Luminous HII Regions Research Articles

A Green Bank Telescope Survey of Large Galactic H ii Regions

As part of our ongoing HII Region Discovery Survey (HRDS), we report the Green Bank Telescope detection of 148 new angularly-large Galactic HII regions in radio recombination line (RRL) emission. Our targets are located at a declination greater than -45deg., which corresponds to 266deg. > l > -20deg. at b = 0deg. All sources were selected from the WISE Catalog of Galactic HII Regions, and have infrared angular diameters >260''. The Galactic distribution of these "large" HII regions is similar to that of the previously-known sample of Galactic HII regions. The large HII region RRL line width and peak line intensity distributions are skewed toward lower values compared with that of previous HRDS surveys. We discover 7 sources with extremely narrow RRLs <10 km/s. If half the line width is due to turbulence, these 7 sources have thermal plasma temperatures <1100 K. These temperatures are lower than any measured for Galactic HII regions, and the narrow line components may arise instead from partially ionized zones in the HII region photo-dissociation regions. We discover G039.515+00.511, one of the most luminous HII regions in the Galaxy. We also detect the RRL emission from three HII regions with diameters >100 pc, making them some of the physically largest known HII regions in the Galaxy. This survey completes the HRDS HII region census in the Northern sky, where we have discovered 887 HII regions and more than doubled the previously-known census of Galactic HII regions.

A Statistical Comparison of the Near Infrared Emission with the Hα Emission from the HII regions of NGC4321, NGC4736 and NGC4254

AbstractWe report on our ongoing project “Statistical studies of HII regions in the nearby extra-galaxies”. We present an overview of our detailed study of warm dust in the nearby Galaxy NGC 4321 (M100), measuring the flux values in the 4 Spitzer-IRAC bands of some 275 HII regions in M100. In addition, we present new measurements of the flux values in the 4 Spitzer-IRAC bands of a complete sample of 70 isolated luminous HII regions in NGC 4736 and 157 regions in NGC 4254. We study the relations between the Hα luminosity and the near-IR luminosity and temperature of HII regions in the three galaxies. We estimate the near-IR luminosities and compare them with the Hα luminosities from archive and literature sources. We find a linear relation between the Hα luminosity and the IRAC luminosity for the HII regions, but no apparent relation between the luminosity and the colour temperature of the regions in any of the three galaxies. The colour temperatures of regions especially in M100 and NGC4254 are confined to a surprisingly narrow range, with a small fraction forming a higher temperature tail to the distribution. These results give new insight into the size function and the 3D distribution of the dust in these regions, and we propose scenarios to explain them.

The ionizing sources of luminous compact HII regions in the RCW106 and RCW122 clouds

Given the rarity of young O star candidates, compact HII regions embedded in dense molecular cores continue to serve as potential sites to peer into the details of high-mass star formation. To uncover the ionizing sources of the most luminous and compact HII regions embedded in the RCW106 and RCW122 giant molecular clouds, known to be relatively nearby (2-4 kpc) and isolated, thus providing an opportunity to examine spatial scales of a few hundred to a thousand AU in size. High spatial resolution (0.3"), mid-infrared spectra (R=350), including the fine structure lines [ArIII] and [NeII], were obtained for four luminous compact HII regions, embedded inside the dense cores within the RCW106 and RCW122 molecular cloud complexes. At this resolution, these targets reveal point-like sources surrounded by nebulosity of different morphologies, uncovering details at spatial dimensions of <1000AU. The point-like sources display [ArIII] and [NeII] lines - the ratios of which are used to estimate the temperature of the embedded sources. The derived temperatures are indicative of mid-late O type objects for all the sources with [ArIII] emission. Previously known characteristics of these targets from the literature, including evidence of disk or accretion suggest that the identified sources may grow more to become early-type O stars by the end of the star formation process.

