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T. M. Bania

Publications and source records attributed to T. M. Bania.

At least 19 recordsLinked to original sources
arXiv2024

The Most Sensitive Radio Recombination Line Measurements Ever Made of the Galactic Warm Ionized Medium

Diffuse ionized gas pervades the disk of the Milky Way. We detect extremely faint emission from this Galactic Warm Ionized Medium (WIM) using the Green Bank Telescope to make radio recombination line (RRL) observations toward two Milky Way sight lines: G20, $(\ell,{\it b}) = (20^\circ, 0^\circ)$, and G45, $(\ell,{\it b}) = (45^\circ, 0^\circ)$. We stack 18 consecutive Hn$α$ transitions between 4.3-7.1 GHz to derive ${\rm \langle Hnα\rangle}$ spectra that are sensitive to RRL emission from plasmas with emission measures EM >10 ${\rm \,cm^{-6}\,pc}$. Each sight line has two Gaussian shaped spectral components with emission measures that range between $\sim$100 and $\sim$300 ${\rm \,cm^{-6}\,pc}$. Because there is no detectable RRL emission at negative LSR velocities the emitting plasma must be located interior to the Solar orbit. The G20 and G45 emission measures imply RMS densities of 0.15 and 0.18$\,{\rm cm^{-3}}$, respectively, if these sight lines are filled with homogeneous plasma. The observed ${\rm \langle Hnβ\rangle}$/${\rm \langle Hnα\rangle}$ line ratios are consistent with LTE excitation for the strongest components. The high velocity component of G20 has a narrow line width, 13.5 km s$^{-1}$, that sets an upper limit of <4,000 K for the plasma electron temperature. This is inconsistent with the ansatz of a canonically pervasive, low density, $\sim$ 10,000 K WIM plasma.

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arXiv2023

Methods for Averaging Spectral Line Data

The ideal spectral averaging method depends on one's science goals and the available information about one's data. Including low-quality data in the average can decrease the signal-to-noise ratio (SNR), which may necessitate an optimization method or a consideration of different weighting schemes. Here, we explore a variety of spectral averaging methods. We investigate the use of three weighting schemes during averaging: weighting by the signal divided by the variance ("intensity-noise weighting"), weighting by the inverse of the variance ("noise weighting"), and uniform weighting. Whereas for intensity-noise weighting the SNR is maximized when all spectra are averaged, for noise and uniform weighting we find that averaging the 35-45% of spectra with the highest SNR results in the highest SNR average spectrum. With this intensity cutoff, the average spectrum with noise or uniform weighting has ~95% of the intensity of the spectrum created from intensity-noise weighting. We apply our spectral averaging methods to GBT Diffuse Ionized Gas (GDIGS) hydrogen radio recombination line (RRL) data to determine the ionic abundance ratio, y+, and discuss future applications of the methodology.

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arXiv2023

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

The Green Bank Telescope (GBT) Diffuse Ionized Gas Survey (GDIGS) traces ionized gas in the Galactic midplane by observing radio recombination line (RRL) emission from 4-8 GHz. The nominal survey zone is $32.3^{\circ}> {\ell} > -5^{\circ}$, $|b|<0.5^{\circ}$. Here, we analyze GDIGS Hn$α$ 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 (DIG). 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 $μ$m 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.

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arXiv2022

Do All Low-Mass Stars Undergo Extra Mixing Processes?

