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Yu-Hsuan Teng

Publications and source records attributed to Yu-Hsuan Teng.

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arXiv2024

PHANGS-JWST: Data Processing Pipeline and First Full Public Data Release

The exquisite angular resolution and sensitivity of JWST is opening a new window for our understanding of the Universe. In nearby galaxies, JWST observations are revolutionizing our understanding of the first phases of star formation and the dusty interstellar medium. Nineteen local galaxies spanning a range of properties and morphologies across the star-forming main sequence have been observed as part of the PHANGS-JWST Cycle 1 Treasury program at spatial scales of $\sim$5-50pc. Here, we describe pjpipe, an image processing pipeline developed for the PHANGS-JWST program that wraps around and extends the official JWST pipeline. We release this pipeline to the community as it contains a number of tools generally useful for JWST NIRCam and MIRI observations. Particularly for extended sources, pjpipe products provide significant improvements over mosaics from the MAST archive in terms of removing instrumental noise in NIRCam data, background flux matching, and calibration of relative and absolute astrometry. We show that slightly smoothing F2100W MIRI data to 0.9" (degrading the resolution by about 30 percent) reduces the noise by a factor of $\approx$3. We also present the first public release (DR1.1.0) of the pjpipe processed eight-band 2-21 $μ$m imaging for all nineteen galaxies in the PHANGS-JWST Cycle 1 Treasury program. An additional 55 galaxies will soon follow from a new PHANGS-JWST Cycle 2 Treasury program.

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arXiv2023

Resolved Measurements of the CO-to-H$_2$ Conversion Factor in 37 Nearby Galaxies

We measure the CO-to-H$_2$ conversion factor ($α_\mathrm{CO}$) in 37 galaxies at 2~kpc resolution, using dust surface density inferred from far-infrared emission as a tracer of the gas surface density and assuming a constant dust-to-metals ratio. In total, we have $\sim790$ and $\sim610$ independent measurements of $α_\mathrm{CO}$ for CO (2-1) and (1-0), respectively. The mean values for $α_\mathrm{CO~(2-1)}$ and $α_\mathrm{CO~(1-0)}$ are $9.3^{+4.6}_{-5.4}$ and $4.2^{+1.9}_{-2.0}~M_\odot~pc^{-2}~(K~km~s^{-1})^{-1}$, respectively. The CO-intensity-weighted mean for $α_\mathrm{CO~(2-1)}$ is 5.69, and 3.33 for $α_\mathrm{CO~(1-0)}$. We examine how $α_\mathrm{CO}$ scales with several physical quantities, e.g.\ star-formation rate (SFR), stellar mass, and dust-mass-weighted average interstellar radiation field strength ($\overline{U}$). Among them, $\overline{U}$, $Σ_{\rm SFR}$, and integrated CO intensity ($W_\mathrm{CO}$) have the strongest anti-correlation with spatially resolved $α_\mathrm{CO}$. We provide linear regression results to \aco for all quantities tested. At galaxy integrated scales, we observe significant correlations between $α_\mathrm{CO}$ and $W_\mathrm{CO}$, metallicity, $\overline{U}$, and $Σ_{\rm SFR}$. We also find that the normalized $α_\mathrm{CO}$ decreases with stellar mass surface density ($Σ_\star$) in the high surface density regions ($Σ_\star\geq100~{\rm M_\odot~pc^{-2}}$), following the power-law relations $α_\mathrm{CO~(2-1)}\proptoΣ_\star^{-0.5}$ and $α_\mathrm{CO~(1-0)}\proptoΣ_\star^{-0.2}$. The power-law index is insensitive to the assumed dust-to-metals ratio. (abridged)

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arXiv2023

Surveying the Whirlpool at Arcseconds with NOEMA (SWAN)- I. Mapping the HCN and N$_2$H$^+$ 3mm lines

