The GBT Diffuse Ionized Gas Survey (GDIGS): Discrete Sources

The Green Bank Telescope Diffuse Ionized Gas Survey (GDIGS) traces ionized gas in the Galactic midplane by observing radio recombination line (RRL) emission from 4 to 8 GHz. The nominal survey zone is 32.°3 > ℓ > −5°, ∣ b ∣ < 0.°5. Here, we analyze GDIGS Hnα ionized gas emission toward discrete sources. Using GDIGS data, we identify the velocity of 35 H ii regions that have multiple detected RRL velocity components. We identify and characterize RRL emission from 88 H ii regions that previously lacked measured ionized gas velocities. We also identify and characterize RRL emission from eight locations that appear to be previously unidentified H ii regions and 30 locations of RRL emission that do not appear to be H ii regions based on their lack of mid-infrared emission. This latter group may be a compact component of the Galactic Diffuse Ionized Gas. There are an additional 10 discrete sources that have anomalously high RRL velocities for their locations in the Galactic plane. We compare these objects’ RRL data to 13CO, H i, and mid-infrared data, and find that these sources do not have the expected 24 μm emission characteristic of H ii regions. Based on this comparison we do not think these objects are H ii regions, but we are unable to classify them as a known type of object.


INTRODUCTION
Ionized gas in the plane of the Milky Way can be decomposed into discrete clumps, particularly H II regions, and a diffuse component known as the "Diffuse Ionized Gas" (DIG).H II regions are zones of ionized gas that form around spectral type O and B stars, where the star's immense energy output ionizes the surrounding medium.These massive stars only live ∼ 10 Myr.H II regions are therefore useful indicators of active massive star formation.In contrast to H II regions, the DIG is spread throughout the Galactic disk, has a lower electron density near n e ∼ 10 −2 cm −3 (Lyne et al. 1985) compared to densities of n e ∼ 10 2 cm −3 or greater found in H II regions (Lockman & Brown 1975), and is not typically associated with specific stars.
We can study Galactic ionized gas using transitions of excited hydrogen atoms.Recombination lines are produced after electrons and ions recombine into an excited state.For nearby plasma, Hα line observations such as those made by the Wisconsin Hα Mapper (WHAM) survey (Reynolds et al. 1998;Haffner et al. 2003) provides sensitive observations that can determine gas properties.The Hα line, although bright, is attenuated by dust and so is of limited use for distant H II regions.Radio recombination lines (RRLs) are less affected by extinction compared to Hα and so are the preferred tracer for more distant H II regions and the DIG.
Previous targeted RRL surveys have been instrumental in assembling the catalog of known H II regions.In particular, the H II Region Discovery Survey (HRDS; Anderson et al. 2011a) and Southern H II Region Discovery Survey (SHRDS; Wenger et al. 2021) have more than doubled the number of known H II regions (Anderson et al. 2018a).An ionized gas spectroscopic detection determines the source's velocity relative to the local standard of rest (LSR).Such pointed observations, however, have an inherent weakness: they are by nature biased toward sources meeting the criteria for inclusion in the study.
Large-scale mapped RRL surveys allow for the study of ionized gas across the Galaxy.The Green Bank Telescope (GBT) Diffuse Ionized Gas Survey (GDIGS) is a RRL survey that was conducted with the goal of studying ionized gas in the Galactic midplane.GDIGS offers several advantages over previous blind RRL surveys in the survey zone.Such surveys lacked the sensitivity and resolution necessary to detect many new discrete sources.The Survey of Ionized Gas in the Galaxy, made with the Arecibo Telescope (SIGGMA), an RRL survey near 1.4 GHz, has a sensitivity of ∼ 1 mJy beam −1 at 5 km s −1 spectral resolution and ∼ 3. ′ 4 spatial resolution (Liu et al. 2013;Liu et al. 2019).The H I Parkes All Sky Survey (HIPASS) RRL survey, has 6.4 mJy beam −1 sensitivity at 20 km s −1 spectral resolution and 14. ′ 4 spatial resolution, also near 1.4 GHz (Alves et al. 2015).As detailed subsequently in Section 2, GDIGS improves upon both of these surveys in sensitivity, spatial resolution, and spectral resolution.We can use GDIGS to partially correct for the intrinsic coverage bias of pointed surveys by locating otherwise unidentified sources within the survey zone, 32.3 IGS is a blind survey, and is consequently sensitive to populations that targeted surveys miss.This feature of mapped surveys like GDIGS is especially relevant for studying clumps of the DIG, which the inclusion criteria of targeted surveys ignore.Using mapped surveys to study H II populations has been shown to be effective in Hou et al. (2022).Hou et al. (2022) mapped the Galactic plane in RRL emission near 1 GHz over 55 • > ℓ > 33 • , | b | < 2.0 • with a sensitivity of 0.25 mJy beam −1 , at 2.2 km s −1 spectral resolution and ∼ 3 ′ spatial resolution using the Five-hundred-meter Aperture Spherical radio Telescope (FAST).This survey does not overlap with the GDIGS survey area.With GDIGS we are therefore able to obtain a more complete census of H II regions and assorted DIG features in the observed section of the Galactic disk than with previous targeted surveys.
Here we use GDIGS data to investigate new sources of RRL emission and to re-analyze previously known sources in ways that only a blind survey allows.We distinguish the emission from known H II regions using version 2.4 of the WISE Catalog of Galactic H II regions (Anderson et al. 2014, hereafter the "WISE Catalog").The WISE Catalog is statistically complete for H II regions ionized by O-type stars (Armentrout et al. 2017).One goal with this study is to improve the WISE Catalog by providing RRL detections and associated information, particularly velocity measurements, for as many sources previously lacking such observations as possible within the GDIGS survey area.We also intend to locate any previously unreported H II regions in the survey area.In the course of the study we encountered numerous regions of discrete RRL emission that do not fit the previously described characteristics of H II regions.We report these in Sections 3.2.2 and 3.2.3.

