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At least 775 records · Page 43
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DOE OSTI2017

THE GOULD’S BELT DISTANCES SURVEY (GOBELINS). II. DISTANCES AND STRUCTURE TOWARD THE ORION MOLECULAR CLOUDS

We present the results of the Gould’s Belt Distances Survey of young star-forming regions toward the Orion Molecular Cloud Complex. We detected 36 young stellar objects (YSOs) with the Very Large Baseline Array, 27 of which have been observed in at least three epochs over the course of two years. At least half of these YSOs belong to multiple systems. We obtained parallax and proper motions toward these stars to study the structure and kinematics of the Complex. We measured a distance of 388 ± 5 pc toward the Orion Nebula Cluster, 428 ± 10 pc toward the southern portion L1641, 388 ± 10 pc toward NGC 2068, and roughly ∼420 pc toward NGC 2024. Finally, we observed a strong degree of plasma radio scattering toward λ Ori.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

ALMA MEASUREMENTS OF CIRCUMSTELLAR MATERIAL IN THE GQ LUP SYSTEM

We present Atacama Large Millimeter/submillimeter Array observations of the GQ Lup system, a young Sun-like star with a substellar-mass companion in a wide-separation orbit. These observations of 870 μ m continuum and CO J = 3–2 line emission with beam size ∼0.″3 (∼45 au) resolve the disk of dust and gas surrounding the primary star, GQ Lup A, and provide deep limits on any circumplanetary disk surrounding the companion, GQ Lup b. The circumprimary dust disk is compact with an FWHM of 59 ± 12 au, while the gas has a larger extent with a characteristic radius of 46.5 ± 1.8 au. By forward-modeling the velocity field of the circumprimary disk based on the CO emission, we constrain the mass of GQ Lup A to be M {sub *} = (1.03 ± 0.05) ∗ ( d /156 pc) M {sub ⊙}, where d is a known distance, and determine that we view the disk at an inclination angle of 60.°5 ± 0.°5 and a position angle of 346° ± 1°. The 3 σ upper limit on the 870 μ m flux density of any circumplanetary disk associated with GQ Lup b of <0.15 mJy implies an upper limit on the dust disk mass of <0.04 M {sub ⊕} for standard assumptions about optically thin emission. We discuss proposed mechanisms for the formation of wide-separation substellar companions given the non-detection of circumplanetary disks around GQ Lup b and other similar systems.

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DOE OSTI2017

Spatially Resolved CO SLED of the Luminous Merger Remnant NGC 1614 with ALMA

We present high-resolution (1.″0) Atacama Large Millimeter/submillimeter Array (ALMA) observations of CO (1–0) and CO (2–1) rotational transitions toward the nearby IR-luminous merger NGC 1614 supplemented with ALMA archival data of CO (3–2) and CO (6–5) transitions. The CO (6–5) emission arises from the starburst ring (central 590 pc in radius), while the lower-J CO lines are distributed over the outer disk (∼3.3 kpc in radius). Radiative transfer and photon-dominated region (PDR) modeling reveals that the starburst ring has a single warmer gas component with more a intense far-ultraviolet radiation field (n{sub H{sub 2}}∼10{sup 4.6} cm{sup −3}, T{sub kin}∼42 K, and G{sub 0}∼10{sup 2.7}) relative to the outer disk (n{sub H{sub 2}}∼10{sup 5.1} cm{sup −3}, T{sub kin}∼22 K, and G{sub 0}∼10{sup 0.9}). A two-phase molecular interstellar medium with a warm and cold (>70 and ∼19 K) component is also an applicable model for the starburst ring. A possible source for heating the warm gas component is mechanical heating due to stellar feedback rather than PDR. Furthermore, we find evidence for non-circular motions along the north–south optical bar in the lower-J CO images, suggesting a cold gas inflow. We suggest that star formation in the starburst ring is sustained by the bar-driven cold gas inflow and that starburst activities radiatively and mechanically power the CO excitation. The absence of a bright active galactic nucleus can be explained by a scenario where cold gas accumulating on the starburst ring is exhausted as the fuel for star formation or is launched as an outflow before being able to feed to the nucleus.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

