We measure the COtoH_{2}conversion factor (
We use ALMA observations of CO(2–1) in 13 massive (
 NSFPAR ID:
 10362450
 Publisher / Repository:
 DOI PREFIX: 10.3847
 Date Published:
 Journal Name:
 The Astrophysical Journal
 Volume:
 925
 Issue:
 2
 ISSN:
 0004637X
 Format(s):
 Medium: X Size: Article No. 153
 Size(s):
 ["Article No. 153"]
 Sponsoring Org:
 National Science Foundation
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Abstract α _{CO}) in 37 galaxies at 2 kpc resolution, using the dust surface density inferred from farinfrared emission as a tracer of the gas surface density and assuming a constant dusttometal ratio. In total, we have ∼790 and ∼610 independent measurements ofα _{CO}for CO (2–1) and (1–0), respectively. The mean values forα _{CO (2–1)}andα _{CO (1–0)}are and ${9.3}_{5.4}^{+4.6}$ , respectively. The COintensityweighted mean is 5.69 for ${4.2}_{2.0}^{+1.9}\phantom{\rule{0.25em}{0ex}}{M}_{\odot}\phantom{\rule{0.25em}{0ex}}{\mathrm{pc}}^{2}\phantom{\rule{0.25em}{0ex}}{(\mathrm{K}\phantom{\rule{0.25em}{0ex}}\mathrm{km}\phantom{\rule{0.25em}{0ex}}{\mathrm{s}}^{1})}^{1}$α _{CO (2–1)}and 3.33 forα _{CO (1–0)}. We examine howα _{CO}scales with several physical quantities, e.g., the star formation rate (SFR), stellar mass, and dustmassweighted average interstellar radiation field strength ( ). Among them, $\overline{U}$ , Σ_{SFR}, and the integrated CO intensity ( $\overline{U}$W _{CO}) have the strongest anticorrelation with spatially resolvedα _{CO}. We provide linear regression results toα _{CO}for all quantities tested. At galaxyintegrated scales, we observe significant correlations betweenα _{CO}andW _{CO}, metallicity, , and Σ_{SFR}. We also find that $\overline{U}$α _{CO}in each galaxy decreases with the stellar mass surface density (Σ_{⋆}) in highsurfacedensity regions (Σ_{⋆}≥ 100M _{⊙}pc^{−2}), following the powerlaw relations and ${\alpha}_{\mathrm{CO}\phantom{\rule{0.25em}{0ex}}(2\u20131)}\propto {\mathrm{\Sigma}}_{\star}^{0.5}$ . The powerlaw index is insensitive to the assumed dusttometal ratio. We interpret the decrease in ${\alpha}_{\mathrm{CO}\phantom{\rule{0.25em}{0ex}}(1\u20130)}\propto {\mathrm{\Sigma}}_{\star}^{0.2}$α _{CO}with increasing Σ_{⋆}as a result of higher velocity dispersion compared to isolated, selfgravitating clouds due to the additional gravitational force from stellar sources, which leads to the reduction inα _{CO}. The decrease inα _{CO}at high Σ_{⋆}is important for accurately assessing molecular gas content and star formation efficiency in the centers of galaxies, which bridge “Milky Way–like” to “starburstlike” conversion factors. 
Abstract We present^{13}CO(
J = 1 → 0) observations for the EDGECALIFA survey, which is a mapping survey of 126 nearby galaxies at a typical spatial resolution of 1.5 kpc. Using detected^{12}CO emission as a prior, we detect^{13}CO in 41 galaxies via integrated line flux over the entire galaxy and in 30 galaxies via integrated line intensity in resolved synthesized beams. Incorporating our CO observations and optical IFU spectroscopy, we perform a systematic comparison between the line ratio and the properties of the stars and ionized gas. Higher ${\mathit{\ue23e}}_{12/13}\equiv I{[}^{12}\mathrm{CO}(J=1\to 0)]/I{[}^{13}\mathrm{CO}(J=1\to 0)]$ values are found in interacting galaxies compared to those in noninteracting galaxies. The global ${\mathit{\ue23e}}_{12/13}$ slightly increases with infrared color ${\mathit{\ue23e}}_{12/13}$F _{60}/F _{100}but appears insensitive to other hostgalaxy properties such as morphology, stellar mass, or galaxy size. We also present azimuthally averaged profiles for our sample up to a galactocentric radius of 0.4 ${\mathit{\ue23e}}_{12/13}$r _{25}(∼6 kpc), taking into account the^{13}CO nondetections by spectral stacking. The radial profiles of are quite flat across our sample. Within galactocentric distances of 0.2 ${\mathit{\ue23e}}_{12/13}$r _{25}, the azimuthally averaged increases with the star formation rate. However, Spearman rank correlation tests show the azimuthally averaged ${\mathit{\ue23e}}_{12/13}$ does not strongly correlate with any other gas or stellar properties in general, especially beyond 0.2 ${\mathit{\ue23e}}_{12/13}$r _{25}from the galaxy centers. Our findings suggest that in the complex environments in galaxy disks, is not a sensitive tracer for ISM properties. Dynamical disturbances, like galaxy interactions or the presence of a bar, also have an overall impact on ${\mathit{\ue23e}}_{12/13}$ , which further complicates the interpretations of ${\mathit{\ue23e}}_{12/13}$ variations. ${\mathit{\ue23e}}_{12/13}$ 
