We observed five giant molecular clouds (GMCs) in the Large Magellanic Cloud (LMC) in the 12CO J = 1-0 line using the Atacama Large Millimeter/submillimeter Array (ALMA). The sample includes four ...GMCs with some signs of star formation-either YSOs, H ii regions, and/or young clusters-and one quiescent GMC without any sign of massive star formation. The data from the ALMA 12 m, 7 m, and Total-Power arrays are jointly deconvolved to obtain high-fidelity images at high spatial resolution (3″ = 0.7 pc). The four star-forming GMCs show very complex structures with clumps and filaments. The quiescent GMC shows a relatively diffuse, extended emission distribution without prominent clumps or filaments. This difference is similar to that between structured molecular gas in Milky Way spiral arms and unstructured gas in the inter-arm regions. We characterize the difference with the brightness distribution function and brightness distribution index. In conjunction with other ALMA studies of GMCs in the LMC, the five GMCs tentatively form an evolutionary trend: from less structured, quiescent GMCs to more structured, actively star-forming GMCs. A future ALMA study will be able to map molecular clouds over the LMC and reveal the evolutionary sequence of molecular clouds.
Abstract
The Atacama Large Millimeter/submillimeter Array (ALMA) serendipitously detected H2O $J_{K_{\rm a}, K_{\rm c}} = 10_{2,9}$–93, 6 emission at 321 GHz in NGC 1052. This is the first ...submillimeter maser detection in a radio galaxy and the most luminous 321 GHz H2O maser known to-date with the isotropic luminosity of $1090\, L_{\odot }$. The line profile consists of a broad velocity component with FWHM = 208 ± 12 km s−1 straddling the systemic velocity and a narrow component with FWHM = 44 ± 3 km s−1 blueshifted by 160 km s−1. The profile is significantly different from the known 22 GHz 61, 6–52, 3 maser which shows a broad profile redshifted by 193 km s−1. The submillimeter maser is spatially unresolved with a synthesized beam of ${0{^{\prime \prime}_{.}}68} \times {0{^{\prime \prime}_{.}}56}$ and coincides with the continuum core position within 12 pc. These results indicate amplification of the continuum emission through high-temperature (>1000 K) and dense n(H2O) > 104 cm−3 molecular gas in front of the core.
We resolve 182 individual giant molecular clouds (GMCs) larger than 2.5 x 10 super(5) M sub(middot in circle) in the inner disks of 5 large nearby spiral galaxies (NGC 2403, NGC 3031, NGC 4736, NGC ...4826, and NGC 6946) to create the largest such sample of extragalactic GMCs within galaxies analogous to the Milky Way. Using a conservatively chosen sample of GMCs most likely to adhere to the virial assumption, we measure cloud sizes, velocity dispersions, and super(12)CO (J = 1-0) luminosities and calculate cloud virial masses. The average conversion factor from CO flux to H sub(2) mass (or X sub(CO)) for each galaxy is 1-2 x 10 super(20) cm super(-2) (K km s super(-1)) super(-1), all within a factor of two of the Milky Way disk value (~2 x 10 super(20) cm super(-2) (K km s super(-1)) super(-1)). We find GMCs to be generally consistent within our errors between the galaxies and with Milky Way disk GMCs; the intrinsic scatter between clouds is of order a factor of two. Consistent with previous studies in the Local Group, we find a linear relationship between cloud virial mass and CO luminosity, supporting the assumption that the clouds in this GMC sample are gravitationally bound. We do not detect a significant population of GMCs with elevated velocity dispersions for their sizes, as has been detected in the Galactic center. Though the range of metallicities probed in this study is narrow, the average conversion factors of these galaxies will serve to anchor the high metallicity end of metallicity-X sub(CO) trends measured using conversion factors in resolved clouds; this has been previously possible primarily with Milky Way measurements.