GDIGS Data
GDIGS is a survey of ionized gas in the midplane of the Milky Way.The GDIGS RRL data were first published in Anderson et al. (2021).A complete description of the details of data collection and processing can be found there, while a shorter summary is provided here.
The GDIGS data were collected using the C-band receiver on the Green Bank Telescope (GBT) in total power mode.Within the 4-8 GHz bandpass, we tuned to 15 usable hydrogen RRLs, as defined in Anderson et al. (2021).The reduced data of average hydrogen RRL emission have a spatial resolution of 2. ′ 65 and 0.5 km s −1 spectral resolution.The rms spectral noise per spaxel is ∼ 10 mK, with sensitivity to emission measures EM ≳ 1100 pc cm −6 .This emission measure translates to a mean electron density of ⟨n e ⟩ ≳ 30 cm −3 for a 1 pc path length, or ⟨n e ⟩ ≳ 1 cm −3 for a 1 kpc path length.Our data processing methodology is described in Anderson et al. (2021).

MIR Data
Mid-infrared (MIR) observations are an important diagnostic tool used to determine if an ionized gas source is an H II region or another class of object.We use 3.6 µm and 8.0 µm data from the Spitzer GLIMPSE sur-vey (Benjamin et al. 2003;Churchwell et al. 2009) and 24 µm data from the MIPSGAL survey (Carey et al. 2009).These surveys provide the best resolution and sensitivity available.H II regions have strong emission at these wavelengths, particularly 8.0 µm and 24 µm.

The WISE Catalog
We compare the regions discussed in this paper to those of the WISE Catalog.The WISE Catalog is the largest, most complete catalog of Galactic H II regions, and contains all 8416 known H II regions and H II region candidates.The catalog distinguishes H II regions from other ionized gas sources by their characteristic emission.
There are four types of sources listed in the catalog: "known," "candidate," "group," and "radio quiet."Known sources have detected spectroscopic ionized gas emission as well as ∼ 20 µm emission.Candidate sources have radio continuum emission coincident with their ∼ 20 µm emission and are thus likely to be real H II regions, but do not have a spectroscopic ionized gas detection.Radio quiet sources have MIR emission but lack associated radio continuum emission; this may be because the radio continuum emission is too faint, or because existing surveys do not cover the sources.Finally, group sources are located nearby "Known" H II regions and are thought to be part of a larger complex but do not have measured spectroscopic ionized gas emission.The latter three of these categories-"candidate", "group", and "radio quiet" sources-are suspected to be H II regions but require confirmation before they can be moved to the "known" category.The detection of RRL emission from WISE Catalog sources previously lacking such a detection provides both confirmation that the source is an H II region and also an LSR velocity that can be used to derive a kinematic distance.We use the presence of these tracers in our identification of new regions below.
The census of H II regions in the WISE Catalog is lacking information for a large fraction of its 8416 entries, ∼ 2000 of which fall within the GDIGS survey area.Many objects have multiple reported ionized gas velocity components and therefore lack a unique LSR velocity assignment, which is a requirement for calculating a kinematic distance.There are 2361 confirmed H II regions with spectroscopic gas detections.The WISE Catalog lacks spectroscopic gas detection for 72% of its entries, including 626 "group" H II regions, 1681 H II region candidates, and 3748 radio-quiet H II region candidates.

GDIGS DISCRETE SOURCES
GDIGS is a blind survey of ionized gas in the Galactic plane and is therefore a useful dataset both for characterizing previously-known H II regions and for searching for other discrete sources of ionized gas.We examine GDIGS RRL emission from sources in the WISE Catalog to determine the velocity of the source and, where applicable, to confirm that the source is an H II region.We also search the GDIGS survey data for new H II regions that do not appear in the WISE Catalog, and locate numerous locations of compact RRL emission that lack MIR emission.

WISE Catalog Objects
We explore detections in the GDIGS survey data of sources belonging to the WISE Catalog.Due to confusion within the GDIGS beam, we are only able to conclusively associate GDIGS emission with WISE Catalog sources that are isolated or are angularly large.Smaller, more tightly grouped regions are likely to be indistinguishable from each other at the GDIGS spatial resolution.
Some WISE Catalog sources are in close angular proximity with each other and share the same reported velocity or velocities.In these cases both objects were originally observed in the same telescope beam.Following the methodology used to construct the WISE Catalog, we report the same measured velocities for both objects.