We present the first results from "Surveying the Whirlpool at Arcseconds with NOEMA" (SWAN), an IRAM Northern Extended Millimetre Array (NOEMA)+30m large program that maps emission from several molecular lines at 90 and 110 GHz in the iconic nearby grand-design spiral galaxy M~51 at cloud-scale resolution ($\sim$3\arcsec=125\,pc). As part of this work, we have obtained the first sensitive cloud-scale map of N$_2$H$^+$(1-0) of the inner $\sim5\,\times 7\,$kpc of a normal star-forming galaxy, which we compare to HCN(1-0) and CO(1-0) emission to test their ability in tracing dense, star-forming gas. The average N$_2$H$^+$-to-HCN line ratio of our total FoV is $0.20\pm0.09$, with strong regional variations of a factor of $\gtrsim 2$ throughout the disk, including the south-western spiral arm and the center. The central $\sim1\,$kpc exhibits elevated HCN emission compared to N$_2$H$^+$, probably caused by AGN-driven excitation effects. We find that HCN and N$_2$H$^+$ are strongly super-linearily correlated in intensity ($ρ_\mathrm{Sp}\sim 0.8$), with an average scatter of $\sim0.14\,$dex over a span of $\gtrsim 1.5\,$dex in intensity. When excluding the central region, the data is best described by a power-law of exponent $1.2$, indicating that there is more N$_2$H$^+$ per unit HCN in brighter regions. Our observations demonstrate that the HCN-to-CO line ratio is a sensitive tracer of gas density in agreement with findings of recent Galactic studies which utilize N$_2$H$^+$. The peculiar line ratios present near the AGN and the scatter of the power-law fit in the disk suggest that in addition to a first-order correlation with gas density, second-order physics (such as optical depth, gas temperature) or chemistry (abundance variations) are encoded in the N$_2$H$^+$/CO, HCN/CO and N$_2$H$^+$/HCN ratios.

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arXiv2023

Star Formation Efficiency in Nearby Galaxies Revealed with a New CO-to-H2 Conversion Factor Prescription

Determining how galactic environment, especially the high gas densities and complex dynamics in bar-fed galaxy centers, alters the star formation efficiency (SFE) of molecular gas is critical to understanding galaxy evolution. However, these same physical or dynamical effects also alter the emissivity properties of CO, leading to variations in the CO-to-H$_2$ conversion factor ($α_\rm{CO}$) that impact the assessment of the gas column densities and thus of the SFE. To address such issues, we investigate the dependence of $α_\rm{CO}$ on local CO velocity dispersion at 150-pc scales using a new set of dust-based $α_\rm{CO}$ measurements, and propose a new $α_\rm{CO}$ prescription that accounts for CO emissivity variations across galaxies. Based on this prescription, we estimate the SFE in a sample of 65 galaxies from the PHANGS-ALMA survey. We find increasing SFE towards high surface density regions like galaxy centers, while using a constant or metallicity-based $α_\rm{CO}$ results in a more homogeneous SFE throughout the centers and disks. Our prescription further reveals a mean molecular gas depletion time of 700 Myr in the centers of barred galaxies, which is overall 3-4 times shorter than in non-barred galaxy centers or the disks. Across the galaxy disks, the depletion time is consistently around 2-3 Gyr regardless of the choice of $α_\rm{CO}$ prescription. All together, our results suggest that the high level of star formation activity in barred centers is not simply due to an increased amount of molecular gas but also an enhanced SFE compared to non-barred centers or disk regions.

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arXiv2023

Fuelling the nuclear ring of NGC 1097

Galactic bars can drive cold gas inflows towards the centres of galaxies. The gas transport happens primarily through the so-called bar ``dust lanes'', which connect the galactic disc at kpc scales to the nuclear rings at hundreds of pc scales much like two gigantic galactic rivers. Once in the ring, the gas can fuel star formation activity, galactic outflows, and central supermassive black holes. Measuring the mass inflow rates is therefore important to understanding the mass/energy budget and evolution of galactic nuclei. In this work, we use CO datacubes from the PHANGS-ALMA survey and a simple geometrical method to measure the bar-driven mass inflow rate onto the nuclear ring of the barred galaxy NGC~1097. The method assumes that the gas velocity in the bar lanes is parallel to the lanes in the frame co-rotating with the bar, and allows one to derive the inflow rates from sufficiently sensitive and resolved position-position-velocity diagrams if the bar pattern speed and galaxy orientations are known. We find an inflow rate of $\dot{M}=(3.0 \pm 2.1)\, \rm M_\odot\, yr^{-1}$ averaged over a time span of 40 Myr, which varies by a factor of a few over timescales of $\sim$10 Myr. Most of the inflow appears to be consumed by star formation in the ring which is currently occurring at a rate of ${\rm SFR}\simeq~1.8$-$2 \rm M_\odot\, yr^{-1}$, suggesting that the inflow is causally controlling the star formation rate in the ring as a function of time.

