Molecular Clouds and Infrared Stellar Clusters in the Far Outer Galaxy

The outer Galaxy beyond the Outer Arm represents a promising opportunity to study star formation in an environment vastly different from the solar neighborhood. In our previous study, we identified 788 candidate star-forming regions in the outer Galaxy (at galactocentric radii R G ≥ 13.5 kpc) based on the Wide-field Infrared Survey Explorer (WISE) mid-infrared (MIR) all-sky survey. In this paper, we investigate the statistical properties of the candidates and their parental molecular clouds derived from the Five College Radio Astronomy Observatory (FCRAO) CO survey. We show that the molecular clouds with candidates have cloud mass functions with shallower slopes, larger fractions of clouds bound by self-gravity, and a greater density than the molecular clouds without candidates. To investigate the star formation efficiency (SFE) at different R G, we used two parameters: (1) the fraction of molecular clouds with candidates and (2) the monochromatic MIR luminosities of candidates per parental molecular cloud mass. We did not find any clear correlation between SFE parameters and R G at R G of 13.5–20.0 kpc, suggesting that the SFE is independent of environmental parameters such as metallicity and gas surface density, which vary considerably with R G. Previous studies reported that the SFE per year (SFE/yr) derived from the star formation rate surface density per total gas surface density, H i plus H2, decreases with increased R G. Our results might suggest that the decreasing trend is due to a decrease in the conversion of H i gas to H2 gas.

view Abstract Citations (35) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Molecular Clouds in the Outer Galaxy. IV. Studies of Star Formation Mead, Kathryn N. ; Kutner, Marc L. ; Evans, Neal J., II Abstract Star formation has been studied in 17 outer Galaxy molecular clouds using the VLA to perform 6 and 20 cm continuum observations to search for H II regions as evidence of massive star formation. IRAS data are used to measure the far-IR luminosity as an indicator of the total star formation rate. H II regions are found to be associated with each of the clouds. The number of ionizing photons required by radio luminosity ranges from 6 x 10 to the 45th/s to 5.5 x 10 to the 49th/s. Far-IR emission was detected from each of the clouds with the range of luminosities being 500 to one million solar. From the data, it is concluded that there is no strong correlation between star formation activity and cloud mass. Publication: The Astrophysical Journal Pub Date: May 1990 DOI: 10.1086/168710 Bibcode: 1990ApJ...354..492M Keywords: Galactic Structure; Milky Way Galaxy; Molecular Clouds; Star Distribution; Star Formation; Continuous Radiation; Far Infrared Radiation; H Ii Regions; Radio Astronomy; Star Formation Rate; Stellar Luminosity; Astrophysics; GALAXIES: THE GALAXY; INTERSTELLAR: MOLECULES; STARS: FORMATION full text sources ADS | data products SIMBAD (24) Related Materials (3) Part 1: 1987ApJ...312..321M Part 2: 1988ApJS...67..149M Part 3: 1988ApJ...330..399M

The outer Galaxy beyond the Outer Arm provides a good opportunity to study star formation in an environment significantly different from that in the solar neighborhood. However, star-forming regions in the outer Galaxy have never been comprehensively studied or cataloged because of the difficulties in detecting them at such large distances. We studied 33 known young star-forming regions associated with 13 molecular clouds at R G ≥ 13.5 kpc in the outer Galaxy with data from the Wide-field Infrared Survey Explorer (WISE) mid-infrared all-sky survey. From their color distribution, we developed a simple identification criterion of star-forming regions in the outer Galaxy with the WISE color. We applied the criterion to all the WISE sources in the molecular clouds in the outer Galaxy at R G ≥ 13.5 kpc detected with the Five College Radio Astronomy Observatory (FCRAO) 12CO survey of the outer Galaxy, of which the survey region is 102.°49 ≤ l ≤ 141.°54, −3.°03 ≤ b ≤ 5.°41, and successfully identified 711 new candidate star-forming regions in 240 molecular clouds. The large number of samples enables us to perform the statistical study of star formation properties in the outer Galaxy for the first time. This study is crucial to investigate the fundamental star formation properties, including star formation rate, star formation efficiency, and initial mass function, in a primordial environment such as the early phase of the Galaxy formation.

We report the discovery of a small aggregate of young stars seen in high-resolution, deep near-infrared ($JHK_S$) images towards IRAS 06345-3023 in the outer Galaxy and well below the mid-plane of the Galactic disc. The group of young stars is likely to be composed of low-mass stars, mostly Class I young stellar objects. The stars are seen towards a molecular cloud whose CO map peaks at the location of the IRAS source. The near-infrared images reveal, additionally, the presence of nebular emission with rich morphological features, including arcs in the vicinity of embedded stars, wisps and bright rims of a butterfly-shaped dark cloud. The location of this molecular cloud as a new star formation site well below the Galactic plane in the outer Galaxy indicates that active star formation is taking place at vertical distances larger than those typical of the (thin) disc.