Radiatively driven Rayleigh-Taylor instability candidates around a forming massive star system

The formation of the highest mass stars are thought to be dominated by instabilities resulting from gravitation and radiation. Instabilities due to gravitation are commonly demonstrated by observations of fragmentation, but those due to effects of radiation have thus far not been found. Here I report on the NACO adaptive optics and mid-infrared diffraction-limited VISIR imaging data of an extemely luminous ultra-compact HII region G333.6-0.2. Two infrared sources, one bright in the near-infrared(appearing point-like) and another in the mid-infrared (resolved with an elliptical shape) are uncovered through this data, which are located at the heart of this region. These infrared sources appear to be embedded in the waist of a bipolar-shaped nebula and UCHII region, the lobes of which are separated by a dark patch. Dense filamentary features with finger/hook morphology are found; they appear to be connected to the two bright infrared sources and the sizes of these hook features are sharply limited to <5000 AU. The observed properties of this target and a large amount of previous data obtained from the literature are compared together with the results of various numerical simulations of high-mass star formation. This comparison favours the interpretation that the finger/hook-like structures likely represent radiatively driven Rayleigh-Taylor instabilities arising in the outflow cavity of a forming high-mass binary star system.

THE CIRCUMNUCLEAR STAR FORMATION ENVIRONMENT OF NGC 6946: Br γ AND H2RESULTS FROM KECK INTEGRAL FIELD SPECTROSCOPY

We present a 3-dimensional data cube of the K band continuum and the Br-gamma, H2 S(0) and S(1) lines within the central 18.5" x 13.8" (520 pc x 390 pc) region of NGC 6946. Data were obtained using OSIRIS, a near-infrared Integral Field Unit at Keck Observatory, with Laser Guide Star Adaptive Optics. The 0.3" resolution allows us to investigate the stellar bulge and the forming star clusters in the nuclear region on 10 parsec scales. We detect giant HII regions associated with massive young star clusters in the nuclear spiral/ring (R ~ 30pc) and in the principal shocks along the nuclear bar. Comparisons of the Br-gamma fluxes with Pa-alpha line emission and radio continuum indicate A_K ~ 3, A_V ~ 25 for the nuclear star forming regions. The most luminous HII regions are restricted to within 70pc of the center, despite the presence of high gas columns at larger radii (R ~ 200pc). H2 emission is restricted to clouds within R ~ 60pc of the center, and in this sense resembles the distribution of the HCN line emission. We propose that gas-assisted migration of the young star clusters is contributing to the buildup of the nuclear bar and nuclear star cluster (R < 30pc) in this galaxy.

Anatomy of Starbursts in Extragalactic Giant HII Regions: M51 Case Study

AbstractExtragalactic giant HII regions (EGHRs) are sites of active, concentrated star formation, and thus provide excellent labs to analyze the starburst phenomenon. Although they have been known for a long time, ground-based observations cannot resolve the physical structures and stellar content of EGHRs. The high resolution and sensitivity of Hubble Space Telescope (HST) are ideal for detailed studies of EGHRs. We have searched the Hubble Legacy Archives (HLA) and found 17 nearby galaxies, within ∼15 Mpc, with Hα and continuum images; to determine the best methods for analyzing these data, we perform an in-depth analysis of the EGHRs in M51. M51 is a face-on spiral galaxy ∼8.4 Mpc away, with well-resolved multi-wavelength observations in the HLA. We sample the 25 most luminous HII regions in M51, many of which are bonafide EGHRs with an H-alpha luminosity &gt; 1039 ergs s−1. We use the Hα image to study the distribution and physical structure of the gas in each HII region and determine its Hα luminosity and required ionizing flux. We use the continuum images to determine whether super stellar clusters (SSCs) are found in these HII regions, and use photometric measurements to determine the mass and age spread of the resolved stellar population. These are then compared with the interstellar structures. The results help us provide the groundwork for studying EGHRs in multiple galaxies and elucidate the starburst phenomenon by investigating questions such as: What role does environment play in the formation of EGHRs? How do EGHRs evolve? How does star formation proceed in an EGHR?