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arXiv2023

The Physical Drivers and Observational Tracers of CO-to-H2 Conversion Factor Variations in Nearby Barred Galaxy Centers

The CO-to-H$_2$ conversion factor ($α_\rm{CO}$) is central to measuring the amount and properties of molecular gas. It is known to vary with environmental conditions, and previous studies have revealed lower $α_\rm{CO}$ in the centers of some barred galaxies on kpc scales. To unveil the physical drivers of such variations, we obtained ALMA Band 3, 6, and 7 observations toward the inner 2 kpc of NGC 3627 and NGC 4321 tracing $^{12}$CO, $^{13}$CO, and C$^{18}$O lines on 100 pc scales. Our multi-line modeling and Bayesian likelihood analysis of these datasets reveal variations of molecular gas density, temperature, optical depth, and velocity dispersion, which are among the key drivers of $α_\rm{CO}$. The central 300 pc nuclei in both galaxies show strong enhancement of temperature $T_\rm{k}>100$ K and density $n_\rm{H_2}>10^3$ cm$^{-3}$. Assuming a CO-to-H$_2$ abundance of $3\times10^{-4}$, we derive 4-15 times lower $α_\rm{CO}$ than the Galactic value across our maps, which agrees well with previous kpc-scale measurements. Combining the results with our previous work on NGC 3351, we find a strong correlation of $α_\rm{CO}$ with low-J $^{12}$CO optical depths ($τ_\rm{CO}$), as well as an anti-correlation with $T_\rm{k}$. The $τ_\rm{CO}$ correlation explains most of the $α_\rm{CO}$ variation in the three galaxy centers, whereas changes in $T_\rm{k}$ influence $α_\rm{CO}$ to second order. Overall, the observed line width and $^{12}$CO/$^{13}$CO 2-1 line ratio correlate with $τ_\rm{CO}$ variation in these centers, and thus they are useful observational indicators for $α_\rm{CO}$ variation. We also test current simulation-based $α_\rm{CO}$ prescriptions and find a systematic overprediction, which likely originates from the mismatch of gas conditions between our data and the simulations.

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arXiv2023

Star Formation Laws and Efficiencies across 80 Nearby Galaxies

We measure empirical relationships between the local star formation rate (SFR) and properties of the star-forming molecular gas on 1.5 kpc scales across 80 nearby galaxies. These relationships, commonly referred to as "star formation laws," aim at predicting the local SFR surface density from various combinations of molecular gas surface density, galactic orbital time, molecular cloud free-fall time, and the interstellar medium dynamical equilibrium pressure. Leveraging a multiwavelength database built for the PHANGS survey, we measure these quantities consistently across all galaxies and quantify systematic uncertainties stemming from choices of SFR calibrations and the CO-to-H$_2$ conversion factors. The star formation laws we examine show 0.3-0.4 dex of intrinsic scatter, among which the molecular Kennicutt-Schmidt relation shows a $\sim$10% larger scatter than the other three. The slope of this relation ranges $β\approx0.9{-}1.2$, implying that the molecular gas depletion time remains roughly constant across the environments probed in our sample. The other relations have shallower slopes ($β\approx0.6{-}1.0$), suggesting that the star formation efficiency (SFE) per orbital time, the SFE per free-fall time, and the pressure-to-SFR surface density ratio (i.e., the feedback yield) may vary systematically with local molecular gas and SFR surface densities. Last but not least, the shapes of the star formation laws depend sensitively on methodological choices. Different choices of SFR calibrations can introduce systematic uncertainties of at least 10-15% in the star formation law slopes and 0.15-0.25 dex in their normalization, while the CO-to-H$_2$ conversion factors can additionally produce uncertainties of 20-25% for the slope and 0.10-0.20 dex for the normalization.

