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Register a new userThe Molecular Properties of Galactic HII Regions
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We derive the molecular properties for a sample of 301 Galactic HII regions including 123 ultra compact (UC), 105 compact, and 73 diffuse nebulae. We analyze all sources within the BU-FCRAO Galactic Ring Survey (GRS) of 13CO emission known to be HII regions based upon the presence of radio continuum and cm-wavelength radio recombination line emission. Unlike all previous large area coverage 13CO surveys, the GRS is fully sampled in angle and yet covers ~75 square degrees of the Inner Galaxy. The angular resolution of the GRS 46 allows us to associate molecular gas with HII regions without ambiguity and to investigate the physical properties of this molecular gas. We find clear CO/HII morphological associations in position and velocity for ~80% of the nebular sample. Compact HII region molecular gas clouds are on average larger than UC clouds: 2.2 compared to 1.7. Compact and UC HII regions have very similar molecular properties, with ~5K line intensities and ~4 km/s line widths. The diffuse HII region molecular gas has lower line intensities, ~3K, and smaller line widths, ~3.5 km/s. These latter characteristics are similar to those found for quiescent molecular clouds in the GRS. Our sample nebulae thus show evidence for an evolutionary sequence wherein small, dense molecular gas clumps associated with UC HII regions grow into older compact nebulae and finally fragment and dissipate into large, diffuse nebulae.
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We present a new, detailed, analysis of the spatial distribution of Galactic HII regions, exploiting a far richer database than used in previous analyses. Galactocentric distances have been derived for 550 objects. Distances from the Sun could be unambiguously derived from velocity data for 117 of them, lying either outside the solar circle (84) or on a line-of-sight tangential to their orbit (33). For 177 further sources, distance estimates are made possible by the use of auxiliary data. A highly significant correlation between luminosity and linear diameter was found and the corresponding least-square linear relationship in the log-log plane was used to resolve the distance ambiguity for an additional 256 sources. Within the solar circle the thickness of the distribution of HII regions around the Galactic plane was found to be comparable to that of OB stars (Bronfman et al. 2000). At larger galactocentric radii the shape of the distribution reflects that of the warp, and its thickness along the z axis increases with increasing distance from the Galactic centre. We also confirm, for a much larger sample, the previously reported positive gradient of electron temperature with galactocentric distance.
Sgr E is a massive star formation complex found toward the Galactic center that consists of numerous discrete, compact HII regions. It is located at the intersection between the Central Molecular Zone (CMZ) and the far dust lane of the Galactic bar, similar to hot spots seen in external galaxies. Compared with other Galactic star formation complexes, the Sgr E complex is unusual because its HII regions all have similar radio luminosities and angular extents, and they are deficient in ~10micron emission from their photodissociation regions (PDRs). Our Green Bank Telescope (GBT) radio recombination line observations increase the known membership of Sgr E to 19 HII regions. There are 43 additional HII region candidates in the direction of Sgr E, 26 of which are detected for the first time here using MeerKAT 1.28GHz data. Therefore, the true HII region population of Sgr E may number >60. Using APEX SEDIGISM 13CO 2-1 data we discover a 3.0x10^5 Solar mass molecular cloud associated with Sgr E, but find few molecular or far-infrared concentrations at the locations of the Sgr E HII regions. Comparison with simulations and an analysis of its radio continuum properties indicate that Sgr E formed upstream in the far dust lane of the Galactic bar a few Myr ago and will overshoot the CMZ, crashing into the near dust lane. We propose that the unusual infrared properties of the Sgr E HII regions are caused by their orbit about the Galactic center, which have possibly stripped their PDRs.
We present molecular line and 1.4 mm continuum observations towards five massive star forming regions at arcsecond resolution using the Submillimeter Array (SMA). We find that the warm molecular gas surrounding each HII region (as traced by SO_2 and OCS) appears to be undergoing bulk rotation. From the molecular line emission and thermal component of the continuum emission, we independently derived gas masses for each region which are consistent with each other. From the free-free component of the continuum emission we estimate the minimum stellar mass required to power the HII region and find that this mass, when added to the derived gas mass, is a significant fraction of the dynamical mass for that region.
We derive infrared and radio flux densities of all ~1000 known Galactic HII regions in the Galactic longitude range 17.5 < l < 65 degree. Our sample comes from the Wide-Field Infrared Survey Explorer (WISE) catalog of Galactic hii regions citep{anderson2014}. We compute flux densities at six wavelengths in the infrared (GLIMPSE 8 microns, WISE 12 microns and 22 microns, MIPSGAL 24 microns, and Hi-GAL 70 microns and 160 microns) and two in the radio (MAGPIS 20 cm and VGPS 21 cm). All HII region infrared flux densities are strongly correlated with their ~20 cm flux densities. All HII regions used here, regardless of physical size or Galactocentric radius, have similar infrared to radio flux density ratios and similar infrared colors, although the smallest regions ($r<1,$pc), have slightly elevated IR to radio ratios. The colors $log_{10}(F_{24 micron}/F_{12 micron}) ge 0$ and $log_{10}(F_{70 micron}/F_{12 micron}) ge 1.2$, and $log_{10}(F_{24 micron}/F_{12 micron}) ge 0$ and $log_{10}(F_{160 micron}/F_{70 micron}) le 0.67$ reliably select HII regions, independent of size. The infrared colors of ~22$%$ of HII regions, spanning a large range of physical sizes, satisfy the IRAS color criteria of citet{wood1989} for HII regions, after adjusting the criteria to the wavelengths used here. Since these color criteria are commonly thought to select only ultra-compact HII regions, this result indicates that the true ultra-compact HII region population is uncertain. Comparing with a sample of IR color indices from star-forming galaxies, HII regions show higher $log_{10}(F_{70 micron}/F_{12 micron})$ ratios. We find a weak trend of decreasing infrared to ~20 cm flux density ratios with increasing $R_{gal}$, in agreement with previous extragalactic results, possibly indicating a decreased dust abundance in the outer Galaxy.
The conversion of the IR emission into star formation rate can be strongly dependent on the physical properties of the dust, which are affected by the environmental conditions where the dust is embedded. We study here the dust properties of a set of HII regions in the Local Group Galaxy M33 presenting different spatial configurations between the stars, gas and dust to understand the dust evolution under different environments. We model the SED of each region using the DustEM tool and obtain the mass relative to hydrogen for Very Small Grains (YVSG), Polycyclic Aromatic Hydrocarbons (YPAH) and Big Grains (YBG). The relative mass of the VSGs (YVSG/YTOT) is a factor of 1.7 higher for HII regions classified as filled and mixed than for regions presenting a shell structure. The enhancement of VSGs within NGC 604 and NGC 595 is correlated to expansive gas structures with velocities greater than 50 km/s. The gas-to-dust ratio derived for the HII regions in our sample exhibits two regimes related to the HI-H2 transition of the ISM. Regions corresponding to the HI diffuse regime present a gas-to-dust ratio compatible with the expected value if we assume that the gas-to-dust ratio scales linearly with metallicity, while regions corresponding to a H2 molecular phase present a flatter dust-gas surface density distribution. The fraction of VSGs can be affected by the conditions of the interstellar environment: strong shocks of 50-90 km/s existing in the interior of the most luminous HII regions can lead to fragmentation of BGs into smaller ones, while the more evolved shell and clear shell objects provide a more quiescent environment where reformation of dust BG grains might occur. The gas-to-dust variations found in this analysis might imply that grain coagulation and/or gas-phase metals incorporation to the dust mass is occurring in the interior of the HII regions in M33.
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