Standard stellar evolution models that only consider convection as a physical process to mix material inside of stars predict the production of significant amounts of 3He in low-mass stars (M < 2 Msun), with peak abundances of 3He/H ~ few x 10-3 by number. Over the life-time of the Galaxy, this ought to produce 3He/H abundances that diminish with increasing Galactocentric radius. Observations of 3He+ in HII regions throughout the Galactic disk, however, reveal very little variation in the 3He abundance with values of 3He/H similar to the primoridal abundance, (3He/H)p ~ 10-5 . This discrepancy, known as the "3He Problem", can be resolved by invoking in stellar evolution models an extra-mixing mechanism due to the thermohaline instability. Here, we observe 3He+ in the planetary nebula J320 (PN G190.3-17.7) with the Jansky Very Large Array (JVLA) to confirm a previous 3He+ detection made with the VLA that supports standard stellar yields. This measurement alone indicates that not all stars undergo extra mixing. Our more sensitive observations do not detect 3He+ emission from J320 with an RMS noise of 58.8 microJy/beam after smoothing the data to a velocity resolution of 11.4 km/s . We estimate an abundance limit of 3He/H <= 2.75 x 10-3 by number using the numerical radiative transfer code NEBULA. This result nullifies the last significant detection of 3He+ in a PN and allows for the possibility that all stars undergo extra mixing processes.

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arXiv2021

Discovery of a New Population of Galactic HII Regions with Ionized Gas Velocity Gradients

We investigate the kinematic properties of Galactic HII regions using radio recombination line (RRL) emission detected by the Australia Telescope Compact Array (ATCA) at 4-10 GHz and the Jansky Very Large Array (VLA) at 8-10 GHz. Our HII region sample consists of 425 independent observations of 374 nebulae that are relatively well isolated from other, potentially confusing sources and have a single RRL component with a high signal-to-noise ratio. We perform Gaussian fits to the RRL emission in position-position-velocity data cubes and discover velocity gradients in 178 (42%) of the nebulae with magnitudes between 5 and 200 m/s/arcsec. About 15% of the sources also have a RRL width spatial distribution that peaks toward the center of the nebula. The velocity gradient position angles appear to be random on the sky with no favored orientation with respect to the Galactic Plane. We craft HII region simulations that include bipolar outflows or solid body rotational motions to explain the observed velocity gradients. The simulations favor solid body rotation since, unlike the bipolar outflow kinematic models, they are able to produce both the large, > 40 m/s/arcsec, velocity gradients and also the RRL width structure that we observe in some sources. The bipolar outflow model, however, cannot be ruled out as a possible explanation for the observed velocity gradients for many sources in our sample. We nevertheless suggest that most HII region complexes are rotating and may have inherited angular momentum from their parent molecular clouds.

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arXiv2021

The GBT Diffuse Ionized Gas Survey (GDIGS): Survey Overview and First Data Release

The Green Bank Telescope (GBT) Diffuse Ionized Gas Survey (GDIGS) traces ionized gas in the Galactic midplane by measuring $4-8$GHz radio recombination line (RRL) emission. The nominal survey zone is $32.3^{\circ}> l >-5^{\circ}$, $|b|<0.5^{\circ}$, but coverage extends above and below the plane in select fields, and additionally includes the areas around W47 ($l \simeq 37.5^{\circ}$) and W49 ($l \simeq 43^{\circ}$). GDIGS simultaneously observes 22 Hn$α$ (15 usable), 25 Hn$β$ (18 usable), and 8 Hn$γ$ RRLs (all usable), as well as multiple molecular line transitions (including of H$_2^{13}$CO, H$_2$CO, and CH$_3$OH). Here, we describe the GDIGS survey parameters and characterize the RRL data, focusing primarily on the Hn$α$ data. We produce sensitive data cubes by averaging the usable RRLs, after first smoothing to a common spectral resolution of 0.5km/s and a spatial resolution of 2.65' for Hn$α$, 2.62' for Hn$β$, and 2.09' for Hn$γ$. The average spectral noise per spaxel in the \hna\ data cubes is $\sim\!10$mK ($\sim\!5$mJy/beam). This sensitivity allows GDIGS to detect RRLs from plasma throughout the inner Galaxy. The GDIGS Hn$α$ data are sensitive to emission measures $EM \gtrsim 1100$cm$^{-6}$pc, which corresponds to a mean electron density $\langle n_e \rangle \gtrsim 30$cm$^{-3}$ for a 1pc path length or $\langle n_e \rangle \gtrsim 1$cm$^{-3}$ for a 1kpc path length.