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arXiv2023

Wide-field CO isotopologue emission and the CO-to-H$_2$ factor across the nearby spiral galaxy M101

Carbon monoxide (CO) emission is the most widely used tracer of the bulk molecular gas in the interstellar medium (ISM) in extragalactic studies. The CO-to-H$_2$ conversion factor, $α_{\rm CO}$, links the observed CO emission to the total molecular gas mass. However, no single prescription perfectly describes the variation of $α_{\rm CO}$ across all environments across galaxies as a function of metallicity, molecular gas opacity, line excitation, and other factors. Using resolved spectral line observations of CO and its isotopologues, we can constrain the molecular gas conditions and link them to a variation in the conversion factor. We present new IRAM 30-m 1mm and 3mm line observations of $^{12}$CO, $^{13}$CO, and C$^{18}$O} across the nearby galaxy M101. Based on the CO isotopologue line ratios, we find that selective nucleosynthesis and opacity changes are the main drivers of the variation in the line emission across the galaxy. Furthermore, we estimated $α_{\rm CO(1-0)}$ using different approaches, including (i) the dust mass surface density derived from far-IR emission as an independent tracer of the total gas surface density and (ii) LTE-based measurements using the optically thin $^{13}$CO(1-0) intensity. We find an average value of $α_{\rm CO}=4.4{\pm}0.9\rm\,M_\odot\,pc^{-2}(K\,km\,s^{-1})^{-1}$ across the galaxy, with a decrease by a factor of 10 toward the 2 kpc central region. In contrast, we find LTE-based values are lower by a factor of 2-3 across the disk relative to the dust-based result. Accounting for $α_{\rm CO}$ variations, we found significantly reduced molecular gas depletion time by a factor 10 in the galaxy's center. In conclusion, our result suggests implications for commonly derived scaling relations, such as an underestimation of the slope of the Kennicutt Schmidt law, if $α_{\rm CO}$ variations are not accounted for.

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arXiv2023

The ALMOND Survey: Molecular cloud properties and gas density tracers across 25 nearby spiral galaxies with ALMA

We use new HCN(1-0) data from the ALMOND (ACA Large-sample Mapping Of Nearby galaxies in Dense gas) survey to trace the kpc-scale molecular gas density structure and CO(2-1) data from PHANGS-ALMA to trace the bulk molecular gas across 25 nearby, star-forming galaxies. At 2.1 kpc scale, we measure the density-sensitive HCN/CO line ratio and the SFR/HCN ratio to trace the star formation efficiency in the denser molecular medium. At 150 pc scale, we measure structural and dynamical properties of the molecular gas via CO(2-1) line emission, which is linked to the lower resolution data using an intensity-weighted averaging method. We find positive correlations (negative) of HCN/CO (SFR/HCN) with the surface density, the velocity dispersion and the internal turbulent pressure of the molecular gas. These observed correlations agree with expected trends from turbulent models of star formation, which consider a single free-fall time gravitational collapse. Our results show that the kpc-scale HCN/CO line ratio is a powerful tool to trace the 150 pc scale average density distribution of the molecular clouds. Lastly, we find systematic variations of the SFR/HCN ratio with cloud-scale molecular gas properties, which are incompatible with a universal star formation efficiency. Overall, these findings show that mean molecular gas density, molecular cloud properties and star formation are closely linked in a coherent way, and observations of density-sensitive molecular gas tracers are a useful tool to analyse these variations, linking molecular gas physics to stellar output across galaxy discs.

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arXiv2023

Sub-kiloparsec empirical relations and excitation conditions of HCN and HCO+ J=3-2 in nearby star-forming galaxies

