Keywords

We present resolved Giant Metrewave Radio Telescope H i observations of the high gas-phase metallicity dwarf galaxy WISEA J230615.06+143927.9 (z = 0.005) (hereafter J2306) and investigate whether it could be a Tidal Dwarf Galaxy (TDG) candidate. TDGs are observed to have higher metallicities than normal dwarfs. J2306 has an unusual combination of a blue g − r color of 0.23 mag, irregular optical morphology and high-metallicity (12 + log(O/H) = 8.68 ± 0.14), making it an interesting galaxy to study in more detail. We find J2306 to be an H i rich galaxy with a large extended, unperturbed rotating H i disk. Using our H i data we estimated its dynamical mass and found the galaxy to be dark matter (DM) dominated within its H i radius. The quantity of DM, inferred from its dynamical mass, appears to rule out J2306 as an evolved TDG. A wide area environment search reveals J2306 to be isolated from any larger galaxies which could have been the source of its high gas metallicity. Additionally, the H i morphology and kinematics of the galaxy show no indication of a recent merger to explain the high-metallicity. Further detailed optical spectroscopic observations of J2306 might provide an answer to how a seemingly ordinary irregular dwarf galaxy achieved such a high level of metal enrichment.

The research of infall motion is a common means to study molecular cloud dynamics and the early process of star formation. Many works had been done in-depth research on infall. We searched the literature related to infall study of molecular cloud since 1994, summarized the infall sources identified by the authors. A total of 456 infall sources are cataloged. We classify them into high-mass and low-mass sources, in which the high-mass sources are divided into three evolutionary stages: prestellar, protostellar and H ii region. We divide the sources into clumps and cores according to their sizes. The H₂ column density values range from 1.21 × 10²¹ to 9.75 × 10²⁴ cm⁻², with a median value of 4.17 × 10²² cm⁻². The H₂ column densities of high-mass and low-mass sources are significantly separated. The median value of infall velocity for high-mass clumps is 1.12 km s⁻¹, and the infall velocities of low-mass cores are virtually all less than 0.5 km s⁻¹. There is no obvious difference between different stages of evolution. The mass infall rates of low-mass cores are between 10⁻⁷ and 10⁻⁴ M_⊙yr⁻¹, and those of high-mass clumps are between 10⁻⁴ and 10⁻¹ M_⊙yr⁻¹ with only one exception. We do not find that the mass infall rates vary with evolutionary stages.

In this paper, we present a database of class I methanol masers. The compiled information from the available literature provides an open and fast access to the data on class I methanol maser emission, including search, analysis, and visualization of the extensive maser data set. There is information on individual maser components detected with single-dish observations and maser spots obtained from interferometric data. At the moment the database contains information from ∼100 papers, i.e., ∼7500 observations and ∼650 sites of class I methanol masers. Analysis of the data collected in the database shows that the distribution of class I methanol maser sources is similar to that of class II methanol masers. They are mostly found in the molecular ring, where the majority of the OB stars are located. The difference between class I and II distributions is the presence of many class I methanol masers in the nuclear disk region (central molecular zone). Access to the class I methanol maser database is available online at http://maserdb.net.

We present a comparative study of the size–line width relation for substructures within six molecular clouds in the Large Magellanic Cloud (LMC) mapped with the Atacama Large Millimeter/submillimeter Array. Our sample extends our previous study, which compared a Planck detected cold cloud in the outskirts of the LMC with the 30 Doradus molecular cloud and found the typical line width for 1 pc radius structures to be five times larger in 30 Doradus. By observing clouds with intermediate levels of star formation activity, we find evidence that the line width at a given size increases with increasing local and cloud-scale 8 μm intensity. At the same time, the line width at a given size appears to independently correlate with measures of the mass surface density. Our results suggest that both virial-like motions due to gravity and local energy injection by star formation feedback play important roles in determining intracloud dynamics.