Abstract
We study properties of the interstellar medium, an ingredient of stars, and star formation activity, in four nearby galaxy pairs in the early and mid stages of interaction for both a galaxy ...scale and a kpc scale. The galaxy-scale Kennicutt–Schmidt law shows that seven of eight interacting galaxies have a star formation rate within a factor of three compared with the best fit of the isolated galaxies, although we have shown that molecular hydrogen gas is efficiently produced from atomic hydrogen during the interaction in the previous paper (Kaneko et al. 2017 PASJ, 69, 66). The galaxy-scale specific star formation rate (sSFR) and star formation efficiency (SFE) in interacting galaxies are comparable to those in isolated galaxies. We also investigate SFE and the Kennicutt–Schmidt law on a kpc scale. The spatial distributions of SFE reveal that SFE is locally enhanced, and the enhanced regions take place asymmetrically or at off-centre regions. The local enhancement of SFE could be induced by shock. We find that the index of the Kennicutt–Schmidt law for the interacting galaxies in the early stage is 1.30 ± 0.04, which is consistent with that of the isolated galaxies. Since CO emission, which is used in the Kennicutt–Schmidt law, is a tracer of the amount of molecular gas, this fact suggests that dense gas, which is more directly connected to star formation, is not changed at the early stage of interaction.
Abstract
We have investigated properties of the interstellar medium in interacting galaxies in early and mid stages using mapping data of 12CO(J = 1–0) and H i. Assuming the standard CO–H2 conversion ...factor, we found no difference in molecular gas mass, atomic gas mass, and total gas mass (the sum of atomic and molecular gas mass) between interacting galaxies and isolated galaxies. However, interacting galaxies have a higher global molecular gas fraction $f_{\rm {mol}}^{\rm {global}}$ (the ratio of molecular gas mass to total gas mass averaged over a whole galaxy) at 0.71 ± 0.15 than isolated galaxies (0.52 ± 0.18). The distribution of the local molecular gas fraction fmol, the ratio of the surface density of molecular gas to that of the total gas, is different from the distribution in typical isolated galaxies. By a pixel-to-pixel comparison, isolated spiral galaxies show a gradual increase in fmol along the surface density of total gas until it is saturated at 1.0, while interacting galaxies show no clear relation. We performed pixel-to-pixel theoretical model fits by varying metallicity and external pressure. According to the model fitting, external pressure can explain the trend of fmol in the interacting galaxies. Assuming half of the standard CO–H2 conversion factor for interacting galaxies, the results of pixel-to-pixel theoretical model fitting get worse than adopting the standard conversion factor, although $f_{\rm {mol}}^{\rm {global}}$ of interacting galaxies (0.62 ± 0.17) becomes the same as in isolated galaxies. We conclude that external pressure occurs due to the shock prevailing over a whole galaxy or due to collisions between giant molecular clouds even in the early stage of the interaction. The external pressure accelerates an efficient transition from atomic gas to molecular gas. Regarding the chemical timescale, high fmol can be achieved at the very early stage of interaction even if the shock induced by the collision of galaxies ionizes interstellar gas.
We present a giant molecular cloud (GMC) catalog of M33, containing 71 GMCs in total, based on wide-field and high-sensitivity CO(J = 3-2) observations with a spatial resolution of 100 pc using the ...ASTE 10 m telescope. Employing archival optical data, we identify 75 young stellar groups (YSGs) from the excess of the surface stellar density, and estimate their ages by comparing with stellar evolution models. A spatial comparison among the GMCs, YSGs, and H II regions enable us to classify GMCs into four categories: Type A, showing no sign of massive star formation (SF); Type B, being associated only with H II regions; Type C, with both H II regions and <10 Myr old YSGs; and Type D, with both H II regions and 10-30 Myr YSGs. Out of 65 GMCs (discarding those at the edges of the observed fields), 1 (1%), 13 (20%), 29 (45%), and 22 (34%) are Types A, B, C, and D, respectively. We interpret these categories as stages in a GMC evolutionary sequence. Assuming that the timescale for each evolutionary stage is proportional to the number of GMCs, the lifetime of a GMC with a mass >10 super(5) M sub(middot in circle) is estimated to be 20-40 Myr. In addition, we find that the dense gas fraction as traced by the CO(J = 3-2)/CO(J = 1-0) ratio is enhanced around SF regions. This confirms a scenario where dense gas is preferentially formed around previously generated stars, and will be the fuel for the next stellar generation. In this way, massive SF gradually propagates in a GMC until gas is exhausted.