We present new HCN and HCO$^+$ ($J$=3-2) images of the nearby star-forming galaxies (SFGs) NGC 3351, NGC 3627, and NGC 4321. The observations, obtained with the Morita ALMA Compact Array, have a spatial resolution of $\sim$290-440 pc and resolve the inner $R_\textrm{gal} \lesssim$ 0.6-1 kpc of the targets, as well as the southern bar end of NGC 3627. We complement this data set with publicly available images of lower excitation lines of HCN, HCO$^+$, and CO and analyse the behaviour of a representative set of line ratios: HCN(3-2)/HCN(1-0), HCN(3-2)/HCO$^+$(3-2), HCN(1-0)/CO(2-1), and HCN(3-2)/CO(2-1). Most of these ratios peak at the galaxy centres and decrease outwards. We compare the HCN and HCO$^+$ observations with a grid of one-phase, non-local thermodynamic equilibrium (non-LTE) radiative transfer models and find them compatible with models that predict subthermally excited and optically thick lines. We study the systematic variations of the line ratios across the targets as a function of the stellar surface density ($Σ_\textrm{star}$), the intensity-weighted CO(2-1) ($\langle I_\text{CO}\rangle$), and the star formation rate surface density ($Σ_\text{SFR}$). We find no apparent correlation with $Σ_\text{SFR}$, but positive correlations with the other two parameters, which are stronger in the case of $\langle I_\text{CO}\rangle$. The HCN/CO-$\langle I_\text{CO}\rangle$ relations show $\lesssim$0.3 dex galaxy-to-galaxy offsets, with HCN(3-2)/CO(2-1)-$\langle I_\text{CO}\rangle$ being $\sim$2 times steeper than HCN(1-0)/CO(2-1). In contrast, the HCN(3-2)/HCN(1-0)-$\langle I_\text{CO}\rangle$ relation exhibits a tighter alignment between galaxies. We conclude that the overall behaviour of the line ratios cannot be ascribed to variations in a single excitation parameter (e.g. density or temperature).

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arXiv2022

CI and CO in Nearby Spiral Galaxies -- I. Line Ratio and Abundance Variations at ~ 200 pc Scales

We present new neutral atomic carbon [CI](3P1-3P0) mapping observations within the inner ~7 kpc and ~4 kpc of the disks of NGC3627 and NGC4321 at a spatial resolution of 190 pc and 270 pc, respectively, using the ALMA Atacama Compact Array (ACA). We combine these with the CO(2-1) data from PHANGS-ALMA, and literature [CI] and CO data for two other starburst and/or active galactic nucleus (AGN) galaxies (NGC1808, NGC7469), to study: a) the spatial distributions of CI and CO emission; b) the observed line ratio RCICO = I_[CI](1-0)/I_CO(2-1) as a function of various galactic properties; and c) the abundance ratio of [CI/CO]. We find excellent spatial correspondence between CI and CO emission and nearly uniform RCICO ~0.1 across the majority of the star-forming disks of NGC3627 and NGC4321. However, RCICO strongly varies from ~0.05 at the centre of NGC4321 to >0.2-0.5 in NGC1808's starburst centre and NGC7469's centre with an X-ray AGN. Meanwhile, RCICO does not obviously vary with $U$, similar to the prediction of PDR models. We also find a mildly decreasing RCICO with an increasing metallicity over 0.7-0.85 solar metallicity, consistent with the literature. Assuming various typical ISM conditions representing GMCs, active star-forming regions and strong starbursting environments, we calculate the LTE radiative transfer and estimate the [CI/CO] abundance ratio to be ~0.1 across the disks of NGC3627 and NGC4321, similar to previous large-scale findings in Galactic studies. However, this abundance ratio likely has a substantial increase to ~1 and >1-5 in NGC1808's starburst and NGC7469's strong AGN environments, respectively, in line with the expectations for cosmic-ray dominated region (CRDR) and X-ray dominated region (XDR) chemistry. Finally, we do not find a robust evidence for a generally CO-dark, CI-bright gas in the disk areas we probed. (abbreviated)

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arXiv2022

PHANGS-JWST First Results: Stellar Feedback-Driven Excitation and Dissociation of Molecular Gas in the Starburst Ring of NGC 1365?