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arXiv2021

The Southern HII Region Discovery Survey. II. The Full Catalog

The Southern HII Region Discovery Survey (SHRDS) is a 900 hour Australia Telescope Compact Array 4-10 GHz radio continuum and radio recombination line (RRL) survey of Galactic HII regions and infrared-identified HII region candidates in the southern sky. For this data release, we reprocess all previously published SHRDS data and include an additional ~450 hours of observations. The search for new HII regions is now complete over the range 259 deg < Galactic longitude < 346 deg, |Galactic latitude| < 4 deg for HII region candidates with predicted 6 GHz continuum peak brightnesses >30 mJy/beam. We detect radio continuum emission toward 730 targets altogether including previously known nebulae and HII region candidates. By averaging ~18 RRL transitions, we detect RRL emission toward 206 previously known HII regions and 436 HII region candidates. Including the northern sky surveys, over the last decade the HII Region Discovery Surveys have more than doubled the number of known Galactic HII regions. The census of HII regions in the WISE Catalog of Galactic HII Regions is now complete for nebulae with 9 GHz continuum flux densities > 250 mJy. We compare the RRL properties of the newly discovered SHRDS nebulae with those of all previously known HII regions. The median RRL full-width at half-maximum line width of the entire WISE Catalog HII region population is 23.9 km/s and is consistent between Galactic quadrants. The observed Galactic longitude-velocity asymmetry in the population of HII regions probably reflects underlying spiral structure in the Milky Way.

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arXiv2021

The Galactic HII Region Luminosity Function at Radio and Infrared Wavelengths

The Galactic HII region luminosity function (LF) is an important metric for understanding global star formation properties of the Milky Way, but only a few studies have been done and all use relatively small numbers of HII regions. We use a sample of 797 first Galactic quadrant HII regions compiled from the WISE Catalog of Galactic HII Regions to examine the form of the LF at multiple infrared and radio wavelengths. Our sample is statistically complete for all regions powered by single stars of type O9.5V and earlier. We fit the LF at each wavelength with single and double power laws. Averaging the results from all wavelengths, the mean of the best-fit single power law index is $\langleα\rangle=-1.75\,\pm\,0.01$. The mean best-fit double power law indices are $\langleα_1\rangle=-1.40\,\pm\,0.03$ and $\langleα_2\rangle=-2.33\,\pm\,0.04$. We conclude that neither a single nor a double power law is strongly favored over the other. The LFs show some variation when we separate the HII region sample into subsets by heliocentric distance, physical size, Galactocentric radius, and location relative to the spiral arms, but blending individual HII regions into larger complexes does not change the value of the power law indices of the best-fit LF models. The consistency of the power law indices across multiple wavelengths suggests that the LF is independent of wavelength. This implies that infrared and radio tracers can be employed in place of H$α$.

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arXiv2020

Green Bank Telescope Observations of ${\bf ^3He^{\bf +}}$: Planetary Nebulae

We use the Green Bank Telescope to search for $^3He^+$ emission from a sample of four Galactic planetary nebulae: NGC 3242, NGC 6543, NGC 6826, and NGC 7009. During the era of primordial nucleosynthesis the light elements $^2H$, $^3He$, $^4He$, and $^7Li$ were produced in significant amounts and these abundances have since been modified primarily by stars. Observations of $^3He^+$ in H II regions located throughout the Milky Way disk reveal very little variation in the $^3He/H$ abundance ratio -- the "$^3He$ Plateau" -- indicating that the net effect of $^3He$ production in stars is negligible. This is in contrast to much higher $^3He/H$ abundance ratios reported for some planetary nebulae. This discrepancy is known as the "$^3He$ Problem". We use radio recombination lines observed simultaneously with the $^3He^+$ transition to make a robust assessment of the spectral sensitivity that these observations achieve. We detect spectral lines at $\sim$ 1 -- 2 mK intensities, but at these levels instrumental effects compromise our ability to measure accurate spectral line parameters. We do not confirm reports of previous detections of $^3He^+$ in NGC 3242 nor do we detect $^3He^+$ emission from any of our sources. This result calls into question all reported detections of $^3He^+$ emission from any planetary nebula. The $^3He/H$ abundance upper limit we derive here for NGC 3242 is inconsistent with standard stellar production of $^3He$ and thus requires that some type of extra mixing process operates in low-mass stars.

