We present a comprehensive study of the velocity dispersion of the atomic (H i) and molecular (H 2 ) gas components in the disks (R R 25 ) of a sample of 12 nearby spiral galaxies with moderate inclinations. Our analysis is based on sensitive high-resolution data from the THINGS (atomic gas) and HERACLES (molecular gas) surveys. To obtain reliable measurements of the velocity dispersion, we stack regions several kiloparsecs in size, after accounting for intrinsic velocity shifts due to galactic rotation and large-scale motions. We stack using various parameters: the galactocentric distance, star formation rate surface density, H i surface density, H 2 surface density, and total gas surface density. We fit single Gaussian components to the stacked spectra and measure median velocity dispersions for H i of 11.9 ± 3.1 km s −1 and for CO of 12.0 ± 3.9 km s −1 . The CO velocity dispersions are thus, surprisingly, very similar to the corresponding ones of H i, with an average ratio of σ H i /σ CO = 1.0 ± 0.2 irrespective of the stacking parameter. The measured CO velocity dispersions are significantly higher (factor of ∼2) than the traditional picture of a cold molecular gas disk associated with star formation. The high dispersion implies an additional thick molecular gas disk (possibly as thick as the H i disk). Our finding is in agreement with recent sensitive measurements in individual edge-on and face-on galaxies and points toward the general existence of a thick disk of molecular gas, in addition to the well-known thin disk in nearby spiral galaxies.

We analyze the velocity dispersions of individual H i and CO profiles in a number of nearby galaxies from the high-resolution HERACLES CO and THINGS H i surveys. Focusing on regions with bright CO emission, we find a CO dispersion value σ CO = 7.3 ± 1.7 km s −1 . The corresponding H i dispersion σ HI = 11.7 ± 2.3 km s −1 , yielding a mean dispersion ratio σ HI /σ CO = 1.4 ± 0.2, independent of radius. We find that the CO velocity dispersion increases towards lower peak fluxes. This is consistent with previous work where we showed that when using spectra averaged ("stacked") over large areas, larger values for the CO dispersion are found, and a lower ratio σ HI /σ CO = 1.0 ± 0.2. The stacking method is more sensitive to low-level diffuse emission, whereas individual profiles trace narrow-line, GMCdominated, bright emission. These results provide further evidence that disk galaxies contain not only a thin, low velocity dispersion, high density CO disk that is dominated by GMCs, but also a fainter, higher dispersion, diffuse disk component.

We present a systematic search for optical counterparts to 13 gravitational wave (GW) triggers involving at least one neutron star during LIGO/Virgo’s third observing run (O3). We searched binary neutron star (BNS) and neutron star black hole (NSBH) merger localizations with the Zwicky Transient Facility (ZTF) and undertook follow-up with the Global Relay of Observatories Watching Transients Happen (GROWTH) collaboration. The GW triggers had a median localization area of 4480 deg2, a median distance of 267 Mpc, and false-alarm rates ranging from 1.5 to 10−25 yr−1. The ZTF coverage in the g and r bands had a median enclosed probability of 39%, median depth of 20.8 mag, and median time lag between merger and the start of observations of 1.5 hr. The O3 follow-up by the GROWTH team comprised 340 UltraViolet/Optical/InfraRed (UVOIR) photometric points, 64 OIR spectra, and three radio images using 17 different telescopes. We find no promising kilonovae (radioactivity-powered counterparts), and we show how to convert the upper limits to constrain the underlying kilonova luminosity function. Initially, we assume that all GW triggers are bona fide astrophysical events regardless of false-alarm rate and that kilonovae accompanying BNS and NSBH mergers are drawn from a common population; later, we relax these assumptions. Assuming that all kilonovae are at least as luminous as the discovery magnitude of GW170817 (−16.1 mag), we calculate that our joint probability of detecting zero kilonovae is only 4.2%. If we assume that all kilonovae are brighter than −16.6 mag (the extrapolated peak magnitude of GW170817) and fade at a rate of 1 mag day−1 (similar to GW170817), the joint probability of zero detections is 7%. If we separate the NSBH and BNS populations based on the online classifications, the joint probability of zero detections, assuming all kilonovae are brighter than −16.6 mag, is 9.7% for NSBH and 7.9% for BNS mergers. Moreover, no more than <57% (<89%) of putative kilonovae could be brighter than −16.6 mag assuming flat evolution (fading by 1 mag day−1) at the 90% confidence level. If we further take into account the online terrestrial probability for each GW trigger, we find that no more than <68% of putative kilonovae could be brighter than −16.6 mag. Comparing to model grids, we find that some kilonovae must have M ej < 0.03 M ⊙, X lan > 10−4, or ϕ > 30° to be consistent with our limits. We look forward to searches in the fourth GW observing run; even 17 neutron star mergers with only 50% coverage to a depth of −16 mag would constrain the maximum fraction of bright kilonovae to <25%.

The velocity dispersion of cold interstellar gas, σ, is one of the quantities that most radically affect the onset of gravitational instabilities in galaxy discs, and the quantity that is most drastically approximated in stability analyses. Here we analyse the stability of a large sample of nearby star-forming spirals treating molecular gas, atomic gas and stars as three distinct components, and using radial profiles of σ CO and σ HI derived from HERACLES and THINGS observations. We show that the radial variations of σ CO and σ HI have a weak effect on the local stability level of galaxy discs, which remains remarkably flat and well above unity, but is low enough to ensure (marginal) instability against non-axisymmetric perturbations and gas dissipation. More importantly, the radial variation of σ CO has a strong impact on the size of the regions over which gravitational instabilities develop, and results in a characteristic instability scale that is one order of magnitude larger than the Toomre length of molecular gas. Disc instabilities are driven, in fact, by the self-gravity of stars at kpc scales. This is true across the entire optical disc of every galaxy in the sample, with few exceptions. In the linear phase of the disc instability process, stars and molecular gas are strongly coupled, and it is such a coupling that ultimately triggers local gravitational collapse/fragmentation in the molecular gas.

We introduce a new diagnostic for exploring the link between angular momentum and local gravitational instability in galaxy discs. Our diagnostic incorporates the latest developments in disc instability research, is fully consistent with approximations that are widely used for measuring the stellar specific angular momentum, j ⋆ = J ⋆ /M ⋆ , and is also very simple. We show that such a disc instability diagnostic hardly correlates with j ⋆ or M ⋆ , and is remarkably constant across spiral galaxies of any given type (Sa-Sd), stellar mass (M ⋆ = 10 9.5 -10 11.5 M ⊙ ) and velocity dispersion anisotropy (σ z⋆ /σ R⋆ = 0-1). The fact that M ⋆ is tightly correlated with star formation rate (SFR), molecular gas mass (M mol ), metallicity (12 + log O/H) and other fundamental galaxy properties thus implies that nearby star-forming spirals self-regulate to a quasiuniversal disc stability level. This proves the existence of the self-regulation process postulated by several star formation models, but also raises important caveats.