We compare embedded young massive star clusters (YMCs) to (sub-)millimeter line observations tracing the excitation and dissociation of molecular gas in the starburst ring of NGC 1365. This galaxy hosts one of the strongest nuclear starbursts and richest populations of YMCs within 20 Mpc. Here we combine near-/mid-IR PHANGS-JWST imaging with new ALMA multi-J CO (1-0, 2-1 and 4-3) and [CI](1-0) mapping, which we use to trace CO excitation via R42 = I_CO(4-3)/I_CO(2-1) and R21 = I_CO(2-1)/I_CO(1-0) and dissociation via RCICO = I_[CI](1-0)/I_CO(2-1) at 330 pc resolution. We find that the gas flowing into the starburst ring from northeast to southwest appears strongly affected by stellar feedback, showing decreased excitation (lower R42) and increased signatures of dissociation (higher RCICO) in the downstream regions. There, radiative transfer modeling suggests that the molecular gas density decreases and temperature and [CI/CO] abundance ratio increase. We compare R42 and RCICO with local conditions across the regions and find that both correlate with near-IR 2 um emission tracing the YMCs and with both PAH (11.3 um) and dust continuum (21 um) emission. In general, RCICO exhibits ~ 0.1 dex tighter correlations than R42, suggesting CI to be a more sensitive tracer of changing physical conditions in the NGC 1365 starburst than CO (4-3). Our results are consistent with a scenario where gas flows into the two arm regions along the bar, becomes condensed/shocked, forms YMCs, and then these YMCs heat and dissociate the gas.

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arXiv2022

The PHANGS-JWST Treasury Survey: Star Formation, Feedback, and Dust Physics at High Angular resolution in Nearby GalaxieS

The PHANGS collaboration has been building a reference dataset for the multi-scale, multi-phase study of star formation and the interstellar medium in nearby galaxies. With the successful launch and commissioning of JWST, we can now obtain high-resolution infrared imaging to probe the youngest stellar populations and dust emission on the scales of star clusters and molecular clouds ($\sim$5-50 pc). In Cycle 1, PHANGS is conducting an 8-band imaging survey from 2-21$μ$m of 19 nearby spiral galaxies. CO(2-1) mapping, optical integral field spectroscopy, and UV-optical imaging for all 19 galaxies have been obtained through large programs with ALMA, VLT/MUSE, and Hubble. PHANGS-JWST enables a full inventory of star formation, accurate measurement of the mass and age of star clusters, identification of the youngest embedded stellar populations, and characterization of the physical state of small dust grains. When combined with Hubble catalogs of $\sim$10,000 star clusters, MUSE spectroscopic mapping of $\sim$20,000 HII regions, and $\sim$12,000 ALMA-identified molecular clouds, it becomes possible to measure the timescales and efficiencies of the earliest phases of star formation and feedback, build an empirical model of the dependence of small dust grain properties on local ISM conditions, and test our understanding of how dust-reprocessed starlight traces star formation activity, all across a diversity of galactic environments. Here we describe the PHANGS-JWST Treasury survey, present the remarkable imaging obtained in the first few months of science operations, and provide context for the initial results presented in the first series of PHANGS-JWST publications.

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arXiv2022

A 2-3 mm high-resolution molecular line survey towards the centre of the nearby spiral galaxy NGC 6946

The complex physical, kinematic, and chemical properties of galaxy centres make them interesting environments to examine with molecular line emission. We present new $2-4$" (${\sim}75{-}150$ pc at $7.7$ Mpc) observations at 2 and 3 mm covering the central $50$" (${\sim}1.9$ kpc) of the nearby double-barred spiral galaxy NGC 6946 obtained with the IRAM Plateau de Bure Interferometer. We detect spectral lines from ten molecules: CO, HCN, HCO$^+$, HNC, CS, HC$_3$N, N$_2$H$^+$, C$_2$H, CH$_3$OH, and H$_2$CO. We complemented these with published 1mm CO observations and 33 GHz continuum observations to explore the star formation rate surface density ${Σ_{\mathrm{SFR}}}$ on 150 pc scales. In this paper, we analyse regions associated with the inner bar of NGC 6946 $-$ the nuclear region (NUC), the northern (NBE), and southern inner bar end (SBE) and we focus on short-spacing corrected bulk (CO) and dense gas tracers (HCN, HCO$^+$, and HNC). We find that HCO$^+$ correlates best with ${Σ_{\mathrm{SFR}}}$, but the dense gas fraction ($f_{\mathrm{dense}}$) and star formation efficiency of the dense gas (${\mathrm{SFE_{dense}}}$) fits show different behaviours than expected from large-scale disc observations.The SBE has a higher ${Σ_{\mathrm{SFR}}}$, $f_{\mathrm{dense}}$, and shocked gas fraction than the NBE. We examine line ratio diagnostics and find a higher CO(2-1)/CO(1-0) ratio towards NBE than for the NUC. Moreover, comparison with existing extragalactic datasets suggests that using the HCN/HNC ratio to probe kinetic temperatures is not suitable on kiloparsec and sub-kiloparsec scales in extragalactic regions. Lastly, our study shows that the HCO$^+$/HCN ratio might not be a unique indicator to diagnose AGN activity in galaxies.