Multiple-Velocity H II Regions
Many previously-known H II regions have more than one measured RRL emission velocity component, which complicates efforts to calculate the regions' kinematic distances and other physical parameters.In the HRDS, for instance, over 30% of all detected H II regions have more than one RRL velocity (Anderson et al. 2015a).For such regions, one velocity component presumably comes from the H II region itself, and the other component(s) from the DIG along the line of sight.We are confident in this assumption because two H II regions along a single line of sight is statistically unlikely (Anderson et al. 2015a).With their large beams and sensitivity to extended emission, single-dish observations are especially susceptible to detecting emission from diffuse RRL emission in the same pointing.
Many sources discussed in this section, both those newly detected in RRL emission and those previously known, have multiple observed velocity components.We can use GDIGS data with a method similar to the one described in Anderson et al. (2015b) to determine the "correct" velocity for some of these sources.For multiple RRL velocity H II regions in the GDIGS range we prepare Moment 0 maps of each measured velocity component by integrating over a velocity range equal to the full width at half maximum (FWHM) of the component and centered at the reported velocity.Each map is centered spatially on the WISE Catalog source.For each region we visually compare the Moment 0 maps at the multiple velocities; the Moment 0 morphology at the "correct" velocity should match the angular size of the source in the WISE Catalog.The emission at the other velocity component(s) along the same line of sight is expected to be more spatially extended.
In Figure 1 we show an example of this method using the known H II region G010.331−00.287from the WISE catalog.We display the GDIGS Hnα emission for the two detected velocities: 0.8 km s −1 and 33.4 km s −1 (Lockman 1989).The emission centered at the 0.8 km s −1 velocity is diffuse and does not match the expected morphology.The emission centered at the 33.4 km s −1 velocity matches the expected H II region morphology, and we therefore treat this as the "correct" H II region velocity.
As a point of comparison, in Figure 2 we show an example of an unsuccessful use of the method with H II region G019.494−00.150, also from the WISE Catalog.As in Figure 1 we display the GDIGS Hnα emission for the two detected velocities, this time at 32.1 km s −1 and 54.6 km s −1 (Anderson et al. 2011a).The emission observed from both velocity components has a good agreement with the size and location of the WISE Catalog entry; we therefore cannot determine which velocity is the "correct" one for the H II region.We compile a list of all 121 multiple-velocity H II regions in the GDIGS range from the WISE Catalog.We use GDIGS data to identify the LSR velocity of the discrete source for 35 of these 121 sources.We give the H II region velocity for these 35 sources in Table 1, which lists the source name, the H II region RRL velocity, and the other RRL velocities detected along the line of sight.The H II regions G359.717−00.036 and G359.727−00.037 are in close angular proximity and have the same reported velocities.In this case, the velocities reported in the WISE Catalog were obtained by previous observations that observed both objects in the same beam.Following the methodology of the WISE Catalog, we report the same velocity components for both.We were not able to distinguish the "correct" velocity for the remaining sources using our method.We hypothesize that the other velocities come from the DIG or from extended envelopes of other nearby H II regions.
We also use GDIGS data to verify a subset of the H II region velocities reported in Anderson et al. (2015a).In Anderson et al. (2015a) the authors attempted to determine the correct velocities of 117 multiple-velocity H II regions from the HRDS using new GBT RRL observations.We identified 60 H II regions in that paper that fall within the GDIGS survey area and applied the method described earlier in this section.None of these 60 sources showed a disagreement with the analysis of Anderson et al. (2015a).For 45 of these H II regions we successfully confirmed the previously reported velocities.We were unable to determine the velocities of the remaining 15, because we were not able to associate the morphology with that expected from the WISE Catalog.

Group H II Regions
GDIGS can provide RRL detections for "group" H II regions, which are found in complexes with other known  (Lockman 1989).The solid white circles in the lower left corners show the 2. ′ 65 beam size.The 0.8 km s −1 emission is spatially extended, whereas the 33.4 km s −1 emission is spatially centered on G010.331−0.278with an angular extent similar to that reported in the WISE Catalog (white circles).We conclude that the velocity of G010.331−0.278 is 33.4 km s −1 , and the 0.8 km s −1 component is from the DIG.H II regions but have not been detected previously in pointed RRL surveys.There are ∼ 160 WISE Catalog group H II regions in the GDIGS survey zone.Many of these are confused with brighter nearby H II regions, and so their RRL emission cannot be reliably measured.There are 40 unconfused group H II regions that we examine for RRL emission.We consider group H II regions unconfused if they are separated from their nearest neighbor by a beam width or more.For each of these, we extract a GDIGS Hnα spectrum by integrating over a circle of radius given by the WISE Catalog.We then fit a Gaussian profile to each spectrum thus obtained.
We give the Gaussian fit parameters for the examined group H II regions in Table 2, which lists the source name, the Galactic longitude (ℓ) and latitude (b), the line intensity (T L ), the LSR velocity (V LSR ), the FWHM line width (∆V ), the rms noise, and the signal-to-noise ratio (S/N).The errors in Table 2 are 1σ uncertainties from the Gaussian fits.Of the 40 observed H II regions, 18 have multiple RRL velocity components.For those sources, we apply the method described in Section 3.1.1.If the correct velocity can be determined, we report the correct velocity first, followed by the other velocity with an asterisk appended to the name.For sources for which we cannot determine which velocity is correct, we report both, appended with "a" and "b" in order of decreasing Hnα intensity.All listed line parameters have a S/N of at least three, where the S/N as defined by Lenz & Ayres (1992) is where 2.35 is the FWHM in km s −1 of the spectral smoothing kernel we used when generating the spectra.
We expect to detect FWHM line widths between ∼ 10 km s −1 and ∼ 35 km s −1 (Anderson et al. 2011b).Line widths with values lower than 10 km s −1 imply electron temperatures < 1100 K (Anderson et al. 2018a).Line widths over ∼ 35 km s −1 are very unlikely to be single components, so those reported here are likely the result of blended components.

