Galactic Star-forming Regions Research Articles

Sink particle radiative feedback in smoothed particle hydrodynamics models of star formation

We present a new method for including radiative feedback from sink particles in smoothed particle hydrodynamics simulations of low-mass star formation, and investigate its effects on the formation of small stellar groups. We find that including radiative feedback from sink particles suppresses fragmentation even further than calculations that only include radiative transfer within the gas. This reduces the star-formation rate following the formation of the initial protostars, leading to fewer objects being produced and a lower total stellar mass. The luminosities of sink particles vary due to changes in the accretion rate driven by the dynamics of the cluster gas, leading to different luminosities for protostars of similar mass. Including feedback from sinks also raises the median stellar mass. The median masses of the groups are higher than typically observed values. This may be due to the lack of dynamical interactions and ejections in small groups of protostars compared to those that occur in richer groups. We also find that the temperature distributions in our calculations are in qualitative agreement with recent observations of protostellar heating in Galactic star-forming regions.

Class I methanol masers in NGC 253: Alcohol at the end of the bar

We have used the Australia Telescope Compact Array (ATCA) to observe the 36.2-GHz class I methanol maser emission towards NGC253 and find that it is located at the interface between the nuclear ring and both ends of the galactic bar. This is thought to be the location of the inner Linblad resonance and we suggest that the maser emission in this region is likely due to large-scale cloud-cloud collisions. We have detected the first extragalactic 44.1-GHz class I methanol maser and find that it is associated with the 36.2-GHz maser emission. In contrast to the class I methanol masers found in Galactic star formation regions, the 44.1-GHz emission in NGC253 is two orders of magnitude weaker than the 36.2-GHz masers. Both the 36.2- and 44.1- GHz emission is orders of magnitude stronger than expected from typical high-mass star formation regions. This demonstrates that the luminous class I methanol masers observed in NGC253 are significantly different from those associated with Galactic star formation.

What Sets the Massive Star Formation Rates and Efficiencies of Giant Molecular Clouds?

Abstract Galactic star formation scaling relations show increased scatter from kpc to sub-kpc scales. Investigating this scatter may hold important clues to how the star formation process evolves in time and space. Here, we combine different molecular gas tracers, different star formation indicators probing distinct populations of massive stars, and knowledge of the evolutionary state of each star-forming region to derive the star formation properties of ∼150 star-forming complexes over the face of the Large Magellanic Cloud (LMC). We find that the rate of massive star formation ramps up when stellar clusters emerge and boost the formation of subsequent generations of massive stars. In addition, we reveal that the star formation efficiency of individual giant molecular clouds (GMCs) declines with increasing cloud gas mass ( ). This trend persists in Galactic star-forming regions and implies higher molecular gas depletion times for larger GMCs. We compare the star formation efficiency per freefall time ( ) with predictions from various widely used analytical star formation models. While these models can produce large dispersions in similar to those in observations, the origin of the model-predicted scatter is inconsistent with observations. Moreover, all models fail to reproduce the observed decline of with increasing in the LMC and the Milky Way. We conclude that analytical star formation models idealizing global turbulence levels and cloud densities and assuming a stationary star formation rate (SFR) are inconsistent with observations from modern data sets tracing massive star formation on individual cloud scales. Instead, we reiterate the importance of local stellar feedback in shaping the properties of GMCs and setting their massive SFR.

The Star-formation Law in Galactic High-mass Star-forming Molecular Clouds

We study the star formation (SF) law in 12 Galactic molecular clouds with ongoing high-mass star formation (HMSF) activity, as traced by the presence of a bright IRAS source and other HMSF tracers. We define the molecular cloud (MC) associated to each IRAS source using 13CO line emission, and count the young stellar objects (YSOs) within these clouds using GLIMPSE and MIPSGAL 24 micron Spitzer databases.The masses for high luminosity YSOs (Lbol>10~Lsun) are determined individually using Pre Main Sequence evolutionary tracks and the evolutionary stages of the sources, whereas a mean mass of 0.5 Msun was adopted to determine the masses in the low luminosity YSO population. The star formation rate surface density (sigsfr) corresponding to a gas surface density (siggas) in each MC is obtained by counting the number of the YSOs within successive contours of 13CO line emission. We find a break in the relation between sigsfr and siggas, with the relation being power-law (sigsfr ~ siggas^N) with the index N varying between 1.4 and 3.6 above the break. The siggas at the break is between 150-360 Msun/pc^2 for the sample clouds, which compares well with the threshold gas density found in recent studies of Galactic star-forming regions. Our clouds treated as a whole lie between the Kennicutt (1998) relation and the linear relation for Galactic and extra-galactic dense star-forming regions. We find a tendency for the high-mass YSOs to be found preferentially in dense regions at densities higher than 1200 Msun/pc^2 (~0.25 g/cm^2).