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arXiv2021

Molecular Gas Properties and CO-to-H2 Conversion Factors in the Central Kiloparsec of NGC 3351

The CO-to-H$_2$ conversion factor ($α_\rm{CO}$) is critical to studying molecular gas and star formation in galaxies. The value of $α_\rm{CO}$ has been found to vary within and between galaxies, but the specific environmental conditions that cause these variations are not fully understood. Previous observations on $\sim$kpc scales revealed low values of $α_\rm{CO}$ in the centers of some barred spiral galaxies, including NGC 3351. We present new ALMA Band 3, 6, and 7 observations of $^{12}$CO, $^{13}$CO, and C$^{18}$O lines on 100 pc scales in the inner $\sim$2 kpc of NGC 3351. Using multi-line radiative transfer modeling and a Bayesian likelihood analysis, we infer the H$_2$ density, kinetic temperature, CO column density per line width, and CO isotopologue abundances on a pixel-by-pixel basis. Our modeling implies the existence of a dominant gas component with a density of $2{-}3\times10^3$ $\rm{cm^{-3}}$ in the central ${\sim}$1 kpc and a high temperature of 30$-$60 K near the nucleus and near the contact points that connect to the bar-driven inflows. Assuming a CO/H$_2$ abundance of $3\times10^{-4}$, our analysis yields $α_\rm{CO}{\sim}0.5{-}2.0$ $\rm{M_\odot\,(K~km~s^{-1}~pc^2)^{-1}}$ with a decreasing trend with galactocentric radius in the central $\sim$1 kpc. The inflows show a substantially lower $α_\rm{CO} < 0.1$ $\rm{M_\odot\,(K~km~s^{-1}~pc^2)^{-1}}$, likely due to lower optical depths caused by turbulence or shear in the inflows. Over the whole region, this gives an intensity-weighted $α_\rm{CO}$ of ${\sim}1.5$ $\rm{M_\odot\,(K~km~s^{-1}~pc^2)^{-1}}$, which is similar to previous dust modeling based results at kpc scales. This suggests that low $α_\rm{CO}$ on kpc scales in the centers of some barred galaxies may be due to the contribution of low optical depth CO emission in bar-driven inflows.

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arXiv2020

Physical Conditions and Kinematics of the Filamentary Structure in Orion Molecular Cloud 1

We have studied the structure and kinematics of the dense molecular gas in the Orion Molecular Cloud 1 (OMC1) region with the N2H+ 3-2 line. The 6'x9' (~0.7x1.1 pc) region surrounding the Orion KL core has been mapped with the Submillimeter Array (SMA) and the Submillimeter Telescope (SMT). The combined SMA and SMT image having a resolution of ~5.4" (~2300 au) reveals multiple filaments with a typical width of 0.02-0.03 pc. On the basis of the non-LTE analysis using the N2H+ 3-2 and 1-0 data, the density and temperature of the filaments are estimated to be ~10^7 cm-3 and ~15-20 K, respectively. The core fragmentation is observed in three massive filaments, one of which shows the oscillations in the velocity and intensity that could be the signature of core-forming gas motions. The gas kinetic temperature is significantly enhanced in the eastern part of OMC1, likely due to the external heating from the high mass stars in M42 and M43. In addition, the filaments are colder than their surrounding regions, suggesting the shielding from the external heating due to the dense gas in the filaments. The OMC1 region consists of three sub-regions, i.e. north, west, and south of Orion KL, having different radial velocities with sharp velocity transitions. There is a north-to-south velocity gradient from the western to the southern regions. The observed velocity pattern suggests that dense gas in OMC1 is collapsing globally toward the high-mass star-forming region, Orion Nebula Cluster.

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