H II Region Candidates
Within the GDIGS survey range there are ∼ 400 WISE Catalog H II region candidates that have spatially coincident radio continuum and MIR emission but lack a spectroscopic ionized gas detection.Since the GDIGS area has been searched several times by sensitive, pointed RRL surveys (e.g., Anderson et al. 2011bAnderson et al. , 2015c)), new H II region detections using GDIGS are expected only for candidate sources that were skipped in previous surveys.We examine these ∼ 400 locations and find 27 H II region candidates that are detected in the GDIGS Hnα data and are unconfused by the RRL emission from nearby bright H II regions, using the same criteria as in Section 3.1.2.We integrate GDIGS Hnα data over the extent of the sources defined in the WISE catalog and fit Gaussian components.We give the RRL parameters in Table 3. Eight of the nebulae are best fit with two Gaussian components.For these sources with multiple velocity components, we again apply the method described in Section 3.1.1.We use the same table format as that of Table 2 and the same S/N requirement as described in Section 3.1.2.Two candidates, G037.319+00.162 and G037.320+00.168,have low S/N values and appear to have blended peaks.Blended peaks occur where two emission peaks that are broad enough and close enough in velocity appear to merge together into one broad, flattopped peak.We fit these candidates with two Gaussian components each.

Radio-quiet H II Region Candidates
Within the GDIGS survey range there are ∼ 800 WISE Catalog radio quiet H II regions.These nebulae have the characteristic MIR morphology of H II regions, but have no detected radio continuum emission in any extant survey.Given the sensitivity of GDIGS, we therefore do not expect many Hnα RRL detections.In the Galactic longitude range 17.5 • > ℓ > −5 • , however, the quality of public radio continuum data suitable for identifying H II regions is poor so the pointed surveys may not have identified all detectable targets.In this range the GDIGS data may be able to provide RRL detections for some radio-quiet H II regions.
We measure RRL emission from 21 out of the 800 WISE Catalog radio quiet H II regions by integrating GDIGS Hnα data over the extent of the sources defined in the WISE catalog.All but one of these are in the zone 17.5 • > ℓ > −5 • .Four of these nebulae are best fit with two Gaussian components, and one with three Gaussian components.We give the Hnα RRL parameters in Table 4, with the same table format as that of Table 2 and the same S/N requirement as described in Section 3.1.2.

New RRL Detections
We discover 45 discrete sources in GDIGS that are not associated with objects in the WISE Catalog.
Some H II region candidates may have been missed by the WISE Catalog because their MIR emission is especially faint or because it was confused with other H II regions in close proximity.If the locations of discrete RRL emission coincide with MIR emission characteristic of H II regions, we categorize them as newly-discovered H II regions; if not, we characterize them as discrete emission zones.All WISE Catalog objects have coincident MIR emission, so the lack of coincident MIR is an important difference.
In addition, we identify several locations of RRL emission of particular interest because of their anomalously high LSR velocities.Because of these unusual velocities, we analyze this last category separately from the rest of the H II regions and discrete emission zones.

Newly Discovered H II Regions
There are eight compact locations of GDIGS RRL emission that are spatially coincident with MIR emission but are not in the WISE Catalog.We hypothesize that these are H II regions that were likely missed in the WISE Catalog because their MIR emission is faint or confused with that of other nearby regions.Of particular note is the relative faintness of these regions' 8 µm emission compared to that of other H II regions.In all other respects these objects look like previously   a Source names for HII regions with multiple detected hydrogen RRL components are appended with "a" and "b" in order of decreasing Hnα RRL intensity.Source names for additional velocity components for sources with successfully determined "correct" velocities are appended with an asterisk.
known H II regions, and we do not have enough information to determine whether the deficiency in 8 µm emission reflects an inherent difference significant enough to put these objects into a different class.We measure RRL emission from these new H II regions by integrating GDIGS Hnα data over their extents defined visually from 24 µm MIR data.The RRL emission from three of these nebulae is best fit with two Gaussian components, and one with three Gaussian components.We characterize the Hnα GDIGS emission from these locations in Table 5.The table format is the same as that of Table 2, with the addition of the region radius.We use the same method for determining the correct velocity as described in Section 3.1.1and the same S/N requirement as described in Section 3.1.2.

Discrete Emission Zones (DEZs)
We discover 30 locations of GDIGS Hnα emission in the Moment 0 maps that lack the spatially coincident MIR emission expected of H II regions and also lack the expected radio continuum emission of supernova rem-nants (SNRs).We call these 30 locations discrete emission zones (DEZs).MIR emission is present for all known Galactic H II regions, including those discovered in older surveys that targeted radio continuum emission peaks (e.g.Lockman 1989).We likewise expect MIR emission from planetary nebulae (Anderson et al. 2012).Most SNRs lack MIR emission (Fuerst et al. 1987), and because they are non-thermal radio sources, are not expected to be strong in RRL emission (Liu et al. 2019).Any RRL emission from SNRs is expected to occur at frequencies lower than those observed in GDIGS.
With available data, we cannot conclusively categorize the emission sources, and here we only characterize their Hnα emission properties.We measure RRL emission from these locations by integrating GDIGS Hnα data over their extents defined by visually fitting circles using the GDIGS Moment 0 map.Of the 30 nebulae, 11 are best fit with two Gaussian components, and one with three Gaussian components.We give the GDIGS Hnα line parameters locations in Table 6.The table format is the same as that of Table 5.We use the same method for determining the correct velocity as described in Section 3.1.1and the same S/N requirement as described in Section 3.1.2.For these DEZs, the "correct" velocity is the one that best matches the emission seen in the initial Moment 0 maps integrated over all velocities.