astro-ph.GA↗
arXiv2019

The GBT Diffuse Ionized Gas Survey: Tracing the Diffuse Ionized Gas around the Giant HII Region W43

The Green Bank Telescope (GBT) Diffuse Ionized Gas Survey (GDIGS) is a fully-sampled radio recombination line (RRL) survey of the inner Galaxy at C-band (4-8 GHz). We average together ~15 Hn$α$ RRLs within the receiver bandpass to improve the spectral signal-to-noise ratio. The average beam size for the RRL observations at these frequencies is ~2'. We grid these data to have spatial and velocity spacings of 30" and 0.5 km/s, respectively. Here we discuss the first RRL data from GDIGS: a six square-degree-area surrounding the Galactic HII region complex W43. We attempt to create a map devoid of emission from discrete HII regions and detect RRL emission from the diffuse ionized gas (DIG) across nearly the entire mapped area. We estimate the intensity of the DIG emission by a simple empirical model, taking only the HII region locations, angular sizes, and RRL intensities into account. The DIG emission is predominantly found at two distinct velocities: ~40 km/s and ~100 km/s. While the 100 km/s component is associated with W43 at a distance of ~6 kpc, the origin of the 40 km/s component is less clear. Since the distribution of the 40 km/s emission cannot be adequately explained by ionizing sources at the same velocity, we hypothesize that the plasma at the two velocity components is interacting, placing the 40 km/s DIG at a similar distance as the 100 km/s emission. We find a correlation between dust temperature and integrated RRL intensity, suggesting that the same radiation field that heats the dust also maintains the ionization of the DIG.

astro-ph.GA↗
arXiv2019

Metallicity Structure in the Milky Way Disk Revealed by Galactic HII Regions

The metallicity structure of the Milky Way disk stems from the chemodynamical evolutionary history of the Galaxy. We use the National Radio Astronomy Observatory Karl G. Jansky Very Large Array to observe ~8-10 GHz hydrogen radio recombination line and radio continuum emission toward 82 Galactic HII regions. We use these data to derive the electron temperatures and metallicities for these nebulae. Since collisionally excited lines from metals (e.g., oxygen, nitrogen) are the dominant cooling mechanism in HII regions, the nebular metallicity can be inferred from the electron temperature. Including previous single dish studies, there are now 167 nebulae with radio-determined electron temperature and either parallax or kinematic distance determinations. The interferometric electron temperatures are systematically 10% larger than those found in previous single dish studies, likely due to incorrect data analysis strategies, optical depth effects, and/or the observation of different gas by the interferometer. By combining the interferometer and single dish samples, we find an oxygen abundance gradient across the Milky Way disk with a slope of -0.052 +/- 0.004 dex/kpc. We also find significant azimuthal structure in the metallicity distribution. The slope of the oxygen gradient varies by a factor of ~2 when Galactocentric azimuths near 30 deg are compared with those near 100 deg. This azimuthal structure is consistent with simulations of Galactic chemodynamical evolution influenced by spiral arms.