Comparison of NIR and Hα emission from the HII regions of M100

AbstractWe measure the flux values in the 4 Spitzer-IRAC bands, of a sample of 78 isolated luminous HII regions in the grand design galaxy M100. We estimate the near-IR luminosities and compare them with the Hα-luminosities from the Knapen HII-region catalogue. We find a strong, and expected positive correlation between the total IR luminosity and the Hα luminosity, but no significant correlation between the luminosity and the IR temperature of the regions.

Statistical Properties of Highly Luminous HII Regions in the Interacting Zone of the Antennae Galaxies, and Comparison with the Molecular Cloud Component

AbstractWe have used the package CLOUDPROPS (Rosolowsky &amp; Leroy 2008) in order to identify clouds, which are contained in data cubes of molecular/Hα line emission, and extract their properties. The molecular data cube is taken from the ALMA public archive and the ionized hydrogen data cube was obtained with the Fabry-Perot interferometer, GHαFaS, on the 4.2 m WHT in La Palma. In our study of the overlap region of the Antennae galaxies we have identified two populations of molecular clouds, above and below log M/M⊙ = 6.75 (in agreement with Wei et al. 2012 who find the break in mass at log M/M⊙ = 6.5) and two different populations of ionized gas clouds; the break in the luminosity occurs near log L = 37.6 (see Fig. 1). We have measured the masses, velocity dispersions and luminosities of the clouds, and derived relation between them for each population. Population II clouds (both molecular and ionized) tend to be located in the most luminous regions, while population I clouds can be found in the outskirts. This work will be published in Font et al. (2013).

Spatially resolved study of the physical properties of the ionized gas in NGC 595

We present Integral Field Spectroscopy (IFS) of NGC 595, one of the most luminous HII regions in M33. This type of observations allows studying the variation of the principal emission-line ratios across the surface of the nebula. At each position of the field of view, we fit the main emission-line features of the spectrum within the spectral range 3650-6990A, and create maps of the principal emission-line ratios for the total surface of the region. The extinction map derived from the Balmer decrement and the absorbed H-alpha luminosity show good spatial correlation with the 24 micron emission from Spitzer. We also show here the capability of the IFS to study the existence of Wolf-Rayet (WR) stars, identifying the previously catalogued WR stars and detecting a new candidate towards the north of the region. The ionization structure of the region nicely follows the H-alpha shell morphology and is clearly related to the location of the central ionizing stars. The electron density distribution does not show strong variations within the HII region nor any trend with the H-alpha emission distribution. We study the behaviour within the HII region of several classical emission-line ratios proposed as metallicity calibrators: while [NII]/Ha and [NII]/[OIII] show important variations, the R23 index is substantially constant across the surface of the nebula, despite the strong variation of the ionization parameter as a function of the radial distance from the ionizing stars. These results show the reliability in using the R23 index to characterize the metallicity of HII regions even when only a fraction of the total area is covered by the observations.

CHANDRAACIS SURVEY OF M33 (ChASeM33): THE ENIGMATIC X-RAY EMISSION FROM IC131

We present the first X-ray analysis of the diffuse hot ionized gas and the point sources in IC131, after NGC604 the second most X-ray luminous giant HII region in M33. The X-ray emission is detected only in the south eastern part of IC131 (named IC131-se) and is limited to an elliptical region of ~200pc in extent. This region appears to be confined towards the west by a hemispherical shell of warm ionized gas and only fills about half that volume. Although the corresponding X-ray spectrum has 1215 counts, it cannot conclusively be told whether the extended X-ray emission is thermal, non-thermal, or a combination of both. A thermal plasma model of kT_e=4.3keV or a single power law of Gamma=2.1 fit the spectrum equally well. If the spectrum is purely thermal (non-thermal), the total unabsorbed X-ray luminosity in the 0.35-8keV energy band amounts to L_X = 6.8(8.7)x10^35erg/s. Among other known HII regions IC131-se seems to be extreme regarding the combination of its large extent of the X-ray plasma, the lack of massive O stars, its unusually high electron temperature (if thermal), and the large fraction of L_X emitted above 2keV (~40-53%). A thermal plasma of ~4keV poses serious challenges to theoretical models, as it is not clear how high electron temperatures can be produced in HII regions in view of mass-proportional and collisionless heating. If the gas is non-thermal or has non-thermal contributions, synchrotron emission would clearly dominate over inverse Compton emission. It is not clear if the same mechanisms which create non-thermal X-rays or accelerate CRs in SNRs can be applied to much larger scales of 200pc. In both cases the existing theoretical models for giant HII regions and superbubbles do not explain the hardness and extent of the X-ray emission in IC131-se.