Anomalous Velocity Features
We identify 10 emission zones in the GDIGS Hnα Moment 0 maps that are not known to be H II regions and have anomalously high velocities compared to that of other H II regions at the same Galactic longitudes.We call such emission zones anomalous velocity features (AVFs) and estimate their positions and radii by visually fitting circles to the emission in the GDIGS Hnα Moment 0 map.
For each of the 10 AVFs, we extract a GDIGS Hnα spectrum by integrating over the extent of the source.
To each such spectrum we then fit a Gaussian profile.
We designate the 10 identified AVFs by their Galactic latitude and longitude.We provide the Gaussian fit parameters in Table 7.The table format is the same as that of Table 5.We use the same method for determining the correct velocity as described in Section 3.1.1and the same S/N requirement as described in Section 3.1.2.

WISE Catalog H II regions
Because H II regions are formed by short-lived massive stars, a sufficiently large sample of H II regions with known distances can be used to measure the current properties of Galactic massive star formation (Anderson et al. 2018a 8. Together, these 131 new velocity measurements represent a 5.6% increase in the number of WISE Catalog objects with a known velocity. The GDIGS RRL velocities of group H II regions indicate that they are for the most part correctly associated with known H II regions.Of the 40 observed group H II regions, 38 have one observed LSR velocity within 10 km s −1 of the "group velocity" (the velocity measured for the H II region complex) reported in the WISE Catalog; all have one component within 20 km s −1 .In Figure 3, we plot the differences between the measured GDIGS group H II region RRL velocity and the group velocity as reported in the WISE Catalog.H II regions with multiple observed velocities are excluded from this plot.There is a strong correlation between the two velocities, indicating that the original group associations for this sample were largely correct.

DEZs
Ionized gas comprises a significant portion of the Galactic ISM, accounting for more than 20% of the Milky Way's gas mass (Reynolds 1991).Despite this, current understanding of the ionized component of the ISM is far from complete.The identification and characterization here of 30 discrete zones of RRL emission suggests that the Galactic DIG may consist in part of compact emission zones with sizes of a few arcminutes.a Source names for HII regions with multiple detected hydrogen RRL components are appended with "a" and "b" in order of decreasing Hnα RRL intensity.Source names for additional velocity components for sources with successfully determined "correct" velocities are appended with an asterisk.
The Hnα RRL morphology of the DEZs is generally centrally-peaked.Some DEZs have nearly circular RRL emission, some are oblong, and some are irregular in shape.In Figure 4 we show an example Moment 0 map of a typical DEZ, G018.641−00.296.
We examine 21 cm radio continuum emission from the Very Large Array (VLA) Galactic Plane Survey (VGPS) at the locations of the 11 DEZs that fall within the VGPS survey area and found identifiable radio continuum emission for G026.584+00.163 and G027.128+00.010.
One of the DEZs, G012.508−00.127,has very faint 24 µm emission near its center, but lacks the 8 µm emission we also require from H II regions.