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arXiv2018

The Southern HII Region Discovery Survey I: The Bright Catalog

The census of Galactic HII regions is vastly incomplete in the Southern sky. We use the Australia Telescope Compact Array (ATCA) to observe 4-10 GHz radio continuum and hydrogen radio recombination line (RRL) emission from candidate HII regions in the Galactic zone 259 deg < l < 344 deg, |b| < 4 deg. In this first data release, we target the brightest HII region candidates and observe 282 fields in the direction of at least one previously-known or candidate HII region. We detect radio continuum emission and RRL emission in 275 (97.5%) and 258 (91.5%) of these fields, respectively. We catalog the ~7 GHz radio continuum peak flux densities and positions of 80 previously-known and 298 candidate HII regions. After averaging ~18 RRL transitions, we detect 77 RRL velocity components towards 76 previously-known HII regions and 267 RRL velocity components towards 256 HII region candidates. The discovery of RRL emission from these nebulae increases the number of known Galactic HII regions in the surveyed zone by 82%, to 568 nebulae. In the fourth quadrant we discover 50 RRLs with positive velocities, placing those sources outside the Solar circle. Including the pilot survey, the SHRDS has now discovered 295 Galactic HII regions. In the next data release we expect to add ~200 fainter and more distant nebulae.

astro-ph.GA↗
arXiv2018

A Galactic Plane Defined by the Milky Way HII Region Distribution

We develop a framework for a new definition of the Galactic midplane, allowing for tilt (rotation about Galactic azimuth 90deg.), and roll (rotation about Galactic azimuth 0deg.) of the midplane with respect to the current definition. Derivation of the tilt and roll angles also determines the solar height above the midplane. Here we use nebulae from the WISE Catalog of Galactic HII Regions to define the Galactic high-mass star formation (HMSF) midplane. We analyze various subsamples of the WISE catalog and find that all have Galactic latitude scale heights near 0.30deg. and z-distribution scale heights near 30pc. The vertical distribution for small (presumably young) HII regions is narrower than that of larger (presumably old) HII regions (~25pc versus ~40pc), implying that the larger regions have migrated further from their birth sites. For all HII region subsamples and for a variety of fitting methodologies, we find that the HMSF midplane is not significantly tilted or rolled with respect to the currently-defined midplane, and therefore the Sun is near to the HMSF midplane. These results are consistent with other studies of HMSF, but are inconsistent with many stellar studies, perhaps due to asymmetries in the stellar distribution near the Sun. Our results are sensitive to latitude restrictions, and also to the completeness of the sample, indicating that similar analyses cannot be done accurately with less complete samples. The midplane framework we develop can be used for any future sample of Galactic objects to redefine the midplane.

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arXiv2018

Green Bank Telescope Observations of 3He+: HII Regions

During the era of primordial nucleosynthesis the light elements 2H, 3He, 4He, and 7Li were produced in significant amounts and these abundances have since been modified primarily by stars. Observations of 3He+ in HII regions located throughout the Milky Way disk reveal very little variation in the 3He/H abundance ratio---the "3He Plateau"---indicating that the net effect of 3He production in stars is negligible. This is in contrast to much higher 3He/H abundance ratios found in some planetary nebulae. This discrepancy is known as the "3He Problem". Stellar evolution models that include thermohaline mixing can resolve the 3He Problem by drastically reducing the net 3He production in most stars. These models predict a small negative 3He/H abundance gradient across the Galactic disk. Here we use the Green Bank Telescope to observe 3He+ in five HII regions with high accuracy to confirm the predictions of stellar and Galactic chemical evolution models that include thermohaline mixing. We detect 3He+ in all the sources and derive the 3He+/H+ abundance ratio using model HII regions and the numerical radiative transfer code NEBULA. The over 35 radio recombination lines (RRLs) that are simultaneously observed, together with the 3He+ transition provide stringent constraints for these models. We apply an ionization correction using observations of 4He RRLs. We determine a 3He/H abundance gradient as a function of Galactocentric radius of -0.116 +/- 0.022 x 10-5, consistent with stellar evolution models including thermohaline mixing that predict a small net contribution of 3He from solar mass stars.