The Dense Molecular Gas and Nuclear Activity in the ULIRG IRAS 13120–5453

We present new Atacama Large Millimeter/submillimeter Array Band 7 (∼340 GHz) observations of the dense gas tracers HCN, HCO{sup +}, and CS in the local, single-nucleus, ultraluminous infrared galaxy IRAS 13120–5453. We find centrally enhanced HCN (4–3) emission, relative to HCO{sup +} (4–3), but do not find evidence for radiative pumping of HCN. Considering the size of the starburst (0.5 kpc) and the estimated supernovae rate of ∼1.2 yr{sup −1}, the high HCN/HCO{sup +} ratio can be explained by an enhanced HCN abundance as a result of mechanical heating by the supernovae, though the active galactic nucleus and winds may also contribute additional mechanical heating. The starburst size implies a high Σ{sub IR} of 4.7 × 10{sup 12} L {sub ⊙} kpc{sup −2}, slightly below predictions of radiation-pressure limited starbursts. The HCN line profile has low-level wings, which we tentatively interpret as evidence for outflowing dense molecular gas. However, the dense molecular outflow seen in the HCN line wings is unlikely to escape the Galaxy and is destined to return to the nucleus and fuel future star formation. We also present modeling of Herschel observations of the H{sub 2}O lines and find a nuclear dust temperature of ∼40 K. IRAS 13120–5453 has a lower dust temperature and Σ{sub IR} than is inferred for the systems termed “compact obscured nuclei (CONs)” (such as Arp 220 and Mrk 231). If IRAS 13120–5453 has undergone a CON phase, we are likely witnessing it at a time when the feedback has already inflated the nuclear ISM and diluted star formation in the starburst/active galactic nucleus core.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Class II 6.7 GHz Methanol Maser Association with Young Massive Cores Revealed by ALMA

We explored the implication of the association (or lack of it) of 6.7 GHz class II methanol (CH{sub 3}OH) masers with massive dense cores (MDCs) detected (within a sample of ATLASGAL selected infrared quiet massive clumps) at 0.9 mm with Atacama Large Millimeter/submillimeter array. We found 42 out of the 112 cores (37.5%) detected with the Atacama Compact Array (ACA) to be associated with 6.7 GHz CH{sub 3}OH masers. The lowest mass core with CH{sub 3}OH maser association is ∼12 M{sub ⊙}. The angular offsets of the ACA cores from the 6.7 GHz CH{sub 3}OH maser peak positions range from 0.″17 to 4.″79, with a median value of 2.″19. We found a weak correlation between the 0.9 mm continuum (MDCs) peak fluxes and the peak fluxes of their associated methanol multibeam (MMB) 6.7 GHz CH{sub 3}OH masers. About 90% of the cores associated with 6.7 GHz CH{sub 3}OH masers have masses of >40 M {sub ⊙}. The CH{sub 3}OH maser containing cores are candidates for embedded high-mass protostellar objects in their earliest evolutionary stages. With our ACA 0.9 continuum data compared with the MMB 6.7 GHz CH{sub 3}OH maser survey, we have constrained the cores already housing massive protostars based on their association with the radiatively pumped 6.7 GHz CH{sub 3}OH masers.