AVFs
Table 7. Anomalous Velocity Features (AVFs) The ten objects that we call AVFs merit further study.They do not fit neatly into current models of gas behavior in the Galactic disk (Reid et al. 2019), and their existence may require an update to those models.We plan additional observations in order to better under- stand the origins of these objects and the reason for their high velocities.
To investigate the nature of these features, we compare GDIGS RRL data with the MIR data described in Section 2.2, as well as with H I from the Galactic All-Sky Survey (GASS; McClure-Griffiths et al. 2009), and with 13 CO J = 2 → 1 spectra from the Structure, Excitation, and Dynamics of the Inner Galactic ISM survey (SEDIGISM; Schuller et al. 2020) and the J = 1 → 0 from the Galactic Ring Survey (GRS; Jackson et al. 2006).GASS has a spectral resolution of 0.82 km s −1 and a sensitivity of 57 mK.SEDIGISM has 1.0 K 1σ sensitivity, 30 ′′ angular resolution, and 0.35 km s −1 spectral resolution.The GRS has 46 ′′ angular resolution, 0.2 km s −1 spectral resolution, and 0.4 K sensitivity.We compare our RRL observations to 13 CO data from these surveys to search for any associated molecular gas, which is commonly found for Galactic H II regions.(Anderson & Bania 2009).
We show sample spectra, Moment 0 maps, and MIR maps in Figure 5.The complete figure set (30 images) can be found in the online version of this article.Morphologically, the RRL Moment 0 maps of AVFs have areas of bright emission with adjacent diffuse emission.In half of the AVFs the bright emission is concentrated in one clump, while in the other half it is split between two to three smaller clumps connected by diffuse emission.Two of the latter group have a structure resembling a shell.In eight out of ten cases each bright area has diffuse emission in all directions that gradually decreases in intensity.In the remaining two AVFs the intensity of GDIGS emission drops off sharply in at least one direction.The individual clumps vary in shape between compact, oblong, and irregular structures.The wide variety of morphologies seen in the AVFs further confounds attempts to determine their nature and origin, and raises the possibility that the AV Fs might not all be the same type of object.
We compare AVF observations with MIR data to determine if the objects may be H II regions, and find that none have the associated 24 µmemission that is characteristic of H II regions.Nine have 24 µm MIR emission near to the RRL emission, but in those cases it is inconsistent with the RRL emission in spatial location or size and is often associated with other known objects.We therefore do not consider that MIR emission is associated with these nine AVFs, and correspondingly, find no evidence that AVFs are H II regions.
All ten AVF spectra show H I spectrally coincident with the AVF's RRL emission, which signals the presence of associated neutral gas.Since H I emits at all allowed velocities in the Galaxy, this result is expected.Four AVFs have 13 CO spatially coincident with the RRL emission, and six AVF spectra show spectrally coincident 13 CO emission.For these six, however, the RRL and 13 CO emission have different spatial distributions.While the circle defining the limits of each AVF is drawn to fit as tightly as possible, the AVFs are not circular so the RRL emission does not completely fill the circle.In these six cases, 13 CO emission is observed inside the circle over which the spectrum is integrated, but is not spatially coincident with the RRL emission.
Eight AVFs are at least partially co-spatial with objects from the WISE Catalog.Among these eight AVFs, there are 13 known H II regions, 11 H II region candidates, and 12 radio-quiet H II regions fully or partially co-spatial with an AVF.Based on their relative loca-tions, angular sizes, and velocities we do not consider any of the catalog objects to be associated with the AVFs.In the case of the known H II regions, we check the AVF's velocity against the velocity of the H II region reported in the WISE Catalog.If they are not within 20 km s −1 , we consider the AVF and H II region unlikely to be associated.We also compare the position and morphology of the AVF to the reported size and position of the H II region in the WISE Catalog.In all cases, either the velocity does not match, the position and morphology do not match, or both.We require all these criteria to match in order to consider the AVF and H II region to be associated.H II region candidates and radio-quiet sources do not have measured velocities, and as such, we cannot make a velocity comparison between these regions and the AVFs.The position and morphology of the AVFs do not correspond to any such sources in the WISE Catalog.
In Figure 6 we show the location of these AVFs on a longitude-velocity (LV) diagram along with GDIGS data, and known H II regions from the WISE Catalog.We also show rectangles with the approximate longitude and velocity ranges of Bania's Clumps 1 and 2 (Bania 1977).These clumps are large, isolated regions of 12 CO emission observed in the direction of the inner Galaxy.Both are ∼ 1 • wide in Galactic longitude, with Clump 1 centered at ∼ 355 • and Clump 2 centered at ∼ 3 • .Clump 1 spans a velocity range of ∼ 60 − 120 km s −1 while Clump 2 covers a range of ∼ 50 − 150 km s −1 .The velocity range of Clump 1 exceeds that allowed by circular rotation at its Galactic longitude, and therefore much of the its velocity is expected to be noncircular (Bania 1977).Clump 2 is notable for having the second widest velocity range of all Galactic 12 CO features, with only the nuclear disk exceeding it.Bania's Clumps are observed over longitude and velocity ranges comparable to some of the AVFs, and are included for comparison.

Anomalous Velocity Features (AVFs)
The WISE Catalog contains two known H II regions that also have unusually high velocities: G002.614+00.133,which has a velocity of 102.4 km s −1 (Lockman 1989), and G007.700−00.123,which has a velocity of 151.5 km s −1 (Anderson et al. 2018b).The AVFs may be of a related class.Both of the known H II regions, however, have coincident 24 µm emission, whereas the AVFs lack such emission.Roughly 80% of H II regions have associated 13 CO emission (Anderson et al. 2009).Meanwhile, only 6 out of the 10 AVFs' spectra show 13 CO emission at the same velocity as the RRL emission.Only 1 out of the 10 AVFs, G355.612+00.314,shows 13 CO spatially associated with the RRL emission,    et al. (2022) shows some compact emission at velocities between 100 and 150 km s −1 .These features, however, have very narrow linewidths indicating that they are noise.
Below, we discuss noteworthy features of the most interesting individual AVFs.
• AVF G355.612+00.314 is the only AVF observed in the fourth Galactic quadrant.It is presented as an example in Figure 5 because it demonstrates the aspects of AVFs outlined above: multiple bright areas connected by diffuse emission which completely surrounds them, spatially coincident 13 CO emission, 24 µm MIR emission associated with another object, spectrally coincident H I and 13 CO emission, and several coincident but unassociated objects from the WISE catalog.This AVF also falls within the velocity range of Bania's Clump 1, and sits on the edge of the Clump 1 longitude range (Bania 1977), as seen in Figure 6.The AVF may be part of the Clump 1 complex, but according to SEDIGISM 13 CO 2 − 1 data it is not coincident spatially or spectrally with the bulk of the Clump 1 13 CO emission.
• AVF G001.702−00.156 is the closest in longitude to the Galactic Center and, at +232.0 km s −1 , has the highest LSR velocity of all the observed AVFs.Visual inspection of its position on the LV diagram (Figure 6) suggests it may be associated with a Galactic Center structure, likely the nuclear disk.The nuclear disk, also referred to as the nuclear ring or central molecular zone (CMZ), is a zone of high-velocity gas located within the inner ∼ 0.5 kpc of the Milky Way (Sormani et al. 2019).We base this inference on the behavior of the known H II regions and the RRL observations near the Galactic center, which roughly form a slanted spike structure on the LV diagram representing the nuclear disk (Fux 1999).A line drawn to continue the apparent structure would pass near this AVF.This AVF's velocity is outside the velocity range of the 13 CO data used here, so no 13 CO comparison is made.
• AVF G003.158−00.040sits at the edge of the velocity range of Bania's Clump 2 and is within its longitude range (Bania 1977), as seen in Figure 6.Like G355.612+0.314and Clump 1, G003.158−0.040straddles the edge of the box representing the longitude-velocity bounds of Clump 2, and we are likewise unable to conclude whether or not this AVF is associated with the Clump.
• AVF G009.402+00.180has an exceptionally small FWHM, at only 5.1 km s −1 .This value implies a low plasma temperature, an unusually low amount of turbulence, or both.
• AVF G013.782−00.152 is the AVF with the lowest LSR velocity in the first Galactic quadrant, and the second lowest LSR velocity overall after G355.612+00.314.One velocity component of the WISE H II region G013.709−00.243, at 97.8 km s −1 , is within 10 km s −1 of the AVF's velocity, but the region is small in size relative to the AVF and it is far from the AVF's centroid, so we cannot draw any conclusions in regard to a possible association.
• AVF G020.206+00.036has the greatest Galactic longitude by more than 5 • .It is the closest in velocity to the bulk of the H II emission, but is distinct enough to be noticeable on Figure 6.This AVF has a small amount of coincident 24 µm emission, but the morphology does not resemble that expected of an H II region.

