arXiv subjects

Amanda M Lee

Publications and source records attributed to Amanda M Lee.

7 recordsLinked to original sources

arXiv2026

ALMA FACTS III. High-Resolution CO(2-1)/CO(1-0) Maps of Twelve Nearby Galaxies

We present early results from a high-resolution analysis ($\sim$100-200pc) of the CO(2-1)/CO(1-0) line ratio in twelve nearby galaxies. We use new ALMA CO(1-0) observations from the Fundamental CO(1-0) Transition Survey (FACTS), and re-imaged CO(2-1) data from PHANGS. We make empirical classifications based on the optical and molecular gas morphologies, which show clear systematic trends in the variation of $R_{21}$ as a function of galactic structure. The sample includes barred and unbarred, and flocculent galaxies. The barred spiral galaxies follow a general trend when the gas exists significantly: $R_{21}$ is high in the center, low along the bar, increases at the bar ends, and then lowers beyond the bar end or flattens in the outer parts of the disk. The structure dependence suggests the importance of galactic dynamics on molecular gas evolution, and consequently on star formation, in galaxies. $R_{21}$ fluctuates in the spiral arms for both barred and unbarred galaxies. Areas around HII regions in some cases appear to show more high-ratio gas. Together, $R_{21}$ varies systematically as a function of galactic structure, dynamics, and star formation activity.

astro-ph.GA↗

arXiv2025

Dynamically-Driven Evolution of Molecular Gas in M83 Traced by CO 2-1/1-0 Line Ratio Variations

We show the variations of the CO J=2-1/1-0 line ratio (R21) across the barred spiral galaxy M83, using the 46 pc resolution data from ALMA. The R21 map clearly evidences the systematic large-scale variations as a function of galactic structures. Azimuthally, it starts from low R21<~0.7 in the interarm regions and becomes high ~>0.7 in the bar and spiral arms, suggesting that the density and/or kinetic temperature of molecular gas increase by about a factor of 2-3. This evolution is seen even in the parts of spiral arms without star formation, and R21 is often elevated even higher to ~0.8-1.0 when HII regions exist in the vicinity. Radially, R21 starts very high >~1.0 at the galactic center, remains low <~0.7 in the bar region, increases to >~0.7 around the bar end, and again decreases to <~0.7 in the rest of disk where the spiral arms dominate. The evolutionary sequence is synchronized with galactic rotation, and therefore, it is determined largely by the galactic structures and dynamics and is governed by the galactic rotation timescales. The R21 map also shows that the influence of stellar feedback is localized and limited. Massive, large, and non-star forming molecular structures have low R21, which also suggests that the bulk molecular gas in the disk is not regulated by stellar feedback, but more likely by galactic structures and dynamics. These results are consistent with suggestions by the earlier studies of the Milky Way and other barred spiral galaxies, and thus, are likely general among barred spiral galaxies in the local Universe.

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arXiv2024

Whole-disk sampling of molecular clouds in M83

We present a catalog of clouds identified from the $^{12}$CO (1--0) data of M83, which was observed using Atacama Large Millimeter/submillimeter Array (ALMA) with a spatial resolution of $\sim$46 pc and a mass sensitivity of $\sim$10$^4$ $M_{\odot}$ (3 $σ$). The almost full-disk coverage and high sensitivity of the data allowed us to sample 5724 molecular clouds with a median mass of $\sim1.9$ $\times$ $10^5$ $M_{\odot}$, which is comparable to the most frequently sampled mass of Giant Molecular Clouds by surveys in the Milky Way. About 60 percent of the total CO luminosity in M83's disk arises from clouds more massive than 10$^6$ $M_{\odot}$. Such massive clouds comprise 16 percent of the total clouds in number and tend to concentrate toward the arm, bar, and center, while smaller clouds are more prevalent in inter-arm regions. Most $>10^6$ $M_{\odot}$ clouds have peak brightness temperatures $T_{\mathrm{peak}}$ above 2 K with the current resolution. Comparing the observed cloud properties with the scaling relations determined by Solomon et al. 1987 (S87), $T_{\mathrm{peak}}$$>2$ K clouds follow the relations, but $T_{\mathrm{peak}}$$<2$ K clouds, which are dominant in number, deviate significantly. Without considering the effect of beam dilution, the deviations would suggest modestly high virial parameters and low surface mass densities for the entire cloud samples, which are similar to values found for the Milky Way clouds by Rice et al. (2016) and Miville-Desch{ê}nes et al. (2017). However, once beam dilution is taken into account, the observed $α_{\mathrm{vir}}$ and $Σ$ for a majority of the clouds (mostly $T_{\mathrm{peak}}$ $<2$ K) can be potentially explained with intrinsic $Σ$ of $\sim$100 $M_{\mathrm{\odot}}\ \mathrm{pc}^{-2}$ and $α_{\mathrm{vir}}$ of $\sim$1, which are similar to the clouds of S87.

astro-ph.GA↗

arXiv2024

Compact and High Excitation Molecular Clumps in the Extended Ultraviolet Disk of M83