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DOE OSTI2017

Thermal Physics of the Inner Coma: ALMA Studies of the Methanol Distribution and Excitation in Comet C/2012 K1 (PanSTARRS)

We present spatially and spectrally resolved observations of CH{sub 3}OH emission from comet C/2012 K1 (PanSTARRS), using the Atacama Large Millimeter/submillimeter Array on 2014 June 28–29. Two-dimensional maps of the line-of-sight average rotational temperature (T {sub rot}) were derived, covering spatial scales 0.″3–1.″8 (corresponding to sky-projected distances ρ ∼ 500–2500 km). The CH{sub 3}OH column density distributions are consistent with isotropic, uniform outflow from the nucleus, with no evidence for extended sources of CH{sub 3}OH in the coma. The T{sub rot}(ρ) radial profiles show a significant drop within a few thousand kilometers of the nucleus, falling from about 60 to 20 K between ρ=0 and 2500 km on June 28, whereas on June 29, T {sub rot} fell from about 120 to 40 K between ρ = 0 km and 1000 km. The observed T {sub rot} behavior is interpreted primarily as a result of variations in the coma kinetic temperature due to adiabatic cooling of the outflowing gas, as well as radiative cooling of the CH{sub 3}OH rotational levels. Our excitation model shows that radiative cooling is more important for the J=7−6 transitions (at 338 GHz) than for the K=3−2 transitions (at 252 GHz), resulting in a strongly sub-thermal distribution of levels in the J=7−6 band at ρ≳1000 km. For both bands, the observed temperature drop with distance is less steep than predicted by standard coma theoretical models, which suggests the presence of a significant source of heating in addition to the photolytic heat sources usually considered.

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DOE OSTI2017

ALMA Observations of Starless Core Substructure in Ophiuchus

Compact substructure is expected to arise in a starless core as mass becomes concentrated in the central region likely to form a protostar. Additionally, multiple peaks may form if fragmentation occurs. We present Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 2 observations of 60 starless and protostellar cores in the Ophiuchus molecular cloud. We detect eight compact substructures which are >15{sup ′′} from the nearest Spitzer young stellar object. Only one of these has strong evidence for being truly starless after considering ancillary data, e.g., from Herschel and X-ray telescopes. An additional extended emission structure has tentative evidence for starlessness. The number of our detections is consistent with estimates from a combination of synthetic observations of numerical simulations and analytical arguments. This result suggests that a similar ALMA study in the Chamaeleon I cloud, which detected no compact substructure in starless cores, may be due to the peculiar evolutionary state of cores in that cloud.

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DOE OSTI2017

Moving-mesh Simulations of Star-forming Cores in Magneto-gravo-turbulence

Star formation in our Galaxy occurs in molecular clouds that are self-gravitating, highly turbulent, and magnetized. We study the conditions under which cloud cores inherit large-scale magnetic field morphologies and how the field is governed by cloud turbulence. We present four moving-mesh simulations of supersonic, turbulent, isothermal, self-gravitating gas with a range of magnetic mean-field strengths characterized by the Alfvénic Mach number M{sub A,0}, resolving prestellar core formation from parsec to a few astronomical unit scales. In our simulations with the turbulent kinetic energy density dominating over magnetic pressure (M{sub A,0}>1), we find that the collapse is approximately isotropic with B ∝ ρ {sup 2/3}, core properties are similar regardless of initial mean-field strength, and the field direction on 100 au scales is uncorrelated with the mean field. However, in the case of a dominant large-scale magnetic field (M{sub A,0}=0.35), the collapse is anisotropic with B ∝ ρ {sup 1/2}. This transition at M{sub A,0}∼1 is not expected to be sharp, but clearly signifies two different paths for magnetic field evolution in star formation. Based on observations of different star-forming regions, we conclude that star formation in the interstellar medium may occur in both regimes. Magnetic field correlation with the mean field extends to smaller scales as M{sub A,0} decreases, making future Atacama Large Millimeter Array observations useful for constraining M{sub A,0} of the interstellar medium.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

ALMA Observations of the Interaction of a Radio Jet with Molecular Gas in Minkowski's Object

