by $^{13}\rm CO$ emission. We fitted baselines of HISA features to $\rm HI$ emission spectra using first and second order polynomial functions. The CNM identified by this method spatially correlates with the morphology of the molecular gas toward the western region. However, no spatial correlation between HISA and $^{13}\rm CO$ is evident toward the eastern part of the filament. The distribution of HISA peak velocities and line widths agrees well with $^{13}\rm CO$ within the whole filament. The column density probability density functions (N-PDFs) of HISA (CNM) and $\rm HI$ emission (tracing both the CNM and the warm neutral medium, WNM) have a log-normal shape for all parts of the filament, indicative of turbulent motions as the main driver for these structures. The $\rm H_2$ N-PDFs show a broad log-normal distribution with a power-law tail suggesting the onset of gravitational contraction. The saturation of $\rm HI$ column density is observed at $\sim$25$\rm\,M_{\odot}\,pc^{-2}$. We conjecture that different evolutionary stages are evident within the filament. In the eastern region, we witness the onset of molecular cloud formation out of the atomic gas reservoir while the western part is more evolved, as it reveals pronounced $\rm H_2$ column density peaks and signs of active star formation.

astro-ph.GA↗

arXiv2020

The history of dynamics and stellar feedback revealed by the HI filamentary structure in the disk of the Milky Way

We present a study of the filamentary structure in the emission from the neutral atomic hydrogen (HI) at 21 cm across velocity channels in the 40'' and 1.5-km/s resolution position-position-velocity cube resulting from the combination of the single-dish and interferometric observations in The HI/OH/Recombination (THOR) line survey. Using the Hessian matrix method in combination with tools from circular statistics, we find that the majority of the filamentary structures in the HI emission are aligned with the Galactic plane. Part of this trend can be assigned to long filamentary structures that are coherent across several velocity channels. However, we also find ranges of Galactic longitude and radial velocity where the HI filamentary structures are preferentially oriented perpendicular to the Galactic plane. These are located (i) around the tangent point of the Scutum spiral arm, $l \approx 28^{\circ}$ and $v_{\rm LSR}\approx 100$ km/s, (ii) toward $l \approx 45^{\circ}$ and $v_{\rm LSR}\approx 50$ km/s, (iii) around the Riegel-Crutcher cloud, and (iv) toward the terminal velocities. Comparison with numerical simulations indicates that the prevalence of horizontal filamentary structures is most likely the result of the large-scale dynamics and that vertical structures identified in (i) and (ii) may arise from the combined effect of supernova (SN) feedback and strong magnetic fields. The vertical filamentary structures in (iv) can be related to the presence of clouds from extra-planar HI gas falling back into the Galactic plane after being expelled by SNe. Our results indicate that a systematic characterization of the emission morphology toward the Galactic plane provides an unexplored link between the observations and the dynamical behaviour of the interstellar medium, from the effect of large-scale Galactic dynamics to the Galactic fountains driven by SNe.

astro-ph.GA↗

arXiv2020

The relative orientation between the magnetic field and gradients of surface brightness within thin velocity slices of 12CO and 13CO emission from the Taurus molecular cloud

We examine the role of the interstellar magnetic field to modulate the orientation of turbulent flows within the Taurus molecular cloud using spatial gradients of thin velocity slices of 12CO and 13CO antenna temperatures. Our analysis accounts for the random errors of the gradients that arise from the thermal noise of the spectra. The orientations of the vectors normal to the antenna temperature gradient vectors are compared to the magnetic field orientations that are calculated from Planck 353~GHz polarization data. These relative orientations are parameterized with the projected Rayleigh statistic and mean resultant vector. For 12CO, 28 percent and 39 percent of the cloud area exhibit strongly parallel or strongly perpendicular relative orientations respectively. For the lower opacity 13CO emission, strongly parallel and strongly perpendicular orientations are found in 7 percent and 43 percent of the cloud area respectively. For both isotopologues, strongly parallel or perpendicular alignments are restricted to localized regions with low levels of turbulence. If the relative orientations serve as an observational proxy to the Alfvenic Mach number then our results imply local variations of the Alfvenic Mach number throughout the cloud.

astro-ph.GA↗

arXiv2019

The HI/OH/Recombination line survey of the inner Milky Way (THOR): data release 2 and HI overview