The RMS survey

Context: The Red MSX Source (RMS) survey has identified a large sample of candidate massive young stellar objects (MYSOs) and ultra compact (UC) HII regions from a sample of ~2000 MSX and 2MASS colour selected sources. Aims: To search for H2O masers towards a large sample of young high mass stars and to investigate the statistical correlation of H2O masers with the earliest stages of massive star formation. Methods: We have used the Mopra Radio telescope to make position-switched observations towards ~500 UCHII regions and MYSOs candidates identified from the RMS survey and located between 190\degr < l < 30\degr. These observations have a 4$\sigma$ sensitivity of ~1 Jy and a velocity resolution of ~0.4 km/s.} Results: We have detected 163 H2O masers, approximately 75% of which were previously unknown. Comparing the maser velocities with the velocities of the RMS sources, determined from 13CO observations, we have identified 135 RMS-H2O maser associations, which corresponds to a detection rate of ~27%. Taking into account the differences in sensitivity and source selection we find our detection rate is in general agreement with previously reported surveys. Conclusions: We find similar detection rates for UCHII regions and MYSOs candidates, suggesting that the conditions needed for maser activity are equally likely in these two stages of the star formation process. Looking at the detection rate as a function of distance from the Galactic centre we find it significantly enhanced within the solar circle, peaking at ~37% between 6-7 kpc, which is consistent with previous surveys of UC HII regions, possibly indicating the presence of a high proportion of more luminous YSOs and HII regions.

STAR FORMATION IN LUMINOUS H II REGIONS IN M33

We present a multiwavelength (ultraviolet, infrared, optical and CO) study of a set of luminous HII regions in M33: NGC 604, NGC 595, NGC 592, NGC 588 and IC131. We study the emission distribution in the interiors of the HII regions to investigate the relation between the dust emission at 8 micron and 24 micron and the location of the massive stars and gas. We find that the 24 micron emission is closely related to the location of the ionized gas, while the 8 micron emission is more related to the boundaries of the molecular clouds consistently with its expected association with photodissociation regions (PDRs). Ultraviolet emission is generally surrounded by the H-alpha emission. For NGC 604 and NGC 595, where CO data are available, we see a radial gradient of the emission distribution at the wavelengths studied here: from the center to the boundary of the HII regions we observe ultraviolet, H-alpha, 24 micron, 8 micron and CO emission distributions. We quantify the star formation for our HII regions using the integrated fluxes at the set of available wavelengths, assuming an instantaneous burst of star formation. We show that a linear combination of 24 micron and H-alpha emission better describes the star formation for these objects than the dust luminosities by themselves. For NGC 604, we obtain and compare extinction maps derived from the Balmer decrement and from the 24 micron and H-alpha emission line ratio. Although the maps show locally different values in extinction, we find similar integrated extinctions derived from the two methods. We also investigate here the possible existence of embedded star formation within NGC 604.

Molecular Cores in Different Evolutionary Stages near Luminous IRAS Sources and UC HII Regions

We report the results of 12CO and 13CO J=1–0 observations of eight candidates of Ultra-Compact (UC) HII regions with the Purple Mountain Observatory (PMO) Qinghai 13.7 -m telescope, which resulted in revealing 11 molecular cores. Their masses range from 130 to 1.7×104 M⊙, with different spatial scales (1∼ 6 pc). Also presented are the relevant HCO+ J=1–0 maps, which enabled us to investigate more detailed structures of these cores. Further comparisons show that four of the cores deviated from the centers of infrared (MIR) emission of Midcourse Space Experiment (MSX), while others correspond either to bright MIR sources or diffuse MIR background. This indicates various evolutionary phases of the cores, including quite early ones for those without MIR sources.

