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Register a new userRe-analysis of the radio luminosity function of Galactic HII regions
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R. Paladini
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We have re-analyzed continuum and recombination lines radio data available in the literature in order to derive the luminosity function (LF) of Galactic HII regions. The study is performed by considering the first and fourth Galactic quadrants independently. We estimate the completeness level of the sample in the fourth quadrant at 5 Jy, and the one in the first quadrant at 2 Jy. We show that the two samples (fourth or first quadrant) include, as well as giant and super-giant HII regions, a significant number of sub-giant sources. The LF is obtained, in each Galactic quadrant, with a generalized Schmidts estimator using an effective volume derived from the observed spatial distribution of the considered HII regions. The re-analysis also takes advantage of recently published ancillary absorption data allowing to solve the distance ambiguity for several objects. A single power-law fit to the LFs retrieves a slope equal to -2.23+/-0.07 (fourth quadrant) and to -1.85+/-0.11 (first quadrant). We also find marginal evidence of a luminosity break at L_knee = 10^23.45 erg s^(-1) Hz^(-1) for the LF in the fourth quadrant. We convert radio luminosities into equivalent H_alpha and Lyman continuum luminosities to facilitate comparisons with extra-galactic studies. We obtain an average total HII regions Lyman continuum luminosity of 0.89 +/- 0.23 * 10^(53) sec^(-1), corresponding to 30% of the total ionizing luminosity of the Galaxy.
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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 $langlealpharangle=-1.75,pm,0.01$. The mean best-fit double power law indices are $langlealpha_1rangle=-1.40,pm,0.03$ and $langlealpha_2rangle=-2.33,pm,0.04$. 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$alpha$.
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.
We present a determination of the luminosity functions of massive young stellar objects (MYSOs) and compact (C)HII regions within the Milky Way Galaxy using the large, well-selected sample of these sources identified by the Red MSX Source (RMS) survey. The MYSO luminosity function decreases monotonically such that there are few with $Lgtrsim 10^{5}$Lsol, whilst the CHII regions are detected up to ~10$^{6}Lsol. The lifetimes of these phases are also calculated as a function of luminosity by comparison with the luminosity function for local main-sequence OB stars. These indicate that the MYSO phase has a duration ranging from 4x10$^{5}$ yrs for 10$^{4}$Lsol to ~7x10$^{4}$ yrs at 10$^{5}$Lsol, whilst the CHII region phase lasts of order 3x10$^{5}$ yrs or ~3-10% of the exciting stars main-sequence lifetime. MYSOs between 10$^{4} Lsol and ~10$^{5}$ Lsol are massive but do not display the radio continuum or near-IR HI{} recombination line emission indicative of an HII region, consistent with being swollen due to high ongoing or recent accretion rates. Above ~10$^{5}$ Lsol the MYSO phase lifetime becomes comparable to the main-sequence Kelvin-Helmholtz timescale, at which point the central star can rapidly contract onto the main-sequence even if still accreting, and ionise a CHII region, thus explaining why few highly luminous MYSOs are observed.
Context. The derived physical parameters for young HII regions are normally determined assuming the emission region to be optically thin. However, this assumption is unlikely to hold for young HII regions such as hyper-compact HII(HCHII) and ultra-compact HII(UCHII) regions and leads to the underestimation of their properties. This can be overcome by fitting the SEDs over a wide range of radio frequencies. Aims. The two primary goals of this study are (1) to determine the physical properties of young HII regions from radio SEDs in the search for potential HCHII regions, and (2) to use these physical properties to investigate their evolution. Method. We used the Karl G. Jansky Very Large Array (VLA) to observe the X-band and K-band with angular resolutions of ~1.7 and ~0.7, respectively, toward 114 HII regions with rising-spectra between 1-5 GHz. We complement our observations with VLA archival data and construct SEDs in the range of 1-26 GHz and model them assuming an ionization-bounded HII region with uniform density. Results. Our sample has a mean electron density of ne=1.6E4cm^{-3}, diameter diam=0.14pc, and emission measure EM = 1.9E7pc*cm^{-6}. We identify 16 HCHII region candidates and 8 intermediate objects between the classes of HCHII and UCHII regions. The ne, diam, and EM change as expected, but the Lyman continuum flux is relatively constant over time. We find that about 67% of Lyman-continuum photons are absorbed by dust within these HII regions and the dust absorption fraction tends to be more significant for more compact and younger HII regions. Conclusion. Young HII regions are commonly located in dusty clumps; HCHII regions and intermediate objects are often associated with various masers, outflows, broad radio recombination lines, and extended green objects, and the accretion at the two stages tends to be quickly reduced or halted.
We have carried out the largest and most unbiased search for hypercompact (HC) HII regions. Our method combines four interferometric radio continuum surveys (THOR, CORNISH, MAGPIS and White2005) with far-infrared and sub-mm Galactic Plane surveys to identify embedded HII regions with positive spectral indices. 120 positive spectrum HII regions have been identified from a total sample of 534 positive spectral index radio sources. None of these HII regions, including the known HCHII regions recovered in our search, fulfills the canonical definition of an HCHII region at 5 GHz. We suggest that the current canonical definition of HCHII regions is not accurate and should be revised to include a hierarchical structure of ionized gas that results in an extended morphology at 5 GHz. Correlating our search with known ultracompact (UC) HII region surveys, we find that roughly half of detected UCHII regions have positive spectral indices, instead of more commonly assumed flat and optically thin spectra. This implies a mix of optically thin and thick emission and has important implications for previous analyses which have so far assumed optically thin emission for these objects. Positive spectrum HII regions are statistically more luminous and possess higher Lyman continuum fluxes than HII regions with flat or negative indices. Positive spectrum HII regions are thus more likely to be associated with more luminous and massive stars. No differences are found in clump mass, linear diameter or luminosity-to-mass ratio between positive spectrum and non-positive spectrum HII regions.
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