With the $Karl~G.~Jansky$ Very Large Array (VLA) in C-configuration, we observed a large portion of the first Galactic quadrant ($l=14.0-67.4^\circ $ and $\lvert b \rvert \leq 1.25^\circ $) achieving an angular resolution of $\leq 40^{\prime\prime}$. At $L$ Band, the WIDAR correlator at the VLA was set to cover the 21~cm HI line, four OH transitions, a series of H$nα$ radio recombination lines (RRLs; $n=151$ to 186), and eight 128~MHz wide continuum spectral windows (SPWs) simultaneously. The HI emission shows clear filamentary substructures at negative velocities with low velocity crowding. The emission at positive velocities is more smeared-out likely due to higher spatial and velocity crowding of structures at the positive velocities. Comparing to the spiral arm model of the Milky Way, the atomic gas follows the Sagittarius and Perseus Arm well but with significant material in the inter-arm regions. With the C-configuration-only HI+continuum data, we produced a HI optical depth map of the THOR areal coverage from 228 absorption spectra with the nearest-neighbor method. With this $τ$ map, we corrected the HI emission for optical depth and the derived column density is 38% higher than the column density with optically thin assumption. The total HI mass with optical depth correction in the survey region is 4.7$\times10^8~M_\odot$, 31% more than the mass derived assuming the emission is optically thin. If we apply this 31% correction to the whole Milky Way, the total atomic gas mass would be 9.4-10.5$\times 10^9~M_\odot$. Comparing the HI with existing CO data, we find a significant increase in the atomic-to-molecular gas ratio from the spiral arms to the inter-arm regions.

astro-ph.GA↗

arXiv2019

Structure and kinematics of the Taurus star-forming region from Gaia-DR2 and VLBI astrometry

Aims:We take advantage of the second data release of the Gaia space mission and the state-of-the-art astrometry delivered from very long baseline interferometry observations to revisit the structure and kinematics of the nearby Taurus star-forming region. Methods: We apply a hierarchical clustering algorithm for partitioning the stars in our sample into groups (i.e., clusters) that are associated with the various molecular clouds of the complex, and derive the distance and spatial velocity of individual stars and their corresponding molecular clouds. Results: We show that the molecular clouds are located at different distances and confirm the existence of important depth effects in this region reported in previous studies. For example, we find that the L 1495 molecular cloud is located at $d=129.9^{+0.4}_{-0.3}$ pc, while the filamentary structure connected to it (in the plane of the sky) is at $d=160.0^{+1.2}_{-1.2}$ pc. We report B 215 and L 1558 as the closest ($d=128.5^{+1.6}_{-1.6}$ pc) and most remote ($d=198.1^{+2.5}_{-2.5}$ pc) substructures of the complex, respectively. The median inter-cloud distance is 25 pc and the relative motion of the subgroups is on the order of a few km/s. We find no clear evidence for expansion (or contraction) of the Taurus complex, but signs of the potential effects of a global rotation. Finally, we compare the radial velocity of the stars with the velocity of the underlying $^{13}$CO molecular gas and report a mean difference of $0.04\pm0.12$ km/s (with r.m.s. of 0.63 km/s) confirming that the stars and the gas are tightly coupled.

astro-ph.SR↗

arXiv2019

Histogram of oriented gradients: a technique for the study of molecular cloud formation

We introduce the histogram of oriented gradients (HOG), a tool developed for machine vision that we propose as a new metric for the systematic characterization of observations of atomic and molecular gas and the study of molecular cloud formation models. In essence, the HOG technique takes as input extended spectral-line observations from two tracers and provides an estimate of their spatial correlation across velocity channels. We characterize HOG using synthetic observations of HI and $^{13}$CO(J=1-0) emission from numerical simulations of MHD turbulence leading to the formation of molecular gas after the collision of two atomic clouds. We find a significant spatial correlation between the two tracers in velocity channels where $v_{HI}\approx v_{^{13}CO}$, independent of the orientation of the collision with respect to the line of sight. We use HOG to investigate the spatial correlation of the HI, from the THOR survey, and the $^{13}$CO(J=1-0) emission, from the GRS, toward the portion of the Galactic plane 33.75$\lt l\lt$35.25$^{o}$ and $|b|\lt$1.25$^{o}$. We find a significant spatial correlation between the tracers in extended portions of the studied region. Although some of the regions with high spatial correlation are associated with HI self-absorption features, suggesting that it is produced by the cold atomic gas, the correlation is not exclusive to this kind of region. The HOG results also indicate significant differences between individual regions: some show spatial correlation in channels around $v_{HI}\approx v_{^{13}CO}$ while others present this correlation in velocity channels separated by a few km/s. We associate these velocity offsets to the effect of feedback and to the presence of physical conditions that are not included in the atomic-cloud-collision simulations, such as more general magnetic field configurations, shear, and global gas infall.