CO Spectral Line Energy Distributions in Galactic Sources: Empirical Interpretation of Extragalactic Observations∗

Abstract The relative populations in rotational transitions of CO can be useful for inferring gas conditions and excitation mechanisms at work in the interstellar medium. We present CO emission lines from rotational transitions observed with Herschel/HIFI in the star-forming cores Orion S, Orion KL, Sgr B2(M), and W49N. Integrated line fluxes from these observations are combined with those from Herschel/PACS observations of the same sources to construct CO spectral line energy distributions (SLEDs) from 5 ≤ J u ≤ 48. These CO SLEDs are compared to those reported in other galaxies, with the intention of empirically determining which mechanisms dominate excitation in such systems. We find that CO SLEDs in Galactic star-forming cores cannot be used to reproduce those observed in other galaxies, although the discrepancies arise primarily as a result of beam filling factors. The much larger regions sampled by the Herschel beams at distances of several megaparsecs contain significant amounts of cooler gas, which dominate the extragalactic CO SLEDs, in contrast to observations of Galactic star-forming regions, which are focused specifically on cores containing primarily hot molecular gas.

Detections of 2 cm formaldehyde emissions towards Galactic star-forming regions with 6 cm counterpart

We report the detections of H2CO emission at the 2 cm transition towards Galactic star-forming regions with known 6 cm counterpart using the Shanghai Tianma Radio Telescope. One significant detection (in NGC 7538) and two possible detections (in G23.01−0.41 and G29.96−0.02) were made. Comparing with previous observations, we found that there is a time lag of appearance of 2 and 6 cm emissions detected in NGC 7538, contradicting with the prediction of radiative pumping via radio continuum radiation. Combinations of the variability of 6 cm masers in NGC 7538 suggest that collisional pumping via high-velocity shocks could better explain the 6 cm H2CO maser emission. Under this scheme, excitation of the 2 cm maser may require a higher collision energy compared to the 6 cm transition.

SPATIAL VARIATIONS OF PAH PROPERTIES IN M17SW REVEALED BY SPITZER/IRS SPECTRAL MAPPING

ABSTRACT We present Spitzer/IRS mid-infrared spectral maps of the Galactic star-forming region M17 as well as IRSF/SIRIUS Brγ and Nobeyama 45 m/FOREST 13CO (J = 1–0) maps. The spectra show prominent features due to polycyclic aromatic hydrocarbons (PAHs) at wavelengths of 6.2, 7.7, 8.6, 11.3, 12.0, 12.7, 13.5, and 14.2 μm. We find that the PAH emission features are bright in the region between the H ii region traced by Brγ and the molecular cloud traced by 13CO, supporting that the PAH emission originates mostly from photo-dissociation regions. Based on the spatially resolved Spitzer/IRS maps, we examine spatial variations of the PAH properties in detail. As a result, we find that the interband ratio of PAH 7.7 μm/PAH 11.3 μm varies locally near M17SW, but rather independently of the distance from the OB stars in M17, suggesting that the degree of PAH ionization is mainly controlled by local conditions rather than the global UV environments determined by the OB stars in M17. We also find that the interband ratios of the PAH 12.0 μm, 12.7 μm, 13.5 μm, and 14.2 μm features to the PAH 11.3 μm feature are high near the M17 center, which suggests structural changes of PAHs through processing due to intense UV radiation, producing abundant edgy irregular PAHs near the M17 center.

WATER MASERS IN THE ANDROMEDA GALAXY. II. WHERE DO MASERS ARISE?