High Spatial Resolution T-ReCS Mid-Infrared Imaging of Luminous Infrared Galaxies

We present diffraction-limited (FWHM ~ 0.3arcsec) Gemini/T-ReCS mid-infrared (MIR: N-band or narrow-band at 8.7micron) imaging of four Luminous Infrared Galaxies (LIRGs) drawn from a representative local sample. The MIR emission in the central few kpc is strikingly similar to that traced by Pa-alpha, and generally consists of bright nuclear emission and several compact circumnuclear and/or extranuclear HII regions. The central MIR emission is dominated by these powerful HII regions, consistent with the majority of AGN in this local sample of LIRGs contributing a minor part of the MIR emission. The luminous circumnuclear HII regions detected in LIRGs follow the extrapolation of the 8micron vs. Pa-alpha relation found for M51 HII knots. The integrated central 3-7kpc of galaxies, however, present elevated 8micron/Pa-alpha ratios with respect to individual HII regions, similar to the integrated values for star-forming galaxies. Our results show that the diffuse 8micron emission, not directly related to the ionizing stellar population, can be as luminous as that from the resolved HII regions. Therefore, calibrations of the star formation rate for distant galaxies should be based on the integrated 8micron emission of nearby galaxies, not that of the HII regions alone.

The IMF in extreme star-forming environments: Searching for variations vs. initial conditions

Any predictive theory of star formation must explain observed variations (or lack thereof) in the initial mass function. Recent work suggests that we might expect quantitative variations in the IMF as a function of metallicity (Larson 2005) or magnetic field strength (Shu et al. 2004). We summarize results from several on-going studies attempting to constrain the ratio of high to low mass stars, as well as stars to sub- stellar objects, in a variety of di! erent environments, all containing high mass stars. First, we examine the ratio of stars to sub-stellar objects in the nearby Mon R2 region utilizing NICMOS/HST data. We compare our results to the IMF by Kroupa (2002) and to the observed ratios for IC 348 and Orion. Second, we present preliminary results for the ratio of high to low mass stars in W51, the most luminous HII region in the galaxy. Based on ground-based multi- colour images of the cluster obtained with the MMT adaptive optics system, we derive a lower limit to the ratio of high-mass to low-mass stars and compare it to the ratios for nearby clusters. Finally, we present the derived IMF for the R136 region in the LMC where the metallicity is 1/4 solar using HST/NICMOS data. We find that the IMF is consistent with that characterizing the field (Chabrier 2003), as well as nearby star-forming regions, down to 1.0 M! outside 2 pc. Whereas the results for both Mon R2 and R136 are consistent with the nearby clusters, the ratio of high to low mass stars in W51 tentatively indicates a lack of low-mass objects.

Mid and Far IR properties of late-type galaxies in the Coma and A1367 clusters: ISOCAM and ISOPHOT observations

We present Mid (MIR) and Far (FIR) Infrared observations of 18 spiral/irregular galaxies belonging to the Coma and A1367 clusters, carried out with the CAM and PHOT instruments on board the ISO satellite. Complementary photometry from the UV to the Near Infrared (NIR) together with Hα imaging, HI and ^(12)CO line measurements allow us to study the relationships between the IR emission and the star formation properties of these galaxies. Most of the resolved galaxies show extended MIR emission throughout their disks even where no Hα emission is present. This suggests that the Aromatic carriers can be excited by the general interstellar radiation field (ISRF), i.e. by visible photons. Only close to HII regions the UV photons are the principal sources of Aromatic carrier excitation. However, when the UV radiation field becomes intense enough these carriers can be destroyed. The average integrated 15/6.75µ_m ratio of the observed galaxies is ∼1, i.e. the typical value for the photodissociation regions (PDRs). This suggests that, despite the high star formation rate (SFR) and the very luminous HII regions of these galaxies, their integrated MIR emission is dominated by PDR-like regions rather than HII-like regions. A cold dust component with average temperature ∼22 K exists in most of the target galaxies, probably arising from big dust grains (BGs) in thermal equilibrium with the ISRF. The contribution to the BGs heating from the ionizing stars decreases with increasing wavelength. A warmer dust component whose emission dominates the spectrum between 20 and 100µ_m is likely to exist. This is probably due to both Very Small Grains (VSGs) and warm BGs emission. The dust to gas ratio of the target galaxies is comparable to that of the solar neighborhood. There is a weak trend between the dust total mass and both the atomic and molecular gas content. The MIR and FIR properties of the analyzed galaxies do not seem to be affected by the environment despite the fact that most of the targets are interacting with the Intra-Cluster-Medium.