We have revisited the question of the association of CO emission with IRAS sources in the outer Galaxy using data from the FCRAO Outer Galaxy Survey (OGS). The availability of a large-scale high-resolution CO survey allows us to approach the question of IRAS-CO associations from a new direction – namely we examined all of the IRAS sources within the OGS region for associated molecular material. By investigating the association of molecular material with random lines of sight in the OGS region we were able to construct a quantitative means to judge the likelihood that any given IRAS-CO association is valid and to disentangle multiple emission components along the line of sight. The paper presents a list of all of the IRAS-CO associations in the OGS region. We show that, within the OGS region, there is a significant increase (~22%) in the number of probable star forming regions over previous targeted CO surveys towards IRAS sources. As a demonstration of the utility of the IRAS-CO association table we present the results of three brief studies on candidate zone-of-avoidance galaxies with IRAS counterparts, far outer Galaxy CO clouds, and very bright CO clouds with no associated IRAS sources. We find that ~25% of such candidate ZOAGs are Galactic objects. We have discovered two new far outer Galaxy star-forming regions, and have discovered six bright molecular clouds that we believe are ideal targets for the investigation of the earliest stages of sequential star formation around Hii regions. Finally, this paper provides readers with the necessary data to compare other catalogued data sets with the OGS data.

Infrared dark clouds (IRDCs) are extinction features against the Galactic infrared background, mainly in the mid-infrared band. Recently they were proposed to be potential sites of massive star formation. In this work we have made a12CO,13CO, and C18O (J = 1→ 0) survey of 61 IRDCs, 52 of which are in the first Galactic quadrant, selected from a catalog given by Simon and coworkers, while the others are in the outer Galaxy, selected by visually inspecting the MSX images. Detection rates in the three CO lines are 90%, 71%, and 62%, respectively. The distribution of IRDCs in the first Galactic quadrant is consistent with the 5 kpc molecular ring picture, while a slight trace of a spiral pattern is also noticeable, and needs to be further examined. The IRDCs have a typical excitation temperature of 10 K and typical column density of several 1022 cm−2. Their typical physical size is estimated to be several parsecs using angular sizes from the Simon catalog. Typical volume density and typical LTE mass are ~5000 cm−3 and ~5000 M☉, respectively. The IRDCs are in or near virial equilibrium. The properties of IRDCs are similar to those of star-forming molecular clumps, and they seem to be intermediate between giant molecular clouds and Bok globules; thus they may represent early stages of massive star formation.

Jets and outflows are an integral part of the star formation process. While there are many detailed studies of molecular outflows towards individual star-forming sites, few studies have surveyed an entire star-forming molecular cloud for this phenomenon. The 100-deg2 Five College Radio Astronomy Observatory CO survey of the Taurus Molecular Cloud provides an excellent opportunity to undertake an unbiased survey of a large, nearby, molecular cloud complex for molecular outflow activity. Our study provides information on the extent, energetics and frequency of outflows in this region, which are then used to assess the impact of outflows on the parent molecular cloud. The search identified 20 outflows in the Taurus region, eight of which were previously unknown. Both 12CO and 13CO data cubes from the Taurus molecular map were used, and dynamical properties of the outflows are derived. Even for previously known outflows, our large-scale maps indicate that many of the outflows are much larger than previously suspected, with eight of the outflows (40 per cent) being more than a parsec long. The mass, momentum and kinetic energy from the 20 outflows are compared to the repository of turbulent energy in Taurus. Comparing the energy deposition rate from outflows to the dissipation rate of turbulence, we conclude that outflows by themselves cannot sustain the observed turbulence seen in the entire cloud. However, when the impact of outflows is studied in selected regions of Taurus, it is seen that locally outflows can provide a significant source of turbulence and feedback. The L1551 dark cloud which is just south of the main Taurus complex was not covered by this survey, but the outflows in L1551 have much higher energies compared to the outflows in the main Taurus cloud. In the L1551 cloud, outflows can not only account for the turbulent energy present, but are probably also disrupting their parent cloud. We conclude that for a molecular cloud like Taurus, an L1551-like episode occurring once every 105 years is sufficient to sustain the turbulence observed. Five of the eight newly discovered outflows have no known associated stellar source, indicating that they may be embedded Class 0 sources. In Taurus, 30 per cent of Class I sources and 12 per cent of flat-spectrum sources from the Spitzer young stellar object (YSO) catalogue have outflows, while 75 per cent of known Class 0 objects have outflows. Overall, the paucity of outflows in Taurus compared to the embedded population of Class I and flat-spectrum YSOs indicates that molecular outflows are a short-lived stage marking the youngest phase of protostellar life. The current generation of outflows in Taurus highlight an ongoing period of active star formation, while a large fraction of YSOs in Taurus have evolved well past the Class I stage.