FWHM Analysis
The FWHM line width is related to the thermal and turbulent motions in the plasma.We can compare the trends in the FWHM values between known WISE Catalog objects and newly discovered regions to check for additional differences between the categories.To that end, we calculate the mean and standard deviation of the FWHM line widths for known WISE Catalog regions, including those with first spectroscopic detections reported in this paper, and for each category of newly discovered region.We exclude regions with multiple velocity components.These statistics, as well as the number of included sources in each category, are reported in Table 9.There are too few new H II regions and AVFs to reasonably draw conclusions from their statistics.The mean FWHM of the DEZs is greater than that of the WISE Catalog objects, but is within one standard deviation.We therefore cannot conclude any additional distinctions between the DEZs and WISE Catalog regions from FWHM trends alone.We use GDIGS RRL data to identify and characterize RRL features from discrete sources of emission.We summarize these results below.

SUMMARY
• We identify from the GDIGS data the H II region velocity of 35 sources with multiple previouslyreported velocity components.We hypothesize that for H II regions with multiple observed velocities, the velocity or velocities not associated with the H II region itself belong to DIG along the line of sight.
• We detect RRL emission for 40 "group" H II regions, 27 H II region candidates, and 21 radio quiet H II regions, all from the WISE catalog.In total, we move 89 WISE Catalog objects into the "known" H II region category.
• We detect RRL emission from eight locations that we believe to be previously unknown Galactic H II regions.
• We detect 30 GDIGS discrete sources that lack coincident MIR emission, and therefore we conclude they are not H II regions.We detect and characterize via Gaussian fitting the RRL emission from these locations.
• Finally, we identify 10 locations of anomalously high-velocity RRL emission, which we refer to as AVFs.We examine the emission at these locations using 13 CO, H I, and MIR data.Based on this analysis we conclude that the AVFs are likely not H II regions.Beyond that, we do not have sufficient information to determine to what class of object they belong.
We present Gaussian fit parameters for 96 H II regions, 30 DEZs, and 10 anomalous velocity features.Of these, the 30 DEZs, the 10 AVFs, and eight of the H II regions were previously unknown; the rest lacked observed RRL spectra.These parameters, specifically the LSR velocities, allow calculation of the kinematic distance to these objects.The kinematic distance in turn allows determination of other physical parameters including physical extent of the regions.

A. WISE CATALOG
We have updated the WISE Catalog of Galactic H II Regions website1 with results from these observations.This includes the addition of LSR velocities for 88 H II regions; updated velocities for 39 H II regions with multiple velocity components; and 8 new H II regions.

Figure 1 .
Figure1.Hnα Moment 0 images toward source G010.331−0.278.Each image is integrated over a range centered at one of the two detected RRL velocities for this source and equal in width to the corresponding FWHM.The left image is integrated over a FWHM of 14.1 km s −1 with the range centered at 0.8 km s −1 , and the right image is integrated over a FWHM of 27.8 km s −1 with the range centered at 33.4 km s −1(Lockman 1989).The solid white circles in the lower left corners show the 2. ′ 65 beam size.The 0.8 km s −1 emission is spatially extended, whereas the 33.4 km s −1 emission is spatially centered on G010.331−0.278with an angular extent similar to that reported in the WISE Catalog (white circles).We conclude that the velocity of G010.331−0.278 is 33.4 km s −1 , and the 0.8 km s −1 component is from the DIG.