We present a new package for joint deconvolution of ALMA 12 m, 7 m, and Total Power (TP) data, dubbed "Total Power Map to Visibilities (Tp2vis)". It converts a TP (single-dish) map into visibilities ...on the Casa platform, which can be input into deconvolvers (e.g., Clean) along with 12 m and 7 m visibilities. The Tp2vis procedure is based primarily on the one discussed in Koda et al. A manual is presented in the Github repository (https://github.com/tp2vis/distribute). Combining data from the different ALMA arrays is a driver for a number of science topics, namely those that probe size scales of extended and compact structures simultaneously (e.g., protostar outflows and environment, AGB stars and planetary nebulae, molecular clouds and cores, and molecular clouds in galaxies). We test Tp2vis using model images, one with a single Gaussian and another that mimics the internal structures of giant molecular clouds. The result shows that the better uv coverage with Tp2vis visibilities clearly helps the deconvolution process and reproduces the model image within errors of only 5% over two orders of magnitude in flux. In the Appendix, we describe how the model image is generated.
Abstract
We present Atacama Large Millimeter/submillimeter Array (ALMA) imaging of molecular gas across the full star-forming disk of the barred spiral galaxy M83 in CO(
J
= 1–0). We jointly ...deconvolve the data from ALMA’s 12 m, 7 m, and Total Power arrays using the MIRIAD package. The data have a mass sensitivity and resolution of 10
4
M
⊙
(3
σ
) and 40 pc—sufficient to detect and resolve a typical molecular cloud in the Milky Way with a mass and diameter of 4 × 10
5
M
⊙
and 40 pc, respectively. The full disk coverage shows that the characteristics of molecular gas change radially from the center to outer disk, with the locally measured brightness temperature, velocity dispersion, and integrated intensity (surface density) decreasing outward. The molecular gas distribution shows coherent large-scale structures in the inner part, including the central concentration, offset ridges along the bar, and prominent molecular spiral arms. However, while the arms are still present in the outer disk, they appear less spatially coherent, and even flocculent. Massive filamentary gas concentrations are abundant even in the interarm regions. Building up these structures in the interarm regions would require a very long time (≳100 Myr). Instead, they must have formed within stellar spiral arms and been released into the interarm regions. For such structures to survive through the dynamical processes, the lifetimes of these structures and their constituent molecules and molecular clouds must be long (≳100 Myr). These interarm structures host little or no star formation traced by H
α
. The new map also shows extended CO emission, which likely represents an ensemble of unresolved molecular clouds.
In 2017, an Atacama Large Millimeter/submillimeter Array (ALMA) high-frequency long baseline campaign was organized to test image capabilities with baselines up to 16 km at submillimeter (submm) ...wavelengths. We investigated image qualities using ALMA receiver Bands 7, 8, 9, and 10 (285-875 GHz) by adopting band-to-band (B2B) phase referencing in which a phase calibrator is tracked at a lower frequency. For B2B phase referencing, it is expected that a closer phase calibrator to a target can be used, comparing to standard in-band phase referencing. In the first step, it is ensured that an instrumental phase offset difference between low- and high-frequency Bands can be removed using a differential gain calibration in which a phase calibrator is certainly detected while frequency switching. In the next step, comparative experiments are arranged to investigate the image quality between B2B and in-band phase referencing with phase calibrators at various separation angles. In the final step, we conducted long baseline imaging tests for a quasar at 289 GHz in Band 7 and 405 GHz in Band 8 and complex structure sources of HL Tau and VY CMa at ∼670 GHz in Band 9. The B2B phase referencing was successfully applied, allowing us to achieve an angular resolution of 14 × 11 and 10 × 8 mas for HL Tau and VY CMa, respectively. There is a high probability of finding a low-frequency calibrator within 5 4 in B2B phase referencing, bright enough to use an 8 s scan length combined with a 7.5 GHz bandwidth.
A new strategy for the design of a new chromatographic stationary phase via simple modifications of the counter anions in poly(ionic liquid)-grafted silica phase based on ionic self-assembly ...technology is proposed. The phase with methyl orange dye as counter anions exhibits ultra-high selectivity towards shape-constrained isomers.
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