Tails of Gravity: Persistence of Star Formation in the CMZ Environment

TL;DR

This work probes star formation in the Galactic CMZ by deriving dust-based column density maps from multi-wavelength data and characterizing the distribution of gas via N-PDFs. By combining large-scale dust maps with high-resolution ALMA 1.3 mm data, the authors estimate the mass of gravitationally bound gas $M_{ m gas}^{ m bound}$ and the mass of the most massive cores $M_{ m core}^{ m max}$, examining their interrelations. They find that four CMZ clouds conform to the established $M_{ m core}^{ m max}$–$M_{ m gas}^{ m bound}$ relation (also seen in solar neighborhood and distant clouds), and that SFR correlates with the bound gas mass in a manner similar to non-CMZ star-forming regions, supporting a self-regulated star formation scenario even in extreme environments. These results suggest that, once gravity dominates locally, core formation and star formation become relatively insensitive to external CMZ-scale conditions over ≳5–10 pc, with implications for refining the Gao–Solomon framework in dense, high-pressure contexts.

Abstract

We characterize star-forming gas in six molecular clouds (Sgr B1-off, Sgr B2, Sgr C, the 20 km s$^{-1}$ and 50 km s$^{-1}$ molecular clouds, and the Brick) in the Galactic central molecular zone (CMZ), and compare their star-forming activities with those in molecular clouds outside the CMZ. Using multi-band continuum observations taken from ${\it Planck}$, ${\it Herschel}$, JCMT/SCUBA-2, and CSO/SHARC2, we derived 8.5" resolution column density maps for the CMZ clouds and evaluated the column density probability distribution functions (N-PDFs). With the archival Atacama Large Millimeter/submillimeter Array (ALMA) 1.3 mm dust continuum data, we further evaluated the mass of the most massive cores ($M_{\rm core}^{\rm ma x}$). We find that the N-PDFs of four of the selected CMZ clouds are well described by a piecewise log-normal + power-law function, while the N-PDFs of the remaining two can be approximated by log-normal functions. In the first four targets, the masses in the power-law component ($M_{\rm gas}^{\rm bound}$), $M_{\rm core}^{\rm max}$, and star formation rate (SFR) are correlated. These correlations are very similar to those derived from low-mass clouds in the Solar neighborhood and massive star-forming regions on the Galactic disk. These findings lead to our key hypotheses: (1) In the extreme environment of the CMZ, the power-law component in the N-PDF also represents self-gravitationally bound gas structures, and (2) evolution and star-forming activities of self-gravitationally bound gas structures may be self-regulated, insensitive to the exterior environment on $\gtrsim$5-10 pc scales.

Figures (17)

• Figure 1: The 450 ${\rm \mu m}$ (top panel) and 850 ${\rm \mu m}$ (bottom panel) submillimeter maps of the Central Molecular Zone, produced by combining Herschel/ Planck and JCMT-SCUBA2 observations. In the top panel, the orange box highlights the six molecular clouds studied in this work. The supermassive black hole at the center of the Milky Way, Sgr A*, is marked with a cyan asterisk. The field of view of the CSO-SHARC2 350 ${\rm \mu m}$ is outlined with gray dot-dashed lines. The RMS levels of the two images are approximately $\sim$0.8 Jy beam$^{-1}$ (450 ${\rm \mu m}$) and $\sim$0.3 Jy beam$^{-1}$ (850 ${\rm \mu m}$), though it is important to note that due to the stacking of multiple maps, the RMS level is not completely uniform across the field of view. Beam sizes are shown as black circles in the lower-left corner of each panel.
• Figure 2: The CMZ 350 ${\rm \mu m}$ continuum map, produced by stacking the CSO-SHARC2 observations from Bally2010ApJ...721..137B_CSOCMZ (gray outline) with our own data (light yellow outline). Both datasets were first combined with Herschel data before stacking. Three of the selected clouds that are covered by this combined dataset are highlighted with orange dashed circles. The positions of two known star clusters, the Arches and the Quintuplet, are marked with cyan circles. The location of the supermassive black hole, Sgr A*, is denoted by a cyan star symbol.
• Figure 3: SED fitting results. All maps are displayed in Galactic coordinates, with scale bars representing 50 pc. Beam sizes are shown in the lower-left corner of each panel. Top: The 85 resolution ${\rm H_2}$ column density ($N_{\rm H_2}$) map, derived assuming a gas-to-dust ratio of 100. Middle: The 85 resolution dust temperature ($T_{\rm dust}$) distribution map. Bottom: The 14 dust emissivity spectral index ($\beta$) map.
• Figure 4: The pc-scale gas column density maps of the selected sources. The grayscale represents the distribution of gas column density derived from SED fitting of Herschel, JCMT, and CSO data (Section \ref{['subsub:Nmap']}). Light green contours outline the regions of self-gravitationally bound gas. The FOVs of the ALMA data are marked by the orange dotted lines, and the most massive core identified within this area is indicated by the red ellipse. Gray dotted lines represent the last closed contour for each source.
• Figure 5: N-PDFs of the target molecular clouds. The blue step lines show the N-PDFs of each molecular cloud. The left and right y-axes indicate the probability density and the corresponding number of pixels, respectively. Given our pixel size of 25, bins with fewer than $\sim$13 pixels correspond to areas smaller than the beam size. The red dashed line represents the model distribution estimated using the MLE+MCMC method. The orange vertical line and shaded area represent the median of the $N_{\rm threshold}$ posterior distribution and the 3-$\sigma$ credible interval. The N-PDFs of the 50 km ${\rm s^{-1}}$ cloud and the Brick cannot be well described by a log-normal + power-law model, as their high column density ends do not exhibit distinguishable power-law structures. Therefore, we adopted a log-normal model alone for the fitting.