Probing magnetic fields in giant molecular clouds is often challenging. Fortunately, recent simulations show that analysis of velocity gradients (the velocity gradient technique; VGT) can be used to map out the magnetic field morphology of different physical layers within molecular clouds when applied to CO isotopologues with different optical depths. Here, we test the effectiveness of the VGT in reconstructing the magnetic field structure of the molecular cloud Vela C, employing seven chemical tracers that have different optical depths, i.e., ¹²CO, ¹³CO, C¹⁸O, CS, HNC, HCO⁺, and HCN. Our results show good correspondence between the magnetic field morphology inferred from velocity gradients using these different molecular tracers and the magnetic field morphology inferred from BLASTPol polarization observations. We also explore the possibility of using a combination of velocity gradients for multiple chemical tracers to explain the structure of the magnetic field in molecular clouds. We search for signatures of gravitational collapse in the alignment of the velocity gradients and magnetic field and conclude that collapsing regions constitute a small fraction of the cloud.

We report the results from a new, highly sensitive (ΔT_mb ∼ 3 mK) survey for thermal OH emission at 1665 and 1667 MHz over a dense, 9 × 9 pixel grid covering a 1° × 1° patch of sky in the direction of l = 10500, b = +250 toward the Perseus spiral arm of our Galaxy. We compare our Green Bank Telescope 1667 MHz OH results with archival ¹²CO(1–0) observations from the Five College Radio Astronomy Observatory Outer Galaxy Survey within the velocity range of the Perseus Arm at these galactic coordinates. Out of the 81 statistically independent pointings in our survey area, 86% show detectable OH emission at 1667 MHz, and 19% of them show detectable CO emission. We explore the possible physical conditions of the observed features using a set of diffuse molecular cloud models. In the context of these models, both OH and CO disappear at current sensitivity limits below an A_v of 0.2, but the CO emission does not appear until the volume density exceeds 100–200 ${\mathrm{cm}}^{-3}$. These results demonstrate that a combination of low column density A_v and low volume density n_H can explain the lack of CO emission along sight lines exhibiting OH emission. The 18 cm OH main lines, with their low critical density of n* ∼ 1 ${\mathrm{cm}}^{-3}$, are collisionally excited over a large fraction of the quiescent galactic environment and, for observations of sufficient sensitivity, provide an optically thin radio tracer for diffuse H₂.

We present the first results from the Small Magellanic Cloud portion of a new Australia Telescope Compact Array H i absorption survey of both of the Magellanic Clouds, comprising over 800 hr of observations. Our new H i absorption line data allow us to measure the temperature and fraction of cold neutral gas in a low-metallicity environment. We observed 22 separate fields, targeting a total of 55 continuum sources, against 37 of which we detected H i absorption; from this we measure a column-density-weighted mean average spin temperature of $\langle {T}_{{\rm{s}}}\rangle$ = 150 K. Splitting the spectra into individual absorption line features, we estimate the temperatures of different gas components and find an average cold gas temperature of ∼30 K for this sample, lower than the average of ∼40 K in the Milky Way. The H i appears to be evenly distributed throughout the SMC, and we detect absorption in 67% of the lines of sight in our sample, including some outside the main body of the galaxy (N_{H i} $\gt 2\times {10}^{21}$ cm⁻²). The optical depth and temperature of the cold neutral atomic gas show no strong trend with location spatially or in velocity. Despite the low-metallicity environment, we find an average cold gas fraction of ∼20%, not dissimilar from that of the Milky Way.