ABSTRACT We present a comparative multiwavelength analysis of water-maser-emitting regions and non-maser-emitting luminous 24 μm star-forming regions in the Andromeda Galaxy (M31) to identify the sites most likely to produce luminous water masers useful for astrometry and proper motion studies. Included in the analysis are Spitzer 24 μm photometry, Herschel 70 and 160 μm photometry, Hα emission, dust temperature, and star-formation rate. We find significant differences between the maser-emitting and non-maser-emitting regions: water-maser-emitting regions tend to be more infrared-luminous and show higher star-formation rates. The five water masers in M31 are consistent with being analogs of water masers in Galactic star-forming regions and represent the high-luminosity tail of a larger (and as yet undetected) population. Most regions likely to produce water masers bright enough for proper motion measurements using current facilities have already been surveyed, but we suggest three ways to detect additional water masers in M31: (1) reobserve the most luminous mid- or far-infrared sources with higher sensitivity than was used in the Green Bank Telescope survey; (2) observe early-stage star-forming regions selected by millimeter continuum that have not already been selected by their 24 μm emission, and (3) reobserve the most luminous mid- or far-infrared sources and rely on maser variability for new detections.

VLBA SURVEYS OF OH MASERS IN STAR-FORMING REGIONS. I. SATELLITE LINES

ABSTRACT Using the Very Long Baseline Array we performed a high-resolution OH maser survey in Galactic star-forming regions (SFRs). We observed all the ground state spectral lines: the main lines at 1665 and 1667 MHz and the satellite lines at 1612 and 1720 MHz. Due to the exceptionality of finding satellite lines in SFRs, we will focus our discussion on those lines. In our sample of 41 OH maser sources, five (12%) showed the 1612 MHz line and ten (24%) showed the 1720 MHz line, with only one source showing both lines. We find that 1720 MHz emission is correlated with the presence of H ii regions, suggesting that this emission could be used to diagnose or trace high-mass star formation. We include an analysis of the possible mechanisms that could be causing this correlation as well as assessing the possible relationships between lines in our sample. In particular, the presence of magnetic fields seems to play an important role as we found Zeeman splitting in four of our sources (W75 N, W3(OH), W51 and NGC 7538). Our results have implications for current understanding of the formation of high-mass stars as well as on the masing processes present in SFRs.

Observations of Galactic star-forming regions with the Cosmic Background Imager at 31 GHz

Studies of the diffuse Galactic radio emission are interesting both for better understanding the physical conditions in our Galaxy and for minimising the contamination in cosmological measurements. Motivated by this we present Cosmic Background Imager 31 GHz observations of the Galactic regions NGC 6357, NGC 6334, W51 and W40 at $\sim$4$'$.5 resolution and conduct an investigation of the spectral emission process in the regions at 4$'$.5 and 1$^{\circ}$ resolution. We find that most of the emission in the regions is due to optically thin free-free. For 2 sub-regions of NGC 6334 and for a sub-region of W51 though, at 4$'$.5 resolution and at 31 GHz we detect less emission than expected from extrapolation of radio data at lower frequencies assuming a spectral index of $-$0.12 for optically thin free-free emission, at 3.3$\sigma$, 3.7$\sigma$ and 6.5$\sigma$ respectively. We also detect excess emission in a sub-region of NCG 6334 at 6.4$\sigma$, after ruling out any possible contribution from Ultra Compact HII (UCHII) regions. At 1$^{\circ}$ resolution we detect a spinning dust component in the Spectral Energy Distribution (SED) of W40 that accounts for 18$\pm$7 % of the total flux density in the region at the peak frequency of 37 GHz. Comparison with 100 ${\rm \mu m}$ data indicate an average dust emissivity for the sub-regions of $0.5\pm4.4$ $\mu$K(MJy sr$^{-1}$)$^{-1}$. Finally we translate the excess emission in the regions to an Anomalous Microwave Emission (AME) emissivity relative to the optical depth at 250 ${\rm \mu m }$. We find that this form of emissivity is independent of the AME significance and has a value somewhere in the order of 10$^4$ Jy.