Figure 2 .
Figure 2. Hnα Moment 0 images toward source G019.494−00.150.Each image is integrated over a range centered at one of the two detected RRL velocities for this source and equal in width to the corresponding FWHM.The left image is integrated over a FWHM of 16.8 km s −1 with the range centered at 32.1 km s −1 , and the right image is integrated over a FWHM of 15.1 km s −1 with the range centered at 54.6 km s −1 (Anderson et al. 2011a).The solid white circles in the lower right corners show the 2. ′ 65 beam size.For both velocity components we see similar emission, and therefore are unable to discern which velocity component belongs to the H II region.

Figure 4 .
Figure 4. Example Hnα Moment 0 map of the DEZ G018.641−0.296.The image is integrated over a range centered at the velocity detected for this DEZ, 67.1 km s −1 and equal in width to the detected FWHM, 36.8 km s −1 .The white circle marks the estimated extent of the DEZ.The filled white circle in the lower right corner shows the 2. ′ 65 GDIGS beam size.

Figure 5 .
Figure5.Top: Spectra for AVF G355.612+00.314.GDIGS RRL data are shown in black, GASS H I data in red, and SEDIGISM 13 CO data in blue.The GDIGS data are spectrally Gaussian-smoothed with a FWHM of 2.35 km s −1 .The vertical green line marks the estimated velocity of the AVF.Bottom left: Moment 0 map for the same AVF.The color map displays the GDIGS RRL Moment 0 integrated intensity, while the black contours display the SEDIGISM 13 CO Moment 0 integrated intensity.As in Section 3.1.1,the Moment 0 integration is carried out over a velocity range equal to the FWHM and centered on the LSR velocity.The 13 CO data are spatially Gaussian-smoothed with a FWHM of 19 ′′ .The contour levels are at 2σ and 3.5σ, where σ is the standard deviation of the 13 CO Moment 0 intensity across the circle.The white circle marks the estimated extent of the AVF.The filled white circle in the upper left shows the 2. ′ 65 GDIGS beam size.Red circles mark known H II regions, orange circles mark radio-quiet H II regions, and cyan circles mark H II region candidates, all from the WISE Catalog.Of the two known H II regions in the area, the northern one has a velocity of −2.6 km s −1 while the other has velocity components of 3.0 km s −1 and −79.1 km s −1 .We do not think either is associated with the AVF.Bottom right: Example three-color composite image of the same AVF made from Spitzer MIR data; 24 µm emission is represented in red, 8 µm emission is represented in green, and 3.6 µm emission is represented in blue.White contours display the GDIGS RRL Moment 0 intensity from the left panel.The contours on this map are at σ, 1.8σ, and 3.1σ, where σ is the standard deviation of the RRL Moment 0 intensity across the circle.A magenta circle marks the estimated extent of the AVF, and red, orange, and cyan circles mark H II regions, radio-quiet H II regions, and H II region candidates from the WISE catalog.The filled white circle in the upper right again shows the 2. ′ 65 GDIGS beam size.A figure set containing maps and spectra for the 10 AVFs discussed in Section 3.2.3 is available in the online version of this article.

Figure 6 .
Figure 6.Longitude-velocity (LV) diagram of the GDIGS survey data (grayscale).Known H II regions from the WISE catalog are marked with crosses: blue for regions with one velocity component, cyan for each component of those with multiple velocity components.Small green circles mark the centroids of the Gaussian decompositions of the GDIGS data (seeAnderson et al. 2021).Larger red circles mark the AVFs.The orange boxes show the approximate position and dimensions of Bania's Clump 1 and 2(Bania 1977).

Table 1 .
Multiple RRL Velocity Locations

Table 2 .
GDIGS Hnα RRL Parameters from WISE Catalog Group H II Regions a Source names for HII regions with multiple detected hydrogen RRL components are appended with "a" and "b" in order of decreasing Hnα RRL intensity.Source names for additional velocity components for sources with successfully determined "correct" velocities are appended with an asterisk.

Table 3 .
GDIGS Hnα RRL Parameters from WISE Catalog Candidate H II Regions Source names for HII regions with multiple detected hydrogen RRL components are appended with "a" and "b" in order of decreasing Hnα RRL intensity.Source names for additional velocity components for sources with successfully determined "correct" velocities are appended with an asterisk.

Table 4 .
GDIGS Hnα RRL Parameters from WISE Catalog Radio-quiet H II Regions

Table 5 .
GDIGS Hnα RRL Parameters from New H II Regions Source names for HII regions with multiple detected hydrogen RRL components are appended with "a" and "b" in order of decreasing Hnα RRL intensity.Source names for additional velocity components for sources with successfully determined "correct" velocities are appended with an asterisk.
).The present work adds H II regions avail-able to such studies by confirming 27 H II region candidates, 21 radio-quiet H II regions, and 40 group H II regions as known H II regions, adding eight previously unreported H II regions, and confirming the velocities of 35 previously known H II regions.These numbers are tabulated in Table

Table 8 .
New RRL Detections by Category Histogram of differences between the measured GDIGS RRL velocity of 22 group H II regions and the group velocity reported in the WISE Catalog, overlaid with the kernel density estimate (KDE) curve for the same.The 18 H II regions with multiple velocity components have been excluded.The group H II regions all have velocities within 20 km s −1 of the velocities of their H II region complexes.
and that AVF's RRL emission is not associated with MIR emission.Based on the lack of 13 CO and 24 µm emission from these objects, we consider it unlikely that they are H II regions.To what class of object the AVFs belong remains undetermined.The LV diagram of RRL emission presented in Hou

Table 9 .
FWHM Line Width Statistics