Tycho's supernova remnant (SNR), as one of the few historical SNRs, has been widely studied in various wave bands. Observations show evidence that Tycho is expanding in a medium with density gradient and possibly interacting with a dense ambient medium toward the northeast direction. From the FCRAO CO survey of the outer Galaxy, we have identified a patch of molecular clouds in this area and have conducted a follow-up observation with the Nobeyama 45 m radio telescope. The high-resolution (16'') Nobeyama data show that a large molecular cloud surrounds the SNR along the northeastern boundary. We suggest that Tycho's SNR and the molecular cloud are located in the Perseus arm and that the dense medium interacting with the SNR is possibly the molecular cloud. We also discuss the possible connection between the molecular cloud and the Balmer-dominated optical filaments and suggest that the preshock gas may be accelerated within the cosmic-ray and/or fast neutral precursor.

We present an investigation of the properties of 21 molecular clouds in the LMC. Our data consist of 60 μm and 100 μm IRAS images that we compare with fully sampled 12CO (J = 1 → 0) maps. The CO data were taken on the 15 m Swedish-ESO Submillimeter Telescope (SEST), which has a beamwidth at 2.6 mm of 45'', or 10 pc at the distance of the LMC. We use the IRAS high-resolution reprocessed data, with approximate resolutions of 60'' and 75'' at 60 μm and 100 μm, respectively. The clouds mapped are in three regions: two complexes south of 30 Doradus, and one at the H II region N11. We measure the far-infrared luminosities for each cloud that has a good CO-FIR correspondence and compare these results with those from similar studies done on Milky Way molecular clouds. The far-infrared luminosities range from 3.5 × 104 to 2.8 × 106 L☉, with an average value of 4 × 105 L☉. This average is lower than that seen for H II regions in the inner Milky Way, as reported by Scoville & Good in 1989 and slightly higher than that for outer Galaxy clouds as reported by Mead et al. in 1990. Virial masses of these clouds are from 4 × 104 to 1 × 106 M☉. Star formation activity (LFIR/MVIR), defined as the ratio of the luminosity from recently formed stars to the total cloud mass, has a range of 2 orders of magnitude for any MVIR. In addition, we find that LFIR/MVIR is independent of cloud mass over the entire range of measured virial masses. These results indicate that although less luminous in CO and FIR than their inner Milky Way counterparts, LMC molecular clouds are undergoing significant massive star formation. LFIR is used to determine the numbers and types of stars embedded in the GMCs. We compare these findings with those from the Milky Way and discuss the implications for star formation theories.

Results are presented from a systematic search for CO emission from regions around 34 young open clusters in the outer Galaxy. The clusters have well-determined distances ranging from about 1 to 5 kpc and ages not greater than about 100 Myr. It was found that some moderately young clusters have no associated CO emission. All the surveyed clusters younger than about 5 Myr have associated with them at least one molecular cloud more massive than 10,000 solar mass, while the molecular clouds associated with clusters older than about 10 Myr are not more massive than a few thousands solar masses. It was also found that molecular clouds are receding from young clusters at a rate of about 10 km/sec, and that they seem to be destroyed by their interaction with the stars. Sites of ongoing star formation were identified in a number of clouds associated with young clusters.

view Abstract Citations (107) References (58) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Molecular Clouds Associated with Luminous Far-Infrared Sources in the Outer Galaxy Carpenter, John M. ; Snell, Ronald L. ; Schloerb, F. Peter Abstract The stellar content and physical properties of the molecular clouds associated with 21 bright far-IR sources in the outer Galaxy have been determined through C-12O, C-13O, 6-cm radio continuum, and IRAS observations. The molecular cloud masses range from 200 to about 10,000 solar masses. The far-IR luminosity-to-mass ratio for these clouds has a mean value of 6.8 solar luminosity/solar masses and shows no correlation with the cloud mass, a result similar to that found for more massive clouds in the inner Galaxy. The radio continuum survey of the 21 bright far-IR sources indicates that most of these regions probably have a single, massive star providing most of the ionization. The cloud masses derived from virial and LTE analyses are in agreement, supporting the assumptions commonly made in their calculations, and a tight, near-linear correlation is found between the C-12O luminosity and the cloud mass. The H2 column density and integrated C-12O intensity are also correlated on a point-by-point basis, although the scatter is larger than the C-12O luminosity-cloud mass relation. Publication: The Astrophysical Journal Pub Date: October 1990 DOI: 10.1086/169251 Bibcode: 1990ApJ...362..147C Keywords: Far Infrared Radiation; Infrared Sources (Astronomy); Luminosity; Milky Way Galaxy; Molecular Clouds; Carbon Monoxide; Carbon 13; Infrared Astronomy Satellite; Interstellar Matter; Physical Properties; Astrophysics; GALAXIES: THE GALAXY; INTERSTELLAR: MOLECULES; INFRARED: SOURCES full text sources ADS | data products SIMBAD (20)