astro-ph.GA↗

arXiv2018

Feedback in W49A diagnosed with radio recombination lines and models

We present images of radio recombination lines (RRLs) at wavelengths around 17 cm from the star-forming region W49A to determine the kinematics of ionized gas in the THOR survey (The HI/OH/Recombination line survey of the inner Milky Way) at an angular resolution of 16.8"x13.8". The distribution of ionized gas appears to be affected by feedback processes from the star clusters in W49A. The velocity structure of the RRLs shows a complex behavior with respect to the molecular gas. We find a shell-like distribution of ionized gas as traced by RRL emission surrounding the central cluster of OB stars in W49A. We describe the evolution of the shell with the recent feedback model code WARPFIELD that includes the important physical processes and has previously been applied to the 30 Doradus region in the Large Magellanic Cloud. The cloud structure and dynamics of W49A are in agreement with a feedback-driven shell that is re-collapsing. The shell may have triggered star formation in other parts of W49A. We suggest that W49A is a potential candidate for star formation regulated by feedback-driven and re-collapsing shells.

astro-ph.GA↗

arXiv2018

Early science with the Large Millimetre Telescope: An LMT/AzTEC 1.1 mm Survey of Dense Cores in the Monoceros R2 Giant Molecular Cloud

We present a 1.1~mm census of dense cores in the Mon~R2 Giant Molecular Cloud with the AzTEC instrument on the Large Millimeter Telescope (LMT). We detect 295 cores (209 starless, and 86 with protostars) in a two square degree shallow survey. We also carry out a deep follow-up survey of 9 regions with low to intermediate ($3 $10~$M_{\odot}$ are exclusively found in regions with high core number densities and that the CMF of the starless cores has an excess of low-mass cores ($<$5~$M_{\odot}$) compared to the CMF of protostellar cores. We report a power law correlation of index $1.99 \pm 0.03$ between local core mass density and gas column density (as traced by Herschel) over a wide range of size scales (0.3-5~pc). This power law is consistent with that predicted for thermal fragmentation of a self-gravitating sheet.

astro-ph.GA↗

arXiv2017

Early Science with the Large Millimetre Telescope: Fragmentation of molecular clumps in the Galaxy

Sensitive, imaging observations of the 1.1 mm dust continuum emission from a 1 deg^2 area collected with the AzTEC bolometer camera on the Large Millimeter Telescope are presented. A catalog of 1545 compact sources is constructed based on a Wiener-optimization filter. These sources are linked to larger clump structures identified in the Bolocam Galactic Plane Survey. Hydrogen column densities are calculated for all sources and mass and mean volume densities are derived for the subset of sources for which kinematic distances can be assigned. The AzTEC sources are localized, high density peaks within the massive clumps of molecular clouds and comprise 5-15% of the clump mass. We examine the role of the gravitational instability in generating these fragments by comparing the mass of embedded AzTEC sources to the Jeans' mass of the parent BGPS object. For sources with distances less than 6 kpc the fragment masses are comparable to the clump Jeans' mass, despite having isothermal Mach numbers between 1.6 and 7.2. AzTEC sources linked to ultra-compact HII regions have mass surface densities greater than the critical value implied by the mass-size relationship of infrared dark clouds with high mass star formation while AzTEC sources associated with Class II methanol masers have mass surface densities greater than 0.7 g cm^{-2} that approaches the proposed threshold required to form massive stars.

astro-ph.GA↗

arXiv2017

Galactic Supernova Remnant Candidates Discovered by THOR

There is a considerable deficiency in the number of known supernova remnants (SNRs) in the Galaxy compared to that expected. Searches for extended low-surface brightness radio sources may find new Galactic SNRs, but confusion with the much larger population of HII regions makes identifying such features challenging. SNRs can, however, be separated from HII regions using their significantly lower mid-infrared (MIR) to radio continuum intensity ratios. We use the combination of high-resolution 1-2 GHz continuum data from The HI, OH, Recombination line survey of the Milky Way (THOR) and lower-resolution VLA 1.4 GHz Galactic Plane Survey (VGPS) continuum data, together with MIR data from the Spitzer GLIMPSE, Spitzer MIPSGAL, and WISE surveys to identify SNR candidates. To ensure that the candidates are not being confused with HII regions, we exclude radio continuum sources from the WISE Catalog of Galactic HII Regions, which contains all known and candidate H II regions in the Galaxy. We locate 76 new Galactic SNR candidates in the THOR and VGPS combined survey area of 67.4deg>l>17.5deg, |b|<1.25deg and measure the radio flux density for 52 previously-known SNRs. The candidate SNRs have a similar spatial distribution to the known SNRs, although we note a large number of new candidates near l=30deg, the tangent point of the Scutum spiral arm. The candidates are on average smaller in angle compared to the known regions, 6.4'+/-4.7' versus 11.0'+/-7.8', and have lower integrated flux densities. If the 76 candidates are confirmed as true SNRs, for example using radio polarization measurements or by deriving radio spectral indices, this would more than double the number of known Galactic SNRs in the survey area. This large increase would still, however, leave a discrepancy between the known and expected SNR populations of about a factor of two.

astro-ph.GA↗