EXTREMELY BRIGHT SUBMILLIMETER GALAXIES BEYOND THE LUPUS-I STAR-FORMING REGION

We report detections of two candidate distant submillimeter galaxies (SMGs), MM J154506.4$-$344318 and MM J154132.7$-$350320, which are discovered in the AzTEC/ASTE 1.1 mm survey toward the Lupus-I star-forming region. The two objects have 1.1 mm flux densities of 43.9 and 27.1 mJy, and have Herschel/SPIRE counterparts as well. The Submillimeter Array counterpart to the former SMG is identified at 890 $\mu$m and 1.3 mm. Photometric redshift estimates using all available data from the mid-infrared to the radio suggest that the redshifts of the two SMGs are $z_{\rm photo} \simeq$ 4-5 and 3, respectively. Near-infrared objects are found very close to the SMGs and they are consistent with low-$z$ ellipticals, suggesting that the high apparent luminosities can be attributed to gravitational magnification. The cumulative number counts at $S_{\rm 1.1mm} \ge 25$ mJy, combined with other two 1.1-mm brightest sources, are $0.70 ^{+0.56}_{-0.34}$ deg$^{-2}$, which is consistent with a model prediction that accounts for flux magnification due to strong gravitational lensing. Unexpectedly, a $z > 3$ SMG and a Galactic dense starless core (e.g., a first hydrostatic core) could be similar in the mid-infrared to millimeter spectral energy distributions and spatial structures at least at $\gtrsim 1"$. This indicates that it is necessary to distinguish the two possibilities by means of broad band photometry from the optical to centimeter and spectroscopy to determine the redshift, when a compact object is identified toward Galactic star-forming regions.

Neutral carbon and CO in 76 (U)LIRGs and starburst galaxy centers

We present fluxes in both neutral carbon [CI] lines at the centers of 76 galaxies with FIR luminosities between 10^{9} and 10^{12} L(o) obtained with Herschel-SPIRE and with ground-based facilities, along with the J=7-6, J=4-3, J=2-1 12CO and J=2-1 13CO line fluxes. We investigate whether these lines can be used to characterize the molecular ISM of the parent galaxies in simple ways and how the molecular gas properties define the model results. In most starburst galaxies, the [CI]/13CO flux ratio is much higher than in Galactic star-forming regions, and it is correlated to the total FIR luminosity. The [CI](1-0)/CO(4-3), the [CI](2-1) (2-1)/CO(7-6), and the [CI] (2-1)/(1-0) flux ratios are also correlated, and trace the excitation of the molecular gas. In the most luminous infrared galaxies (LIRGs), the ISM is fully dominated by dense and moderately warm gas clouds that appear to have low [C]/[CO] and [13CO]/[12CO] abundances. In less luminous galaxies, emission from gas clouds at lower densities becomes progressively more important, and a multiple-phase analysis is required to determine consistent physical characteristics. Neither the CO nor the [CI] velocity-integrated line fluxes are good predictors of H2 column densities in individual galaxies, and X(CI) conversion factors are not superior to X(CO) factors. The methods and diagnostic diagrams outlined in this paper also provide a new and relatively straightforward means of deriving the physical characteristics of molecular gas in high-redshift galaxies up to z=5, which are otherwise hard to determine.

DETECTION OF A METHANOL MEGAMASER IN A MAJOR-MERGER GALAXY

We have detected emission from both the 4_{-1}-3_{0} E (36.2~GHz) class I and 7_{-2}-8_{-1} E (37.7~GHz) class II methanol transitions towards the centre of the closest ultra-luminous infrared galaxy Arp 220. The emission in both the methanol transitions show narrow spectral features and have luminosities approximately 8 orders of magnitude stronger than that observed from typical class I methanol masers observed in Galactic star formation regions. The emission is also orders of magnitude stronger than the expected intensity of thermal emission from these transitions and based on these findings we suggest that the emission from the two transitions are masers. These observations provides the first detection of a methanol megamaser in the 36.2 and 37.7 GHz transitions and represents only the second detection of a methanol megamaser, following the recent report of an 84 GHz methanol megamaser in NGC1068. We find the methanol megamasers are significantly offset from the nuclear region and arise towards regions where there is Ha emission, suggesting that it is associated with starburst activity. The high degree of correlation between the spatial distribution of the 36.2 GHz methanol and X-ray plume emission suggests that the production of strong extragalactic class I methanol masers is related to galactic outflow driven shocks and perhaps cosmic rays. In contrast to OH and H2O megamasers which originate close to the nucleus, methanol megamasers provide a new probe of feedback (e.g. outflows) processes on larger-scales and of star formation beyond the circumnuclear starburst regions of active galaxies.

