ALMAGAL V. Relations between the core populations and the parent clump physical properties

TL;DR

This study analyzes ALMAGAL ALMA data to relate clump-scale properties ($M$, $ ext{Σ}$, $T$) and evolutionary indicators ($L/M$, $T_ ext{bol}$) to core populations within clumps, using $1.4$ mm continuum fragments to probe fragmentation ($N_ ext{core}$) and the mass of the most massive core ($M_ ext{MMC}$). It finds a robust, density-driven fragmentation pattern: $N_ ext{core}$ correlates with $ ext{Σ}$, while $M_ ext{MMC}$ and core formation efficiency (CFE) rise with evolutionary indicators, suggesting ongoing accretion from the clump (clump-fed) rather than pre-assembled cores (core-fed). Comparisons with Rosetta Stone simulations indicate qualitative agreement in trends but substantial scatter and occasional overprediction of fragmentation, highlighting the roles of turbulence, magnetic fields, and feedback. Overall, the large, statistically robust ALMAGAL sample reveals that density and evolution largely govern fragmentation and core growth, with significant variability due to unquantified physics and observational biases, reinforcing clump-fed scenarios for high-mass star formation.

Abstract

Context. The fragmentation of massive molecular clumps into smaller, potentially star-forming cores plays a key role in the processes of high-mass star formation. The ALMAGAL project offers high-resolution data to investigate these processes across various evolutionary stages in the Galactic plane. Aims. This study aims at correlating the fragmentation properties of massive clumps, obtained from ALMA observations, with their global physical parameters (e.g., mass, surface density, and temperature) and evolutionary indicators (such as luminosity-to-mass ratio and bolometric temperature) obtained from Herschel observations. It seeks to assess whether the cores evolve in number and mass in tandem with their host clumps, and to determine the possible factors influencing the formation of massive cores (M > 24M_\odot). Methods. We analyzed the masses of 6348 fragments, estimated from 1.4 mm continuum data for 1007 ALMAGAL clumps. Leveraging this unprecedentedly large data set, we evaluated statistical relationships between clump parameters, estimated over about 0.1 pc scales, and fragment properties, corresponding to scales of a few 1000 au, while accounting for potential biases related to distance and observational resolution. Our results were further compared with predictions from numerical simulations. Results. The fragmentation level correlates preferentially with clump surface density, supporting a scenario of density-driven fragmentation, whereas it does not show any clear dependence on total clump mass. Both the mass of the most massive core and the core formation efficiency show a broad range and increase on average by an order of magnitude in the intervals spanned by evolutionary indicators such as clump dust temperature and the luminosity-to-mass ratio. This suggests that core growth continues throughout the clump evolution, favoring clump-fed over core-fed theoretical scenarios.

Figures (23)

• Figure 1: Example of ALMAGAL target observed from mid-infrared to millimeter. The ALMAGAL ID (AG323.7408-0.2633), its physical parameters mol25, and the number of cores detected in ALMAGAL continuum observations col25 are reported in the top-left corner of the figure. Panels $a$-$h$ contain the source images ($\sim 80\arcsec \times 45\arcsec$) at wavelengths increasing from 24 $\mu$m to 1.38 mm (the wavelength in $\mu$m is reported in the top-right corner of each panel). Specifically, in panel $a$ the Spitzer-MIPS image of the source (saturated in the center) is shown; in panels $b$-$c$ the Herschel-PACS images; in panels $d$-$f$ the Herschel-SPIRE images; in panel $g$ the ATLASGAL image; finally, in panel $h$ the new ALMAGAL image, in which 15 fragments were detected by col25. Angular resolutions in different panels are the following: $a$: 6; $b$: 10.2tra11; $c$: 13.5; $d$: 23; $e$: 30; $f$: 42; $g$: 19; $h$: 0.5. Being detected at 70 $\mu$m (and also at 24 $\mu$m, in this case), in accordance with the Hi-GAL catalog criteria this source is classified as star-forming.
• Figure 2: Distributions of ALMAGAL target physical properties: $a$) heliocentric distance; $c$) mass (common logarithm); $e$) surface density (common logarithm); $g$) bolometric luminosity over mass ratio (common logarithm); $i$) modified black body temperature $k$) bolometric temperature. Panels $b$, $d$, $f$, $h$, $j$, $l$ show the corresponding cumulative distributions. Three different populations are shown separately: in red, quiescent (70-$\mu$m dark) clumps; in blue, star-forming (70-$\mu$m bright) clumps; in cyan, star-forming clumps associated with an ultra-compact Hii region eli17eli21. All histograms are normalized by their total (116, 817, and 74 sources, respectively), therefore they do not mirror the actual number ratios between clump classes, and units on the $y$ axis are arbitrary. Additionally, in the panels containing the cumulative distributions, the red and dotted curves correspond to the subsamples of quiescent and star-forming clumps with no detection of mm cores in ALMAGAL observations (discussed in Sect. \ref{['zerocores']}); UCHii regions are not shown, as only one of them appears devoid of cores inside.
• Figure 3: Properties of ALMAGAL targets as a function of the number of cores ($N_\mathrm{core}$) revealed in their interior. Top: fluxes in the five Hi-GAL bands (the core-band correspondence is reported in the legend), and their averages in bins of $\Delta N_\mathrm{core}=1$. Middle: temperature determined using a modified-black-body fit to the Hi-GAL SEDs, and its average. Bottom: as in the top panel, but for the product of flux densities by the squared distance. Both in the top and bottom panels, a break in the red curve corresponds to a bin populated by sources without a 500 $\mu$m detection.
• Figure 4: Number of fragments $N_\textrm{core}$ detected in each ALMAGAL target as a function of the target's physical parameters: $a$) heliocentric distance, with the vertical dotted line separating the distances originally assigned to the "near" and the "far" sample, see text; $b$) mass; $c$) surface density (logarithm); $d$) luminosity over mass ratio, with the orange line representing the prediction of the model of leb25 with initial conditions set to $M=500~\mathrm{M}_\odot$, $\mathcal{M}=7$, and $\mu=10$ (presented and discussed in Sect. \ref{['simulations']}); $e$) modified black body temperature; $f$) bolometric temperature, respectively. Red open triangles are used for quiescent clumps, dark blue open diamonds for the star-forming ones, and light blue filled circles for counterparts of a UCHii region. The symbol under each panel label represents the typical error bar associated to data. In this case, the vertical error bar is replaced by an arrow, to indicate that the number of detected cores likely represents an underestimate of the "actual" $N_\textrm{core}$. The green line connects the medians of $N_\textrm{core}$ in bins (whose width is specified in green as well); its dotted parts correspond to bins containing low statistics (< 10 values). In particular, in panel $c$ the medians calculated in bins of surface densities based on deconvolved clump sizes is represented as a dark green solid/dotted line. The mean values are also shown, connected by a magenta line.
• Figure 5: Top: statistical distribution of the fragment number $N_\mathrm{core}$ for the ALMAGAL targets, categorized into quiescent (red histogram), star-forming (blue), and CORNISH-based UCHii regions (light blue). The sum of each histogram is normalized to 1, therefore all histograms subtend the same area and do not reflect the actual numerical proportions among the classes. The three vertical dashed lines indicate the medians of these distributions, calculated excluding cases where $N_\mathrm{core}$=0. Bottom: Cumulative distributions corresponding to the histograms shown in the top panel. The cumulative distribution of $N_\mathrm{core}$ for star-forming targets located within the regions covered by the CORNISH and CORNISH South surveys is also shown (dotted blue), to highlight the potential impact of contamination from UCHii regions (see text).