We use the Atacama Large Millimeter Array (ALMA) to detect and image CO (1-0) emission from Minkowski’s Object, a dwarf galaxy in the cluster Abell 194 that is interacting with a radio jet from a nearby elliptical galaxy. The ALMA observations, which are the first to detect molecular gas in Minkowski’s Object, also image the high-frequency continuum emission from the radio jet, allowing us to study the interaction in detail. We estimate the range in the mass of molecular gas in Minkowski’s Object assuming two different values of the ratio of the molecular gas mass to the CO luminosity, α{sub CO}. For the Milky Way value of α{sub CO}=4.6 M{sub ⊙}(Kkms{sup −1}pc{sup 2}){sup −1} we obtain a molecular gas mass of M{sub H{sub 2}}=3.0×10{sup 7} M{sub ⊙}, 6% of the H I gas mass. We also use the prescription of Narayanan et al. (2012) to estimate an α{sub CO}=27 M{sub ⊙}(Kkms{sup −1}pc{sup 2}){sup −1}, in which case we obtain M{sub H{sub 2}}=1.8×10{sup 8} M{sub ⊙}, 36% of the H I mass. The observations are consistent with previous claims of star formation being induced in Minkowski’s Object via the passage of the radio jet, and it therefore being a rare local example of positive feedback from an active galactic nucleus. In particular, we find highly efficient star formation, with gas depletion timescales ∼5×10{sup 7}--3×10{sup 8} year (for assumed values of α{sub CO}=4.6 and 27 M{sub ⊙}(Kkms{sup −1}pc{sup 2}){sup −1}, respectively) in the upstream regions of Minkowski’s Object that were struck first by the jet, and less efficient star formation downstream. We discuss the implications of this observation for models of jet-induced star formation and radio-mode feedback in massive galaxies.

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DOE OSTI2017

Comprehensive Study of Thermal Desorption of Grain-surface Species by Accretion Shocks around Protostars

We conducted numerical simulations of the dust heating in accretion shocks induced by the interaction between the infalling envelope and the Keplerian disk surrounding a protostar, in order to investigate the thermal desorption of molecules from the dust-grain surfaces. It is thought that the surfaces of the amorphous dust grains are inhomogeneous; various adsorption sites with different binding energies should therefore exist. We assumed that the desorption energy has a Gaussian distribution and investigated the effect of the desorption energy distribution on the desorption-efficiency evaluation. We calculated the desorption fractions of the grain-surface species for wide ranges of input parameters and summarized our results in a shock diagram. The resulting shock diagram suggests that the enhanced line emissions around protostars observed using the Atacama Large Millimeter Array cannot be explained by the thermal desorption in an accretion shock if typical interstellar dust-grain sizes (∼0.1 μm) and a single desorption energy are considered. On the other hand, if significantly smaller dust grains are the main grain-surface species carriers and the desorption energy has a Gaussian distribution, the origin of the enhanced line emission can be explained by the accretion shock heating scenario for all of the three protostars examined in this study: IRAS 04368+2557, IRAS 04365+2535, and IRAS 16293–2422. The small-grain-carrier supposition is quite reasonable when the dust grains have a power-law size distribution because the smaller grains primarily contribute to the dust-grain surface area.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Radial Surface Density Profiles of Gas and Dust in the Debris Disk around 49 Ceti

We present ∼0.″4 resolution images of CO(3–2) and associated continuum emission from the gas-bearing debris disk around the nearby A star 49 Ceti, observed with the Atacama Large Millimeter/Submillimeter Array (ALMA). We analyze the ALMA visibilities in tandem with the broadband spectral energy distribution to measure the radial surface density profiles of dust and gas emission from the system. The dust surface density decreases with radius between ∼100 and 310 au, with a marginally significant enhancement of surface density at a radius of ∼110 au. The SED requires an inner disk of small grains in addition to the outer disk of larger grains resolved by ALMA. The gas disk exhibits a surface density profile that increases with radius, contrary to most previous spatially resolved observations of circumstellar gas disks. While ∼80% of the CO flux is well described by an axisymmetric power-law disk in Keplerian rotation about the central star, residuals at ∼20% of the peak flux exhibit a departure from axisymmetry suggestive of spiral arms or a warp in the gas disk. The radial extent of the gas disk (∼220 au) is smaller than that of the dust disk (∼300 au), consistent with recent observations of other gas-bearing debris disks. While there are so far only three broad debris disks with well characterized radial dust profiles at millimeter wavelengths, 49 Ceti’s disk shows a markedly different structure from two radially resolved gas-poor debris disks, implying that the physical processes generating and sculpting the gas and dust are fundamentally different.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

