Molecular gas and star formation in central rings across nearby galaxies

TL;DR

This study uses ~1'' resolution CO(2-1) data from PHANGS-ALMA to identify and characterize 20 central rings in nearby galaxies, assessing their molecular gas content and star formation while linking these properties to host galaxy bars. For 14 rings, PHANGS-MUSE ΣSFR maps enable ring-level SFRs and depletion times, with ring masses around 10^8 M⊙ and SFRs near 0.2 M⊙ yr^-1, implying a typical depletion time of ~0.5–0.6 Gyr. Relative to the Milky Way CMZ, PHANGS rings occupy similar fractional gas and SFR contributions, suggesting that the processes driving gas inflow and star formation in central rings are broadly similar across nearby galaxies, despite differences in absolute scale. The results show that longer bars correlate with more massive rings, though classical bar strength does not robustly predict ring gas content, and they highlight a stellar-mass threshold for ring formation, with most rings residing in massive galaxies.

Abstract

Nearby galaxies exhibit a variety of structures, including central rings, similar to the MW Central Molecular Zone (CMZ). These rings are common in barred galaxies and can be gas-rich and highly star-forming. We aim to study molecular gas content and star formation rate of central rings within nearby galaxies and link them to global galaxy properties (e.g. bar morphology). We utilize $1\,$'' resolution CO(2-1) PHANGS-ALMA observations, visually identify 20 central rings and determine their properties. For $14$ rings, SFR surface density maps are available. We derive ring geometry, integrated molecular gas masses, SFRs, depletion times, and compare them to host galaxy and bar properties. Molecular gas is a good tracer for central rings: Previous studies used ionized gas and dust tracers to identify central rings in galaxies of similar morphological types as this study. In comparison, we find similar fractions of galaxies hosting central rings and similar radii distributions. The gaseous central rings have typical radii of $400_{-150}^{+250}\,$pc, molecular gas masses of $\log(M_\text{mol}/M_\odot){\sim}8.1_{-0.23}^{+0.17}$, and SFRs of $0.21_{-0.16}^{+0.15}\,M_\odot/\text{yr}$, thus contributing $5.6_{-2.1}^{+4.5}\,\%$ and $13_{-5}^{+10}\,\%$ to their host galaxies' molecular gas mass and SFR. The MW CMZ sits at the lower end of the radius, molecular gas mass, and SFR distribution, but it has a similar molecular gas mass and SFR fraction, and depletion time. Longer bars contain more massive molecular central rings, but we find no correlation between bar strength and the ring's molecular gas content. Although absolute central ring properties likely depend on host galaxy properties, the similarities between the MW CMZ and PHANGS central rings in relative parameters suggest that the processes of gas inflow and star formation are similar for central rings across nearby galaxies.

Figures (10)

• Figure 1: Global SFR as a function of global stellar mass for our sample galaxies from the PHANGS-ALMA sample. The solid and dashed black lines denote the main sequence of star forming galaxies with a scatter of 0.36 dex, as given by Leroy_2019. The PHANGS galaxies with central rings are color coded by the central ring gas mass as obtained in this study and the different symbols denote barred spiral (diamond) and unbarred spiral (open diamond) galaxies, barred early-type galaxies (ETGs, cross) and unbarred ETGs (open cross). The MW is plotted as a black dot with red errorbars.
• Figure 2: Molecular gas distribution of the galaxy NGC 1097. The different ellipses represent the "best" elliptical annulus (red, solid line) and uncertainty ellipses (black, dashed lines), indicating emission which is definitely ring material (narrower annulus, "strict" mask) and emission that might still be ring material (wider annulus, "broad" mask). The white circle in the lower left corner indicates the beam size, and a scalebar is provided in the lower right. The color bar indicates the integrated intensity in terms of the brightness temperature.
• Figure 3: Gallery of central rings identified in a sample of 81 PHANGS-ALMA galaxies. The galaxy name is given at the top of each panel. All panels have the same physical size of $3 \times 3\,\text{kpc}^2$, but with differing color stretches. The minimum and maximum intensity is listed at the bottom right of each panel. The "best" elliptical annulus (see Sect. \ref{['sec:ring-shapes']}) is plotted in red and the beam size is plotted in white in the lower left corner.
• Figure 4: Distributions of central ring parameters (from left to right): radius $r_\text{ring}$, relative radius $r_\text{ring}/r_{25}$, molecular gas mass $M_\text{mol}^\text{ring}$, ring molecular gas mass fraction $M_\text{mol}^\text{ring}/ M_\text{mol}^\text{gal}$, star formation rate ${\rm SFR}_\text{ring}$, ring SFR fraction ${\rm SFR}_\text{ring} / {\rm SFR}_\text{gal}$, depletion time $M_\text{mol}^\text{ring} / {\rm SFR}_\text{ring}$, and relative depletion time $t_\text{dep}^\text{ring} / t_\text{dep}^\text{gal}$. The distributions are shown for the whole sample on the left and for spirals only on the right, thus leading to the different numbers $N= \{...\}$ considered for the distributions. The violins show a smoothed distribution, the boxes contain the $25$th to $75$th percentile of the distributions, the black whiskers span out to $1.5$ times the interquartile range, and the circles mark outliers beyond this range. The median is shown by the black horizontal line. The respective values for the MW CMZ are plotted as black diamonds along with their error bars in red.
• Figure 5: Distribution of the absolute central ring radius (upper panel) and the ring radius relative to the disk size as measured by $r_{25}$ (lower panel). The green histogram shows the sample of central rings in this study while the white histogram shows the ring sample of the "Atlas of Images of NUclear Rings" (AINUR; comeron_ainur_2010).