The extent and nature of molecular material in the outer Galaxy (R > R⊙) has been an open, interesting question since the discovery of molecular material in the outer Galaxy (Kutner & Mead, 1981). From large scale CO surveys, two estimates of the total molecular mass have been made with the same result, M(H2) = 6 × 108 M⊙ (Wouterloot et al., 1990), (Digel et al., 1990). Both estimates have a strong bias towards large, star forming (warm) clouds. Wouterloot et al. used the IRAS point source catalog as a finder chart for their observations and the inner Galaxy mass distribution to scale their result. Digel et al. conducted an unbiased survey with a large (8.8 arcminute) beam which underestimated the amount of material in small clouds due to the effects of beam dilution.

The extent and nature of molecular material in the outer Galaxy (R > R⊙) has been an open, interesting question since the discovery of molecular material in the outer Galaxy (Kutner & Mead, 1981). From large scale CO surveys, two estimates of the total molecular mass have been made with the same result, M(H2) = 6 × 108 M⊙ (Wouterloot et al., 1990), (Digel et al., 1990). Both estimates have a strong bias towards large, star forming (warm) clouds. Wouterloot et al. used the IRAS point source catalog as a finder chart for their observations and the inner Galaxy mass distribution to scale their result. Digel et al. conducted an unbiased survey with a large (8.8 arcminute) beam which underestimated the amount of material in small clouds due to the effects of beam dilution.

The first 1.1 mm continuum survey toward the Small Magellanic Cloud (SMC) was performed using the AzTEC instrument installed on the ASTE 10 m telescope. This survey covered 4.5 deg2 of the SMC with 1σ noise levels of 5–12 mJy beam−1, and 44 extended objects were identified. The 1.1 mm extended emission has good spatial correlation with Herschel 160 μm, indicating that the origin of the 1.1 mm extended emission is thermal emission from a cold dust component. We estimated physical properties using the 1.1 mm and filtered Herschel data (100, 160, 250, 350, and 500 μm). The 1.1 mm objects show dust temperatures of 17–45 K and gas masses of 4 × 103–3 × 105 M ⊙, assuming single-temperature thermal emission from the cold dust with an emissivity index, β, of 1.2 and a gas-to-dust ratio of 1000. These physical properties are very similar to those of giant molecular clouds (GMCs) in our galaxy and the Large Magellanic Cloud. The 1.1 mm objects also displayed good spatial correlation with the Spitzer 24 μm and CO emission, suggesting that the 1.1 mm objects trace the dense gas regions as sites of massive star formation. The dust temperature of the 1.1 mm objects also demonstrated good correlation with the 24 μm flux connected to massive star formation. This supports the hypothesis that the heating source of the cold dust is mainly local star-formation activity in the 1.1 mm objects. The classification of the 1.1 mm objects based on the existence of star-formation activity reveals the differences in the dust temperature, gas mass, and radius, which reflects the evolution sequence of GMCs.

We present new millimeter and infrared spectroscopic observations towards the radio nebula G10.0-0.3, which is powered by the wind of the Luminous Blue Variable star LBV 1806-20, also closely associated with the soft gamma-ray repeater SGR 1806-20, and believed to be located in the giant Galactic HII complex W31. Based on observations of CO emission lines and NH_3 absorption features from molecular clouds along the line of sight to G10.0-0.3, as well as the radial velocity and optical extinction of the star powering the nebula, we determine its distance to be 15.1$^{+1.8}_{-1.3}$ kpc in agreement with Corbel et al. (1997). In addition, this strengthens the association of SGR 1806-20 with a massive molecular cloud at the same distance. All soft gamma-ray repeaters with precise location are now found to be associated with a site of massive star formation or molecular cloud. We also show that W31 consists of at least two distinct components along the line of sight. We suggest that G10.2-0.3 and G10.6-0.4 are located on the -30 km/s spiral arm at a distance from the Sun of 4.5 $\pm$ 0.6 kpc and that G10.3-0.1 may be associated with a massive molecular cloud at the same distance as the LBV star, i.e. 15.1$^{+1.8}_{-1.3}$ kpc, implying that W31 could be decomposed into two components along the line of sight.