ALMA CO Clouds and Young Star Complexes in the Interacting Galaxies IC 2163 and NGC 2207

Atacama Large Millimeter Array (ALMA) observations of CO(1-0) emission in the interacting galaxies IC 2163 and NGC 2207 are used to determine the properties of molecular clouds and their association with star-forming regions observed with the Hubble Space Telescope. Half of the CO mass is in 249 clouds, each more massive than 4.0×10{sup 5} M{sub ⊙}. The mass distribution functions for the CO clouds and star complexes in a galactic-scale shock front in IC 2163 both have a slope on a log–log plot of −0.7, similar to what is observed in Milky Way clouds. The molecular cloud mass function is steeper in NGC 2207. The CO distribution in NGC 2207 also includes a nuclear ring, a mini-bar, and a mini-starburst region that dominates the 24 μm, radio, and Hα emission in both galaxies. The ratio of the sum of the masses of star complexes younger than 30 Myr to the associated molecular cloud masses is ∼4%. The maximum age of star complexes in the galactic-scale shock front in IC 2163 is about 200 Myr, the same as the interaction time of the two galaxies, suggesting the destruction of older complexes in the eyelids.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Detection of Exocometary CO within the 440 Myr Old Fomalhaut Belt: A Similar CO+CO{sub 2} Ice Abundance in Exocomets and Solar System Comets

Recent Atacama Large Millimeter/submillimeter Array observations present mounting evidence for the presence of exocometary gas released within Kuiper Belt analogs around nearby main-sequence stars. This represents a unique opportunity to study their ice reservoir at the younger ages when volatile delivery to planets is most likely to occur. We here present the detection of CO J = 2-1 emission colocated with dust emission from the cometary belt in the 440 Myr old Fomalhaut system. Through spectrospatial filtering, we achieve a 5.4σ detection and determine that the ring’s sky-projected rotation axis matches that of the star. The CO mass derived ((0.65--42)×10{sup −7} M{sub ⊕}) is the lowest of any circumstellar disk detected to date and must be of exocometary origin. Using a steady-state model, we estimate the CO+CO{sub 2} mass fraction of exocomets around Fomalhaut to be between 4.6% and 76%, consistent with solar system comets and the two other belts known to host exocometary gas. This is the first indication of a similarity in cometary compositions across planetary systems that may be linked to their formation scenario and is consistent with direct interstellar medium inheritance. In addition, we find tentative evidence that (49±27)% of the detected flux originates from a region near the eccentric belt’s pericenter. If confirmed, the latter may be explained through a recent impact event or CO pericenter glow due to exocometary release within a steady-state collisional cascade. In the latter scenario, we show how the azimuthal dependence of the CO release rate leads to asymmetries in gas observations of eccentric exocometary belts.