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arXiv2018

Hydrogen Radio Recombination Line Emission from M51 and NGC628

We report the discovery of hydrogen radio recombination line (RRL) emission from two galaxies with star formation rates (SFRs) similar to that of the Milky Way: M51 and NGC628. We use the Green Bank Telescope (GBT) to measure $\sim$15 Hn$α$ recombination transitions simultaneously and average these data to improve our spectral signal-to-noise ratio. We show that our data can be used to estimate the total ionizing photon flux of these two sources, and we derive their SFRs within the GBT beam: $Ψ_{\rm OB} = 3.46$ M$_{\odot}$ yr$^{-1}$ for M51 and $Ψ_{\rm OB} = 0.56$ M$_{\odot}$ yr$^{-1}$ for NGC628. Here, we demonstrate that it is possible to detect RRLs from normal galaxies that are not undergoing a starburst with current instrumentation and reasonable integration times ($\sim$12 hr for each source). We also show that we can characterize the overall star-forming properties of M51 and NGC628, although the GBT beam cannot resolve individual HII region complexes. Our results suggest that future instruments, such as the Square Kilometre Array and the Next Generation Very Large Array, will be able to detect RRL emission from a multitude of Milky Way-like galaxies, making it possible to determine SFRs of normal galaxies unaffected by extinction and to measure global star formation properties in the local universe.

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arXiv2018

Kinematic Distances: A Monte Carlo Method

Distances to high mass star forming regions (HMSFRs) in the Milky Way are a crucial constraint on the structure of the Galaxy. Only kinematic distances are available for a majority of the HMSFRs in the Milky Way. Here we compare the kinematic and parallax distances of 75 Galactic HMSFRs to assess the accuracy of kinematic distances. We derive the kinematic distances using three different methods: the traditional method using the Brand & Blitz (1993) rotation curve (Method A), the traditional method using the Reid et al. (2014) rotation curve and updated Solar motion parameters (Method B), and a Monte Carlo technique (Method C). Methods B and C produce kinematic distances closest to the parallax distances, with median differences of 13% (0.43 kpc) and 17% (0.42 kpc), respectively. Except in the vicinity of the tangent point, the kinematic distance uncertainties derived by Method C are smaller than those of Methods A and B. In a large region of the Galaxy, the Method C kinematic distances constrain both the distances and the Galactocentric positions of HMSFRs more accurately than parallax distances. Beyond the tangent point along longitude=30 degrees, for example, the Method C kinematic distance uncertainties reach a minimum of 10% of the parallax distance uncertainty at a distance of 14 kpc. We develop a prescription for deriving and applying the Method C kinematic distances and distance uncertainties. The code to generate the Method C kinematic distances is publicly available and may be utilized through an on-line tool.

astro-ph.GA↗
arXiv2017

Carbon Monoxide Observations Toward Star Forming Regions in the Outer Scutum-Centaurus Spiral Arm

The Outer Scutum-Centaurus arm (OSC) is the most distant molecular spiral arm known in the Milky Way. The OSC may be the very distant end of the well-known Scutum-Centaurus arm, which stretches from the end of the Galactic bar to the outer Galaxy. At this distance the OSC is seen in the first Galactic quadrant. The population of star formation tracers in the OSC remains largely uncharacterized. Extragalactic studies show a strong correlation between molecular gas and star formation, and carbon monoxide (CO) emission was recently discovered in the OSC. Here we use the Arizona Radio Observatory (ARO) 12-m telescope to observe the $^{12}$CO J = 1-0 and $^{13}$CO J = 1-0 transitions toward 78 HII region candidates chosen from the WISE Catalog of Galactic HII Regions. These targets are spatially coincident with the Galactic longitude-latitude ($\ell, b$) OSC locus as defined by HI emission. We detect CO emission in $\sim 80$% of our targets. In total, we detect 117 $^{12}$CO and 40 $^{13}$CO emission lines. About 2/3 of our targets have at least one emission line originating beyond the Solar orbit. Most of the detections beyond the Solar orbit are associated with the Outer Arm, but there are 17 $^{12}$CO emission lines and 8 $^{13}$CO emission lines with LSR velocities that are consistent with the velocities of the OSC. There is no apparent difference between the physical properties (e.g., molecular column density) of these OSC molecular clouds and non--OSC molecular clouds within our sample.

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