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We report observations of the ground state transitions of¹²CO,¹³CO, C¹⁸O, HCN, and HCO⁺at 88–115 GHz in the inner region of the nearby galaxy IC 342. These data were obtained with the 16 pixel spectroscopic focal plane array Argus on the 100 m Robert C. Byrd Green Bank Telescope (GBT) at 6″–9″ resolution. In the nuclear bar region, the intensity distributions of¹²CO(1–0) and¹³CO(1–0) emission trace moderate densities, and differ from the dense gas distributions sampled in C¹⁸O(1–0), HCN(1–0), and HCO⁺(1–0). We observe a constant HCN(1–0)-to-HCO⁺(1–0) ratio of 1.2 ± 0.1 across the whole ∼1 kpc bar. This indicates that the HCN(1–0) and HCO⁺(1–0) lines have intermediate optical depth, and that the corresponding$n_{{\mathrm{H}}_2}$of the gas producing the emission is of order 10^4.5−6cm⁻³. We show that HCO⁺(1–0) is thermalized and HCN(1–0) is close to thermalization. The very tight correlation between the HCN(1–0) and HCO⁺(1–0) intensities across the 1 kpc bar suggests that this ratio is more sensitive to the relative abundance of the two species than to the gas density. We confirm an angular offset (∼10″) between the spatial distribution of molecular gas and the star formation sites. Finally, we find a breakdown of theL_IR–$L_{\mathrm{HCN}}$correlation at high spatial resolution due to the effect of incomplete sampling of star-forming regions by HCN emission in IC 342. The scatter of theL_IR–$L_{\mathrm{HCN}}$relation decreases as the spatial scale increases from 10″ to 30″ (170–510 pc), and is comparable to the scatter of the global relation at a scale of 340 pc.

 

Around 400 Myr after the big bang, the ultraviolet emission from star-forming galaxies reionized the Universe. Ionizing radiation (Lyman continuum, LyC) is absorbed by cold neutral hydrogen gas (H i) within galaxies, hindering the escape of LyC photons. Since the H i reservoir of LyC emitters has never been mapped, major uncertainties remain on how LyC photons escape galaxies and ionize the intergalactic medium. We have directly imaged the neutral gas in the nearby reionization-era analogue galaxy Haro 11 with the 21 cm line to identify the mechanism enabling ionizing radiation escape. We find that merger-driven interactions have caused a bulk offset of the neutral gas by about $6\,$ kpc from the centre of the galaxy, where LyC emission production sites are located. This could facilitate the escape of ionizing radiation into our line of sight. Galaxy interactions can cause both elevated LyC production and large-scale displacement of H i from the regions where these photons are produced. They could contribute to the anisotropic escape of LyC radiation from galaxies and the reionization of the Universe. We argue for a systematic assessment of the effect of environment on LyC production and escape.

 

ABSTRACT Previous work has argued that atomic gas mass estimates of galaxies from 21-cm H i emission are systematically low due to a cold opaque atomic gas component. If true, this opaque component necessitates a $\sim 35{{\ \rm per\ cent}}$ correction factor relative to the mass from assuming optically thin H i emission. These mass corrections are based on fitting H i spectra with a single opaque component model that produces a distinct ‘top-hat’ shaped line profile. Here, we investigate this issue using deep, high spectral resolution H i VLA observations of M31 and M33 to test if these top-hat profiles are instead superpositions of multiple H i components along the line of sight. We fit both models and find that ${\gt}80{{\ \rm per\ cent}}$ of the spectra strongly prefer a multicomponent Gaussian model while ${\lt}2{{\ \rm per\ cent}}$ prefer the single opacity-corrected component model. This strong preference for multiple components argues against previous findings of lines of sight dominated by only cold H i. Our findings are enabled by the improved spectral resolution (0.42 ${\rm km\, s^{-1}}$), whereas coarser spectral resolution blends multiple components together. We also show that the inferred opaque atomic ISM mass strongly depends on the goodness-of-fit definition and is highly uncertain when the inferred spin temperature has a large uncertainty. Finally, we find that the relation of the H i surface density with the dust surface density and extinction has significantly more scatter when the inferred H i opacity correction is applied. These variations are difficult to explain without additionally requiring large variations in the dust properties. Based on these findings, we suggest that the opaque H i mass is best constrained by H i absorption studies. 

Abstract We present PHANGS–ALMA, the first survey to map CO J = 2 → 1 line emission at ∼1″ ∼100 pc spatial resolution from a representative sample of 90 nearby ( d ≲ 20 Mpc) galaxies that lie on or near the z = 0 “main sequence” of star-forming galaxies. CO line emission traces the bulk distribution of molecular gas, which is the cold, star-forming phase of the interstellar medium. At the resolution achieved by PHANGS–ALMA, each beam reaches the size of a typical individual giant molecular cloud, so that these data can be used to measure the demographics, life cycle, and physical state of molecular clouds across the population of galaxies where the majority of stars form at z = 0. This paper describes the scientific motivation and background for the survey, sample selection, global properties of the targets, Atacama Large Millimeter/submillimeter Array (ALMA) observations, and characteristics of the delivered data and derived data products. As the ALMA sample serves as the parent sample for parallel surveys with MUSE on the Very Large Telescope, the Hubble Space Telescope, AstroSat, the Very Large Array, and other facilities, we include a detailed discussion of the sample selection. We detail the estimation of galaxy mass, size, star formation rate, CO luminosity, and other properties, compare estimates using different systems and provide best-estimate integrated measurements for each target. We also report the design and execution of the ALMA observations, which combine a Cycle 5 Large Program, a series of smaller programs, and archival observations. Finally, we present the first 1″ resolution atlas of CO emission from nearby galaxies and describe the properties and contents of the first PHANGS–ALMA public data release.