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DOE OSTI2017

Multiple Disk Gaps and Rings Generated by a Single Super-Earth

We investigate the observational signatures of super-Earths (i.e., planets with Earth-to-Neptune mass), which are the most common type of exoplanet discovered to date, in their natal disks of gas and dust. Combining two-fluid global hydrodynamics simulations with a radiative transfer code, we calculate the distributions of gas and of submillimeter-sized dust in a disk perturbed by a super-Earth, synthesizing images in near-infrared scattered light and the millimeter-wave thermal continuum for direct comparison with observations. In low-viscosity gas ($\alpha \lesssim {10}^{-4}$), a super-Earth opens two annular gaps to either side of its orbit by the action of Lindblad torques. This double gap and its associated gas pressure gradients cause dust particles to be dragged by gas into three rings: one ring sandwiched between the two gaps, and two rings located at the gap edges farthest from the planet. Depending on the system parameters, additional rings may manifest for a single planet. A double gap located at tens of au from a host star in Taurus can be detected in the dust continuum by the Atacama Large Millimeter Array (ALMA) at an angular resolution of ~$0\buildrel{\prime\prime}\over{.} 03$ after two hours of integration. Ring and gap features persist in a variety of background disk profiles, last for thousands of orbits, and change their relative positions and dimensions depending on the speed and direction of planet migration. Candidate double gaps have been observed by ALMA in systems such as HL Tau (D5 and D6) and TW Hya (at 37 and 43 au); we submit that each double gap is carved by one super-Earth in nearly inviscid gas.

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DOE OSTI2017

A Radio-to-mm Census of Star-forming Galaxies in Protocluster 4C23.56 at Z = 2.5: Gas Mass and Its Fraction Revealed with ALMA

We investigate gas contents of star-forming galaxies associated with protocluster 4C23.56 at z = 2.49 by using the redshifted CO (3–2) and 1.1 mm dust continuum with the Atacama Large Millimeter/submillimeter Array. The observations unveil seven CO detections out of 22 targeted Hα emitters (HAEs) and four out of 19 in 1.1 mm dust continuum. They have high stellar mass (M{sub ⋆}>4×10{sup 10} M {sub ⊙}) and exhibit a specific star-formation rate typical of main-sequence star-forming galaxies at z∼2.5. Different gas-mass estimators from CO (3–2) and 1.1 mm yield consistent values for simultaneous detections. The gas mass (M{sub gas}) and gas fraction (f{sub gas}) are comparable to those of field galaxies, with M{sub gas}=[0.3,1.8]×10{sup 11}×(α{sub CO}/(4.36×A(Z))) M{sub ⊙}, where α{sub CO} is the CO-to-H{sub 2} conversion factor and A(Z) is the additional correction factor for the metallicity dependence of α{sub CO}, and 〈f{sub gas}〉=0.53±0.07 from CO (3–2). Our measurements place a constraint on the cosmic gas density of high-z protoclusters, indicating that the protocluster is characterized by a gas density higher than that of the general fields by an order of magnitude. We found ρ(H{sub 2})∼5×10{sup 9} M{sub ⊙} Mpc{sup −3} with the CO(3–2) detections. The five ALMA CO detections occur in the region of highest galaxy surface density, where the density positively correlates with global star-forming efficiency (SFE) and stellar mass. Such correlations possibly indicate a critical role of the environment on early galaxy evolution at high-z protoclusters, though future observations are necessary for confirmation.

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DOE OSTI2017

Formation and Atmosphere of Complex Organic Molecules of the HH 212 Protostellar Disk

HH 212 is a nearby (400 pc) Class 0 protostellar system recently found to host a “hamburger”-shaped dusty disk with a radius of ∼60 au, deeply embedded in an infalling-rotating flattened envelope. We have spatially resolved this envelope-disk system with the Atacama Large Millimeter/submillimeter Array at up to ∼16 au (0.″04) resolution. The envelope is detected in HCO{sup +} J = 4–3 down to the dusty disk. Complex organic molecules (COMs) and doubly deuterated formaldehyde (D{sub 2}CO) are detected above and below the dusty disk within ∼40 au of the central protostar. The COMs are methanol (CH{sub 3}OH), deuterated methanol (CH{sub 2}DOH), methyl mercaptan (CH{sub 3}SH), and formamide (NH{sub 2}CHO, a prebiotic precursor). We have modeled the gas kinematics in HCO{sup +} and COMs and found a centrifugal barrier (CB) at a radius of ∼44 au, within which a Keplerian rotating disk is formed. This indicates that HCO{sup +} traces the infalling-rotating envelope down to the CB and COMs trace the atmosphere of a Keplerian rotating disk within the CB. The COMs are spatially resolved for the first time, both radially and vertically, in the atmosphere of a disk in the earliest, Class 0 phase of star formation. Our spatially resolved observations of COMs favor their formation in the disk rather than a rapidly infalling (warm) inner envelope. The abundances and spatial distributions of the COMs provide strong constraints on models of their formation and transport in low-mass star formation.

