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Abstract:

The star formation efficiency (SFE) has been shown to vary across different environments, particularly within galactic starbursts and deep within the bulges of galaxies. Various quenching mechanisms may be responsible, ranging from galactic dynamics to feedback from active galactic nuclei (AGNs). Here, we use spatially resolved observations of warm ionized gas emission lines (Hβ, [O iii] λλ4959,5007, [N ii] λλ6548,6583, Hα and [S ii] λλ6716,6731) from the imaging Fourier transform spectrograph SITELLE at the Canada-France-Hawaii Telescope (CFHT) and cold molecular gas (12CO(2-1)) from the Atacama Large Millimeter/sub-millimeter Array (ALMA) to study the SFE in the bulge of the AGN-host galaxy NGC 3169. After distinguishing star-forming regions from AGN-ionized regions using emission-line ratio diagnostics, we measure spatially resolved molecular gas depletion times (τdep 1/SFE) with a spatial resolution of ≈100 pc within a galactocentric radius of 1.8 kpc. We identify a star-forming ring located at radii 1.25 ± 0.6 kpc with an average τdep of 0.3 Gyr. At radii <0.9 kpc, however, the molecular gas surface densities and depletion times increase with decreasing radius, the latter reaching approximately 2.3 Gyr at a radius ≈500 pc. Based on analyses of the gas kinematics and comparisons with simulations, we identify AGN feedback, bulge morphology and dynamics as the possible causes of the radial profile of SFE observed in the central region of NGC 3169.

Abstract:

We provide new planetary nebula luminosity function (pnlf) distances to 19 nearby spiral galaxies that were observed with VLT/MUSE by the PHANGS collaboration. Emission line ratios are used to separate planetary nebulae (pne) from other bright [O, III] emitting sources like compact supernovae remnants (snrs) or H ii regions. While many studies have used narrowband imaging for this purpose, the detailed spectral line information provided by integral field unit (ifu) spectroscopy grants a more robust way of categorizing different [O, III] emitters. We investigate the effects of snr contamination on the pnlf and find that we would fail to classify all objects correctly, when limited to the same data narrowband imaging provides. However, the few misclassified objects usually do not fall on the bright end of the luminosity function, and only in three cases does the distance change by more than 1¦Ò. We find generally good agreement with literature values from other methods. Using metallicity constraints that have also been derived from the same ifu data, we revisit the pnlf zero-point calibration. Over a range of 8.34 < 12 + log (O/H) < 8.59, our sample is consistent with a constant zero-point and yields a value of M? = -4.542+0.103-0.059, mag, within 1¦Ò of other literature values. MUSE pushes the limits of pnlf studies and makes galaxies beyond 20 Mpc accessible for this kind of analysis. This approach to the pnlf shows great promise for leveraging existing archival ifu data on nearby galaxies.

Abstract:

We use high-resolution maps of the molecular interstellar medium (ISM) in the centres of eighty-six nearby galaxies from the millimetre-Wave Interferometric Survey of Dark Object Masses (WISDOM) and Physics at High Angular Resolution in Nearby GalaxieS (PHANGS) surveys to investigate the physical mechanisms setting the morphology of the ISM at molecular cloud scales. We show that early-type galaxies tend to have smooth, regular molecular gas morphologies, while the ISM in spiral galaxy bulges is much more asymmetric and clumpy when observed at the same spatial scales. We quantify these differences using non-parametric morphology measures (Asymmetry, Smoothness and Gini), and compare these measurements with those extracted from idealised galaxy simulations. We show that the morphology of the molecular ISM changes systematically as a function of various large scale galaxy parameters, including galaxy morphological type, stellar mass, stellar velocity dispersion, effective stellar mass surface density, molecular gas surface density, star formation efficiency and the presence of a bar. We perform a statistical analysis to determine which of these correlated parameters best predicts the morphology of the ISM. We find the effective stellar mass surface (or volume) density to be the strongest predictor of the morphology of the molecular gas, while star formation and bars maybe be important secondary drivers. We find that gas self-gravity is not the dominant process shaping the morphology of the molecular gas in galaxy centres. Instead effects caused by the depth of the potential well such as shear, suppression of stellar spiral density waves and/or inflow affect the ability of the gas to fragment.

Abstract:

Star formation in galaxies is governed by the amount of molecular gas and the efficiency that gas is converted into stars. However, assessing the amount of molecular gas relies on the CO-to-H2 conversion factor (alpha_CO), which is known to vary with molecular gas conditions like density, temperature, and dynamical state -- the same conditions that also alter star formation efficiency. The variation of alpha_CO, particularly in galaxy centers where alpha_CO can drop by nearly an order of magnitude, thus causes major uncertainties in current molecular gas and star formation efficiency measurements. Using ALMA observations of multiple low-J 12CO, 13CO, and C18O lines in several barred galaxy centers, we found that alpha_CO is primarily driven by CO opacity changes and therefore shows strong correlations with observables like velocity dispersion and 12CO/13CO line ratio. Motivated by these results, we have constructed a new alpha_CO prescription which accounts for emissivity effects in galaxy centers and verified it on a set of barred and non-barred galaxies with measured alpha_CO values from dust. Applying our new prescription to 65 galaxies from the PHANGS-ALMA survey, we found an overall three times higher star formation efficiency in barred galaxy centers than in non-barred galaxy centers, and such a trend is obscured when using a constant alpha_CO or other existing prescriptions. Our results suggest that the high star formation rate observed in barred galaxy centers is due to an enhanced star formation efficiency compared to non-barred galaxy centers or the disk regions, rather than a substantially increased amount of molecular gas in barred galaxy centers

Abstract:

[[abstract]]The relative distribution of molecular gas and star formation in galaxies gives insight into the physical processes and timescales of the cycle between gas and stars. In this work, we track the relative spatial configuration of CO and H¦Á emission at high resolution in each of our galaxy targets and use these measurements to quantify the distributions of regions in different evolutionary stages of star formation: from molecular gas without star formation traced by H¦Á to star-forming gas, and to H ii regions. The large sample, drawn from the Physics at High Angular resolution in Nearby GalaxieS ALMA and narrowband H¦Á (PHANGS-ALMA and PHANGS-H¦Á) surveys, spans a wide range of stellar masses and morphological types, allowing us to investigate the dependencies of the gas?star formation cycle on global galaxy properties. At a resolution of 150 pc, the incidence of regions in different stages shows a dependence on stellar mass and Hubble type of galaxies over the radial range probed. Massive and/or earlier-type galaxies in our sample exhibit a significant reservoir of molecular gas without star formation traced by H¦Á, while lower-mass galaxies harbor substantial H ii regions that may have dispersed their birth clouds or formed from low-mass, more isolated clouds. Galactic structures add a further layer of complexity to the relative distribution of CO and H¦Á emission. Trends between galaxy properties and distributions of gas traced by CO and H¦Á are visible only when the observed spatial scale is ?500 pc, reflecting the critical resolution requirement to distinguish stages of the star formation process.[[notice]]ÑaÕýÍê