The extended ultraviolet (XUV) disks of nearby galaxies show ongoing massive star formation, but their parental molecular clouds remain mostly undetected despite searches in CO(1-0) and CO(2-1). The recent detection of 23 clouds in the higher excitation transition CO(3-2) within the XUV disk of M83 requires an explanation. We test the hypothesis: the clouds in XUV disks have a clump-envelope structure similar to those in Galactic star-forming clouds, having star-forming dense clumps (or concentrations of multiple clumps) at their centers, which predominantly contribute to the CO(3-2) emission, surrounded by less-dense envelopes, where CO molecules are photo-dissociated due to the low-metallicity environment there. We utilize new high-resolution ALMA CO(3-2) observations of a subset (11) of the 23 clouds in the XUV disk. We confirm the compactness of the CO(3-2)-emitting dense clumps (or their concentrations), finding clump diameters below the spatial resolution of 6-9~pc. This is similar to the size of the dense gas region in the Orion A molecular cloud, the local star-forming cloud with massive star formation. The dense star-forming clumps are common between normal and XUV disks. This may also indicate that once the cloud structure is set, the process of star formation is governed by the cloud internal physics rather than by external triggers. This simple model explains the current observations of the clouds with ongoing massive star formation, although it may require some adjustment, e.g., including an effect of cloud evolution, for a general scenario of star formation in molecular clouds.

astro-ph.GA↗

arXiv2024

The CO-to-H$_2$ Conversion Factor in the Barred Spiral Galaxy M83

We analyze the CO-to-H$_2$ conversion factor ($α_{\rm{CO}}$) in the nearby barred spiral galaxy M83. We present new HI observations from the JVLA and single-dish GBT in the disk of the galaxy, and combine them with maps of CO(1-0) integrated intensity and dust surface density from the literature. $α_{\rm{CO}}$ and the gas-to-dust ratio ($δ_{\rm{GDR}}$) are simultaneously derived in annuli of 2 kpc width from R = 1-7 kpc. We find that $α_{\rm{CO}}$ and $δ_{\rm{GDR}}$ both increase radially, by a factor of $\sim$ 2-3 from the center to the outskirts of the disk. The luminosity-weighted averages over the disk are $α_{\rm{CO}} = 3.14$ (2.06, 4.96) M$_{\odot}$ pc$^{-2}$[K$\cdot$ km s$^{-1}$]$^{-1}$ and $δ_{\rm{GDR}}$ = 137 (111, 182) at the 68% (1$σ$) confidence level. These are consistent with the $α_{\rm{CO}}$ and $δ_{\rm{GDR}}$ values measured in the Milky Way. In addition to possible variations of $α_{\rm{CO}}$ due to the radial metallicity gradient, we test the possibility of variations in $α_{\rm{CO}}$ due to changes in the underlying cloud populations, as a function of galactic radius. Using a truncated power-law molecular cloud CO luminosity function and an empirical power-law relation for cloud-mass and luminosity, we show that the changes in the underlying cloud population may account for a factor of $\sim 1.5-2.0$ radial change in $α_{\rm{CO}}$.

astro-ph.GA↗

arXiv2023

Diverse Molecular Structures Across The Whole Star-Forming Disk of M83: High fidelity Imaging at 40pc Resolution

We present high-fidelity CO(1-0) imaging of molecular gas across the full star-forming disk of M83, using ALMA's 12m, 7m, and TP arrays and the MIRIAD package. The data have a mass sensitivity and resolution of 10^4Msun and 40 pc. The full disk coverage shows that the characteristics of molecular gas change radially from the center to outer disk. The molecular gas distribution shows coherent large-scale structures in the inner part, including the central concentration, bar offset ridges, and prominent molecular spiral arms. In the outer disk, the spiral arms 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 (~>100Myr). 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 (~>100Myr). These interarm structures host little or no star formation traced by Halpha. The new map also shows extended CO emission, which likely represents an ensemble of unresolved molecular clouds.

astro-ph.GA↗

arXiv2022

First Detection of the Molecular Cloud Population in the Extended Ultraviolet (XUV) Disk of M83

We report a CO(3-2) detection of 23 molecular clouds in the extended ultraviolet (XUV) disk of the spiral galaxy M83 with ALMA. The observed 1kpc^2 region is at about 1.24 times the optical radius (R25) of the disk, where CO(2-1) was previously not detected. The detection and non-detection, as well as the level of star formation (SF) activity in the region, can be explained consistently if the clouds have the mass distribution common among Galactic clouds, such as Orion A -- with star-forming dense clumps embedded in thick layers of bulk molecular gas, but in a low-metallicity regime where their outer layers are CO-deficient and CO-dark. The cloud and clump masses, estimated from CO(3-2), range from 8.2x10^2 to 2.3x10^4 Msun and from 2.7x10^2 to 7.5x10^3 Msun, respectively. The most massive clouds appear similar to Orion A in star formation activity as well as in mass, as expected if the cloud mass structure is universal. The overall low SF activity in the XUV disk could be due to the relative shortage of gas in the molecular phase. The clouds are distributed like chains up to 600 pc (or longer) in length, suggesting that the trigger of cloud formation is on large scales. The universal cloud mass structure also justifies the use of high-J CO transitions to trace the total gas mass of clouds, or galaxies, even in the high-z universe. This study is the first demonstration that CO(3-2) is an efficient tracer of molecular clouds even in low-metallicity environments.

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