THE STRUCTURAL EVOLUTION OF FORMING AND EARLY STAGE STAR CLUSTERS

We study the degree of angular substructure in the stellar position distribution of young members of Galactic star-forming regions, looking for correlations with distance from cluster center, surface number density of stars, and local dynamical age. To this end we adopt the catalog of members in 18 young ($\sim$1-3 Myr) clusters from the Massive Young Star-Forming Complex Study in Infrared and X-ray (MYStIX) Survey and the statistical analysis of the Angular Dispersion Parameter, $\delta_{\rm ADP}$. We find statistically significant correlation between $\delta_{\rm ADP}$ and physical projected distance from the center of the clusters, with the centers appearing smoother than the outskirts, consistent with more rapid dynamical processing on local dynamical, free-fall or orbital timescales. Similarly, smoother distributions are seen in regions of higher surface density, or older dynamical ages. These results indicate that dynamical processing that erases substructure is already well-advanced in young, sometimes still-forming, clusters. Such observations of the dissipation of substructure have the potential to constrain theoretical models of the dynamical evolution of young and forming clusters.

Astrometry of Galactic star-forming regions G48.99−0.30 and G49.19−0.34 with VERA: Astrometric accuracy verification of VERA

Abstract We conducted astrometry of H2O masers in Galactic star-forming regions G48.99−0.30 and G49.19−0.34 with VLBI Exploration of Radio Astrometry (VERA). The observational results yield the parallaxes of G48.99−0.30 and G49.19−0.34 to be 0.178 ± 0.017 mas ($5.62^{+0.59}_{-0.49}\:$kpc) and 0.211 ± 0.016 mas ($4.74^{+0.39}_{-0.33}\:$kpc), respectively. We discuss in detail the sources of astrometric errors based on the observations of the two sources. The positional systemic errors in RA and Dec of (23, 31) μas for G49.19−0.34 are dominated by a tropospheric contribution from the Earth's atmosphere. Those of (23, 53) μas for G48.99−0.30 are attributed to a tropospheric contribution and a maser structure effect. The formal positional error from thermal noise is significantly smaller than these systemic positional errors. VERA phase-referencing observations could achieve a positional accuracy of a few tens of μas in both RA and Dec directions for point-like and bright sources at high elevation.

A SURVEY OF THE MOLECULAR ISM PROPERTIES OF NEARBY GALAXIES USING THEHERSCHELFTS

The 12CO J=4-3 to J=13-12 lines of the interstellar medium from nearby galaxies, newly observable with the Herschel SPIRE Fourier Transform Spectrometer (FTS), offer an opportunity to study warmer, more luminous molecular gas than that traced by 12CO J=1-0. Here we present a survey of 17 nearby infrared-luminous galaxy systems (21 pointings). In addition to photometric modeling of dust, we modeled full 12CO spectral line energy distributions from J=1-0 to J=13-12 with two components of warm and cool CO gas, and included LTE analysis of [CI], [CII], [NII] and H2 lines. CO is emitted from a low-pressure/high-mass component traced by the low-J lines and a high-pressure/low-mass component which dominates the luminosity. We found that, on average, the ratios of the warm/cool pressure, mass, and 12CO luminosity are 60 +/- 30, 0.11 +/- 0.02, and 15.6 +/- 2.7. The gas-to-dust-mass ratios are < 120 throughout the sample. The 12CO luminosity is dominated by the high-J lines and is 4 $\times 10^{-4}$ LFIR on average. We discuss systematic effects of single-component and multi-component CO modeling (e.g., single-component J < 3 models overestimate gas pressure by ~ 0.5 dex), as well as compare to Galactic star-forming regions. With this comparison, we show the molecular interstellar medium of starburst galaxies is not simply an ensemble of Galactic-type GMCs. The warm gas emission is likely dominated by regions resembling the warm extended cloud of Sgr B2.

Multimolecular studies of Galactic star-forming regions

Molecular emission-line observations of isolated Galactic star-forming regions are used to model the physical properties of the molecular interstellar medium in these systems. Observed line ratios ...