The Origin of the Diffuse Hα in Spirals

Using high-quality Hα images of five spiral galaxies, we have studied the luminosity and distribution of the emission from diffuse ionized gas (DIG). The estimated DIG luminosities account for 25–60%of the total Hα emission in each galaxy and analysis of the distribution has shown that the DIG is highly correlated geometrically with the most luminous HII regions of the galaxies. The power required to ionize the DIG is very high. The mean ionization rates per unit surface area of a galaxy disc are of the order of 107 cm-2 s-1. Lyman continuum photons (Lyc) from OB asociations are the most probable sources of this ionization. Here we propose a specific model for these sources: we show that the Lyman photon flux that leaks out of the density-bounded HII regions of the galaxies is more than enough to ionize the measured DIG in the five galaxies analysed.

Internal Kinematics of the most Luminous HII Regions in M100

AbstractWe have used the TAURUS Fabry–Perot mapping spectrometer on the William Herschel telescope (WHT) to produce a complete kinematic map of the disk of M100 in Hα. Here we show how the internal velocity dispersion (σ) of the principal emission components of the brightest regions varies with their Hα luminosity. The plot shows ample scatter, but an upper envelope in σ is clearly linear (in the log–log plane) with a slope of 2·6, a result which agrees precisely with an earlier graph by Arsenault et al., who selected instead the regions of highest surface brightness. We show that this result, which differs from the conventional prediction from the virial theorem, is consistent with virialisation if the H II regions are density bounded, and thus offers evidence in support of the density bounding hypothesis for the most luminous regions in disk galaxies.

Optical and near-IR spectrophotometry of the galaxy NGC 3310

Spectrophotometric information in the range λλ 3500–9200 A has been obtained for 16 of the star forming regions in the circumnuclear ring of the galaxy NGC 1068. In 8 of these regions, the Hα luminosity is higher than 4 1039 erg s−1 cm−2, so they can be placed among the most luminous HII regions observed. In 10 of the 16 studied regions, the CaT infrared absortion lines are definitely present. In 7 of these regions the measured equivalent width is lower than 7 A, while in the other three, higher values are obtained (7.3, 8.6, 8.7). In these three cases, the lines might have their origin in a population of red supergiant stars. If we compare the CaT and the Ha equivalent width with theoretical models (Leitherer & Heckmann 1995, ApJS, 96, 9; Leitherer et al. 1999, ApJSS, 123, 3L), we find that the regions with low values of the CaT equivalent width are on the line standing for a 6.5 Myr single burst model of Salpeter initial mass function. For those with the highest values of the equivalent width, the data could be reproduced by a mixed population of young stars (2 – 8 Myr) ionizing the gas, and older ones (15 – 20 Myr) responsible for the CaT observed.

On the Stability of Ionization Fronts

The boundaries between HII and HI regions often have shapes which suggest that the motion of the interstellar gas is unstable there. A characteristic shape of this kind is the so-called “elephant's trunk,” a long tongue of dark material (presumably an HI region) seen superimposed on the luminous HII region, and outlined by a thin rim brighter than the luminous background. A detailed study of these shapes has recently been made by Pottasch; earlier observations had been made by Duncan and by Struve.