Abstract We measure the thermal electron energization in 1D and 2D particleincell simulations of quasiperpendicular, lowbeta (
β _{p}= 0.25) collisionless ion–electron shocks with mass ratiom _{i}/m _{e}= 200, fast Mach number –4, and upstream magnetic field angle ${\mathcal{M}}_{\mathrm{ms}}=1$θ _{Bn}= 55°–85° from the shock normal . It is known that shock electron heating is described by an ambipolar, $\stackrel{\u02c6}{\mathit{n}}$ parallel electric potential jump, ΔB ϕ _{∥}, that scales roughly linearly with the electron temperature jump. Our simulations have –0.2 in units of the preshock ions’ bulk kinetic energy, in agreement with prior measurements and simulations. Different ways to measure $\mathrm{\Delta}{\varphi}_{\parallel}/(0.5{m}_{\mathrm{i}}{{u}_{\mathrm{sh}}}^{2})\sim 0.1$ϕ _{∥}, including the use of de Hoffmann–Teller frame fields, agree to tensofpercent accuracy. Neglecting offdiagonal electron pressure tensor terms can lead to a systematic underestimate ofϕ _{∥}in our lowβ _{p}shocks. We further focus on twoθ _{Bn}= 65° shocks: a ( ${\mathcal{M}}_{\mathrm{s}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}4$ ) case with a long, 30 ${\mathcal{M}}_{\mathrm{A}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}1.8$d _{i}precursor of whistler waves along , and a $\stackrel{\u02c6}{\mathit{n}}$ ( ${\mathcal{M}}_{\mathrm{s}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}7$ ) case with a shorter, 5 ${\mathcal{M}}_{\mathrm{A}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}3.2$d _{i}precursor of whistlers oblique to both and $\stackrel{\u02c6}{\mathit{n}}$ ;B d _{i}is the ion skin depth. Within the precursors,ϕ _{∥}has a secular rise toward the shock along multiple whistler wavelengths and also has localized spikes within magnetic troughs. In a 1D simulation of the , ${\mathcal{M}}_{\mathrm{s}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}4$θ _{Bn}= 65° case,ϕ _{∥}shows a weak dependence on the electron plasmatocyclotron frequency ratioω _{pe}/Ω_{ce}, andϕ _{∥}decreases by a factor of 2 asm _{i}/m _{e}is raised to the true proton–electron value of 1836. 
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z ≲ 2.6 (z _{mean}= 2.30), all chosen independent of emissionline strength. These galaxies have a median stellar mass of and a median star formation rate of $\mathrm{log}{({M}_{*}/{M}_{\odot})}_{\mathrm{med}}={8.29}_{0.43}^{+0.51}$ . We measure the faint electrontemperaturesensitive [O ${\mathrm{S}\mathrm{F}\mathrm{R}}_{\mathrm{H}\alpha}^{\mathrm{m}\mathrm{e}\mathrm{d}}={2.25}_{1.26}^{+2.15}\phantom{\rule{0.25em}{0ex}}{M}_{\odot}\phantom{\rule{0.25em}{0ex}}{\mathrm{y}\mathrm{r}}^{1}$iii ]λ 4363 emission line at 2.5σ (4.1σ ) significance when considering a bootstrapped (statisticalonly) uncertainty spectrum. This yields a directmethod oxygen abundance of ( $12+\mathrm{log}{(\mathrm{O}/\mathrm{H})}_{\mathrm{direct}}={7.88}_{0.22}^{+0.25}$ ). We investigate the applicability at high ${0.15}_{0.06}^{+0.12}\phantom{\rule{0.33em}{0ex}}{Z}_{\odot}$z of locally calibrated oxygenbased strongline metallicity relations, finding that the local reference calibrations of Bian et al. best reproduce (≲0.12 dex) our composite metallicity at fixed strongline ratio. At fixedM _{*}, our composite is well represented by thez ∼ 2.3 directmethod stellar mass—gasphase metallicity relation (MZR) of Sanders et al. When comparing to predicted MZRs from the IllustrisTNG and FIRE simulations, having recalculated our stellar masses with more realistic nonparametric star formation histories , we find excellent agreement with the FIRE MZR. Our composite is consistent with no metallicity evolution, at fixed $(\mathrm{log}{({M}_{*}/{M}_{\odot})}_{\mathrm{med}}={8.92}_{0.22}^{+0.31})$M _{*}and SFR, of the locally defined fundamental metallicity relation. We measure the doublet ratio [Oii ]λ 3729/[Oii ]λ 3726 = 1.56 ± 0.32 (1.51 ± 0.12) and a corresponding electron density of ( ${n}_{e}={1}_{0}^{+215}\phantom{\rule{0.33em}{0ex}}{\mathrm{cm}}^{3}$ ) when considering the bootstrapped (statisticalonly) error spectrum. This result suggests that lowermass galaxies have lower densities than highermass galaxies at ${n}_{e}={1}_{0}^{+74}\phantom{\rule{0.33em}{0ex}}{\mathrm{cm}}^{3}$z ∼ 2. 
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