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DOE OSTI2017

ALMA Observations of N83C in the Early Stage of Star Formation in the Small Magellanic Cloud

We have performed Atacama Large Millimeter/submillimeter Array (ALMA) observations in the {sup 12}CO(J=2−1), {sup 13}CO(J=2−1), C{sup 18}O(J=2−1), {sup 12}CO(J=3−2), {sup 13}CO(J=3−2), and CS(J=7−6) lines toward the active star-forming region N83C in the Small Magellanic Cloud (SMC), whose metallicity is about one-fifth of the Milky Way (MW). The ALMA observations first reveal subparsec-scale molecular structures in {sup 12}CO(J=2−1) and {sup 13}CO(J=2−1) emissions. We found strong CO peaks associated with young stellar objects (YSOs) identified by the Spitzer Space Telescope, and we also found that overall molecular gas is distributed along the edge of the neighboring H ii region. We derived a gas density of ∼10{sup 4} cm{sup −3} in molecular clouds associated with YSOs based on the virial mass estimated from the {sup 12}CO(J=2−1) emission. This high gas density is presumably due to the effect of the H ii region under the low-metallicity (and accordingly small-dust content) environment in the SMC; far-UV radiation from the H ii region can easily penetrate and photodissociate the outer layer of {sup 12}CO molecules in the molecular clouds, and thus only the innermost parts of the molecular clouds are observed even in {sup 12}CO emission. We obtained the CO-to-H{sub 2} conversion factor X{sub CO} of 7.5×10{sup 20} cm{sup −2} (K km s{sup −1}){sup −1} in N83C based on virial masses and CO luminosities, and it is four times larger than that in the MW, 2 × 10{sup 20} cm{sup −2} (K km s{sup −1}){sup −1}. We also discuss the difference in the nature between two high-mass YSOs, each of which is associated with a molecular clump with a mass of about a few ×10{sup 3} M{sub ⊙}.

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DOE OSTI2017

The Circumgalactic Medium of Submillimeter Galaxies. II. Unobscured QSOs within Dusty Starbursts and QSO Sightlines with Impact Parameters below 100 kpc

We present Atacama Large Millimeter/submillimeter Array (ALMA) 870 μm observations of 29 bright Herschel sources near high-redshift QSOs. The observations confirm that 20 of the Herschel sources are submillimeter-bright galaxies (SMGs) and identify 16 new SMG−QSO pairs that are useful to studies of the circumgalactic medium (CGM) of SMGs. Eight out of the 20 SMGs are blends of multiple 870 μm sources. The angular separations for six of the Herschel-QSO pairs are less than 10″, comparable to the sizes of the Herschel beam and the ALMA primary beam. We find that four of these six “pairs” are actually QSOs hosted by SMGs. No additional submillimeter companions are detected around these QSOs, and the rest-frame ultraviolet spectra of the QSOs show no evidence of significant reddening. Black hole accretion and star formation contribute almost equally in bolometric luminosity in these galaxies. The SMGs hosting QSOs show similar source sizes, dust surface densities, and star formation rate surface densities to those of other SMGs in the sample. We find that the black holes are growing ∼3× faster than the galaxies when compared to the present-day black hole/galaxy mass ratio, suggesting a QSO duty cycle of ≲30% in SMGs at z∼3. The remaining two Herschel-detected QSOs are undetected at 870 μm, but each has an SMG “companion” only 9″ and 12″ away (71 and 95 kpc at z = 3). They could be either merging or projected pairs. If the former, they would represent a rare class of “wet−dry” mergers. If the latter, the QSOs would, for the first time, probe the CGM of SMGs at impact parameters below 100 kpc.

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