Millimeter molecular line observations have been conducted toward the young (∼900 yr) bipolar planetary nebula (PN) K4-47, using the 12 m antenna and the Submillimeter Telescope of the Arizona Radio Observatory, and the Institut de Radioastronomie Millimétrique 30 m Telescope. Measurements at 1, 2, and 3 mm of multiple transitions were carried out to ensure the accuracy of all molecular identifications. K4-47 was found to be unusually chemically rich, containing three complex species, CH₃CN, H₂CNH, and CH₃CCH, which have never before been observed in a planetary nebula. In addition, HC₃N, N₂H⁺, H₂CO, c-C₃H₂, and SiO have been identified in this object, as well as a variety of ¹³C-substituted isotopologues (${{{\rm{H}}}_{2}}^{13}$CO, c-¹³CCCH₂, c-CC¹³CH₂, ${{\mathrm{CH}}_{3}}^{13}$CN, ¹³CH₃CN, ${{\mathrm{CH}}_{3}}^{13}$CCH, and ¹³CH₃CCH), including all three doubly¹³C-substituted varieties of HC₃N—the first known object in which all three species have been detected. After CO and H₂, the most abundant molecules in K4-47 are CCH and CN, which have abundances of f  ∼ 8 × 10⁻⁷, relative to molecular hydrogen. Surprisingly, the next most abundant molecule is CH₃CCH, which has f  ∼ 6 × 10⁻⁷, followed by HCN with an abundance of ∼5 × 10⁻⁷. The results suggest that K4-47 is the most chemically complex planetary nebula currently known. The molecular content of K4-47 closely resembles that of the C-star IRC+10216, but with lower abundances, except for HCO⁺, H₂CO, and CH₃CCH. The PN also chemically and morphologically resembles the bipolar protoplanetary nebula CRL 618, with similar enrichments of ¹³C, ¹⁵N, and ¹⁷O, suggestive of an explosive process at the end of the asymptotic giant branch.

We have introduced a new method of estimating the electron temperature and density of H ii regions by using single-dish observations. In this method, multiple hydrogen radio recombination lines of different bands are computed under the assumption of low optical depth. We use evolutionary hydrodynamical models of H ii regions to model hydrogen recombination line emission from a variety of H ii regions and assess the reliability of the method. According to the simulated results, the error of the estimated temperature is commonly <13%, and that of the estimated density is <25% for a <1% uncertainty of the observed line fluxes. A reasonable estimated value of electron density can be achieved if the uncertainty of the line fluxes is lower than 3%. In addition, the estimated values are more representative of the properties in the relatively high density region if the gas density gradient is present in the H ii region. Our method can be independent of the radio continuum observations. But the accuracy will be improved if a line-to-continuum ratio at millimeter wavelengths is added to the estimation. Our method provides a way to measure the temperature and density in ionized regions without interferometers.

We report Arecibo 21 cm absorption-emission observations to characterize the physical properties of neutral hydrogen (H i) in the proximity of five giant molecular clouds (GMCs): Taurus, California, Rosette, Mon OB1, and NGC 2264. Strong H i absorption was detected toward all 79 background-continuum sources in the ∼60 × 20 square degree region. Gaussian decompositions were performed to estimate temperatures, optical depths, and column densities of the cold and warm neutral medium (CNM and WNM). The properties of individual CNM components are similar to those previously observed along random Galactic sightlines and in the vicinity of molecular clouds, suggesting a universality of cold H i properties. The CNM spin temperature (T_s) histogram peaks at ∼50 K. The turbulent Mach numbers of CNM components vary widely, with a typical value of ∼4, indicating that their motions are supersonic. About 60% of the total H i gas is WNM, and nearly 40% of the WNM lies in thermally unstable regime 500–5000 K. The observed CNM fraction is higher around GMCs than in diffuse regions, and increases with increasing column density (${N}_{{\rm{H}}{\rm{I}}}$) to a maximum of ∼75%. On average, the optically thin approximation (${N}_{{\rm{H}}\,{\rm{I}}}^{* }$) underestimates the total column density by ∼21%, but we find large regional differences in the relationship between ${N}_{{\rm{H}}{\rm{I}}}$ and the required correction factor, f = ${N}_{{\rm{H}}{\rm{I}}}/{N}_{{\rm{H}}\,{\rm{I}}}^{* }$. We examine two different methods (linear fit of f versus log₁₀(${N}_{{\rm{H}}\,{\rm{I}}}^{* }$) and uniform T_s) to correct for opacity effects using emission data from the GALFA-H i survey. We prefer the uniform T_s method because the linear relationship does not produce convincing fits for all subregions.