DETECTION OF 36 GHz CLASS I METHANOL MASER EMISSION TOWARD NGC 253

We have used the Australia Telescope Compact Array (ATCA) to search for emission from the $4_{-1} \rightarrow 3_{0}E$ transition of methanol (36.2 GHz) towards the center of the nearby starburst galaxy NGC253. Two regions of emission were detected, offset from the nucleus along the same position angle as the inner spiral arms. The emission is largely unresolved on a scale of 5 arcsec, has a full-width half maximum (FWHM) line width of < 30 km s$^{-1}$, and an isotropic luminosity orders of magnitude larger than that observed in any Galactic star formation regions. These characteristics suggest that the 36.2 GHz methanol emission is most likely a maser, although observations with higher angular and spectral resolution are required to confirm this. If it is a maser this represents the first detection of a class I methanol maser outside the Milky Way. The 36.2 GHz methanol emission in NGC253 has more than an order of magnitude higher isotropic luminosity than the widespread emission recently detected towards the center of the Milky Way. If emission from this transition scales with nuclear star formation rate then it may be detectable in the central regions of many starburst galaxies. Detection of methanol emission in ultra-luminous infra-red galaxies (ULIRGs) would open up a new tool for testing for variations in fundamental constants (in particular the proton-to-electron mass ratio) on cosmological scales.

AN ALL-SKY SAMPLE OF INTERMEDIATE-MASS STAR-FORMING REGIONS

We present an all-sky sample of 984 candidate intermediate-mass Galactic star-forming regions color-selected from the Infrared Astronomical Satellite (IRAS) Point Source Catalog and morphologically classify each object using mid-infrared Wide-field Infrared Survey Explorer (WISE) images. Of the 984 candidates, 616 are probable star-forming regions (62.6%), 128 are filamentary structures (13.0%), 39 are point-like objects of unknown nature (4.0%), and 201 are galaxies (20.4%). We conduct a study of four of these regions, IRAS 00259+5625, IRAS 00420+5530, IRAS 01080+5717, and IRAS 05380+2020, at Galactic latitudes |b| > 5 degrees using optical spectroscopy from the Wyoming Infrared Observatory along with near-infrared photometry from the Two-Micron All Sky Survey to investigate their stellar content. New optical spectra, color-magnitude diagrams, and color-color diagrams reveal their extinctions, spectrophotometric distances, and the presence of small stellar clusters containing 20-78 solar masses of stars. These low-mass diffuse star clusters contain 65-250 stars for a typical initial mass function, including one or more mid-B stars as their most massive constituents. Using infrared spectral energy distributions we identify young stellar objects near each region and assign probable masses and evolutionary stages to the protostars. The total infrared luminosity lies in the range 190 to 960 solar luminosities, consistent with the sum of the luminosities of the individually identified young stellar objects.

A multi-scale filament extraction method:getfilaments

Far-infrared imaging surveys of Galactic star-forming regions with Herschel have shown that a substantial part of the cold interstellar medium appears as a fascinating web of omnipresent filamentary structures. This highly anisotropic ingredient of the interstellar material further complicates the difficult problem of the systematic detection and measurement of dense cores in the strongly variable but (relatively) isotropic backgrounds. Observational evidence that stars form in dense filaments creates severe problems for automated source extraction methods that must reliably distinguish sources not only from fluctuating backgrounds and noise, but also from the filamentary structures. A previous paper presented the multi-scale, multi-wavelength source extraction method getsources based on a fine spatial scale decomposition and filtering of irrelevant scales from images. In this paper, a multi-scale, multi-wavelength filament extraction method getfilaments is presented that solves this problem, substantially improving the robustness of source extraction with getsources in filamentary backgrounds. The main difference is that the filaments extracted by getfilaments are now subtracted by getsources from detection images during source extraction, greatly reducing the chances of contaminating catalogs with spurious sources. The intimate physical relationship between forming stars and filaments seen in Herschel observations demands that accurate filament extraction methods must remove the contribution of sources and that accurate source extraction methods must be able to remove underlying filamentary structures. Source extraction with getsources now provides researchers also with clean images of filaments, free of sources, noise, and isotropic backgrounds.